High Speed Bearings for Machine Tool Spindles

A major car manufacturer reports that the greatest part of machines down time is due to spindle bearings failure, despite the fact that their loads are generally very low. It is Shown in this paper that the main reason for these early failures is thermal instability of the bearings/spindle/spindle housing assembly, that leads to uncontrolled hertzian stresses and hence to bearing destruction. It is also shown that the destructive process takes place during the first minutes after spindle start-up, but failure may appear days, weeks or even months later. The simplified equations and block diagrams describing the thermal behaviour of the spindle/bearing assembly are given. Finally, technological solutions are shown that completely eliminate the causes of thermal instability, while increasing bearing stiffness and decreasing its power consumption, by a complete control of hertzian stresses, hence greatly increasing bearings reliability and performances.