Lesson 18. Working principle of differential system and its construction

When an automobile travels around a corner, the distance traveled by the outside wheels is greater than that traveled by the inside wheels. If the wheels are mounted on dead axles so that they turn independently of each other, like the front wheels of an ordinary passenger vehicle, they will turn at different speeds to compensate for the difference in travel. But, if the wheels are driven positively by the engine, a device is necessary which will permit them to revolve at different speeds without interfering with the propulsion system. To accomplish this purpose a system of gears called the differential is provided.

The driving axle is one of the cross members which supports the load of the tractor, and has the driving wheels at its ends. The driving axle consists of a housing, a differential, two axle shafts (half axles), and final drives (if required).

The differential is an important component of the driving axle. The main functions performed by the differential system are:

  1. Further reduces the rotations coming from the gear box before the same are passed on to the rear axles.

  2. Changes the direction of axis of rotation of the power by 90o i.e. from being longitudinal to transverse direction.

  3. To distribute power equally to both the rear driving axles when the tractor is moving in straight ahead direction.

  4. To distribute the power as per requirement to the driving axles during turning i.e. more rotations are required by the outer wheel as compared to the inner wheel – during turns.

The main components of the differential (Fig. 18.1) are:

  1. Input pinion gear

  2. Crown wheel gear

  3. Differential cage

  4. Differential star

  5. Differential axle (sun) gear

differential diagram 



Crown wheel

Sun Gears

Star Gear

One wheel jacked and rotated, Gear engaged

Engine rotates


Only jacked side rotates

All rotate

Two wheels jacked and one rotated

Other wheel will rotate in opposite direction

Does not rotate

Both rotate

All rotate

Tractor moving in straight ahead position

Both wheels rotate with same speed

Rotates with the cage

Both rotate with the cage

Do not rotate independently but with the cage

Tractor turning left or right

Turning side rotates with slow speed. Other wheel rotates faster


Turning side rotates slower, other side rotates faster

All rotate

Diff locked (side dog coupling engaged with crown-wheel

Both wheels rotate with same speed

Rotates with the cage

Both gears rotate with same speed (with the cage)

Do not rotate independently but with the cage

Last modified: Monday, 3 March 2014, 4:30 AM