Research On Static And Dynamic Characteristics And Stability Of The Hybrid Air Bearing System

The hybrid air-bearing is widely used in the field of high-speed and/or highprecision machining due to its advantages of low friction,high precision and no pollution.The high-speed/high-precision hybrid air-bearing spindle built with hybrid air bearings is the essential component for high-precision machine tools,which is the basis determining machining accuracy and efficiency of the machine tool.However,the hybrid air-bearing has issues often including low stiffness,loading capacity and dynamic stability.Those issues sometimes hinder the applications of the air-bearing spindle in the field of ultraprecision manufacturing.Therefore,around the two key scientific issues of the accurate theoretical model of the static performance of the hybrid air-bearing main shaft and the stability research of the hybrid air-bearing,this doctoral dissertation discusses the static and dynamic performance and stability of the hybrid air bearing-rotor system at both the theoretical and experimental levels.Stability and other comprehensive performances were systematically studied to clarify the influence of the design parameters of the hybrid air bearing spindle on the dynamic and static characteristics of the main shaft system,and to provide a theoretical basis for further design and development of the hybrid air bearing spindle systems with the application to ultra-high speed and high precision machining.In this dissertation,a static model for high-speed hybrid radial air-bearings is presented.For the problem of inaccurate discharge coefficients under high-speed conditions,based on the finite element method,a flow coefficient correction formula is proposed,and the effect of the flow coefficient on the static performance of hybrid radial air bearings is studied.Using the modified flow coefficient model,a5-degree-of-freedom hydrostatic axial / radial bearing group is established to compute the static performance of the bearing system,and a 5-degree-of-freedom hybrid air bearing group angular stiffness calculation method is proposed,which is used to clarify the influences of the structural parameters of the bearings on the static performance of the bearing system.The dynamic characteristics of hybrid air bearing are the foundation of the stability study of the bearing-rotor system.For this purpose,this dissertation establishes a dynamic performance calculation model of hybrid air bearing and describes the influence of bearing parameters on dynamic stiffness and dynamic damping coefficients.For the developed spiral groove micro-structured hybrid air-bearing,the Reynolds equation is transformed and improved by taking account of the spiral groove micro-structure as proposed.Numerical computational method to establish dynamic and static models of spiral groove micro-structured hybrid air bearing,and to explain the mechanism of spiral groove and optimize spiral groove microstructure parameters;Based on the calculated dynamic stiffness and dynamic damping of hybrid air bearing,a FEM model of the rotor system is developed to calculate the natural frequency of the rotor system and to provide a data basis for calculating the critical speed of the hybrid air bearing-rotor system.Based on the study of dynamic characteristics of the hybrid air-bearing,this dissertation systematically studies the stability characteristics of general hybrid air bearing-rotor system and hybrid air bearing-rotor system with spiral groove micro-structure,respectively.Moreover,this dissertation analyzes the influence of the bearing structural parameters and groove structure parameters respectively on the stability of the spindle system.Based on the final design parameters,the critical speeds of the two systems are calculated.Based on the non-steady-state Reynolds equation,the rotational error prediction model for a hybrid air bearing-rotor system is established to analyze the typical nonlinear dynamic behavior influenced by factors such as the spindle speed,load,unbalance and rotor quality,and to further explain the influence of bearing structural parameters on the spindle rotational error.Finally,a high-speed hybrid air-bearing spindle is developed,and an experimental platform for the dynamic and static characteristics of the spindle is designed and built to measure the dynamic vibration of the high-speed hybrid air-bearing spindle,and to verify the critical speed and stability characteristics of the hybrid air-bearing spindle system;the dynamic error is measured to verify the effectiveness of the spindle rotational error model.A high-precision hybrid air bearing spindle for a high precision internal grinding machine is developed and its static performance is tested to verify the simulation model for assessing the dynamic performance of the static and dynamic pressure air bearing;By experimentally measuring and testing the dynamic characteristics of the high precision air-bearing spindle,the hybrid air-bearing stability and the spindle rotational error model is proved,and furthermore,the design specifications for the entire spindle system are verified in satisfaction with the original design expectation.