Key Performance Indicator Analysis And Assessment Of Linear Motion Rolling Guide

The linear motion rolling guide(LM)is one of key execution units in numerical control(CNC)machine tools,affecting the machine precision,precision stability and precision maintenance.Recently,main factories' LM products have achieved rapid progress in static stiffness and overall performance.However,LMs' precision maintenance is not bettter than the foreign products',which will restrict Chinese CNC machine tools to choose the domestic LMs.Most factory applications and tests show that,lack of static stiffness,preload failure,instability dynamic performance and low reliability are the main reasons affect the precision maintenance.In this paper,based on research and development of general technology for LMs of high-grade CNC machine tools(2012ZX04002021),we focus on four questions:preload analysis and measurement,static stiffness evaluation,vibration reduction of linear damper,reliability evaluation assessment.Finite element method,circuit model,dynamic model and numerical calculation are used in this study.Combining tests,LMs'four key performance indicators:preload,static stiffness,vibration and reliability are analyzed and evaluated.The main research contents are as follows:(1)ANSYS finite element method(FEM)is used to investigate the initial preload deviation.Based on the deviation rate of theoratical static stiffness and testing one,the preload deviation,?,is evaluated.In the test,one kind of carriage anti-deformation structure is used.Study shows that,the over-size increases,? increases.When the anti-deformation force decreases,? decreases.With 80 kN force to resist the carriage deformation,? comes up to 7.3%.Once the rigidity of the carriage is improved,the stability of the load performance will be improved,which is crucial to the CNC machine tools.According to the structure characteristic of the LMs,a circuit model is abstracted,and based on the Hertz theory,GW model and Ohm's law,the electric contact resistance RLM is calculated.The preload electric contact resistance test results show that,with the over-size of the rolling elements increasing,the RLM decreases.The RLM of roller type is lower than ball one in the same over-sized level.For the LM running 2000 km under 20%rated dynamic load,the RLM value is higher than new one implying the preload failure.(2)In traditional studies of LMs,the static stiffness is a main indicator to evaluate the mechanical behavior.However,the local contact profile between the roller and raceway surface,although very important to the precision maintenance,cannot be reflected in the measured value of static stiffness.In this chapter,a new indicator,named the real contact total length(RCTL),is proposed based on the Greenwood-Williamson model and the Hertz contact theory.Test results show that the error rate between theoretical L value and test one in non-preload LM is 8.1%.And the L increases with the load increasing.For the preload LM,L values are divided into two parts,and grow in linear condition with the load.In same load,roughness Ra increases,L decreases.Also,increasing of roller length will increase the L,which are directly related to the mechanical behavior of LM in the design and manufacture stages.(3)Dynamics analysis is used to build SDOF model of LM system combined linear damper.Based on the model,response vibration equation under simple harmonic excitation is built.And a test bench is used to move the damper for a several kilometers to examine the damping maintenance.According to the theoretical calculation and test results,we find that first order natural frequency of linear damper is around 650 Hz.The frequency deviation rate for the new oil groove structure is 9.4%.The increase of the oil viscosity will lead to the response vibration acceleration peak value decreasing,which agrees well with the theoretical model.And the increase of the oil viscosity will improve the damping maintenance of the damper.(4)Based on acceleration reliability theory,LM reliability tests are operated.After single/double level failure terminated testing,MTBF are calculated.Test results and calculation show that,double level terminated tests cut down the testing time,reduce the test cost,and improve the testing efficiency..This method is applied in JB/ZXXXX-201X LM relibility and life part 3:LM relibility testing standard.Based on series system reliability calculation,quality assessment Q is proposed.Q is composed of raceway,rolling element,overspill and other quality.We test four kinds of LMs from four different manufacturers.Study shows that although all LMs are failure-free,there are still quality differences.This study provides one extra assessment in reliability test.