New backward whirl phenomena in intact and cracked rotor systems

Based on several published studies and relevant Campbell diagrams, it is well known that the passage through the critical backward whirl (BW) rotational speeds in rotor systems should theoretically precede the passage through the critical forward whirl (FW) rotational speeds. Theoretically, this means that the location of the zone/zones of the BW orbits should be captured before acquiring and exceeding the critical FW rotational speed under the effect of an unbalanced force excitation. However, for rotor systems that exhibit recurrent startup and coast down operations, the associated equations of motion take the form of linear time-varying (LTV) equations for which the traditional Campbell diagram is no longer valid for predicting the related critical FW and BW rotational speeds. Accordingly, herein, it is numerically and experimentally verified that new zones of BW rotational speeds appear immediately after the critical FW rotational speed is acquired and exceeded during startup and coast down operations in the cases of intact and cracked rotor systems. These numerical and experimental investigations have been carefully conducted to provide robust evidence about the appearance of these new BW zones. These discovered BW zones are found to be affected by the crack damage in the shaft and were accompanied with a stiffness asymmetry in the bearings. Unlike the case where the critical FW rotational speed is acquired and exceeded, the vibration during the case at which speeds acquire and exceed these new BW zones of rotational speeds was found to be associated with an abrupt reduction in whirl amplitudes to minimum values for both start-up and coast down operations.