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The differential is the thing that works both drive axles at the same
time, but lets them rotate at different speeds so that the car can
make turns. When a car makes a turn, the outer wheel has to turn
faster than the inner wheel, due to the difference in the length of
the paths they take. The differential is located between the two
wheels, and is attached to each wheel by a half-shaft rotated through
a bevel gear. Four-wheel drive cars have a separate differential for
each pair of wheels.
A grooved, or splined, axle side gear is positioned on the splined
end of each axle. The side gears are driven by "spider" gears, which
are little gears mounted on a shaft attached to the differential
case. As it is supported by the differential case, the side gear can
turn inside the case.
The differential case can be turned, revolving around the axle gears.
The differential pinion (a pinion is a small gear that either drives
a larger gear or is driven by one) shaft turns the ring gear, which
is fastened to the differential case. The propeller shaft (drive
shaft) connects the transmission output shaft to the differential
pinion shaft. The turning differential case is mounted on two large
bearing holders. These bearings are called carrier bearings.
The propeller shaft rotates the ring gear pinion, and the pinion
turns the ring gear. The ring gear then turns the differential case
and pinion shaft, but the axle side gears will not turn. By passing
the differential pinion shaft through two differential pinion gears
that mesh with the side gears, the case will turn and the axle side
gears will turn with it. During turns, the side gears turn at rates
dictated by the radius of the turns, and the spider gears then turn
to allow the outer wheel to turn faster than the inner one.
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