axle housing and drive axle designs will be different when
the vehicle has independent rear suspension. Also, when
the rear axle assembly is equipped with a limited-slip
differential, it will contain more parts. These features will
be discussed later in this chapter.
Differential Assembly
The differential assembly in a rear-wheel drive vehicle
has three functions. The first, and most obvious, is to
redirect the power flow to drive the rear wheels. The
power flow must make a 90° turn between the drive shaft
assembly and the rear wheels. This is accomplished in the
differential assembly by the drive pinion and ring gears.
The second function of the differential assembly is
to multiply engine power, reducing speed at the output
in the process. If there were no gear reduction (1:1 gear
ratio), the vehicle would accelerate very slowly. In some
cases, the engine would be unable to move the vehicle.
At the very least, gas mileage would be harmed, since
the engine would not reach its most efficient rpm range.
For this reason, the ring and drive pinion assembly, by
design, provides a reduced speed at its output. The
reduction is between 2:1 and about 5:1, depending on
the engine size, vehicle weight, and intended use of the
vehicle.
The third function of the differential assembly is to
allow the vehicle to make turns. If the assembly did not
make allowances for the different speeds of the rear wheels
during turns, one tire would lose traction with the ground
as the vehicle turned corners. The differential assembly
allows the vehicle to make smooth turns.
The differential assembly consists of numerous parts,
including the differential drive gears (ring and drive
pinion gears), pinion bearings, differential case, spider
and side gears, and side bearings. See Figure 16-3. These
parts and their function are described in detail in the
following section.
Differential Drive Gears
The differential drive gears also called the ring and
pinion gearset, consist of the ring and drive pinion gears,
Figure 16-4. These hypoid gears redirect power flow by
90° and multiply engine power. The number of teeth in the
ring gear compared to the number of teeth in the drive pin-
ion gear sets the rear axle ratio. For instance, if the ring
gear has 40 teeth and the pinion gear has 10 teeth, the
ratio is 40:10, or 4:1. The ring gear always has more teeth
than the drive pinion gear. Rear axle ratios can always be
determined by dividing the number of teeth on the ring
gear by the number of teeth on the drive pinion gear.
Drive pinion gear
The drive pinion gear is a hardened-steel gear with an
integral shaft, Figure 16-5. It is machined to mesh with and
rotate the ring gear. The end of the shaft opposite the gear
has external splines that fit the internal splines of the differ-
ential pinion yoke/flange. The gear is supported by two
tapered roller bearings, called pinion bearings.
By design, the axial centerline of the drive pinion gear
lies below that of the ring gear. With this design, the pinion
gear is placed lower in the rear axle housing. This is done
to lower the drive shaft and, therefore, the drive shaft hump
in the vehicle passenger compartment. The spiral design of
the gear teeth allows the gears to mesh with a sliding
motion, creating a smooth power transfer. As a result of the
sliding action, the gears must have a good supply of the
proper lubricant. Gears of this type are called hypoid gears.
312 Manual Drive Trains and Axles
Pinion bearing
Drive
pinion gear
Adjusting
nut
Side
bearing
Side gears
Ring gear
Spider
gears
Differential
shaft
Case
Side bearing
Adjusting
nut
Figure 16-3. Relative positions of parts of a differential assembly.
The interaction of the various parts of the differential may be
more easily understood by studying this illustration. (Subaru)
Rear axle
housing
Pinion
preload shim
Differential
pinion yoke
Pinion
bearings
Pinion depth shim
Drive
pinion gear
Ring
gear
Side
bearing
Differential
case
Differential
bearing shims
(endplay/preload)
Side
bearing
Figure 16-4. The positions of the drive pinion gear and the ring
gear are always about the same. The two pinion bearings and
two side bearings are always tapered roller bearings that must
be carefully adjusted. Some drive pinion gears have a third
bearing—a pinion pilot bearing—for support. Bearings and
adjusting devices (shims or adjusting nuts) are usually located
as shown. (DaimlerChrysler)