Research On Design Method Of Multi-throttle Aerostatic Guideway With Recesses And Grooves Based On Modified Discharge Coefficient

Air bearing has got the advantages of high precision,high speed,small friction and long service life,which is the key technology for the construction of linear motion reference in high precision and high speed motion field.However,the air bearing has the problems of mutual restriction of stiffness and stability,low bearing capacity and stiffness,especially in extreme conditions.Therefore,it is of great theoretical and practical significance to study the bearing capacity and stiffness of the air bearing and its influencing factors,so as to improve the performance of the air bearing and expand its application in high-end manufacturing equipment and precision measurement equipment.In order to meet the requirements of high-speed and high acceleration air floating guidance technology in high-speed variable aperture system,focusing on the problem that it is difficult to improve the rotational stiffness of the guideway under the limited space volume,the technical research on the groove composite throttle guide with high rotational stiffness is carried out from the aspects of theoretical modeling,numerical analysis,design method and experimental verification,so as to characterize the micro structures.The influence of discharge coefficient and guideway performance is the focus,which provides a new principle and method for the structure design and performance improvement of guideway.Based on the flow balance condition,the local flow field near the throttling device and the micro structure of the recess and groove is related to the global flow field,and the pressure distribution after throttling and the whole field pressure distribution are obtained.On this basis,the analytical equations of stiffness and rotational stiffness are derived under the eccentric and inclined states of the closed guideway,and the analytical model of the mechanical properties of the composite throttle guide is established.The results show that the influence of recess and groove microstructure on the performance of guideway is caused by throttling pressure,and the key factors affecting the pressure are throttle area and throttle coefficient.The effective throttling area can be identified by the characteristic size of microstructure,but the analytical relationship between discharge coefficient and microstructure is not clear,but it can be further studied by numerical method.In order to solve the flow field of a narrow and long film with microstructure,a typical impinging jet model was taken as the starting point,and the relationship between gas viscosity and wall height was redescribed,and a dimensionless velocity function was obtained,including the linear part for laminar flow calculation and the logarithmic part for turbulence calculation.In order to meet the requirement of dimensionless grid height in numerical calculation of gas viscosity,an adaptive near wall mesh capturing and subdivision method is proposed.The effects of the minimum element size,partition mode and adaptive cycle times of the initial mesh are studied.The limited conditions for the application of the mesh adaptive method to the volume finite element calculation of composite flow field are obtained.The accuracy and feasibility of the method are verified.It is shown that the proposed mesh self-adaptive method can be used to study the influence of microstructural variation on discharge coefficient and guideway performance It provides a solution for studying the influence of the micro structure parameters of the cavity.The influence of six parameters,including the diameter of the recess,the depth of the recess,the depth of the equalizing groove,the width of the equalizing groove,the sectional area of the guiding groove and the depth width ratio of the guiding groove on the performance of the guideway are revealed by using the mesh adaptive method.In order to ensure that the shallow cavity depth matches the local gas film thickness,a shallow cavity microstructure with gradient depth is designed,which can significantly improve the rotational stiffness of the guide rail and reduce the working point of the gas film thickness.The results show that the working point of film thickness is adjusted in the design of microstructure parameters,which provides basic data for the modification of throttle coefficient and the design of guide structure.The analytical expression of discharge coefficient and microstructure parameters is established.The modified discharge coefficient is combined with the analytical model of guideway performance.Through the effective throttling area and the modified discharge coefficient,the microstructure characteristics are introduced into the guideway performance calculation,and the CFD analytical hybrid design method of the local flow field in the groove cavity guided by numerical value solution is proposed.The method is a high rotational stiffness groove cavity composite joint The design of flow guide provides theoretical guidance.Using this method,a groove compound throttle guide is designed,and the experimental device is built to verify the effectiveness of the design method.The experimental results show that the rotational stiffness of the guideway reaches Kθ=1.03×10~4 Nm/rad in an extremely limited space volume(the actual air floating working area is only 84mm×20 mm).The new compound throttle guide is applied to the guide mechanism of high-speed variable aperture system.The test results show that the scanning acceleration reaches a0=50 m/s~2,which meets the requirements of scanning motion performance.