Vibration Case Study Shorts

1. 3801 VIBRATING CONVEYOR

The support structure for this conveyor system is over 10 m tall and utilises anti-vibration mounts (AVMs). The requirement was to establish that these AVM’s were fit for purpose and that their transmitted vibration would not lead to fatigue cracks in the structure. This involved a mix of vibration theory and finite element analysis. Special attention was required for the design of bracing as it had to clear various items of plant and still resist the vibration forces.

The figures show the vibratory stresses in the structure and show a detail of the bracing and beam connections:

2. HOTEL BEDROOM VIBRATION

A well-known London hotel was getting complaints of vibration in the rooms on its top floor. We were asked to investigate and quickly identified that the vibration was the result of poor mounting of ventilation fans and it was one of those instances where we were able to apply an immediate fix. Unfortunately, we have no photographs, though we would have liked to show you the beaming smile on the Manager’s face. He was delighted, telling us that he would recoup our fees that night by upgrading the rooms from economy to de lux.

3. OFFICE FLOOR VIBRATION

This is an example of plant installed in the basement of an office building producing unacceptable vibrations on the ground floor. While every effort had been made to install anti-vibration mounts for both floor and ceiling supported equipment, items such as ventilation ducts were not because they were assumed to have no vibration. However, we discovered that some equipment was suspended from these ducts and that was the route the vibration was taking.

In another instance we measured the vibration characteristics in an office meeting room to discover that it tallied with a synchronous electric motor speed. We found the culprit – a fan impellor heavily caked with dust making it severely out of balance. It was some distance away from the area of complaint. If it had not been for our monitoring equipment, it is unlikely that the cause would have been so quickly located.

4. VIBRATION SENSITIVE EQUIPMENT INSTALLATION

Hospitals, universities and electronic manufacturers are a small sample of the users of equipment that is very sensitive to vibration. For existing buildings we offer a simple service of installing self-powered vibration monitoring equipment which we can leave for up to a week to record the ambient vibration in the horizontal and vertical directions. This is then processed and assessments made. In one instance we found that the vibrations at the equipment of concern were greater than anywhere else in the building, because that was where there was the post people traffic.

If the building has not yet been erected, still on the drawing board, we can model the building and its soil. With that model we can determine how vibrations will be transmitted, either from outside (e.g. road traffic, passing trains) or from within (plant room, machinery, pedestrians, etc.)

5. DR WHO

Cognoscenti of Dr Who will know that he occasionally made quips about earthquakes in Cardiff. Those television programmes were filmed close to the Welsh Assembly Building which was renowned for its tremors. Another building nearby had also suffered vibrations – so severe that large panes of glass had fallen, fortunately without anyone being hurt. We instrumented that building and established that the source of the vibration was about 100 metres at an old in-filled dock. Construction traffic moving over a small patch of poorly consolidated ground were shaking everything around.

Dr Who now has to fall back on jokes about Daleks and stairs.