Vibration Analysis of a Bent Shaft.
Vibration Analysis of a Bent Shaft.
A bent shaft predominantly causes high 1X axial vibration. The dominant vibration is normally at 1X if the bend is near the centre of the shaft, however, you will see 2X vibration if the bend is closer to the coupling.
The axial measurement is the key indicator.
Spectrum: Look for a high 1X peak in the axial plane. If the bend is close to the coupling then you will see a higher 2X peak. You may also observe higher than-normal 1X and 2X peaks in the radial direction.
The phase is also a good indicator to diagnose a bent shaft. The phase at 1X measured in the axial directions at opposite ends of the component will be 180° out of phase. Please note if measurements are taken at the face of the component they will be in phase.
TimeWave Form: The TWF is not a good indicator, however, a sinusoidal waveform is expected in the axial plane.
Bent Shaft Spectrum in the axial direction. Please note the 2Xrpm will tend to appear if the shaft bend is closer to the coupling.
Recommended Corrective Action:
Bent Shaft: A notification to be raised to check the shaft runout or eccentricity with a dial indicator. The OEM will usually give a maximum permissible run-out tolerance on the shaft extension. This is usually called the TIR (Total Indicator Run Out) Usually it is the halfway point on the extended shaft length. See diagram below.
If the tolerance is exceeded then the bent shaft (rotor) will need to be replaced but the OEM should always be consulted.
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