Experimental Investigations on Torsional Vibrations of a Rotor During a Rotor-Stator Rub

Theoretical investigations on rotor-stator rub phenomenon are available on wide range of problems. However the experimental studies are limited in numbers. Further, the measurement studies on torsional vibration during rub are hardly any. The present work reports and discusses measurement of rotor’s torsional vibrations during its contact with the stator and examines the possible rub diagnostic features. A simple rotor bearing system is made to interact with a stator pin mounted on a stiffer stator frame. The rotor assembly, with two discs and two support bearings, intends to model a turbopump where the central disc simulates the impeller and the overhanging disc is meant to represent turbine. A flexible coupling between the driver and the driven unit is provided to reduce the effects of any misalignment remaining in the system and to isolate the rotor from the motor. The measurement of torsional vibration is carried out using a torsional laser vibrometer. The measured signals during rub and no rub conditions are compared in time as well as in frequency domain to bring out the torsional vibrations features related to rub. Excitation of various torsional modes of the rotor system is experienced due to the frictional torque caused by the occurrence of rub. The presence of rotor’s torsional mode frequencies, rotational harmonics and the bending natural frequency in the spectrum of torsional velocity signals are indicative of rub.