Differential
(mechanical device)
Differential is a
car parts, variety of
gearbox, almost always used in one of two ways. In one of these, it
receives one input and provides two outputs; this is found in every
automobile. In the other, less commonly encountered, it combines two
inputs to create an output that is the sum of the
inputs.
In car and other wheeled
vehicles, the differential allows each of the driving wheels to rotate
at different speeds, while supplying equal torque to each of them. In
automotive applications, the differential and its housing are sometimes
collectively called a "pumpkin".
Purpose
The vehicles’ wheels rotate at
different speeds, especially when turning corners. The differential is
designed to drive a pair of wheels with equal force, while allowing them
to rotate at different speeds. In vehicles without a differential, such
as karts, both driving wheels are forced to rotate at the same speed,
usually on a common axle driven by a simple chain-drive mechanism. When
cornering, the inner wheel travels a shorter distance than the outer
wheel, resulting in the inner wheel spinning and/or the outer wheel
dragging.
Functional
description
The following description of a
differential applies to a "traditional" rear- or front-wheel-drive car
or truck:
Power is supplied from the engine
car parts, via the transmission or gearbox, to a drive shaft, which
runs to the differential. A spiral bevel pinion gear at the end of the
propeller shaft is encased within the differential itself, and it meshes
with the large spiral bevel ring gear. The ring gear is attached to a
carrier, which holds what is sometimes called a spider, a cluster of
four bevel gears in a rectangle, so each bevel gear meshes with two
neighbors and rotates counter to the third, that it faces and does not
mesh with. Two of these spider gears are aligned on the same axis as the
ring gear and drive the half shafts connected to the vehicle's driven
wheels. These are called the side gears. The other two spider gears are
aligned on a perpendicular axis which changes orientation with the ring
gear's rotation. These two gears are just called pinion gears, not to be
confused with the main pinion gear. As the carrier rotates, the changing
axis orientation of the pinion gears imparts the motion of the ring gear
to the motion of the side gears by pushing on them rather than turning
against them, but because the spider gears are not restricted from
turning against each other, within that motion the side gears can
counter-rotate relative to the ring gear and to each other under the
same force.
Thus, for example, if the car is
making a turn to the right, the main ring gear may make 10 full
rotations. During that time and the right wheel will make fewer
rotations as it has less distance to travel. The side gears will rotate
in opposite directions relative to the ring gear by, say, resulting in
the left wheel making 12 rotations, and the right wheel making 8
rotations.
The rotation of the ring gear is
always the average of the rotations of the side gears. This is why if
the wheels are lifted off the ground with the engine off, and the drive
shaft is held manually rotating one wheel causes the other to rotate in
the opposite direction by the same amount.
When the vehicle is traveling in a
straight line, there will be no differential movement of the planetary
system of gears other than the minute movements necessary to compensate
for slight differences in wheel diameter, undulations in the road etc.
