Contact Characteristics Of Orthotropic Joint With Lubrication Medium

Most of the existing studies on joint for pure joint,in which joint need to be cleaned with alcohol or acetone in experimental.However,in actual mechanical equipment,usually does not exist such completely pure joint.Even between the non-sliding fixed joint,often with lubrication medium.Existing research has shown that the oil film between joint has a certain influence on performance.In this thesis,a fractal model for the contact characteristics of orthotropic joint with lubrication medium is proposed.Based on the fractal law of asperity horizontal distance and height difference profile,the Hertz contact theory with elliptic correction factor and the stick-slip theory are adopted,the probability density function of contact angle for non-centering contact of asperity is introduced,according to the Reynolds equation with direction parameter of surface textures and Newtonian viscous law,the contact stiffness of solid considering non-centering contact of asperity and the stiffness of fluid are obtained separately.In terms of the analytical relationship between solid stiffness and fluid stiffness,the normal and tangential contact stiffness models of orthotropic joint with lubrication medium are established,and the model’s rationality and effectiveness are verified by experiment and finite element simulation.This paper has explored the following aspects of research:(1)The distribution law of asperity horizontal distance and height difference profile are studied.The Leica confocal microscope DCM3 D is utilized to acquire the three-dimensional profile data of the specimen surface by sampling along both the horizontal and height directions at varying resolutions.Then,the data is processed for the first time based on the definition of the "three-point peak",and the data is processed for the second time according to the concept of the asperity horizontal distance and height difference to obtain profile data for asperity horizontal distance and height difference.Following the flow of PSD method for solving fractal parameters,the PSD curve is plotted in a double logarithmic coordinate system.These curves are then subjected to linear least squares fitting,and the corresponding slopes and intercepts are acquired,which were used to determine the distribution law of the asperity horizontal distance and height difference profile.The results show that the asperity horizontal distance and height difference profile at different sampling intervals both have fractal law,so the fractal parameters of the profiles can be obtained by solving the formula.This conclusion provides a theoretical basis for modelling the contact characteristics of joint considering non-centering contact of asperity.(2)A normal contact stiffness model of orthotropic joint with lubrication medium is established.Starting from the microscopic contact characteristics of the joint,based on the asperity deformation mechanism,the Hertz contact theory with elliptic correction factor and the stick-slip theory are used to analyze the non-centering contact deformation process of asperity,and the contact mechanics model of asperity is proposed.According to the fractal law of asperity horizontal distance and height difference profile,the probability density function of contact angle for non-centering contact of asperity is derived,and the modeling of the normal contact stiffness of solid considering non-centering contact of asperities is completed.The normal stiffness of fluid is obtained by solving the Reynolds equation with direction parameter of surface textures.Then,on the basis of the spring parallel model,the normal contact stiffness of solid and the normal stiffness of fluid are connected,the normal contact stiffness model of orthotropic joint with lubrication medium is modeled.The simulation results show that the normal contact stiffness of joint decrease first and then increase with the increase of the fractal dimensional,and first increase and then decrease with the increase of the fractal roughness parameter.In addition,the normal contact stiffness of joint also increase with the increase of the plastic index and the normal contact load of joint.(3)A tangential contact stiffness model of orthotropic joint with lubrication medium is established.A solid tangential contact stiffness model considering non-centering contact of asperities is constructed by applying the contact mechanics model of asperity and combining with the probability density function of contact angle for non-centering contact of asperity.Based on the Newtonian viscous law,the tangential stiffness model of fluid subject to tangential contact load only at joint is improved,the total frictional force and total tangential deformation of the lubrication medium layer are solved,and the tangential stiffness of fluid is further derived according to the definition of stiffness.The tangential contact stiffness of solid and fluid are combined,and the tangential contact stiffness model of orthotropic joint with lubrication medium is established.The simulation analysis results indicate that the effect of the tangential stiffness of fluid on the tangential contact stiffness of joint is minimal,and the tangential contact stiffness of joint mainly comes from the tangential contact stiffness of solid,and the effect of fluid can be ignored in the modelling of the tangential stiffness of joint.(4)Experimental and finite element simulation validations are conducted for the theoretical model.Experimental testing is performed on the constructed testing platform using the hammer impact method to obtain the first three orders of natural frequencies and vibration shapes under different preloading conditions.According to finite element analysis,the normal and tangential contact stiffness of joint is imported into the virtual material contact layer by running the APDL code,thus establishing a finite element model of the whole machine including a virtual material contact layer.Subsequently,static analysis is conducted on the model to determine the contact layer stiffness under working load,and this value is substituted into the modal analysis of the linear model of the entire machine to obtain the first three orders of natural frequencies and vibration shapes under different preloading conditions.Comparison of the two results show that the experimental vibration shapes and finite element simulation vibration shapes are consistent under the condition of lubricated medium,and the relative error between the experimental natural frequencies and finite element natural frequencies is 4.1%.In summary,the model proposed in this paper is reasonable and can predict the contact performance of joint more accurately.