The term "axle cover" can actually refer to several components on an automobile or truck. In some cases, this term may refer to the differential cover; in other instances, it may refer to covers that resemble hubcaps. In still other applications of the term, the axle cover may refer to the axle itself. Generally, all of these parts are made from metal, usually a high grade of steel that can resist heat damage as well as impacts from road debris.
The differential is a component that is part of the drive axle. Several gears are mounted within the differential to help transfer power from the drive shaft to the wheels, and these gears are placed in an oil bath to help lubricate and protect the gears from excess friction and heat. The differential cover is sometimes referred to as an axle cover because it is often positioned in the center of an axle. This cover is an important component because it helps contain the oil within the differential and protect the gears from impacts from road debris or other objects. The axle cover in this case must be removed or otherwise loosened in order to change the differential oil; a gasket is pressed between the cover and the rest of the differential housing to help prevent leaks.
Another type of axle cover is mounted on the outside of truck wheels. This cover helps protect axle components that would otherwise be exposed to dirt, grime, grit, and even impacts from rocks or other solid objects. Within the ends of the axles, grease is often packed to help facilitate smooth movements of axle components. The axle cover can help prevent grit and grime from entering the grease, or prevent the grease from leaking out of the axle itself.
Sometimes the term "axle cover" refers to the outer part of the axle itself. It is the solid part of the axle that is secured to the vehicle's frame; contained within the cover are other axle components that will move freely thanks to bearings pressed into the cover or housing. The differential is part of the axle that is contained within the housing or cover, as are the axle spindles that will attach to the wheels of the vehicle. In this case, axle covers will not move when the vehicle is in motion; instead, the components within the covers will move.