Dynamic Modeling And Experimental Study Of Hydrostatic Bearing Considering Fluid Compressibility

The requirement of the precision are becoming higher for the complex parts of the highend equipment.Meanwhile,the demand for the precision heavy /super heavy machine tools is becoming urgent.The motion pair of hydrostatic bearing is one of the key supporting ways for precision and heavy machine tools,and its dynamic characteristic affect the dynamic performance and stability of the machine tools.Although scholars at home and abroad have extensively studied the static bearing characteristics and pressure distribution of the hydrostatic bearing,the research results of the influence mechanism of the weak compression of fluid on the its dynamic performance are lack.The dynamic characteristics of the hydrostatic bearings are studied by using numerical calculation method.Based on the developed experimental platforms,the mechanism and modeling of the hydrostatic bearing are studied in detail.The main content is as follows:(1)Considering the weak compression of fluid,introducing the equation of the liquid state,the coupled dynamic model of the hydrostatic bearing is established by combining the N-S equation,the continuous equation and the Newtonian equation of the supporting surface.For the large dynamic displacement of the worktable,a dynamic mesh method is put forward to study the influence of boundary speed on the flow of the whole basin.The implementation of the coupled dynamics model are successfully calculated when the code is embedded into the CFD solver.What's more,the transient response of the hydrostatic bearings for dimensions and shapes of the pocket are discussed under different bulk modulus of fluid in the paper.It is found that the compressibility of fluid affects the flow rate and the damping force of the fluid.This reveals the mechanism of the overshoot of the vibration amplitude.(2)To improve the calculation efficiency,a dynamic modeling method is proposed based on the finite element method.Combined with the experimental result,the effective bulk modulus of the fluid is deterimined in the paper.Firstly,based on the pressure results and the parameters of the pipeline,one-dimensional dynamics of the pipe is applied to calculate the outlet flow of the pipeline,and it is used as the inlet boundary condition of the hydrostatic bearing.Secondly,the dynamic displacement is obtained by solving the dynamic model of the hydrostatic bearing and the dynamic equation of the worktable.Finally,the effective volume elastic modulus of the system is determined by comparing the theoretical displacement with the experimental displacement.This method can not only quickly calculate the dynamic response of hydrostatic bearing,but also determine the effective bulk modulus of the actual system.By comparing the theoretical calculation and experimental results,the feasibility and effectiveness of the method are verified.(3)According to the mechanism of the response of the hydrostatic bearings,a piecewise nonlinear dynamic model is proposed in the paper.Meanwhile,the identification method of the dynamic model is studied.Based on the experimental results and the proposed nonlinear dynamic model,the equivalent stiffness and equivalent damping of the dynamic model are estimated by applying the method of the Levenberg-Marquardt.(4)Considering the friction,inclination and machining error,an experimental platform of the hydrostatic bearing is designed in the paper.When the traditional excitation method can't effectively drive the vibration of the oil film in actual working conditions,displacement excitation method is introduced.The dynamic displacement is obtained by using the developed experimental platform under different working conditions.Meanwhile,the effect of initial compression displacement,inlet flow rate and number of joints on the dynamic characteristics of the hydrostatic bearings is discussed.