# Bent Shaft

General Symptoms:

1. High overall vibration in the axial direction in displacement and velocity parameters
2. Generally we would get 1N in the axial direction if the bend in at the centre of the shaft
3. We may also get 2N in the axial direction if the bend in near to the coupling.
4. Vertical and Horizontal axis measurements will also often reveal peaks at 1N and 2N but the key to catch a bent shaft is to pay attention to what we get in the axial direction.

Reasons of bending:

1. Excessive heat. E.g. in motors that are overheated for various reasons, like for example, loose connections of the terminals. Also refer to the problem of Rotor Bow .. here.
2. Physically bent or run out
3. Sag of a long shaft — also called catenary. For example — turbine shaft.
4. Half critical speed — a phenomenon seen in horizontal machines operating close to the earth’s resonant frequency

Phase:

Phase measurement is an effective test to confirm presence of bent shaft. Phase at 1N measured in the axial direction at opposite ends of the components will be 180 degrees out of phase.

However, if the phase measurements are taken around the shaft we would find that they are all in phase since the shaft will appear to be moving back and forth in the axial direction.

Spectrum:

In addition to the prominent presence of 1N and 2N in the axial direction we would also find higher than normal 1N and 2N peaks in the radial directions.

Time waveform:

In this case time waveform would not prove to be a good indicator for bent shaft. However, a sinusoidal waveform is expected in the axial direction if the vibration is predominately 1N. In the case of a predominate presence of 2N there would be a “wobble” depicting the classic “M” or “W” pattern depending on the phase angle, if the bend is closer to the coupling.

by

Dibyendu De

dde@rgbwaves.com

9836466678