Simulation, Test, and Mitigation of ½× Forward Whirl following Rotor Drop onto Auxiliary Bearings

1/2× forward whirl repeatedly occurred after a test rotor spinning at 5800 rpm was dropped onto ball bearing type auxiliary bearings (AB), utilized as a backup for magnetic bearings (MB). The measured contact forces that occurred between the rotor and the AB during the ½× subsynchronous vibration were about thirteen times larger than the static reaction force. The vibration frequency coincided with the rotor-support system natural frequency with the rotor at rest on the AB, an occurred at ½ of the rotor spin speed when dropped. The test rig provided measurements of rotor-bearing contact force, rotor orbit (vibrations), and rotational speed during rotor drop events. A simulation model was also developed and demonstrated that parametric excitation in the form of a Mathieu Hill model replicated the measured ½× forward whirl vibrations. The simulation model included a nonlinear, elastic-thermal coupled, ball bearing type AB model. The transient model successfully predicted the ½× vibration when the rotor was passing 5800 RPM as well, and the simulation results quantitatively agreed well with the test results in the frequency domain. Several approaches for mitigating the 1/2× forward whirl were presented such as adding an elastomer O-ring or waviness spring in the AB support system. Measurements confirmed that adding AB dampers effectively mitigated the ½ subsynchronous forward whirl and significantly reduced the contact forces.