Integrated Thermal Analysis of Aerostatic Bearing System

由于空气粘度低，空气轴承被认为很少或没有热量产生，并且主要应用于超精密机床，很少注意热效应。然而，随着高速加工需求的增加，由于轴承特性对轴承间隙非常敏感，并且由于主轴和外壳的热变形而发生变化，因此气膜中产生热量的影响已变得不可忽视。主轴受离心力的膨胀也对其有影响。研究的最终目标是评估实际运行条件下的空气静压轴承性能，该项目已承担开发有限元程序来计算气膜中的压力分布（无压雷诺兹方程）以及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，实验测量了温度分布、压力和主轴挠度。轴承间隙内的气流可以被认为是由运动部件（主轴旋转）引起的库埃特流和由压力引起的泊泽维尔流的组合，关于这些流动的影响，计算显示出非常有趣的结果。在空气等压缩流体中，温度分布与非压缩流体不同。一般情况下，无论是非压缩流体还是压缩流体，库特流都会引起温升。另一方面，在非压缩流体中，泊泽维尔流引起温升，而在压缩流体中则没有。当压力梯度为正时，温度总是升高。然而，当压力梯度为负时，温度会下降。这些计算结果表明，在空气静压轴承中，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.