The bearing
defect frequency identified is just over 3x RPM. Notice that there are
no significant peaks at 1x, 2x or 3x rpm on Figure 1 (there were on the
velocity spectrum). There are, however, extremely significant peaks
at 1x, 2x and 3x the impact frequency - in this case a bearing defect frequency
(there are other impact sources). The enveloping signal provides the following:
The impact
frequency:
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This piece of
information can be used on the velocity or acceleration spectrum to help
determine the condition of the bearing (how bad is it ?).
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After identifying
the defect frequency from Figure 1, inspect your velocity or acceleration
plot and place your cursor on that same impact frequency and turn on your
harmonics.
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If you are able
to relate, through the harmonics, the high frequency peaks to this impact
frequency, you have confirmed the presence of a bearing defect.
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You can then make
an assessment of condition based on the amplitudes present, noise level,
etc.
The intensity
of the impacts:
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This piece of
information can be used to help determine how quickly a bearing can be
expected to deteriorate since the impacts are so destructive.
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You can compare
this to hitting a small pothole in your car or hitting a huge, sharp edged
pothole - the first is annoying, the second can destroy your wheel.
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The assessment
can be made by displaying the amplitudes on a 'dB' scale (see Figure 2)
and comparing the peak amplitude to the surrounding 'carpet' level (which
is affected by lubrication and load, among other things).
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Figure 2 - Enveloping Spectrum
From
Figure 1 on dB Scale
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In Fig. 2 (which
is the same plot as Figure 1 except the amplitudes were on a linear scale
in Fig. 1 and are on a dB scale in Fig. 2), the amplitude on the peak is
about 125 dB.
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The surrounding
carpet level, which is an estimate of the surrounding amplitudes, is in
the 100 - 102 dB range. The following guidelines can be used:
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Difference of
12-18 dB is a significant level of impacting and should be watched closely.
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Difference of
> 18 dB is a severe level - intense impact energy, very destructive.
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