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electricpete

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Reply with quote  #1 

I have seen several cases (*) where elevated vibration damaged sleeve bearings in a short period of time.    The damage appearance has either been fatigue or smearing/wiping (melted babbit).  In at least one case we changed bearings and saw smearing/wiping bearing damage again in a short period of time (demonstrating that the damaged bearings were an effect of the vibration, not a cause). 

Fatigue cracking makes some sense, since the bearing sees a cyclical loading (although it's tough to estimate the magnitude of that loading).

But smearing/wiping / melted babbit at first doesn't make as much sense.  After thinking about it, I conclude that it must be a result of the shaft / bearing relative movement disrupting the oil film somehow.   There might be many ways this can happen, but attached depicts where a 1x forward whirl orbit would generate pressures that tend to oppose the normal oil wedge pressure from rotation. That's the best way for me to visualize what might be happening although I'm sure it can be much more complex. 

What do you think, is this a reasonable explanation for why a bearing may tend to wipe / smear the babbit when subject to excess vibration?  

* I'm sure you guys are way more interested in the case details than in the speculation about the mechanisms.  I'll try to put together a presentation showing what we saw recently.  But it'll take me some time.

 
Attached Files
pdf OrbitInterferesWithOilFilm4.pdf (35.87 KB, 23 views)

OLi

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Reply with quote  #2 
Wow, must be a significant vibration level? I would in theory see a touch in the bearing, local shaft hot spot, shaft bending and wipes bearing but a vibration that by itself reduce the oil film looks like magic to me. On the other hand I measured data a few weeks ago on a likely repeating coupling lockup on a 1500RPM 50MW turbine that went from steady state to 100+ microns and 9mm/s in less than 0.2s and generator bearings were damaged after that even if it tripped instantly and then dropped vibration it did that 5+ times very reproducing, if damage was due to hand cranking a few minutes w/o jack oil or the vibration is still argued but my bet is not on the hand cranking. Bearing on the other side of the coupling in the gbx was fine and didn't vibrate that much either, very strange happening, likely supported by a resonance also and vibration was 2xRPM so no 1xRPM bending anyway that I would expect. So maybe if you let it run like that for a while......
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John from PA

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Reply with quote  #3 
An old but worthwhile read on the subject can be found at http://oaktrust.library.tamu.edu/bitstream/handle/1969.1/163584/T18127-136.pdf?sequence=1&isAllowed=y
electricpete

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Reply with quote  #4 

Quote:
but a vibration that by itself reduce the oil film looks like magic to me

From an intuitive level (not necessarily experience), I look at it the opposite way.  When we first learn about hydrodyamic bearings it is a little mysterious how they can support load without contacting, but it is explained (for the rotating but not orbiting case) based on the pressure distribution of the oil which results as the viscous oil is sheared between two sliding surfaces in a particular geometry (converging wedge).    I can comprehend that. But what seems magic to me (again from intuition, not experience) is to expect that load-supporting oil film based on pressure to continue when the assumed geometry of a static converging wedge dramatically changes.    By and large, experience suggests I think most shaft relative vibrations encountered do not cause babbit damage, but it's not easy for me to understand how/why that is. Whatever the explanation for supporting the load of the orbitting shaft without metal to metal contact, it must be significantly different than the static explanation... maybe something like a squeeze film effect?

Quote:
An old but worthwhile read on the subject can be found at

Thanks, that is an interesting read.  I have always wondered about the fact that some sources suggests as a ballpark relative vibration alarm limit that peak-to-peak displacement should not exceed some fraction (let's say 50%) of diametral bearing clearance and I think the story is that bearing damage from the vibration might be expected as that relative motion goes beyond that.  I don't know why damage would be expected but I have been telling myself the story that it might have to do with oil film (along with fatigue.. two different mechanisms).   The article seems to provide a basis for thinking about limits based on static and dynamic eccentricity ratios, but mostly all in terms of loading, nothing whatsoever about oil film dynamics. So it does steer me toward thinking maybe damage that occurs at high relative vibrations is more related to the loading rather than the oil film dynamics that I postulated in op. 

========================= 

We have one family of 1800rpm 2500 hp sleeve bearing motors that have experienced three different signficant rotor-related vibration episodes: one in 2003, another in 2010, and most recently in 2019.  The 2003 and 2019 events both resulted in some babbit smearing in a short period of time.

  • Attached is a powerpoint describing the 2003 event:
  1. overview in slide 1.  
  2. The chronology is shown reading left to right accross the top of slide 2.
  3. There's a lot going on. It was thermal-induced vibration (see slide 15 for perfect correlation between vib and winding temperature) that got significantly worse when we installed a stator that ran significantly hotter, even hotter at no-load than the old stator ran under load (I think it was because the stator had non-magnetic wedges installed when OEM used magnetic wedges).
  4. Side 12 shows wiping of a portion of the bearing after 30 minutes run time.  The progression of orbits during that 30 minute runtime is shown on slide 11. Yeah, we recorded 15 mils relative when the diametral clearance was only 8 mils... I don't particularly trust that 15 mils number but it safe to say it was really moving. The portion of the bearing that is wiped is a stripe down the center of the bearing which (come to think of it) does correspond to a housing rib that clamps the bearing on the outside.
  • I've got a ton of history on the 2019 event. That is worth a discussion all on it's own. I'll post that as a separate thread when I get to it.

 
Attached Files
pdf horizontalPump13_2003_for_post.pdf (1.83 MB, 27 views)

George D

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Reply with quote  #5 
Thanks for posting Pete.

I don't think I can contribute insightfully on sleeve bearing failure modes?  But your Horizontal Pump case history triggered some curiosity that has troubled me for awhile.

I've seen displacement amplitudes, especially through criticals, on our 5400 rpm turbine driven feed pumps that exceed the installed lift-check bering clearance values recorded during installation.  My intuition doesn't allow me to think that is even possible?  Is it?  Could a padded bearing support flex enough to allow this to occur, or am I just seeing evidence of the actual diametral clearance that may not have been accurately recorded during installation?  My curiosity was triggered when I read the portion of your case history that cited a displacement of almost 200% of the bearing's diametral clearance.  Do you recall if they recorded a lift check once disassembled to determine if the bearing was wiped enough to create clearance to allow displacements of 200% of the installed diametral clearance?  Or is there another reason for this?


electricpete

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Reply with quote  #6 

I agree it gets your attention when indicated peak to peak movement exceeds diametral clearance.   For our specific case I don't have any records of as-found bearing clearance of lift check after the measurement, but I'm pretty sure we did not lose 4 mils on both halves of the bearing (to give 8 mils total).

I'm not a guy that is well versed in prox probes, others may have better suggestions.  Here's my thoughts about what might have happened (maybe):

Imagine an imbalance at the axial center of the rotor that forces the shaft into a bow shape. Then the shaft crosses at an angle through each bearing.    For my case 8 mils diametral bearing clearance, at a particular point in time the shaft at inboard edge of the bearing might be 4 mils above centerline and the shaft centerline at outboard edge of the bearing might be 4 mils below bearing centerline.  Let's say bearing width is 4" and the sensor reads another 4" beyond the inboard end... then if that same "slope" per axial distance is continued out, it could be 8 mils below bearing centerline at that sensor location.  One half revolution later everything flips and it is 8 mils above bearing centerline at that sensor location.  16 mils peak to peak.

I should also point out that the machine is not normally monitored by prox probes.  We brought in a consultant from Bently Nevada and they set up temporary prox probes which gave us this data.  I don't have any photo of exactly how/where they mounted those prox probes.. the details are fuzzy in my memory but I'm wondering if there may have been a flimsy bracket.  If the sensor mass on flimsy bracket is a base excited SDOF with a resonance far below running speed, then the bracket moves opposite direction of the housing and (assuming housing is moving in phase with shaft) the relative reading increases.

again, others may have better ideas about this than me. 

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