Vibration response of a cracked rotor in presence of rotor–stator rub

Fatigue crack and rotor–stator rub are two important faults in rotating machinery. Researchers have mostly studied the vibration behavior of a rotor with crack and rotor–stator rub separately. However, once the crack is developed in a rotor, the rotor is more likely to make contact with stator under tight clearance conditions, due to increased vibration level. The present study is aimed to examine vibration response of the cracked rotor in presence of common rotor faults such as unbalance and rotor stator rub. Numerical and experimental investigations are carried out and steady-state vibration analysis is presented. Experimental investigation for a multifault rotor system is attempted for the first time. The full spectrum analysis has been used effectively to extract the distinctive directional features of these rotor faults. The investigation focuses on directional nature of the higher harmonics for identification of rub in the cracked rotor. The study reveals that spectrum rich in spectral lines is a rub symptom. However, these higher harmonics are weaker than the 1X response. Rub in uncracked rotor excites forward and backward whirling frequency components almost equally. Cracked rotor without rub exhibits strongly forward whirling vibrations. Rotor rub in the cracked rotor reveals different response compared with the uncracked rotor, particularly the nature of 2X and higher harmonics at corresponding subharmonic resonances. Backward whirling nature of 2X frequency component as well as that of higher harmonic (that matches with the bending natural frequency) at corresponding subharmonic resonances, has been proposed for diagnosis of rotor rub in cracked rotor.