Experimental Evaluation of 200mm Hybrid Air Foil Bearing Aimed for MW Scale Turbomachinery

Abstract The nonlinear characteristics of foil bearings often add complexities to analysis and design of foil bearings over wide size ranges. Previously, scaling laws for bearing clearance and areal stiffness of supporting foil structure for differently sized foil bearings were proposed to address these challenges. The scaling laws enable to design large foil bearing by scaling up existing small foil bearing. To verify the proposed scaling law and demonstrate the feasibility of a large foil bearing, a 200mm (in diameter) hybrid air foil bearing with potential use for 2MW generator was designed and manufactured by scaling up an existing 50mm foil bearing following the scaling laws. The 200mm hybrid foil bearing was tested for its static performance. The bearing features a three-pad offset preload with total six hydrostatic injection ports to improve the load carrying capacity at low speed. A great effort was put into designing a test rig to demonstrate the bearing static performance. The test rig is configured with a series of ball bearings-supported test rotor and the hybrid foil bearing is floating on the rotor. A pneumatic actuator is used to apply external load to the foil bearing and a torque rod measures the friction torque of the foil bearing. The test rig was designed with maximum allowable speed of 18,000rpm and the rotordynamics of the test rig was analyzed to ensure its integrity under the testing conditions. Two hybrid modes, i.e., with all injection ports open and only bottom injection ports open, were compared. The hybrid foil bearing can withstand up to 2000N at zero speed under 5 bar absolute supply pressure for both hybrid modes. Twenty start/stop cycles test at 1000rpm over extended period under 2000N in hybrid modes show the bearing temperature changes very little, demonstrating very robust bearing function. The hybrid foil bearing was also tested in hydrodynamic mode at 4400rpm under 2000N, verifying the full functionality of the bearing in hydrodynamic mode. A preview of a 2MW permanent magnet generator featuring the 200mm hybrid foil bearing is also presented as one of the potential applications.