Analysis of backward whirling characteristics of a dual-rotor system caused by unbalance

•Backward whirling motion of dual-rotor systems caused by unbalanced excitation.•The effects of rotation direction, unbalanced excitation, speed ratio, etc. on the whirling characteristics.•Experimental verification of backward whirling characteristics of dual-rotor systems. In this paper, a dual-rotor system with five-point support is established, considering the nonlinearity of the inter-shaft bearing. Based on the full-spectrum analysis theory, the whirling characteristics of the dual-rotor system are analyzed. In a counter-rotating dual-rotor system, the backward whirling phenomenon caused by the unbalanced excitation is found, and the effects of rotation direction, unbalanced excitation, speed ratio, and inter-shaft bearing on this phenomenon are discussed. Finally, the simulation results are verified based on the dual-rotor test rig. The results show that a negative speed ratio is a necessary condition to generate the backward whirling of the rotor; in the counter-rotating dual-rotor, a large unbalance of one rotor is an important factor causing the backward whirling of the other rotor. The larger the unbalance is, the more likely the backward whirl will occur; changing the speed ratio will mainly affect the initial speed and speed range at which backward whirl occurs; The radial clearance and stiffness of the intermediate bearing mainly affect the coupling degree of the inner and outer rotors, thereby affecting the whirl characteristics of the dual-rotor. The above research helps understand the whirl characteristics of the counter-rotating dual-rotor deeply.