# Research Developments of Aerostatic Thrust Bearings: A Review
## 7 Conclusions and Discussions  
In this paper, the research developments of ATBs are summarized from several aspects: theoretical models and experimental methods, static performance, dynamic performance, and the applications of ATBs. In addition, the development trends of ATBs are discussed. Based on the above analysis, the following conclusions can be drawn:  
1. Accurate theoretical models and experimental methods. The flow in aerostatic bearings is very complex. There are various phenomena such as turbulence, supersonic flow, velocity slip effect, pneumatic hammer, and shock waves. These factors have a non-negligible impact on bearing performance. There are still some challenges in theoretical calculation. The multiple physical fields need to be coupled with the increasing complexity of the bearing system. More refined theoretical models are required to ensure accuracy in the design and verification of ATBs. Experimental technology plays an important role in studying bearing performance, verifying theoretical models, and exploring phenomenon mechanisms. The experimental methods applied to ATBs are limited by tiny bearing clearances. The tedious experimental equipment and test process also limit the performance measurement of ATBs. Thus, more accurate and advanced experimental test methods are necessary. 1. High-performance ATBs with large stiffness and load capacity. The applications of ATBs are affected by some demerits, such as low load capacity and stiffness. The stiffness and load capacity of ATBs are far less than with oil bearings. The development of high-speed machinery and high-precision machinery has demanded higher requirements for the performance of ATBs. Therefore, it is urgent to develop high-performance ATBs with significant stiffness and load capacity. 1. The applications of ATBs in special conditions. 2. At present, there is still limited literature on the study of ATBs working in extreme-temperature environments, and the adopting of special gases as a lubricating medium. With increasing applications of ATBs under special conditions, it is necessary to spend more research time on ATBs adopting special gases, and ATBs under extreme-temperature conditions. At the same time, the influence of extreme-temperature conditions and special gases on bearing performance should be considered. 3. The applications of ATBs in special conditions. 4. At present, there is still limited literature on the study of ATBs working in extreme-temperature environments, and the adopting of special gases as a lubricating medium. With increasing applications of ATBs under special conditions, it is necessary to spend more research time on ATBs adopting special gases, and ATBs under extreme-temperature conditions. At the same time, the influence of extreme-temperature conditions and special gases on bearing performance should be considered. 5. Pneumatic hammer of ATBs. With the development of ATB technology, theory study has become more and more evolved. The pneumatic hammer is a key problem in the study of the dynamic performance of ATBs, which can easily lead to the instability or failure of ATBs. The pneumatic hammer has been studied from different perspectives. Corresponding research results have been obtained. However, there is still no consistent theoretical explanation for the pneumatic hammer. Further in-depth research is needed on the study of the pneumatic hammer. **Author Contributions:** Literature investigation, Q.Z., M.Q. ; supervision, T.L., Y.H. and S.C.; validation, Q.Z. and T.L. ; writing--original draft, Q.Z. ; writing--review and editing, Q.Z. and T.L. All authors have read and agreed to the published version of the manuscript. **Funding:** This project was funded by the National Key R&D Program of China (2019 YFB1504600) and the Youth Innovation Team of Shaanxi Universities. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** Not applicable. **Conflicts of Interest:** The authors declare no conflict of interest.