Design of a suspended handle to attenuate rock drill hand-arm vibration: model development and validation

Abstract

Pneumatic jackleg drills are widespread percussion tools used in the mining industry. Hand-arm vibration frequency-weighted exposure levels evaluated within the 6.3–1250 Hz frequency range have been found to be on the order of 25 m/s 2 when operating this tool in typical mining conditions. This study concerns the development of a suspended handle designed to provide attenuation of the vibration and shocks being generated at the tool blow frequency, which occurs between 35 and 45 Hz on most types of pneumatic drills. The results of the first phase of development of such a handle are presented in this paper. They are based on the development and validation of a model combining the suspended handle and the hand-arm system. For that purpose, the hand-arm system is represented by a four-degree-of-freedom lumped parameter model, referred to as model 2 in the ISO 10068 standard. As part of this investigation, a model is developed to represent two different types of suspended handles: one incorporating helicoidal springs, the other viscoelastic mounts. These combined hand-arm-suspended handle models are then validated by comparing the model predictions with the measurements of vibration transmissibility realized while exciting the suspended handles on an electrodynamic shaker system. These measurements involved the use of human subjects holding the handles while applying a push force varying from 0 to 80 N, and a grip force ranging from 20 to 50 N. For values of grip and push forces set at 50 N, good agreement was achieved between the model predictions and the measurements, especially at frequencies above 35 Hz. The hand was found to have a significant influence on the vibration transmissibility responses of both suspended handles. When the values of suspension stiffness were selected to provide a resonant frequency of 25 Hz for the free handles, the vibration attenuation achieved at a frequency of 35 Hz was on the order of 30% when gripping the handle, while a slight amplification was noted at that frequency with the free handle. As the frequency increased towards 45 Hz, even more important attenuation could be realized (on the order of 50%) for the handle-hand combination. When the resonant frequency of the free suspended handle was set at a higher frequency (i.e., 67 Hz), the hand-arm system was found to provide additional damping at the handle resonant frequency, while not introducing any significant influence at the lower frequencies as compared with the behavior observed for lower natural frequency suspended handles.