How to know when a shaft is bent

With overall vibration and spectral analysis, a bent shaft problem usually appears identical to a misalignment problem. Phase measurements are needed to distinguish between the two.

Bent shaft causes

• Cold bow – As a result of gravity, a shaft with a high length to width ratio can, at rest, develop a bend.
• Improper handling during transportation
• High torque

Effects on bearings

As with imbalance, a bent shaft usually causes the bearing to carry a higher dynamic load than its design specification, which in turn causes the bearing to fail due to fatigue.

Diagnosis

To diagnose a bent shaft, use overall vibration measurements, spectral analysis and phase measurements.

Spectrum analysis

A bent shaft typically produces spectra that have misalignment type characteristics. A higher than normal 1x/2x amplitude may occur. A high 2x amplitude can vary from 30% of the 1x amplitude to 100 to 200% of the 1x amplitude.

Phase analysis

Phase measurements are essential when diagnosing a bent shaft. It’s important to note that all phase values are ±30°.

• Radial phase measurements (vertical and horizontal) typically appear “in phase”
• Axial phase measurements are typically 180° out of phase

If both the above conditions are true, the problem is probably a bent shaft.

Summing it up

If the primary vibration plane is in the axial direction, and there is a dominant 1x peak, and there is a 180° phase difference in the axial direction across the machine, then there may be a bent shaft.