Integrated Thermal Analysis of Aerostatic Bearing System

由于空气的低粘度，空气轴承一直被认为产生的热量很少或没有产生热量，主要应用于超精密机床，对热效应的关注很少。然而，随着高速加工需求的增加，气膜发热的影响已经变得不容忽视，因为轴承特性对轴承间隙非常敏感，并且由于主轴和轴承座的热变形而发生变化。离心力引起的主轴膨胀也会对其产生影响。本研究的最终目的是评估气体静压轴承在实际运行条件下的性能，并开发有限元程序来计算气膜内的压力分布(不可压缩雷诺方程)以及气膜和壳体的温度分布(能量方程)。建立了一套气体静压轴承系统，对温度分布、压力和主轴挠度进行了实验测量。轴承间隙中的气流可以看作是由运动部件(主轴旋转)引起的Couette流和由压力引起的Poiseuille流的组合，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，温度分布与在不可压缩流体中不同。一般来说，无论是不可压缩流体还是压缩流体，Couette流都会引起温升。另一方面，Poiseuille流在非压缩流体中引起温升，而在压缩流体中不引起温升。每当压力梯度为正时，温度总是升高。然而，在压力梯度为负的情况下，温度可能会下降。这些计算结果表明，在气体静压轴承中，如果压力梯度较大且为负，则存在温度下降的可能性。

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.