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Nagesh

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Reply with quote  #1 
Hi everyone,

So I have a high vibration issue in an horizontal induction motor with sleeve bearings (coupled to a BFW pump) which is 3.5 MW and provided with proximity probes. The vibration amplitude is very much less than the alarm limits but are in an increasing trend which is my concern. I have a query regarding the shaft centerline plot.

Now let us say the bearing clearance is 0.2 mm. Now during a start-up (or a coast down) I was looking at a shaft centerline plot and the vertical axis shows that the shaft has lifted 0.19 mm. So by these readings, the maximum distance between the shaft and the bearing is just 0.01 mm or 10 micron. Now I notice that the shaft vibration shown by both the proximity probes is 50 micron. Since the shaft has stabilized at this position, it will be vibrating with a peak to peak amplitude of 50 micron from its position. So even if I just consider the peak amplitude of vibration which is 25 micron, the shaft will rub on the bearing as the  available gap between the shaft and bearing is just 10 micron. Am I correct in assuming this?

The reason I ask this question is I don't find any other indication of shaft to bearing rubbing. The full spectrum and the orbit aren't conclusive. The frequency of vibration is purely 1X. Pump vibrations are normal and steady. And the metal analysis of oil doesn't indicate any Babbitt material. Appreciate any input on this. 

Regards,
Nagesh.
John from PA

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Reply with quote  #2 
What you say can happen and a combination of the direct orbit (properly scaled) and overlayed on the shaft centerline plot can portray the picture (example attached).  But I would not make the assumption that damage is necessarily occuring without looking at some further data.  Other data should support the conclusion and it appears this may not be the case.  The shaft centerline plot in particular is susceptible to thermal growth effects on the proximity probes.  This can be significant, especially from a cold start.    If it is possible the plot should be based on shutdown information and reference data chosen carefully, also from the shutdown.

orbit overlay.jpg 


John from PA

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Quote:
Originally Posted by John from PA
The shaft centerline plot in particular is susceptible to thermal growth effects on the proximity probes.  This can be significant, especially from a cold start.    If it is possible the plot should be based on shutdown information and reference data chosen carefully, also from the shutdown.


I should also have mentioned that one major issue with shaft centerline data acquired from a cold start is alignment changes due to thermal effects will also likely occur.  
mtitone

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Reply with quote  #4 
For all vibration work, always be mindful of what you are measuring.  For a proximity probe, you are measuring the distance from the probe tip to the target (shaft, most of the time).

The probes for motors are often mounted on extender rods (stingers) through the bearing cover.  Therefore, if the bearing is moving relative to the bearing cover, the probe sees the total shaft movement as vibration and position changes and the total shaft movement is movement within the bearing plus the bearing movement.

If the bearing cover distorts due to thermal or other forces, the plots show this as shaft movement within the bearing, but it is really just cover movement.  As John mentioned, thermal growth of the probe and other components can also confuse the picture.

Another consideration applies to tilting pad bearings.  Due to the tilting of the pads, the shaft can move more than the actual bearing clearance.

Finally, sometimes the probes are mounted a significant distance from the bearing.  In this case some of the previously mentioned effects can be magnified.

To echo what John says, use hot coastdown data when possible, and use that coastdown data for compensation.

SCL plots are valuable, but learning to use them takes some practice.  Good Luck.

Michael Titone
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