Differential Gears

When a car turns a corner, the driving wheels (the driven road wheels) rotate at different speeds, since the inside wheel has to travel a much shorter distance than the outside one.

For this reason the power must not be passed straight to the two wheels mounted on one continuous axle, but must, instead, act through a device called a differential gear, which drives two separate half shafts - one to each driving wheel - with the same torque, but at different speeds of rotation.

The propeller drive shaft, which forms the connection from the gearbox (or transmission case) to the rear axle, is provided at its rear end with a bevel pinion, which drives a larger gear wheel called the crown wheel; the latter is loosely mounted on one of the half shafts of the rear axle but is firmly connected to a box in which the four differential pinions are mounted.

When this whole box rotates (as a result of the rotation of the crown wheel to which it is bolted), the differential pinions whose shafts are rotatably mounted in the box, will be carried round with it on their shafts.

When the vehicle is travelling straight ahead, the box rotates, but the differential pinions will then not rotate on their shafts. They drive the bevel wheels which are rigidly connected to the inner ends of the driving shafts (the ‘half shafts’ to the road wheels) and which now rotate at the same speed.
When the vehicle is travelling in a bend: the right-hand half shaft is rotating at a slower speed than the left-hand half shaft.

Now the differential pinions will rotate on their respective shafts, with the result that they retard the bevel wheel of the right-hand half shaft and at the same time accelerate the left-hand bevel wheel. The principle of this operation: If the two toothed racks of the bevel wheels of the half shafts both move to the right at the same speed, the pinion will form a rigid connection between them.

But if the bottom rack is retarded a little, the pinion will roll along it and, consequently, rotate in relation to both racks.