Analyzing
a trend is not typically a difficult thing to do . . . providing you know
what it is you are trending.
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Are you trending
high frequency vibration ?
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Not if you only
use displacement.
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Are you trending
the typical, general equipment speed ranges of 1000 - 3600 rpm and associated
mechanical problems.
-
Not with displacement
or acceleration - only with velocity.
-
Are you trending
very low frequency vibration - below 100 rpm, for instance ?
-
Only with displacement
and, if the frequency is low enough, only by changing type of transducer.
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With vibration,
a trend headed in the wrong direction almost certainly means it is increasing.
With a properly set up program, properly collected data and proactive personnel,
a program can be successful with collecting little more than trend values.
For whatever reason, however, that is rarely the case. |
In any event,
trending can be useful but it is not usually enough to make specific judgements.
A determination of the exact frequencies involved is usually required and
should be attempted in any event to avoid a possibly embarassing and expensive
mistake. A determination of frequencies can be made two separate ways -
viewing the data as a raw time domain plot or putting it through an FFT
process to generate a spectrum. An overview of the process of analyzing
a spectrum will now be presented followed by an in-depth look at how, in
a practtical sense, the FFT process works. Understanding that will help
you view the spectrum with an appropriate amount of respect and
skepticism. |