The differential gear is an assembly of gears that connects a divided shaft and permits the two sections of the shaft to rotate at different speeds or varying relative speeds. The most common application of the differential gear is in the automobile differential. It connects the divided rear axle, giving equal driving force to the two rear wheels while permitting them to rotate at different speeds when the automobile rounds a curve and the outer wheel tracks a longer arc than the inner wheel. This prevents undue slippage of the tires on the pavement, which would cause excessive tire wear, and reduces strain on the axle.
An ordinary automobile differential is shown in the accompanying diagram. The driving wheels are mounted on the outer ends of the divided rear axle. Power is transmitted to them from the drive shaft through the differential. As the drive shaft rotates, its pinion gear turns the ring gear and the spider, which is rigidly attached to the ring gear. As the spider rotates, its pinion gears turn the side gears, which are rigidly fixed to the inner ends of the divided rear axle.
When the automobile is moving straight ahead, the side gears and the spider's pinion gears do not move relatively to each other but rotate as a unit. However, when the automobile is rounding a curve, the side gears turn at different speeds, allowing one wheel to turn faster than the other. The difference in the speeds of the side gears is compensated for by the spider's pinion gears, which rotate so that the increase in speed of one side gear is offset by a corresponding decrease in speed of the other. The spider's pinion gears rotate at speeds that are the algebraic sum of the rotation speeds of the side gears.