A rear axle assembly is a very large and heavy piece of steel and iron that is used to propel the vehicle by converting rotational force into linear motion. The drive shaft sends rotational power from the engine and transmission to the rear axle assembly differential. That assembly uses a ring and pinion gear set to convert the rotational power to linear motion, by turning the axles, tires and wheels. The differential component of the rear axle assembly can be a separate and removable component from the axle housing, or it can be a permanently attached component of the rear axle with a removable cover to allow access to the gear set.
In most vehicles, the rear axle assembly is the second largest and heaviest component of the vehicle, second only to the engine block. The reason for the heft of the rear axle assembly is that this component not only supports the weight of the rear of the vehicle, but it also supplies the propulsion aspects of the vehicle. The method of powering any wheeled vehicle is typically the same.
Rotational power from the engine's spinning crankshaft is transferred through the transmission and sent to the rear axle assembly in a rear-wheel drive application. The drive shaft connects to the pinion yoke, which is actually the snout of the pinion gear. The pinion gear applies the rotational power to the ring gear. This is the first step in the conversion to linear movement.
As the ring gear is powered around the center of the rear axle package, it turns the axles, wheels and tires in a driving force. The differential can be equipped with one of several types of differential packages, each providing a particular feature. The most common type of differential gear carrier is the open carrier. This powers one drive tire, while the other remain free and un-powered. This type of differential is placed in the average rear axle assembly of the family sedan.
The next step up in the performance-type rear axle assembly is the limited slip differential. This allows both rear tires to receive power if the vehicle becomes stuck. The most performance-oriented of all factory axle packages is the locker or locked carrier. This differential provides power to both rear tires equally and at all times. This differential only unlocks to allow the vehicle to turn a corner. In racing applications, the rear axle assembly will commonly have a spool installed to permanently lock both axles at all times.