An axle cap is a device used to seal the hub on a floating axle when the vehicle is being towed. Manufactured from various metals and composites, some of the most popular styles in axle cap designs are made from plastic. Once the axle is pulled from the wheel hub assembly, the axle cap is placed over the axle-mounting studs and secured by tightening the retaining nuts finger-tight. The installation of the cap allows the oil to remain inside of the hub as the vehicle is towed, thereby protecting the wheel bearings, the oil seals and even the wheels by preventing the hub oil from being slung out of the hub and covering the wheels.
When towing buses, heavy trucks and virtually any vehicle equipped with full floating wheel hubs, one or more of the drive axles must be removed from the differential to allow the wheels to spin freely during towing. One of the problems with this is that the axle actually seals the hub and prevents oil from leaking out as the wheel rotates during the towing process. Without the assistance from an axle cap, tow truck drivers were forced to place cardboard and other materials in the open hub to stop the oil from escaping. This often failed, resulting in burned wheel bearings, burned grease seals and very messy wheels as the oil would cover the wheels and tires. Occasionally, fires would result from the loss of oil combined with extreme heat related to operating the bearings while dry.
By placing an axle cap over the exposed hub opening, the loss of oil is kept to a minimum, and often none is lost at all. Rubber and cork gaskets placed on the hub behind the axle cap seal the hubs and maintain the oil level inside of the hub. Some models of axle cap include an oil fill access plug to allow tow truck operators to top off the hub prior to towing the vehicle. Once the tow is complete, the axle cap can be removed and the axle replaced. The cap can be wiped off and stored in the tow vehicle until it is needed again. Some axle cap designs include multiple bolt patterns to allow the cap to be used on a variety of different hubs.
By utilizing the covers, the towing company often gains a level of professionalism in the eyes of the vehicle owners. Preventing the need to wash oil off of the vehicle generated from towing with an improperly-sealed hub, the customer can save money on the cost of the tow. The covers also prevent wasted time for the towing company by eliminating the need to stop on the road because of burning wheel bearings on the vehicle in tow.