Reaction Force Exposure for Tightening Tool Users: An Experimental Study on Nutrunners

Assembly workers within for example the motor vehicle industry, are subject to physically demanding work tasks. One common type of load exposure for assembly workers, are reaction forces generated by tightening tools, such as nutrunners. The tool users counteract these forces through repetitive muscular force exertions, which in the long-term can pave the way for musculoskeletal disorders. As with other well-acknowledged load exposures commonly occurring in the assembly line environment, such as vibrations from tools, or adverse postures, reaction loads generated by nutrunners need thoroughly founded recommendations regarding acceptable exposure, in order to enable sustainable physical workloads for the tool users. The aim of this study was to lay a foundation for such recommendations. An experimental study was carried out in a simulated workstation, where assembly workers performed tightenings with an electric angle nutrunner, at different tool settings, joint stiffnesses and work-paces. Through a psychophysical approach, the participants judged the maximum amount of reaction load exposure which they perceived would enable a sustainable workload. The highly dynamic tool setting resulted in the highest chosen tightening torque levels, i.e. 35.0 and 37.7 Nm at five and eight tightenings per minute, respectively. This corresponded to a peak reaction force exposure of 71.6 and 69.8 N, respectively. Despite the statistically significant differences in impulse between the highly dynamic tightenings on hard joints and continuous drive tightenings on medium joints, no statistically significant difference between the chosen torque levels was found, suggesting that additional factors may contribute to the experience of reaction load exposure.