What is the
source of the vibration at 1,800 cpm ?
-
Obvious choice
- 1x rpm: Since it is a direct drive machine and we will make the assumption
that there is no other vibration feeding into this machine from another,
you have a very good probability of being right (note that it is not a
100% probability) if you say that peak is the vibration 1x rpm.
What is the source
of the vibration at 3,600 cpm ?
-
Obvious choice
- 2x rpm: Since the machine is running around 1800, chances are again fairly
good you will be right if you say this peak is related to 2x rpm.
What is the source
of the vibration at 7,200 cpm ?
-
Obvious choice
- 4x rpm: This could be the 4x running speed harmonic and many people would
think that is the obvious choice - but that would be very risky because
there is another vibration source very close to this range. 2x AC line
frequency in the U.S. is 7200 cpm (6000 cpm in countries with 50 Hz power
supply) and could very well be any proportion of that amplitude.
-
For example, if
the machine is running 1780 rpm (1480 rpm in 50Hz countries), 4x that is
7120 cpm (5920 cpm in 50Hz countries). That is 80 cpm away from 2x line
frequency (7200 cpm or 6000 cpm) - much less than the 300 cpm/line spectrum
resolution. That means the energy from both of the vibrations will be quantified
in the same line of resolution (i.e. combine into one, single
peak).
Are you monitoring
bearing defect frequency harmonics sufficiently ?
-
Yes. With a machine
running 1800, collecting an FFT to 120kcpm is sufficient to monitor the
full range of frequencies in which bearing-related peaks will appear.
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