Ergonomics - Atlas Copco

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In the GTG 40 grinding machine, this has
been solved by locking the guard with com-
pressed air when the machine is running.
From our simulations we could see that
a rigid support handle with a mass at the
end could also increase the inertia. On the
other hand, an excessively large mass at the
end caused increased vibration of the throt-
tle handle. The alternative of isolating the
support handle from the machine housing
was also considered.
Isolation between source and hand
The most common way to do this is to iso-
late the handles with a mass spring system.
Here two cases can be studied – one with
a dominant direction of vibration, and the
other with a complex vibration pattern in
three directions and rotation as well.
Designing vibration isolation for the sup-
port handle is done by placing a spring element
between the body of the machine and the han-
dle. The handle itself is the mass. Thus, from a
dynamic viewpoint, we disconnect the weight
and moment of inertia of the handle from the
machine. Accordingly, the center of rotation of
the combined system is moved and, more im-
portantly, the restraining effect of the handle’s
moment of inertia disappears. The machine
will thus vibrate more than previously.
This effect can be studied by analysis in
running mode. Vibration is measured at a
number of different points on the machine
and a computer animation of the tool’s
geometry is obtained based on the values
gained. During animation, the movement
of the tool is greatly magnified, permitting
study of the movements of different parts of
the machine in relation to each other.
Here the handle is running out of phase
with the machine and the vibration actually
increases due to the dynamic disconnection
of the support handle. The dynamic proper-
ties of the complete system, machine and
handle, must be considered to avoid the
resonance frequency of the total system
matching the rotational frequency of the
machine, because this will amplify the
vibration.
Whether the rigid handle or the isolated
handle is best from the point of view of
hygiene is an interesting question.
However, covering a handle with a layer
of visco-elastic material and claiming a
reduction in vibration is wrong. The cut-off
frequency for this design is above 300 Hz
and most of the vibration energy in a power
tool handle is below that frequency – thus,
there is no reduction in vibration.
How far should we go?
Measures of this kind have resulted in
a tool with vibration emission values below