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How
Amplitude Units Relate to Different Failure Modes
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The reason has to do with each amplitude
unit's sensitivity to different modes of machinery failures. In other words,
each unit has a specific usefulness in monitoring machinery health. There
are three types of failure causing effects that we are monitoring with
vibration:
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Stress (bending
a component excessively causes it to fail)
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Fatigue (something
simply wears out over time)
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Force (the 'pushing'
and/or 'striking' actions being applied to cause the movement)
The graph below shows the sensitivity
of each amplitude unit to the likelihood of a failure over a wide range
of frequencies. |
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Notice that at
low frequencies (primarily below 300 cpm or 5 Hz), displacement is the
most sensitive unit to the likelihood of a failure. That is due to the
fact that a stress failure (something being bent back and forth until it
breaks) is the most likely failure mode at those low frequencies - the
fatigue and applied forces become, as frequency approaches 0, simply too
low to cause a failure.
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Once you get above
300 cpm, the most likely failure mode increasingly becomes the 'fatigue'
mode (to which velocity is the most sensitive unit).
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Fatigue
failures basically occur when a component simply wears out - it tires of
the repeated back and forth movement (even a relatively small total distance)
over an extended period of time and many, many cycles. Between about 300
cpm and 120,000 cpm (5 - 2000 Hz), fatigue is the most likely failure mode
and therefore velocity is the most effective and reliable amplitude unit
to monitor with.
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Once you reach
120,000 cpm (2000 Hz), the most likely failure mode is 'force'-related.
What is a force-related failure ? When you reach these very, very high
frequencies (remember, you are dealing with moving an entire rotor structure
back and forth 2000 times per second or more), you are dealing
with massive amounts of force to move that structure back and forth even
a tiny distance at such a tremendously high frequency. Therefore, it is
that tremendous pushing or striking action that causes the failure.
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| It must be understood that there
are areas of the chart where the units overlap and two conditions (stress
and fatigue effects, for instance) exist. |
