INTRODUCTION
Learning Objective: Identify the components and explain the functions and the maintenance procedures for a drive line assembly, differentials, drive axles, a transfer case, and a power takeoff unit. Describe the different types of universal and constant velocity joints. Explain the adjustments and measurements of the ring and pinion gears. Describe the procedures for removing and replacing axle bearings and seals.

One important function of the power train is to transmit the power of the engine to the wheels. In a simple situation, a set of gears or a chain could perform this task, but automotive vehicles usually are not designed for such simple operating conditions. They are designed to have great pulling power, move at different speeds, operate forward and reverse, and travel on rough as well as smooth surfaces. To meet these widely varying conditions, a number of units have been added. In this chapter we will discuss drive lines, differentials, drive axles (rear and front drive), and power train accessories (transfer cases and power takeoffs).

DRIVE LINE ASSEMBLY
Learning Objective: Identify the parts and the functions of different types of drive lines. Describe the different types of universal joints.

The drive line assembly has several important functions. It must perform the following:

Send turning power from the transmission to the rear axle assembly.

Flex and allow up-and-down movement of the rear axle assembly.

Provide a sliding action to adjust for changes in drive line length.

Provide a smooth power transfer.

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.

The drive line assembly (fig. 5-1) consists of the following:

SLIP YOKE- connects the transmission output shaft to the front universal joint.

FRONT UNIVERSAL JOINT- the swivel connection that fastens the slip yoke to the drive shaft.

DRIVE SHAFT- a hollow metal tube that transfers turning power from the front universal joint to the rear universal joint.

REAR UNIVERSAL JOINT- a flex joint that connects the drive shaft to the differential yoke.

REAR YOKE- holds the rear universal joint and transfers torque to the gears in the rear axle assembly.

SLIP YOKE (JOINT)
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.

Figure 5-1.- Drive line assembly.