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Rang

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Reply with quote  #16 
I've monitored a machine that was trending above 50g-s, the client refused the believe the data (our accelerometer was 'soaking' due to high amplitudes). It took 2 years to fail, but when it did, what a mess! It would have killed someone if they were standing next to it.

The machine was a Steam Feed Pump and the vibration of interest was the gearmesh, obviously running at its natural frequency.

The question is 'Why keep running it?'. This is something we are bad at communicating to client/operator/production team.
RustyCas

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Reply with quote  #17 
You can have a large spall in the outer race, but the rest of the bearing could be in pretty good shape (relatively). Did you notice any air flow from the motor cooling fan? (assuming TEFC) Hot motor frame?
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Noknroll

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Reply with quote  #18 
Hey Rusty
Hope you weren't losing sleep over waiting for my reply [smile] I was back on site again today and yes there is cooling air flow.
I usually measure this bearing radially opposite the load zone, just out of curiosity today i took an extra reading radially in the load zone and vibration was lower? go figure
electricpete

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Reply with quote  #19 
Interesting case study.

I'll chime in on the last bit. Since it's an outer race defect and the vibration is highest at a location other than the load zone, the straightforward interpretation might be that's where the outer race defect is currently located  (although outer ring might creep slowly in the housing over time).  But who knows, there may be other more subtle factors at work (like the how the internal construction affects transmission path between the bearing and the monitoring point).   I realize your intent was just to figure out the highest measurable vib to show the client, so it doesn't really matter why it's higher one place than the other. 

Noknroll

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Reply with quote  #20 
For those that appreciate a conclusion, this is what was found when it was finally decide to pull the bearing, considering the amplitude of the acceleration i was surprised to see such a small amount of spalling. Note the comet trail
Marking on the inner race interface with shaft has me intrigued the bearing was quite loose it had axial internal clearance of 0.5 mm

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Vibe-Rater

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Reply with quote  #21 
Good work Nok,

Short sharp spalls like that can cause very high G values. No surprise. Every mount has a natural frequency and the spall is close to an impact (has all frequencies) so whichever harmonic falls on the resonance will be amplified accordingly.

As well as that too much clearance in the fit of inner race on shaft. As you mentioned.

Had a paper machine press roll bearing once with a nice deep spall. At 1 machine speed a particular harmonic was super high, another machine speed a different harmonic was high. In each case coinciding with the fixed Hz press nip resonance.  One of the few times when you can see it all line up with a Hz frequency axis. Yes that old chestnut again. See how I sneak it in everytime! LOL.
Vibe-Rater

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Reply with quote  #22 
BTW Nok, 10 out of 10 for following up to get old bearing. That is often really tough and doesn't happen anywhere near enough. I find the bearings tell the truth as opposed to opinions.  Put 3 engineers in a room and you get 5 opinions... Great calibration tool - the bearing that is. rgds
RustyCas

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Reply with quote  #23 
Has anyone ever been able to have the outer race “indexed” as to top-dead-center? All it would take is a single punch mark and you’d then know exactly where the spalls were,
and which side was out. Have asked for this a number of times, and never been done. My theory is that motor shops don’t really want to “fix” motors, for obvious reasons. There is one shop one of my customers uses, and I swear that every rebuild they do comes back with some little mechanical problem which no one will ever notice, except a vibration guy. They are either consistently inept, or perhaps just devious.

And as to your original question, yes I have several that just will not die! I’ll post some data later.

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Danny Harvey

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Reply with quote  #24 
"Rebuild" is a very loosely used term.
fburgos

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Reply with quote  #25 
Quote:
Originally Posted by Noknroll
For those that appreciate a conclusion, this is what was found when it was finally decide to pull the bearing, considering the amplitude of the acceleration i was surprised to see such a small amount of spalling. Note the comet trail
Marking on the inner race interface with shaft has me intrigued the bearing was quite loose it had axial internal clearance of 0.5 mm


thanks great example.
Dave Reynolds

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Reply with quote  #26 
Noknroll,

Thank you for sharing the data and findings. I agree with your original post of monitoring G values and spectral changes as the bearing degrades. ONE reason why your G's are high is related to shaft turning speed, the faster the shaft turns the more acceleration it can produce. So the bearing you took out with that spall, if it were only going say 1500 rpm the G values would be less with the same fault.

Have used peakvue as a confirming technology that there truly is a bearing fault, both readings should show high G values if there is truly a fault along with the fault frequency being present.

The attached, one motor had been in service for quite some time before it decided to go south in a hurry found a cracked cage, other is a large gearbox with a short period of run time, when I got the tapered cup from the customer that was bad, it looked brand new, but when you ran your finger around the race, there was a "bump" right in the roller path. The customer chose to change the bearing, I recommended to trend, but it was "new" to them and they wanted it to be available 100% of the time.

Dave

 
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pdf Vibration Publication.pdf (675.32 KB, 17 views)
pdf Vibration Publication 2.pdf (445.50 KB, 14 views)

trapper

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Reply with quote  #27 
Quote:
Originally Posted by RustyCas
Has anyone ever been able to have the outer race “indexed” as to top-dead-center? All it would take is a single punch mark and you’d then know exactly where the spalls were, and which side was out. Have asked for this a number of times, and never been done. My theory is that motor shops don’t really want to “fix” motors, for obvious reasons. There is one shop one of my customers uses, and I swear that every rebuild they do comes back with some little mechanical problem which no one will ever notice, except a vibration guy. They are either consistently inept, or perhaps just devious.


I too ask for it all the time. I'm fairly friendly with the motor shop guys who do all the work (and their GM) and I don't think it's because they don't really want to "fix" motors. I think it tends to be forgotten because they are just too dang busy.

The three times they replaced a bearing on site (motors > 1,000 HP), one if us in our shop was able to catch when they got the bearing cover off and took pictures of the outer race positioning in-situ before they pulled the bearing. All the other times, it goes to the motor shop and, even though we get the bearings back from the shop, they never have positioning marks. [frown]

Noknroll

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Reply with quote  #28 
Dave Reynolds
Thanks for your input and case studies

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