NONLINEAR VIBRATION OF HORIZONTAL JEFFCOTT ROTORS SUPPORTED BY OIL FILM DAMPER BEARINGS HAVING NO CENTERING SPRINGS - CALCULATION OF NONLINEAR IMBALANCE RESPONSE AND FORCED DAMPED NATURAL FREQUENCY

In this paper, nonlinear imbalance response and forced damped natural frequency are calculated for horizontal Jeffcott rotors supported by oil film damper bearings having no centering springs. The former calculation reveals that there exists four types of synchronous imbalance responses. In one of the four types, the vibration amplitide can become infinitely large, and this dangerous type of vibration can appear easily with increasing imbalance. The latter calculation reveals that the stability margin, which is determined from the calculated complex eigenvalues, greatly depends on the type of synchronous response that happens to appear.