Analysis of a ball screw with a preload and lubrication

Theoretical analyses of the kinematics of a single-nut double-cycle ball screw are made in the present study. An adjustment in the preload can be achieved by generating an offset λλ on the center pitch of two ball tracks. The critical load, which is very slightly higher than the preload, becomes the border of two axial load subregions. The contact kinematics for the axial load applied in each of these two load subregions are analyzed for these two ball tracks. Several parameters pertinent to the frictional forces formed at the ball–nut contact areas and the mechanical efficiency of a ball screw can thus be derived for these two axial load subregions. The theoretical frictional forces and thus the resulting mechanical efficiencies obtained by considering the oil lubrication are compared with that without oil lubrication. Experimental results of mechanical efficiency were also obtained from the performance tests of a ball screw; they show the behavior varying with an axial load quite similar to that exhibited in the theoretical solutions. The gaps in mechanical efficiency formed in a ball screw without oil lubrication are greatly reduced when considering the oil lubrication. This feature is consistent with the experimental results.