Preload degradation mechanism of double-nut ball screws revealed by an innovative wear distribution model

The preload of ball screws is generally used to confirm the ball screws with a high axial contact stiffness and a high positioning accuracy. Aiming at the preload degradation due to wear on the raceway of double-nut ball screws, a wear distribution model was proposed by calculating the contact stress distribution and the inhomogeneous relative sliding velocity distribution on the contact region to determine the wear profile of the raceway. Firstly, the contact stress distribution and the relative sliding velocity distribution on the contact surface were determined using the kinematics analysis and the force balance analysis. Subsequently, a wear distribution model was established to determine the wear depth distribution of the contact surface based on elastic-plastic contact mechanics of asperity with consideration of the microscopic characteristics of the raceway. Finally, the three-level step-stress accelerated degradation test based on the simulated model was designed and implemented on a self-designed bench to validate the proposed model. Experimentally measured friction torques of the ball screws were used to reflect the preload degradation, and they were compared with the results of the simulated model. The results showed that the degradation curves of the experimental friction torques and the wear profiles of screw raceways were in good agreement with the numerical results, confirming the correctness and effectiveness of the proposed model. The results indicated that the wear profile of the raceway could be accurately calculated based on the proposed model, clearly describing the wear process of the raceway surface and providing a more precise approach to predict preload degradation for ball screws.