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44 HAND ARM VIBRATION See more like this online at: www.hsmsearch.com IMPROVING ASSESSMENT Svantek’s Jacek Kuczynski discusses how the development of new, very small MEMs sensors have made it possible to locate force sensors next to vibration accelerometers. When in contact with the surface of a vibrating tool or machine, mechanical vibrations are transmitted directly to the human body, affecting individual tissues or even the whole body. In practice, the most dangerous hand-arm vibration can cause pathological changes in the nervous, vascular (cardiovascular) and osteoarticular system.  Hand-arm vibration occurs when one or both of the hands are in contact with a vibrating surface and the characteristic feature is the variability in time. Very often, measurement results depend on the point in time that monitoring takes place. The EC Physical Agents Directive – Vibration defines both the test methods and measurements describing this kind of vibration and limits the duration of exposure to vibration. Changes in the human body resulting from contact with mechanical vibrations are recognised as an occupational disease called ‘vibration syndrome’ or ‘white finger disease’. There is no cure and symptoms manifest by the fading fingertips of one or more fingers. Measurements are currently made using ‘human vibration meters’ equipped with vibration acceleration sensors. Not every device is suitable for measuring the vibration affecting workers, which is why ISO 8041 outlines that a human vibration meter should meet certain minimum requirements. The majority of human vibration meters use piezoelectric accelerometers. Drawbacks of these sensors include fragility, high price and DC-shift effect problems. Any measurements showing signs of DCshift should be disregarded (according to ISO 5349-2). The introduction of MEMS technology offered significant advantages including reducing the overall cost of the instrument, low power consumption, small size, resistance to mechanical shocks, full electromagnetic compatibility and no DC-shift effect. MEMS accelerometers can be attached to the hands without any impact to the performance of everyday activities even underneath anti-vibration gloves - giving the true results of vibration exposure. A force sensor can then be installed next to the accelerometer, allowing measurement of the contact force simultaneously to the tri-axial acceleration. ISO 5349-2 The evaluation of vibration exposure described in ISO 5349-1 is solely based on the measurement of vibration magnitude at the grip zones or handles, and exposure times. Additional factors, such as gripping and feed forces applied by the operator, the posture of the hand and arm, the direction of the vibration and the environmental conditions, etc, are not taken into consideration. ISO 5349-2 does not offer guidance to evaluate these additional factors. However, it is recognised that reporting of all the relevant information is important for the development of improved methods for the assessment of vibration risk. Improved methods of handarm vibration exposure measurement have been defined by ISO 5439 as the ones using additional factors such as contact force in order to decrease the uncertainty of exposure time. At the time the ISO standard was written it was practically impossible to perform force measurements together with tri-axial vibration measurements due to hardware limitations. Now, very small transducers can be fitted right next to the MEMS technology-based vibration accelerometer in a form of hand-arm adapter as specified in ISO 5349-2 and ISO 10819. With such an effective solution, it has become possible to perform continuous measurements through the whole working day which decreases the uncertainty of the sample limitation. The time history of contact force values proves important in determining the exposure time by simple selection of the force threshold level. Svantek’s newly launched SV103 instrument is the world’s first personal hand-arm vibration exposure level meter. It meets ISO 8041:2005 and performance measurements in accordance to ISO 5349-1 and ISO 5349-2 with special adapters mounted on the operator’s hand. Inside the hand adapter is the latest MEMS accelerometer and a contact force sensor. Contact forces act between the hand and vibration surface: the push / pull force and the gripping force. The need for simultaneous assessment of the contact forces and vibration magnitudes has been universally recognised and reflected in ISO 15230. Both acceleration and contact force values are displayed clearly on an OLED screen. Contact force detection has helped set a new benchmark in hand-arm vibration measurements. Simultaneous monitoring of coupling forces and vibration is necessary because varying coupling forces applied by operators on handheld vibrating tools influence differently the level of vibration energy to the hand/arm system. Coupling forces modify exposure to vibration and the health effects it causes. Additionally the synergic impact of force and vibration on the cardiovascular system, nervous system and joints and muscles should be considered. It is clear that the future evaluation of the occupational exposure limits for hand/arm vibration should also consider coupling forces exerted on vibrating tools. Tel: 01296 682040 “In practice, the most dangerous hand-arm vibration can cause pathological changes in the nervous, vascular (cardiovascular) and osteoarticular system...”


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