A method for identifying the whirling motion of a rotating shaft has been developed. This method is based on the phase unwrapping of two strain gages' signals. These two stain gages are being attached ninety degrees apart onto the rotating shaft thus observe the shaft's motion in a rotating coordinate with speed the same as that of the shaft. With such arrangement of the strain gages, the whirling motions of the shaft can be identified provided that the rotation speed of the shaft is known. These whirling motions include forward whirl and backward whirl. Equations are given to show how to interpret the strain measure- ments. Examples from an experiment of a vertical rotating shaft are also given to show the use of this method. The proposed method is shown to be able to identify the whirling motions, which can not be achieved using only proximity probes.

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