Combination Resonance of Dual Rotor Systems with Pedestal Looseness Using a Modified Model

In the bearing-support structure of aero engine, there is often a fit clearance between bearing outer ring and pedestal, which is called pedestal looseness, causing potential combination resonance under the unbalance excitation of the high-pressure and low-pressure rotors in dual rotor systems. In this paper, the phenomenon of combination resonance in dual rotor systems with pedestal looseness is investigated. A modified model of pedestal looseness is proposed to accurately describe the interaction between the bearing and pedestal. An analytical expression of the nonlinear force between the bearing and pedestal under multi-frequency excitations is derived, theoretically revealing the mechanism of combination frequencies induced by pedestal looseness. A method based on the harmonic balance technique is developed to solve for the combination frequency response of rotor system with pedestal looseness. It is further demonstrated that when the combination frequencies approach the modal frequencies, combination resonance occurs, significantly exacerbating the dynamic response. Finally, the modified model of pedestal looseness is applied in dual rotor systems with an inter-shaft bearing. Experimental verifications are conducted to study potential combination frequency phenomenon. The results indicate that when the combination frequencies approach the coupled modal frequencies of the dual rotor system, the amplitude of the combination frequency in the response increases significantly, leading to combination resonance.