The drive line assembly has several important functions. It must perform the following:
The assembly provides a path through which power is transmitted from the transmission to the drive axle assemblies or auxiliary equipment. Vehicles having a long wheelbase are equipped with a drive shaft that extends from the transmission or transfer case to a center support bearing and a drive shaft that extends from the center support bearing to the rear axle.
Figure 10-1 — Drive line assembly.
The drive line assembly (Figure 10-1) consists of the following:
The type of transmission (manual or automatic) determines how the slip joint is connected to the drive shaft. On a manual transmission, the slip yoke is splined to the drive shaft with the yoke for the universal joint directly behind the transmission or transfer case, whereas with the automatic transmission, the slip yoke is splined to the output shaft. Either way they serve the same purpose—to provide the necessary telescopic action for the drive shaft. As the axle housing moves forward and backward, the slip joint gives freedom of movement in a horizontal direction and yet is capable of transmitting rotary motion.
The slip yoke used with an automatic transmission has the outer diameter machined smooth. This smooth surface provides a bearing surface for the bushing and rear oil seal in the transmission. The transmission rear oil seal rides on the slip yoke and prevents fluid leakage out of the rear of the transmission. The seal also keeps dirt out of the transmission and off the slip yoke.
The drive shaft, also called a propeller shaft, is commonly a hollow steel tube with yoke(s) welded on the end. The tubular design makes the drive shaft strong and light. Most vehicles use a single, one-piece drive shaft. However, many trucks have a two- piece drive shaft. This cuts the length of each shaft to avoid drive line vibration.
Since a drive shaft spins at full engine speed in high gear, it must be straight and perfectly balanced. If NOT balanced, the shaft can vibrate violently. To prevent this vibration, drive shaft balancing weights are welded to the shaft at the factory. Small metal weights are attached to the light side to counteract the heavy side for smooth operation.
The drive shaft can be either open or enclosed, depending on the type of drive used. The Hotchkiss drive has an open drive shaft that operates a rear axle assembly mounted on springs (Figure 10- 2). The Hotchkiss drive requires that the springs be rigid enough to withstand the twisting action (torque) of the rear axle and the driving and braking forces that the springs transmit to the frame.
Figure 10-2 — Hotchkiss drive assembly.
Another type of drive is a torque tube. Torque tubes differ from the Hotchkiss design in that a solid drive shaft is enclosed in a hollow torque tube and rotates within a support bearing to prevent whipping. One universal joint is used at the front of the drive shaft, and the rear of the drive shaft is attached to the axle drive pinion through a flexible coupler.
A universal joint, also called a U-joint, is a flexible coupling between two shafts that permits one shaft to drive another at an angle to it (Figure 10-3). The universal joint is flexible in a sense that it will permit power to be transmitted while the angle of the other shaft is continually varied. A simple universal joint is composed of three fundamental units consisting of a journal (cross) and two yokes. The two yokes are set at right angles to each other and their open ends are connected by the journal. This construction permits each yoke to pivot on the axis of the journal and also permits the transmission of rotary motion from one yoke to the other. As a result, the universal joint can transmit power from the engine through the shaft to the rear axle, even though the engine is mounted in the frame at a higher level than the rear axle, which is constantly moving up and down in relation to the engine.
Figure 10-3 — Universal joint.
A peculiarity of the conventional universal joint is that it causes a driven shaft to rotate at a variable speed in respect to the driving shaft. There is a cyclic variation in the form of an acceleration and deceleration of speed. Two universal joints are placed in a drive shaft to eliminate the speed fluctuations of the shaft while the shaft is at an angle to the power source. The universal joints are placed at a 90-degree angle to each other, and one counteracts the action of the other while in motion.
Three common types of automotive drive shaft universal joints are used on rear-wheel drive vehicles: cross and roller, ball and trunnion, and double-cardan (constant velocity) universal joints.
The cross and roller design is the most common type of drive shaft U-joint. It consists of four bearing caps, four needle roller bearings, a cross or journal, grease seals, and snap rings (Figure 10-4).
Figure 10-4 — Cross and roller universal joint.
The bearing caps are held stationary in the drive shaft yokes. Roller bearings fit between the caps and the cross to reduce friction. The cross is free to rotate inside the caps and yokes. Snap rings usually fit into grooves cut in the caps or the yoke bores to secure the bearing caps and bearings.
There are several other methods of securing the bearing caps in the yokes. These are bearing covers, U-bolts, and bearing caps.
The ball and trunnion universal joint is a T-shaped shaft that is enclosed in the body of the joint (Figure 10-5). The trunnion ends are each equipped with a ball, mounted in needle bearings, and move freely in grooves in the outer body of the joint, in effect, creating a slip joint. Compensating springs at each end of the drive shaft hold it in a centered position.
Figure 10-5 — Ball and trunnion universal joint.
Variations in length are permitted by the longitudinal movement of the balls in the body grooves. Angular displacement is allowed by outward movement of the balls on the trunnion pins. This type of universal joint is recognized easily by the flexible dust boot that covers it.
The double-cardan universal joint uses two cross and roller joints in tandem to form a single joint (Figure 10-6). The joints are linked through a centering yoke that works in conjunction with a specially designed spring-loaded centering ball. The components are contained within the centering coupling yoke.
Figure 10-6 — Double-cardan universal joint.
As the shafts rotate, the action of the centering ball and yoke maintains an equally divided drive angle between the connected shafts, resulting in a constant drive velocity.
The speed fluctuations caused by the conventional universal joints do not cause much difficulty in the rear-wheel drive shaft where they have to drive through small angles only. In front-wheel drives, the wheels are cramped up to 30 degrees in steering. For this reason velocity fluctuations present a serious problem. Conventional universal joints would cause hard steering, slippage, and tire wear each time the vehicle turns a corner. Constant velocity joints eliminate the pulsations because they are designed to be used exclusively to connect the front axle shaft to the driving wheels.
Basic operation of a CV joint is as follows:
The constant velocity joints you will normally encounter are the Rzeppa, Bendix-Weiss, and tripod types (Figure 10-7).
Figure 10-7 — Rzeppa CV Joint.
The Rzeppa constant velocity (CV) joint is a ball bearing type in which the balls furnish the only points of driving contact between the two halves of the coupling. A Rzeppa CV joint consists of a star-shaped inner race, several ball bearings, bearing cage, outer race or housing, and a rubber boot.
The inner race (driving member) is splined to the inner axle shaft. The outer race (driven member) is a spherical housing that is an integral part of the outer shaft; the balls and ball cage are fitted between the two races. The close spherical fit between the three main members supports the inner shaft whenever it is required to slide in the inner race, relieving the balls of any duty other than the transmission of power.
The movement of the balls is controlled by the ball cage. The ball cage positions the balls in a plane at right angles to the two shafts when the shafts are in the same line. A pilot pin located in the outer shaft moves the pilot and the ball cage by simple leverage in such a manner that the angular movement of the cage and balls is one half of the angular movement of the driven shaft. For example, when the driven shaft is moved 20 degrees, the cage and balls move 10 degrees. As a result, the balls of the Rzeppa joint are positioned, from the top view, to bisect the angle formed.
A tripod or ball and housing CV joint consists of a spider, usually three balls, needle bearings, outer yoke, and boot. The inner spider is splined to the axle shaft with the needle bearings and three balls fitting around the spider. The yoke then slides over the balls. Slots in the yoke allow the balls to slide in and out and also swivel.
During operation, the axle shaft turns the spider and ball assembly. The balls transfer power to the outer housing. Since the outer housing is connected to the axle stub shaft or hub, power is sent through the joint to propel the vehicle.
When two or more drive shafts are connected in tandem, their alignment is maintained by a rubber bushed center support bearing (Figure 11-8). The center support bearing bolts to the frame or underbody of the vehicle. It supports the center of the drive shaft where the two shafts come together.
Figure 11-8 — Center support bearing.
A sealed ball bearing allows the drive shaft to spin freely. The outside of the ball bearing is held by a thick, rubber, doughnut-shaped mount. The rubber mount prevents vibration and noise from transferring into the operator’s compartment.
A bearing similar to the center support bearing is often used with long drive lines containing a single drive shaft. This bearing is called a pillow block bearing. It is commonly used in drive lines that power auxiliary equipment. Its purpose is to provide support for the drive shaft and maintain alignment. When used at or near the center of the shaft, it reduces the whipping tendency of the shaft at high speed or when under heavy loads. The construction of pillow blocks varies. The simplest form is used on solid power takeoff drive shafts, which is no more than a steel sleeve with a bronze bushing.
A drive line is subjected to very high loads and rotating speeds. When a vehicle is cruising down the road, the drive shaft and universal joints or constant velocity joints may be spinning at full engine rpm. They are also sending engine power to either the front or rear axle assemblies. This makes drive line maintenance very important.
The drive shafts must be perfectly straight and the joints must be unworn to function properly. If any component allows the drive shafts to wobble, severe vibration, abnormal noises, or even major damage can result.
When operating a vehicle to verify a complaint, keep in mind that other components could be at fault. A worn wheel bearing, squeaking spring, defective tire, transmission, or differential troubles could be at fault. You must use your knowledge of each system to detect which component is causing the trouble.
Drive shaft noises are usually caused by worn U-joints, slip joint wear, or a faulty center support bearing. Drive shaft noises and possible causes are as follows:
Any other abnormal sound should be traced using your knowledge of mechanics, a stethoscope, and the vehicle’s service manual troubleshooting chart.
To inspect the drive shaft for wear or damage, raise the vehicle and place it on jack stands. Look for undercoating or mud on the drive shaft. Check for missing balance weights, cracked welds, and other drive shaft problems.
To check for working U-joints, wiggle and rotate each U-joint back and forth. Watch the universal joint carefully. Try to detect any play between the cross and the yoke. If the cross moves inside the yoke, the U-joint is worn and needs to be replaced.
Also, wiggle the slip yoke up and down. If it moves in the transmission bushing excessively, either the yoke or the bushing is worn. Inspect the rear yoke bolts for tightness. Make sure the rear motor mount is NOT broken. Look at any condition that can upset the operation of the drive shaft.
If after a thorough check of the drive shaft you fail to determine the problem, notify the shop supervisor. The drive shaft may require detailed measuring (drive shaft run out and drive shaft angle) or may need to have its balance checked.
The universal joints on many automotive vehicles are factory lubricated. However, construction equipment has universal joints that have lubrication fittings that should be lubricated at regular intervals.
Service to universal joints that are factory lubricated is limited to replacement when signs of excessive wear are present. The universal joints provided with lubrication fittings are lubricated only with a hand operated low-pressure grease gun. Use of a high-pressure grease gun will damage the seals, resulting in early failure of the universal joint.
Another area to be concerned with when servicing the universal joints is the slip yoke (joint). Slip yokes may be lubricated from the transmission or through a lubrication fitting.
Always consult the manufacturer’s service manual for lubrication intervals and proper lubricants to be used.
A worn universal joint is the most common drive line problem, causing squeaking, grinding, clunking, or clicking sounds. The grease inside the joint can dry out. The roller bearings will wear small indentations in the cross. When the bearings try to roll over these dents, a loud metal-on-metal grinding or chirp sound can result.
Quite often, a worn U-joint is discovered when the transmission is placed in reverse. When the vehicle is backed up, the roller bearing is forced over the wear indentation against normal rotation. When this occurs, the rollers will catch on the sharp edges in the worn joint, causing even a louder sound.
The universal joint may require removal and disassembly to enable you to check the condition of the join physically. Steps for the removal and disassembly of a U-joint are as follows:
Do NOT allow the full weight of the drive shaft to hang from the slip yoke. Support the drive shaft to prevent damage to the extension housing, rear bushing, and front U-joint.
Wear safety glasses to protect your eyes in case the snap rings fly out of the universal joint during removal.
Figure 10-9 — Universal joint disassembly.
Normally, a universal joint is replaced anytime it is disassembled. However, if the joint is relatively new, you can inspect, lubricate, and reassemble it.
During the inspection, clean the roller bearings and other parts in solvent. Then check the cross and rollers for signs of wear. If you find the slightest sign of roughness or wear on any part, replace the U-joint.
Once you have cleaned, inspected, and found the U-joint to be in a serviceable condition, you must reassemble it. Steps for reassembling a U-joint are as follows:
If the bearing cap fails to press into place with normal pressure, disassemble the joint and check the roller bearings. It is easy for a roller bearing to fall and block cap installation. If you try to force the cap with excess pressure, the universal and drive shaft could be damaged.
After assembly, check the action of the U-joint. Swing it back and forth into various positions. The joint should move freely, without binding. Double check that all snap rings have been installed properly. Once the U-joint has been checked and is working properly, reinstall the drive shaft back into the vehicle as follows:
Wipe off the outside slip yoke and place a small amount of grease on the internal splines. Align the marks and slide the yoke into the rear of the transmission.
Constant velocity joint service requires disassembly of the joint. Refer to the service manual for the vehicle when servicing a CV joint. The manual will give special detailed directions that are required depending on the type of joint.
Once the CV joint is disassembled, obtain a CV joint repair kit (usually includes new joint components, grease, boot, and bootstraps). When the joint is being assembled, refer back to the service manual for detailed directions.
Always use the recommended type of grease on a CV joint. The wrong type of grease will cause boot deterioration and joint failure. CV joint kits provide the correct type and amount of grease required.
After reassembling the CV joint, fit the boot over the joint. Make sure the boot ends fit into their grooves. Install the bootstraps. Do not over tighten the straps, as they may cut the boot or break.
The center support bearing is normally pre-lubricated and sealed at the factory. However, some support bearings have lubrication fittings and require lubrication at regular intervals. Even though lubrication extends the useful life of the bearing, they eventually wear out. The first indication of support bearing failure is excessive chassis vibration at low speed. This is caused by the bearing turning with the drive shaft in the rubber support.
When a faulty bearing is suspected, it should be inspected for wear and damage. If the rubber support shows any evidence of hardening, cracking, or tearing, it should be replaced.
Should you encounter a faulty support bearing, replacement procedures are usually limited to separating the drive shafts, unbolting the bearing support from the frame or cross member, and sliding the bearing and support assembly from the shaft.
If only the bearing is available from the parts room, disassemble the unit by gently prying the bearing out of the rubber support. Next, remove the dust shield from the bearing. Clean all parts that are to be reused. When the bearing is being replaced, some manufacturers recommend that waterproof grease be placed on both sides of the bearing, not for a lubricant but to exclude water and dust from the bearing. Install the dust shield and press the new bearing into the support.
Before securing the bearing support to the frame or cross member, check the service manual to determine if shims are required for alignment purposes. When reassembling support bearings, you should exercise care to ensure that proper alignment of the drive line is maintained. This will prevent abnormal wear of the universal joints.
1. Which is NOT a function of a drive line assembly?