An axle boot is a component that is commonly found on constant velocity (CV) axles. The primary purpose of these boots is to keep axle grease from escaping when a vehicle is in motion, though they can also stop various contaminants from entering the joints. If cracks or tears develop in an axle boot, it must be replaced or the joint it protects will eventually fail. A variety of materials can be used in the construction of axle boots, though rubber and other pliable substances can allow them a full range of motion when the wheels are turned. Typical CV axles have two of these boots because one is required for each joint.
There are several different types of axle configurations that can be used in cars and trucks, though constant velocity joints are commonly found in vehicles that have front-located engines and front wheel drive, or rear-located engines with rear wheel drive. These joints can use a few different methods to allow the transmission of a car or truck to rotate the wheels at a constant velocity regardless of whether they are turned or the suspension is moved up or down. Grease is applied to these joints and then sealed into place by axle boots during manufacture.
The two main functions of an axle boot are to keep grease in and keep contaminants out. CV joints rotate at significant speeds whenever a vehicle is driven, so the grease contained within them tends to be slung out during use. Since an axle boot is sealed to both the joint and the axle shaft, that grease is retained and can continue to lubricate. If an axle boot is cut or torn by a foreign object, or simply weakens and develops cracks due to age, the lubricating grease can be lost, which will lead to excess heat in the joint and eventual failure. A joint may also fail if abrasive materials are allowed to enter a cracked boot and interfere with the internal bearings.
Due to the fact that a torn axle boot can result in a failed CV joint, these components are often replaced at the first sign of wear or damage. It is still possible to replace a boot that has fully torn, though if the joint was damaged it may fail at a later time. Axle boot replacement typically involves the removal of the CV joint, and often also requires that the entire axle assembly be removed from the vehicle.