Discomfort from pneumatic tool torque reaction: Acceptability limits

Pneumatic shut-off nut runners may produce large reaction forces to the operator's hand, especially at the end of the securing of threaded fasteners. These reaction forces depend on the shut-off mechanism, the joint hardness, tool torque level and to some extent the physical properties of the tool. The objective of this study was to find acceptability limits for the discomfort from pneumatic tool torque reaction forces that could be related to technical test measures of the tools.

In a study at a truck assembly industry 38 workers participated. Reaction forces, tool handle displacements and subjective discomfort ratings were measured. The tools were first tested according to ISO 6544 in the laboratory and the tool torque impulse was calculated. Strong correlations between tool handle displacements (r = 0.952), reaction forces (r = 0.981, vertical force) and ratings were found. Acceptability limits for ratings, tool handle displacements and reaction forces were also determined. No subject would accept to work a whole workday at a discomfort level over 9 on a 20-point scale and all would work a whole workday at a discomfort level of 2. These limits could then be correlated to the tool torque impulse measures from the technical test, thus making it possible to predict how many of the assemblers would work within acceptance limits.