Effects of DC-powered pistol grip tool location and orientation on operator upper extremity stiffness and damping

In automotive assembly lines, workers routinely used DC-powered pistol grip tools for the installation of threaded fasteners. The stiffness and damping offered by the hand-tool system dictates the handle displacement due to the reaction torque. The aim of the study was to predict the typical ranges of stiffness and damping offered by the upper extremity in different wrist orientations and locations while operating a pistol-grip hand tool. The hand-tool system was represented using a single degree-of-freedom torsional model and a deterministic approach was adopted to identify the system parameters. Tightening tasks were executed by ten experienced hand-tool operators at three torque levels (5 Nm, 7.5 Nm, and 10 Nm) and at four different fastener locations corresponding to varying wrist orientations. At 5 Nm, 7.5 Nm, and 10 Nm torques, the mean operator stiffnesses were 645 N/m, 879.5 N/m, and 1019 N/m respectively with a mean damping being 22.88 N/m, 15.14 N/m and 12.38 N/m respectively. The stiffness coefficients were different between wrist positions but not the damping coefficients. The research demonstrates the approach to model pistol grip hand tool operation and determine the stiffness and damping parameters. This approach could be used for determining optimal torque ranges and positions to minimize rotary tool handle displacement due to reaction torque, thereby reducing the risk of injury.