The effects of power hand tool dynamics and workstation design on handle kinematics and muscle activity

Reaction force and workstation design aspects of right angle nutrunner operation were studied in order to better understand their effects on handle kinematics and muscle activity. Tool reaction force factors included spindle target torque (25 Nrn and 50 Nm) and joint hardness (35 ms and 900 ms build-up time). Workstation factors included orientation (horizontal and vertical) and operator distance (10 cm and 35 cm) from the tool. Dependent variables included handle displacement and velocity, work done on the tool-hand system, power involved in doing work, and EMG activity in the forearm and upper arm muscles. Isometric and eccentric strength corresponding to exertions against the tool for velocities of 0 m/s , 0.084 m/s , 0.251 m/s , 0.503 m / s and 0.754 m/ s were measured and the relationship between strength and handle kinematics during tool operation was studied. Six inexperienced volunteers (four males and two females) participated. Subjects operated a 58.5 cm long, 3.6 kg right angle nutrunner on a fastener 120 cm off the floor using the fight hand while standing. The handle was most stable (defined as minimum average peak velocity and displacement) when torque was 25 Nm, when vertical workstations were closest (10 cm) to the operator, or when horizontal workstations were farthest (35 cm) from the operator. Greater handle stability was observed for the horizontal workstation than for the vertical workstation. The hard joint (35 ms build-up) resulted in 307% greater peak handle velocity and 195% greater average power acting against the operator compared to the soft joint, however total work against the operator was 134% less for the hard joint. Little correlation was observed between static or dynamic strength and handle kinematics. EMG latency was measured from the onset of torque build-up. The average latency was 38 ms for the hard joint and 171 ms for the soft joint. Relevance to industry Ergonomic aspects of right angle nutrunner parameters are important for designing workstations and selecting power hand tools in assembly operations that maximize performance and quality, while minimizing physical stress. These parameters include process factors (torque and joint hardness) and workstation design factors (orientation and distance from the operator).