Forensic Engineering Analysis of a Wheel Spindle Failure Due to Pre-Load and Fatigue
Typically, most vehicles equipped with non-powered wheels use a spindle that not only supports the weight of the vehicle but also allows the rotation of the tire. The rotation of the hub and wheel on the spindle is accomplished with the use of either a tapered or double row ball bearing. Bearings are mounted between the spindle and hub/wheel assembly, then secured with a castle nut set to a specified torque. Tapered bearings are chosen for this application because they are designed for applications where forces are generated radially (vertically) and axially (laterally) during use. Although tapered bearings are ideally suited for use in wheel and spindle assemblies, they must be installed properly to perform as designed. As part of that installation, the spindle nut must be properly torqued in order to apply a sufficient pre-load to the tapered bearings. Without the proper pre-load, the bearings can either generate too much friction or ride improperly on the spindle, generating forces that are not properly distributed. This paper will discuss the failure of a spindle and wheel assembly that experienced fatigue due to improper pre-load of the spindle nut.