At present, the axis orbit (whirling) and the instantaneous angular speed (spinning) are important symptoms in the condition monitoring of rotor systems. However, because of the lack of research of the transient characteristics of axis orbit within a whirl cycle, the axis orbit cannot reflect the instantaneous characteristics of the rotation during one whirling cycle like the instantaneous angular speed. Therefore, in this paper, a new concept of instantaneous whirling speed of axis orbit within a whirling cycle is proposed and defined. In addition, the transient characteristics of instantaneous whirling speed are studied. Meanwhile, the response mechanisms are qualitative analyzed through the study of the work of the additional stiffness excitation and the conversion relationship between the kinetic energy and the potential energy. Then, the minimum of the relative instantaneous whirling speed (RWS) is proposed as a potential monitoring index for crack severity. The instantaneous whirling speed is a new attribute of axis orbit and a new perspective for the vibration analysis of cracked rotors. The addition of this new attribute significantly increases the effect of axis orbit for distinguishing normal and cracked rotors. The new analysis perspective and the new diagnosis index are potential supplements for crack diagnosis.
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