Rotordynamics Performance of Hybrid Foil Bearing Under Forced Vibration Input

Foil bearings (FB) are one type of hydrodynamic air/gas bearings but with a compliant bearing surface supported by structural material that provides stiffness and damping to the bearing. The hybrid foil bearing (HFB) in this paper is a combination of a traditional hydrodynamic foil bearing with externally pressurized air/gas supply system to enhance load capacity during the start and to improve thermal stability of the bearing. The HFB is more suitable for relatively large and heavy rotors where rotor weight is comparable to the load capacity of the bearing at full speed and extra air/gas supply system is not a major added cost. With 4448–22,240 N thrust class turbine aircraft engines in mind, the test rotor is supported by HFB in one end and duplex rolling element bearings (REB) in the other end. This paper presents experimental work on HFB with diameter of 102 mm performed at the U.S. Air force Research Laboratory (AFRL). Experimental works include: measurement of impulse response of the bearing to the external load corresponding to rotor's lateral acceleration of 5.55 g, forced response to external subsynchronous excitation, and high-speed imbalance response. A nonlinear rotordynamic simulation model was also applied to predict the impulse response and forced subsynchronous response. The simulation results agree well with the experimental results. Based on the experimental results and subsequent simulations, an improved HFB design is also suggested for higher impulse load capability up to 10 g and rotordynamics stability up to 30,000 rpm under subsynchronous excitation.