A Thermal-Based Spindle Bearing Load Controller: Preliminary Experimental Evaluation

Thermally induced bearing loads are a serious problem for high-speed machining spindles used in metal cutting. Ideally, the bearing loads can be set to provide the necessary spindle stiffness to achieve the desired dynamic performance, while at the same time achieving long bearing life. Unfortunately, this is difficult to do because many process conditions affect these loads, sometimes in difficult to predict ways with significant degradation of spindle performance and life. The purpose of this paper is to describe a preliminary experimental evaluation of controlling bearing loads by controlling the heat generated by a thermal actuator placed around the spindle housing. A box spindle modified with an electric heating tape placed around its front ball bearings is used for this evaluation. The results show that significant control over the bearing load can be achieved for the conditions tested. However, it is discovered that asymmetrical placement of the heater with respect to the pair of back-to-back angular-contact bearings makes it difficult to regulate the loads on each bearing to the set point. Solutions to this problem are discussed and further research outlined. Nevertheless, the feasibility of the control of thermally induced bearing spindle loads with an externally mounted heater appears promising.