Theoretical And Experimental Investigation Of The Degradation Reliability Of Ball Screws

As key components of computer numerical control(CNC)machine tools,ball screws play a significant role in determing the positiong accuracy and reliability of CNC machine tools.Under the guidance and support of the National Science and Technology Major Projects of China in recent 10 years,we have achieved rapid development for domesticallymade ball screws in recent years.As the direct manifestation of the overall performance for ball screws,reliability has significant influence on the market occupation for ball screws.However,the MTBF of domestically-made ball screws and imported ball screws cannot reach 2/3 of that for foreign ball screws,leading to the low market share for domesticallymade ball screws in the high precision CNC machine tools maket.Ball screws are typical product with small sample and long service life.Hence,it is difficult to achieve accurate reliability assessment through enough failure samples in the traditional reliability technology.In this paper,based on the reliability growth project for key components of CNC machine tools(2018ZX04039001),the degradation reliability technology was investigated.We carried out in-depth theoretical and experimental research on the degradation mechanism,performance degradation model,degradation reliability evaluation,and the remaining useful life prediction for ball scres,which provides new method to solve the difficulties in the reliability of ball screws.The specific research contents are as follows:(1)Formulate the framework of degradation reliability.Traditional reliability technology only concentrates on the failure information,which ignores the performance degradation information.Differently,the degradation information is is made full use of to provide a set of standardized workflows from performance degradation mechanism analysis to performance degradation modeling,reliability experiments,reliability modeling and asseseement,remaning useful life prediction and optimal maintance decision in this paper.The framework is significant in guiding the reliability work for ball screws.(2)Establish the performance degradation model for ball screws.Based on the load distribution model and sliding speed model,the modified Archard wear model is proposed.Then the performance degradation mechanism model under time-varying working conditions is established.Moreover,introducing the precison coefficient,the modified calculation method of the effective ball number for ball screws is proposed,and the testing method is also proposed.The experimental results show that the relative error of the modified calculation method in this paper is within 7.14%,which verifies the two-stage degradation for ball screws.Based on the two-pahase performance degradation mechanism model,the performance degradation index(D)is constructed after the preload being selected as the performance parameter.Considering the commom attributes of products in the same population,the individual differences of products caused by random factors,and the measure error with heteroscedasticity caused by random factors,a two-phase degradation model is established for the performance degradation index.(3)Propose the paramerter estimation method for the perfoemance degradation model.The degradation reliability test benches for light-load working conditions medium-load working condition,and heavy-load working condition were developed respectively.Then the degradation reliability experiments under three working conditions were conducted and the performance degradation data were obtained.To estimate the model parameters of the complicated performance degradation model,the estimation method based on the SIC algorithm and the EM algorithm for two-phase degradation model is proposed.(4)Establish the degradation reliability evaluation model considering the fatigue failure for ball screws.Based on the performance degradation model,the degradation reliability model is proposed.According to the competitive failure criterion,the degradation reliability evaluation model considering the fatigue failure for ball screws is proposed.Moreover,the reliability evaluation of ball screws under three different application conditions are carried out and verified respectively.(5)Verify the degradation reliability model and propose the application method of degradation reliability for ball screws.Vliadation experiments under light-load,mediumload,and heavy-load working conditions are conducted respectively.The results show that the the performance degradation data are within the 95% confidence interval of the established performance degradation model.It verifies the accuracy of the performance degradation model and the accuracy of the established degradation reliability model.Considering the uncertainty of the turning point,the integrated RUL prediction method is proposed,which combines the degradation mechanism model and the data-driven method.With the proposed method,we can predict the remaing useful life for the two-phase degradation ball screws.As the focus points of different application fields of ball screws are different,the predictive maintenance strategy centered on reliability and the optimal maintenance strategy centered on the economic cost are propose respectively.The sample analysis results show that,the predictive maintenance strategy centered on reliability can increase 67.94% of the average life for ball screws,and he optimal maintenance strategy centered on the economic cost can reduce 20.45% of the maintanence cost for ball screws.