# Research Developments of Aerostatic Thrust Bearings: A Review
## 1 Introduction  
In aerostatic thrust bearings (ATBs), high-pressure gas is introduced between the moving surface and the static surface. A high-pressure gas film with a certain bearing capacity and stiffness is formed between the friction pairs. Thus, aerostatic bearings have the following advantages, such as low friction, high speed, high precision, and long service life [1]. A schematic photograph of aerostatic thrust bearings (ATBs) is depicted in Figure 1a. The pressurized gas is fed from an external source. It enters the bearing gap through the restrictor and finally flows to the atmosphere at the outer boundary of the bearing, as shown in Figure 1b. The pressure redistribution and throttling effect of gas are achieved in this process. The pressure distribution in the gas film of the ATB is shown in Figure 1c. The pressure near the orifice is higher, and the gas pressure gradually decreases when it is far away from the orifice. The velocity contour near the orifice is shown in Figure 1d [1]. Along the radial direction, the velocity decreases gradually. Aerostat thrust bearings are widely used in many fields. The typical applications are high-speed machinery, and high-precision applications, as shown in Figure 2. In high-speed applications, common applications are aerostatic spindles and turbo machines. For aerostatic spindles, a high rotational speed is required to maintain machine spindle speed [2, 3]. Large load capacity is expected during the fly-cutter milling process. For turbomachines [4], more gas can be processed while running at high speed. In high-precision applications, common applications are precision machine tools, precision measuring equipment and lithography-associated production equipment [5, 6]. Aerostatic guideways are applied to ensure precision linear motion with high repeatability and high resolution [7]. With the development of precision machining, the accuracy of motion is required to be higher,even at the sub-nanometer level [8]. Therefore, higher requirements are placed on the performance of ATBs, such as load-carrying capacity, stiffness, and stability. It is hoped to improve the stability of ATBs while still ensuring high static performance. The performance analysis of ATBs is mainly categorized into two types, static and dynamic characterization. In the static performance of ATBs, the main physical parameters are load capacity, static stiffness, and the mass consumption of pressurized gas. They are usually evaluated as a function of gas film clearance. The dynamic performance of ATBs mainly focuses on stability study. In stability evaluation, the main physical parameters are dynamic stiffness and damping coefficients. It is necessary to evaluate the margins of stability of ATBs [9]. Figure 1: Schematic of ATB, pressure distribution, and velocity distribution: (**a**) schematic of ATB, (**b**) operation principle of ATB, (**c**) pressure distribution in ATB [1], (**d**) velocity contour near the orifice [1]. Figure 2: Applications of aerostatic thrust bearings [3; 4; 5; 7]. In the static and dynamic performance of ATBs, much progress has been achieved. At present, there are still some challenges in the application process. The first limitation is the low stiffness and load capacity of ATBs, which are only about 1/1000 of oil bearings. The second limitation is the dynamic performance of aerostatic bearings, which determines the dynamic response of equipment, to a large extent. The difficulties lie in how to obtain stable dynamic performance by designing bearings with appropriate parameters and operating conditions. In addition, the \"pneumatic hammer\" is a key problem in the application of ATBs, which has a great influence on the stability of aerostatic thrust bearings. At present, a significant improvement in the performance of ATBs has been achieved, but the limitations of ATBs have not been completely overcome. In order to promote the development of high-performance ATBs, it is necessary to make a comprehensive summary of ATBs.