Ergonomics - Atlas Copco

Fig. 3.16 The autobalancer automatically
offsets wheel imbalance.
Imbalance forces
For grinding machines, the imbalance of
the grinding wheel produces the dominat-
ing force which generates vibration. A unit
has been developed to compensate for the
imbalance. It consists of a number of steel
balls that can move freely in a groove. This
groove is integrated into the back flange
and its center line aligns with the center
line of the spindle.
The resonance frequency of the machine
system held by human hands is about 15
Hz. Therefore, the frequency of the oscil-
lating forces when the machine runs will
always exceed critical. A rotating unbal-
anced mass will try to move the center of
rotation away from the center line towards
the unbalanced mass.
When this happens the balls in the
groove will move in the opposite direction
and offset the imbalance. This only takes a
fraction of a second. If the imbalance changes
during the grinding process, the balls will
quickly find new positions to compensate for
the new imbalance.
Although this could offset the imbalance
there are still forces acting on the machine,
due to the fact that the balls are not acting
in the same plane as the imbalance.
Handling the remaining forces
Two design options are available in this
case with a rotating vector. One option is to
increase the moment of inertia as much as
possible. The other option is to isolate the
handles from the machine.
Increase the inertia
We mentioned earlier that it is an advantage
for the center of gravity of the tool to be as
near the wheel as possible. One section of the
tool situated near the wheel is, of course, the
guard. The guard’s mass is also located away
from the center of gravity, giving high inertia.
When designing the guard, steps are
taken to ensure that this component is tightly
attached to the machine, and rigid enough to
have a resonance frequency above the rota-
tional frequency of the machine, i.e., in phase
with the motion of the machine. At the same
time, the guard must be adjustable so that
the operator can easily change its position
depending on the type of task to be performed.
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