Modal characteristics analysis for a flexible rotor with non-smooth constraint due to intermittent rub-impact

Intermittent rub-impact, during which the contact between rotor and stator is characterized by a “bouncing” or intermittent type of behavior, is one of the most common rubbing forms in rotating machinery. When the intermittent rub-impact occurs, the non-smooth constraint, which is the phenomenon that the system stiffness changes with respect to the state of contact and noncontact, will appear. The paper aims at discovering the possible effects of the non-smooth constraint on the flexible rotor’s modal characteristics by theoretical and experimental methods. The qualitative description for non-smooth constraint is given for the intermittent rub-impact process, and the dynamic modeling for a rotor system with non-smooth constraint is carried out. Meanwhile, the analysis method is developed by Floquet theory and Hill’s method to obtain the rotor’s modal characteristics. The results reveal that the non-smooth constraint produced by the intermittent rub-impact will increase the modal frequencies and critical speeds of the rotor system significantly. Due to the time-varying features of the constraint stiffness, the modal frequencies for the intermittent rub-impact rotor present fluctuant changes with the increase of rotation speed, which is different from the general linear rotor system. The non-smooth constraint is possible to lead the rotor’s instability, and the rotor’s instable regions can be expanded significantly for the increase of average constraint stiffness, constraint amplitude and contact time ratio. Non-smooth constraint could also expand the resonance speed and resonance sideband of the rotor system, which sometimes results in amplitude jump phenomenon. Keywords: Flexible rotor, Intermittent rub-impact, Instability, Modal characteristics, Non-smooth constrain