Lesson 2/Learning Event 2
When the differential side gears rotate at two different speeds, they cause the pinions or
spider gears to rotate on their shafts. The pinions walk around the slower side gear and
force the other side gear to turn faster or speed up. The faster side gear must turn at the
speed of the final drive ring gear plus whatever rotation is being caused by the rotating
pinion. The pinion rotates as it travels between the two side gears. The pinions continue
to rotate on their shafts as long as the side gears are at different speeds. If the vehicle
turns in the opposite direction, the pinions will again rotate on their shafts but in the
opposite direction.
When the inner wheel slows down in a turn, the outer wheel speeds up the same amount.
For example, let us say the ring gear and differential case are rotating at 100 revolutions
per minute (RPM) as the vehicle makes a turn that causes the inside wheel to slow down
to 70 RPM. This is 30 RPM slower than the ring gear. The outer wheel therefore has to
be rotating 30 RPM faster than the ring gear, or 130 RPM.
If torque is being applied to the rear axle, such as when going up a hill, the torque enters
the rear axle through the pinion and is then increased as it passes through the ring gear. It
is then transferred to the differential case and the differential pinion shaft and pinions.
The pinions then apply equal torque to each axle side gear, and the torque is transferred
to the wheels by the axle shafts.
If the tire on one side of an axle with a conventional differential loses its grip on the road
surface, the wheel will spin. With traction or grip on the road gone on one side, that
wheel becomes very easy to turn. The opposite wheel that still has good traction is hard
to turn. The power flow takes the path of least resistance and goes to the slipping wheel.
(Because of the differential gearing, one of the side gears is hard to turn and the other
very easy. The driving differential pinions walk around the hard-to-turn side gear and at
the same time drive the easy-to-turn side gear faster than normal.) This is an undesirable
feature, but it is not enough of a problem to cause a change to the types of axles used on
military vehicles.
A great amount of force is needed to move a heavy vehicle when it is stopped. When
engine torque is applied to the rear axle, there are forces attempting to move in many
directions. As the pinion gear tries to turn the ring gear, the two gears will tend to be
forced apart. Keep in mind that as the engine tries to move the vehicle, the vehicle will
resist and try to remain at rest. If the pinion gear is of the straight bevel type, the force
being applied will try to push the driving pinion to the
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