The drive axle assembly transmits torque smoothly and efficiently from the engine and transmission to drive the vehicle's wheels. The purpose of a drive axle is to change the direction of the power flow, multiply torque, and allow different speeds between the two drive wheels. Drive axles are used for both front-wheel- and rear-wheel-drive vehicles.

FRONT-WHEEL-DRIVE (FWD) AXLES

Front-wheel-drive axles, also called axle shafts and front-drive shafts, transfer engine torque generally from the transaxle differential to the front wheels. One of the most important components of FWD axles is the constant velocity (CV) joint. These joints are used to transfer uniform torque and a constant speed while operating through a wide range of angels.

One front- or four-wheel-drive cars, operating angles of as much as 40 degrees are common. The drive axles must transmit power from the engine to the front wheels that have to drive, steer, and cope with the severe angles caused by the up-and-down movement of the vehicle's suspension. To accomplish this, these cars must have a compact joint that ensures the driven shaft is rotated at a constant velocity, regardless of angle. CV joints not only need to bend, they also must permit the length of the axle assembly to change as the wheel travels up and down.

FRONT-WHEEL-DRIVE APPLICATIONS

In a typical front-wheel-drive application, two CV joints are used on each half shaft. The outboard joints can be fixed Rzeppa or tripod, and the inner ones plunging tripod, double-offset, or cross groove joints.

Front-wheel-drive half shafts can be solid or tubular, of equal or unequal length, and come with or without damper weights. Equal length shafts are used in some vehicles to help reduce torque steer (the tendency to steer to one side as engine power is applied). In these applications, an intermediate shaft is used as a link from the transaxle to one of the half shafts. This intermediate shaft can use an ordinary Cardan universal joint to a yoke at the transaxle. At the outer end is a support bracket and bearing assembly. Looseness in the bearing or bracket can create vibrations. These items should be included in any inspection of the drivetrain components. The small damper weight, called a torsional damper, that is sometimes attached to one half shaft serves to dampen harmonic vibrations in the drivetrain and to stabilize the shaft as it spins, not to balance the shaft.

Regardless of the application, outer joints typically wear at a higher rate than inner joints, because of the increased range of operating angles to which they are subjected. Inner joint angles may change only 10 to 20 degrees as the suspension travels through jounce and rebound. Outer joints can undergo changes of up to 40 degrees in addition to jounce and rebound as the wheels are steered. That, combined with more flexing of the outer boots, is why outer joints have a higher failure rate. On average, nine outer CV joints are replaced for every inner CV joint. That does not mean the technician should overlook the inner joints. They wear too. Every time the suspension travels through jounce and rebound, the inner joints must plunge in and out to accommodate the different arcs between the driveshafts and suspension. Tripod inner joints tend to develop unique wear patterns on each of the three rollers and their respective tracks in the housing, which can lead to noise and vibration problems.

REAR-WHEEL-DRIVE (RWD) AXLES

Starting at the front or transmission end of the rear-wheel-drive shaft, there is a slip yoke, universal joint, drive axle yoke, and drive shaft. At the rear or differential end, there is another drive axle yoke and a second universal joint connected to the differential pinion flange.

In addition to these basic components, some drivetrain systems employ a center carrier bearing for added support. Large cars with long drive shafts often use a double U-joint arrangement, called a constant velocity U-joint, to help minimize driveline vibrations. A U-joint should not be confused with the types of CV joint found on front-wheel-drive vehicles, which is described previously in this chapter. The function of these various components follows.

SLIP YOKE The most common slip or sliding yoke design features an internally splined, externally machined bore that lets the yoke rotate at transmission output shaft speed and slide at the same time (hence the name slip yoke). While the need for rotation is obvious, without the linear flexibility, the drive shaft would bend like a bow the first time the suspension jounced.

DRIVE SHAFT AND YOKES The drive shaft is nothing more than an extension of the transmission output shaft. The drive shaft, which is usually made from seamless steel tubing, transfers engine torque from the transmission to the rear driving axle. The yokes, which are either welded or pressed onto the shaft, provide a means of connecting two or more shafts together. At the present time, limited numbers of vehicles are equipped with fiber composite - reinforced fiberglass, graphite, and aluminum - drive shafts. The advantages of fiber composite drive shaft are weight reduction, torsional strength, fatigue resistance, easier and better balancing, and reduced interference from shock loading and torsional problems. Some drive shafts are fitted with a torsional damper to reduce torsional vibrations.

The drive shaft, like any other rigid tube, has a natural vibration frequency. If one end was held tightly, it would vibrate at its own frequency when deflected and released. It reaches its natural frequency at its critical speed. Critical drive shaft speed depends on the diameter of the tube and its lengths. Diameters are as large as possible and shafts as short as possible to keep the critical speed frequency above the driving speed range. It should be remembered that since the drive shaft generally turns three to four times faster than the tires, proper drive shaft balance is required for vibration-free operation.

Several different methods have been designed to reduce the effects of vibrations and noise transfer. One example is a drive shaft with cardboard liners that strengthen the drive shaft's axial length. Drive shaft performance has also been improved by placing biscuits between the drive shaft and cardboard liner. These biscuits are simply rubber inserts that reduce noise transfer within the drive shaft. The tube-in-tube construction, is  another method of reducing drive shaft problems. This design eliminates much of the vibration and also reduces the clicking sound heard when the driveline is stressed with directional rotation changes. What makes the design different is that the input driving yoke has an input shaft that fits inside the hollow drive shaft. Rubber inserts are bonded to the outside diameter of the input shaft and to the inside diameter of the drive shaft.

As discussed later in this website, there are several methods of balancing drive shafts and drive axles. One of the most common techniques employed by vehicle manufacturers to reduce vibrations is to balance the drive shaft by welding balance weights to the outside diameter.