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Driveshafts and Drive Axles, Page 1 of 3

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Driveshafts
See Figures 1 and 2

In a conventional longitudinally mounted front-engine/rear wheel drive vehicle, a driveshaft is used to transfer the torque from the engine, through the transmission output shaft, to the differential in the axle, which in turn transmits torque to the wheels. The driveshaft can be made out of steel or aluminum and can be either solid or hollow (tubular).

A splined slip yoke assembly, either as an integral part of the shaft or utilizing a splined transmission output shaft, permits the driveshaft to move forward and rearward as the axle moves up and down. This provides smooth performance during vehicle operation.

Figure 1 Cut-away view of a typical solid driveshaft and related components.
 Cut-away view of a typical solid driveshaft and related components.

On some four wheel drive vehicles, a front driveshaft connects the power flow from the transfer case to the front drive axle.

The driveshaft uses flexible joints, called Universal joints (U-joints) or Constant Velocity joints (CV-joints) to couple the transmission/transaxle to the drive axle/drive wheels. Refer to the Universal and Constant Velocity joints section for more information.

Front wheel drive vehicles also utilize driveshafts, although they are usually referred to as halfshafts. The halfshafts are usually equipped with CV-joints on each end which allow the wheels to turn as well as move up and down while still smoothly transferring engine power to the wheels. Front wheel drive vehicles typically use a transaxle (a combination TRANSmission and drive AXLE)

Figure 2 Exploded view of a typical front wheel drive halfshaft assembly using CV joint components on both ends.
Exploded view of a typical front wheel drive halfshaft assembly using CV joint components on both ends.

Some rear and four wheel drive vehicles use halfshafts. These vehicles will usually have a rigidly mounted differential and an independent suspension with halfshafts linking the differential to the drive wheels. For example, the 1998 Chevrolet Corvette--not only does it use halfshafts to drive the rear wheels, the rigidly mounted transaxle is actually in the rear of the vehicle with a driveshaft connecting the front mounted engine to the transaxle! As another example, the four wheel drive Subaru models use a modified front wheel drive transaxle assembly with an additional power output. A driveshaft couples the front transaxle to the rear differential with four halfshafts driving the front and rear wheels.

Universal and constant velocity joints
See Figures 3, 4, 5, 6 and 7

Because of changes in the angle between the driveshaft or halfshaft and the axle housing or driven wheel, U-joints and CV-joints are used to provide flexibility. The engine is mounted rigidly to the vehicle frame (or sub-frame), while the driven wheels are free to move up and down in relation to the vehicle frame. The angle between the driveshaft or halfshaft and the axle housing or driven wheels changes constantly as the vehicle responds to various road conditions.

Figure 3 U-joints are necessary to compensate for changes in the angle between the driveshaft and the drive axle.
 U-joints are necessary to compensate for changes in the angle between the driveshaft and the drive axle.

To give flexibility and still transmit power as smoothly as possible, several types of U-joints or CV-joints are used.

The most common type of universal joint is the cross and yoke type. Yokes are used on the ends of the driveshaft with the yoke arms opposite each other. Another yoke is used opposite the driveshaft and, when placed together, both yokes engage a center member, or cross, with four arms spaced 90° apart. A bearing cup is used on each arm of the cross to accommodate movement as the driveshaft rotates.

Figure 4 Exploded view of a typical cross and yoke universal assembly.
Exploded view of a typical cross and yoke universal assembly.

The second type is the ball and trunnion universal, a T-shaped shaft that is enclosed in the body of the joint. The trunnion ends are each equipped with a ball mounted on needle bearings that move freely in grooves in the outer body of the joint, in effect creating a slip-joint. This type of joint is always enclosed.

Figure 5 Cut-away view of a typical enclosed ball and trunnion type U-joint.
Cut-away view of a typical enclosed ball and trunnion type U-joint.

A conventional universal joint will cause the driveshaft to speed up or slow through each revolution and cause a corresponding change in the velocity of the driven shaft. This change in speed causes natural vibrations to occur through the driveline necessitating a third type of universal joint -- the double cardan joint. A rolling ball moves in a curved groove, located between two yoke-and-cross universal joints, connected to each other by a coupling yoke. The result is uniform motion as the driveshaft rotates, avoiding the fluctuations in driveshaft speeds.

Figure 6 Exploded view of a typical double cardan U-joint assembly.
Exploded view of a typical double cardan U-joint assembly.

The CV-joints, which are most commonly associated with front wheel drive vehicles, include the Rzeppa, the double offset, Tri-pod and Birfield joint.

The Rzeppa and double offset are similar in construction. They use a multi-grooved cross which is attached to the shaft. Balls ride in the cross grooves and are retained to the cross by a cage. The entire assembly then slides into an outer housing which has matching grooves for the balls to ride in.

Figure 7 Exploded view of a CV-joint equipped halfshaft. CV-joints shown are the Rzeppa/double offset style and the Tri-pod.
Click on picture to enlarge view

The Tri-pod design is similar to the ball and trunnion design, except it has three needle bearing mounted balls inside the housing spaced evenly apart (thus its name).

The newest of the CV-joints is called the Birfield. This joint is primarily found on import vehicles although some domestic vehicles are starting to use it as well. This joint is not serviceable and the manufacturers give no pictures or descriptions of its construction.

Front-wheel drive
See Figure 8

Front-wheel-drive vehicles are the more common arrangement for most cars and mini-vans these days. These vehicles do not have conventional transmissions, drive axles or driveshafts. Instead, power is transmitted from the engine to a transaxle, or combination of transmission and drive axle, in one unit. Refer to the Automatic or Manual Transmission/Transaxle Section for more information on the transaxle.

A single transaxle accomplishes the same functions as a transmission and drive axle in a front-engine/rear-drive axle design. The difference is in the location of components.

In place of a conventional driveshaft, a front wheel drive design uses two driveshafts, usually called halfshafts, which couple the drive axle portion of the transaxle to the wheels. Universal or constant velocity joints are used just as they would be in a rear wheel drive design.

Figure 8 Example of a typical transverse engine, front-wheel drive system. Notice that the components are similar to the rear-wheel drive systems, except for location.
Click on picture to enlarge view

Rear-wheel drive
See Figure 9

Rear-wheel-drive vehicles are mostly trucks, very large sedans and many sports car and coupe models. The typical rear wheel drive vehicle uses a front mounted engine and transmission assemblies with a driveshaft coupling the transmission to the rear drive axle. The rear axle assembly is usually a solid (or live) axle, although some import and/or performance models have used a rigidly mounted center differential with halfshafts coupling the wheels to the differential.

Figure 9 View of the typical rear-wheel drive axle system with leaf springs.
Typical rear wheel drive axle system with leaf springs.

Some vehicles do not follow this typical example. Such as the older Porsche or Volkswagen vehicles which were rear engine, rear drive. These vehicles use a rear mounted transaxle with halfshafts connected to the drive wheels. Also, some vehicles were produced with a front engine, rear transaxle setup with a driveshaft connecting the engine to the transaxle, and halfshafts linking the transaxle to the drive wheels.

Four-wheel drive
See Figure 10

When the vehicle is driven by both the front and rear wheels, two complete axle assemblies are used and power from the engine is directed to both drive axles at the same time. A transfer case may be attached to, or mounted near, the rear of the transmission/transaxle and directs the power flow to the rear and/or front axles through two driveshafts. Since the angles between the front and rear driveshafts change constantly, slip joints are used on the shafts to accommodate the changes in distance between axles and transfer case.

Another form of four or All Wheel Drive (AWD) design may use a front mounted engine and modified front wheel drive transaxle assembly with an additional power output. Two halfshafts connect the front wheels to the transaxle. Some models may have a transfer case connected to the transaxle's additional power output. A driveshaft couples the front transaxle or transfer case to the rear differential with two halfshafts driving the rear wheels.

Shifting devices attached to transfer cases disengage the front drive axle when four wheel drive capability is not needed. However, some newer transfer cases are in constant mesh and cannot be totally disengaged. These are known as "full-time" four wheel drive and are just what the name says, four wheel drive operating all the time. This is made possible by either a differential in the transfer case or through the use of a hydraulic viscous coupling.

Jeep& vehicles use a full-time system called Quadra-Trac, which is full-time four wheel drive with a limited slip differential in the transfer case. All you have to do is drive.

Figure 10 Typical transmission and transfer case design four wheel drive system. The shaded area represents the power flow.
Typical transmission and transfer case design four wheel drive system. The shaded area represents the power flow.

Viscous coupling transfer case

Back in the early '80s, American Motors created a full-time four wheel drive system that requires no action by the driver to activate the system, and take advantage of the improved traction and handling of four wheel drive.

Since this time other similar systems have been implemented both in domestic and import vehicles.

The heart of these systems is a transfer case, which distributes the torque between front and rear axles by means of a viscous or fluid coupling. The coupling provides a slip-limiting action and absorbs minor driveline vibrations, giving smoother and quieter operation.

When the front and rear driveshafts turn at the same speed, as they do when the vehicle drives straight down the road, there is no differential action. In a turn or other maneuvers where front and rear wheels must travel slightly different distances, differential action is required because the driveshafts must be able to rotate at slightly different speeds. When this happens, the fluid in the coupling-a liquid silicone-permits normal differential action.

Greater variations in speed between the driveshafts, such as occur when a wheel or pair of wheels encounter reduced traction and tend to spin, bring the viscous coupling's slip-limiting characteristics into action. The action of the viscous coupling is velocity-sensitive, permitting the comparatively slow movements typical of normal differential action but quickly building up resistance and effectively transmitting available torque to the axle with the best traction.

The action of the fluid between the plates in the coupling could be compared to the action of water against a body when wading across a pool. In waist-deep water, a person can walk with comparatively little effort as long as he moves slowly and gently. However, when he tries to hurry, the additional effort that is required is proportionate to the increase in speed he attempts to achieve. Therefore, it is with the viscous coupling. However, instead of water, there is liquid silicone with a viscosity nearly the consistency of honey.

This four wheel drive system is more efficient than other automatic four wheel drive systems because there is no "open" differential (as opposed to a limited-slip differential) between the driveshafts. In the "open" differential system, the loss of traction at one wheel results in no torque being delivered to the other wheels, since it is the nature of the open differential to deliver motion to the "easy" shaft-the one that is slipping. When using a viscous coupling, the loss of traction at one wheel on the rear axle brings the slip-limiting character of the viscous coupling into action, causing drive torque to be transferred to the front axle.

In addition to the differential function, the viscous coupling also improves braking effectiveness. It acts as a skid deterrent, tending to equalize drive-shaft speeds when the wheels at one end or the other want to lock and slide.

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©1998 W. G. Nichols - Chilton's Easy Car Care