Another unit of the power transmission system which must be used is the differential. It is a well-known fact that in turning a curve the outer wheels travel faster than the inner ones. To compensate for this a differential is used, sometimes called a compensating or equalizing gear. .

If the driving wheels were solidly connected to each other by the axle and that axle driven by a chain or shaft from the center or thereabouts, great stress would be placed upon the transmission tires and other parts, owing to the fact that both wheels in turning a corner would travel at the same speed, but owing to the fact that the outside one must travel faster than the inner one, the latter would be forced around or skidded on the road.

The layman often experiences difficulty in understanding the differential, as it is always enclosed in a casing and is entirely out of sight and unless he has a chance to see it in the repair shop or factory, he has little chance to familiarize himself with its action by actual observation.

The power of the motor is applied either to a centrally divided rear axle (usually termed a live axle) or to a jack shaft, thence by chains and sprockets to the rear wheels, turning loose on a dead rear axle. The first condition applies to all shaft-driven vehicles, such as the level gear, internal gear, double reduction and worm drive, the second applies to chain-driven vehicles. The action of the differential is identical for either of the above-mentioned drives. It is mounted in the rear axle housing for all shaft-driven types, while for the chain-driven types it is mounted in the jack shaft housing.

The object of the differential is to permit of equally dividing the driving effort of a single source of motive power between the two driving wheels and to allow cars driven through wheels on opposite sides to be freely steered. In turning a corner, the driving effort must be divided in such a way that one wheel may rotate faster than the other and still receive the same driving force as the other. In other words, the driving force must be equally divided between the driving wheels under all conditions for both forward and backward motion.

The division of the driving effort is very desirable, except when the driving wheels are on ground with greatly different coefficient of adhesion. For instance, if one wheel got into a mud hole, it might not have sufficient adhesion to prevent it from spinning under the turning effort impressed by the differential gear and it would then be impossible to propel the car except by locking the differential, because the turning effort of the wheel standing on solid ground would be no greater than the turning effort corresponding to the coefficient of adhesion of the wheel in the mud hole. For this reason differential locks are often provided.

This lock may be left engaged as long as all four wheels move in a perfectly straight path. When, however, a vehicle is to be moved in a curved path as in turning a corner the driving wheels must revolve at different speeds, since the outer one has to cover a longer distance in the same time than does the inner wheel which is on the inside of the curve.

There are three types of gear differentials in common use, viz., the bevel type, the spur type and the helical type.

The bevel pinion type of differential, which is probably the most common form is illustrated in Fig. 104. It consists of the following parts: a housing A, which is usually made in halves, two bevel gears B and C secured to the differential or driving shafts D and E respectively; a two, three or four-armed spider F, on the radial arms of which are carried bevel pinions G, G-l, G-2. G-3 meshing with both of the bevel gears B and C. To the housing A of the differential is usually bolted a bevel gear H for driving it, but this gear forms no part of the differential proper. It will be understood that the differential shafts D and E, either have secured to their outer ends road wheels or sprockets, which drive the road wheels through chains. Gear H meshes with the bevel driving pinion.

Bevel Gear Type Differential.

Fig. 104. Bevel-Gear Type Differential.

The action of the differential may be explained as follows: The spider F is held to the housing A and the turning effort or torque which is applied to the housing by means of the driving or ring gear H, and its pinion is equally divided by the bevel pinion G between the gears B and C. As long, therefore, as the resistance to motion of the two bevel gears B and C (in other words to the motion of the driving road wheels of the car), is the same, the two wheels will turn at the same speed. If, however, the steering wheel is turned so as to move the car to one side or the other, the driving wheel on the side to which the car turns is automatically held back with a force equal to the rond adherence. The differential gear, B or C as the case may be, on that side will then turn slower and its mate on the other side will turn correspondingly faster, while the bevel pinion G between them will turn on their journals at a speed corresponding to the difference in speed of rotation of the two bevel gears B and C. While a vehicle is turning a curve, although the speeds of the two driving wheels are unequal, the tangential forces acting at their circumference are equal.