Summary

It is possible to use a VSA™ Vibration Spectrum Analyzer to diagnose misalignment between two rotating devices with an inline interconnection.

Details

A pump in a manufacturing facility had always been noisy but levels had gradually increased to the point of being intolerable.

The pump was removed, stripped, checked and reassembled with new bearings. This did not fix problem. In fact it was even noisier. At this point a vibration consultant was called in.

The consultant was asked to diagnose the problem without shutting the pump down, as the pump was vital in the production process.

The first step was to measure radial vibration on the pump. The vibration spectrum measured in terms of velocity showed 1 X rotational speed vibration to be high, and the 2 X rotational speed vibration was found to be at a similar level.

The second step was to measure radial vibration on the drive motor casing. Likewise, the vibration spectrum measured in terms of velocity showed 1 X rotational speed vibration to be high, and the 2 X rotational speed vibration was at a similar level.

Hypothesis: the problem was likely to be in the coupling between pump and motor, since the vibration characteristics of both the pump and the motor were similar.

The third step was to measure axial vibration. The vibration spectrum measured in terms of velocity showed 2 X rotational speed vibration—double that of 1 X rotational speed vibration.

The 2 X rotational speed vibration being higher meant that there was a high probability that problem was coupling misalignment. Given the fact that the noise increased after the pump rebuild, the assumption was made that the alignment was worse after reassembly.

At this point there was a solid justification for stopping the pump.

The pump and coupling were removed. It was discovered that the rubber bushings in the coupling were hard and partially disintegrated, resulting in looseness since there was no damping of vibration.

The bushings were replaced with temporary inserts. Before reassembly, the motor shaft center height was measured. The pump shaft was found to be 0.03” low, so shims of this thickness were manufactured. The system was reassembled and aligned horizontally as closely as possible for a test.

The test run confirmed that the noise problem had disappeared. A technician was instructed to realign the pump once new bushings were received from the OEM. Two weeks later the new bushings were installed and the pump system was realigned. The system was started up and ran successfully, with no need for a follow up visit from the vibration consultant.

Derek Norfield
Director of Applications
Datastick Systems, Inc.