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Technical data are subject to change without notice.
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• Box 4 • S-645 21 Strängnäs • Sweden
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Measurement
If a fan is out of balance, it will shake at its speed of
rotation, i.e. move backwards and forwards once per
revolution. The number of vibrations per time unit is
the
vibration frequency
, measured in Hz (Hertz =
cycles per second).
The rotational speed of any piece of plant is known as
its
fundamental frequency
. For a fan with a speed of
1 500 r.p.m. the fundamental frequency is 25 Hz (1
500 r.p.m.
÷
60).
In practice, machine vibration usually consists of many
different frequency components. For a general as-
sessment of machine condition one uses
wide fre-
quency band measurements
, that is all vibrations
within a large frequency range are measured simulta-
neously.
Cyclic movement can be measured and described in
three different ways, as
·
displacement
·
acceleration
·
velocity
Displacement means the actual distance the object
moves, measured either from its rest position in one
direction (peak) or as the total movement in both
directions (peak to peak). Displacement is usually meas-
ured in mils.
A part that is moving from rest, speeding up, slowing
down and stopping twice per cycle is obviously accel-
erating and decelerating continuously. Acceleration is
measured in g (1 g = 32.17 ft/sec
2
).
The third measuring parameter is the speed at which
the object moves, the vibration velocity. Velocity is
expressed in in/sec.
Both acceleration and speed are constantly changing.
One can measure a peak value of either, but a mean
value often gives a better indication of the forces
involved in the movement. Most instruments measure
the
RMS value
(root mean square value) of the move-
ment and use a scaling factor to indicate the peak
levels if they are given at all.
Frequency
Hz
Displacement
mil
Acceleration
g
Velocity
in/s
