Theoretical And Experimental Investigation Of Friction And Wear Characteristics Of Linear Motion Ball Guides

As a key functional part of advanced CNC machine tools,a linear motion ball guide（LMBG）is one of the main factors restricting the accuracy of the principal machines.In recent years,with the support of the National Major Projects of Science and Technology（Advanced CNC Machine Tools and Basic Manufacturing Equipment）,the LMBG products from domestic leading enterprises have been significantly improved in basic performance,and have gradually entered the market of middle and high-end products of rolling functional parts.However,there is still a certain gap between domestic and foreign well-known products especially in precision retain ability.From the feedback of the field performance from CNC machine tool manufacturers,combined with the preliminary test research of major domestic manufacturers of LMBGs,it was found that the rapid loss of preload caused by friction and wear is one of the main reasons leading to the short time of accuracy retention of the domestic LMBGs.The rapid decline of accuracy results in low service life and reliability of LMBGs,which makes them difficult to meet the requirements of high-grade CNC machine tools for high accuracy and high reliability.As a result,the domestic high-grade CNC machine tools still largely use imported LMBG products,and the market share of domestic products is low.The reason lies in the late development of the LMBG manufacturing in our country and the lack of research on common,fundamental technical problems.Therefore,in this thesis,based on the national science and technology major project（2016ZX04004008）,the theoretical and experimental research has been carried out systematical to investigate the friction,wear and other basic characteristics of LMBGs.A comprehensive prediction model of preload degradation of LMBGs is established,which makes up for the lack of basic research on common technology,and provides theoretical and technical support for improving the service life and reliability of domestic LMBG products.It also lays a foundation for narrowing the gap between domestic and imported high-end products,and then improving the proportion of domestic rolling functional parts in high-grade CNC machine tools.The preload level of LMBGs can be charactered by friction state,and the preload degradation is the wear process.Therefore,this thesis mainly investigates the friction and wear characteristics of LMBGs.On this basis,the degradation model of preload is established by taking the preload degradation as the quantitative characterization of friction and wear characteristics.Firstly,a contact load model of LMBGs under different load conditions is established.Then considering the torque loads,the machining errors and operation conditions,the friction and wear characteristics of LMBGs are systematically studied combining with corresponding tests.Next,based on the friction model（the measurement basis）and the wear model（the theoretical basis）,the comprehensive prediction model of preload degradation is established.Finally,based on the above analysis,the optimization strategy for preload degradation is proposed and verified experimentally.The specific research contents are listed as follows:（1）Considering the skirt deformation of the carriage,the contact load model of an LMBG under external loads is established,and the load distribution under different torque loads is analyzed.The influence of machining errors on contact load is further investigated.The above studies are the basis for the subsequent analysis of dynamic characteristics such as friction and wear.（2）Considering the effects of lubrication conditions and raceway morphology,a friction force model for LMBGs is developed according to the equilibria of force and moment accounting for the variation of friction parameter,0)_{（8）,with operating conditions.Based on the modified lumped capacitance method（MLCM）,combined with finite element analysis,a thermodynamic model is established,which indirectly verifies the applicability of the proposed friction calculation model in high-speed conditions.In particular,under operating conditions of light load and high velocity,the difference between calaulated friction force and the theoretical value falls within 12%,which is much better than the conventional model.Overall,the proposed model has a wider applicability than the conventional model.（3）Based on the traditional Archard wear model,the contact coefficient64}^′5)（9）?))is introduced to reflect the actual contact condition of LMBGs.A wear coefficient measurement method of LMBGs is proposed and the wear rule is summarized.Based on the modified Archard wear model and the measured wear coefficient,a two-stage wear model of LMBGs is established considering the influence of machining errors.The variation of wear depth and initial wear rates of each raceway groove under different model parameters are analyzed in detail,which provides a direct theoretical basis for the prediction of preload degradation.（4）Based on the two-stage wear model,a comprehensive prediction model of preload degradation of LMBGs is established,including the key parameters such as the initial point,turning point of a two-stage degradation,critical point and failure point.Based on the friction model,the complete degradation curve of the preload of an LMBG is obtained by measuring the preloading drag force at the corresponding mileage.According to the rigidity curve,it is shown that the selection of the critical point in the model is reasonable.The results show that the deviation of the predicted value of the theoretical model from the measured value is less than 18.61%,which proves that the model can accurately predict the preload degradation during operation.