Study On Some Key Techniques Of Ultra-Precision Aerostatic Bearing Axes System

Study on ultra-precision aerostatic bearing axes system is always one of the major research subjects of ultra-precision machine tools. With the extensive use of aerostatic slideways and spindle in ultra-precision machine tools, aerostatic bearing axes system has become an object to be designed with higher stiffness, higher accuracy, more simple construction and better damping capacity. Orofice restrictor and porous restrictor are one of the main feeding types with which this kind of bearings have such disadvantages as low stiffness and difficult to be made at present. As a new feeding type, partial porous restrictor has the characteristics of orifice restrictor and porous restrictor. Partial porous aerostatic bearing has been concerned for its advantages of simple construction, higher stiffness and easy to be produced. It is a pity that few research works on this kind of bearings have been made. Moreover, there are defects to assess the spindle radial error motions in the appreciation theory. For these problems, the design theory study on partial porous aerostatic bearing and measurement appreciation study on the aperiodic spindle radial error motions in one circle are chose as the main content, in detail as follows:With the usual assumptions of a thin lubricating film and the isothermal laminar flow conditions, the pressure distribution in the clearance space of an air lubricated partial porous bearing is given by the modified Reynolds equation derived from simplified Navier-Stokes equations satisfying the velocity slip boundary condition, following the Beavers-Joseph slip velocity model. Then the flow motion in the porous media is given by the equations governed by Darcy's law with the equation of continuity. Based on the two nonlinear equations, the theoretical analysis is presented to predict the steady state performance characteristics of partial porous aerostatic bearing.Galerkin weighted residual method and Green's theorem are used to transform the above two equations into the corresponding weak integral forms. Then by the finite element method, the numerical calculation results show that the parameters have important effect on the static characteristic regularly, including diameter, height, permeability, number, position layout of porous restrictor and so on. Moreover, high load capacity or high stiffness can be achieved by choosing right parameters only according to the rules. By testing serialization partial porous aerostatic thrust bearings prototype workpiece, good agreement is achieved between the results from the present model and the experiment results. The comparison results not only proved that the present mathematical model and the numerical calculation method are valid, but also that there are some advantages of higher stiffness, easy to be made and so on for this kind of bearings.The convergence is analyzed to obtain the pressure distribution in the clearance space of a partial porous aerostatic bearing during the process of solving the FEM equation. An improved proportional division method and the proportional division factor G are provided to get the true convergent roots during the whole working clearance space.There are very few methods to appreciate extensively aperiodic spindle error motions. A new appreciation model is provided to asses spindle radial error motions, after the spindle radial error motions set transformed. Based on the minimax theory, the uniform evaluation criterion of Minimum Zone Reference Circles method is then established, and also the effects surface uniform evaluation criterion. Moreover, a uniform step equation is obtained to improved the speed of appreciation. The test of appreciation algorithm has proved that both the appreciation precision and the efficiency are improved greatly. Then the appreciation theory is completed for the spindle radial error motions.