# Research Developments of Aerostatic Thrust Bearings: A Review
## 3 Advances in Static Performance
### Structure Improvements  
#### 3.3.1 Introduction of Restrictors  
The restrictors in ATBs are mainly classified into four types: orifice restrictors, porous restrictors, annular restrictors, and slot restrictors, as shown in Figure 8[61]. The orifice restrictor and annular restrictor are the most common types. The annular restrictor (or called the inherently compensated restrictor) is formed by the feed hole. The orifice restrictor is formed with a feed pocket. The orifice is produced with a twist drill and its diameter commonly ranges from 0.15 to 0.4 mm.  
1 Micro feed holes  
Recently, with the development of laser beam machining and micro drills, it is possible to manufacture feed holes with less than 0.1 mm diameter. The use of micro holes is beneficial to improving the performance of aerostatic bearings. There are two main reasons for adopting micro-holes:  
* The pressure can be improved, and the gas flow can remain stable. The load capacity of the bearing is enhanced.
* A restricted area independent of the gas clearance is formed.  
Aerostatic thrust bearings with micro holes show great potential in improving static performance. [PERSON] fabricated an aerostatic bearing using microfabrication technology [33]. The configuration of the several different forms is shown in Figure 9a, and the number of holes is 25,100 and 400, respectively. The surface pressure distribution of the bearing with 25 micro holes is shown in Figure 9b. The cross-sectional area of each micro-rope is 30 um\({}^{2}\). The gas film pressure is symmetrically distributed. The pressure in the central region is relatively high, showing a diamond distribution. Aerostatic thrust bearings with micro holes have a smooth and uniform pressure distribution. The load capacity has been greatly improved. [PERSON] [62] conducted research on small feed holes of less than 0.05 mm diameter with CFD and DFM models. The results indicated that higher stiffness could be obtained by adopting micro feed holes. [PERSON] [63] investigated the static performance of ATBs with micro feed holes of less than 0.1 mm diameter. The results showed that the load capacity and stiffness of the aerostatic bearings can be optimized by modifying the number and diameter of aerostatic bearings. In order to  
Figure 8: Various types of restrictors. (**a**) orifice restrictor (**b**) annular restrictor (**c**) slot restrictor (**d**) porous restrictor.  
reduce the nano-scale vibration of aerostatic bearings, [PERSON] [64] proposed a novel design scheme for an array micro hole restrictor (AMR). Different from the traditional orifice restrictor, the AMR had uniformly distributed micron-sized holes distributed in the form of arrays, so that the total restricted area was the same as the single-hole restrictor. For a more uniform aperture, AMRs were manufactured with laser drilling technology.  
2. Multi-orifice series  
Compared with the traditional orifice-type restrictor, the static performance of the bearing can be improved by adopting the multi-orifice type restrictor. [PERSON] proposed a novel pocket orifice-type restrictor with multi-orifice series [65]. The aerostatic bearings with this restrictor had higher maximal bearing stiffness than the traditional bearings. The bearing stiffness increased by \(11\%\) with the restrictor of four sub-orifices in the series.  
3. Multi-loop Coupling Pocket  
[PERSON] [66] proposed an aerostatic bearing with a double-loop coupling pocket and studied the performance of this bearing. It was found that this type of aerostatic bearing had a higher static load capacity, which provided practical guidance for the design of high-performance aerostatic bearings.