The purpose of this paper is to examine the factors which initiate, contribute to, and subsequently cause the occasional loss of tractor or trailer wheels while under way on the nation's highways. Specifically, these are wheel loss failures which are characterized by deterioration of the axle nut system. Of major significance is the explanation of how the axle nut will degenerate to the extent the wheel can no longer be retained on the axle spindle.There are a number of reasons which cause the separation of either the hub or the wheel from the axle spindle. Initially, the various types of axle nut systems will be categorized and identified by type. The variations and their potential for wheel separation failure are discussed and evaluated.Of interest for examination are the following axle nut systems: 1. The Single Castellated Axle Nut With Its Means Of Securement Provided By A Cotter Pin. 2. The Conventional Double Jam Nut System. 3. Modified Jam Nuts Having Interlock Capability. 4. Modified Jam Nuts Having Interlock Capability With Variations Of Bearing Adjustment. Each of the four listed axle nut systems will be reviewed after identifying isolated problems with which they are distinctly associated. With additional insight provided by the identification and explanation of the predominant modes of axle nut failure, engineers involved with the future design of trailer axles and their axle nut retention systems can objectively select the most appropriate configuration needed to satisfy their individual or unique requirements for reliable axle nut securement and wheel bearing adjustment.