Analysis of stator vibration response for the diagnosis of rub in a coupled rotor-stator system

Abstract

The present day rotors are more susceptible to rub due to reduced clearance that adversely affects the performance of the machines. In most of the theoretical investigations on rub, the structural connectivity between rotor and the stator casing is ignored and the dynamics of the rotor are studied with stator interaction as external to the system. Thus, in the present study, a coupled rotor-stator system is modelled where the vibrating rotor transmits the force to the stator structure through the bearing support and during the direct contact. The rub interaction is defined using contact mechanics based Lagrange multiplier method. The model ensures dynamic rotor-stator contact boundary and more realistic contact constraints in contrast to most of the earlier approaches. The stator thus responds through its own vibratory motion and the instantaneous clearance decides the nature of interaction between them. The stator vibration data at the casing location is explored to provide conclusive indication of the rub phenomenon. The intermittent rub interactions that are non-synchronous to rotational frequency cause the system to exhibit quasi-periodic response and the transient excitation due to impacting rotor generates natural frequencies in the system response. In the stator vibration spectrum, the presence of sub harmonic frequency components and stronger presence of harmonics of rotational frequency near the natural frequencies of the system indicate the occurrence of rub. Only low amplitude rotational frequency component is observed otherwise. Strongly correlated with rotor vibration response, the stator vibration response and its rub specific features are proposed for the rub diagnosis.