Factors Which Affect the Transient Behaviour of Preloaded Ball Bearing Assemblies

Abstract

The continuing development of cutting tool materials allows further increases in the speed of machine tool spindles to be made. The majority of rolling bearings used in mainspindle assemblies are preloaded to maintain adequate stiffness and lack of play at low speeds. However, unacceptable thermal distortion of machine structures may occur if the speed range is simply extended. In the last decade research into the lubrication of high pressure sliding contacts has identified and quantitatively evaluated the parameters which govern the frictional losses in such situations, and the study of elastohydrodynamic lubrication has evolved. Practical applications of this work have been made, but little has been reported on the transient variation of the preload and accompanying frictional losses in mainspindle assemblies under operating conditions. This paper describes an experimental rig developed to measure the variation of the preload and friction torque in a pair of angular contact ball bearings. The results illustrate the effects of spindle speed, oil flow rate, type of oil, housing design and housing cooling. The variation of friction torque is discussed in the light of elastohydrodynamic theory.