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An axle assembly on a car or truck is the set of components that allow the wheels to rotate freely. Two wheels are usually connected by a shaft known as an axle; it sits inside an axle housing, and is held in place by bearings and/or bushings that allow it to rotate within the axle housing. Damage to the axle itself can cause the entire assembly to fail, so the housing acts as a protective layer for the spinning axle. Lubrication of the axle is also made possible because of the housing.
The axle housing may also contain other components that allow for steering, driving, or load bearing. If an axle housing is intended primarily for bearing a load it may be called a dead axle because it is not used to propel the vehicle forward. Many front wheel drive vehicles feature a rear dead axle that is meant only for load bearing and for keeping the left and right rear wheels on track. If the axle is part of the drive system, it may be known as a drive axle.
Sometimes an axle is not a solid piece, but instead two pieces that connect within the axle housing. This allows the wheels to rotate at different speeds; the gears that drive these wheels will be contained within the axle housing, as will the two separate axle pieces. A driveshaft may also enter the housing at its front, usually in the center, to connect with any drive gears for the wheels. The housing allows these components to be protected from impacts, and it also allows lubrication to be contained within the space, preventing premature breakdown of the axle components.
Larger trucks may feature more than one axle to help support the weight of the vehicle without causing excess strain on the axle itself. One axle is devoted entirely to supporting the truck's weight, while the other axle is not weight-bearing at all and allows the axle shaft to spin with far less stress. This is sometimes known as a full float axle system, and it is common on dump trucks, tractor trailers, and other large, load-bearing vehicles. Semi-floating axles will still end up supporting some of the weight of the vehicle, and non-floating axles are usually responsible for supporting the entire load of the vehicle.