Theoretical Investigations on Tribological Properties of Air Foil Thrust Bearings during Start-Up Process

During the start-up process, air foil thrust bearings (AFTBs) come into contact and are prone to localized overheating and even failure. Frequent starts and stops are the major sources of wear. In this study, a transient mixed lubrication model considering the air rarefaction effect is established. In the model, asperity contact between the top foil of the air foil thrust bearing (AFTB) and the rotor surface is considered using a deterministic approach, and the deformation of the foil structure is considered using the elastic-aerodynamic coupling. The influences of bearing compliance, external load, acceleration time and contact stiffness on the tribological properties during the start-up process are investigated. The results show that the lift-off speed increases with the increase of the bearing compliance and of the external load. Specifically, the lift-off speed of AFTBs increases from 1950 rpm to 5325 rpm when the bearing compliance increases linearly from 0.2 to 1.0. When the external load increases from 10 N to 80 N, the lift-off speed is increased by approximately 30 times, from 450 rpm to 13,575 rpm. Furthermore, a decrease in the acceleration time of the rotor generates a decrease in the lift-off time and an increase in the contact stiffness of the bearing leads to a decrease in the lift-off speed of the bearing. By exploring the effects of the bearing parameters and working conditions on the tribological performance of AFTBs during start-up, the results presented in this study can provide theoretical support for improving start-up performance and designing high-reliability and long-life AFTBs.