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Hmmm.....well, I still feel strongly about the looseness. The question is where and why. 

Its difficult to tell you exactly where the looseness is located. Did you open the fan housing and look for any type of looseness of the fan on the shaft? Are there any signs of cracks in the weldments?  With power secured, does the shaft turn smoothly with no rubbing or abnormal sounds?

Also, while its running, look closely at all "interfaces"  that is, housing support to base, base to foundation, etc. Just checking bolt tightness is not always enough. In cases like this, I use my fingers to sense around for the highest impacting.

Don't give up.      Jim P

Looseness is probable. Looseness could be between parts that are inspectable externally or it could be an internal problem that can only be verified  by opening the parts. Example is bearing inner race OD and shaft.

In my word, amplitudes like 24 mm/sec are too high to keep the machine running or keep the operators check the machine. Safety is important.

Have you inspected the lubricant  (drained the oil to check it)? Any observations like unstable reading, noise, high temperature were noticed?

Regards- Ali M. Al-Shurafa 

To prove you do not have loose fits, use a long bar/block of wood/dial indicator. Put indicator at top dead center of the shaft and lift the shaft. If you have more than .003-.005" of up and down play, something is worn/loose

The harmonics in you vibration data indicates looseness, sources of looseness could be bearing fit to shaft or housing, loose bearing housing feet, loose fan base, worn coupling, fan wheel loose on shaft to name a few areas of interest

Have seen 3x come from loose bearing fits and coupling issues


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