Operating temperatures in mist lubricated rolling element bearings for gas turbines

In addition to geometry and environmental factors, the operating temperatures of bearings are directly affected by the applied load, the operating speed, the friction torque, and the lubricant type and viscosity. If heat dissipation is not adequate, the bearing system temperature will increase to the point that the lubricant is distressed, ultimately leading to bearing failure. Current studies are investigating both mist and vapor phase lubrication which have the potential to increase lubricant operating temperature from 200°C for a conventional recirculating polyolester liquid to as high as 600°C for a tertiary-butyl phenylphosphate. The primary benefit of these lubrication methods is the potential to reduce the lubricant on-board storage requirements, in particular for expendable engine applications. However, this technique removes the primary mode for heat rejection, and the bearing operating temperature may become the critical factor for life considerations. A process has been developed to numerically investigate the operating temperature of a rolling element bearing using the finite element method. Predicted steady-state temperature responses for a 30 mm ball bearing operating with mist lubricant delivered at 325°C for three different shaft operating speeds are presented. This analysis process can be used to evaluate thermal conditions and to determine cooling requirements for future bearing system designs