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 comer, 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.
Some trucks are equipped with a differential lock to
prevent one wheel from spinning. This lock is a simple
dog clutch, controlled manually or automatically, that
locks one axle shaft to the differential case and bevel
drive gear. This device forms a rigid connection
between the two axle shafts and makes both wheels
rotate at the same speed. Drivers seldom use it,
however, because they often forget to disengage the
lock after using it.
Several automotive devices are available that do
almost the same thing as the differential lock. One that
is used extensively today is the high-traction
differential. It consists of a set of differential pinions
and side gears that have fewer teeth and a different
tooth form from the conventional gears. Figure 13-19
shows a comparison between these and standard gears.
The high-traction differential pinions and side gears
depend on a variable radius from the center of the
differential pinion to the point where it comes in
contact with the side gear teeth, which is, in effect, a
variable lever arm. While there is relative motion
between the pinions and side gears, the torque is
unevenly divided between the two driving shafts and
wheels; whereas, with the usual differential, the torque
is evenly divided always. With the high-traction
differential, the torque becomes greater on one wheel
and lesson the other as the pinions move around, until
both wheels start to rotate at the same speed. When
that occurs, the relative motion between the pinion and
side gears stops and the torque on each wheel is again
equal. This device helps to start the vehicle or keep it
rolling when one wheel encounters a slippery spot and
loses traction while the other wheel is on a firm spot
and has traction. It will not work however, when one
wheel loses traction completely. In this respect, it is
inferior to the differential lock.
With the no-spin differential (fig. 13-20), one wheel
cannot spin because of loss of tractive effort and
thereby deprive the other wheel of driving effort. For
example, one wheel is on ice and the other wheel is on
dry pavement. The wheel on ice is assumed to have no
traction. However, the wheel on dry pavement will pull
to the limit of its tractional resistance at the pavement.
The wheel on ice cannot spin because wheel speed is
Figure 13-19.-Comparison of high-traction differential gears and standard differential gears.