What is a Differential on a car?

Outer and inner wheels go different distance on the same time interval by a car turning. The inner wheel goes less way on the inside line distance then outer wheel.  So the outer wheel rotation moment should quicker than inner.

A similar phenomenon occurs with linear movement if the rear wheels of the car have different diameters, which is quite possible with uneven load distribution on the car body, non-uniformity tire wear, different internal tire pressures and when driving on rough roads.

The Purpose of Differential is providing rotation of a drive wheels with different speed.

Types of differential in automobile

There are three main types of Differential in automobile:

• gear differential;
• gearless differential;
• worm gear differential.

Differential can be usual or controlled central differential (limited slip differential). Gear differentials are usual, and the gearless and worm gear differentials are limited slip differential.

Differentials are divided symmetric and asymmetric depending on whether it distributes the torque between the half-axles equally or not equally.

How does a differential work?

In order to understand what does a differential do, you can see Figure 1. The figure below displays the workings for the Differential.

a – when the moving of straight trajectory; b -when driving a car in a bend; 1 and 8 – axle shafts; 2 and 6 – axle shafts gear wheels; 3 – driven wheel of a main gear; 4 – satellite axis; 5 and 9 – satellites; 7 – differential box.

Gear symmetric Differential

Figure 2 shows components of symmetric differential that install between axle shafts of drive wheels. Two differential cups 1 and 5 joined with bolts 6. There is driven gear wheel of main gear is on the differential box, is providing drive to gearbox rotation.

Figure 2. Details of symmetric differential: 1 and 5 differential cups; 2 and 7 – washers; 3 – semi axial gear wheels; 4 – satellites; 6 – bolts; 8 – differential cross.

A cross 8 is clamped between the differential cups, on the spikes of which the spur bevel gears, the so-called satellites 4, meshed with two bevel gears 3 can rotate freely and can rotate.

Cogwheels with internal slots are connected to the splined ends of the axle shafts, freely passing through the holes in the differential box. At the outer ends of the axles are wheels. Washers 2 and 7 are placed under the end surfaces of the satellites and semi-axial gears to reduce friction.

When the differential box 7 rotates, it rotates the axle shafts 1 and 8 through semi-axial gears 2 and 6 and the satellites 5 and 9.

The transmission of torque occurs in the following order: through the driven wheel 3 of the main gear to the differential box 7, to the axis 4 of the satellites, to the satellites 5 and 9, to the semi-axial gears 2 and 6 and to the semi-axle shafts 1 and 8. Satellites, in addition, can rotate on their own axles, so they can change the frequency of rotation of the semi-axial gears relative to the differential box.

If the satellites do not rotate on the axis, then both semi axels shafts rotate at the same speed. This occurs when the car moves on a straight and level road, when the rear wheels with the same rolling resistance pass the same path. And therefore they have the same speed rotation.

When the car turns to the right, the satellites, rotating on their axles, run around the axle gears and increase the rotational speed of the semi axial gear wheel 2 and the associated axle shafts 1 and wheels. At this time, the rotational speed of the semi-axial gear 6 is reduced. This reduces the rotational speed of the axle shaft 8 and the wheel associated with the gear wheel 6. The speed of the differential box always remains half the sum of the rotational speeds of the left and right half shafts.

Center differential

In vehicles with two drive axles the center differential is used. The center differential case is attached to the main gear housing of the intermediate axle. The front center differential cup is bolted to the rear cup. A differential mechanism is placed inside, which includes satellites with a cross, a bevel gear wheel of the intermediate axle drive and a rear axle drive wheel.

The gear wheel with splines is constantly engaged with the bevel gear of the main gear of the intermediate axle, and the wheel with the shaft transmitting the rotation of the main gear of the rear axle. The gear wheel has external teeth with which the internal gear clutch and the differential lock clutch are in constant engagement.

Center Differential Construction

a – center differential construction; b – locking mechanism switch; 1 – driven cone gear wheel; 2 – main gear case; 3 – driving cylindrical gear wheel; 4 – satellite support washer; 5 – satellite; 6 – satellite bronze bushing; 7 – semi-axial gear wheel; 8 – support washer of semi-axial gear wheel; 9 – cross; 10 – driven cylindrical gear wheel; 11 – flange; 12 – axle differential housing; 13 – front cup of differential; 14 – microswitch; 15 – fork of coupling lock; 16 – locking mechanism activating; 17 – drive cone gear wheel; 18 – rear axle drive shaft; 19 – nut lock; 20 – spacer sleeve; 21 – lock coupling; 22 – inside gear coupling; 23 – cone gear wheel of drive intermediate axle; 24 – cone gear wheel of drive rear axle; 25 – rod; 26 – housing; 27 – pressure spring; 28 – return spring; 29 – cup of rod; 30 – membrane.

How does center differential work

When the fork moves the coupling forward, the last slides along the outer teeth of the inner coupling and engages with the outer teeth of the right differential cup, connecting the gear wheel to the differential housing to locking the center differential.

In order to prevent the locking mechanism from turning off, the internal gear coupling has two gears rims on the outside, and the teeth thickness of the outer gear rim is greater 0.4 mm than the thickness of the teeth of the inner gear rim. The activate of the locking mechanism, the driver must open the valve, directing compressed air between the cover and the membrane of the locking mechanism. The membrane is compressed, bends and overcomes the resistance of the spring, acting on the cup of the rod through the spring and moves the rod, and with it the plug.

At the same time, the contacts of the microswitch are closed, which include a control lamp on the instrument panel. Forced blocking of differential is used when driving on slippery and soggy dirt roads. The presence of a differential in the drive to the drive wheels of the car sometimes negatively affects its flotation. If one of the drive wheels hits a slippery section of the road, and the other rolls along a dry section, then it`s not possible transmitting significant torque to the wheel moving along the dry section due to the differential.

In this case, the wheel located on a slippery section will slip, and the other will stand motionless.

This occurs as a result that each satellite is an equal-arm beam, distributing the force acting on it between the semi-axial gears equally. If one wheel hits a slippery section of the road, then the semi-axial gear connected to it provides less resistance to the satellite and perceives the entire transmitted moment, while the other wheel remains stationary.This disadvantage eliminates by locking the differential.

In this case, both semi-axial gears will rotate at the same speed forcibly. This is ensured by connecting the gear wheels together.

Differential service

Differential parts must be lubricated because they are load with significant forces. In order to improve the supply of lubricant to these parts and increase the wear resistance of the satellite support washers, an oil trap is installed on the differential box. Differentials of passenger cars usually have two satellites, and trucks and buses have four satellites.