| With the development of modern mechanical manufacturing technology towards intellectualization,flexibility,integration and high automation,CNC machine tools are required to maintain high accuracy,low noise,high reliability and long life under high speed and heavy load.Ball screw nut pair is one of the critical components of the feed system of CNC machine tools.Its friction and dynamic contact characteristics directly affect the performance of CNC machine tools.When the ball screw pair is running at high speed,because of the friction heating and contact deformation of the joint surface,the machining accuracy,reliability and life will be reduced.The main contradiction to improve the machining accuracy of CNC is studying on the friction and dynamic contact characteristics of the ball screw pair.Based on Hertz contact theory,deformation compatibility condition and micro-displacement theory,the quasi-static contact model of ball screw pair with assembly error is established,and the influence of additional force caused by misalignment error between the screw and motor axis on contact characteristics of ball screw pair is analyzed.At the same time,based on machine tool test data,the friction regression model is established.The assembly error of CNC machine tools is identified.The specific research contents are as follows.(1)The statically indeterminate beam model with the extended end screw shaft is established,and the formula for calculating the additional contact force of the screw nut pair caused by the misalignment error between the servo motor shaft and the screw is deduced.The influence of misalignment error on the additional force of the ball screw nut is analyzed by numerical calculation.Based on Hertz contact theory,micro-displacement theory and deformation compatibility condition,a quasi-static contact model of ball screw pair with additional force is established.Through numerical calculation,the effects of axial load,misalignment error and table position on contact normal force,contact angle and friction force of ball screw pair are analyzed.(2)A real-time acquisition system for the position of the worktable and the torque and current of the servo motor in the working process of the feed system was established,and the main feed system of HTC1635 and HTC2050i CNC were tested respectively.Based on the experimental data and cubic spline fitting,the relationship between the friction force of the two feeding systems and the position of the worktable is extracted,and the variation rule of the instantaneous mean value of friction force with the feed speed and the position of the worktable is analyzed.Then,the change signal of friction force around the instantaneous mean value of the main feed system of HTC1635 is extracted and its FFT spectrum analysis is carried out.The spectrum components obtained are compared with the frequency spectrum of the instantaneous signal of friction force calculated by(1)numerical calculation,and the feed system has misalignment error confirmed between servo motor and screw.(3)Based on empirical modal analysis and wavelet analysis,the friction signals of the main feed system of HTC2050i are decomposed,and the power spectrum analysis and correlation coefficient of the decomposition components are calculated.The comparison shows that the wavelet decomposition method is more propitious for analyzing the frequency doubling characteristics of the friction force of the feed system,and the spectrum is used to analyze the frequency doubling characteristics of the friction force of HTC2050i main feed system.Position-frequency analysis of each wavelet decomposition component is carried out by spectrum moment,and the characteristic curve of friction force varying with position of feed system is given.(4)A multivariate linear regression model for the relationship among friction force,axial force and misalignment error of feed system is established.The parameters of the regression model are identified by the numerical results of(1),and the validity of the regression model is confirmed by residual analysis and correlation test methods. |
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