The 2002 failure of Control Moment Gyroscope (CMG) 1 in the International Space Station, and the subsequent anomalies in the remaining CMGs, have led to an interest in prognostics for the ISS CMGs. The bearing outer races can slide axially in the housing and are preloaded by spring washers. The prevailing failure theory is that one of the outer races became stuck in the housing, and that subsequent housing motion caused intermittent loss of preload, leading to bearing failure. A system of three coupled differential equations is presented that describes the axial dynamic motion of the flywheel/bearing assembly. Simulation results are presented supporting the theory that loss of preload can occur if one bearing becomes stuck.