Resonance and post-resonance backward whirl in intact and cracked rotor systems with rub-impact

The whirling behavior in rotor systems is an indicator for rotor health monitoring. The abnormal whirling is the first sign for faults appearance in rotor systems. The excitation of backward whirling is associated with the shaft or bearing stiffness content. Nonsynchronous whirl near resonance speeds significantly affects rotor faults in rotor systems. The response of intact and cracked rotors in the presence of rotor-stator rubbing is significantly altered by the nonsynchronous whirling which further complicates the dynamical behavior of the rotor during its transient operation. In cracked rotors, the post-resonance backward whirl (Po-BW) excitation was proven to exist due to crack propagation and bearing anisotropy. Therefore, the influence of nonsynchronous whirl in association with rub-impact and breathing crack on the presence of Po-BW in intact and cracked rotor systems is numerically and experimentally investigated in the present research. The Jeffcott rotor and the finite element models are employed for numerical simulation during transient operation conditions. According to the obtained numerical and experimental whirl response results, the backward whirl phenomena can be now classified into three categories - pre-resonance, resonance and post-resonance backward whirls. Robust experimental evidence of the presence of Resonance-BW and Po-BW excitations in cracked and healthy rotor systems with rub-impact is analyzed which provides further insights on the effect of multiple faults on BW excitations.