Integrated Thermal Analysis of Aerostatic Bearing System

空气轴承由于空气的低粘度而被认为很少或没有发热，并且主要应用于超精密机床，很少关注热效应。然而，随着高速加工需求的增加，气膜中的热产生的影响已变得不可忽略，因为轴承特性对轴承间隙非常敏感，并且由于主轴和壳体的热变形而发生变化。主轴膨胀的离心力也会影响它。研究的最终目标是评估空气静压轴承的性能在实际运行条件下，该项目已承担开发有限元程序来计算在空气膜的压力分布（不可压缩雷诺方程）和空气膜和外壳的温度分布（能量方程）。建立了一个空气静压轴承系统，对轴承的温度分布、压力分布和主轴变形进行了实验测量。轴承间隙中的空气流动可以被认为是由运动部件（主轴的旋转）引起的Couette流动和由压力引起的Poiffille流动的组合，并且计算显示了关于这些流动的影响的非常有趣的结果。在压缩流体中，如空气，温度分布不同于那些在不可压缩的流体。通常，无论是不可压缩流体还是可压缩流体，库埃特流动都引起温度升高。另一方面，Poiffille流在不可压缩流体中引起温度升高，而在压缩流体中不引起温度升高。当压力梯度为正时，温度总是升高。然而，在压力梯度为负的情况下，温度可以降低。这些计算结果表明，在气体静压轴承中，如果压力梯度是大的和负的，有可能的温度下降。

Air bearings have been considered to have little or no heat generation due to the low viscosity of air, and have been applied mainly to ultra precision machine tools with paying a little attention to the thermal effects. With increasing demand for high speed machining, however, the effect of heat generation in the air-film has become not negligible, because the bearing characteristics are very sensitive to the bearing clearance and it changes due to the thermal deformations of the spindle and housing. The spindle expansion by the centrifugal force also affects it. A final goal of the study is to evaluate aerostatic bearing performances under actual operating conditions, and this project has been undertaken to develop finite element programs to calculate pressure distribution in the air-film (incompressive Reynolds equation) and the air-film and housing temperature distributions (the Energy equation). An aerostatic bearing system has been built to measure temperature distributions, pressures and the spindle deflections experimentally. The air flow in the clearance of the bearing can be considered as the combination of Couette flow caused by the moving part (the rotation of the spindle) and Poiseuille flow caused by the pressure, and the calculations show very interesting results regarding to the effects of these flows. In compressive fluid like air, the temperature distributions are different from those in incompressive fluid. Generally, Couette flow always induces temperature rise regardless of incompressive and compressive fluid. On the other hand, Poiseuille flow induces temperature rise in incompressive fluid, but not in compressive fluid. Whenever the pressure gradient is positive, the temperatures always increase. However, the temperatures could decrease in case the pressure gradient is negative. These calculation results suggest that, in aerostatic bearings, there is a possibility of the temperature decrease if the pressure gradient is large and negative.