on the outside of the turn must rotate faster and travel a
greater distance than the wheels on the inside. This
causes no difficulty for the front wheels on the usual
passenger car because each wheel rotates independently.
However, for the rear wheels to be driven at different
speeds, the differential is needed. It connects the
individual axle shaft for each wheel to the bevel drive
gear; therefore, each shaft can turn at a different speed
and still be driven as a single unit. Refer to the
illustration in figure 11-17 as you study the following
discussion on differential operation.
The bevel drive pinion, connected to the propeller
shaft, rotates the bevel drive gear and the differential
case which is attached to it. Within the case, the
differential pinions are free to turn on individual pivots
called trunnions. Power is transmitted to the axle shafts
through the differential pinions and the side gears. The
axle shafts are splined to the side gears and keyed or
bolted to the wheels.
When the resistance is equal on each rear wheel, the
differential pinions, side gears, and axle shafts all rotate
as ONE UNIT with the drive gear. In this case, there is
no relative motion between the pinions and the side
gears in the differential case; that is, the pinions do not
turn on the trunnions, and their teeth will not move over
the teeth of the side gears.
When the vehicle turns a corner, one wheel must
turn faster than the other. The side gear driving the
outside wheel will run faster than the side gear
connected to the axle shaft of the inside wheel. To
compensate for this difference in speed and to remain in
mesh with the two side gears, the differential pinions
must then turn on the trunnions. The average speed of
the two side gears, axle shafts, or wheels is always equal
to the speed of the bevel drive gear.
To overcome the situation where one spinning
wheel might be undesirable, some trucks are provided
with a DIFFERENTIAL LOCK. This is a simple dog
clutch, controlled manual] y or automatically, which
locks one axle shaft to the differential case and bevel
drive gear. Although this device forms a rigid connection
between the two axle shafts and makes both wheels
rotate at the same speed, it is used very little. Too often,
the driver forgets to disengage the lock after using it.
There are, however, automatic devices for doing almost
the same thing. One of these, which is used rather
extensively today, is the high-traction differential. It
consists of a set of differential pinions and side gears
Figure 11-17.Typical differential and axle assembly with ring and pinion.