Wheel bearings are most often the first thing a mechanic will think of when the term 'bearing' is used. In modern vehicles, they've largely been integrated into other systems of the axles, knuckles and joints. They can still fail, however, and require maintenance to prevent that and replacement should it occur. Wheel bearings are not the only type of bearings on a vehicle.
What Are Bearings?
Bearings are devices that allow things to spin. They appear in nearly every moving part of a vehicle, from engine accessories like the alternator to the wheels on the chassis. Any spinning part on the vehicle, be it an electric motor, a pump, or a tire and wheel, will have bearings.
Bearings are usually made up of a metal ring into which round or oval balls have been placed. With heavy lubrication, they allow the ring around the balls and a shaft down the center to spin independently while remaining connected.
Every bearing will fail if not properly maintained. If lubrication dries up or leaks out, if foreign objects or grit get into the bearing, or something similar happens, the bearing will eventually fail due to friction tearing it apart. In this case, it must be replaced.
Most modern bearings are sealed to prevent the lubricant from leaking and thus require little or no direct maintenance to keep in order. Component parts, such as the shafts and pulleys that may be connected, do require some maintenance to keep them from causing the bearing's failure. If the pulley becomes bent, the shaft becomes bent, or the whole is damaged in an accident then the bearing will (often quickly) begin to fail.
Replacing bearings is most often a simple, straight-forward process, but getting to them to make the replacement is not always easy.
In fact, wheel bearings are usually the easiest to replace as they require only that the tire/wheel and possibly a cover or cap be removed from the axle. Others, such as the bearing in an air conditioning pump, may be much more involved.
Regardless, most of them can be done by a good DIY mechanic at home.