Differential

Differential

When both rear wheels are connected to a common driving shaft rapid wear of\rear tyre, and difficulty in steering from the straight-ahead position are soon experienced. It can be seen in Fig. 26.43 that the outer wheel must travel a greater distance than the inner wheels during cornering of the vehicle. Hence, if the wheels are interconnected, the tyres have to 'scrub' over the road surface and tend to keep the vehicle moving straight ahead. These problems can be minimized by driving one wheel and allowing the other to run free. But this provides unbalanced driving thrust and unequal cornering speeds due to which the arrangement was not accepted. The problem was solved in 1827 by Pequeur of France who invented the differential. This mechanism rotates the wheels at different speeds, while maintaining a drive to both wheels.

Example 26.4. The steering set of a, vehicle provides a turning-circle radius of 6.6 m with a wheel-track width of 1.2 m. The effective road wheel rolling diameter is 0.72 m. Calculate the number of revolutions made by the inner and outer wheels for one turning circle.

Fig. 26.43. Need for differential.

and revolutions completed by inner wheels = 12pi/0.72pi = 16.6 revolutions

The differences = (20 - 16.6) = 3.4 revolutions between ^he inner and outer wheels for one complete turning circle of wheels. This difference must be absorbed by tyre scrub or accom­modated by some mechanical device.

Principle

Consider the two discs, illustrated in Fig. 26.44A, are joined by shafts to the wheels and interconnected with a lever. When a force, F, is put to C at the centre of the lever, each disc receives half the applied force. The movement of the discs depend on the resistances, R, opposing the motion of the shafts. If a larger resistance acts on disc 'B', the lever tilts, and pushes disc 'A' forward a greater amount. This condition is illustrated in plan view in Fig. 26.44B and the increase in distance travelled by A equals decrease in distance travelled by B, and increase in speed of A equals decrease in speed of B.

Therefore, A + B = 2C.

The disc system is replaced by bevel gears in Fig. 26.44C, called sun wheels (disc) and planets (levers). The drive, applied to the cross pin, pushes the planet gears forward and exerts an equal

Fig. 26.44. Action of differential.

torque on each sun wheel irrespective of the speed. When the vehicle negotiates corner, the inner wheel slows down and the planets rotate on their own axis allowing the outer wheel to speed up. During straight-ahead movement of the vehicle the whole unit rotates at the same speed.

A differential unit is shown in Fig. 26.44D in which a crown wheel is bolted on to a differential cage. This cage also carries the sun wheels on plain bearings and transmits the drive to the cross pin. Two planet gears are sufficient for light vehicles but four gears are necessary in heavier vehicles to reduce tooth pressures. The differential is lubricated by the final-drive oil, which splashes through holes in the differential cage.