This paper presents a method of determining the locations of unbalances on a rotor when the unbalances occur abruptly during rotor running. The method makes use of journal vibrations, because, generally, there is a relation between the locations of unbalances and the unbalance vibration modes of a rotor. At critical speeds, where this relation is particularly significant, the phase angle of an unbalance vibration shifts 90 deg, and unbalance distributions excite vibration modes. Which mode or modes is excited depends on the location of an unbalance on a rotor. In this paper, a rotor is assumed to be divided lengthwise into three sections on which unbalances are assumed to be distributed. A simple algorithm, which is derived from the characteristics of the foregoing vibrations and assumptions, allows four unbalance distributions to be determined on a rotor, at the center, on the either side (right or left side), on the right and left sides (out of phase), and on the right and left sides (in phase) of a rotor.

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