During the several decades past, Ball screw play an important role in the feed drive system of CNC machines. With the development of manufacturing industry, CNC machine require higher precision delivering motion due to the higher requirement of products quality. Therefore, the reliability and the work performance are crucial standard for ball screw. The wear of ball screw will decrease the preload, which is the mean reason for the failure of ball screw. As the preload can avoid axial backlash and excessive heat generation, So suitable preload can improve the precision and lifecycle of ball screw. Predictive maintenance should be applied to avoid the failure of ball screw.