A Dynamic Mechanical Model for Hand Force in Right Angle Nutrunner Operation

A deterministic mechanical model based on physical tool parameters was used for estimating static and dynamic hand forces from kinematic measurements. We investigated the effects of target torque (25, 40, and 55 Nm) and threaded fastener joint hardness (35-, 150-, 300-, 500-, and 900-ms torque buildup time) on hand force. Estimated hand force was affected by target torque and joint hardness. Peak and average dynamic hand force was least for the hard joint (35-ms buildup) and greatest for the medium hardness joint (150-ms buildup). Tool inertia played the major role in reducing hand reaction force. Estimated hand force decreased when the inertial force component increased. Inertial force decreased by 366% when buildup time increased from 35 to 300 ms. Static modeling overestimated hand force; the error ranged from 10% for a soft joint to 40% for a hard joint. Results from direct hand force measurements using a strain gauge dynamometer showed that the dynamic model overestimated peak hand force by 9%. However, average hand force and force impulse were not significantly overestimated.