CHAPTER 2
p. 41
CHAPTER 2 CHASSIS AND ATTACHING SYSTEMS INTRODUCTION The chassis of a vehicle provides the strength necessary to support the vehicular components and the payload placed upon it. The suspension system, part of the chassis, contains the springs, shock absorbers, tires, and wheels. The steering mechanism is also part of the chassis and suspension, and it provides the operator with a means of controlling the direction of travel. The transmission, propeller shaft, universal joints, differential, axles, wheels, tires, brakes, and steering system are part of the chassis. The body or outer portion of the vehicle encloses the mechanical parts and passenger compartment, but is not considered part of the chassis. The components that make up the chassis of the vehicle are held together in proper relation to each other by the frame. NOTE: Basic Machines , NA VEDTRA 14037, has an entire chapter devoted to the power train. This chapter covers those components that are not adequately covered in Basic Machines , and some of the applications and different arrangements of power trains in support equipment. Most chassis repair jobs require work by mechanics under hoisted or jacked up vehicles. When new personnel report to your shop, never assume that they have been instructed in the safety precautions that must be observed when working under a vehicle. Personnel should be instructed in the use of jacks, safety stands, and other safety devices used in your shop. After a person has been pinned under a falling vehicle, it is too late to give safety instructions. FRAMES LEARNING OBJECTIVES : Identify the purpose of the frame. Describe the working relationship between the frame and the chassis. Identify procedures for inspecting, checking, and adjusting the frame. The frame is generally constructed of steel, and it is built rigid and strong so that it can withstand shocks, twists, vibrations, and other strains (fig. 2-1). The side members or rails are the heaviest parts of the frame. The crossmembers are attached to the side members strongly enough to prevent twisting of the frame. For added strength, angular pieces of metal (gusset plates) are riveted or welded at the points where members are joined. Frames normally require very little or no maintenance or service. Frames that have been bent, twisted, or broken, however, may be repaired if the damage is not too severe. Frame alignment can be checked in several ways. One way of checking the frame for forward alignment is by using frame gauges. Frame gauges (each gauge has a sight mounted on its center crossbar) are hung from the vehicle’s frame in three places. Frame alignment is checked by sighting from the front of the vehicle toward the rear. If the sights on the center of the gauges do not line up, the frame is out of alignment. If the frame is out of line, it is usually permissible to straighten it, provided the lack of alignment is not too great. You should consult the appropriate technical manual before you attempt to straighten the frame. When frame members have been broken or so badly distorted that they require replacement, new members can be installed with nuts and bolts or welded. The preferred method is to use hot rivets. When the front suspension crossmember has been damaged, you normally replace it. The front suspension crossmember is manufactured to such close tolerances that it is practically impossible to restore it to perfect alignment by straightening. If the front crossmember is not in line, the front wheels cannot be aligned, and the result is poor steering and rapid tire wear. For frame and chassis repairs that require welding and in-depth skin and rivet replacement, refer to the applicable equipment technical manuals. Other pertinent manuals include the Aeronautical and Support Equipment Welding Manual, NA V AIR 01-1A-34; General Manual for Structural Repair, NA V AIR 01-1A-1; andStructural Hardware, NA V AIR 01-1A-8. 2-1
p. 42
Q2-1. Before you attempt to repair a bent of broken frame, you should first consult which of the following resources? 1. The SE division chief 2. The airframes shop 3. The manufacturer of the equipment 4. The appropriate technical manuals Q2-2. What is the preferred method of installing a new member in a frame that has been badly distorted? 1. Join them together with nuts and bolts 2. Use hot rivets to join the two pieces 3. Spot weld the joint 4. Cold weld all joints on both sides AXLES LEARNING OBJECTIVES : Identify the categories of axles. Describe the relationship between the axle and the chassis. Although axles used on support equipment are of different sizes, shapes, and lengths, they are classified into two distinct categories—driving and nondriving (fig. 2-2). The driving axles, also known as live axles, transmit power from the transmission to the driving wheels of the vehicle. The driving wheels can be the front or rear of the vehicle. Nondriving axles, also 2-2 A. AUTOMOTIVE TYPE FRAME B. TOW TRACTOR FRAME ASf02001 Figure 2-1.—Typical frames. KNUCKLE ASf02002 I BEAM (NONDRIVING) (DRIVING) Figure 2-2.—Axles (driving and nondriving).
p. 43
known as dead axles, are mounted to the frame of the chassis and remain stationary. This type of axle may run the entire width of the vehicle or may be only a short axle mounted to the side of the frame or chassis. Some nondriving axles will have wheels mounted directly on the end of the axle, such as the rear wheels of a trailer. Others, such as the front wheels of a trailer, will have the wheels mounted on nonsteering knuckles, which, in turn, are mounted on the ends of the axle. Axles require very little maintenance. However, nondriving axles may require straightening or replacement after accidents or rough handling. Whenever a vehicle has a bent or damaged nondriving axle, the appropriate technical manual must be consulted for the repair limits and procedures. If trouble develops with a driving axle, the only solution is to replace it. Normally, the only trouble encountered with driving axles results from defective wheel bearings or a complete break of the axle. Driving axles are encased in a housing; therefore, they are isolated from the outside elements. Replacement of a driving axle requires removal of several parts from the axle housing. For semi-floating or full-floating axles, only the axle is unbolted, and it can be removed without removing the wheel assembly. As the parts are removed, you must take care not to further damage seals, threads, splines, or bearing surfaces. Never strike the end of an axle shaft to loosen it, as this may damage the bearings or differential. Most all of your axle work will result from scheduled maintenance requiring the removal, cleaning, and repacking of wheel bearings, as shown in figure 2-3. It is very important to properly pack wheel bearings because the grease must be forced between the rollers. If the grease is only smeared on the outside of the bearing, the bearing may overheat and destroy itself. Also, ensure that the bearings are completely dry before they are packed with lubricant. Water or condensation will cause the bearings to rust. WARNING Never spin a bearing by blowing compressed air across it. This may damage the bearing by causing it to seize up or fly apart, injuring the person holding the bearing. Q2-3. What are the two types of axles used on SE? 1. Moving and stationary 2. Internal and external 3. Driving and nondriving 4. Ridged and flexible 2-3 ASf02003A. B. Figure 2-3.—Packing wheel bearings.
p. 44
Q2-4. How are drive axles secured to an item of SE? 1. They are encased in a housing 2. They are bolted to the frame with U-bolts 3. They are welded to the frame 4. They are attached by the springs and shocks Q2-5. Ensuring grease is forced between the rollers of a wheel bearing is a very important step to prevent the bearing from overheating. 1. True 2. False SUSPENSION SYSTEMS LEARNING OBJECTIVES : Identify the normal components of a suspension system. Identify the springs used on suspension systems. Describe the shock absorbers used on a suspension system. The suspension systems for support equipment differ from most highway-driven vehicles. Most of the support equipment is not designed to travel over 15 to 20 miles per hour, and is only required to move short distances. The normal components of a suspension system are the springs and shock absorbers. However, some tow tractors have no rear springs, as the axles are bolted to the frame. Support equipment with suspension systems have either leaf or coil springs. SPRINGS Springs support the frame above the axle and the body of the vehicle, as well as the equipment mounted on the vehicle. They provide a flexible connection between the wheels and the frame and allow the vehicle to withstand the shocks of uneven surfaces. The best spring is one that absorbs shock rapidly and returns to its normal position slowly. Since the spring cannot perform this function alone, it is assisted by a shock absorber. Very flexible springs allow too much movement of the frame, while stiff springs do not allow enough movement. The springs do not actually support the weight of the wheels and axles. These parts make up the unsprung weight of the vehicle, which decreases the action of the springs. Therefore, the unsprung weight is kept to a minimum to permit the springs to support the vehicle frame and load. Coil springs (fig. 2-4) are used on most independent suspension systems because of low cost and maintenance. The main disadvantage of coil springs is that their frictionless action results in too much pitching of the vehicle. This pitching is dampened by the action of the shock absorber. Figure 2-5, view A, shows how a coil spring is mounted. The spring seat and hanger are shaped to fit the coil ends and hold the spring in place. Spacers made of rubberized fabric are placed at each end of the coil to prevent squeaking. The rubber bumper, mounted in the spring supporting member, prevents metal-to-metal contact when the spring is compressed. Coil spring systems require torque rods or a stabilizer shaft to prevent the axle from moving forward and back. Springs require very little maintenance; normally they are changed only when they fail. Leaf springs (fig. 2-5, view B) present no problem to change. The unit is raised until the tires are off the ground; then jack stands are placed under the frame. Next, a floor jack is placed under the axle, and it is raised until a no-load condition is obtained. Then, the bolts or pins holding the spring in place are removed, the spring is unbolted from the axle and removed, and the new springs are installed. Coil springs (fig. 2-5, view A) are replaced in about the same manner, but must also be compressed by a spring compressor prior to removal and installation. WARNING Consult the applicable technical manual before trying to repair a faulty coil spring—they can be very dangerous if removed incorrectly. SHOCK ABSORBERS Springs alone are not always satisfactory in a vehicle suspension system. A stiff spring gives a hard ride because it does not flex and rebound when the vehicle passes over a bump. On the other hand, a spring that is too flexible rebounds too much, and causes the vehicle to ride rough. Shock absorbers are used to smooth the riding qualities of the vehicle. They prevent excessive jolting of the vehicle by balancing spring stiffness and flexibility. They allow the springs to return to rest slowly after having been compressed. Double-acting shock absorbers (fig. 2-6) check spring compression as well as rebound. Most shock absorbers used at present are of the double-acting type because they permit the use of the more flexible springs. 2-4
p. 45
Most shock absorbers used today are hydraulically operated, and their operation is easy to understand. When the springs are flexed, liquid hydraulic fluid within the shock absorber is forced through a small opening by a piston. Since liquids cannot be compressed, the movement of the piston is controlled by the rate of flow of the hydraulic fluid through the opening. The tube shock absorber, as shown in figure 2-6, is used on automotive equipment and most types of support equipment requiring shocks. The tube shock absorber is a self-contained unit that cannot be repaired; it can only be replaced when it becomes inoperative. Shock absorbers are usually attached to the vehicle, as shown in figure 2-5, view A. Rubber mountings are used to fasten shock absorbers to the frame and axle to eliminate wear and noise. Most shocks can be replaced by removing the hardware that holds them in place. The hardware may be bolts at both ends of the shock absorber or, in some styles, the shafts are threaded and washers and nuts hold them in place. 2-5 LOWER CONTROL ARMS (LOWER WISHBONE) ASf02004 LOWER SPRING SEAT COMPRESSION BUMPER KING PIN REBOUND BUMPER UPPER SPRING SEAT UPPER CONTROL ARMS (UPPER WISHBONE) SHOCK ABSORBER FRONT CROSSMEMBER BRACE COIL SPRING STEERING KNUCKLE STEERING KNUCKLE SUPPORT STEERING KNUCKLE ARM DUAL TIE RODS STEERING GEAR PITMAN ARM GREASE FITTINGS STABILIZER ATTACHMENT TO SIDE RAIL (RUBBER-INSULATED) STABILIZER SHAFT GREASE FITTINGS SIDE RAIL STABILIZER SHAFT MOUNTING AT LOWER SPRING SEAT GREASE FITTINGS TOP MOUNT OF SHOCK FRONT CROSSMEMBER Figure 2-4.—Coil spring suspension.
p. 46
2-6 FRAME AXLE RUBBER BUMPER TORQUE ROD SHOCK ABSORBER COIL SPRING SPRING SEAT BUMPER SPRING HANGER BUSHING LUBE FITTING REBOUND CLIPS SPRING CENTER BOLT U-BOLTS LUBE FITTINGS SPRING SHACKLE SPRING HANGER BUSHING ASf02002 A. COIL SPRING B. LEAF SPRING Figure 2-5.—Coil and leaf springs. RING AND CUP ASSEMBLY ASf02006 CHECK VALVE SPRING RESERVOIR TUBE PISTON BAFFLE ROD GUIDE OIL SEAL GASKET SEAL RETAINER PISTON ROD OIL SEAL CAP OIL SEAL SPRING COMPRESSION RELIEF VALVE CHECK VALVE CHECK VALVE SEAT CYLINDER TUBE PISTON NUT DUST SHIELD PISTON ROD OIL SEAL Figure 2-6.—Double-acting shock absorbers.
p. 47
Q2-6. Because they cannot absorb shock rapidly and they return to their normal position slowly, what component is used to assist the springs of a suspension system? 1. Rubber mounts 2. Keeper mounts 3. Shock absorbers 4. Torsion bars Q2-7. The two types of springs used on SE are coil and leaf springs? 1. True 2. False Q2-8. Most shock absorbers are operated by which of the following methods? 1. They are pneumatically operated 2. They are hydraulically operated 3. They are electrically operated 4. They are battery operated STEERING LEARNING OBJECTIVES : Identify the parts and purpose of a steering assembly. Identify procedures for troubleshooting a steering assembly. Identify procedures for repairing or replacing a steering assembly. Identify procedures for maintenance of a steering assembly. Though steering may be a simple operation, the steering mechanism is rather complex. Figures 2-7 and 2-8 show diagrams of a steering mechanism. STEERING ASSEMBLIES All steering mechanisms have the same basic parts. The steering linkage ties the front wheels together and connects them to the steering gear case at the lower end of the steering column, which, in turn, connects the gear case to the steering wheel. The arms and rods of the steering linkage have ball, or ball and socket, ends to provide a swivel connection between them. These jointed ends are provided with grease fittings, dust seals, or boots. Many of them have end-play adjustment devices. These joints and devices must be adjusted and lubricated regularly. The arms, rods, and joints of steering linkages in your equipment may be arranged differently from those shown in figure 2-7. But, you will most likely find them in the same general location in the front or rear and underneath the vehicle. The tie rod (fig. 2-7), for example, is usually located behind the axle and keeps the front wheels in proper alignment. To provide for easier steering and maximum leverage, the tie rod may be separated into two lengths and connected to the steering gear near the center of the vehicle. The rod (drag link) connecting the steering arm and the pitman arm may be long or short, depending on the installation. The pitman arm (fig. 2-8) is splined to the shaft extending from the steering gear case and moves forward and backward—depending on which way the wheels are turned. It is approximately vertical when the front wheels are straight ahead. Therefore, the length of the connecting rod is determined by the distance between the steering arm and the vertical position of the pitman arm. Unlike the tie rods, the length of the connecting rod is not adjustable. 2-7 ASf02007 STEERING KNUCKLE PIVOTS STEERING KNUCKLE ARMS DRAG LINK (CONNECTING ROD) PITMAN ARM STEERING GEAR BOX STEERING COLUMN STEERING WHEEL TIE ROD Figure 2-7.—Diagram of a steering mechanism. ASf02008 KNUCKLE ARM FRAME FRONT CROSS MEMBER INTERMEDIATE KNUCKLE ARM DRAG LINK (CONNECTING ROD) PITMAN ARM STEERING GEAR BOX STEERING COLUMN Figure 2-8.—Steering assembly.
p. 48
The steering gear case contains the gears that control the movement of the pitman arm and steering linkage. Figure 2-9 shows a typical steering gear assembly, the location of the adjustment screw, and the filler plug for gear lubricant. The principal parts of any steering gear unit are the worm gear and sector. A sector is a portion of a full gear. The sector gear and worm provide a ratio between the number of turns of the steering wheel to one turn of the pitman arm. Because the pitman arm does not turn a complete revolution, only a section of one gear is used in the gear arrangement. The gear ratio of the steering column and the cross shaft that carries the pitman arm varies from 4 to 1 to 18 to 1 in automotive equipment. The high steering gear ratios are used in vehicles that are hard to steer or are slow moving. The low steering gear ratios are used in vehicles that are easy to turn. To provide easier and more efficient steering, roller and ball bearings have been added to the steering gear units. The design of the worm gear and sector has been changed, and even the names of these parts are different. In figure 2-9, the worm gear is called a cam and the sector is called a stud. These parts work on the same principle as a worm gear and sector. Another form of steering gear is the recirculating ball and nut gear (fig. 2-10). In this assembly the nut is mounted on a continuous row of balls on the worm to reduce friction. The ball nut is fitted with tubular ball guides to return the balls diagonally across the nut to recirculate them. As the nut moves up and down, the pitman arm turns, and the vehicle’s wheels turn with it. On equipment that is steered by the rear wheels, such as forklifts, the steering components and operation are the same except that a longer drag link is necessary to permit the driver to face forward. Because the steering column must be pointed forward to permit the operator to face forward, the distance from the gearbox and pitman arm to the steering knuckle arm is greater than on a front steering vehicle. Thus, a longer drag link is required. POWER STEERING Automotive power steering hydraulic systems consist of three units: the pump (including reservoir), the power cylinder or cylinders, and the control valve. The power steering pump is driven by the engine or an engine-driven accessory, by a belt, a shaft, or gears. Lines and hoses connect the three units, and all systems are constructed so that the vehicle can be steered manually, should the power steering system fail. One type of power steering system has the power cylinder and control valve built into the gearbox or case at the base of the steering column. As the steering wheel is turned to the right or left, the control valve directs hydraulic fluid pressure to one side or the other of a piston in the power cylinder that is connected to the 2-8 ASf02009 JACKET STEERING GEAR SHAFT VENT NUTS UPPER COVER SHIMS OIL FILLER PLUG CAM STUDS (TAPERED) NUT AND LOCK WASHER PITMAN ARM LOCK NUT ADJUSTING SCREW LEVER SHAFT Figure 2-9.—Steering gear unit of the cam and lever type.
p. 49
pitman shaft and arm. When the driver returns the steering wheel to the neutral or straight ahead position, the pressure on both the right and left turn sides of the piston is equalized, and the vehicle travels straight ahead. The pump is usually adjacent to a fluid reservoir where the excess hydraulic fluid is stored. Figure 2-11 is a diagram of this type of system. In the other type of power steering system, the control valve is on the pitman arm at the base of the steering column. The power cylinder is mounted on the tie rod between the wheels. In the system shown in figure 2-12, the power cylinder is double acting. When the steering wheel is turned, the pitman arm turns and routes fluid under pressure to one side of the cylinder, which assists in turning the wheels. When the steering wheel is neutral, equal pressure is applied to both sides of the power cylinder. 2-9 J H E K G F C D A B ASf02010 A. Lash adjuster screw locknut F. Pitman arm B. Lash adjuster screw G. Worm C. Worm bearing adjusting screw locknut H. Jacket D. Worm bearing adjusting screw J. Streeing gear shaft E. Recirculating balls K. Ball nut Figure 2-10.—Recirculating ball-type steering gear. POWER STEERING UNIT CONVENTIONAL STEERING LINKAGE HYDRAULIC PUMP AND RESERVOIR ASf02011 Figure 2-11.—Diagram of a power steering unit.
p. 50
STEERING SYSTEM MAINTENANCE Maintenance of the steering system consists of regular inspections, maintaining proper wheel alignment, lubrication, and occasional adjustment and replacement of parts to compensate for wear. The driver can usually sense steering and alignment troubles, as well as detect hard steering or play in the steering system. But, it is the responsibility of the AS to find the trouble and to remedy it. Some play in the steering wheel is normal and provides for easier steering of the vehicle. A large amount of play, however, means a freer movement of the steering wheel without a corresponding movement of the front wheels. Too much wheel play is caused by improper adjustment or wear of the steering linkage, steering knuckle plates, or loose wheel bearings. Hard steering may be caused by very tight adjustments, mechanical difficulties in the steering gear or linkages, not enough air in the tires, or improper wheel alignment. Sometimes the driver may say that the vehicle “wanders.” However, it may be that the driver tends to over-steer the vehicle. Nevertheless, the vehicle should be checked for low tire pressures, tight or loose wheel and brake adjustments, or improper front wheel alignment. “Pulling” of the vehicle when braking could be caused by grabbing brakes. If the vehicle “pulls” when driven, check for proper toe-in and toe-out in addition to the other causes already mentioned. Steering shocks, caused by sharp and rapid movements of the steering wheel, may be the result of driving over a rough surface or hitting objects on the surface. When the vehicle does not steer properly, it should be checked for sagging springs, defective shock absorbers, or looseness in the steering gear or linkage. Uneven tire inflation also could be the cause. Inspections Steering components, springs, and shock absorbers should be checked daily before operation of the vehicle. This preoperational inspection consists of a visual inspection for signs of lubricant leakage, corrosion of steering or suspension components, and loose parts, such as steering or spring components. The preoperational inspection is normally the responsibility of the operator. When inspecting the steering mechanism, you may need someone to assist you by turning the steering wheel back and forth through the free play while you check the linkage and connections. This will allow you to more easily determine if the steering gear is secured rigidly to the frame and no excessive looseness of the linkage or gearbox is present. A slight amount of free play may seem insignificant, but if allowed to remain, the free play will quickly increase and result in poor steering control. Wheel Alignment Steering control depends greatly upon the position of the wheels in relation to the rest of the vehicle and the surface over which it travels. Any changes from the specified setting of the wheels affect steering and the riding control of the vehicle. Therefore, the proper wheel alignment is important for vehicle control. 2-10 CHECK VALVE (IN CONTROL VALVE HOUSING) ASf02012 PUMP RESERVOIR POWER CYLINDER CONTROL VALVE Figure 2-12.—Power steering system.
p. 51
Front-end geometry is the term manufacturers use to describe steering and front wheel alignment. Front end geometry includes PIVOT INCLINATION, WHEEL CASTER, WHEEL CAMBER, TOE-IN, and TOE-OUT. These terms refer to angles in the front wheel alignment that may change because of driving over rough terrain, striking stationary objects, accident damage, and wear. PIVOT INCLINATION (sometimes called kingpin angle) is the number of degrees that the kingpin is tilted toward the center of the vehicle from a vertical position (fig. 2-13). Pivot inclination keeps the wheel spindles pointed outward and in line with the axle, and helps to make steering easier. WHEEL CASTER is the number of degrees that the steering knuckle is tilted to the rear, or to the front. Caster tends to keep the front wheels pointed straight ahead and brings them back to a straightforward position after a turn. The front wheel of a bicycle is castered and permits the rider to steer without using his hands. When the castered wheel of a bicycle is turned from the straight-ahead position by leaning sideways, the front end is slightly raised. After the turn is made, the weight of the bicycle forces the front end down and helps straighten the wheels. Caster in automotive vehicles with leaf-type springs is obtained by inserting wedges or shims between the front axle and the spring so that the steering knuckle pivots are tilted slightly backward from the vertical. However, most modern automotive vehicles do not have leaf-type springs. Vehicles without leaf-type springs use shims between the upper suspension arm and the frame to obtain the desired caster. If the knuckle pivots (kingpins) are tilted forward, the caster is said to be negative (fig. 2-14). The caster is said to be positive when the knuckle pivots are tilted backward. Most vehicles have positive caster, but some modern vehicles have negative caster. Caster is measured in degrees and varies from approximately 0.5° to approximately 3° on modern vehicles. The technical manual for the vehicle should list the correct settings. WHEEL CAMBER is the number of degrees that the wheels are tilted in or out at the top (fig. 2-13). Wheels having camber are closer together at the bottom than they are at the top. Camber, together with pivot inclination, reduces side thrust on the kingpin bearings in the steering knuckle and support, thus permitting easier steering and less wear of parts. Camber angle, in today’s vehicles, very seldom exceeds 1°, and is obtained by tilting the wheel spindles slightly downward on the steering knuckles. Camber brings the wheels perpendicular to the surface of the road, permitting better rolling contact. Wheels that have camber must also have toe-in and toe-out. NOTE: In modern vehicle design, greater pivot inclination reduces the need for excessive camber. TOE-IN is the number of inches that the front wheels point in toward the center of the vehicle (fig. 2-15). When forced to follow a straight path by motion of the vehicle, cambered wheels tend to slip away from each other. But toe-in wheels tend to travel toward each other and, therefore, balance the effect of camber. If your shop has floating turntables, use them to check steering errors. Run the vehicle up on the tables, and then turn the front wheels with the steering wheel. Each floating table turns with the wheel on it and registers the angle of the turn. When one wheel turns 20°, the other should turn about 23°, or to the specifications for your unit. Before checking front wheel alignment, be sure that the front tires are properly inflated and that 2-11 CAMBER ASf02013 PIVOT INCLINATION PIVOT AXIS KING PIN AXLE WHEEL AXIS WHEEL SPINDLE CENTER OF TIRE CONTACT STEERING KNUCKLE VERTICAL LINES TURNING RADIUS ASf02013 CAMBER Figure 2-13.—Pivot inclination and camber. PIVOT LINE NEGATIVE CASTER ANGLE VERTICAL POSITIVE CASTER ANGLE PIVOT LINE VERTICAL FORWARD FORWARD ROAD CONTACT POINTS ASf02014 Figure 2-14.—A. Negative caster; B. positive caster.
p. 52
steering knuckles and linkages, shock absorbers, and wheel bearings are correctly adjusted. If any of the above items cannot be adjusted back to proper specifications, then that part or assembly must be replaced. There are a number of devices that may be used for testing wheel alignment. One of the devices used for measuring toe-in is the measuring pole (fig. 2-16). Each pole has a pointer and a gauge on one end and can be lengthened and shortened like a curtain rod. With the vehicle resting on a level floor and the wheels in a straight-ahead position, push the vehicle forward a few feet to remove all play in the axle assembly. Put pencil marks on the inside walls of the tires at equal distances from the floor, and at both the front and rear of the tire. Place the pole between the two marks at the front of the tire, and set the pointer at zero. Then, use the pole to measure the distance between the two rear marks. The distance between the two rear marks should conform to the manufacturer’s specifications. If not, it is necessary to adjust the toe-in. TOE-OUT is the difference in the turning of the inner wheel, with the outer wheel turned at a 20-degree angle. Toe-out is necessary because of the different turning radii of the front wheels and the necessity for preventing slipping of the front wheels when turning. Power Steering Maintenance Since pressure builds up in the power steering system, the fittings, gaskets, and lines used in the system should be inspected frequently for leaks. When a fitting is found leaking, tighten or replace it. Gaskets and packing should be replaced when it is determined that they are leaking or faulty. The level of the fluid in the power steering system should be checked regularly and refilled as necessary. You should exercise care when adding fluid, ensuring no foreign matter enters 2-12 ASf02016 Figure 2-16.—Measuring pole. SPRING SEATS CENTER LINE OF WHEELS AXLE I-BEAM FRONT STEERING ARMS TIE ROD TIE ROD ENDS TOE-IN IS DISTANCE B-A MEASURED IN FRACTIONS OF AN INCH A B ASf02015 Figure 2-15.—Toe-in.
p. 53
the system to block the control valve and cause the system to malfunction. In addition to linkage maintenance, there are other items in power or hydraulically assisted steering systems that must be inspected for condition and operability (fig. 2-17). If the pump is belt driven, the first thing to check is the condition of the drive belt. Make sure that it is sufficiently tight to prevent slippage, yet loose enough to prevent damage to the shaft bearings. If in doubt as to the proper adjustment, check the manufacturer’s specification in the operator’s or maintenance manual. The fluid level in the reservoir should be checked at each maintenance cycle and kept above the add or low mark. Inspect the hoses and connections for signs of leakage. Loss of even a small amount of fluid can damage the pump, so if leaks are found, make repairs as soon as possible. The power steering system will absorb much of the looseness or slack that would be readily apparent 2-13 CONTROLLER PUMP ACTUATOR ASf02017 Figure 2-17.—Power steering system.
p. 54
in manual steering. Because of this, wear of the linkage will normally progress to a dangerous point before the operator typically becomes aware of the problem. Should hard steering occur, check the pressure of the steering pump to determine if that is the problem. Use a suitable pressure gauge to determine the pressure of the system. Follow the manufacturer’s procedure, and compare the results to the specifications in the technical manual. If within limits, adjust the pressure by altering spring tension in the relief valve. If the results are not within limits and cannot be adjusted, a faulty component is indicated, and it should be replaced. Other Maintenance Procedures Lubrication involves the greasing and oiling that is performed at the time the entire vehicle is greased. Figure 2-18 shows the tie rod end grease fitting. Consult the manufacturer’s service instructions for the proper lubricant to use. Depending on the steering arrangement, a drag link or idler arm rod may be used in the linkage to connect the pitman arm and the remainder of the steering linkage. This portion of the linkage is usually constructed so that to remove it from the vehicle, one or both ends must be disassembled or loosened. The adjusting plug (fig. 2-19) is used to remove any free play between the drag link and the connecting parts of the linkage. Occasionally, adjustment of the plug is needed to compensate for wear of the ball and seats or weakening of the spring. The tie rod is equipped with a ball-type socket at each end to allow for movement of the connecting parts of the steering linkage. These sockets, called TIE ROD ENDS, must be checked for wear or slack. The linkage should pivot at the ball socket without allowing free movement between the socket and ball. A slight drag is considered the optimum condition of the ball joints. In addition to the flexible end connections, the linkage is designed so that an adjustment can be made when performing wheel alignment. The tie rod is normally connected directly to the steering knuckle or spindle arm and used to transmit the steering effort to the wheel via the knuckle or spindle. You can check worn or improperly adjusted linkage connections by jacking up the front end of the vehicle, grasping each wheel (front and rear of the wheel), and moving the wheel in and out to check for excessive movement. At the same time, you can check 2-14 KNUCKLE ARM STUD DUST SEAL BALL SOCKET TIE-ROD END CLAMP BOLT ADJUSTING THREADS TIE ROD ASf02018 LUBRICANT FITTING Figure 2-18.—Tie rod end showing clamp bolt and adjusting threads. ASf02019 COTTER PIN ADJUSTING PLUG SPRING SAFETY PLUG DRAG LINK COTTER PIN ADJUSTING PLUG SPRING PLUG SPRING BALL SEATS LUBRICATION FITTINGS BALL SEATS Figure 2-19.—Drag link assembly—exploded view.
p. 55
for worn steering knuckle parts and loose wheel bearings by grasping the top and bottom of each wheel and shaking it to determine the amount of wobble. Test the steering gear by watching the pitman arm while someone turns the steering wheel. If considerable movement of the steering wheel is required to set the pitman arm in motion, the steering gear is either worn or out of adjustment and requires adjustment to the manufacturer’s specifications. Adjust the length of the tie rod to increase or decrease the toe-in. If the vehicle has two tie rods, it is necessary to adjust each equally. In figure 2-15, the right-hand tie rod is adjusted for the proper setting. Figure 2-18 shows a cross section of one of the ends. Before making any adjustments, count the number of exposed threads at the ends of the tie rod. One end of the tie rod has a right-hand thread, and the other end has a left-hand thread that screws into the fitting. Turning the rod in one direction so that more of these threads enter the fitting shortens the rod. Turning the rod in the opposite direction exposes more threads, and the rod becomes longer. Very little turning is required to change the length of the rod. To increase or decrease toe-in, loosen the clamp bolts and turn the rod in the direction that will give you the proper adjustment. Use a pipe wrench and make one turn at a time. If the tie rod is behind the axle, lengthening the tie rod increases toe-in, and shortening it decreases toe-in. If the tie rod is in front of the axle, then lengthening it decreases toe-in, and shortening it increases toe-in. Always tighten the clamp bolts after making an adjustment. WARNING While repairing or adjusting the steering system and the wheel alignment, be sure the vehicle is stationary. At least one wheel should be blocked on both sides, even if the equipment is on a level surface. Aboard ship a four to eight point tie down may be required Q2-9. Which of the following components keep the front wheels in proper alignment? 1. The pitman arm 2. The tie rod 3. The king pin 4. The connecting rod Q2-10. The pitman arm connects the steering gearbox to which of the following components? 1. The tie rod ends 2. The king pins 3. The axle 4. The connecting rod Q2-11. What are the principal parts of a steering gear unit? 1. The worm gear and sector 2. The worm gear and connecting rod 3. The sector and pitman arm 4. The sector and the Recirculating balls Q2-12. Which of the following conditions can cause excessive play in the steering system? 1. Improper adjustment of the steering gear unit 2. Improper adjustment of the steering linkage 3. Improper adjustment of the brakes 4. Improper wheel alignment Q2-13. Front-end geometry does NOT include which of the following terms? 1. Caster 2. Camber 3. Pinion tilt 4. Pivot inclination Q2-14. Which of the following statements best describes wheel caster? 1. The number of turns required to adjust the connecting rod one inch 2. The number of inches the front wheel points in towards the center of the unit 3. The number of degrees the wheel is pointed in or out at the top 4. The number of degrees the steering knuckle is tilted to the rear or front BRAKES LEARNING OBJECTIVES : Identify the types of brake systems. Identify procedures for inspecting, checking, testing, troubleshooting, and repairing brake systems. Describe the relationship between the brake system and the chassis. 2-15
p. 56
Efficient, reliable brakes are just as important to powered support equipment as the engine. They are required not only to stop the vehicle, but to stop it in as short a distance as possible, and then hold it in place after it is stopped. Because brakes are expected to decelerate a vehicle at a faster rate than the engine can accelerate it, they must control a greater power than that developed by the engine. The requirement for good brakes is not limited to powered equipment. Nonpowered equipment, such as oxygen- and nitrogen-servicing trailers, air-conditioning units, bomb trailers, and work stands, requires some type of braking system. While the primary purpose of the brakes on powered equipment is to stop the vehicle, their primary purpose on nonpowered equipment is to secure the equipment after it has been pushed or towed to the desired location. A brake absorbs mechanical energy by transferring it into heat through friction. Friction is the resistance to relative motion between two surfaces in contact with each other. Thus, when a stationary surface is forced into contact with a moving surface, the resistance to relative motion, or the rubbing action between the two surfaces slows down the moving surface. In nearly all brake systems, the brake drums provide the moving surface, and the brake shoes are the stationary surface. The friction between the brake drum and the brake shoe slows the wheel, and the friction between the tires and the road surface brings the vehicle to a complete stop. This braking action is accomplished through rods and cables in a mechanical brake system, a liquid coupling (brake fluid) in a hydraulic brake system, and air pressure in a pneumatic brake system. A combination of hydraulics and pneumatics (or vacuum) is used to operate the brakes of some equipment. Although electrical systems are used to operate some brake systems, they are not commonly used on support equipment. The AS is responsible for the maintenance of the brakes and brake systems on all support equipment. This includes servicing, inspecting, adjusting, and repair. To accomplish these tasks, you must have a thorough knowledge of the various types of brakes and brake systems and how they operate. TYPES OF BRAKES Powered support equipment is equipped with either two- or four-wheel brakes. In either case, individual brake assemblies are provided for each braking wheel and are operated by a foot pedal. Powered equipment also has an emergency or parking brake, which is operated by a separate pedal or lever. This may be a separate brake assembly, such as a transmission parking brake, or it may simply be a secondary method of controlling the wheel brake assemblies. The brake assemblies on nonpowered equipment are similar to those on powered equipment, but are usually provided only on two wheels. Drum Brakes Drum-type brake assemblies may be classified into two general types—external contracting and internal expanding. There are different designs of the internal expanding type, of which the conventional shoe is the most common type used on support equipment. The expander tube brake, another design of the internal expanding type, is used on some types of equipment. There are three types of drum brake assemblies—external contracting, internal expanding (shoes), and internal expanding (expander tube). EXTERNAL CONTRACTING BRAKES .— External contracting brakes (fig. 2-20) are rarely used for wheel brakes. However, they are often used as parking brakes. Figure 2-21 shows the external contracting brake used as a transmission parking brake. The brake drum is located at the point where the drive shaft is attached to the rear of the transmission. INTERNAL EXPANDING (SHOE) .—Internal expanding brakes are used almost exclusively as wheel brakes. This type permits a more compact and economical construction. The brake shoe and brake operating mechanism are supported on a backing plate or brake shield, which is attached to the axle flange in the case of nondriving axles or to the axle housing in the case of driving axles. The brake drum, attached to the rotating wheel, acts as a cover for the shoe and operating mechanism and furnishes a frictional surface for the brake shoes. Figures 2-22 and 2-23 show the arrangement of the brake shoe and operating mechanism of a typical wheel brake assembly. In operation, the brake shoe of an internal expanding brake is forced outward against the drum to 2-16
p. 57
produce the braking action. One end of the shoe is hinged to the backing plate by an anchor pin, while the other end is unattached and can be moved in its support with the operating mechanism. When force from the operating mechanism is applied to the unattached end of the shoe, the shoe is forced (expanded) against the drum and brakes the wheel. A retracting spring returns the shoe to the original position when braking action is no longer required. The brake-operating linkage alone does not provide sufficient mechanical advantage for positive braking. Some means of supplementing the physical application of the braking system must be used to increase pressure on the brake shoes. A self-energizing action is very helpful in accomplishing this, once setting of the shoes is started by physical effort. While there are variations of this action, it is always obtained by the shoes themselves, which tend to revolve with the revolving drum. Figure 2-24 illustrates the self-energizing action of a brake shoe. As shown, the drum is revolving counterclockwise. When the shoe is forced against the drum, it tends to rotate with the drum. As the initial braking pressure is increased on the cam, the wedging action increases and the shoe is forced more tightly against the drum to increase friction. This self-energizing action results in more braking action than could be obtained with the actuating force alone. Brakes making use of this self-energizing principle to increase pressure on the braking surface are known as servo brakes. (Servo is the action or device used to convert a small movement into a greater movement or force.) The operator controls the self-energizing action; that is, as the operator adds pressure to the brake pedal, the self-energizing action increases. Thus, it is most important that the operator control the total braking action at all times. The amount of self-energizing action available depends mainly upon the location of the anchor pin. This is determined by the manufacturer and is located at the point outward of the braking surface where the operator has the maximum control of the braking action. If the anchor pin is located too near the center of the drum, the shoe will automatically lock when the brakes are initially applied. When two shoes are anchored on the lower part of the brake shield, self-energizing action is effective on only one shoe. The other shoe tends to revolve away from its pivot, which reduces its braking action. When 2-17 ASf02020 BRAKE BAND BRAKE LEVER FULCRUM DRUM BRAKE LINING Figure 2-20.—External contracting brake. LINKAGE CONTROL LEVER BRAKE DRUM BRAKE BAND ASf02021 Figure 2-21.—External contracting transmission parking brake.
p. 58
the wheel is revolving in the opposite direction, the self-energizing action is produced on the opposite shoe. With the arrangement shown in figure 2-25, both shoes are positioned to develop self-energizing action with forward movement of the vehicle. For this reason, the arrangement is used mostly on the front wheels where maximum braking effort is required during normal forward movement. In reverse, this arrangement would not have any servo action. Two shoes are usually arranged so that self-energizing action is effective on both, regardless of the direction of drum rotation. This is accomplished by pivoting the shoes to each other and leaving the pivot free of the brake shield. The only physical effort required is for operating the first, or primary, shoe. Both shoes then apply additional pressure to the braking surfaces with no increase in the pressure on the operating linkage. The anchor pins are fitted into slots in the free ends of the brake shoes. 2-18 BRAKE SHOE CAM TO EXPAND BRAKE SHOES DRUM ANCHOR PINS BRAKE LINING ASf02022 Figure 2-22.—Internal expanding brake. 11 ASf02023 1 2 3 4 6 5 12 8 12 4 3 1 10 11 13 10 7 5 6 3 4 12 82 7 5 6 3 4 9 12 1 1. Brake shoe and lining 5. Pin spring 9. Anchor pin lock 2. Front brake shoe anchor plate assembly 6. Pin 10. Front brake shoe return spring 3. Front brake shoe link pin lock 7. Pin lock 11. Front brake shoe wheel cylinder 4. Front brake shoe link pin 8. Anchor pin 12. Front brake shoe link Figure 2-23.—Wheel brake assembled and exploded view.
p. 59
This method of anchoring allows the movement of the shoes necessary to expand against the drum when the shoes are forced against the drum, and the self-energizing action of the primary shoe is transmitted through the pivot to the secondary shoe (fig. 2-26). Both shoes tend to revolve with the drum and wedge against the drum through the one anchor pin. The other anchor pin causes a similar action when the wheel is revolving in the opposite direction. Disc Brakes The disc brake assembly (fig. 2-27) consists of a metal disc and caliper assembly. The disc, which is bolted to the wheel hub, rotates with the wheel of the vehicle. The caliper assembly, which remains stationary, attaches to the steering knuckle on the front wheels or to the rear axle housing on the back wheels. The disc is usually solid when used on a lightweight vehicle and vented (for cooling) on a heavy vehicle. Both sides of the disc are machined to provide a smooth friction surface. The caliper contains one or more hydraulic pistons, which cause the brake shoes (one on either side of the disc) to squeeze the disc in a viselike manner. Figure 2-28 illustrates the operation of a typical disc brake assembly. In actual application, the brake shoes (friction pads) are held in light contact with the disc when the brakes are released by a piston return spring. Some disc brake shoes have telltale tabs that contact the disc when lining wear has reached a predetermined point. This results in a scraping noise when the vehicle is operated, warning the operator that the brake shoes are badly worn and should be replaced. Disc brakes require no adjustment. However, you will occasionally have to add brake fluid to the master cylinder reservoir, which supplies fluid for the disc portion of the braking system. This is necessary because the piston return springs, by keeping the shoes against the disc as wear occurs, create a larger cavity for the fluid in the caliper assembly. A booster assembly is often used in disc brake systems, as they have no self-energizing feature. Some features of the disc brake make it more desirable than the drum brake; namely, braking action is instantaneous when pressure is applied to the caliper assembly, fading caused by the heat is eliminated, and the brakes are not affected when water is splashed onto the disc and linings. SERVICING DISC BRAKES .—Anytime a vehicle with disc brakes is scheduled for maintenance, the disc should be inspected for scoring and hard spots. Slight scoring results from normal braking. A disc that is scored less than 0.015 inches can be used without machining if the overall thickness of the disc is still within the manufacturer’s specifications. Heavy scoring or hard spots require the disc to be machined. The rust ridges that build up as wear occurs are of no concern unless new shoes are to be installed. Placing new shoes on a disc that has rust ridges causes the shoes to seat on the ridge, resulting in poor braking. These ridges should be removed by grinding or machining prior to installing new shoes. A special lathe is normally used for machining discs. However, if 2-19 ASf02024 DIRECTION OF DRUM ROTATION CAM SHOE ANCHOR PIN DRUM Figure 2-24.—Brake shoe self-energizing action. PISTON SHOE RETRACTING SPRING ANCHOR PIN PISTON WHEEL CYLINDER WHEEL CYLINDER STEM ASf02025 SHOE BRAKE LINE ANCHOR PIN WHEEL CYLINDER Figure 2-25.—Front wheel brake assembly.
p. 60
none is available, the metal lathe in the machine shop and a grinding attachment will do the job. A test should be made when shoes are replaced to determine if the disc has excessive runout (out of round) or thickness variation. Either condition will cause erratic braking similar to that caused by a warped drum on conventional brakes. Runout or wobble of the disc as it rotates must be checked with a dial indicator. Thickness variation is determined by measuring the thickness of the disc in at least three places approximately 1 inch from the outer edge of the disc. Should either of these tests indicate a faulty disc, the disc must be machined. If it is worn excessively, it must be replaced. REPLACING DISC BRAKE LININGS .—Disc brakes have flat linings bonded to a metal plate or pad (shoe). The pad is not rigidly mounted inside the caliper assembly, thus it is said to float. The pads are held in position by retainers or internal depressions (pockets machined into the caliper). Figure 2-29 shows a disc brake assembly. To remove brake pads, raise the front of the vehicle and remove the wheels. Next, remove approximately two-thirds of the fluid from the master cylinder and 2-20 SLOTS CAM ANCHOR PINS DRUM PRIMARY SHOE PIVOT SECONDARY SHOE ASf02026 Figure 2-26.—Primary and secondary brake shoes—self-energizing action. CALIPER SHOE AND LINING ANTI-RATTLE SPRING DISK PISTON SPLASH SHIELD ASf02027 Figure 2-27.—Disc brake assembly. CONNECTING TUBE FRICTION PADS CYLINDER PISTONS BRAKE DISK HYDRAULIC PRESSURE FROM MASTER CYLINDER APPLIED POSITION ASf02028 RELEASED POSITION Figure 2-28.—Sectional view of a disc brake in released and applied position.
p. 61
discard. Do not remove all of the fluid or bleeding of the system will be required on reassembly. Then, remove the shoes following the manufacturer’s procedure. The procedure may vary depending on the manufacturer and model. Some models allow removal of the shoes with the caliper mounted on the vehicle, while other designs require removal of the caliper before the shoes can be extracted. In either case, the pistons should be bottomed in the bores of the caliper assembly (fully pressed into the caliper) to release any tension on the shoes. This allows clearance for any rust buildup (ridge) at the outer edge of the disc. NOTE: The rust buildup near the edge of the disc may have to be removed for the new shoes to seat properly on the disc. The next step is to remove the old shoes and insert the new shoes. Remount the caliper or replace the shoe retainers, torquing all bolts according to the manufacturer’s specifications. Refill the master cylinder and apply the brakes a few times. Then, check for any leaks at the pistons in the caliper. If there are no leaks, replace the wheels and lower the vehicle. A road test should be made to insure that the brakes are working properly and also to seat the new shoes on the disc. Several (3 or 4) heavy brake applications at approximately 3 to 5 miles per hour will work. If the brakes function normally after the braking test, the job is complete. SERVICING CALIPER ASSEMBLIES .— Servicing disc brake caliper assemblies usually 2-21 ASf02029 BRAKE LINING (PAD) BRAKESHOE INNER CALIPER HOUSING SPLASH SHIELD VENTILATING LOUVERS NUT LOCK COTTER PIN BEARING HUB OUTER CALIPER HOUSING BRAKING DISC TYPICAL DISC BRAKE ASSEMBLY PISTON SEAL STRETCHED PISTON CYLINDER BORE BRAKES APPLIED DUST BOOT CALIPER HOUSING PISTON SEAL RELAXED NONVENTILATED (SOLID) DISC ROTOR VENTILATED DISC OR ROTOR BRAKE PLATE (SHOE) AND LINING ASSEMBLY BRAKE PLATE AND LINING (PAD) ASSEMBLY BRAKE PAD (LINING) WEAR INDICATOR 0.005 BRAKES RELEASED PISTON BEING RETRACTED BRAKES APPLIED Figure 2-29.—Disc brake assembly.
p. 62
involves the replacement of pistons, seals, and dust boots. To perform this type of service, it is necessary to remove the caliper assembly from the vehicle. The procedures for this type of service will be listed in the manufacturer’s maintenance manual. Internal Expanding (Expander Tube) Brakes The expander tube brake is used on some types of support equipment. For example, the A/S32K-1C/1D weapon loaders are equipped with expander tube brakes. The technical manual frequently refers to this brake as the aircraft-type expander tube brake because the expander tube brake was designed originally for aircraft. Several models of naval aircraft are equipped with brakes of this type. An exploded view of an expander tube brake is shown in view A of figure 2-30. View B shows the brake completely assembled, and view C shows a cross-sectional view of the assembled brake. The main parts of the brake are the spider, frames, expander tube, brake blocks, and return springs. The spider, sometimes referred to as a flange or torque flange, is the basic unit around which the brake is built. The main part of this spider is secured to the wheel support. The detachable metal frames form a groove around the outer circumference into which the expander tube, brake blocks, springs, and so on, are fitted. The expander tube is made of neoprene reinforced with fabric, and has a metal nozzle through which hydraulic fluid enters and leaves the tube. The brake blocks are made of materials quite similar to that used in the linings of shoe-type brakes. The actual braking surface is strengthened by a metal backing plate. The blocks are held in place around the spider and are prevented from rotating by the torque bars, which are secured to the frames. The size of the brake assembly varies with different types of equipment. As the size of the assembly changes, the number of blocks per assembly changes. The brake return springs are semi-elliptical or half-moon in shape. One is fitted between each separation in the brake blocks. The ends of the springs are designed to fit into slots in the brake frames. The bowed center section of the spring pushes inward, holding the blocks firmly against the expander tube (fig. 2-30, view C). This prevents the blocks from dragging against the drum when the brake is released. The expander tube brake assembly is hydraulically operated and may be used with any of the conventional hydraulic brake systems. When the brake pedal is applied, the fluid is forced into the expander tube. The spider and frames prevent expansion inward or to the sides. Thus, the pressure of the fluid in the tube overcomes spring tension and forces the blocks radially outward against the brake drum, creating friction. The tube shields prevent the expander tube from extruding between the blocks, and the torque bars prevent the blocks from rotating with the drum. Friction created by the brake is directly proportional to brake line pressure. When the brake pedal is released, the return springs return the blocks inward, compressing the expander tube and forcing the fluid back to the brake control unit. BRAKE LININGS There are several important qualities that are desired of the material for brake linings. First, of course, it must have high frictional qualities. Second, the material must have the ability to withstand high temperatures. Since there is a lack of cooling facilities around the brake assembly, it is necessary that the brake lining be able to dissipate the heat rapidly. In addition, the material must be durable and moisture-resistant. The two main types of brake linings are organic and metallic. One form of organic lining is woven material composed of asbestos fiber, cotton fibers, and copper or bronze wire. It is treated with a mineral-based chemical to resist the effects of oil and water. It is then pressed and undergoes a baking process that compresses the fibers into a dense material, helping the lining to resist the effects of heat from friction. This material has a very high frictional quality but a low rate of heat transfer or dissipation. Also, it is severely affected by oil, even after treatment. Therefore, the woven material is used mostly on transmission brakes. Another form of the organic type is the molded lining. This lining is made of a dense, hard, compact material, and is cut into various forms to fit different types of shoes. Generally, the compact materials are resins and mineral fibers, mixed as a semi-liquid, molded, pressed, and baked. The result is an extremely dense material. Very often copper or bronze wires are added to the mixture. 2-22
p. 63
The frictional qualities of this type of lining are lower because of the smooth surface. However, this type of lining dissipates heat rapidly and wears longer than the woven type. As a result, the molded lining is more suitable for application on the wheel brakes than are the woven types. Because of the density of the molded material, it is less affected by oil and water. Metallic brake linings are made of finely powdered iron, copper, or graphite, and lesser amounts of inorganic fillers and friction modifiers. The mixture is put through a molded block process and compressed and baked into the desired form. Metallic brake linings are used when extreme braking conditions exist. The frictional characteristics of metallic linings are more constant that those of organic linings. 2-23 ASf02030 2 3 4 5 6 7 8 9 10 11 3 34975 10 1 23 5 6 11 VIEW A VIEW B VIEW C 1 1. Brake frame bolt 4. Expander tube 7. Return spring 10. Inlet 2. Torque bar bolt 5. Brake block 8. Return spring shield 11. Spider 3. Frame 6. Torque bar 9. Tube shield Figure 2-30.—Expander tube brake. A. exploded view; B. assembled view; C. cross-sectional view.
p. 64
The lining may be secured to the shoes by riveting or by a bonding process in which the linings are glued to the shoe. In the bonding process, pressure and heat are applied to make a secure bond between the lining and the shoe. The bonding process allows the lining to be worn comparatively thin without danger of cutting or scarring the drum. When brass rivets are used to secure the lining to the shoe, the lining must be replaced when it is worn to a specified amount to keep the rivets from scarring the drum. WARNING Many types of linings contain asbestos, and dust from normal wear may accumulate in the brake drum assembly. For this reason, special procedures are followed, including the wearing of a suitable respirator, when working on asbestos brakes. Do not use compressed air to blow out the drum or brake assembly, and use a suitable solvent for cleaning parts. Remember that asbestos dust is treated as hazardous waste. BRAKE DRUMS Brake drums are made of pressed steel, cast iron, or a combination of the two metals. Cast iron drums dissipate the heat generated by friction more rapidly than steel drums, and have a higher coefficient of friction with any particular brake lining. However, cast iron drums are heavier and are not as strong as steel. Quite often, steel drums have a cast iron liner fused into them to provide the necessary strength and heat-dissipating qualities (fig. 2-31). To further add to the strength and heat dissipation qualities, cooling ribs are sometimes used on the outside of the drums. BRAKE ADJUSTMENTS Due to normal wear of the brake linings, brake assemblies (except for disc brakes) require periodic adjustment. Normal wear can result in excessive clearance between the lining and drum and can lead to poor and uneven braking. One possible outcome of poorly adjusted brakes is that during hard braking, one wheel may lock before the others are stopped, causing the driver to lose steering control of the vehicle. It is important, therefore, that brake adjustments be made to provide equal distribution of brake action to all wheels. When a brake is correctly adjusted, the lining attached to the brake shoes fits evenly against the brake drums when the brakes are applied. Also, the lining will be free of the drum when the brakes are released. The linings must not drag against the drum, yet they must be near enough to give the proper leverage between the operating mechanism and the friction surfaces. The backing plate of some brake assemblies contains small slots for the purpose of checking these clearances with a feeler gauge. Most automobiles and some support equipment have a mechanism that automatically adjusts the brakes. The adjustment takes place when the brakes are applied as the vehicle is moving backward. As the brakes are applied, friction between the primary shoe and the brake drum forces the primary shoe against the anchor pin (fig. 2-32). Then, hydraulic pressure from the wheel cylinder forces the upper end of the secondary shoe away from the anchor pin and downward. This causes the adjuster lever to pivot on the secondary shoe so that the lower end of the lever is forced against the sprocket on the adjuster screw. If the brake shoes have worn enough, the adjuster screw turns a full tooth. This spreads the lower ends of the brake shoes a few thousandths of an inch, or enough to compensate for the shoe wear. Some brake assemblies and even autoadjusting brakes require first time adjustment in the same manner as nonself-adjusting brakes. Many have a slot in the rear of the backing plate to allow a thin blade tool to turn the star wheel. Brakes with this type of adjustment require you to turn the star wheel until the 2-24 ASf02031 COOLING RIBS CAST IRON LINER STEEL Figure 2-31.—Sectional view of a brake drum.
p. 65
wheel is locked by the brake shoes, and then back off the star wheel a certain number of notches or until no dragging noise is heard between the drum and the shoes. NOTE: Unless thoroughly familiar with the type of brakes to be adjusted, you should carefully follow the procedures outlined in the manufacturer’s service manual. WARNING When working on brakes it is usually necessary to jack up the vehicle. When doing so, be sure to observe these precautions: (1) Place the proper rated jack stands under the vehicle when it is raised. (2) Never work under a unit using a jack only. (3) Before using a hydraulic jack, make sure it is filled with fluid and has no apparent leaks. (4) Aboard ship, tie or chain the unit down while it is on jack stands. BRAKE INSPECTION AND MAINTENANCE Frequent brake inspections are necessary to ensure safe operating conditions. Brake inspections are not made just to comply with regulations but to ensure safety of personnel and equipment. Defective brakes are a contributing factor to many accidents that might have been avoided with frequent and thorough brake inspections. Like most components of support equipment, brake assemblies require both operational and visual inspections. The interval of these inspections is usually specified in the applicable instruction manual and maintenance requirements cards (MRCs). An operational check is usually conducted daily. On nonpowered equipment, this can be accomplished by attempting to move the vehicle with the brakes set. Similarly, emergency or parking brakes of powered equipment can be checked by applying power to the wheels with the brakes set. A road test is usually conducted for the operational check of the wheel brakes of powered equipment. The brakes must stop a moving vehicle in a reasonable distance. After the vehicle is stopped, the brake marks on the roadway should be inspected to see if there is an indication of any one wheel braking more than the others. If the brakes do not stop the vehicle within the prescribed distance, are not equalized, grab, or do not hold, the necessary adjustments or repairs must be made. The wheel brake assembly must be visually inspected at specified intervals. To do this, you must remove the wheel and brake drum. Clean the dust and crud out of the assembly, taking care not to breathe the asbestos dust. You should use an asbestos vacuum cleaner or a wet spray-type cleaner and respirator. Ensure you meet all EPA, LOCAL COMMAND, and NA VOSH REQUIREMENTS prior to doing brake work. The brake assembly should be checked for loose or broken brake shoe retracting springs, worn clevis and cotter pins in the brake operating mechanism, and indications of grease or oil leaks at the wheel bearing 2-25 ASf02032 RETURN SPRING PRIMARY SHOE WHEEL CYL. BOLT LINK ADJUSTER (UPPER) SPRING, AUTOMATIC ADJUSTER NUT, PIVOT R.H. ADJUSTER LEVERNUT, PIVOT L.H. SECONDARY SHOE RETURN SPRING Figure 2-32.—Brake assembly, self-adjusting.
p. 66
grease retainer. All parts of the self-adjusting brake systems must be free of rust and corrosion for them to work properly. While checking springs, clevis and cotter pins, you should clean the star adjustor threads with a hand-held wire brush and apply a thin film of oil to ensure proper operation. In the case of hydraulic brake systems, the hydraulic brake cylinder should be checked at this time. The condition of the brake linings and brake drum must be checked thoroughly at this time. Brake linings that are riveted to the brake shoe should be replaced if worn to less than 40 percent of their original thickness. Also, the shoes must be replaced if a rivet head is riding or is about to make contact with the brake drum. If the lining is bonded to the shoes, the remaining lining must be at least 1/16-inch thick. Brake linings that pass inspection for wear, must be securely fastened to the brake shoes and free from grease and oil. Small grease or oil spots can be removed from the lining with an approved cleaning fluid. Do not use a petroleum-based cleaner, as it leaves an oily residue on linings. Most shops recommend dry cleaner solvent. If the lining is saturated with grease or oil, it must be replaced. The source of grease and oil on the lining must be located and remedied. WARNING When using cleaners, you must wear safety glasses for eye protection. If the brake lining needs replacing, the brake shoes must be removed. Before the shoes are removed, the front and rear shoes should be marked so that they will be replaced in their original position. The brake shoes may be removed by first removing the brake shoe return spring and the anchor pin locks. (See fig. 2-23.) For good braking action, brake drums should be perfectly round and have a uniform surface. Excessive pressure exerted by the brake shoes and heat developed by their application often cause the brake drums to become out of round. Drums should be inspected for distortion, cracks, scores, roughness, and excessive glaze, which lowers braking efficiency. Light score marks can be removed with emery cloth. When the surface becomes badly scored or out of round, it may be reground in a lathe to a true and smooth surface. Excessive grinding will remove too much material from the drum, leaving it too thin and weak. (All drums are stamped with the maximum allowable area to be removed.) When this occurs, the drum must be replaced. Also, a drum that is cracked or badly distorted should be replaced. BRAKE SYSTEMS Although brake assemblies and brake drums are similar on all equipment, the operating mechanisms may differ greatly. The two most common types of brake systems used on support equipment are mechanical and hydraulic. Mechanical Brake Systems Brakes operated entirely by mechanical linkages from the brake pedal or lever are called mechanical brakes. The mechanical linkages may consist of levers, rods, cables, or any combination of these. APPLICATION.—Equalizing brakes, so that all brakes will be applied at the same time, has always been a problem of mechanical brake systems, particularly on vehicles with brakes on all four wheels. For this reason, mechanical brake systems are seldom used for wheel brakes on powered support equipment. They are used, however, for transmission or parking brakes on these vehicles. In fact, most external contracting brakes are operated by mechanical systems. Mechanical brakes will stay set better than hydraulic brakes, so they are very good for holding a parked vehicle. In regard to support equipment, the major use of mechanical brake systems is on nonpowered equipment. Almost all items of nonpowered equipment that require brakes are equipped with some form of mechanical brake system. COMPONENTS AND OPERATION .—Figure 2-33 illustrates a typical mechanical system for the operation of parking or emergency brakes. This is simply a secondary means of controlling the rear wheel brakes. Variations of this arrangement are used to operate the parking brakes on some powered support equipment. A cable assembly is the main working element of this system. A single cable connects the control lever to the intermediate lever and spreader. From here, two cables complete the connection to the two rear wheel brake assemblies. Large portions of the cable slide within a flexible conduit. This conduit not only serves 2-26
p. 67
as a protective housing for the cable but also provides a means for securing the cable to the frame with clamps. When the brake lever is pulled, the cables move to operate levers in the two rear-wheel mechanisms. The levers, as they operate, force the brake shoes apart and in contact with the drum. The intermediate lever and spreader equalize the tension of the two cables to the brakes. This action, in turn, tends to equalize the braking action of the two brakes. When the hand lever is moved in the opposite direction, the cable moves and releases the brakes. Transmission parking brakes (fig. 2-21) are usually operated by a similar cable mechanism. However, only a single cable, such as the one labeled “front cable and conduit” in figure 2-33, is necessary for this operation. Cable assemblies are also used to operate the brakes on some nonpowered support equipment. However, many of this type of equipment are provided with mechanical brake systems similar to the arrangement shown in figure 2-34. To apply the brakes, the brake lever is moved to the right, which, in turn, moves the front rod forward. This rotates the cross shaft clockwise, moving the rear rods forward. This action rotates a cam in each brake assembly (fig. 2-22), forcing the brake shoes apart and in contact with the drum. The brakes are released when the hand lever is moved in the opposite direction. There are many variations of this arrangement found on support equipment. Cables are sometimes used instead of the front and rear rods. On some equipment the hand lever and cross shaft may be located directly over the wheels. The control device on some equipment may be a foot pedal rather than a hand lever. On some models of towed equipment, the control linkage for the brakes is attached to the tow bar. With the tow bar in the horizontal position, the brakes are released. When the tow bar is placed in the vertical position, the linkage moves and applies the brakes. These models are usually equipped with a hand lever or foot pedal to apply the brakes when the tow bar is in the horizontal position. INSPECTION AND MAINTENANCE .— Mechanical brake systems require very little maintenance; however, they must be inspected periodically. Inspections include an operational check of the control device and visual inspection of the mechanical linkage. You should pay special attention to such possible problems as frayed cables, bent rods and shafts, and loose fittings and clamps. The entire 2-27 ASf02033 HAND BRAKE LEVER IMMEDIATE LEVER AND SPREADER REAR CABLE AND CONDUIT FRONT CABLE AN CONDUIT CLAMP Figure 2-33.—Mechanical brake system—cables. ASf02034 BRAKE HAND LEVER FULCRUM FRONT ROD CROSS SHAFT REAR RODS Figure 2-34.—Mechanical brake system—rods and shaft.
p. 68
system should be inspected for signs of rust and corrosion. Maintenance includes replacement of defective parts, servicing with required lubricants, securing loose clamps and fittings, and straightening bent rods and shafts. Upon replacement of any component, the brakes require adjustment to ensure both brakes are applied equally, that they hold the unit in place when the brakes are applied, and that they release fully when the brake level is in the released position. Hydraulic Brake Systems A hydraulic brake system is primarily a liquid connection or coupling between the brake pedal and the individual brake assemblies. APPLICATION.—The effectiveness and reliability of hydraulic brake systems have been proven through their extensive use on automobiles for over 50 years. As a result, hydraulic systems are the most common systems used for the operation of the wheel brakes on powered support equipment. The hydraulic brake systems on some late model equipment are provided with a vacuum or air booster. The latest hydraulic brake systems used on support equipment tractors are true high-pressure hydraulic systems. These systems are powered by engine-driven hydraulic pumps. COMPONENTS AND OPERATION .—Figure 2-35 shows a typical arrangement of a hydraulic brake system. The system consists of a master cylinder connected by tubing and flexible hose to the wheel cylinders. The master cylinder serves as a fluid reservoir, the system pump, and the control valve. The wheel cylinders are the actuators. (An actuator is a device that transforms fluid pressure into mechanical force to move a mechanism.) A nonpetroleum-based fluid is normally used in the brake systems of support equipment. The applicable technical manual should be consulted to ensure that the correct brake fluid is used. Figure 2-36 shows the operation of a hydraulic brake system. Depressing the brake pedal moves the piston within the master cylinder, thus developing fluid pressure. This buildup of pressure forces fluid from the master cylinder into the fluid lines and the wheel cylinders. Like all liquids, brake fluid for all practical purposes is noncompressible. Therefore, the pressure originated in the master cylinder is transmitted equally to the wheel cylinders. This pressure, applied at the wheel cylinders, causes the pistons to move outward. 2-28 ASf02036 BRAKE PEDAL WHEEL CYLINDERS WHEEL CYLINDER PISTONSMASTER CYLINDER MASTER CYLINDER PISTON FORCE Figure 2-36.—Operation of hydraulic brake system. ASf02035 FRONT WHEEL CYLINDER -- RIGHT FRONT WHEEL BRAKE HOSE FRONT AXLE BRAKE HOSE BRAKE TUBE FRONT AXLE TEE REAR WHEEL CYLINDER -- RIGHT BRAKE TUBE REAR AXLE TEE REAR AXLE BRAKE HOSEREAR WHEEL CYLINDER -- LEFTFRONT WHEEL CYLINDER -- LEFT MASTER CYLINDER BRAKE PEDAL SIGNAL LAMP SWITCH FRONT WHEEL BRAKE HOSE TUBE UNION Figure 2-35.—Typical hydraulic brake system.
p. 69
This pressure overcomes the tension of the retracting springs and forces the shoes against the drums. As the pressure on the foot pedal is increased, greater pressure is built up within the wheel cylinders and, consequently, greater force is exerted against the shoes. When the pressure on the pedal is released, the brake shoe retracting springs return the brake shoes to their normal or released position. The return movement of the brake shoes, in turn, causes movement of the wheel cylinder pistons toward their released position since the force from the master cylinder is removed. The displaced fluid returns to the reservoir. Master Cylinder .—A cutaway view of a typical master cylinder is shown in figure 2-37. The unit consists basically of an iron casting that contains a reservoir for the fluid and a machine-finished precision cylinder that houses the piston, return spring, and two-way valve assembly. There are two ports leading from the reservoir (supply tank) to the cylinder—the breather port and the compensating port. Both ports serve to furnish fluid to the cylinder for the braking stroke. In addition, the breather port allows the fluid on the rod side of the piston to escape to the reservoir during the release stroke. This prevents a fluid lock when the brakes are released. The compensating port allows fluid to flow to and from the reservoir to allow for thermal expansion and contraction. Thus, a constant volume is maintained in the system at all times. The reservoir is sealed at the top with a combination filler and breather cap, which permits atmospheric pressure on the fluid at all times. The piston is a spool-like member with a rubber or leather cup seal at either end. These seals are referred to as the primary and secondary cups. The primary cup is held against the piston by the return spring and acts against the brake fluid during the braking stroke. The secondary cup is located at the other end of the piston and prevents external leakage of the brake fluid. An explanation of the two-way valve can be made with reference to figure 2-38. As pressure is applied to the foot pedal and the piston is forced into the cylinder, the fluid pressure is applied to the inner and outer segments of the two-way valve. The spring, acting against the inner segment, is comparatively light. As the fluid pressure opens this valve, additional fluid enters the hydraulic lines, applying pressure to the pistons in the wheel cylinders. As the brake pedal is released, fluid pressure in the wheel cylinders decreases. The brake shoe retracting springs, acting against the wheel cylinder pistons, cause a slight back pressure in the fluid lines. This pressure overcomes the tension of the return spring and forces the entire two-way valve assembly off its seat. This allows some of the fluid from the lines to enter the reservoir. When the pressure in the lines decreases to approximately 6 to 16 psi, the return spring closes the valve unit against its seat. As a result the system remains under a small pressure. This pressure does not cause the shoes to drag, but it does assure a positive 2-29 ASf02037 PISTON STOP RETAINER SPRING LINK BOOT SECONDARY CUP PISTON PRIMARY CUP RETURN SPRING CHECK VALVE VALVE SEAT SUPPLY TANK FILLER CAP COMPENSATING PORTBREATHER PORT Figure 2-37.—Hydraulic brake master cylinder.
p. 70
seal at the wheel cylinder cup packing and prevents air from entering the system. Wheel Cylinder.—The wheel cylinder may be of almost any design or exterior shape to suit the need, but all wheel cylinders work on the same basic principle and fulfill the requirements of moving the brake shoes into contact with the drum. There are two basic designs—a single-piston type and a double-piston type, sometimes called uniservo and duoservo cylinders, respectively. Different combinations of these two types of cylinders are used on different models of equipment. Figure 2-39 illustrates a double-piston wheel cylinder. The single-piston is similar, only smaller because it has only one piston. This unit, regardless of whether single- or double-piston type, changes the applied fluid pressure into mechanical force to move the brake shoes. The wheel cylinder housing, made from a casting, is bolted to the brake backing plate. The two pistons in the cylinder move in opposite directions under hydraulic pressure. Through a short stem, the pistons push the shoes against the drum. These stems are connected directly to the shoes. Rubber cup seals fit tightly in the cylinder bore against each piston to prevent the escape of fluid. Between the cups is a light spring to keep the cups in position against the pistons. The open ends of the cylinder are fitted with rubber boots to keep out foreign matter. Brake fluid enters the cylinder from the brake line connection between the pistons. A bleeder port and valve are located at the top of the cylinder between the pistons. This provides a means for releasing air from the system. Various applications of wheel cylinders are used in support equipment, depending upon the manufacturer’s design. Some may have one single-piston cylinder or a dual-piston cylinder per wheel, each operating two shoes. Others may use a combination of one single and one dual piston or two dual pistons per wheel. When a dual system is used, each cylinder is mounted diametrically opposite the other, and each operates one end of two shoes. INSPECTION AND MAINTENANCE .— Hydraulic brake systems must be inspected at specified intervals. A visual inspection includes checking the fluid level in the master cylinder reservoir, the security of mounting bolts and clamps, the condition of tubing and hoses, and the entire system for leaks. When the wheels and drums are removed for inspection of the brake assemblies, the wheel cylinder should be inspected for leaks. The boots may be pulled 2-30 ASf02038 CHECK VALVE CAGE SEAT BRAKE LINE VALVE SEAT RETURN SPRING PRIMARY CUP PISTON PISTON PRIMARY CUP RETURN SPRING VALVE CAGE BRAKE LINE COMPENSATING PORT PRESSURE APPLIED PRESSURE RELEASED BREATHER PORT Figure 2-38.—Hydraulic brake master cylinder—operation of two-way valve. ASf02039 BOOT CUPPISTON RETURN SPRING CYLINDER HOUSING BLEEDER SCREW Figure 2-39.—Hydraulic brake wheel cylinder.
p. 71
from around the ends of the cylinder to check for leakage between the pistons and the cylinder wall. An operational inspection of a hydraulic brake system may be accomplished during the road test. However, several checks may be made by operating the brake pedal while the vehicle is parked. If the brake pedal bottoms when pushed down, a further check must be made of the system. This condition may be caused by worn brake linings, but more often by a defective hydraulic system. Insufficient fluid in the reservoir is a prime cause of this problem. If the fluid is low, the reservoir should be checked for external leaks. Internal leakage from the lines through the master cylinder may also cause the pedal to bottom. Air in the hydraulic system should be suspected when the operation of the pedal is soft or spongy. The air must be removed to obtain a solid pedal. This process is called bleeding. The maintenance of hydraulic brake systems consists of correcting the problems found during scheduled inspections and those that occur during normal operations. This includes servicing the system, repairing and replacing tubing and flexible hose, repairing the master cylinder and wheel cylinders, and bleeding the system. Servicing .—As applied to hydraulic brake systems, servicing consists of checking the fluid level and adding necessary fluid to the master cylinder reservoir. When adding hydraulic fluid, be sure it is the kind recommended by the manufacturer. Some manufacturers use natural rubber seals in the operating systems, and others use synthetic rubber or other materials. Unless the recommended brake fluid is used, the seals deteriorate quickly, causing possible failure of the brake system. Dirt and other foreign matter that accumulates around the filler opening can also affect brake operation. Even a small particle of dirt may find its way into the operating mechanism and close a vent or prevent a valve from sealing properly. All dirt and foreign matter must be removed before removing the filler cap. The fluid level should be approximately 1/4 to 1/2 inch from the filler opening. This distance is usually specified in the applicable service technical manual. This space compensates for thermal expansion of the brake fluid. Master Cylinder Repair .—When the master cylinder requires repair, it must be removed from the vehicle. If a stoplight switch is mounted on the cylinder, the wires must be disconnected first. Next, the hydraulic line and the pedal linkage are disconnected from the cylinder. Then, loosen the hold-down bolts and remove the cylinder. The exterior surface of the cylinder should be thoroughly cleaned before disassembly. Figure 2-37 identifies the various components and parts of the master cylinder. After the exterior surface of the cylinder is cleaned, remove the filler cap of the reservoir and pour out and discard the fluid. Be sure to follow the proper procedure for disposal of the fluid. Clamp the master cylinder, boot upward, in a vise, and then remove the boot and pedal rod. Next, remove the spring retainer (snap ring). With this removed, the piston stop (a thick steel washer) is free to be removed with the piston. Remove the master cylinder from the vise and up-end the cylinder to allow the piston, spring, and valve assembly to slide out. The secondary cup, primary cup, piston spring, and valve assembly should all be replaced with new parts. These items are usually available in a repair kit. After the parts are removed, the cylinder walls should be cleaned and inspected. If there are deep pits or scratches in the bore, the master cylinder should be replaced. The cylinder bore may be honed with a suitable stone to remove rust, scores, and shallow pits and scratches. After honing a cylinder, be sure that all abrasive dust is removed; then, lubricate the bore with new, clean brake fluid. Assemble the parts into the cylinder in the logical order. First, install the valve assembly on the end of the piston return spring. Check again to ensure that the bore of the cylinder is clean and lubricated with clean fluid. Lubricate the new primary and secondary cups and piston. Guide the lips of the primary cup into the bore and use the piston to force the cup into the cylinder. As the secondary cup is installed, be sure that the lips do not fold over when they contact the bore. Then, place the piston stop on the piston, and with a drift punch inserted in the hole at the back of the piston, force the piston into place. Hold the piston in place and install the spring retainer. Replace the pedal rod and a new boot. Install the filler cap only after ensuring that the vent hole is open. Reinstall the master cylinder on the vehicle, reconnecting the brake line and the wire to the stoplight switch. Then connect the pedal rod to the brake pedal. The applicable technical manual should 2-31
p. 72
be consulted for adjustment procedures for the pedal linkage. The master cylinder must be serviced and the system bled before the brakes are ready for service. However, before this is accomplished, all necessary repairs should be made to the wheel cylinders. Wheel Cylinder Repair .—Wheel cylinders are rebuilt in much the same manner as a master cylinder. To do this, it is seldom necessary to remove the cylinder assembly from the brake backing plate. However, before the cylinder can be repaired, the brake assemblies must be removed. To disassemble a wheel cylinder with two pistons (fig. 2-39), pull the boots from the cylinder and push the pistons, cups, and spring out of the cylinder. After the parts are removed, clean the cylinder wall and check for pits and accumulation of rust. A small quantity of pits or rust at the exact center of the cylinder should not affect the operation of the wheel cylinder. If the rust or pits are just inside the outermost polished areas of the cylinder bore, they must be removed by honing. Cylinders containing deep pits or scratches must be replaced. After honing a cylinder, remove all the abrasive dust and lubricate the cylinder walls with clean, new hydraulic fluid. The pistons are usually made of aluminum, and unless badly scored, they may be reused indefinitely. However, they should not be sanded—only cleaned with an approved solvent or clean, hydraulic brake fluid. After the cylinder bore is satisfactorily cleaned and lubricated, lubricate the cup seals. Insert a cup into the end of a cylinder. Do not push the cup through the bore. As soon as the lips of the cup are in the bore, use the piston to move the cup into place. Installing the other cup and piston may be more difficult, since the cup retaining spring will push against the cup and piston already installed. With one hand, hold the one piston from pushing out, and with the other hand, install the spring, cup, and piston. Then, install the boots and brake shoe links, rods, or slugs. Install a wheel cylinder clamp or use a cord or wire to tie a loop around the cylinder to hold the components in the cylinder until the brake shoes are installed. Bleeding Hydraulic Brake Systems .—During repair of the master brake cylinder or wheel cylinders, or anytime a brake line is disconnected, air will enter the hydraulic brake system. Also, when the fluid level in the reservoir is allowed to become too low, air will enter the brake lines. Most hydraulic systems are equipped with return lines between the actuating units and the reservoir. The fluid circulates from the reservoir, through the supply lines, through the actuating units, and back to the reservoir. This allows any air in the system to escape through the reservoir vent during circulations. Brake hydraulic systems, however, are not equipped with return lines; therefore, there is no means for the air to escape. Air in the system will cause the action of the brake pedal to feel soft and spongy because air is compressible. The hydraulic brake system must be bled to expel this air. There are two common methods of bleeding a hydraulic brake system—the pressure method and the manual method. The pressure method employs a brake bleeder tank, which delivers fluid under pressure to the master cylinder (fig. 2-40). Before pressurizing the bleeder tank for use, ensure that the tank has an adequate supply of the required type of brake fluid and that the valve on the discharge line is closed. Manufacturers usually recommend that the master cylinder reservoir be filled with hydraulic fluid before connecting the pressure tank. Make sure there is a tight seal between the adapter cap and the master cylinder filler port. Then, apply pressure to the master cylinder by opening the valve on the discharge line of the tank. 2-32 ASf02040 PRESSURE TANK Figure 2-40.—Bleeding hydraulic system—pressure method.
p. 73
Most authorities recommend that the system should be bled starting with the longest line and working successively to the shortest. Other authorities prefer the opposite method; that is, starting with the shortest line and finishing with the longest. So, you should always consult the technical manual for the unit you are servicing to see if there is a preferred method. If you suspect that air is trapped in one specific brake line, bleed that line first. For example, assume that the right rear wheel cylinder of a vehicle has been disconnected for maintenance and repair. After the repair has been completed and the cylinder connected, this brake line should be bled first. In fact, it is possible that all of the air that entered the line while it was disconnected may be bled from the one bleeder valve. If at all possible, use a length of flexible tubing and a clean glass bottle or jar to trap the brake fluid expelled from the wheel cylinder bleeder valve. Before attaching the tube, be sure to clean the bleeder valve end. To bleed the line, first loosen the bleeder valve screw. This allows fluid to flow into the jar. Keep the end of the tube submerged in the fluid. By observing the flow from the tube, you will notice air bubbles as they appear. When the air bubbles stop, all the air has been expelled from that section of line and the wheel cylinder. Then, tighten the bleeder valve screw. Repeat this procedure with each bleeder valve. The use of the tube and jar is recommended regardless of the method used to force fluid through the lines. You can use a manual method that requires two persons. One person operates the brake pedal, pumping until the pedal action is hard—hydraulic pressure in the system. The other person opens the bleeder valve screw, allowing the air and fluid to escape as the brake pedal is still forced downward. As soon as the brake pedal nears bottom, the operator signals the other person so that person can close the bleeder valve. As soon as the valve is closed, the brake line pressure is again pumped up and the bleeder valve opened, repeating the process until the air is expelled. When bleeding a hydraulic brake system that consists of two wheel cylinders, an upper and a lower on each wheel, the operation is slightly different. In that case, bleed the upper cylinder at each wheel first, and then bleed the lower cylinder. High-Pressure Hydraulic Brake Systems The latest type of hydraulic brake system is the style used on the aircraft tow tractor, Model A/S32A-37 (fig. 2-41). This system has a brake pedal (called a brake actuator in this equipment), but not a master cylinder. In place of the master cylinder, there is a spool valve in the actuator that controls the flow of hydraulic fluid to apply pressure on the disc brakes. 2-33 ASf02041 ACCUMULATOR ACTUATOR PUMP FLUID RESERVOIR Figure 2-41.—Hydraulic system of the A/S32A-37 tow tractor.
p. 74
When the brake is not pressured, excess hydraulic fluid is returned to the hydraulic reservoir. The major difference in this system is that it has an open hydraulic system pressurized to 2200 psi with a 600 pound precharge in the accumulator to provide braking power even when the engine is not running. (Good for a limited number of uses.) The rest of the brake components are similar in appearance and operation to those of any disc brake system. Power Brake Systems The brake system most commonly used on powered support equipment is the hydraulically operated system. However, the increase in size and weight of aircraft has required heavier and more powerful support equipment, especially the aircraft-towing tractors. With this increase in vehicle power and weight, it is necessary to have a brake system that is more effective and less strenuous for the operator. It was almost impossible for the operator to apply sufficient braking action to control a heavy, yet comparatively small, vehicle. To compensate for this, some hydraulic brake systems are equipped with a form of power system to assist the force of the operator’s foot in applying the brakes. Power brake systems use the principle of the hydraulic brake to operate the wheel brake cylinders and produce braking action. In addition, these systems use the energy of air pressure, either to apply the necessary pressure to the hydraulic fluid or to assist in this application. Atmospheric pressure provides this energy in some power systems, while a compressor is required in others. Many of the Navy’s aircraft-towing tractors are equipped with some type of power brake system. Some are equipped with a form of vacuum-boost system, while others use an air-over-hydraulic system. Vacuum Systems Air has weight (atmospheric pressure), and this weight results in a pressure of approximately 14.7 psi at sea level. It is this air pressure that is used in the operation of vacuum brake systems. It is impossible to create a perfect vacuum, but by pumping air from a container, it is possible to obtain several pounds per square inch difference in pressure between the outside and the inside of the container. If the container were suddenly opened, outside air (atmospheric pressure) would rush into the container to equalize the pressure. It is upon this principle that the power cylinder of a vacuum brake system operates. There are many varieties of vacuum-powered brake systems, and it is impossible to cover them all within the scope of this course. However, the system most commonly used on support equipment is called Hydrovac. The Hydrovac combines into one assembly a hydraulically actuated control valve, a tandem piston vacuum power cylinder, and a hydraulic slave cylinder (fig. 2-42). The vacuum power brake cylinder is connected hydraulically to both the master cylinder and the wheel cylinders. The vacuum source for this system, as for all vacuum brake systems, is the intake manifold of the engine. Reciprocating engines, particularly gasoline engines, generate a substantial vacuum in this area except when running at full power. A check valve maintains a vacuum within the system, even after the engine is stopped, by closing the intake manifold when the pressure in the manifold rises above the vacuum pressure within the system. A vacuum reservoir is usually required so that a substantial source of vacuum is available. Once air is pumped out of the vacuum reservoir through the intake manifold, the resulting 2-34 ASf02042 VALVE CYLINDER FITTING PLUGVACUUM POWER CYLINDER HOSE CONNECTIONS BLEEDER SCREWS CONTROL VALVE END PLUG SLAVE CYLINDER Figure 2-42.—Hydrovac vacuum power brake cylinder.
p. 75
vacuum is diminished only by operating the power cylinder. The vacuum power cylinder is divided into four compartments by the front and rear pistons and the center plate (fig. 2-43). The vacuum source is directly connected to the compartment between the center plate and rear piston. The vacuum is connected from this compartment, by means of the vacuum line, to the relay or control valve. From the control valve, the vacuum is connected to the front compartment by a passage in the valve body. In the released position, the control valve diaphragm plate and vacuum valve seat is held down by the valve spring. This keeps the vacuum valve open and the atmospheric valve closed. In this position, the vacuum is connected through the vacuum valve and the atmospheric control line to the compartment between the center plate and front piston and, through the parts in the hollow piston rod, to the rear compartment. Therefore, vacuum is present in all compartments in the released position, and both pistons remain inoperative. The piston return spring holds the pistons in the OFF position. The push rod, in the released position, maintains the bypass (check) valve off its seat, permitting a direct hydraulic connection from the master cylinder, through the hydraulic slave cylinder, to the wheel cylinders. With this construction, foot pedal pressure can be applied to the wheel cylinders for braking action should vacuum or Hydrovac failure make the power cylinder inoperative. The relay valve diaphragm has vacuum on both sides, and is held in the OFF position by the valve spring. When the vacuum in the Hydrovac is the same as, or greater than, the source vacuum, the poppet valve in the vacuum check valve rests on its seat and, in the event of engine failure or rapid acceleration, traps the vacuum in the Hydrovac system in readiness for brake application. As the foot pedal is depressed, fluid is forced from the master cylinder through the open bypass (check) valve to the slave cylinder and on to the wheel cylinders (fig. 2-44). The fluid is also forced through the drilled bypass passage to the relay valve hydraulic piston, which is forced outward against the pressure of the valve spring. This gradually forces the diaphragm plate and vacuum valve seat toward the brakes-applied position. The movement of the diaphragm first closes the vacuum valve against its seat, sealing off the vacuum from the atmospheric control line. After the vacuum valve is seated, further motion of the diaphragm causes the atmospheric valve to leave its seat. This permits air from the air cleaner to enter the atmospheric control line, and then to the compartment between the center plate and the front piston. It then flows through the hollow piston rod to the rear compartment. With the 2-35 ASf02043 PISTON DRILLED HOLES BY-PASS VALVE (OPEN POSITION) HYDRAULIC SLAVE CYLINDER PISTON RETURN SPRING SLAVE CYLINDER PISTON DRILLED PASSAGE CONNECTION SLAVE CYLINDER TO RELAY VALVE RELAY VALVE HYDRAULIC PISTON DIAPHRAGM PLATE AND VACUUM VALVE SEAT DIAPHRAGM RELAY VALVE BRAKE LINE ATMOSPHERIC FROM AIR CLEANER WHEEL CYLINDERATMOSPHERIC VALVE (CLOSED)VACUUM CHECK VALVE ENGINE INTAKE MANIFOLD VACUUM VALVE (OPEN) MASTER CYLINDER BRAKE PEDAL LOW PRESSURE HYDRAULIC BRAKE LINE VACUUM INLET LINE CENTER PLATE REAR PISTON PISTON ROD PIPE PLUG PISTON RETURN SPRING PISTON ROD DRILLED HOLES FRONT PISTON VACUUM LINE PUSH ROD VALVE SPRING VACUUM ATMOSPHERIC PRESSURE RESIDUAL MASTER CYLINDER HYDRAULIC PRESSURE ATMOSPHERIC CONTROL LINE FRONT { Figure 2-43.—Hydrovac operation—released position.
p. 76
vacuum still present on the front side of both pistons and atmospheric pressure on the rear sides of both pistons, the pistons are forced toward the slave cylinder. Movement of the pistons and push rod toward the slave cylinder closes the bypass (check) valve, and then causes the slave-cylinder piston to move outward. This forces fluid under pressure into the wheel cylinders to apply the brakes. The foot pedal pressure, acting through the master cylinder, also acts against the slave cylinder piston, assisting the vacuum pistons and push rods. The pressure at the wheel cylinders (that is, the total braking effort) is the sum of the output of the vacuum pistons in the Hydrovac and the foot pedal pressure at the master cylinder. Release of foot pedal pressure removes the fluid pressure from below the relay valve hydraulic piston. This allows the valve spring in the relay or control valve to return the atmospheric and vacuum valves to the released position. The atmosphere is exhausted from the rear sides of both pistons, making them inoperative and allowing the piston return spring to move the pistons to the released position. When the foot pedal movement stops at some intermediate point between the released and fully applied position, the pistons will move slightly toward the applied position. This reduces the fluid pressure under the relay valve hydraulic piston the necessary amount to allow the diaphragm to drop and close both the atmospheric and vacuum valves in the control valve. Thereafter, the slightest foot pedal movement, either toward the released or applied position, will result in opening either the vacuum or atmospheric valve, and will partially release or further apply the brakes. With the exception of the vacuum system and particularly the power cylinder, the inspection and maintenance procedures for vacuum brake systems are similar to those required for hydraulic brake systems. To check the vacuum brake system, first shut off the engine and apply the brakes several times to bleed all vacuum from the system. This may require as many as 20 or 30 applications on systems equipped with vacuum reservoirs. Spongy or soft action of the brake pedal indicates air in the hydraulic system. If this occurs, the system must be bled. With the pedal held at the normal braking pressure, start the engine. If the pedal lowers toward the floorboard when the engine starts, the vacuum system is operating properly. If the pedal fails to move, the vacuum system is at fault. Figure 2-45 shows a type of air-over-hydraulic brake system that is used on some of the newer tow tractors. The air control valve controls the flow of compressed air to an air chamber in the master cylinder. The air control valve is mechanically linked to the brake pedal. 2-36 ASf02044 ATMOSPHERIC VALVE (OPEN) ENGINE INTAKE MANIFOLDMASTER CYLINDER BRAKE PEDAL PISTON RETURN SPRING VACUUM ATMOSPHERIC PRESSURE MASTER CYLINDER HYDRAULIC PRESSURE SLAVE CYLINDER HYDRAULIC PRESSURE ATMOSPHERIC CONTROL LINE FRONT CENTER PLATE (STATIONARY) REAR PISTON VACUUM VALVE (CLOSED) VACUUM LINE FRONT PISTON VACUUM CHECK VALVE REACTIONARY FORCE ATMOSPHERIC FROM AIR CLEANER DIAPHRAGM RELAY VALVE HYDRAULIC PISTON DRILLED PASSAGE CONNECTION SLAVE CYLINDER TO RELAY VALVE { SLAVE CYLINDER PISTON HYDRAULIC SLAVE CYLINDERBRAKE LINE TO WHEEL CYLINDER PISTON RETURN SPRING BY-PASS VALVE (CLOSED POSITION) WHEEL CYLINDER PISTON ROD DRILLED HOLES PISTON ROD (ARROWS INDICATE DIRECTION OF MOTION OF THE VARIOUS UNITS AND DIRECTION OF THE FORCES ACTING WITHIN THE HYDROVAC) Figure 2-44.—Hydrovac operation—applied position.
p. 77
Brake application is relative to the pressure applied by the operator on the brake pedal. The brakes of the vehicle may be partially released at any time by slightly relieving pressure on the pedal or entirely released by removing all pressure from the pedal. One of the ports of the air control valve is an exhaust port, which releases air from the air chamber of the master cylinder during the release action. The master brake cylinder assembly is the point in this system where the pneumatic system and hydraulic system join. Figure 2-46 shows a partial cutaway view of the assembly. Only the air chamber is shown cut away because the design and operation of the master cylinder part of the assembly are similar to any other hydraulic master brake cylinder. The piston and piston rod of the air chamber are connected to the push rod of the master cylinder. When the brakes are applied, the air control valve allows air under pressure to enter the air pressure port of the air chamber. This pressure forces the diaphragm to move the piston and piston rod in the air chamber and compresses the return spring. This movement, in turn, moves the push rod and piston in the master cylinder, forcing fluid pressure to the wheel cylinders. When the brakes are released, the air control valve stops the flow of air to the air chamber and at the same time opens the exhaust port. This allows the air to flow out of the chamber, through the line, and out the exhaust port of the control valve. The return spring returns the piston and piston rod to the release position. This movement 2-37 1 2 3 4 5 6 7 8910 11 12 1 HYDRAULIC LINE PRESSURE LINE APPLY LINE ASf02045 1. Wheel cylinder connectors 5. Control valve 9. Master brake cylinder 2. Tank drain valve 6. Control air line 10. System governor 3. System safety valve 7. Air compressor 11. Hydraulic line 4. Compressed air line 8. Air chamber 12. Air reservoir Figure 2-45.—Air-over-hydraulic brake system. ASf02046 DIAPHRAGM PISTON AIR PRESSURE PORT PISTON ROD SLEEVE MASTER CYLINDER BREATHER PORT RETURN SPRING Figure 2-46.—Master cylinder assembly—air-over-hydraulic brake system.
p. 78
releases the brakes through the action of the master cylinder. The inspection and maintenance procedures described earlier for hydraulic and pneumatic brake systems apply as well to the hydraulic and pneumatic portions of the air-over-hydraulic system. You should consult the appropriate technical manual for required maintenance of the master cylinder assembly. Q2-15. On a drum type brake system, which of the following malfunctions, if any, will occur if the anchor pin is too near the center of the drum? 1. The shoe will not firmly press against the drum when brakes are applied 2. The shoe will automatically lock when brakes are initially applied 3. The brake pedal will go to the floor when brakes are applied 4. None Q2-16. A disc brake assembly consists of a metal disc and which of the following components? 1. A caliper 2. A wheel cylinder 3. An actuating cylinder 4. A brake drum Q2-17. What is the main advantage of bonding a brake pad to the brake shoe? 1. Bonding reduces manufacturing costs 2. Bonding allows for the use of thicker pads to be used 3. Bonding reduces the amount to galvanic corrosion 4. Bonding allows the pad to be worn thin without scarring the drum Q2-18. Brake linings that are riveted to the brake shoe should be replaced if worn to less than what percent of their original thickness? 1. 10 percent 2. 20 percent 3. 30 percent 4. 40 percent Q2-19. What method of bleeding a brake system employs a bleeder tank? 1. The conventional method 2. The pressure method 3. The universal method 4. The manual method POWER TRAINS LEARNING OBJECTIVE : Identify the components of a basic power train. Aviation support equipment that is powered and mobile must have a method of transferring the power from the engine to the wheels. This is the basic power train, which consists of the transmission, propeller shaft, universal joints, differential, and axles. Figure 2-47 shows one type of power train. Consider the power train required in a four-wheel (rear) drive vehicle. In the process of transmitting power from the engine to the driving wheels, the power train provides the following: /c183A means of engaging the engine to the drive wheel /c183Several different gear ratios between the engine and the drive wheels 2-38 ASf02047 DIFFERENTIAL CARRIER UNIVERSAL JOINTS AXLE HOUSING PROPELLER SHAFT TRANSMISSION Figure 2-47.—Power train.
p. 79
/c183A means of changing the direction of rotation of the drive wheels (forward and reverse) /c183A means of permitting one drive wheel to turn at a different speed than the other In addition to the basic components of a power train, one or more of the following additional components may also be part of a drive train: fluid couplings, torque converters, transfer cases, and auxiliary transmissions. NOTE: Basic Machines, NA VEDTRA 14037, has an entire chapter devoted to the power train, in which most of the components are described and illustrated. As a result, these components are described only briefly in this discussion. Emphasis is on those component that are not adequately covered in Basic Machines and some of the applications and different arraqngements of power trains in support equipment. Therefore, for a better understanding of the functions and operations of the components of the power train, the appropriate chapter is Basic Machines should be studied in conjunction with this text. Q2-20. What are the components of a basic power train? 1. Transmission, propeller shaft, universal joints, differential, and axles 2. Transmission, propeller shaft, universal joints, axles, and torque converters 3. Torque converter, transfer case, fluid couplings, and auxiliary transmission 4. Torque converter, axles, transmission, fluid couplings, and transfer case TRANSMISSIONS LEARNING OBJECTIVES : Identify the components of support equipment transmissions. Identify procedures for inspecting, checking, testing, and adjusting support equipment transmissions. Identify procedures for troubleshooting support equipment. Identify procedures for repairing, removing, and replacing support equipment transmissions. If the power requirements between the engine and the drive wheels were relatively constant, a power train consisting of a drive shaft and some type of clutch to obtain gradual application of the load on the engine would be sufficient. Speed could be regulated by the engine throttle. However, this is not the case with self-propelled automotive vehicles. Whether the vehicle is the family automobile, an aircraft tow tractor, a fire truck, or a weapons loader, a great deal more power is required to start the vehicle in motion than to keep it in motion. Also, power requirements vary with the load on the vehicle. In addition, both forward and reverse movements are required. Therefore, some type of speed and power changing device is required in the power train. This device is the transmission. In a discussion of the transfer of power from the engine to the drive wheels, we must consider the effects of TORQUE and SPEED. Torque is a twisting force that tends to produce rotation or torsion. The engine power applies this twisting force to the shafts and other rotating members of the power train. Consider a small gear (10 teeth) keyed to a shaft (driving), extending from the crankshaft, and meshed with a larger gear (20 teeth), which is keyed to a second shaft (driven). The driving shaft is parallel to the driven shaft, and power from the engine applies torque to the driving shaft. As the driving shaft turns, the teeth of the smaller gear apply torque force to the teeth of the larger gear, which, in turn, apply torque to the driven shaft. Since the distance from the center to the rim of the larger gear is greater than that of the smaller gear, the twisting force or torque applied on the driven shaft is greater than the torque of the driving shaft. Therefore, the torque applied by the engine has been increased. In the process of increasing the torque, something must be lost or decreased. It requires two revolutions of the small gear (10 teeth) for one revolution of the large gear (20 teeth), which is a gear ratio of 2 to 1. Therefore, as the torque increases, speed decreases. By altering the sizes of the gears on either or both shafts, almost any combination of speed and torque can be obtained, within the capabilities of the engine. Basically, this is what a transmission accomplishes by means of gears or other methods. NOTE: Basic Machines , NA VEDTRA 14037, contains detailed information about the types of gears and the manner in which gears may be arranged to change the speed, torque, or direction of rotation of a shaft. There are many different types of transmissions; however, most can be placed into one of two classes—standard and automatic. With a standard transmission, the operator must manually shift the transmission from one speed/torque ratio to another using a manually operated clutch. With an automatic 2-39
p. 80
transmission, the operator selects neutral, reverse, or one of several ranges of forward speed. The transmission automatically shifts from one forward speed/torque ratio to another based on the rpm of the engine and the speed of the vehicle. The automatic transmission does not require a manually operated clutch. All support equipment used by the Navy have automatic transmissions. Most of the fundamentals of hydraulics are put to work in one form or another in the automatic transmission. Automatic transmissions use such hydraulic devices as relief valves, shifter valves, pressure regulators, governors, and servo pistons. In most cases, the transmissions are used with fluid couplings or hydraulic torque converters. FLUID COUPLINGS A fluid coupling, sometimes called a “fluid clutch,” is precisely what its name implies. When it is placed between an engine at the power-input end and some other mechanism, such as a transmission at the output end, it couples the two hydraulically, and there is absolutely no mechanical connection between them. A simple sort of fluid coupling can be made with two electric fans, as shown in figure 2-48. If the fans are placed a few inches apart, facing each other, and one fan is plugged in so that it runs, the current of air from the running fan will cause the blades of the other fan to turn. In this case, the air takes the place of the fluid. Since the two fans are not enclosed or closely coupled, this sort of coupling is not very efficient. To make a more efficient coupling, oil is used, and the two members (driving member and driven member) are mounted very close together and enclosed in a housing. In a fluid coupling, the driving member is called the PUMP and the driven member the TURBINE. This terminology is not universal. Some manufacturers call the pump the IMPELLER or DRIVER, and the turbine may be called the RUNNER. Often the two members are called the FRONT TORUS and the REAR TORUS. (A torus is doughnut-shaped and turns on an axis.) You should become familiar with all these terms so that you can understand the manufacturers’ technical manuals. Note that in figure 2-49, the pump and turbine are torus-shaped with fins extending radially from its center. The pump of the fluid coupling is connected to the engine and is rotated by the crankshaft. Usually, the pump is bolted directly to the flywheel. The turbine is made exactly like the pump, but it is connected to the transmission input shaft. The two members of the coupling face each other within a housing that is filled with the driving fluid (generally oil). When the pump goes into motion, oil is forced outward by centrifugal force around the entire circumference of the pump and hurled against the blades of the turbine. A continuous flow of oil against the turbine blades is necessary to transfer sufficient kinetic energy to keep a vehicle in motion. The centrifugal force of the oil as it leaves the pump gives the oil the velocity it needs. The faster the pump operates, the more velocity the oil has when leaving the pump. The design of the coupling permits the oil to return to the pump as soon as it has delivered its energy to the turbine. Where the vehicle has not started to move, the turbine is stationary. For instance, the engine may be rotating the pump at 900 rpm. The pump is consequently imparting energy to the fluid, which, in turn, imparts energy to the turbine. By the time the oil returns to the pump, the pump has moved some distance, making it impossible for the oil to re-enter the pump through the same set of vanes it left. Consider one drop of oil as it leaves the pump, goes through the 2-40 ASf02048 AIR IS COUPLING FLUID Figure 2-48.—Basic fluid coupling, using air in place of fluid. ASf02049 TURBINE PUMP Figure 2-49.—Fluid coupling pump and turbine.
p. 81
turbine, and back to the pump. The drop would follow a path that looks something like a string wound around a doughnut through the hole (fig. 2-50). This path of oil is called a vortex and is the path of the stream of oil that drives the turbine. There are as many vortex streams in a fluid coupling as there are vanes. As the turbine begins to turn, the difference in speed between the pump and the turbine decreases. As the speed difference decreases, the coils of the vortex become closer together. A LOW VORTEX exists when the pump and turbine are traveling at nearly the same speed. A HIGH VORTEX exists when the pump and turbine speeds differ greatly. The higher the vortex, the greater is the driving power of the oil. When the vortex is high, the oil tends to strike the fins on the turbine at nearly a right angle. The degree of vortex is continually changing and is determined by the difference in speed between the members of the coupling. A condition known as “zero vortex” or “fluid coupling stage” exists only when the two members of the fluid coupling are turning at exactly the same speed. When this happens the fluid coupling actually has no driving power. The pump, turbine, and the fluid within the coupling are all turning as one unit in a rotary motion. The fluid coupling stage seldom exists because the turbine usually lags a little behind the pump when there is a load on the vehicle. The fluid coupling stage exists momentarily when the vehicle begins to coast or reduce speed. As soon as the engine slows down, the momentum of the vehicle causes the turbine to throw a vortex of oil at the pump, thus permitting the engine to help reduce the speed of the vehicle by creating a drag. With fluid couplings, shock loads can never be transmitted into the engine. Sudden gear-breaking jerks are impossible. If a vehicle is overloaded, the fluid coupling slips and never allows the engine to become overloaded. Thus, harmful low-speed lugging of the engine is impossible. Vibrations and irregularities of the engine can be harmful to the rest of the vehicle’s power train, but with fluid couplings, it is impossible for these engine irregularities to be transmitted to the power train. Since a fluid coupling is nothing more than a sort of connecting link, it can deliver only the torque or “twist” delivered to it; it cannot increase torque or power. To get this added feature requires the use of a torque converter. TORQUE CONVERTERS At first glance, torque converters appear very similar to fluid couplings and, in fact, are similar in several ways. Both have a driving member and a driven member. Both transmit torque (or power) by passing oil from the vanes of the pump to the vanes of the turbine. However, the fluid coupling is essentially a special form of clutch that transmits torque at maximum efficiency when both members are turning at close to the same speed. When the pump turns appreciably faster than the turbine, the efficiency coupling of torque into the turbine is lowered. Figure 2-51 shows a fluid coupling with the driving member (pump) turning much faster than the 2-41 VORTEX FLOW IN A FLUID ASf02050 Figure 2-50.—Schematic of vortex flow in a liquid. ASf02051 DRIVING MEMBER DRIVEN MEMBER FLOW OF OIL Figure 2-51.—Fluid coupling oil flow “bounce back” effect.
p. 82
driven member (turbine). Notice that the vanes of the pump are radial to the shaft and set straight. The turbine vanes are exactly the same as those of the pump. Therefore, when the pump is turning much faster than the turbine, the oil is thrown onto the vanes of the turbine with considerable force, as shown by the heavy arrows. The oil strikes the vanes of the turbine and splashes, or bounces back, as shown by the smaller arrows. The “bounce back” effect actually opposes the oil flow from the pump and causes inefficiency, or torque loss. Thus, when there is a big difference in driving and driven speeds, a good share of the driving torque is used in overcoming the bounce back effect. This bounce back effect becomes less and less as the speeds of the pump and turbine come closer together. At the fluid coupling stage, the bounce back effect is practically gone, resulting in a very good torque coupling between the pump and turbine. The torque coupling, on the other hand, is greatly different in the torque converter. The torque converter is designed to prevent, or reduce to a minimum, the effects of oil bounce back. This is accomplished by the use of one or more members, in addition to the pump and turbine. The vanes of all the members are curved to either aid in the torque coupling from one member to another, or at least not to be a hindrance to this coupling. As a result of the torque converter design, there is no torque loss when there is a large speed difference between the pump and turbine. Quite the contrary—when there is a large speed difference, the torque is increased, or multiplied, in the torque converter. The torque converter may be thought of as a special form of fluid coupling that acts, in a sense, like a gear transmission with a large number of gearshift positions. That is, it can transmit torque ata1t o1 ratio (direct drive); or under certain conditions, the converter can increase the torque so that more torque is delivered than is applied. However, just as in a standard transmission, if there is a torque increase there is a speed reduction. The torque converter provides varying drive ratios between the pump and turbine, thereby providing varying amounts of torque increase. This is accomplished by the use of curved vanes in the pump and turbine and by the use of one or more extra members (elements). These additional members are placed between the driving and driven members. The vane curvature can be seen in the torque converter cutaway shown in figure 2-52. As in the fluid coupling, the pump hurls the oil into the vanes of the turbine, but because of the curvature of the vanes, the change in the direction of the oil is gradual, thus reducing the effects of bounce back. As the oil passes through the turbine, its direction is changed by the curved vanes so that it leaves the trailing edges of the turbine vanes in a direction to oppose the pump direction. This, in effect, would be worse than the bounce back effect of the fluid coupling. However, the oil is again redirected before it reaches the pump by the addition of a third member, a STATOR. The stator is a curved vane that redirects the oil from the turbine into the pump in a way that aids rather than hinders its rotation. It is this aid caused by the stator that produces torque multiplication in the torque converter. Torque converter stator operation and the ability of the torque converter to multiply torque may be better understood by studying figure 2-53. Figure 2-53 illustrates the effects of a jet of oil on a flat piece of metal and on a bucket attached to a wheel. The oil jet will impart a terrific force against the flat piece of metal, as shown in view A; however, notice the bounce 2-42 ASf02052 OIL FLOW TURBINE PUMP 5 43 2 1 Figure 2-52.—Torque converter, multi-element.
p. 83
back of oil, which is wasted power. If the oil enters and leaves a curved bucket, as in view B, the push imparted to the bucket and wheel is small. However, if a curved vane is used to redirect the oil leaving the bucket back into the bucket as in view C, the torque is increased. Detail views B and C act like a torque converter with a pump, turbine, and stator. Normally, a torque converter has at least three members. However, some have more than three members. Refer again to figure 2-52. This converter has five members, and each member is numbered in sequence as oil passes through it; thus, (1) is the primary pump, (2) is the turbine, (3) is the secondary stator, (4) is the primary stator, and (5) is the secondary pump. The five members are mounted on shafts and hubs in such a way that each one has some degree of independent rotation. The stator elements are mounted through overrunning clutches to a stationary hub shaft, while the secondary pump is mounted through an overrunning clutch to the pump shaft. If there is a secondary turbine element, it will be mounted on the turbine shaft. When the vehicle is first started or is under heavy load (fig. 2-52), the primary pump (1) is turning at a relatively high speed while the turbine (2) is turning slowly. As shown by the oil flow arrow, the oil is passing from the primary pump (1) to and through the turbine (2). As the oil leaves the turbine (2), it flows into the stators (3 and 4). Notice that these stators change the direction of the oil flow to aid the primary pump (1). However, the secondary pump (5) is between the primary stator (4) and the primary pump (1). Note how the vanes of the secondary pump (5) oppose the oil flow from the stators to the primary pump (1). The secondary pump (5) is not needed during heavy load, hard acceleration operation. To keep it from hindering the action of the torque converter at this time, the secondary pump is allowed to overrun, or spin faster than the primary pump. This moves it out of the path of the oil flow into the main pump. The secondary pump is mounted on an overrunning clutch to make this possible. The overrunning clutch allows the secondary pump to spin faster than the primary pump when the oil is hitting the back faces of its vanes, but locks or “clutches” when the secondary pump tries to rotate more slowly than the primary pump. More information is given about overrunning clutches later in this section. After start and during normal operation, turbine (2) picks up speed and thus approaches the speed of pump (1). Under these conditions the total oil mass between pump (1) and turbine (2) rotates (a low coil vortex condition as described earlier develops), and secondary pump (5) overruns. During this time, the oil, as it leaves the trailing edges of the turbine, is not thrown back with as great a thrust against the vanes of stator (3). As a result, the rotating oil mass will cause stator (3) to begin to rotate (it is also mounted on an overrunning clutch). As stator (3) begins to rotate, the oil leaving the turbine vanes hits the back side of the stator vanes and overruns to move out of the oil flow path. At the time this is happening, secondary pump (5) tends to slow down as the oil no longer strikes the back side of its vanes with any appreciable thrust. It “clutches” or locks and actually helps the primary pump impart some forward motion or driving force to the oil. Primary stator (4) remains stationary and changes the oil flow direction only slightly. When the turbine and primary pump reach the “coupling stage”, the torque converter acts just like a fluid coupling. At this time, both stators (3) and (4) are freewheeling, or rotating with the mass of rotating oil. Neither is contributing anything to the converter operation. The secondary pump, however, is locked to the primary pump, and thus helps impart the required driving force to the oil under light loads or high speeds. 2-43 ASf02053 AB C Figure 2-53.—Effects of oil jetstream on a flat surface and on a curved surface.
p. 84
The foregoing discussion of torque converters is general, and is adaptable to the operational understanding of any hydraulic torque converter. Actually, the changeover from stall speed to coupling speed in the converter, as outlined above, is not sudden but gradual and is determined by the power demands of the load and/or operator. When the vehicle begins to move, there is one power demand. Then, as the turbine speed begins to increase, the oil flow direction in the converter begins to shift. The shifting oil pattern effects the torque multiplication. This effectively reflects changing power demands, and is similar to a gear-type transmission with an infinite number of gear ratios. The torque converter, like the fluid coupling, can be used as a coupling between the engine and a mechanical clutch. However, since the torque converter can effectively vary torque ratios, a transmission is not always required. There is no reverse or neutral on the torque converter; therefore, a transmission is normally used to meet the demands required of most moving vehicles. Some converters also incorporate a lockup clutch and a retarding device within the converter hydraulic system. Converter Hydraulic System The converter hydraulic (oil) system usually consists of a reservoir, a supply pump, a filter, and a cooler. When the converter is used in conjunction with an automatic transmission, the sump of the transmission is used as the reservoir, since the oil is circulated to the converter and other hydraulic units of the transmission by the supply pump. The cooler, or heat exchanger, is used to dissipate the heat that is generated by the converter. The oil is constantly subjected to agitation and motion when the vehicle is operating, thus it becomes very hot. Some converters have fins on the pump housing similar to the fins on an air-cooled engine to dissipate the heat. Others use the circulating coolant of the vehicle’s engine for cooling, while still others use a heat exchanger of the radiator type. A typical radiator-type heat exchanger is shown in figure 2-54. This type of cooler is mounted with the radiator for the vehicle’s engine. Overrunning Clutch For the torque converter to function properly, the stator or stators and the secondary members must be able to overrun. This is made possible by mounting the members on an overrunning clutch. In most cases, either an overrunning (one-way) clutch of the roller type or a sprag unit type is used. A typical roller-type overrunning clutch is shown in figure 2-55. Notice the shape of the spaces surrounding the rollers in the clutch cam. When the member overruns, the rollers move against light springs into the larger spacing in the cam, thus allowing freewheeling. If the member slows down, the rollers are wedged into the smaller spaces by light springs and clutch hub rotation. This action locks the rotating member to the shaft. A sprag unit assembly is shown in figure 2-56. This assembly consists of an inner and outer race, thrust washers, snap ring, and the required number of sprag units. The sprags (fig. 2-56, view A) are usually held in alignment and on a slight angle between the inner and outer race by an energizing spring. The operation of a sprag assembly can be seen in detail in of figure 2-56, view B. As long as the inner or 2-44 TRANSMISSION ENGINE COOLER RADIATOR ASf02054 Figure 2-54.—Typical radiator-type oil cooler. ASf02055 OVERRUNNING CLUTCH CAM TORQUE CONVERTER ROTATION OVERRUNNING CLUTCH ROLLEROVERRUNNING CLUTCH SPRING OVERRUNNING CLUTCH CAM RIVET OVERRUNNING CLUTCH HUB TURBINE SHAFT OVERRUNNING CLUTCH HUB SHAFT STATOR Figure 2-55.—Roller-type overrunning clutch.
p. 85
outer race rotation is with the slight angle of the sprag, the unit will overrun. However, if the inner or outer race slows down and tends to rotate against the angle of the sprag, it will jam or wedge between the two races and cause them to turn as one unit. PLANETARY GEAR SYSTEM Automatic transmissions use a system of planetary gears to enable the torque from the engine, coupled through torque converters or fluid couplings, to be used as efficiently as possible in meeting the power demands placed on the vehicle either by the operator or pull loads. The planetary gear units are the heart of the modern automatic transmission. Therefore, an understanding of gears and the planetary gear system is essential for an understanding of the automatic transmission. Gears One of the first things that should be noted about gears is the direction of rotation. The direction that the driven gear turns is opposite to that of the driving gear when only two meshing gears are used. To have the driven gear turn in the same direction as the driving gear, an idler gear is used between the driving and driven gears. Figure 2-57 shows the direction of gear travel using an idler gear. By simply covering one of the outside gears, shown in figure 2-57, direction of gear travel can be seen for two meshed gears. The gear ratio for the gears in figure 2-57 is 1 to 1, since each 2-45 ASf02056 STYLUS SPRAG SPRAG UNIT OUTER FACE ENERGIZING SPRING FRONT OUTPUT SHAFT DRIVEN GEAR SPRAG POSITIONING MARKS OUTER RACE SPRAG INNER RACE INNER RACE ROTATES FASTER THAN OUTER RACE. SPRAGS ARE FREE. OUTER RACE ATTEMPTS TO ROTATE FASTER THAN INNER RACE. SPRAGS ARE WEDGED. NOTCHES FOR SPRINGSOVERRUNNING LOCKUP B A Figure 2-56.—Typical sprag assembly, overrunning clutch.
p. 86
gear has the same number of teeth. Witha1t o1 ratio, the driven gear rotates at the same speed as the driving gear. Two meshed spur gears are shown in figure 2-58, with the larger gear having twice the number of teeth as the smaller gear. This arrangement produces a speed ratio of 2 to 1 with the small gear driving, since the small gear rotates twice as fast as the larger one. Thus, the gear ratio between two meshing gears is a comparison of the rpm of one gear to the rpm of the other gear. Gears are not only used to produce speed ratios (same speed, increased speed, or reduced speed), they are also used because the mechanical advantage of gears is directly related to the gear ratio of the driving gear to the driven gear, as is the speed ratio. Therefore, if the small gear drives the gear twice its size, the mechanical advantage is 2 to 1, since the small gear must exert its torque twice the distance (two revolutions) to turn the large gear one revolution (fig. 2-58). If the large gear drives the small gear, the mechanical advantage is 1 to 2. The turning effort required to rotate the gears is called torque. The torque ratio between gears varies with the mechanical advantage. Thus, if a small gear drives a larger gear, the speed is decreased, but the torque is increased; if a large gear drives a small gear, the speed is increased, but the torque is decreased. This shows that through a gear train, speed can be obtained by sacrificing torque, or torque can be increased by sacrificing speed. There are numerous types of gears used throughout the vehicle power train. The most common gears found in automatic transmissions are the spur and helical gears. The gears shown in figures 2-57 and 2-58 are SPUR gears. The HELICAL gear differs from the spur gear in that its teeth are cut at an angle to the sides of the gear, while the spur gear teeth are cut straight and at right angles to the side of the gear. 2-46 ASf02057 Figure 2-57.—Direction of gear rotation. ASf02058 2 TURNS DRIVER DRIVEN 1 TURN Figure 2-58.—Gear speed ratio.
p. 87
Helical gears in mesh have more tooth contact area in contact and operate quieter than spur gears. Another gear common in the planetary gear system of automatic transmissions is the RING gear or internal gear. In this gear the teeth face toward the center of the gear instead of outward. The teeth can be of the spur or helical configuration. Planetary Gears The planetary gear set consists of three separate, but interconnected, rotating members or gears. Figure 2-59 shows a planetary gear set. Notice that the outer gear, or ring gear, is an internal gear because the gear teeth point inward toward the center. The inner gear is called the “sun” gear. The gears between the internal gear and sun gear are called “planet pinion” gears; they are held in place by the planet pinion carrier. This gear system is called the planetary gear system because the planet gears can rotate and at the same time revolve around the sun gear. Drums, hubs, and shafts can be used to put torque (power) into any one of the three members and, at the same time, hold other members so the gear ratio through the system can be increased or decreased. In addition, by the proper turning and holding arrangement, the system can reverse rotation. Only one of the members can be the input, another the output, and one must be stationary. If any two members are locked together, the entire planetary gear system is locked out, and the input shaft and the output shaft must turn at the same speeds, thus producing a direct drive or ratio of 1 to 1. If no member is held stationary and no two members are locked together, then the system will not transmit power at all. The input shaft may turn but the output shaft will not. The most common method of holding the members of a planetary gear system is through the use of clutches or bands. There are several combinations of speed/torque ratios available through a planetary gear system. Figure 2-60 shows a chart containing six different conditions that can result in the planetary gear system by holding or turning the various members. For example, the column under condition 1 shows that holding the sun gear while turning the pinion carrier causes the ring gear to turn faster than the pinion carrier. When the pinion carrier is turned, the pinion must walk around the sun gear because the gears are meshed with the sun gear. The pinion gears are also 2-47 ASf02059 PLANETARY SUN GEAR PLANETARY PINION GEAR PLANETARY PINION CARRIER PLANETARY RING GEAR Figure 2-59.—Members of a simple planetary gear system. CONDITION 1 2 3 4 5 6 SUN GEAR H H T IR I T PINION CAGE T L L H T H RING GEAR I T H T H LR H - HOLD T - TURN R - REVERSE I - INCREASE OF SPEED L - REDUCTION OF SPEED ASf02060 Figure 2-60.—Chart of the speed ratio combinations in a simple planetary gear system.
p. 88
meshed with the ring gear; as they walk around the sun gear (while rotating) on their shafts, they force the ring gear to rotate at a speed increase. Not all of the conditions shown in figure 2-60 are used in vehicle transmissions, but they should be studied for full understanding of the planetary gear system. Figure 2-61 shows the six conditions referred to in fig. 2-60. Notice condition 6 in both figures 2-60 and 2-61; this is a very common condition found in transmissions, since speed reduction is desired in a reverse situation. There are two additional conditions. One condition is DIRECT DRIVE, whereby any two members are locked together giving a drive ratio of 1 to 1. The other is NEUTRAL, whereby torque is simply blocked because no member is held stationary (the gears rotate but there is no torque transmittal). 2-48 ASf02061 STATIONARY DRIVEN MEMBER (OUTPUT) DRIVING MEMBER (INPUT) MORE SPEED LESS TORQUE (1) STATIONARY DRIVEN MEMBER (INPUT) DRIVEN MEMBER (OUTPUT) LESS SPEED MORE TORQUE (2) STATIONARY DRIVEN MEMBER (OUTPUT) DRIVING MEMBER (INPUT) LESS SPEED MORE TORQUE (3) DRIVEN MEMBER (OUTPUT) (4) STATIONARY DRIVING MEMBER (INPUT) DRIVEN MEMBER (OUTPUT) STATIONARY DRIVING MEMBER (INPUT) LESS SPEED MORE TORQUE MORE SPEED LESS TORQUE (5) (6) DRIVING MEMBER (INPUT) STATIONARY DRIVEN MEMBER (OUTPUT) LESS SPEED MORE TORQUE Figure 2-61.—Views of six speed/torque ratios for a simple planetary gear system.
p. 89
Complex Planetary Gear System As in the case of simple machines, complex planetary systems are merely combinations of two or more of the simple planetary units. Planetary units can be arranged to provide several different conditions or gear ratios. However, only two of these conditions can be used for each unit—either direct drive and a gear reduction, or reverse and direct drive. This shows that a single planetary unit is actually a two-speed transmission. All that is needed to operate the unit is a way to hold any one member to provide a reduction in speed (torque increase), and a way to lock any two members together for direct drive. Of course, the single unit would not provide the tractive torque and variable speeds necessary on some types of support equipment. For this reason most automatic transmissions contain two or more planetary units (complex planetary system) arranged to provide the required tractive torque for starting and moving heavy loads, and also providing variable forward speeds as well as a reverse gear. There are inherent advantages to the planetary gear system. For example, since each gear of the planetary unit is in contact with at least two other gears of the unit, there is a lot of gear tooth contact to carry the load. Another advantage is that the gears are always in mesh, and there is no tooth damage due to tooth clash or partial engagement. However, the big advantage, and the one that makes it so popular, is the ease of shifting gears, which can be done automatically. CONSTRUCTION OF AN AUTOMATIC TRANSMISSION The components of an automatic transmission can be divided into four groups—the torque converter, the range or gearing section, the operating units, and the hydraulic control units. These four groups are shown in figure 2-62. The torque converter and range or gear section incorporating the use of a planetary gear train (two or more planetary units) were discussed earlier. Therefore, only the operating units and hydraulic control units are discussed in depth in this section. Operating Units The operating units of the transmission are the servos and clutches. The servo unit (fig. 2-63) consists of the servo body (containing the cylinder, piston, and return spring) and the friction band (attached to the body), which is used to stop or hold a rotating drum or retainer. The servo simply converts the hydraulic pressure applied to the piston into mechanical force, thus controlling the application of the brake (friction) band. When the brake band is applied, a member of a planetary gear unit is held stationary, providing a specific gear ratio. The clutches, like the servo units, normally use hydraulic pressure for their actuation and a return spring for release. Clutches can be of various designs, but most are the multiple friction disc type (fig. 2-64). The clutch discs can be wet or dry—the major 2-49 TORQUE CONVERTER ASf02062 HYDRAULIC OPERATING UNITS RANGE SECTION HYDRAULIC CONTROL UNIT Figure 2-62.—Representative transmission groups. DRUM OR RETAINER ASf02063 FRICTION BAND SPRING PISTON CYLINDER (SERVO BODY) Figure 2-63.—Servo unit. CLUTCH DISC ASf02064 CLUTCH PLATE CLUTCH RETAINER PISTON APPLY PRESSURE RELEASE SPRING SHAFT AND DISC HUB Figure 2-64.—Multiple disc clutch unit.
p. 90
difference being that the disc of the wet disc must be bathed in oil to function. The disc face of either type (wet or dry) is covered with friction material, generally a bonded sintered bronze facing. The discs are splined to a hub, and the hub is splined to a shaft. The steel clutch plates are mounted between the friction discs and are splined to a clutch housing or retainer. The clutch plates and disc are commonly referred to as a clutch-pack. The clutch retainer contains the actuating piston and return or release spring. When hydraulic pressure is applied to the clutch piston, the discs and plates are squeezed together and rotate as a unit. The release spring is also compressed at this time. When hydraulic pressure is removed from the piston, it is returned to the release position by the release spring. This causes the discs and plates to separate. The clutch hub is attached to one member of a planetary gear unit, while the clutch retainer is attached to another member of the same planetary unit. Therefore, when the clutch discs and plates are squeezed together causing these two units to rotate as one unit, the planetary gear unit is in direct drive (1 to 1 ratio). The clutch is used for holding two planetary members, while the servo unit holds only one. Hydraulic Control Units In the manual selective type of transmission, the operator must select the gear ratio he or she thinks is best for engine load and vehicle speed. However, in the automatic transmission, it is the job of the hydraulic control units to make the right selection (gear ratio) for the engine load and vehicle speed and to make the selection at the right time. A typical system of hydraulic control units consists of many components. Some of these components sense vehicle speed, others sense engine power, some supply hydraulic pressure, and still others regulate and control the hydraulic oil and pressure. A typical system of hydraulic control units is shown in figure 2-65. Most of the transmission’s valves and oil passages are housed in the control units. The control units are usually made up of several valve bodies bolted together, or the valve bodies are mounted on a metal plate that has the oil passages machined or cast into the plate and valves. The front pump, driven by the torque converter at or near engine crankshaft speed, is usually mounted on the transmission input shaft at the front of the transmission. The pump delivers oil from the sump (transmission oil pan), through a filter, to the proper hydraulic control units for routing to the operating units as directed by the various valves. The regulator valve maintains a controlled oil pressure to the control units. The manual valve permits the operator to manually select the desired operating range. The shift valve initiates the upshifting or downshifting as determined by the governor and throttle pressures. The throttle valve is positioned by the accelerator linkage; therefore, it is sensitive to engine speed. 2-50 ASf02065 FILTER FLUID SOURCE (SUMP) CHECK VALVES REGULATOR VALVE TORQUE CONVERTER CONTROL VALVE MANUAL SELECTOR VALVE SHIFT VALVE THROTTLE VALVE ORIFICES KICKDOWN VALVE CUSHIONING DEVICES RESTRICTIONS HYDRAULIC CONTROL UNIT TORQUE CONVERTER OPERATING UNITS RANGE SECTION REAR PUMP OUTPUT SHAFT GOVERNOR VALVE FRONT PUMP Figure 2-65.—Typical hydraulic control unit and it’s components.
p. 91
Figure 2-66 shows a typical throttle valve and linkage. The throttle valve delays the upshifting and regulates oil pressure proportional to the accelerator pedal position or carburetor throttle opening. The governor valve causes the upshifting and regulates oil pressure proportional to the transmission output shaft speed. The kickdown valve provides a means of forcing a downshift to a lower gear ratio when it is desirable, such as when accelerating rapidly. The torque converter control valve controls the pressure and flow of oil to the torque converter and to the lubricating passages of the transmission. Valves, metering orifices, or restrictions are added to the hydraulic system to help time and smooth out the operations of the transmission. HYDRAULIC CONTROL SYSTEM The hydraulic control system makes the transmission fully automatic, and to do its job completely, must consist of a fluid source, pressure supply system, hydraulic operating units, and control units for regulating both fluid flow and pressure. The fluid source is the fluid contained in the transmission oil pan, normally referred to as a sump. The fluid is strained through a filter as it flows from the sump to the pump or pumps. The hydraulic system must maintain the oil under pressure for its various components to function properly. It is the job of the pump or pumps to remove oil from the sump and deliver it under pressure to the systems. The pumps can be of several different designs, however, most manufacturers use the internal gear, constant displacement pump. In some transmissions only one oil pump may be used, and it is mounted at the front of the transmission. However, most manufacturers of support equipment use a pump at both the front and rear of the transmission. This allows the vehicle to be “push started,” whereas with the single pump, “push-starting” cannot be accomplished, since the engine must be operating to drive the torque converter and the pump. WARNING Pushing or pulling a vehicle equipped with an automatic transmission is extremely dangerous and can result in damage to the equipment and injury or death to personnel. A vehicle equipped with an automatic transmission should not be pushed or pulled except in an extreme emergency, and then only after all possible precautions are taken to eliminate the dangers involved. Pressure Regulator Oil from the pumps must be delivered to the operating units under regulated pressure. To obtain this regulated pressure, a pressure-regulating valve is incorporated into the system pressure line. Figure 2-67 shows an oil pressure regulator. Oil from the pump or pumps flows to the chamber between two lands of a spool valve. Movement of the valve in one direction is limited by a mechanical stop. Movement in the other direction is limited by a spring calibrated to the required system pressure (usually around 80 to 90 psi). As shown in figure 2-67, oil flows from the chamber of the regulator valve to the manual valve and back to the rear of one large land in the regulator valve. This area of the regulator valve is known as the secondary reaction area. Also notice there is an oil flow from the pumps to the rear of the small land of the secondary reaction area. This area is known as the primary reaction area. In looking at the two reaction areas, notice the working areas of the lands. There will be no regulated pressure (at least not 80 to 90 psi) until oil has reached the secondary reaction area of the valve. After the oil has entered the secondary reaction area, the force of the oil acts on both the large and small lands in the area. When the pumps have supplied sufficient pressure, the resulting force acting on the large land of the secondary reaction area moves the valve to the left against the spring. This movement 2-51 ASf02066 THROTTLE VALVE OUTPUT SHAFT GOVERNOR ACCELERATOR PEDAL CARBURETOR HYDRAULIC CONTROL UNIT Figure 2-66.—Typical throttle valve and linkage.
p. 92
uncovers a vent port that bypasses oil back to the suction side of the pump or the sump. The valve seeks and stabilizes at a pressure that balances the setting of the spring. The oil that is delivered to the control system by the regulator valve is known as the line or main line pressure. Manual Selector Valve The oil pump (or pumps) and pressure regulator provide the hydraulic system with the fluid and pressure necessary to operate the transmission operating units. However, to control the transmission, a valve controlled by the operator is required. This operator-controlled valve is commonly called the “manual valve.” The manual valve can be positioned by the operator to provide low-gear ratios, or low- and high-gear ratios, reverse, and neutral; on some transmissions an intermediate range of ratios can be selected as well as park. The operator may use a selector lever or a push button to control the valve (fig. 2-67). The manual valve acts as a directional control valve. It blocks fluid flow and pressure to the operating units while in the neutral position; in other positions, it directs oil to one or more of the operating units. Governor A governor is normally mounted on the output shaft of the transmission, rotating with it; therefore, it is sensitive to the speed of the vehicle. Because the governor is sensitive to the speed of the vehicle, it is used to transmit fluid under pressure, proportional to the vehicle speed. There are two types of governors in general use—the centrifugally operated spool valve and the fluid velocity type. Figure 2-68 shows both a fluid velocity and a centrifugally operated governor. The centrifugal governor usually consists of a weight and valve assembly enclosed within the governor housing. The spool valve is free to move inside the cylinder or bore of the housing within the 2-52 ASf02067 MANUAL SELECTOR VALVE SPRING REGULATOR VALVE VENT FROM OIL PUMP PRIMARY REACTION AREA STOP SECONDARY REACTION AREA PUSH-BUTTON UNIT R N D L SELECTOR LEVER N R 1 2 D Figure 2-67.—Oil regulator . ASf02068 REGULATED OIL PRESSURE OUTPUT SHAFT VENT MAIN LINE OIL PRESSURE WEIGHT AND SPRING ASSEMBLY A. CENTRIFUGAL GOVERNOR PITOT TUBE DRUM CAN OUTPUT SHAFT VANES B. FLUID VELOCITY GOVERNOR Figure 2-68.—Centrifugal and fluid velocity governors.
p. 93
limitations of the spring-loaded weight. The governor housing rotates with the output shaft. At no speed or low speed, the spring is able to keep the valve positioned to prevent fluid flow or restrict it to a very small amount. As vehicle speed increases, so does the centrifugal force acting on the weight. When sufficient force acts on the weight, it moves outward against the spring and allows more fluid flow (greater pressure). As more pressure is built in the governor output line, it is also being built up between the spool lands of the governor valve. The result is to position the valve by spring pressure and pressure acting on the spool lands (one larger than the other) to balance the centrifugal force acting on the weight. The fluid velocity governor consists of a pilot tube that is fixed (stationary), and usually sits between two sets of vanes in an oil-filled drum. The oil drum (can) is mounted on the transmission output shaft and rotates with it. The pilot tube is a small tube open at both ends and curved on the end inserted into the oil. This tube does not rotate with the oil drum. As the drum rotates, its vanes propel the oil into the stationary curved pilot tube. The velocity of the oil entering the pilot tube is directly proportional to the speed of the output shaft. Thus, it is capable of transmitting hydraulic pressure sensitive to the vehicle speed. The governor’s oil pressure causes the upshifting of the transmission (higher gear ratio), but the throttle valve pressure (fig. 2-66) opposes or delays the upshifting. However, the pressures from the throttle and governor valves are not sufficient to operate the servos and clutches effectively. Therefore, these pressures are used to control another valve, which, in turn, controls a regulated pressure sufficient to operate the servos and clutches. The valve controlled by the throttle and governor pressures is called a shift valve. Shift Valve The shift valve, located in the hydraulic control unit, moves to up shift the transmission gear ratio when governor pressure becomes greater than throttle pressure. As shown in figure 2-69, with the shift valve in the up shift position, ports in the valve body are uncovered, allowing oil from a regulated pressure source (normally line pressure) to pass unrestricted to one or more of the operating units. Energizing one or more of the operating units causes a change in the planetary gear ratio. By using throttle pressure (reflecting engine speed), governor pressure (reflecting vehicle speed), and a shift valve to control the operating units, gear shifting is automatic. The shift valve shown in fig. 2-69 is a spring-loaded, spool-type valve. It has two lands of equal area and one larger area land. When the transmission is in neutral (no hydraulic oil going to any of the control units), the shift valve spring moves the valve to the right, blocking the clutch port. This means that the valve is always set so the vehicle will start forward in the low position of the shift valve, and only up shift when governor pressure exceeds throttle pressure. As the vehicle moves forward, governor pressure will be directed to the rear area of the large land in the shift valve. When the governor 2-53 ASf02069 TORQUE CONVERTER ACCELERATOR PEDAL OPERATING UNITS RANGE SECTION GOVERNOR SERVO UPSHIFT DOWNSHIFT THROTTLE VALVE LINE PRESSURE VENT CLUTCH LAND GROOVE Figure 2-69.—Shift valve.
p. 94
pressure force is greater than the throttle pressure force, the shift valve will move, uncovering the clutch port and at the same time closing the servo pressure port. This causes the clutch to actuate and place the transmission in an up shift position. Both the servo and clutch will have some method of venting trapped oil back to the sump. The method of exhausting the trapped fluid varies with the many manufacturers. Some use ball-check valves, others use metered orifices, and still others cut grooves around the valve lands for oil return. (Notice the servo port land in figure 2-69 for an illustration of a grooved land for oil return.) Throttle Valve The governor valve causes an upshift in the transmission. If there was no opposing or balancing oil pressure, the transmission would always upshift at the same vehicle speed. However, to give the operator more control over the shifting of the transmission and to lessen overloading of the engine, a throttle valve is incorporated into the hydraulic control system (fig. 2-70). This valve directs oil pressure to oppose the governor pressure, and thus delays upshifting. The throttle valve is operated manually by the operator through the accelerator pedal linkage. One end of the valve contains a spring and a moveable plug. When the accelerator pedal moves the linkage, the plug is moved and thereby varies the tension on the valve spring. The force of the spring acting on the spool valve increases with an increasing carburetor throttle opening. In operation the throttle valve receives oil flow from the manual selector valve. Oil enters the area between the large lands of the throttle valve and also goes into the shift valve oil passage. Oil is also allowed to enter a reaction area at the small end of the valve. Oil pressure builds up in the throttle valve circuits until the oil pressure in the reaction area is sufficient to force the valve against the valve spring. The valve moves against the spring until the vent (return) port is opened enough to balance the forces acting against the reaction area land and the spring tension acting on the other end of the valve. The regulated oil pressure from the throttle valve is directed to the spring-loaded end of the shift valve. Thus, the throttle oil pressure adds to the shift valve spring pressure to hold the shift valve in the servo position. This action is opposed by the governor oil pressure. Therefore, an increased throttle pressure setting requires an increased governor oil pressure (greater vehicle speed) to cause an up shift. It is sometimes desirable while operating in direct drive (up shift), and even at full throttle, to downshift for better acceleration, load control, emergencies, and so on. To accomplish this downshift, the hydraulic control system uses a kickdown valve. Kickdown Valve The shift valve at this point has two oil pressures (throttle and governor), which act to control the time of transmission shifting according to vehicle speed and throttle opening. What is needed now is a method of getting the required oil pressure to an area of the shift valve that will cause the shift valve to move to a downshift position, even at full throttle. To get the required oil pressure and control its flow to the shift valve, the hydraulic system uses what is commonly called a “kickdown valve.” The kickdown valve, like all other valves found in the automatic transmission, can vary in design from manufacturer to manufacturer. It may be an integral part of the throttle valve, or a separate valve arrangement. It may use piston-type valves or ball-check valves. For our purpose, a separate valve arrangement containing a spring and ball is used (fig. 2-71). The oil comes from the manual selector valve into a chamber or housing, where the spring-loaded ball stops oil flow. The ball is connected to the throttle linkage. The throttle linkage can unseat the ball and allow oil flow into an oil passage leading to the shift valve. The throttle valve, the governor valve, and the kickdown valve directly control the shift valve. Figure 2-72 shows how these valves function to control the shift valve. As shown in figure 2-72, the oil (when the kickdown valve ball is unseated by the rod) is allowed to flow from the kickdown valve through a passage to a chamber of the shift valve. At this time the throttle 2-54 ASf02070 TO SHIFT VALVE VENT SPRING MOVEABLE PLUGLINE OIL PRESSURE PRIMARY REACTION AREA Figure 2-70.—Throttle valve.
p. 95
valve is fully open (maximum throttle oil pressure). At the same time, the governor pressure is high (higher than the throttle pressure because the kickdown valve is needed for downshift). Notice in the figure that the kickdown oil pressure, while acting on both a small and large surface area of the shift valve, will have a resulting force that acts in direct opposition to the governor oil pressure. What we have is kickdown oil pressure, throttle oil pressure, and the spring tension of the throttle end of the shift valve acting against governor oil pressure. The shift valve moves to uncover the servo port (downshift position) and at the same time release the clutch. Reseating of the kickdown ball, by releasing the accelerator pedal, will cause the transmission to again up shift. The kickdown valve is unseated normally just a little beyond full throttle. There is usually a device installed to indicate to the operator that the accelerator pedal has reached the point of kickdown. The device is generally a spring or hydraulically loaded plug, which offers resistance to accelerator pedal movement after reaching full throttle. This is a very desirable feature, since the vehicle may at times have to be operated at full throttle with kickdown operation. When the operator feels the resistance, he or she knows the accelerator is at full throttle. Further movement, called “going through detent,” causes kickdown. Operating Ranges The hydraulic control system will shift automatically depending on vehicle speed and load conditions, except for reverse operation. The operator is able to control the timing of transmission shifts and make a forced downshift when necessary. For reverse operation, another planetary gear unit is added. To operate the reverse planetary unit, another operating unit (servo) is added. The different gear ratio changes discussed are D for direct drive, L for low range, N for neutral, and R for reverse operation. NEUTRAL RANGE .—The neutral position (N) is the no-power-flow position of the transmission. The neutral position is selected by the operator by placing the manual selector lever or push button in N. Most vehicle engines will not start unless the N position has been selected. 2-55 ASf02072 FROM GOVERNOR TO APPLY AREA OF SERVO TO CLUTCH THROTTLE VALVE VENT LINKAGE ROD KICKDOWN VALVE MANUAL SELECTOR VALVE SHIFT VALVE Figure 2-72.—Functional shift valve controls. ASf02071 FROM MANUAL SELECTOR VALVE TO THROTTLE VALVE BALL LINKAGE K.D. VALVE ROD TO SHIFT VALVESPRING Figure 2-71.—Kickdown valve (separate valve arrangement).
p. 96
There is no power flow through the transmission in neutral. However, the oil pump is functioning to provide oil for the torque converter and, at the same time, developing a regulated line oil pressure to be used when other transmission ranges are selected. Figure 2-73 shows the neutral position for a typical transmission. Notice in figure 2-73 that there are two pumps—a front pump and a rear pump. The front pump operates any time the engine is running and furnishes oil flow and pressure for all of the hydraulic circuits except the governor. The rear pump, driven by the output shaft, furnishes oil for the governor and assists the front pump, when necessary. Check valves are used to 2-56 ASf02073 DIRECT CLUTCH PLANET CARRIER INPUT SHAFT REVERSE BAND KICKDOWN SUN GEAR KICKDOWN RING GEAR REVERSE PLANET GEAR REVERSE SUN GEAR OUTPUT SHAFT REVERSE RING GEAR POWER FLOW IN NEUTRAL CLUTCH REVERSE SERVO THROTTLE VALVE KICKDOWN (LOW) SERVO KICKDOWN CHAMBER SHIFT VALVE TORQUE CONVERTER T.C. CONTROL VALVE COOLER SECONDARY REACTION AREA REGULATOR VALVE VENT PRIMARY REACTION AREA TO LUB FRONT PUMP OIL FILTER REAR PUMP OUTPUT SHAFT WEIGHT GOV. VALVE LDNR MANUAL VALVE LINKAGE KICKDOWN VALVE Figure 2-73.—Neutral (N) operating range.
p. 97
prevent one pump from trying to drive the other in case of failure. (If the check valves were not there and a pump failed, the other pump would pump its oil through the failed pump and into the sump.) Oil from the front pump is directed to the torque converter (TC) control valve. The TC valve regulates the oil flow and pressure for the torque converter and the transmission lubrication circuits. Oil is also directed from the pump to the regulator valve and the manual selector valve. Oil is directed from the manual selector valve back to the primary reaction area of the regulator valve. When the oil pressure is high enough in the reaction area, it will move the regulator valve against the valve spring and, at the same time, vent some of the oil flow back to the sump. When the valve has positioned itself to balance the spring tension and the oil pressure in the reaction area, the line oil pressure is regulated. Figures 2-73, 2-74, 2-75, 2-76, and 2-77 show a drawing of the clutch, kickdown (low) servo band, reverse servo band, and planetary gear sets (including the reverse planetary unit). Notice in the drawings how the planetary units are held or driven by use of hubs, housings, and hollow shafts. As each operating range is discussed, take note of the power flow through the planetary units. LOW RANGE.—Low range (L) is the gear ratio selected by the operator when it is advisable to keep the transmission in low gear for an extended period of time. Low range is selected manually, by placing the selector lever or push button in the L position. This operating range is shown in figure 2-74. Through linkage from the selector lever, the manual selector valve has been moved to a position that opens oil passages going to the shift valve and then to the servo. One oil passage goes to a check valve and then to the kickdown chamber of the shift valve, which causes the shift valve to remain in a downshift position. The other oil passage passes oil through the shift valve servo passage to the low/kickdown servo unit. Thus, the transmission is in low gear and will remain in this condition as long as the selector lever is in the L range. Governor oil pressure is developed during this operation, but it is not able to move the shift valve against the main-line oil pressure in the kickdown chamber. Throttle valve oil pressure is also applied to the shift valve, which aids the kickdown chamber oil pressure in opposing the governor oil pressure. DRIVE RANGE.—The operator, by moving the selector lever to the drive (D) range, allows the transmission to start in low range and automatically up shift to a higher range at the proper time. In selecting the drive position, the manual selector valve is moved to cover the oil passage that feeds line oil pressure into the kickdown valve chamber of the shift valve, as shown in figure 2-75. The shift valve will remain in the low range position (held by spring tension and throttle valve oil pressure) until the governor oil pressure is sufficient to move the shift valve against the throttle valve oil pressure. Governor oil pressure increases proportional to the vehicle speed. When governor oil pressure is high enough, the shift valve will move. Movement of the shift valve closes the servo oil passage and uncovers the clutch oil passage. Notice in figure 2-75 that when the clutch is engaged there is also a release of oil pressure applied to the servo. This prevents the clutch from engaging before the servo is completely released. With the clutch engaged, the transmission is operating i na1t o1 gear ratio, or direct drive. If, while operating in direct drive, it is desirable to make a forced downshift—that is, go into kickdown—the operator simply pushes the accelerator pedal through full throttle (detent). When the throttle is forced through detent, mechanical linkage moves the kickdown valve rod, forcing the kickdown valve ball off its seat, as shown in figure 2-76. When the kickdown ball is unseated, main-line oil pressure is directed to the kickdown chamber of the shift valve. At the same time, maximum throttle oil pressure is acting against the up shift end of the shift valve. The combined force of line pressure from the kickdown valve, assisted by throttle pressure and spring force at the up shift end of the shift valve, is now sufficient to overcome the high governor oil pressure. Therefore, the shift valve is forced into the downshift position, the clutch is released, and the low-speed servo band applied. The transmission is then in gear reduction, or low range. When the accelerator is moved to a position less than full throttle, the kickdown ball is again seated by spring force. Oil pressure is no longer directed to the kickdown chamber, and once again the shift valve is moved by governor pressure to an up shift position. REVERSE RANGE .—Most transmissions operate efficiently at a regulated line pressure of 80 to 90 psi. This is true for forward drive conditions only. For reverse operation the working line pressure must be increased to handle (prevent slipping) the high 2-57
p. 98
torque loads that are imposed on the operating unit—that is, the clutch or servo used to drive or hold the reverse member of the planetary gear set. This oil pressure increase is accomplished by movement of the manual selector valve and use of a smaller reaction area in the regulator valve. To operate the vehicle in reverse, the operator moves the manual selector lever to the reverse (R) position, as shown in figure 2-77. Linkage moves the manual selector valve, opening a passage leading to the reverse operating unit. At the same time, an oil passage to the regulator primary reaction area is closed, and an 2-58 ASf02074 KICKDOWN BAND APPLIED KICKDOWN RING GEAR CLUTCH REVERSE SERVO THROTTLE VALVE KICKDOWN (LOW) SERVO SHIFT VALVE TORQUE CONVERTER T.C. CONTROL VALVE COOLER SECONDARY REACTION AREA REGULATOR VALVE VENT PRIMARY REACTION AREA TO LUB FRONT PUMP OIL FILTER REAR PUMP KICKDOWN CHAMBER WEIGHT GOV. VALVE LDNR MANUAL VALVE LINKAGE KICKDOWN VALVE REVERSE BAND DRUM REVERSE SUN GEAR OUTPUT SHAFT REVERSE PLANET GEAR REVERSE RING GEAR REVERSE PLANET CARRIER KICKDOWN SUN GEAR STATIONARY INPUT SHAFT POWER FLOW IN BREAKAWAY, KICKDOWN OR LOW Figure 2-74.—Low (L) operating range.
p. 99
oil passage directing oil to the smaller (or secondary) reaction area of the regulator valve is opened. As shown in figure 2-77, the oil passage to the primary reaction area of the regulator is closed off. With this condition, and having oil from the pump directed to the small land area of the regulator valve, more working oil pressure must be applied to the small land to cause the valve to operate. Therefore, a greater pump pressure is required and produced to move the regulator valve against the valve spring. The regulated oil pressure in the reverse range is usually between 200 and 300 psi. This increased oil pressure is directed to the reverse servo unit; all 2-59 KICKDOWN BAND POWER FLOW IN DIRECT DRIVE ASf02075 CLUTCH REVERSE SERVO THROTTLE VALVE KICKDOWN (LOW) SERVO SHIFT VALVE TORQUE CONVERTER T.C. CONTROL VALVE COOLER SECONDARY REACTION AREA VENT PRIMARY REACTION AREA TO LUB FRONT PUMP OIL FILTER REAR PUMP KICKDOWN CHAMBER WEIGHT GOV. VALVE LDNR MANUAL VALVE LINKAGE KICKDOWN VALVE DIRECT CLUTCH APPLIED INPUT SHAFT PLANET CARRIERS REVERSE BAND OUTPUT SHAFT REVERSE SUN GEAR KICKDOWN RING GEARKICKDOWN SUN GEAR APPLY PRESSURE RELEASE PRESSURE Figure 2-75.—Drive (D) operating range.
p. 100
other oil passages not concerned with reverse have been closed off. DEVICES REQUIRED FOR SMOOTH OPERATION The transmission discussion to this point has covered the required essentials of a basic automatic transmission. However, the performance would leave much to be desired. For instance, shifting would be rough and erratic, and the life of the transmission would be shortened. Therefore, each manufacturer adds certain refinements or devices in the form of valves, orifices, check valves, metering devices, timing devices, and so on, to smooth out the operation 2-60 POWER FLOW IN BREAKAWAY, KICKDOWN OR LOW ASf02076 CLUTCH REVERSE SERVO THROTTLE VALVE KICKDOWN (LOW) SERVO SHIFT VALVE TORQUE CONVERTER T.C. CONTROL VALVE COOLER SECONDARY REACTION AREA PRIMARY REACTION AREA TO LUB FRONT PUMP OIL FILTER REAR PUMP KICKDOWN CHAMBER WEIGHT GOV. VALVE LDNR MANUAL VALVE LINKAGE KICKDOWN VALVE KICKDOWN BAND APPLIED KICKDOWN RING GEAR INPUT SHAFT KICKDOWN SUN GEAR STATIONARY REVERSE PLANET CARRIER REVERSE PLANET GEAR OUTPUT SHAFT REVERSE SUN GEAR REVERSE BAND DRUM REGULATOR VALVE Figure 2-76.—“Kickdown” forced shift position.
p. 101
of the transmission. For example, a device is needed that will cushion the application of the operating units (clutches and servos) to prevent harsh engagement. The device used by most manufacturers to cushion (time) the operating units is the shuttle valve. The timing devices are necessary to regulate the time of filling and venting the operating units. On some transmissions the manufacturer uses a device to prevent the transmission from being shifted into reverse while the vehicle is moving forward. This 2-61 POWER FLOW IN REVERSE ASf02077 TO LUB FRONT PUMP OIL FILTER REAR PUMP CLUTCH PLANET CARRIERS STATIONARY KICKDOWN (LOW) SERVO INPUT SHAFT REVERSE BAND APPLIED REVERSE RING GEAR OUTPUT SHAFT REVERSE SUN GEAR KICKDOWN SUN GEAR IDLING KICKDOWN PLANET GEAR IDLING KICKDOWN RING GEAR REVERSE SERVO THROTTLE VALVEKICKDOWN VALVE LINKAGE SHIFT VALVE KICKDOWN CHAMBER TORQUE CONVERTER T.C. CONTROL VALVE COOLER SECONDARY REACTION AREA REGULATOR VALVE PRIMARY REACTION AREA VENT GOV. VALVE WEIGHT LDNR MANUAL VALVE Figure 2-77.—Reverse (R) operating range.
p. 102
device, known as a blocker valve, is usually hydraulically operated. The blocker valve normally requires the vehicle speed to be below a certain speed before reverse can be selected. Some transmissions use a lockup clutch with the torque converter. This clutch is usually located between the turbine and the pump (impeller) drive housing and is applied by oil pressure controlled by the transmission hydraulic control units. When the converter reaches the coupling stage, oil pressure is directed to the lockup clutch, causing it to engage. The lockup clutch locks the turbine and pump together at coupling stage speed for a positive 1 to 1 ratio. Transmissions with several forward speeds and a reverse normally use a device known as the high and low splitter. This device consists of a planetary gear set, and hydraulic control units are used to lock two members of the planetary gear set together for direct drive, or to hold one member for low gear ratio. The output from the splitter assembly is passed to the other planetary units, and thereby gives several gear ratios for each combination of planetary units. Another device used on some automatic transmissions is known as the hydraulic retarder. This device is used on heavy duty vehicles to assist the brake system in slowing down the vehicle, such as when descending steep grades. The retarder, usually located at the front of the transmission behind the torque converter, is controlled by a foot pedal in the operator’s area. Through linkage, the pedal is connected to a control valve mounted on the side of the transmission that cuts off or supplies oil to the retarder. The retarder consists of a cavity or housing with vanes cast into the housing and a rotor with integral blades. The rotor is connected to and turned with the output (turbine) shaft of the torque converter. When the operator depresses the foot pedal, oil is directed into the cavity surrounding the rotor. The oil source for the retarder unit is from the torque converter. The oil strikes the rotor blades in a direction opposite to that in which the rotor is turning. This produces the same effect as turning a paddle wheel in the opposite direction from that in which a stream of water is turning the wheel. That is, the oil is churned by the action of the rotor and the vanes in the cavity. This churning of oil tends to put a drag on the output shaft of the torque converter. Therefore, anytime the retarder is used, it tends to help the vehicle brake system. CAUTION If the retarder is used for a long period of time, it tends to overheat the transmission oil. Usually a warning light is mounted on the operator’s instrument panel to indicate high oil temperature. When the light comes on, the retarder should be disengaged to allow the oil to cool. TRANSMISSION MAINTENANCE Maintenance and repair of automatic transmissions may sound like a complicated job, but with the proper tools, test equipment, replacement materials, and “know-how,” these units can be maintained properly. Before disassembly of a transmission due to internal malfunction, you should investigate the availability of a quick engine change (QEC) for that unit, ensure sufficient parts and technical information are available, and ensure the urgency of need is there to justify the teardown. It is quite time-consuming to disassemble an engine, make a repair, and reassemble the engine only to find the engine requires additional internal repairs. The automatic transmission, like any other hydraulic system, depends, as a very important first step, on its oil supply. The oil must be of the correct type, must be at the proper level, and most important, must be clean. Contaminated oil can clog oil lines and passageways and restrict valve movement in the hydraulic control unit. A high or low oil level usually causes the transmission to overheat and results in eventual failure. Some transmission failures have been a direct result of oil leaks; therefore, all oil leaks should be corrected as soon as possible after being detected. There have been instances when the transmission was removed from the vehicle for repair unnecessarily; the defect could have been corrected without transmission removal. For examin the transmission output shaft seal, speedometer drive pinion, oil pan gasket, or oil cooler can normally be corrected without transmission removal. On some transmissions the servo units and governors may also be repaired or replaced without removing the entire assembly. In all cases every possible effort should be made to make repairs without transmission removal from the vehicle. The manufacturers of automatic transmissions normally recommend a step-by-step procedure for testing the transmission in the vehicle. These 2-62
p. 103
procedures are designed to determine proper functioning and to assist in isolating and diagnosing transmission troubles. Figure 2-78 shows a typical automatic transmission, showing the range or gearing section, operating units, hydraulic control unit (valve body), torque converter, pumps, governor, and transmission case. Optional devices such as lockup clutches, high-low splitters, and retarder units are not shown. You should be familiar with several terms used by the manufacturers in describing maintenance procedures for various transmissions. The term apply-passage means a passage through which oil pressure is applied to an operating unit. Air-check simply means substituting air pressure for oil pressure and applying it to the apply-passage for checking an operating unit. Another term is gauging-hole, which refers to holes provided in adjacent parts to hold them in alignment or in a certain position by a gauge pin or rod. While the parts are held by the gauge pin, certain adjustments can be made. A detailed step-by-step discussion of the possible repairs that can be performed or how they are performed for the many automatic transmissions is beyond the scope of this training course. However, there should be a manufacturer’s repair manual available in the work center for the specific transmissions for which you have maintenance responsibility. These service and repair manuals should be followed very closely in performing all transmission maintenance and repair. The maintenance discussed in the following paragraphs is general in nature and presented to give you a better understanding of overall transmission maintenance. Adjustments In maintaining the automatic transmission, adjustments are necessary for satisfactory performance. Adjustments are required after the repair of a transmission and periodically while in service. The manufacturer usually specifies the time interval between most adjustments, and these recommendations should be followed. The adjustments discussed in the following paragraphs are the ones most commonly found on automatic transmissions. Control linkage adjustment is the adjustment of the selector linkage to the manual control valve. The 2-63 INPUT SHAFT ASf02078 FRONT CLUTCH ASSEMBLY FRONT BAND REAR BAND TRANSMISSION CASE REAR OIL PUMP GOVERNOR ASSEMBLY OUTPUT SHAFT OUTPUT SHAFT DRIVE PLANET PINION CARRIER ASSEMBLY OIL STRAINER HYDRAULIC CONTROL UNIT INTERMEDIATE SHAFT REAR CLUTCH ASSEMBLY FRONT OIL PUMP TORQUE CONVERTER Figure 2-78.—Typical automatic transmission.
p. 104
adjustment varies for different vehicles; therefore, the technical manual for the vehicle should be consulted for this adjustment. Throttle linkage adjustment is the adjustment of the control rod from the throttle valve on the transmission to the accelerator linkage at the engine carburetor. This adjustment is very difficult to set until the engine has been properly adjusted, because the engine speed affects the throttle valve pressure. Band adjustments are made periodically as specified by the manufacturer. Some transmissions allow band adjustments from the outside of the transmission case, while others require removal of the oil pan before the bands can be adjusted. In all cases, for trouble-free operation the bands should be adjusted at the time interval and by the procedures specified by the manufacturer. Oil Checks When checking an automatic transmission, most manufacturers suggest you start with the oil level in the reservoir. Next, make an oil pressure check on the various hydraulic circuits. For example, to check the main line or regulated oil pressure, connect a pressure gauge to the specified check point on the transmission and operate the engine at the specified rpm with the transmission in neutral (N). This test can be used to check the condition of the pressure regulator or pump. OIL LEVEL CHECKS.—This check is made to determine if there is sufficient oil in the reservoir to supply the demands of the torque converter or fluid coupling plus the other needs of the transmission. To make the test, most manufacturers recommend operating the engine and transmission until they have achieved the normal operating temperature. Usually, when the engine temperature is normal, so is the transmission oil temperature. Before taking a reading of the oil level, operate the transmission through its ranges and then place the selector in the N position. Remove the bayonet gauge (dipstick) from the transmission and check to see if the oil is at the right level. If the level is low, fill it with the correct type of oil to the correct level. If the level is too high, it should be drained until the correct level has been reached. Different transmission manufacturers may recommend different positions for the shift lever during this check. In case the manufacturer recommends that the lever be in the D position, the wheels should be chocked and the brakes set during the oil level check. OIL PRESSURE CHECKS .—Oil pressure checks are performed to determine if sufficient pressures are available to the operating units to cause them to operate properly. The main thing in making oil pressure tests is to use a pressure gauge that will handle, without damage, the pressures being tested. Another important point is to be sure of the pressure check point on the transmission, as each hydraulic circuit may require a different pressure for a certain operation. Torque Converters and Fluid Couplings Torque converters and fluid couplings normally require very little maintenance. However, oil seals and gaskets may fail occasionally, thus requiring maintenance. For maintenance other than oil leaks, the fluid coupling or torque converter is replaced as a unit, if it is a sealed unit. If it is not a sealed unit, limited repair and parts replacement may be authorized. If maintenance requires the disassembly of a unit, it should be thoroughly inspected. In most cases all seals and gaskets should be replaced with new ones during reassembly. Components of the unit, such as a pump, turbine, stator, one-way clutch, and so on, should be replaced if they show signs of excessive wear or are defective. Range Section The range section (planetary sets) of a disassembled transmission should be inspected for chipped teeth, wear patterns, and worn support bearings. The thrust washers should also be inspected, and replaced if they show signs of excessive wear. If the bearings are defective, or any member of the planetary set is defective, replace the gear set. Operating Units A clutch is usually used to hold two members of a planetary gear set, and a servo band is used to hold one member. Normally, a defective clutch or servo (slippage) can be detected by road testing the vehicle. The manufacturer’s recommendations should be followed for road testing a specific vehicle. If a clutch or servo is defective, it normally will require transmission removal from the vehicle to perform maintenance (except for servo band adjustment). 2-64
p. 105
A “no drive” condition might exist in a transmission, even with correct oil pressure, because of inoperative clutches or bands. Therefore, before removing a clutch assembly or a servo unit from the transmission, check it with air pressure. The front or rear clutches and kickdown (low) servos may be tested by applying clean, moisture-free compressed air of 30 to 100 psi to the apply-passages. Figure 2-79 shows a typical oil passage arrangement as seen after the removal of the hydraulic control unit (valve body). Apply air pressure to the apply-passage and listen for a dull thud, which indicates that the clutch or servo is operating. In some cases the thud may not be heard in the clutches; in this case, place the fingertips on the clutch housing, and movement of the clutch piston can be felt when the clutch is applied. If, by applying air pressure, the clutches and servos operate properly, but allow slippage under road testing, then the trouble is probably in the band, band adjustment, or the clutch disc. If air can be heard escaping through the clutch or servo unit, a seal or gasket is defective and must be replaced. A defective clutch disc usually requires replacement of the entire clutch assembly. Hydraulic Control Unit Foreign matter in the hydraulic control unit is one of the major causes of an automatic transmission malfunction. A particle of dirt or a piece of lint could cause a valve within the unit to partially stick or seize in the valve body. This would cause the transmission to operate improperly, depending upon which valve was sticking. If a malfunction or troubleshooting indicates a defect in the hydraulic control unit, the entire unit should be disassembled, cleaned, and inspected. All defective parts should be replaced. While the inspection of parts is mostly visual, the springs contained in the unit should be checked for proper length and pressure and for such defects as distortion and broken coils. Upon disassembly, it is good practice to place the parts on a large piece of hard-surfaced paper. The nomenclature of each removed part should be written adjacent to the part. This is to prevent interchanging parts that look alike. A 2-65 LINE PRESSURE TO ACCUMULATOR ASf02079 REAR SERVO APPLY FRONT SERVO APPLY FRONT SERVO RELEASE FRONT PUMP SUCTION FRONT PUMP PRESSURE FRONT CLUTCH APPLY REAR CLUTCH APPLY TO TORQUE CONVERTER FROM TORQUE CONVERTER TO COOLER REAR PUMP PRESSURE GOVERNOR PRESSURE REAR PUMP SUCTION Figure 2-79.—Typical transmission oil passages.
p. 106
typical hydraulic control unit (valve body) is shown in figure 2-80. Other hydraulic system components such as pumps, lockup clutches, retarders, high-low splitters, governors, and oil pressure regulator valves, if separate from the unit, are disassembled, cleaned, inspected, and repaired in a very similar manner to the components of the hydraulic control unit. Before reassembly of the control unit, all parts should be cleaned with the proper solvent and blown dry with clean, moisture-free, low-pressure compressed air. If compressed air is not available, the parts should be drip-dried. You should NEVER wipe the parts with any type of material that may leave lint. Table 2-1 is a troubleshooting chart that lists many of the typical problems that occur with automatic transmissions. The table also includes probable causes for the problems and suggested remedies. The intent is always to try to solve the problem before tearing down the unit for repair or replacing it. Transmission Removal The procedures for removal and replacement of transmissions vary depending on the model that you are dealing with. Therefore, an in-depth discussion will not be provided here, just a few things that are common to all transmissions. First, in some models it is easier and faster to remove the engine and transmission as one unit. After the engine and transmission are removed, the transmission can be disconnected from the engine. Prior to removing the transmission, it should be drained of fluid, all lines disconnected and plugged, all linkages disconnected, and all electrical wires disconnected, if required. The drive shaft must be removed, and the tail shaft plugged or bagged and taped to prevent fluid from spilling as the unit is removed. Automatic transmissions are very heavy. Many models weigh over 200 pounds, so you must have the proper sling or transmission jack to raise or lower the unit. Upon installation, make sure that all mounting brackets are in place, and that hardware is secured prior to removing the supporting unit (sling or jack). Then, reconnect the lines, wires, linkage, and drive shafts according to the technical manual for the transmission. Service Diagnosis and Repair of Automatic Transmissions The transmission should not be removed nor disassembled until a careful diagnosis is made, the definite cause determined, and all possible external corrections performed. In diagnosing any abnormal 2-66 ASf02080 ONE LARGE STEEL BALL FRONT PUMP CHECK VALVE FIVE SMALL STEEL BALLS Figure 2-80.—Typical hydraulic control unit.
p. 107
2-67 TROUBLE PROBABLE CAUSE REMEDY Automatic shifts at too high a speed 1. 2. Governor valve stuck Modulator cable adjustment 1. 2. Clean or replace governor Adjust cable Automatic shifts at too low a speed at full throttle 1. 2. G governor valve stuck Governor spring weak 1. 2. Clean or replace governor Replace spring Low main pressure in all ranges 1. 2. 3. 4. 5. 6. 7. Low oil level Oil filter element clogged Sealring on oil pickup tube leaking or missing Main-pressure regulator Valve spring weak Control valve body leaking Valves sticking Oil pump worn or damaged 1. 2. 3. 4. 5. 6. 7. Add oil to proper level Replace filter Install new sealring Replace transmission Replace transmission Replace transmission Replace transmission Low main pressure in first gear, normal pressure in other forward ranges 1. 2. First gear circuit of control valve body leaking Excessive leaking at first and reverse piston seals 1. 2. Replace transmission Replace transmission Intermittent buzzing noise 1. 2. 3. 4. Low oil level Air leak at oil intake pipe Clogged filter Aerated oil 1. 2. 3. 4. Add oil to proper level Replace intake pipe seal and filter Replace filter Improper oil level, or improper or contaminated oil Excessive creep in first and reverse gears 1. Engine idle speed too high 1. Adjust idle speed Low lubrication pressure 1. 2. 3. Oil level too low Excessive internal oil leakage Cooler lines restricted or leaking 1. 2. 3. Add oil to proper level Check other pressures above; check valve body mounting volts Check for kinks, leakage; replace lines if necessary Oil leaking into converter housing 1. 2. 3. 4. Leaking engine crankshaft rear oil seal Charging oil pump, lip-type seal at torque converter, leaking Leaking sealring around body of oil pump Cracked weld in converter assembly leaking 1. 2. 3. 4. Refer to vehicle service manual Replace pump seal Replace transmission Replace converter assembly Transmission heating up in all ranges 1. 2. 3. 4. Oil level low Oil level high Engine cooling system restricted Oil cooler lines restricted 1. 2. 3. 4. Add oil to proper level Drain oil to proper level Refer to vehicle service manual Clean or replace lines Table 2-1.—Allison Four-Speed Automatic Transmission Troubleshooting Chart
p. 108
2-68 TROUBLE PROBABLE CAUSE REMEDY High stall speed 1. 2. 3. 4. Oil level low Clutch pressure low Forward clutch slipping (forward) First and reverse clutch slipping 1. 2. 3. 4. Add oil to proper level Refer to low main pressures above Replace transmission Replace transmission Low stall speed 1. 2. Engine not performing efficiently (may be due to high altitude) Broken converter parts 1. 2. Refer to engine manufacturer’s manual or vehicle service manual Replace converter assembly Rough shifting 1. 2. 3. Manual selector linkage out of adjustment Control valves sticking Modulator cable binding or improperly adjusted 1. 2. 3. Adjust linkage Replace transmission Replace cable or adjust to specification Engine overspeeds on full throttle upshift 1. 2. 3. 4. Piston seals leaking or clutch plates slipping in range involved Forward clutch piston seals or clutch plates slipping (all upshifts) Broken sealrings on front support hub Sticking governor valve 1. 2. 3. 4. Replace transmission Replace transmission Replace transmission Clean or replace governor Excessive slippage and clutch chatter in one range (not in all ranges) 1. 2. 3. Clutch slippage in that range clutch Excessive oil leakage in range piston seals Oil leakage in valve components for that particular range 1. 2. 3. Replace transmission Replace transmission Replace transmission Dirty oil 1. 2. 3. 4. Failure to change oil at proper interval Excessive heat Clutch failure Damaged oil filter 1. 2. 3. 4. Change oil; install new filter Check oil cooler lines Replace transmission Replace filter Oil leak at output shaft 1. Faulty or missing seal at output flange 1. Install new lip-type seal in bearing retainer Slippage in all forward gears 1. 2. 3. Low oil level Low clutch pressure Forward clutch slipping 1. 2. 3. Add oil to proper level Refer to low main pressures above Replace transmission Slippage in fourth and reverse gears only 1. 2. Fourth clutch slipping Broken sealrings on support assembly hub 1. 2. Replace transmission Replace transmission Slippage in reverse and first gears; proper function in other forward gears 1. First-and-reverse clutch slipping 1. Replace transmission Vehicle moves forward in neutral 1. 2. Range selector linkage out of adjustment Forward clutch failed and dragging 1. 2. Adjust linkage Replace transmission Table 2-1.—Allison Four-Speed Automatic Transmission Troubleshooting Chart—Continued
p. 109
shift condition, ensure that the engine is properly tuned, the idle speed is correct, all band are adjusted correctly, and the control linkage is as specified. Always make the hydraulic pressure tests before any removal or disassembly. Refer to the applicable repair manual for in-depth troubleshooting, pressure check valves, and hydraulic fluid flow charts. In recent years the Navy has introduced a program whereby an engine that requires internal engine repair is swapped out as a complete assembly (with its transmission, if applicable). The Navy supply system has contract provisions where complete engine/transmission assemblies are rebuilt and dynamometer tested by a civilian source and reintroduced back into the supply system. This concept is referred to as QEC (quick engine change), and is offered for approximately 80 percent of the SE power plant/drive train configurations, with more QECs added as the fleet requirements dictate. Before disassembly of an engine/transmission assembly due to internal malfunction, you should investigate the availability of a QEC for that unit. If a QEC is not available, then before disassembling the engine/transmission assembly, ensure that sufficient parts and technical information are available and that the urgency of need is there to justify the teardown. It is quite time-consuming to disassemble an engine/transmission assembly, make a repair, and reassemble the engine/transmission assembly only to find the engine requires additional internal repairs. If a QEC is available for the type of equipment you are troubleshooting and you can still use the equipment until the replacement assembly is received, your best option is to order the QEC. A rebuilt transmission, if part of the QEC, will accompany the engine. The engine will be totally rebuilt with all new components, and the complete package will be dynamometer tested. Eventually most aviation ship’s A VCAL and COSAL listings will authorize a QEC package to be stored on board for each type of equipment under this concept for immediate receipt and installation. Refer to the applicable NA V AIR and NA V AIRW ARCEN (NAWC) instructions for the program policies and a listing of the QEC kits available. As previously mentioned, the transmissions are part of the QEC program and will be exchanged rather than overhauled in the fleet. NA V AIRINST 13610.2 (series) (QEC instruction) lists the procuring procedures as well as the depth of allowable repairs by the user. Q2-21. Which of the following components can transmit torque at a ration of 1 to 1 (direct drive) or under certain conditions, can increase torque so that more torque is delivered than applied? 1. The transmission 2. The differential 3. The torque converter 4. The hydraulic multiplier Q2-22. What type of gear system is used by automatic transmissions? 1. A direct drive gear system 2. An opposing gear system 3. A multiplying gear system 4. A planetary gear system Q2-23. The gears that are located between the internal gear and the sun gear are referred to as what type of gears? 1. The planet pinion gears 2. The internal connecting gears 3. The ring gears 4. The orbiting gears Q2-24. Which of the following components in an automatic transmission is responsible for making the right gear ratio selection for the engine load and vehicle speed? 1. The shifter valve 2. The hydraulic control units 3. The kick-down valve 4. The speed-sensing valve Q2-25. The governor is normally mounted in which of the following locations on the transmission? 1. On the input shaft 2. On the output shaft 3. On the front pump 4. On the rear pump Q2-26. Which of the following valves directs oil pressure to oppose the governor pressure, and thus delays upshifting? 1. The load control valve 2. The kick-down valve 3. The throttle valve 4. The governor valve 2-69
p. 110
Q2-27. Which of the following components is NOT a common adjustment that can be made to an automatic transmission? 1. The hydraulic control unit 2. The throttle linkage 3. The control linkage 4. The bands TRANSFER CASES LEARNING OBJECTIVES: Recognize the purpose of transfer cases. Identify the components of the transfer case. Identify procedures for inspecting, checking, testing, and adjusting transfer cases. Transfer cases are placed in the power trains of vehicles driven by all wheels. Their purpose is to provide the necessary offsets for additional propeller shaft connections to drive the wheels. Some transfer cases contain an overrunning sprag unit (or units) on the front output shaft. (A sprag unit is a form of overrunning clutch; power can be transmitted through it in one direction but not in the other.) On these units the transfer case is designed to drive the front axle slightly slower than the rear axle. During normal operation when both front and rear wheels turn at the same speed, only the rear wheels drive the vehicle. However, if the rear wheels should lose traction and begin to slip, they tend to turn faster than the front wheels. As this happens, the sprag unit automatically engages so that the front wheels also drive the vehicle. The sprag unit simply provides an automatic means of engaging the front wheels in drive whenever additional tractive effort is required. There are two types of spring unit transfers—a single and a double. Essentially, both types work in the same manner. The first indication of trouble within a transfer case, as with other components of the power train, is usually noisy operation. If an operator reports trouble, make a visual inspection before removing the unit from the vehicle. Check for such things as oil level, oil leakage, and water in the oil. Make sure the shift lever linkages are inspected. If the shift lever linkages are bent or improperly lubricated, it is hard to shift the transfer case. In some cases, this condition makes shifting impossible; make sure other possible troubles such as clutch slippage, damaged propeller shaft, and damaged axles have been eliminated. Worn or broken gears, worn bearings, and excessive end play in the shafts cause noise operation of the transfer case. When you have decided that the trouble is within the transfer case, remove the unit from the vehicle for repairs. Make sure the transfer case is thoroughly cleaned before disassembly of the unit begins. When the unit is disassembled, clean each part with an approved cleaning solvent. Inspection of the individual parts should follow the same procedure as outlined for transmission. Avoid waste by reusing old parts that are in good condition. If you are not thoroughly familiar with a particular make and model of transfer case, you should check the manufacturer’s repair manual to ensure that proper adjustments and assembly procedures are followed. Q2-28. During normal operation when both the front and rear wheels are turning at the same speed, the transfer case operates in what specific manner? 1. It applies torque to the front wheels only 2. It applies torque to the rear wheels only 3. It applies torque to both sets of wheels at the same time 4. It applies torque to neither the front nor the rear until there is a change of speed Q2-29. What is the first indication that a transfer case is not operating properly? 1. The front wheels fail to turn 2. The rear wheels fail to turn 3. The vehicle will not move 4. The transfer case will produce unusual noises PROPELLER SHAFT ASSEMBLIES LEARNING OBJECTIVES : Identify components of propeller shaft assemblies. Identify procedures for repairing or removing and replacing propeller shaft assemblies. The propeller shaft (drive shaft) assembly consists of a propeller shaft, a slip joint, and one or more universal joints (fig. 2-81). The propeller shaft is a driving shaft that transmits the power from the transmission to the differential. Propeller shafts may be solid or tubular. Tubular shafts are used when the 2-70
p. 111
transmission is some distance from the differential. Solid shafts are usually used on tow tractors because the shaft is very short. The slip joint is necessary because the movement between the transmission and the differential requires that the propeller shaft be able to shorten or lengthen itself. On vehicles having rear springs, the differential moves up and down as the rear wheels move over uneven surfaces. This up and down movement lengthens and shortens the distance between the transmission and the differential. On vehicles such as tow tractors that have no rear springs, the slip joint is necessary because the distance between the transmission and the differential still increases and decreases when the vehicle moves over uneven surfaces. Vibration of the engine on its shock mounts and expansion and contraction also necessitate a slip joint. Because the differential is situated below the level of the transmission, universal joints are necessary to permit the change of direction of drive. The usual type of slip joint consists of a splinted shaft that fits into a splinted sleeve, as shown in figure 2-81. The splints permit the continuing transmission of power as the sleeve moves back and forth on the shaft. A universal joint is essentially a double-hinged joint consisting of a Y-shaped yoke on the driven shaft, another Y-shaped yoke on the driving shaft, and a cross-shaped member called the spider. Figure 2-81 shows a common universal joint. Two of the four arms (trunnions) of the spider fit into bearings in the end of the driving shaft yoke, and the other two arms (trunnions) are assembled in the end of the driven shaft yoke. When the two shafts are at an angle to each other, the bearings in the yokes permit the yokes to swing on the trunnions with each revolution. (Universal joints may have roller bearings around the trunnions.) Slip joints and universal joints require little maintenance, except for those that need lubrication where fitted with grease fittings. Others may be factory packed with grease and not need lubrication. Repair is normally limited to removal and replacement of worn bearings, which are pressed out and replaced with new ones, as illustrated in figure 2-82. 2-71 ASf02081 SNAPRING ROLLER BEARING UNIVERSAL JOINT ASSEMBLY CORK GASKET SLIP JOINT SPLINE SNAPRING OIL SEAL ROLLER BEARING OIL SEALSNAPRING ROLLER BEARING PROPELLER SHAFT FLANGE YOKE NUTS JOURNAL U-BOLT LOCK PLATE DUST CAP JOURNAL FLANGE YOKE FIXED YOKE SPLINED SLIDING YOKE ASSEMBLED IN SAME PLANE Figure 2-81.—Propeller shaft assembly. ASf02082 DRIVER SOCKET RECEIVER SOCKET Figure 2-82.—Universal joint removal.
p. 112
Q2-30. The purpose of the slip joint is to allow the propeller shaft to shorten or lengthen so that it can be used in several different types of SE? 1. True 2. False Q2-31. What is the purpose of the universal joint? 1. It permits the change of direction of drive 2. It can be used in several different types of equipment 3. It allows the differential to apply power to one set of wheels at a time 4. It can shorten or lengthen depending on movement of the rear axle DIFFERENTIALS LEARNING OBJECTIVES : Identify the purpose of differentials. Identify procedures for inspecting, checking, and adjusting differentials. Identify troubleshooting procedures for differentials. Identify maintenance procedures for differentials. The differential is connected to the propeller shaft by the final drive. The final drive consists of a pinion gear driven by the propeller shaft. The pinion turns a ring gear that is part of the differential. The function of the ring gear and pinion is to change the direction of the power transmitted through the propeller shaft by 90° in order to drive the axles. The ring gear and pinion also provide fixed reduction between the speed of the propeller shaft and the axles. The gear ratio is determined by dividing the number of teeth on the ring gear by the number of teeth on the pinion. Most aviation support equipments have bevel gears (fig. 2-83) in the final drive. Straight bevel gears are very noisy; therefore, spiral bevel gears are used on most equipment. The ring gear and pinion are housed in the differential housing and lubricated by gear oil. The purpose of the differential is to adjust for the difference in distance the driven wheels travel when the vehicle turns. For example, if a 90-degree turn was made on a 20-foot radius, the inner wheel would travel about 31 feet and the outer wheel would travel nearly 39 feet. The differential permits each axle to turn at a different rate and still be driven as a single unit. CONVENTIONAL DIFFERENTIALS The drive pinion, connected to the propeller shaft, rotates the drive ring gear and the differential case that is attached to it (fig. 2-84). When both wheels are rotating at the same speed, as they do on a smooth straight surface, the differential pinions do not rotate on their trunnions but serve to lock the drive ring gear and differential case with the differential side gears and axles, making them turn as one unit. In this case there is no relative motion between the drive ring gear and the axles, and the teeth of the differential pinions do not move over the teeth of the differential side gears. When the vehicle turns, one wheel must turn faster than the other. The differential side gear driving the outside wheel through the axle turns faster than the side gear of the inside wheel. For the drive ring gear to remain meshed with the two differential side gears, each turning at different speeds, the differential pinions must turn on their trunnions. The amount by which the differential pinions cause the inside side gear to slow in the rate of turn is the amount by which they will cause the outside side gear to increase the rate of turn. The average speed of the two side gears is always equal to the speed of the drive ring gear. For 2-72 DRIVE PINION ASf02084 DRIVE RING GEAR WHEEL AXLE FRAME DIFFERENTIAL CASE DIFFERENTIAL PINION TRUNNION DIFFERENTIAL PINION AXLE DIFFERENTIAL SIDE GEARS DIFFERENTIAL PINION DIFFERENTIAL PINION TRUNNION PROPELLER SHAFT Figure 2-84.—Rear axles and differential. ASf02083 SPIRAL BEVEL GEAR SPUR BEVEL GEAR Figure 2-83.—Ring gear and pinion.
p. 113
example, if the drive ring gear makes four revolutions and the inner side gear, axle, and wheel make one revolution, the outside wheel will rotate seven times. If one wheel spins free from traction on a vehicle using a conventional differential, the other wheel loses power because the differential pinions are revolving around the side gear of the stationary wheel and applying all the power to the spinning wheel. This result would be entirely unsatisfactory in towing tractors; therefore, a no-spin differential is employed. NO-SPIN DIFFERENTIALS To provide the means of improving tractive effort of the driving wheels when one wheel slips from loss of traction, the differential must prevent actual slippage and apply torque power to the driving wheels only to the extent that the wheels can use the torque without slipping. The no-spin differential (fig. 2-85) uses a pair of toothed clutches to do this. It does not contain side gears as does the conventional differential. Instead it contains a spider attached to the drive ring gear through four differential pinions turning on the spider trunnions, plus two driven clutch members with side teeth that are indexed by spring pressure with side teeth in the spider. Two side members are splinted to the wheel axles and, in turn, are splinted into the driven clutch members. The center cam (fig. 2-86) in the spider is held in place by a snap ring that permits the center cam to rotate, but does not permit it to move laterally. When making a right turn, the right-driven clutch member remains fully engaged with the spider clutch teeth (fig. 2-87). The spider clutch teeth (the driving teeth) drive the right (inside) wheel at drive ring gear speed. The left wheel (outside) covers a greater distance and must turn faster than the drive ring gear speed. The differential must permit this action. As the left wheel begins to turn faster, the left-driven clutch member also turns faster than the drive ring gear and spider speed. As the left-driven clutch member begins to turn faster, the cam lobes or ramps on its edge ride up on the cam lobes on the center cam. This action pushes the left-driven clutch member away from the spider so the clutch teeth disengage (fig. 2-88). As the crest of the ramp is passed, spring pressure forces the teeth of the driven clutch member back into full engagement with the teeth on the spider. This action is repeated as long as the left wheel turns more rapidly than the right wheel. Full drive is applied to the right wheel; no drive is applied to the left wheel. As soon as the vehicle completes the turn and the left wheel slows down to the right-wheel speed, driving power is applied equally to 2-73 ASf02085 SPRING DRIVEN CLUTCH MEMBER SPIDER AND CENTER CAM ASSEMBLY DRIVEN CLUTCH MEMBER SPRING SIDE MEMBER SPRING RETAINER SPRING RETAINER SIDE MEMBER Figure 2-85.—No-spin differential disassembled. ASf02086 CENTER CAM SPIDER SNAP RING Figure 2-86.—Construction of center cam.
p. 114
both. For a left turn, the action is similar except that full drive is applied to the left wheel; the right wheel turns more rapidly than the left wheel. With the no-spin differential, one wheel cannot spin because of loss of traction, and thereby deprives the other wheel of its driving effort. For example, suppose that one wheel is on ice and the other wheel is on dry pavement. The wheel on ice is assumed to have no traction. The wheel on ice cannot spin because wheel speed is governed by the speed of the wheel applying tractive effort. Therefore, the wheel on dry pavement will pull to the limit of its tractional resistance at the pavement. MAINTENANCE The gear oil in the differential should be periodically checked and brought to the proper level if needed. The area under the differential should be checked after the vehicle has been parked to determine that the gaskets in the differential housing are not leaking. When gaskets are leaking, you should drain the gear oil, support the housing, remove the bolts and old gasket, clean the surfaces, and install a new gasket. When operating the vehicle in cold weather, the manufacturer’s specifications should be consulted to determine the differential gear oil to be used. It may be necessary to change gear oil with the seasons. The first clue to existing trouble in a differential is usually a noise. Defective universal joints, rough gear wheel bearings, or tire noises may be improperly diagnosed as differential trouble. To properly determine the trouble, the source of the noise must be found, and the operating conditions under which the noise is most pronounced must be noted. A clue may be gained as to the cause of the trouble by noting whether the noise is a growl, a hum, or a knock; whether it is heard when the vehicle is operating on a straight road, or on turns only; and whether the noise is most noticeable when the engine is driving the vehicle or when it is coasting. A humming noise in the differential usually means that the drive ring gear or drive pinion needs adjusting. An improperly adjusted ring gear or pinion prevents normal tooth contact between the gears and produces gear noise and wear. The humming noise gradually takes on a growling characteristic if the trouble is not corrected, and the ring gear and pinion eventually need replacing. The manufacturer’s specifications and instructions should be consulted, studied, and followed for any adjustment. Tire noises may be mistaken for differential noises. Tire noises vary according to the type of pavement the tires are on while differential noise will not. To determine whether the noise is caused by tire or differential, drive the vehicle over several types of pavement, and if the noise changes with the type of pavement, tires are the cause. 2-74 DRIVEN CLUTCH MEMBER AND SPIDER REMAIN LOCKED AND TRAVEL AT SAME SPEED ASf02087 SIDE MEMBER SPIDER RIGHT HAND TURNDRIVEN CLUTCH MEMBER SIDE MEMBER DRIVEN CLUTCH MEMBER ELEVATED BY CAMS DISENGAGES FROM SPIDER CLUTCH TEETH AND TRAVELS AT FASTER SPEED SPIDER CLUTCH TEETH DRIVE DRIVEN CLUTCH MEMBER DRIVEN CLUTCH MEMBER Figure 2-87.—Operation of no-spin differential during a turn. ASf02088 DRIVEN CLUTCH MEMBER BECOMES DISENGAGED AND TRAVELS FASTER THAN SPIDER SPIDER SPIDER AND DRIVEN CLUTCH MEMBER TRAVEL AT SAME SPEED DRIVEN CLUTCH MEMBER DRIVEN CLUTCH MEMBER TEETH DRIVEN BY SPIDER CENTER CAM MEMBER CENTER CAMS SERVE AS RAMPS TO ELEVATE THE DRIVEN CLUTCH MEMBERS THROUGH CONTACT WITH THEIR FIXED CAMS Figure 2-88.—Action of a center cam during a turn.
p. 115
A noise that is present in the differential only when the vehicle is rounding a corner is usually caused by trouble in the differential case assembly. The differential pinion gears may be too tight on their trunnions, or the differential side gears may be tight. Damaged or worn gears can produce a noise when the vehicle turns. If bearings or gears are damaged, a knocking noise can result. The quick engine change (QEC) program will guide you in the repair and replacement of differentials. Check the latest instructions for guidelines and level of authorized maintenance. Q2-32. What is the purpose of a differential? 1. To adjust for the difference in the distance the driven wheels travel when the vehicle turns 2. To adjust for the difference in torque to the driven wheels 3. To shorten or lengthen the propeller shaft 4. To connect the propeller shaft to the transmission Q2-33. What is the function of the side gears in a differential? 1. They connect the planetary gear and the sun gear 2. They connect the shaft to the spline 3. They allow for one wheel to turn faster than the other when the vehicle is turning 4. They multiply torque on both drive wheels independently depending on load conditions Q2-34. Using the conventional differential, if one wheel spins free from traction, it will cause which of the following effects to the other wheel? 1. Applies all the power to it 2. Causes it to lose power 3. Spins free from traction 4. Rotate at the same speed Q2-35. The no-spin differential uses what type of component to prevent the loss of traction? 1. Overrunning clutches 2. Spline gears 3. Pinion gears 4. Toothed clutches DRIVE AXLE ASSEMBLIES LEARNING OBJECTIVES : Identify the components of drive axle assemblies. Identify procedures for removing, replacing, and repairing drive axle assemblies. The drive axle conveys the torque power from the differential to the wheel. It is made of steel and is solid in construction. Most aviation support equipment has full-floating drive axles (fig. 2-89). The wheel fits over the end of the axle housing, and the weight is carried by two roller bearings between the wheel and the axle housing. The outer end of the axle has a flange that bolts to the wheel. Thus, the axle goes through the axle housing and wheel and applies turning power to the outside of the wheel. The wheel is held on the axle housing by adjustment nuts (fig. 2-90). The wheel hub, roller bearings, and axle shaft are lubricated by the same gear oil that lubricates the differential. The axle can be removed by removing the flange bolts, breaking the flange seal, and pulling the axle from the housing. This can be done without removing the wheel. When replacing an axle, the proper size and thickness of the gasket should be used as specified in the manufacturer’s instructions. 2-75 ASf02089 OUTER WHEEL BEARING BRAKE DRUM AXLE HOUSING DIFFERENTIAL HOUSING AXLE SHAFT INNER WHEEL BEARING WHEEL HUB WHEEL RIM Figure 2-89.—Cross section of rear hub in full floating axle. ASf02090 Figure 2-90.—Bearing mounting and adjustment nuts of full floating construction.
p. 116
Some aviation support equipments, such as tow tractors, have rear axle planetary gears at the wheel end of the axle. These gears further reduce the power between the axle and the wheel. The axle has a gear on the outer end, which turns three planetary pinion gears; these turn an internal ring gear bolted to the wheel. This system is all lubricated by the same gear oil that lubricates the differential. For adjustment and servicing, consult the manufacturer’s instructions. SAFETY While transmissions, propeller shaft assemblies, differentials, or rear axle assemblies are not usually dangerous to work on or service, all safety precautions related to mechanical work must be followed to prevent injury. These assemblies are heavy when removed as units, and the simple act of dropping one of them can cause a serious injury. In addition to the usual safety precautions, it should be reemphasized that proper blocking of the vehicle being worked on is always a safe practice. In addition, when one end of a vehicle is raised from the ground, a minimum of two safety stands should be used in place of the jack or lifting device that was used to raise the vehicle. It is never a safe practice to be under a vehicle when it is held off the ground only by lifting devices. Q2-36. What types of drive axles are most commonly used in SE? 1. Direct drive axles 2. Independent drive axles 3. Ridged drive axles 4. Full-floating drive axles EXHAUST SYSTEMS LEARNING OBJECTIVES : Identify the purpose of exhaust systems. Identify the components of an exhaust system. Identify procedures for removing and repairing exhaust system components. The purpose of the exhaust system is to channel harmful exhaust gasses to the rear of the vehicle. This system was developed to prevent the driver and passenger(s) from inhaling the harmful gasses and becoming ill. The exhaust system attaches from the exhaust side of the engine head, but it must be supported elsewhere as the system is made of light weight pipe and will not support itself. The most logical place to put the exhaust system is under the frame assembly where it can hang by hangers, called muffler hangers. They not only support the weight of the system but also help absorb the vibrations that could cause metal fatigue. Although the exhaust system is under the frame it normally does not hang below the lowest part of the chassis. This helps prevent it from being ripped off by flight deck arresting cables or road hazards on shore bases. Most exhaust systems are made up several pieces of pipe and a muffler, as shown in figure 2-91. Some original equipment may be one-piece systems, but replacement parts are made up of several pieces. To remove and replace the exhaust system, you should remove the section to be replaced by unbolting the attaching hardware and removing the items to be replaced with proper replacement items. Tighten the system hardware to the manufacturer’s specifications. NOTE: Always refer to the Maintenance Instruction Manual (MIM) when performing any maintenance on support equipment. 2-76 ASf02091 MUFFLER HANGERS TAIL PIPE MUFFLER HEADER PIPE Figure 2-91.—Typical exhaust system.
p. 117
Q2-37. What is the primary purpose for the automotive exhaust system? 1. It removes the gasses from the engine compartment and prevents them from reentering the engine intake 2. It creates a back pressure in the engine to increase fuel economy 3. It channels the gasses to the rear of the vehicle to prevent the driver from breathing harmful gasses 4. It allows for a means of connecting a muffler to reduce noise levels TIRES AND WHEELS LEARNING OBJECTIVES : Identify the components of tire and wheel assemblies. Identify the procedures for inspecting, checking, and adjusting tire and wheel assemblies. Identify troubleshooting procedures for wheel assemblies. Identify procedures for removing, repairing, and replacing tire and wheel assemblies. As you have already learned, tires play a major role in braking, steering, and alignment. The S upport Equipment Tire and Wheel Assemblies manual, NA V AIR 17-1-129, provides maintenance, inspection, repair and safety procedures for tire/wheel and bearing assemblies used on support equipment. This manual should be reviewed to ensure a complete understanding of these types of repair actions, as well as the technical manual for the model of equipment you are repairing. This same manual is further intended to develop guidelines and procedures for training and certification of SE personnel as tire/wheel maintenance technicians. The most important rule for an AS on tire and wheel maintenance is that you are not authorized to perform any maintenance until you are tire and wheel certified in writing by your aircraft maintenance officer. Figure 2-92 is a sample certificate and figure 2-93 is a sample requirements list. 2-77 ASf02092 This is to certify has successfully completed all established requirements for SE/AWSE tire/wheel tear down/build-up and is qualified to perform tire/wheel servicing/handling on: Program Manager Signature Date: (Type Assembly) (MO Initials) (Type Assembly) (MO Initials) (Type Assembly) (MO Initials) (Type Assembly) (MO Initials) (Type Assembly) (MO Initials) (Type Assembly) (MO Initials) (Type Assembly) (MO Initials) (Type Assembly) (MO Initials) MO Signature: Date: Original to: Individual's Qualification/Certification Record Program Manager Recommendation Figure 2-92.—Support Equipment tire/wheel maintenance certification.
p. 118
2-78 Trainee Initials Command: Name: W/C Supervisor Indoc Signature: 1. Required Reading: (applicable sections) A. OPNA VINST 4790.2 B. OPNA VINST 5100.19, VOL 1 Chapters A3, B5, B6, B12 C. OPNA VINST 5100.23 Chapters 7, 10, 15, 19, 20 D. NA V AIR 00-80T-96 E. NA V AIR 01-1A-20. para 5-4 a/b F. NA V AIR 17-1-123 G . NA V AIR 17-1-125 H. NA V AIR 17-1-129 I. NA V AIR 17-600-174-6-1 J. NA V AIR 19-1-55 K. Applicable MIMs/MRCs (List each applicable publication) Supervisor Signature Date 2. Safety Films: Servicing Multi-Piece Wheel Rims (OSHA A110) (Recommended) Servicing Single Piece Wheel Assemblies (OSHA A113) (Recommended) High Pressure Gases In Aviation (24795DN) (Required) Aircraft Tire Maintenance (25784DN) (Required) NOTE: All required reading and viewing of video shall be accomplished prior to starting the tire/wheel maintenance OJT. 3. Completed course of instructions: Nitrogen Servicing Equipment Phase 1 Completion Date: Phase II Completion Date: 4. OJT: A certified technician will sign off/date each area of OJT each time the individual performs a task under supervision (applicable for A through I). NOTE: Each OJT area requires a minimum of three tasks for each type assembly. A. Bearings Removal/Cleaning/ Inspection/Handling/Lubrication/ Installation (if applicable) Trainee Initials Supervisor Signature Date Signature Date W/C Date: Rate/Rank: SSN: Date: 1 A. Tear Down/Build-Up/Inflation of Solid Rim Assembly C. Tear Down/Build-Up/Inflation of Split Rim Assembly D. Tear Down/Build-Up/Inflation of Demountable Flange Assembly E. Tear Down/Build-Up/Inflation of SD-2 Spotting Dolly (if applicable) F. Use of Bend Breaker (if applicable) G . Use of Inflation Gage H. Operation of Nitrogen/Air Servicing Equipment/Remote Inflator Assembly I. Documentation Procedures 5. Certification: A QAR, certified in tire and wheel maintenance, will sign each area only after th individual has demonstrated proficiency and awareness of all procedures and safety precautions. A. Bearing Handling/Lubrication Procedures (if applicable) Signature: Date: B. Tear Down/Build-Up/Inflation of Solid Rim Assembly Signature: Date: 2 ASf02093 C. Tear Down/Build-Up of Split Rim Assembly Signature: Date: D. Tear Down/Build-Up/Inflation of Demountable Flange Assembly Signature: Date: E. Tear Down/Build-Up Fnflation of SD-2 Spotting Dolly Assembly (if applicable) Signature: Date: F. Use of Bead Breaker (if applicable) Signature: Date: G . Use of Inflation Gage Signature: Date: H. Operation of Air Servicing Equipment/Remote Inflator Assembly Signature: Date: I. Documentation Procedures Signature: Date: WRITTEN TEST SCORE (Minimum 90 percent): QAR Signature: Date: 3 Figure 2-93.—Support equipment tire/wheel maintenance requirements.
p. 119
WHEEL RIMS The three types of rims most commonly used on SE are the solid rims, split rims, and demountable flange rims. Solid Rims Solid rims are made in one piece and are permanently fastened to the wheel hub (fig. 2-94). They feature a well in the center that permits mounting and demounting of the tire. This type of rim is often referred to as a drop center rim, and is generally used on smaller vehicles and light trucks. Split Rims Split rims consist of two, usually identical, halves secured together by tie bolts. They are usually mounted on handling equipment, small trailers, and front wheel assemblies of towing tractors. When assembling split rim wheels, you must ensure that steel and aluminum halves are not mixed. Mixing different types of wheel halves may cause cracks and wheel failure. Demountable Flange Rims The demountable flange rim consists of two parts: the rim base and the demountable flange. The demountable flange (split ring) holds the tire by interlocking with the rim base when the tire is inflated. This rim is used on rear axles of towing tractors and similar heavy duty vehicles. Rims are mounted with the demountable flange outboard on single wheel installations, and facing each other on dual wheel installations. TIRES The types of tires most commonly used on SE are bias ply, bias belted, radial, tubeless, and solid rubber tires. Bias Ply In bias ply tires, tire cords are arranged in two or more (even number) plies, depending on the strength desired in the finished tire. The cords, or plies, cross the tire circumference at an angle, usually 30 to 40 degrees. This design provides rigidity in both sidewall and tread. A disadvantage is that bias ply tires squirm more and tend to run hotter than belted bias or radial tires. Bias Belted Constructed similarly to bias ply tires, belted tires will have an additional two or more layers of fabric (belts) under the tread. The cords in the belt also run at an angle, about 25 degrees to the circumference. This construction provides the sidewall stiffness of the bias ply with increased strength and stiffness in the tread. 2-79 ASf02094 1. LOCKWASHER 2. NUT 3. OUTER RIM HALF 4. VALVE STEM HOLE 5. VALVE STEM 6. TIRE BALANCE MARK 7. TIRE 8. INNER RIM HALF 9. TIE-BOLT 3 4 9 5 6 7 8 1 2 B. SPLIT RIMA. SOLID RIM 1 2 3 1 1. TIRE 2. RIM 3. FLANGE Figure 2-94.—Solid and split rims.
p. 120
The bias belted tires squirm less, run cooler, and give better mileage than the bias tire. Radial Radial tires are constructed with one to three plies of the body cords running at right angles to the circumference. Over this radial section is added a belt, or belts, of fabric or steel. The design provides a tire with flexible sidewalls but great stiffness and strength in the tread area. It has minimum squirm, runs cool, and provides long wear. Tubeless Tires Tubeless tires may be bias ply, bias belted, or radial. In addition, they have a thin rubber lining, and the bead area is grooved to form an airtight seal with the rim. Solid Rubber Tires Solid rubber tires are used primarily on slow moving, heavy loading equipment. They provide far less cushioning than pneumatic tires but give loading capacity far greater than comparable sized pneumatic tires. WHEEL AND TIRE MAINTENANCE The types of wheel assemblies found on support equipment are split rim, demountable flange, and solid rim (automotive) types. Of the three types of wheel assemblies, the demountable flange possesses the highest potential for explosive separation. The destructive potential of air, or nitrogen, under pressure is tremendous. As an example of the explosive force produced, a 10.00 x 20 tire inflated at 105 psig creates in excess of 40,000 pounds of pressure against the rim flange. In a controlled test, this force accelerated the locking to 130 mph, and raised a 215-pound dummy 10 feet upward from a wheel resting horizontally on the deck. This explosive potential requires that all inflated, or partially inflated, tires be handled with the same care and precautions given live ordnance. Nondestructive Inspection A nondestructive test/inspection (NDI) of split rim (steel) wheels and tie-bolts are required by applicable periodic maintenance requirements cards (MRCs). (Example, front wheel assembly, P/N HA-1321, A/S32A-31A tow tractor.) Rims requiring NDI and meeting visual inspection are paint stripped and subjected to magnetic particle or liquid penetrant inspection. Personnel performing NDI inspection must hold current certification according to requirements of OPNA VINST 4790.2 for MIL-STD-6866 and/or MIL-STD-1949 inspection methods. Removing and Replacing Tires The following procedures involve removing a wheel from a vehicle, removing the tire from the rim, putting the tire back on the rim, reinflating the tire, and putting the wheel back on the vehicle. Use a “deflated tire flag” to show that a tire has been deflated and the valve core removed. Maintenance should never be conducted on a wheel/tire assembly where a deflated tire flag is not installed. The flag tells you that the tire is safe. Deflated tire flags are usually made locally, and they are made so that the pin extends at least 1/4 inch beyond the open end of the valve cap, with the cap shouldered against the pin. WARNING Short deflated tire flag pins (less than 3/4 inch) may enable the flag to be screwed on the valve stem with the valve core still installed. Always check to be sure that the core has been removed. REMOVING WHEEL ASSEMBLIES .—Use the following steps for removing a wheel from a vehicle: 1. Chock both wheels at the end opposite the end being jacked. 2. Jack the vehicle and place jack stands under the frame or axle, as required by the technical manual. Lower the vehicle to the jack stands. If aboard ship, install tie-down chains. 3. Deflate the tire to be removed, remove the valve core, and install a deflated tire flag (fig. 2-95). If the valve stem is equipped with a valve extension, remove the extension valve core and extension first. Wheels being removed to facilitate other maintenance need only be deflated to storage pressure, which is one-half the service pressure or 15 psig, whichever is less. 2-80
p. 121
NOTE: On dual wheel installations, the wheel not being serviced should be deflated to 15 psig prior to loosening any wheel clamps or mounting bolts. 4. Remove the wheel clamps using a crisscross pattern. 5. Remove the wheel. DEMOUNTING A TIRE .—The use of power-operated equipment, when available, for demounting tires is recommended. When using power-operated equipment, follow the manufacturer’s operating instructions carefully. When demounting is to be done by hand, proceed as follows: 1. Loosen both tire beads from the rim flanges by inserting a bead breaking iron between the rim flange and the tire bead (fig. 2-95). Work progressively around the rim, rotating the bead breaking iron down and forcing the bead completely into the center well area. Turn the wheel over and repeat the procedure. NOTE: Ensure that the tire is fully deflated before dislodging the tire beads. Use care in dislodging the tire beads to avoid damaging the bead seats. If the tire bead requires excessive force to break free, apply tire lubricant to the bead area. 2. With the demountable flange side facing up, insert the lock ring tire iron, curved side up, into prying notch (fig. 2-95). With the iron, pry the flange partly out of the rim. Insert a flat tire iron adjacent to the lock ring tire iron and work both irons progressively around the rim until the flange is completely removed. 3. If the tire has a tube, push the valve stem through the hole in the rim. Shift the tire and tube on the rim to keep the stem away from the edges of the valve hole. 4. Hold the upper tire bead down into the well area of the rim (fig. 2-95). On the opposite side of the rim, insert a flat tire iron between the upper bead and the rim. Pull the tire iron down, lifting the tire bead partly off the rim. Use a second tire iron to work progressively around the rim, completely removing the upper tire bead. 5. Stand the wheel upright. Push down on the rim so the inner tire bead is positioned in the well area (fig. 2-95). Insert a flat tire iron between the inner tire bead and the rim. Pry the tire free of the rim. 6. If the tire has a tube, remove the flap and tube from the tire (fig. 2-95). Inspect all parts for wear, deterioration, or other defects. If the rim is to be reused, it should be cleaned, coated, and finished according to maintenance instructions. NOTE: To prevent damaging wheel components, areas for servicing tires and wheels normally have rubber matting on the deck. MOUNTING A TIRE .—Use the following procedure to mount a tire on a rim: 1. If you are installing a tube in the tire, dust it with talcum powder ZZ-T-416 to assist in assembly and to reduce chafing. Insert the tube and flap into the tire (fig. 2-95). Align the valve stem with the balance mark on the side of the tire. Inflate the tube slightly to hold the tube and flap in place and to prevent it from becoming wrinkled or pinched. 2. Apply rubber lubricant to both tire beads and the rim flange area. 3. Place the rim on rubber matting with the demounting flange side up (fig. 2-95). Place the tire on the rim, and align the valve stem with the valve hole. If necessary to force the tire bead down into the well area, tap it in using a rubber hammer. Be careful not to damage the tire bead seat. Use a flat tire iron to force the lower tire bead completely into the rim well area. Guide the valve stem through the valve hole in the rim. 4. Use a flat tire iron to pry the upper tire bead over the edge of the rim (fig. 2-95). Force the bead completely into the well area of the rim. 5. Place the end of the demountable flange, without the prying notch, into the gutter of the rim (fig. 2-95). Insert the tire iron into the notch of the flange and under the gutter of the rim. Pry the flange over the edge of the rim. Hold the flange, and work progressively around the rim until the flange is completely down into the gutter. Shift the tire and the tube on the rim to center the valve stem in the rim valve hole. 6. Inspect the wheel to insure that the flange is fully seated under the rim gutter (fig. 2-95). The opening between the flange ends should be 3/32 inch to 5/16 inch (1/4 inch is optimum). WARNING Because of its design, the demountable flange rim is potentially the most dangerous of all the rims handled in the aviation support equipment community. Personnel servicing this rim/wheel assembly must adhere to all safety precautions, listed or inferred, in NAVAIR 17-1-129 and other applicable technical manuals. 2-81
p. 122
2-82 ASf02095 1. VALVE STEM2. VALVE CORE 3. VALVE EXTENSION4. DEFLATED TIRE FLAG (RED) 1 23 3/4"MIN VALVE STEM ASSEMBL Y LOOSENING TIRE BEAD 1 2 1. BEAD BREAKING IRON 2. RIM FLANGE 1. PRYING NOTCH2. FLANGE 3. FLAT TIRE IRON4. LOCK RING TIRE IRON5. VALVE STEM2 1 3 4 5 REMOVING DEMOUNT ABLE FLANGE 1. RIM2. TIRE BEAD 3. FLAT TIRE IRON 1 3 2 UPPER TIRE BEAD REMOV AL RIM REMOV AL 1. RIM2. TIRE BEAD 3. FLAT TIRE IRON4. TIRE13 2 4 DEMOUNT ABLE FLANGE SEATING CORRECT INCORRECT INSTALLING THE DEMOUNT ABLE FLANGE 1. TIRE IRON2. VALVE HOLE 3. RIM 4. FLANGE4 1 2 3 INSTALLING THE UPPER TIREBEAD 3 2 1 1. WELL AREA2. LOWER TIRE BEAD 3. VALVE STEM 4. FLAT TIRE IRON5. VALVE HOLE 6. TIRE MOUNTING OF TIRE AND TUBE 3 2 1 4 5 6 1. TIRE2. TUBE 3. FLAP 4. VALVE STEM 5. BALANCE MARK 3 2 1 4 5 FLAP AND TUBE REMOV AL / INSTALLATION 4 Figure 2-95.—Demountable flange teardown and buildup.
p. 123
INFLATING A TIRE AFTER MOUNTING.— Inflating a support equipment tire involves the use of two very specific pieces of equipment. These two pieces of equipment are a tire inflator assembly and an inflation safety cage. Tire Inflator Assembly Kits .—A tire inflator assembly kit, commonly called an inflator, is a tool for inflating aircraft and support tires (fig. 2-96). The kit comes with all of the fittings and gauges required to service vehicle and aircraft wheel assemblies. Because of the numerous types and sizes of support equipment tires used, tire shops must have an assortment of the kits to cover a variety of tire pressures. Note in figure 2-96 that the two gauging elements in this kit cover the ranges of 10 to 150 psig and 50 to 600 psig. Always use the proper kit for the job, which is the one whose relief pressure valve is set closest to, but greater than, the service pressure of the tire. Tire Inflation Safety Cage .—Tires/rim assemblies sometimes explode violently during initial inflation after being mounted. Normally, this is the result of defects in the materials or improper assembly. A tire inflation safety cage is a device for containing a tire during inflation (fig. 2-97). Current regulations require that after mounting, any tire must be placed in a safety cage before it is inflated to its service pressure. The sole purpose of the safety cage is to protect you. 2-83 ASf02096 ACTUATING LEVER BLOCK ASSY. GAUGING ELEMENT (10 TO 150 PSI) REMOTE CONTROLLER GAUGING ELEMENT (50 TO 600 PSI) SERVICING HOSE (10 FT.) HOSE ASSEMBLY FITTING BLEEDER PETCOCK CARRYING CASE RELIEF VALVE DETACHABLE FITTINGS TIRE VALVE ADAPTOR Figure 2-96.—Tire inflator assembly kit. ASf02097 Figure 2-97.—Safety cage for inflation of tires.
p. 124
WARNING Never repair or adjust tires alone. And, under no circumstances should you reach into, or enter, the tire inflating cage for the purpose of servicing or adjusting equipment, except in the presence of someone capable of rendering aid. While an inflation cage is in operation, do not allow any person to rest or lean against the cage. Do not place any equipment on, or lean any equipment against, the cage. Inflating the Tire .—The unintended explosive separation of tire and rim components is the primary cause of accidents during tire/wheel servicing. For this reason, servicing should only be conducted by fully certified personnel, or personnel under the direct supervision of certified personnel. Use the following steps to inflate a tire after mounting it on a rim: 1. Remove the valve core (or deflated tire flag), and attach the tire valve adapter to the valve stem (fig. 2-95). 2. Attach the servicing hose fitting to the valve adapter. 3. Place the wheel in the inflation safety cage (fig. 2-96). Close and secure the door of the safety cage. 4. Select the proper gauging element from the tire inflator assembly, and attach it to the remote controller. 5. Connect the servicing hose to the remote controller, and connect the remote controller to the shop air supply. 6. Inflate the tire to a maximum of 10 psig, and allow it to fully deflate. This should cause the tube to center itself in the tire, and the tire bead should seat onto the bead seat. Then, inflate the tire with just enough pressure to seat both tire beads against the rim flanges. The beads should seat before reaching the service pressure for the tire. 7. Continue inflating until the service pressure for the tire is reached. Then, leave the tire in the safety cage for 10 minutes. After 10 minutes, recheck the pressure. If no loss is detected, reduce the pressure in the tire to 50 percent of service pressure, or 15 psig, whichever is the lesser. 8. Remove the wheel from the inflation cage. Remove the servicing hose fitting and valve adapter, and install a valve cap, finger tight. The wheel is now ready to be placed in storage or mounted on a vehicle. If the Beads Fail to Seat During Inflation.—Occasionally beads will fail to seat due to friction between the bead and the rim. If the beads will not seat, deflate the tire, remove it from the cage, and apply fresh tire lubricant. Return the tire to the cage and repeat the inflation procedure. WARNING Inflated tires should be inspected for proper flange and tire bead seating while still contained within the inflation cage. However, do not try to correct the seating of tire beads or rim flange by hammering, striking, or otherwise forcing the components while the tire is pressurized. If a Significant Pressure Loss is Noted After Inflating the Tire .—If a significant pressure loss is noted during the 10 minute wait following inflation of a tire, use the following steps to correct the problem: 1. Reduce pressure 50 percent of service pressure, or 15 psig, whichever is the lesser. 2. Remove the wheel from the inflation cage. 3. Remove the servicing hose and valve adapter. 4. Determine the cause of the leak and make repairs. Some common causes are: /c183A defective or incorrect valve core /c183A loose valve stem or damaged seal (tubeless) /c183A defective rim/tire seal area (tubeless) /c183A defective tire inner liner (tubeless) /c183A puncture /c183A defective or cut valve stem (tube) 2-84 CAUTION Never exceed tire service pressure while seating the beads. The beads may strike the flange with enough force to break, jump the flange, or otherwise damage the tire.
p. 125
/c183A damaged or deteriorated inner tube After the cause of the leak is corrected, inflate the tire. INSTALLING THE WHEEL ON THE VEHICLE.—When wheels are to be used on dual installations, they should be matched in size and tread wear. Improperly matched tires cause rapid, uneven wear, and may also cause transfer case and differential failures. If dual tires are used in a situation where permissible differences in measurement are allowed, the larger tire should be mounted outboard. Table 2-2 lists some instances where differences in measurement are allowed. Use the following steps for installing a wheel on a vehicle: 1. Ensure that the vehicle is firmly supported on jack stands and chocked. (For our discussion, assume that you are aboard ship, and tie-down chains are required.) 2. Place the wheel on the vehicle. /c183Install single wheels with the demountable flange outboard. /c183For dual wheel assemblies, mount the inner wheel with the demountable flange facing outboard. Install a spacer and then the outer wheel. For the outer wheel, the demountable flange faces inboard. 3. Install the wheel bolts/clips. Use a crisscross pattern, and apply the correct amount of torque. When torque values are not listed in the technical manual or MRC, use the values listed in table 2-3. NOTE: Threads should not be oiled, as the torque values listed are derived with oil-free threads. 4. Install the tire valve adapter to the valve stem, and connect the fitting to the adapter. 5. Attach the gauge to the remote controller. Ensure that the relief pressure valve setting of the remote controller is no higher than necessary. 6. Connect the servicing hose and shop air to the remote controller. 7. Inflate the tire to the prescribed service pressure. Use short bursts to allow frequent pressure checks. 8. Remove the fitting and valve adapter, and install the valve cap, finger tight. 9. Loosen the tie-downs, and jack the vehicle enough to remove the jack stands. Lower the vehicle, remove the jack, and retighten the tie-downs. WARNING When inflating a tire outside of a safety cage, ensure that all personnel are clear of the rim/tire trajectory area. NOTE: We elected to base our discussion here on the demountable flange rim because it is potentially the most dangerous to work with. However, the procedures for working with solid and split rims is very similar. Consult NA V AIR 17-1-129 and other applicable technical manuals for procedures for working with these types of rims. Repairing Wheels by Welding Wheels should be repaired by welding only when new wheels are not available. Such repairs are limited to one weld of a crack that is no longer than 1 inch, or two welds of cracks that are no longer than 1/2 inch. Further, no weld can be within 1 inch of any bolt hole. 2-85 Outside Diameter of Tire Permissible Difference Diameter Circumference Under 30 inches From 30 to 40 inches From 40 to 50 inches Over 50 inche 1/4 inch 3/8 inch 1/2 inch 3/4 inch 3/4 inch 1-1/8 inch 1-1/2 inch 1-3/4 inch Table 2-2.—Tolerances in Matching Dual Tires FINE THREAD SERIES Bolt Size Torque (lbs in.) 5/16 - 24 100 - 140 3/8 - 24 160 - 190 7/16 - 20 450 - 500 1/2 - 20 480 - 690 9/16 - 18 800 - 1000 5/8 - 18 1100 -1300 Table 2-3.—Torque Ranges by Bolt Size
p. 126
Wheels repaired by welding must be NDI inspected, and must be replaced as soon as possible. SAFETY REVIEW Here are a few safety precautions to remember when working with support equipment tire and wheel assemblies: Always place wheels in an inflation cage or an OSHA approved equivalent restraining device for initial inflation. Wheel assemblies sometimes explode violently during initial inflation due to defects or improper assembly. For aircraft tires used on support equipment, such as the spotting dolly, only an approved aircraft tire inflation cage should be used. (Aircraft tires are typically inflated to 200 pounds, or more.) On dual wheel installations, the wheel not being serviced should be deflated to 15 psig prior to loosening any wheel clamps. Ensure that a tire is fully deflated before dislodging the tire beads. Never exceed tire service pressure to seat beads. Beads may strike flanges with enough force to break, jump the flange, or otherwise damage the tire. For vehicles with disc brakes, wheels should be installed using a torque wrench only. Impact wrenches may distort the brake disc when used to tighten lug nuts/bolts. Q2-38. Which of the following types of rims are NOT used on SE? 1. Split rims 2. Open-faced rims 3. Solid rims 4. Demountable flange rims Q2-39. In what direction should the lock rings of the demountable flange rim be installed on the hub of a tow tractor that has more than one wheel per side? 1. They should be installed so that they are facing each other 2. They should be installed so that they are facing away from each other 3. They should be installed so that both rings are facing outboard 4. They should be installed so that both rings are facing inboard Q2-40. Which of the following components must be installed on a tire prior to removing it from the item of SE? 1. The safety tag 2. The demountable flange 3. The valve stem 4. The deflated tire flag Q2-41. What is the minimum required length of a deflation tire flag pin? 1. 1.0 in. 2. 1/4 in. 3. 1/2 in. 4. 3/4 in. Q2-42. What are the two very specific pieces of equipment that are required to inflate a tire after it has been assembled? 1. High pressure air and a tire inflator assembly 2. An inflation cage and a tire inflator assembly 3. An inflation cage and a pressure gauge 4. A tire inflator assembly and nitrogen cart Q2-43. There are procedures you must follow if there is significant tire pressure loss after how many minutes? 1. 10 min 2. 20 min 3. 30 min 4. 40 min Q2-44. When it becomes necessary to weld a wheel assembly, repairs are limited to which of the following specifications? 1. Limited to 1 weld, no longer than ½ inch 2. Limited to 2 welds, no longer than 1 inch each 3. Limited to 1 weld, no longer than 1 inch 4. Limited to 2 welds, no longer than 1/3 inch each 2-86