Vibrations of a rotating shaft containing a transverse crack. Variations of a resonance curve due to the angular position of an unbalance at the major critical speed.

In order to find some way to detect a transverse crack due to fatigue in a rotating machine, we have investigated experimentally and theoretically the change in the resonance curve caused by the occurrence of a crack. The experimental apparatus is a vertical rotating shaft system in which an elastic shaft with a disk is supported simply at both ends. In experiments, the amplitude curve and the phase angle curve were measured in the neighborhood of the major critical speed for various angular positions of the rotor unbalance. The results show that the shape of the resonance curve changes extremely due to the direction of the unbalance, and an unstable region appears when the unbalance exists in the same direction as the crack. It is shown that this phenomenon is explained theoretically by combining the characteristics of the unsymmetrical shaft and that of the symmetrical shaft or by considering rotating nonlinear spring characteristics.