The drive axle is a special type of transmission that reduces the speed of the drive from the engine and transmission, then divides the power to the wheels. Power enters the axle from the driveshaft via a companion flange, which is mounted on the drive pinion shaft. The drive pinion shaft and gear which carry the power into the differential turn at engine speed. The gear on the end of the pinion shaft drives a large ring gear whose axis of rotation 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 determined by dividing the number of pinion gear teeth into the number of ring gear teeth.
The ring gear drives the differential case. This case provides the two mounting points for the ends of a pinion shaft on which are mounted two pinion gears. The pinion gears drive the two side gears, one of which is located on the inner end of each axle shaft.
By driving the axle shafts through this arrangement, the differential allows the outer drive wheel to turn faster than 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.
Proper adjustment of the relationship between the ring and pinion gears is critical. It should be attempted only by those with extensive equipment and/or experience.
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. During turning on a dry pavement, the effects of the clutches are overcome, and each wheel turns at the required speed. When slippage occurs at either wheel, however, the clutches will transmit some of the power to the wheel which has the greater amount of traction. Because of the presence of clutches, limited-slip units require a special lubricant.