On Elimination of the Unstable Region of the Major Critical Speed in a Rotating Shaft System Carrying an Unsymmetrical Rotor : Dynamical Effect of a Flexible Pedestal or Added Mass on the Shaft

A rotating shaft system carrying an unsymmetrical rotor and supported by rigid bearing pedestals at both shaft ends is a four-degree-of-freedom system, at it has the so called "static unstable region"in the neighborhood of the major critical speed. Such an unstable region can be eliminated by adoption of flexible bearing pedestal at one shaft end, or by mounting an additional mass on the shaft. In the above two methods for elimination of the unstable region, the degree of freedom increases by 2, while dynamic effects of these methods are quite different from that of a dynamic absorber used in recti-linear vibratory system. In this paper, obtained analytical results of these methods are compared with several series of experiments.