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Executive Report: Problems of rotation EHN reports on why great care must be taken when measuring HAV levels of breakers and percussive tools to avoid erroneous readings. The most significant outcome of EHN’s HAV Conference last October was a consensus among hirers, manufacturers and contractors to seek testing of equipment to the BS 5349 standard under conditions reflecting real-life usage. The standard describes how vibration can be measured in three directions, or axes, as recommended by the Health & Safety Executive, thereby enabling a more accurate ‘tri-axial’ figure to be calculated. The ‘Y’ axis should be across the palm of the operator’s hand, the ‘Z’ axis should be aligned with the dominant axis of vibration, co-axial with the tool’s piston, and the ‘X’ axis should be mutually perpendicular to the other two. Measurement is undertaken using sensitive meters, which record the vibration registered by accelerometers (sensors) secured to the tool’s handles. Great care has to be taken to ensure accuracy. Some professionals with an interest in HAV are concerned that some readings could be of dubious quality. David Allan, who was for many years Managing Director of Macdonald Air Tools Ltd and now works as a consultant, believes that one potential problem when measuring percussive tools is imperceptible ‘rotation’ of the accelerometers, however tightly clamped to the handles they might appear to be. “This results in an amplification of the acceleration values, which do not truly represent the actual vibration energy being transmitted to the operator’s hands,” he states. “In percussive tools, such as breakers and hammer drills, the Z-axis reading should normally be the highest of the three. If one of the others is significantly higher, it may indicate an error. According to my research, moving the accelerometer by 90° on the handles, and making a second set of calculations, can give a better indication. You then take the Y and Z readings from the first test, and the X reading from the second test for the final calculation. I would suggest that taking two sets of readings like this should be common practice.” BS 5349 contains information about test procedures and identifies potential causes of erroneous readings, including rotation. It also highlights certain practical difficulties, such as the placing of the accelerometers. The logical place to fasten them is half-way across the handle, but this can hamper normal gripping. Adapter plates can be fitted in some cases, but it shows how complex taking measurements can be. “Manufacturers should design tools that reduce the likelihood of rotation,” contends David Allan. “For example, in hand-held scabblers, ensuring that the centre of gravity of the cylinder block is coaxial (in the same plane) as the inlet connectors can eliminate the problem. Air tool manufacturers began tackling issues like this many years ago, as soon as HAV became more widely publicised. This sort of equipment was perceived as being more likely to cause noise and vibration. More recently, attention has been focused on electric power tools, and some manufacturers have been surprised by readings on certain products that are higher than expected. “What is important is that tool testing should be carried out consistently, to stipulated criteria. It would also be helpful to include a representation of productivity, to help buyers and users to make meaningful comparisons. This might give details of the number of holes drilled in a certain time, or the amount of concrete broken. Operators must also be trained correctly, since how a tool is used can greatly affect the vibration level.” David Allan states that developing tools with low-vibration technology may lead to higher costs, but believes that hirers (and their customers) will pay a sensible premium since they have to meet Health & Safety obligations. However, he also points out that vibration figures should not be seen in isolation. “Productivity is an important consideration. A tool with a higher vibration emission may be more productive within the permitted limits, and it could therefore represent a better option.” • Executive Hire News › Archives › July 2006 › Executive Report › Problems of rotation
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