Ergonomics - Atlas Copco

Shocks into a
mechanical system
The tool and the hand-arm system can be
considered as a mechanical system. The
theory of vibration and shocks tells us that
the response from a mechanical system ex-
posed to a shock is a function of the duration
of the impulse and the natural frequency
of the tool/hand-arm system. The latter
depends on the posture of the hand and is
therefore closely related to the design of the
workstation.
The response
The response can be defined as the amplitude
of the motion of the tool handle due to the
shock. An operator’s subjective perception of
the shock reaction is closely related to the am-
plitude of the motion of the tool handle. The
displacement of the handle during tightening
could be a measure of the probable subjective
response from the operator using the ma-
chine. The displacement is, however, not easy
to measure during actual operation.
International Standard
Another method of quantifying shock re-
action is proposed in the ISO 6544 standard
of 1981. According to this standard, the tool
is held with its handle resting against a stop.
Torque during tightening is measured with
an in-line transducer and the impulse is
calculated by integrating the torque signal
versus time for various joints.
This type of test set-up ignores the
dynamic properties of both the tool and the
operator.
Machine design
The mechanical development of angle nut-
runners has followed two separate paths
– pneumatic tools and electric tools.
In pneumatic tools, the design concept
has been to minimize the impulse. A key
element here is the clutch. Pneumatic angle
nutrunners are provided with an extremely
fast clutch. When the tool reaches its preset
final torque, the clutch disengages the
motor from the gear train in 3.5 ms. This
fast action improves the accuracy of the tool
and the part of the impulse resulting from
the disengagement action is very small.
In practice, this means that when the
tools are used on hard joints the operator
can hold the tool handle between his thumb
and index finger and not experience any
shock because it will be absorbed by the
inertia of the tool.
For softer joints it takes longer to build
up the torque, which increases the impulse
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