LINEAR VIBRATION
TESTING AND ADJUSTING
Figure 2
The total distance traveled by the weight, that is, from
one peak to the opposite peak, is referred to as the
peak-to-peak displacement.
Figure 3
This measurement is usually expressed in mils, where
one mil equals one thousandth of an inch (0.001 in). It
can be used as a guide in judging vibration severity.
Both average or root-mean-square (rms) are sometimes
used to measure vibration. rms = 0.707 times peak of
vibration. These readings are meaningful in theoretical
discussions, but are of limited practical value.
Another
popular
method
used
to
determine
the
magnitude of vibration is to measure the vibration
velocity. Note that the example is not only moving, but
also changing direction. This means that the speed of
the weight is also constantly changing. At its limit of
motion, the speed of the weight is "0." As it passes
through the neutral position, its speed or velocity is
greatest.
Figure 4
The velocity is an extremely important characteristic of
vibration but, because of its changing nature, a single
point has been chosen for measurement. This is the
peak velocity and is normally expressed in inches per
second peak.
Vibration acceleration is another important characteristic
of vibration. It is the rate of change of velocity. In the
example, not that peak acceleration is at the extreme
limit of travel where velocity is "0." As the velocity
increases, the acceleration decreases until it reaches "0"
at the neutral point.
Acceleration is normally referred to in units of "g" (peak)
where one "g" equals the force of gravity at the earth's
surface. (980.665 cm/se, 386.09 in/s2, 32.174 ft/s2
Displacement, velocity, and acceleration a all used by
vibration experts to diagnose particular problems.
Displacement measurements tend to be a better
indication of vibration under condition of dynamic stress
and are, therefore, most commonly used. Caterpillar
has standardized on displacement measurement.
Velocity is a direct measure of vibration and, as such,
provides
the
best
overall
indicator
of
machinery
condition. It does not, however, reflect the affect of
vibration on brittle material.
The relationship between peak velocity and peak-to-
peak displacement can be found by the following
formula:
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