The drive axle is a special type of transmission that reduces the speed of the drive from the engine and transmission and divides the power to the wheels. Power enters the axle from the driveshaft via the companion flange, which is mounted on the drive pinion shaft. From there, the drive pinion shaft (which turns at engine/transmission speed) and gear carry the power into the differential. The gear on the end of the pinion shaft drives a large ring gear, the axis of rotation of which is 90 degrees away from that of the pinion. The pinion and gear reduce the gear ratio of the axle, and change the direction of rotation to turn the axle shafts which drive both wheels. The axle gear ratio is found by dividing the number of pinion gear teeth into the number of ring gear teeth.
The ring gear drives the differential case. The case provides the two mounting points for the ends of a pinion shaft, on which are mounted two pinion gears. These pinion gears drive the two side gears, each attached to the inner end of an axle shaft.
By driving the axle shafts through this arrangement, the differential allows the outer drive wheel to turn faster that the inner drive wheel in a turn.
The main drive pinion and the side bearings, which bear the weight of the differential case, are shimmed to provide proper bearing preload, and to position the pinion and ring gears properly.
Limited-slip differentials include clutches which tend to link each axle shaft to the differential case. Clutches may be engaged either by spring action or by pressure produced by the torque on the axles during a turn. When turning on dry pavement, the effects of the clutches are overcome and each wheel turns at the required speed. When slippage occurs at the either wheel, however, the clutches will transmit some of the power to the wheel with the greater amount of traction. Because of the clutches, limited-slip units often require a special lubricant.