• Car wheels spin at different speeds, especially when turning, each wheel travels a different distance through the turn, and the inside wheels travel a shorter distance than the outside wheels. Since speed is equal to the distance travelled divided by the time it takes to go that distance, the wheels that travel a shorter distance travel at a lower speed.
• For the non-driven wheels on a car this is not an issue, as there is no connection between them, so they spin independently. But the driven wheels are linked together so that a single engine and transmission can turn both wheels.
• If the car did not have a differential, the wheels would have to be locked together, forced to spin at the same speed. This would make turning difficult and hard on the car. For the car to be able to turn, one tire would have to slip. A great amount of force is required to make a tire slip, putting a heavy strain on the axle components.
• The differential is a device that splits the engine torque two ways, allowing each output to spin at a different speed.

Construction:

• Torque is supplied from the engine, via the transmission, to a drive shaft, which runs to the final drive unit that contains the differential.
• A spiral bevel pinion gear (P) takes its drive from the end of the propeller shaft, and is encased within the housing of the final drive unit. This meshes with the large spiral bevel ring gear (C), known as the crown wheel.

• The crown wheel gear is attached to the differential carrier or cage, which contains the sun gears (A & B) and planet pinions (C &D), which are a cluster of four opposed bevel gears in perpendicular plane.

• The two sun wheel gears drive the axle half shafts connected to the vehicle's driven wheels. The other two planet gears are aligned on a perpendicular axis which changes orientation with the ring gear's rotation.

Working:

• The Planet pinions are stationary when the vehicle is going a straight road, and the speeds of both the sun gears are equal, thus torque transmitted is the same.
• Suppose if the vehicle is making a turn to the right, the main crown wheel may make N full rotations.
• During that time, the left wheel will make more rotations because it has further to travel, and the right wheel will make fewer rotations as it has less distance to travel.
• Now because of the resistance, the right sun gear will rotate at a lesser speed, say ‘n’ rotations less. The left wheel will rotate at ‘n’ more than the input speed due to the action of the pinion gears.
• This will give the resultant speed on left wheel as (N+n) rpm and on the right wheel as (N+n) rpm.

When the car is taking a turn, the outer wheel have to travel more distance as compared to the Inner wheels in the same time. If car has solid rear axle only & no other drive, there will be tendency for the wheels to skid. To avoid this some mechanism must be incorporated which reduces the speed of inner wheel & increase of spend of outer wheels when taking a turn. It should at the same time keep the speed of all wheels same when going straight ahead. Such a device which serves the above function is called differential.
To understand the principle on which differential works, consider the simplest differential called as open differential as shown in fig 12.1.

To the crown wheel of the final drive is attached to cage, which carries a cross pin (in case two planet pinions are employed) or a ‘spider’ (in case four planet pinion are used in differential). Two sun gears mesh with the two or four planet pinions. Axle half shafts are splined to each of these sun gears. The crown wheel is free to rotate on half shaft.

Working of Differential :
When the vehicle is going straight the cage & the inner gears rotate as a single unit & the two half shaft rotate at the same speed. In this situation there is no relative movement among the various differential gears when the vehicle is stationary. Then turning one sun gear will cause other to rotate in the opposite direction. That means if left - sun rotates ‘n’ times in a particular time, the right sun gear will rotate 'p' times but of course in opposite direction. The rotation is superimposed on the normal wheel when the vehicle is taking turn. Thus for example, consider a vehicle with wheel speed N rpm going straight, when it takes a turn towards right. At this time, there will be resistance to motion of the right wheel & as result of differential action if the right wheel rotates back at 'n' rpm than the left sheet will rotate forward at 'n' rpm. This will give the resultant