An impact
can occur due to a number of different problems - bearing defects being
#1 on the list (most common and most important). But because of
exactly how the FFT works, the impact frequency gets filtered
out of the displacement, velocity or acceleration spectra. Figure 1 shows
a outer race defect occurring on a bearing rotating about 1200 rpm.
Figure 1 - A Signal Showing An
Impact Occurring
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But why does
the impact frequency get filtered out ? It is simply because there is no
sinusoid at the impact frequency (no sinusoid associated
with the rate at which the impacts are occurring). The only sinusoids present
are:
-
The 1x rpm sinusoid
(we see 3+ cycles of that);
-
The sinusoid associated
with the ringdown occurring just after the impact occurs (similar
to a bell ringing down after being struck).
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| So how does
the FFT process the signal shown ? The 1x rpm sinusoid is no problem but
how does it account for the periodic impacts occurring ? |
