A biomechanical model is presented to estimate user hand/arm force exertion with two pneumatic nail gun trigger systems. The sequential actuation trigger (SAT) is safer than the contact actuation trigger (CAT) but increases the user's exertion of force because the trigger must be actuated after the safety tip is held pressed against the workpiece. Time integrated hand force was calculated for a single user based on direct measurement of nail gun tip force against the workpiece (tip contact) and from estimated force to support the tool weight during transfer between nails and during idle holding. The model shows that hand/arm force increases when nailing with the SAT (relative to CAT) and with a vertically-oriented workpiece (relative to horizontal). Expressed per nail fired, the user exerted 0.13 Ns (horizontal orientation) and 2.88 Ns (vertical orientation) integrated hand force during tip contact with CAT compared to 26.15 Ns (horizontal) and 46.08 Ns (vertical) with SAT. Depending upon idle holding duration, integrated hand force during tip contact was estimated to have been 1-3% of 48-132 Ns total hand force with CATand 21-44% of 83-167 Ns total hand force with SAT (average of horizontal and vertical orientations). Based on standard time allowances from work measurement systems it is proposed that efficient application of hand force during tip contact with SAT can reduce this contribution to 6-15% of 55-139 Ns total hand force. The model is useful for considering differences in hand/arm force exertion between the SAT and CAT systems Relevance to industry: This paper presents a model of hand/arm force associated with two types of pneumatic nail gun actuation (trigger) systems. The model clarifies differences in user force exertion with the sequential actuation and contact actuation triggers to inform nail gun trigger selection decisions.
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