Ergonomics - Atlas Copco

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Fig. 3.3 The same feed force applied by the operator causes a different load on the wrist depending on posture.
elbow level (Fig. 3.3). The same feed force in
a tool with a straight handle used on a hori-
zontal workpiece is limited by the operator’s
muscular capacity to push the tool down (ul-
nar push force). The strength of the person’s
grip can also be a limiting factor, as there is a
tendency for the hand to slip on the handle.
Similarly, the reaction torque in a pistol
handle tool used on a vertical workpiece
applies an axial rotation torque to the
operator’s forearm. Therefore the forearm’s
muscular capacity to rotate, supination or
pronation, will determine the maximal allow-
able installed torque of a pistol-type tool. An
intolerable torque in a pistol handle tool may
well be tolerable in an angular-type tool, due
to the advantage of pulling muscle strength
as well as the extra leverage provided by the
angular tool (Fig. 3.4 ).
In summary, feed force and reaction
torque during hand tool operations demand
different actions from the hand-arm system,
depending on the tool design and the tasks
being performed.
Fig. 3.4 When using pistol grip tools, torque applied by the operator is limited by the operator’s
supination torque capacity. For higher torque an angle nutrunner is a better alternative utilizing
the pulling back force to balance the installed torque.