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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 13, Additional Vibration Sources

Section 3, To Determine a Bent Shaft (using Phase)

A bent shaft creates vibration of 1 x rpm. The phase symptoms are usually the same for a bend between bearings and overhung bearings.

Sometimes the axial vibrations due to a bent shaft are not as pronounced as shown. Most often a "couple" type axial motion will be the symptom. Sometimes it will not show. Then, the other symptoms will have to be the guides for analysis.

"Bent shaft" symptoms will often be the result of resonant whirl rather than a permanent bend. It will be "bent" in one of the modes of that particular resonance, such as at first critical speed, or second, and so on. But, it will not be bent when either not running or at a non-resonant speed.

A bent shaft will show the axial vibration as a rocking motion as indicated. If severely bent, the axial rocking at one end will be 180° out-of-phase with the rocking at the other end.



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