They transfer power to the drive axles and rear wheels.
Side gears are also called axle end gears.
Some heavy-duty differentials contain four spider
gears and two pinion shafts. In this design, there is a center
hole in one of the shafts. The other shaft passes through it.
The side gears are splined to the drive axle. On some
differentials, the side gears contain C-locks, which hold
the axles in place. See Figure 16-11.
The spider and side gears are bevel gears. Power
transfer through the bevel gears causes them to be forced
away from each other. This causes high thrust forces on the
backs of the gears, where they contact the differential case.
Hardened-steel washers are usually installed between the
back of the gears and the case. These washers provide a
sliding surface and reduce wear. See Figure 16-12.
Figure 16-13 shows the operating states of the differ-
ential while driving straight ahead and while driving
around a corner. In Figure 16-13A, the vehicle is moving
straight ahead and both wheels are traveling at the same
speed. The spider and side gears rotate with the case but
do not move in relation to it. The entire case assembly
rotates as a unit.
When the vehicle makes a turn, the axles and the side
gears begin turning at different speeds. The outer wheel—
the left wheel, in the case of a right turn—turns faster than
the inner wheel, and the left side gear turns faster than the
right side gear. See Figure 16-13B. As a result of the
different axle speeds, the spider gears begin to rotate. The
left side gear, which is moving faster than the right side
gear, drives the spider gears, causing them to rotate on, or
walk around, the right side gear.
Note that the differential case speed on turns is the
average of the side gear speeds. This is because one side
gear is rotating faster than the case and the other side gear
is rotating slower than the case. In Figure 16-14, when the
vehicle makes a turn, the action of the differential allows
the outer wheel to turn at 110% of case speed, while the
inner wheel turns at 90% of differential case speed. These
percentages will vary with the radius of the turn.
Locking differential
The standard differential works well in most situa-
tions. However, on very slippery surfaces, such as icy or
muddy roads, lack of traction can cause the rear wheels to
slip. This is because the standard differential will drive the
wheel with the least traction.
If one drive wheel is on dry pavement and the other
is on ice or mud, the ring gear and differential case will
drive the spider gears. However, the spider gears will not
drive both side gears. When the spider gears are driven by
the differential case, they will walk around the side gear
related to the wheel on dry pavement. As a result, the
spider gears drive the slipping wheel, and the vehicle will
not move. The standard differential sends almost all engine
power to the slipping wheel.
To overcome this problem, locking differentials are
used. Locking differentials overcome traction problems by
sending some power to both wheels, while allowing the
vehicle to make normal turns. There are several different
types of locking differentials, including limited-slip, ratchet,
and Torsen® differentials.
The two most common types of limited-slip
differential are the clutch-plate differential and the cone
differential. The clutch-plate differential uses several fric-
tion discs that look like small manual clutch discs. The
cone differential uses a cone-shaped clutch that engages a
matching cone-shaped receptacle. Limited-slip differen-
tials have various brand names, including Positive
Traction, Sure-Grip, Anti-Spin, Traction-Lok, and TXT.
Many technicians refer to limited-slip differentials as
Positraction differentials, although this is actually a
General Motors brand name dating back to the 1950s.
Due to their complexity and higher cost, limited-slip
differentials are used only on high performance versions of
rear-wheel drive automobiles. Limited-slip differentials are
commonly found on modern trucks and SUVs. Many SUVs
and some trucks have limited-slip differentials on the front
and rear axles. Some companies make aftermarket limited-
slip differentials to replace original equipment designs or
to convert standard differentials to limited slip units.
An example of a common clutch-plate differential is
shown in Figure 16-15. The most obvious difference
between this limited-slip differential and a standard differ-
ential is the clutch packs placed between the side gears
and the differential case.
The clutch friction discs are made of steel covered
with a friction material. The clutch plates are made of
steel. The discs and plates are alternately splined to the
side gear and dogged (meaning tabs fit into grooves) to the
differential case, Figure 16-16. Grooves in the discs or
plates are for better grabbing power.
Figure 16-17 shows the moving parts of a clutch-plate
differential. The spider gears, side gears, and other parts
are very similar to those used in a standard differential. The
Chapter 16 Rear Axle Assembly Construction and Operation 315
Drive axle
Drive axle
Ring gear
Side gears
Case
Spider gears
Figure 16-10. The basic components of a differential case
assembly. The ring gear is bolted to the case, and the spider
gears and side gears are mounted inside. On most differential
assemblies, side bearings are pressed onto the case. All
differentials contain the same general parts.