Introduction
The rear axle assembly is used on rear-wheel drive
vehicles. This assembly is the final leg of the drive train. It
is often called the final drive or rear end. The rear axle
assembly is often mistakenly called the differential. The
differential is only part of the rear axle assembly.
The basic design of rear axle assemblies has been
adopted by all manufacturers for many years. There are
several variations, but all operate according to the same
basic principles. The major difference between rear axle
assemblies depends on whether the vehicle has solid-axle
rear suspension or independent rear suspension. Solid-axle
rear suspension incorporates rigid and nonflexing drive
axles and axle tubes; both wheels move as one solid unit
in response to bumps and potholes. Independent rear
suspension incorporates jointed drive axles (no axle tubes)
that allow for flexibility and independent axle movement.
This chapter is designed to identify and explain the con-
struction and operation of various rear axle assemblies. The
material in this chapter provides a basis for understanding
how to properly troubleshoot and repair rear axle assemblies.
Construction and Operation Overview
The rear axle assembly includes the differential
assembly, the rear drive axles, and the rear axle housing.
Rear axle assemblies are subjected to heavy loads from the
engine and road. They are ruggedly constructed and
seldom fail. The most common rear end failures are axle
bearing failures. A typical rear axle assembly is shown in
Figure 16-1.
In a rear axle assembly, engine power enters the drive
pinion gear from the drive shaft assembly and differential
pinion yoke/flange. The drive pinion gear, which is in
mesh with the ring gear, causes the ring gear to turn. The
interaction of the ring and drive pinion gears turns the
power flow at a 90° angle. The difference in the number of
teeth on the ring and pinion gears causes a reduction gear
ratio. This reduces turning speed, while increasing torque.
Power from the ring gear flows through the differential
case, spider gears, and side gears to the drive axles. The
drive axles transfer power from the differential assembly to
the rear wheels.
The bearings and rear axle housing are key com-
ponents of the rear axle assembly. They are designed to
support and align the differential assembly and the drive
axles. Notice that the bearings and axle housing are large,
heavy-duty parts. This is to ensure they will stand up under
hard usage.
Seals and gaskets are also very important to the
operation of the rear axle assembly. Seals are used at the
differential pinion yoke/flange and at the outer drive axles.
Gaskets are used at housing interfaces, such as between
the differential cover and the housing, to provide a tight
seal from the outside.
Figure 16-2 is an exploded view of a common type of
rear axle assembly. Notice the relationship of the internal
parts to the housing and to each other. Note that the rear
310 Manual Drive Trains and Axles
Wheel, or
axle, bearing
Axle seal
Axle
flange
Side
bearings
Differential
case
Inspection cover
Ring gear
Drive axle
Pinion
bearings
Companion
flange
Drive
pinion gear
Pinion or
spider gears
Side gears
Rear axle
housing
Drive
axle
Figure 16-1. Most rear axle assemblies contain the same parts as shown in this cutaway. Note that some drive axles differ from this
basic design. (Ford)