Ergonomics - Atlas Copco

External load
When using a hand-held power tool,
the operator’s hand, arm and shoulder
system are subjected to the forces gen-
erated by the tool in use and the weight
of the tool. Excessive load of this type
may cause fatigue or, at worst, result in
damage to the operator’s musculo-
skeletal system. To minimize this risk,
the level, frequency and duration of the
load must be taken into consideration
when designing the workstation and
selecting the tools.
Forces from hand-held
power tools
Typical external load includes feed force and
reaction torque. Feed force is the force needed
to push a tool towards a working object – for
instance, the force needed to push a drill bit
into a workpiece when drilling.
Feed force usually acts along the longi-
tudinal axis of a pistol- or straight-type tool,
and perpendicular to the longitudinal axis
of an angular type tool. Many tools employ
the rotation principle. During operation,
the tool generates a rotational torque which
overcomes resistance from the workpiece.
Meanwhile, the rotational movements may
generate a dynamic torque. These two
torques together must be balanced by a
reaction torque applied by the operator’s
hand in the opposite direction.
The effect of the forces on the operator’s
body depends on the type of tool, the handle
design, and the task performed. The same
feed force and reaction torque may influ-
ence the operator’s arm, hand and shoulder
differently, and place different load require-
ments on the muscles used.
For instance, when used on a vertical
workpiece, the feed force from a tool with
a pistol handle is limited by the operator’s
muscular capacity to push the tool forward.
The same feed force may subject the wrist
to greater bending torque when operating
the tool at shoulder level than operation at
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