A Study Of Contact Characteristic Parameters Of Machine Joint Interfaces Based On Fractal Theory

Contact characteristic parameters contain contact stiffness,contact damping,static friction coefficient,contact thermal resistance,contact electrical impedance and so on,and the first three contact parameters directly affect the static and dynamic characteristics of the machine tools.Contact stiffness and contact damping derived from joint interfaces of a whole machine tool played a vital role compared with material itself,and these two contact parameters become the key points to study the static and dynamic characteristics of the machine tools.So,studying the contact characteristic parameters of the joint interfaces had contributed to investigate the structure dynamic optimal design of the whole machine tool,and to improve the performance of it.However,because of the nonlinear characteristic for the contact characteristic of the joint interfaces,the mechanism of the contact characteristic parameters hadn't been investigated explicitly for a long time,and this led to establishing the dynamic model of the joint interfaces precisely in theory become more difficult.Therefore,this dissertation will mainly devote in building analytical models for contact characteristic parameters,and provide theoretical basis for dynamic optimal design of mechanical structure.The main achievements are presented as follows:An analytical contact mechanics model of joint interfaces based on fractal theory was proposed.An improved 3D WM fractal function was used for the contact surface characteristic,starting with contact characteristic at the micro level,the contact behavior of elastic deformation,elastoplastic deformation and plastic deformation of asperities were analyzed.Based on Hertz contact theory and fractal theory,contact load and contact area equations for elastoplastic deformation of asperities were established,contact mechanics model of asperities and joint interfaces were proposed,the influence factors were also analyzed.By comparing the present model to other four classical contact models and test data,proved the validity and universality of the present model.An analytical model of normal and tangential contact stiffness parameters was established.Using the contact mechanics model and Mindlin contact theory,normal and tangential contact stiffness of asperities in elastic and elastoplastic contact regimes were obtained.Based on fractal theory,normal and tangential contact stiffness models of the whole joint interfaces were built.Meanwhile,vibration differential equation of spherical to plane contact problem was analyzed,linear static stiffness of asperities was derived,and the relationship between dynamic stiffness and static stiffness was also established.On the other hand,stiffness characteristic under lubrication was also studied.By establishing average Reynolds equation between rough surface contact under partial lubrication based on fractal theory,analytical solution for the average film pressure was derived based on separated variable method.An analytical equivalent stiffness parameter model considering solid contact stiffness and oil film stiffness was established,and the influence factors were analyzed.An analytical model of normal and tangential contact damping parameters was also put forward.Based on fractal theory,Sabot vibration theory and Mindlin contact theory,the normal and tangential stored energy and dissipated energy per cycle of joint interfaces were deduced,the normal and tangential contact damping parameters model be also built.On the other hand,damping characteristic under lubrication was also studied.The normal oil film damping was solved by average Reynolds equation under partial lubrication,an analytical equivalent damping parameter model considering solid contact damping and oil film damping was established,and the influence factors were analyzed.An analytical model of static friction coefficient parameter of the joint interface was put forward.Based on the third strength theory and fractal theory,solved maximum static friction force of one asperity,and worked out the maximum static friction force and static friction coefficient of the joint interface.Meanwhile,the friction characteristic of the joint interface under lubrication was discuss,the expression of fluid friction and static friction coefficient under partial lubrication was solved out on the basis of Newton's law of viscous fluid and the mean pressure solution.Finally,the contact fractal parameters were identified by the means of surface profiles test analysis for test specimen.The modal parameters and frequency response functions curve were identified according to the results of the modal test.In the meantime,build a test-bed system to carry out experiments for fraction force,and static friction coefficient parameter was acquired.The experiment results show the validity of the theory.