Vibration diagnostic analysis, interpretation and practical solutions
The use of vibration analysis and interpretation to diagnose problems is often considered to be something of a “black art”. However, when combined with extensive practical experience and an intuitive understanding of the way machines and structures respond, it can be a very powerful tool for applications such as condition monitoring, quality control, fault finding/diagnosis and for identifying and solving structural vibration problems.
This can even be the case where conventional wisdom might conclude that there is insufficient information available for an accurate diagnosis – therein lies both the art and the “thrill” for the engineer.
Dynamic Vibration Absorbers – controlling plant vibration
Instead of charging users for the disconcerting "ride", the Millennium Bridge was retro-fitted with vibration absorbers to eliminate the 2Hz sway excited by pedestrians.
We have used the same principles to develop novel dynamic vibration absorbers that can be used to eliminate noise and vibration problems on plant and structures such as pipe-work (reducing fatigue etc), motors and pumps, dynamometers, support structural steel, engines, castings…
We have developed an elegant dynamic vibration absorber that can simply be bolted to pipe work or other structures to reduce vibration by 90-99%. Technically, this is very satisfying as the absorber is small, efficient and “sexy”…
Installed on petrochem pipe work attached to a reciprocating compressor generating high levels of vibration (illustrated here), this not only killed the vibration (eliminating wear and potential fatigue problems), but also produced a constant stream of curious staff wanting to see how such a small device could have such a big effect!
High levels of vibration on this water pumping station unit had caused maintenance problems and necessitated motors being sent away for refurbishment. The dominant element was at the 10.7Hz pump running speed with an amplitude of over 8mm/sec.
After detailed diagnosis, a pair of tuned dynamic vibration absorbers were designed that could be bolted to the motor whilst it was still in service. Once tuned, they all but eliminated the vibration – not only a very elegant and effective technique, but there is also a hint of magic about it…
“When I went down there afterwards, it was so smooth I thought the motor wasn’t running…” Alan England, maintenance engineer
The noise from this 115T Bliss power press was reduced by 90% (10dB) by analysing the flywheel vibration and then bolting £15 worth of small steel blocks mounted on gasket springs at specific points. These tuned dynamic vibration dampers cancel out the flywheel vibration, fit inside the existing guards and were installed in a few hours. Compare this with the cost and access problems associated with the only alternative noise control measure – a high cost conventional acoustic enclosure.
Similar dynamic vibration dampers fitted to the main gear wheels on a wire draw bench reduced the overall noise by 5dB(A) – again with no effect on normal operation, access or maintenance and at a cost saving of c £13000.
Vibration analysis indicated that the compressor was surging which would cause damage and probably require a strip-down. However, by identifying precisely what was happening mechanically within the compressor and discussing the results with an AC expert, the fault was diagnosed as a gas shortage causing liquid to flash across the expansion valve – cured by re-charging. This example illustrates the power of non-invasive fault identification.
Persistent propshaft failures were compromising engine test programmes. Vibration analysis was used to diagnose the problem as strong coupling between the response of the engine on its mounts and a structural mode of the engine pallet. The solution was designed and fitted within an hour (a simple steel channel welded across the pallet was all that was required).
A gravure coating line was producing striations on the thin aluminium web (due to film thickness variations). As the marking wavelength was independent of line speed, a resonance was diagnosed. This was traced (using a non-contacting probe) to web vibration at a specific point in the coating head. The solution designed was a simple, low cost web damper.
Bearing failures on a food processing mill continually recurred despite careful overhauls. Vibration signature comparisons with a second mill indicated a loose rear bearing. This diagnosis was confirmed in strip-down – the rear bearing was found to be turning in its housing.
The motor/gearbox on a large fermentation vessel suffered varying motor current and intermittent noise. Downtime would be costly due to the production schedule. Single point vibration analysis indicated excessive input shaft float – very unlikely (due to the construction) but confirmed later on partial strip-down (25 thou v 6 thou specification). Pinpoint accuracy.
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