📖 Field Diaries : Episode 5

When High 1× Vibration Isn't an Imbalance

One of the easiest mistakes to make in vibration analysis is assuming that a dominant 1× running speed on a fan automatically points to an impeller imbalance.

During a routine inspection of a supply fan, vibration analysis identified elevated vibration levels dominated by the fan running speed. As the fan is pulley driven with a step-up in speed, it was immediately clear from the motor vibration that this was no ordinary or straightforward imbalance diagnosis.

Although the fan running speed was dominant, the motor and fan vibration displayed several tell-tale signs that something else was going on. In particular, multiple harmonics of the motor running speed, together with a prominent 2× motor running speed frequency, suggested that the fault was not simply the result of an unbalanced fan.

At first glance, balancing would have seemed like the obvious recommendation.

However, the vibration signatures suggested there was a deeper mechanical issue that required further investigation..

A decision was made to investigate the mechanical condition of the fan.

With the drive belts removed, a dial test indicator (DTI) inspection was carried out.

The results

• Fan shaft run-out: 0.20 mm

• Pulley sheave run-out: 0.40 mm

The measurements confirmed excessive run-out within the fan shaft assembly.

The vibration spectra, dominated by running speed and multiple harmonics, supported the mechanical findings and confirmed that excessive shaft run-out was driving the elevated vibration levels.

The lesson

This investigation highlights the importance of looking beyond the obvious.

Vibration analysis clearly indicated that a mechanical defect was present, but confirming the root cause required a simple mechanical inspection. In this case, a DTI inspection verified excessive shaft run-out within the fan assembly, giving confidence in the diagnosis and ensuring the correct repair could be planned.

Takeaway

Vibration analysis tells you where to look. Good engineering confirms why it's happening.