Design And Performance Study Of Self-compensated Hydrostatic Bearing For Ultra-precision Machine Tool

Ultra-precision machines need bearing systems with better static and dynamic characteristics with the development of Ultra-precision technology. Hydrostatic bearings are the only type of bearings that effectively combine high accuracy, high stiffness and high damping ratio, thus they find an irreplaceable place in high performance precision machines. Hydrostatic rotary tables and spindles are key functional units in precision machines, and it is meaningful to develop them. In this dissertation a self-compensated hydrostatic bearing(SHB) was designed for rotary tables and spindles, and its static and dynamic characteristics were theoretically studied with finite element method. Finally it was applied to a hydrostatic rotary table and experiments were carried out to test its stiffness and motion accuracy. The dissertation aims to systematically study the design & simulation method and performance parameters of the self-compensated conical hydrostatic bearing and offer theoretical guidance to the development of hydrostatic rotary tables and spindles with high accuracy, high stiffness and high damping characteristics. The main contents of the study are as follows:1. A new type of SHB was designed by introducing the restricting unit into angled-surface SHB, thus the lubricant diffusion across the restrictor surface was avoided without increasing the axial length, which was the disadvantage of opposed pad SHB. The designed bearing is of compact profile and simple parts, which is prone to be modularly designed and manufactured. It is possible to achieve very high stiffness and accuracy for its performance is not influenced by external restrictor, so it is potentially applicable to precision machine motion systems.2. Theory model for self-compensated conical hydrostatic bearing was built, and the simulation method of static and dynamic characteristics was studied using perturbation method. On the basis of this the influence of restrictor on the load carrying capacity, flow rate, stiffness and damping coefficients was studied. The results show that the SHB exhibits larger load carrying capacity than the fixed-restrictor hydrostatic bearings(FHB); the former has larger stiffness coefficients than the latter under small eccentricity, but the eccentricity has more significant influence on the stiffness of the SHB; a radial SHB has larger damping coefficients than FHBs, while the damping coefficients of axial SHB are between those of capillary/slot restricted bearing and orifice restricted bearing.3. The basic criterion on stiffness optimization of the SHB was studied with flow equivelance theory, which focuses on choosing proper restricting ratio. The optimal restrictirng ratio in axial motion for the designed hydrostatic bearng is 2, and that in radial motion is function of inner flow parameter, which has a value smaller than 2. The way how to reduce the consumed power of the designed bearing was discussed according to the power equations, and it reveals that both the bearing clearance and the lubricant viscocity can not be designed too small.4. The influence of the initial restricting coefficient, the conical angle and the pocket size on the static and dynamic characteristics of the self-compensated hydrostatic rotary bearing was studied. The results show that the damping coefficients decrease with the increment of the restricting coefficient, and the effect of the restricting coefficient on the load carrying capacity and stiffness is not significant when its value is larger than some critical one; the larger the conical angle is, the better the axial load carrying capacity, the axial stiffness and damping coefficients are; the smaller the conical angle is, the better the radial load carrying capacity is, so does the damping coefficients and the stiffness under small eccentricity; the larger the pocket size is, the bigger the load carrying capacity and stiffness are, but the bearing consumes more lubricant and the damping coefficients get smaller.5. The velocity characteristic of the SHB under high-speed rotation was studied. The results show that the fluid inertia reduces the fluid film force while the dynamic effect increases the fluid film force, and with the increment of the rotation speed the load carrying capacity and the attitude angle increase and fluid flow rate decreases; both the cross stiffness coefficients and the direct damping coefficients increase with the speed, and negative direct stiffness coefficient may occur in the loading direction under high speed and large eccentricity; it is better to set the value of restrictring coefficient around the optimal value, otherwise it may be detrimental to load carrying capacity, stiffness and damping, but its influence on the stability threshold is not significant; the SHB has larger mass threshold value of stability than capillary or orifice restricted bearings, but its stability is worse than the latters under large eccentricity.6. The effect of the manufacturing errors on the performance of SHB was studied, and the results indicate that the dimension error, the disalignment of the conical bearings and the tilting error of the restricting ring all reduce the load carrying capacity. A hydrostatic rotary table was fabricated and its stiffness and motion accuracy was tested. The rotary table exhibits axial stiffness of 220N/ μm, radial stiffness of 120N/ μm and motion accuracy better than 0.4 μm under supply pressure of 1 MPa. It is necessary to improve the manufacturing accuracy for the SHB to fully exihibit its advantage in stiffness and accuracy.