Dynamics of a bowed rotor with a transverse surface crack

Abstract

The rotors in industrial installations have various faults that could simultaneously exist and sometimes cannot be removed altogether, although efforts are always made to keep them within the acceptable limits (e.g., unbalance, misalignment, etc). Researchers in the past have mostly modelled and analysed individual fault separately and assumed a simplified rotor model and support conditions. In this paper, the equations of motion of the rotor with a transverse surface crack with a bow are formulated and steady state and transient response analysis of the rotor is attempted. The purpose of the study is to assess the effect of the residual bow on the stiffness characteristic of the rotating cracked shaft and changes if any, qualitative and/or quantitative that a bow may bring about in the dynamics of a cracked rotor. It has been observed that the usual level of bow may not significantly influence the stiffness variation and the nonlinear nature of crack response is not significantly altered. However, the bow completely masks the sensitivity of orbital response of cracked rotor to unbalance phase at half the critical speed and the use of influence of unbalance phase on orbital response at half critical speed of cracked rotor cannot be used for the detection purposes. The directional nature of higher harmonics such as 3× frequency component and their amplitude do not significantly alter. Due to the presence of crack and under the influence of gravity, the zero response at the self-balancing speed of the bowed rotor for a specific phase of unbalance is not observed.