Harmonics of a side band pattern tend to put a lot of highlighters into the spectrum, which can be
confusing. There is also a strong possibility that the multiples of the basic pattern overlap.
In the symptom shown below (H, I), the number of side bands has therefore been reduced to two on
either side of the center line.
The pattern in (H) is made clearer by placing marker on top of the BPFI line and selecting ‘Show har-
monics’ (I).
Please note: While the presence of side bands and multiples in a spectrum is significant, the actual
number of such elements is not important. The job of a symptom is to point out relevant data, not
to find ‘everything’.
Zooming in on the spectrum and sweeping a cluster of lines reveals how close together significant
lines can be.
The BPFI factor of this bearing is 7.75, so the harmonics of BPFI are spaced 7.75 * RPM / 60 apart. The
symptom ‘Bearing, BPFIM’ looks for four harmonics but only two side bands.
H
I
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