Theoretical And Experimental Investigation Of The Preload Loss Of Ball Screws

Ball screws,which directly determine the machining accuracy and precision stability of CNC machine tools,are the key part for the CNC machine tools to realize the aim of'precise,fast and overloading'.In the last decade,with the support of the National Major Projects of Science and Technology,homemade ball screws have made a significant improvement in the stiffness and comprehensive performance.However,there still exists a great gap between homemade and foreign high-end ball screws in precision stability,which has become a serious bottleneck restricting the localization of China's high-end CNC machine tools.Large number of testing shows that,it is the rapid loss of preload that mainly leads to the poor precision stability of homemade ball screws,which leads to the leading position of the imported products in the ball screws used in the middle and high-grade CNC machine tools in our country.Therefore,the study of preload loss of balls screws becomes the core factor to catch up foreign products.This paper,based on the national science and technology major project(2012ZX04002021),has carried out an in-depth theoretical and experimental research on the calculating method of load distribution,preload,preload failure criterion and preload decline process of ball screws.More importantly,the optimization strategy of the preload degradation is proposed,which provides a theoretical guidance for the important engineering problems(such as the prolongation of the service life of the ball screw pair and the improvement of the reliability)and lays a foundation for the localization of China's high-end CNC machine tools.In order to solve the problem as aforementioned,the degradation characteristics of the preload of ball screws must be clarified.To get a complete degradation curve of the preload loss,the first problem is to calculate the initial value preload accurately(the start of the curve:calculating method of the preload),and then we need to know the critical value of the decreasing preload(the end of the curve:calculating method of the failure criterion of the preload),and the most important thing is to master the decline law of the preload(the intermediate process of the curve:calculating method of the decreasing preload).And all the above study must be based on the force analysis of ball screws(the theoretical basis of the curve:calculating method of the load distribution).The specific research contents are as follows:(1)Usually,the load of each ball in a ball screw mechanism is considered to be the same.While in this dissertation,considering the deformation of the nut,screw shaft and other elastic units,a load distribution model is built under both the static and dynamic condition,which proves the load of each ball is different and the misuse of the correlation between the axial and normal displacements of the ball-raceway contact in the literature.In addition,the effects of the lead,conformity,ball circle,axial load and rotational speed on the load distribution of the ball screws are calculated and analyzed,which lays a foundation of further studies on the prediction of preload loss of ball screws.(2)After the analysis of two different kinds of traditional formula(the NSK formula of Japan and the common formula in domestic)of the preload and no-load torque for ball screws a new correlation between the preload and no-load drag torque is derived,and a novel preload-adjustable ball screw mechanism and a drag torque measuring system are constructed to obtain the coefficient of friction(COF)of ball screws under different rotational speeds and preloading levels.Based on the above studies,a new calculating method of the preload of ball screws is built.(3)Some scholars think the decompression condition would happen when the axial load is 2.83 times of the preload,while others think the decompression condition has already happened when the axial load is equal to the preload.To resolve this dispute,based on the load distribution analysis of no-preloaded ball screws,the load distribution model of preloaded ball screws is built and the calculating methods of the critical axial load and preload are derived.Based on the novel preload adjustable ball screw mechanism and the stiffness test bench,both the preload and the decompression condition are detected in the experiment,which proves the new methods are valid.(4)Based on a modified coefficient Kn.introduced especially for ball screws to reflect the actual contact condition,the modified Archard theory is applied to ball screws to build a new wear model for ball screws.Meanwhile,the whole process of the reciprocating motion of the preloaded ball screws with or without axial load is analyzed,which proves that the above two conditions can be equivalent to each other.Then,a new test method for the whole life operation test for the preloaded ball screws under no axial load is proposed to obtain the wear law and wear coefficient,which can significantly simplify the test and improve the experimental efficiency.Based on the wear model,the critical axial load,the wear law and the wear coefficient,a new prediction model of the preload for ball screws is built.(5)Based on a new operation system,the variation of the preload,travel and travel deviation of a typical ball screw(R40-16K5-FDC)are measured.The experimental results show that,the relative error of the comprehensive model of preload loss built in this paper is within 5.53%.More importantly,new strategies for the optimization of preload loss are put forward,which has improved the performance of the optimized ball screw significantly.