On-Line Preload Monitoring for Anti-Friction Spindle Bearings of High-Speed Machine Tools

Catastrophic and premature bearing failure caused by excessive thermally-induced bearing preload is a major design problem for spindle bearings of high-speed machine tools. Due to a lack of a low cost and easy to maintain on-line preload measuring technique, the traditional solution is to limit the maximum spindle speed and the initial bearing preload. This solution is incompatible with the need to increase machining productivity, which requires increasing the spindle speed, and product quality (surface finish, dimensional accuracy), which requires increasing (or at least not decreasing) the preload to keep the spindle system stiff. This paper proposes a solution that is compatible with increased productivity and quality. A dynamic state observer has been developed based on a preload model derived from physical laws of heat transfer and thermoelasticity to estimate the spindle bearing preload via low cost thermocouples attached to the bearing outer ring and the spindle housing. A systematic procedure to determine the observer gains is developed to account for modeling errors, unknown parameters, nonlinearities, and measurement noise. The modeling errors are particularly significant in this problem because factors like bearing skidding, wear, and lubricant deterioration are not easily modeled, and the observer must be designed to be insensitive to them. A unique modeling error compensator idea is introduced to null-out the modeling errors. The preload observer has been successfully validated on two different bearing configurations for a wide range of speeds and running conditions.