Vibration reduction in unbalanced hollow rotor systems with nonlinear energy sinks

Reduction of whirling vibration amplitudes in rotordynamic systems resulting from unbalance force during the passage through the critical speeds is very important for preventing propagation of fatigue cracks, excessive noise and instability problems. In this paper, numerical investigations of passive targeted energy transfer (TET) to mitigate the whirling vibration amplitude in rotor systems at the critical speeds are performed by applying nonlinear energy sinks (NESs) to the rotor systems. The three-degree-of-freedom hollow shaft-NES system has been modeled based on Lagrange's method. Numerical simulations have been performed to optimize the NES parameters in order to obtain the optimum performance for vibration reduction. The influence of critical parameters, including the angle between the unbalance mass and the NES, the NES damping and the unbalance mass on the NES performance in whirling vibration reduction, is also investigated. Additionally, the application of the NES is compared with that of the tuned mass damper (TMD). Numerical results show that the NES is able to efficiently reduce the resonant amplitude of the rotor systems when the stiffness and mass ratios are optimized appropriately. Furthermore, it is found that, unlike the NES, the TMD requires an offset distance from the shaft centerline to be able to reduce the resonant amplitude; otherwise, it fails in mitigating the whirling amplitude. Identifying this offset for the TMD requires pre-knowledge of the exact unbalance value and its eccentricity from the shaft centerline. However, the NES does not require a priori knowledge of the unbalance value which is an advantage of the NES compared to the TMD. The TMD, applied here with an offset distance able to mitigate the resonant vibration, only works as a balancer when positioned opposite to the location of the unbalance mass, which limits its application in vibration reduction of rotor systems.