Optimal design of an opposite aerostatic thrust bearing considering rarefaction effect

Abstract

Aerostatic bearings have been widely used in ultra-precision machine tools. As the key component, the performance of the air bearing has a dramatic impact on the machining accuracy. Traditional static design often takes load-carrying capacity and static stiffness as optimization objectives, while the vibration of the spindle is mainly resisted by the dynamic stiffness in the actual machining process. In this paper, the air film thickness and orifice size are optimized to improve the dynamic stiffness of the aerostatic thrust bearing by using the non-dominated sorting genetic algorithm-II (NSGA-II). The orifice diameters of upper and lower thrust plate are independent. As the optimal solutions tend to low air film thickness, the hypothesis of continuum flow between spindle and plate is no longer applicable. Thus, the rarefaction effect is considered by integrating a slip flow model into the Reynolds equation. It is found that the preferable parameters corresponding to static design and dynamic design are different. Compared with the conventional design with same orifice size, both static stiffness and dynamic stiffness can achieve a considerable improvement with independent orifice diameters. The research results are expected to provide suggestions for the design of opposite air thrust bearings.