Ergonomics - Atlas Copco

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We can start by looking at the machine itself,
not taking the process noise into account. In a
pneumatic grinder, for example, there are sev-
eral sources. If the grinder is equipped with a
vane motor, this is the loudest source. Usually
it is most efficient to start noise control work
at the main source.
The vane motor
A vane motor consists of a rotor with a set
of vanes rotating eccentrically in a cylinder.
The motor has an inlet for compressed air
and at least one outlet, normally in the form
of holes or slits in the cylinder mantle.
When a vane passes the outlet, com-
pressed air passes out at high speed. This
creates a pressure pulse outside the cylinder,
Fig. 3.22 The outlet
pulse from the vane
motor is one of the
main acoustic sources
for the motor.
moving away from the cylinder at the speed
of sound (340 m/s). The next vane starts up
the next pulse in the same manner.
When the motor runs, a noise will be
created with a fundamental frequency equal
to the rotor speed multiplied by the number
of vanes. A narrow band analysis of this
noise will show the fundamental frequency
and a number of overtones. In other words,
the signal is not a sine wave but a more
complex one. However, the fundamental fre-
quency is often dominant. One way to treat
this noise is to arrange the rows of vanes in
the rotor asymmetrically. Another option
that can be chosen at the same time is to de-
sign the outlet in a way that lowers the air
flow gradient, since the noise level decreas-
es when the change in flow decreases.
Muffler
When the motor has been optimized ac-
cording to all parameters, including power,
efficiency, weight, etc., the remaining noise
can only be muffled away.
The motor is protected by a housing.
Part of the housing is designed to act as a
muffler. If a pipe is connected to the outlet,
the noise from the open end of the pipe can
be controlled by the length of the pipe. If
the pipe length is equal to a wave length
or a half wave length of the fundamental