Paper No. 4
J. C. Wachel, Short Course in Rotordynamics of Turbomachinery, Texas A&M University, May 1981, 1982.
Rotor instability vibrations in compressors and turbines have occurred more frequently in recent years and have caused sever failures and costly downtime for several large projects. Rotor instabilities can occur in flexible shaft units which operate above their first critical speed. The whirling instability frequency is usually near one of the shaft critical speeds and can be caused by many factors, including hydrodynamic bearings, seals, internal friction, aerodynamic cross coupling, and torsional coupling. The whirling motion can be subsynchronous or supersynchronous, and may be forward or backward precession; however, most serious problems are subsynchronous and have forward whirl.