Diagnosing a Bent Shaft

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Portable balancer & Vibration analyzer Balanset-1A

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Vibration sensor

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Optical Sensor (Laser Tachometer)

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Balanset-4

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Magnetic Stand Insize-60-kgf

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Reflective tape

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Dynamic balancer “Balanset-1A” OEM

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1. The Nature of a Bent Shaft

A Bent Shaft is a condition where a machine’s rotor has been plastically deformed, causing it to deviate from a straight line. This can be the result of thermal stress (e.g., a hot turbine rotor that was not properly cooled), mechanical damage (e.g., from a dropped rotor), or as a sympathetic failure from a severe unbalance or misalignment condition.

A bent shaft will produce vibration that is very similar to unbalance or misalignment, but it has one key distinguishing characteristic: high axial vibration.

2. The Vibration Signature of a Bent Shaft

The primary vibration signature for a bent shaft is a high-amplitude peak at 1X the running speed of the shaft. This is because the bend in the shaft behaves like a large “heavy spot,” creating a centrifugal force similar to unbalance.

However, the key diagnostic indicators are:

• High Axial Vibration: The most important sign of a bent shaft is high 1X vibration in the axial (thrust) direction. As the bent shaft rotates, it forces the components connected to it (like couplings and bearings) to move back and forth along the shaft’s axis. Amplitudes of axial vibration that are greater than 50% of the radial (horizontal or vertical) vibration are a strong indicator of a bent shaft or severe misalignment.
• Similar Radial Vibration: Like unbalance, the 1X radial vibration will be high.
• Dominant 1X Frequency: The spectrum is often dominated by the 1X peak, although sometimes a 2X peak will also be present, especially if the bend is near the center of the shaft.

3. Differentiating a Bent Shaft from Misalignment

A bent shaft and shaft misalignment can produce very similar vibration signatures, as both often cause high axial vibration. The key is to use phase analysis to distinguish between them.

• Procedure: Take radial and axial phase measurements at both the inboard and outboard bearings of the machine (four readings in total).
• Bent Shaft Indication: If the shaft is bent, the axial phase readings taken at the same radial position (e.g., top of both bearings) will be approximately 180 degrees out of phase with each other. This is because as one end of the machine is pushed forward by the bend, the other end is pulled backward.
• Misalignment Indication: In a classic case of angular misalignment, the axial phase readings will be approximately in-phase (or close to 0 degrees apart).

Taking phase readings across the coupling can also provide definitive evidence to distinguish between the two fault conditions.

4. Differentiating a Bent Shaft from Unbalance

While both conditions create high 1X radial vibration, unbalance will typically produce very little axial vibration. If you have a high 1X peak and significant axial vibration, the problem is unlikely to be unbalance alone.

Additionally, a bent shaft cannot be corrected by balancing. Attempting to balance a bent shaft may reduce the vibration at one location but will often increase it at another. If a rotor is difficult or impossible to balance, it is a strong sign that the problem is a bent shaft, not unbalance.

5. Confirmation

The ultimate confirmation of a bent shaft is to place the rotor on V-blocks or in a lathe and use a dial indicator to measure the shaft’s runout. A significant runout reading will confirm that the shaft is physically bent and needs to be straightened or replaced.

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