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OLi

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Reply with quote  #1 
I am back on my favorite big motors. I have one problem motor left now running full load but having indications of rubb. One seal inspected was brass and had lost like 1.5mm and the other is a plastic thingy air seal that made 1+ mm grove in the shaft. Anybody seen orbits or other data in connection with plastic seals? We have constrained orbits and levels on 1xRPM increase, phase shift etc. and it now seems to stabilize around 80+ microns after running 5 hours on load. End user like me to balance and maybe it is worth a try as the vectors get stable finally and if lucky it may reduce rubbing. Absolute bearing vibration are low, less than 2mm/s peak in most directions. 
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John from PA

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Reply with quote  #2 
OLi, we (and you likely know who) preach that a rub is often a secondary malfunction in that some primary malfunction is the root cause of high vibration and the amplitude is sufficient to set off a rub.  So a balance shot might be worthwhile, especially if the correction planes are readily accessible.  I would make the shot based on proximity probes since you make the comment that absolute levels (I assume you mean casing) are low.

I don't think I've ever encountered plastic causing rub symptoms but I do know of a class of STG where carbon brushes at the generator (perhaps 20 tons?) set up all the classic symptoms of a rub.  It was hard to believe but simply lifting the brushes caused the vibration to drop and dropping the brushes onto the shaft caused the vibration to return. 
OLi

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Reply with quote  #3 
We will check that, it has been discussed but not checked. We will start balancing tomorrow pending seal inspection. It was already the plan and our order to come here was "balancing in case of" and now it is "in case"..... Problem is that the vector from the mechanical unbalance seen at low speed are overthrown by thermal part at full speed, full load and a few hours operation by 150% in the likely opposite phase angle so it could be a no win situation, acceptable at full speed and load would give less good status at startup, things are never easy, still the data are reasonably stable and repeatable so it may be possible to do something, time will tell. Thank's for the input.
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Good Vibrations since early 1950's, first patented vibrometer 1956 in the US.
http://www.vtab.se
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