Thermo-Mechanical Model of Ball Screw With Non-Steady Heat Sources

Abstract

A ball screw system is widely used for rapid translating precise motion because of its high efficiency and long lifetime. However, a high-speed ball screw drive system naturally produces heat through friction at contact areas, which thereby causes thermal expansion, which adversely affects machining accuracy. In order to achieve high accuracy and great stiffness, the ball screw preload is common solution of removing the backlash of the ball screw. Consequently the applied preload on screw shaft produces more heat through friction. It results in temperature rise and thermal deformation in the ball screw drive system. Thus a thermally induced preload problem is introduced $the heat source in the bearing becomes non-steady because of the coupling between the generated friction heat and the axial force of screw shaft changed by thermal deformation. A closed loop finite element analysis of ball screw drive system was carried out taking the non-steady heat sources in bearings into account. The developed numerical model confirms significant influence of bearings preload upon thermal stabilization of the ball screw drive system. The results of simulation are in accordance with measured data. The slight discrepancies are probably caused by fact that the heat flux from heated nut is not included in the model during idle time. Further the uniform heat transfer coefficient along the ball screw is applied to the FEA which is not fully in accordance with measurement conditions. Nevertheless, the numerical model sufficiently simulates thermo-mechanical behaviour of ball screw, so it is suitable for estimation of bearing thermal stability