Text: A vibration of the self-excited (sometimes called free) vibration category occurring in rotor/fluid lubricated bearing/seal systems. This malfunction causes the shaft centerline dynamic precessional motion to be forward, and usually circular, and at a frequency proportional to the shaft rotative speed and in the range of 40-49 percent of the shaft rotative speed. The whirl mechanism is not limited to oil-lubricated bearings but can occur when any fluid (steam, process fluid, air, etc.) is between two rotating concentric cylindrical surfaces.

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