Oil Whirl is a condition in which a strong vibration occurs at between 0.38X and 0.48X. It never shows up at precisely 0.5X, but is always a little lower in frequency. It is caused by excessive clearance and light radial loading, which results in the oil film building up and forcing the journal centerline to migrate around in the bearing opposite the direction of rotation at less than one-half RPM. Oil whirl is a serious condition and needs to be corrected when found, for it can deteriorate fairly quickly to the point where metal-to-metal contact occurs in the bearing.
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