Simulation of vibration of handheld machinery

For assessing the exposure to vibration to which users of handheld machinery are subjected, companies use emission measurements taken in situ under real use conditions, in compliance with Directive 2002/44/EC. Since such measurements are time-consuming, and therefore costly, the assessment may, under certain conditions, be made on the basis of the “machine” vibration emission values declared by the manufacturer. However, the declared values result from standardised testing conducted in the laboratory and reproducing specific conditions that cannot represent all of the conditions encountered out in the field. Since the declared values are, by nature, not representative of the field values, an empirical coefficient of correction is applied in order to obtain, ultimately, exposure values that are analogous to those obtained by measuring in situ.
In order to reduce the loss of assessment objectivity related to the use of these empirical factors, we propose to demonstrate that it is possible to predict, by numerical simulation, the vibration level emitted by a machine under real use conditions, and to quantify the maximum error between the predicted value and the value actually emitted.
The numerical method used is the finite element method. All of the calculations are made using the software LS-DYNA3D, which makes it possible to model the propagation of the shock waves and to resolve the dynamics equations. The models developed in that environment will be validated on the basis of observations and of measurements performed on machinery tested under repeatable laboratory conditions. In order to guarantee that the experimental conditions are maintained as constant as possible throughout the testing, the operator holding the machinery will be replaced by a mechanical device, whose properties are similar to those of a hand-arm system. The test conditions will be chosen so as to represent a broad sample of real use conditions. The capacity of a model to predict the vibration emitted by machinery under real use conditions will be assessed by taking measurements at work stations in the field, by simulating such situations and by comparing the numerical results with the measurements taken in situ.
It is also proposed to conduct thinking on the methods of transferring and of deploying these models that could be implemented subsequently in order to make them accessible to CARSATs (occupational health and pension insurance funds), and to companies who wish to assess the exposure of their employees to vibration. Actions will be conducted to encourage manufacturers to develop their own models of machinery in order to contribute to generalising the method. In the long term, the method should enable occupational safety & health specialists to assess the exposure to vibration of their employees who use handheld vibrating machinery.