Axle bearings are devices used to allow an axle to spin without resistance, wear and seizure. Typically comprised of roller-type bearings, common axle bearings provide a wider contact surface than the usual ball bearing can provide. Lubricated by the differential oil, the axle bearings differ from a front wheel bearing that is commonly packed with high-temperature bearing grease. The axle bearing is located near the end of the axle tube in what is known as a bearing cup. A rumble or grinding noise coming from the wheel area is often the first indication that a bearing is needing to be replaced.
The differences between front wheel bearings and axle bearings are significant. Wheel bearings are installed in pairs consisting of both inner and outer bearings, whereas axle bearings are installed as a single bearing and only support the outer end of the axle shaft. The axle bearings are usually pressed onto an axle shaft by means of a hydraulic press. The axle has an area machined near the axle flange that provides a flat and smooth area for the bearing to rest, also allowing the axle to spin true inside the axle housing without vibration or wobble. A wheel bearing rides upon a bearing race that is installed in the wheel hub with a race driver and a hammer.
The axle bearings are held in position partially by the axle and with some assistance from a bearing retainer. The retainer is placed on the axle before the bearing is installed. Once the axle has been slid into place within the axle housing, the bearing retaining plate is bolted into position on the end of the axle housing's axle tube. This is the extent of the axle being secured in some styles of axles, while others utilize a steel C-clip that is installed in a groove on the inside of the axle shaft. The axles are pushed slightly inward and the C-clips are placed into the groove in the axle.
In some racing applications, the C-clips are not used and are replaced with C-clip eliminators that fasten the axle to the axle housing, close to the axle bearings. There are front and rear sides of the axle bearings, and placing the bearing with the wrong side in or out can be grounds for premature bearing failure. Maintaining proper axle lubrication depth is critical to ensure proper bearing lubrication and long life of the bearing.