Origin of Split Resonance and Backward Whirl in a Simple Rotor

Split resonance and backward whirl in a simple rotor, were investigated both analytically and experimentally. A two degree-of-freedom rotor model was developed to simulate the steady state, lateral vibration characteristics of a simply supported, single disk rotor. This model included the effects of direct and cross coupled, linear damping and stiffness. The computer model was used to quantify the influence of bearing characteristics on rotordynamic response. In the absence of gyroscopic effects, split resonance is due to separate and distinct natural frequencies in the two orthogonal lateral directions created by non-equal direct stiffnesses. Backward whirl can occur between these two frequencies if the direct damping is sufficiently low. The model was able to predict the observed response of a simply supported rotor, including split resonance and backward whirl. The cause of the asymmetric direct stiffnesses in the experimental apparatus, which created split resonance and backward whirl was investigated. In particular, the influence of geometric imperfections in the plain bearing sleeve, gravitational forces, degree of imbalance and bearing support stiffness asymmetries were isolated using the experimental apparatus. It was determined that the bearing asymmetry was caused by the gravitational influence. However, larger imbalances increased the asymmetry and large damping was able to suppress backward whirl.