Experimental Study on Internal Friction Induced Vibrations of a High-Speed Turbine Rotor-Bearing System

Internal friction may induce instability of a high-speed turbine rotor-bearing system. The effects of relative slippage between the inner ring of a ball bearing and the shaft on the vibration and stability of a practical high-speed turbine are experimentally studied. The features of the rotor include a cantilevered large mass and the rigid body mode of the first order critical speed. Clearance and transition fits of the bearing and the shaft are considered, together with the variations of tightening torque acted on the locking nut. The dynamic features of vibrations under each condition are analyzed in detail. For clearance fit with standard tightening torque, a “switch” rotating speed is observed at which both the dominant vibration frequency and amplitude change dramatically. Half-frequency whirl entrainment is captured. The “switch” speed and predominate subharmonic frequency induced by internal friction are all increased with amplified tightening torque. For transition fit, instability is also observed for standard tightening torque. In such case, vibrations are always dominated by the fundamental frequency component, however, two “switch” speeds are captured in terms of vibration amplitudes. The time waveforms, spectrums and orbits vary significantly within the range of the two “switch” speeds. With speed increasing above the second “switch” speed, the rotor becomes stable again. Using larger tightening torque is able to suppress the internal friction. The present studies provide elaborated features of internal friction induced vibration, which are highly potentially useful for rotor fault diagnosis.