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Author Topic:   failed ball bearing inspection - overheated grease and brinneling marks
electricpete
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posted 04-20-2004 09:56 AM     Click Here to See the Profile for electricpete   Click Here to Email electricpete     Edit/Delete Message   Reply w/Quote
This is the same motor I talked about in my thread on TEFC. There was in fact some warning in our old vib data that we didn't seee.

We replaced bearings and found the inboard bearing is the one with the problem. (even though we could feel the vib very much stronger on outboard fan shroud when rotated by hand than any place else on motor).

Before we disassembled we compared the bearings and it feels like inboard bearing has more looseness than outboard bearing. When you try to tilt inner race compared to outer race you can see the movement ? 20 mils?

Inspection of inboard bearing showed: http://reliability-magazine.com/pub/gc22Slide1.JPG

http://reliability-magazine.com/pub/gcSlide2.JPG

Clearly there was overheating as indicated by baked-on brown grease deposits on outer race (after it is cleaned) seen on rightmost picture at at slide 2.

Grease is black, dried/caked as shown in slide 1. We are left with a question of whether:
A - heat (from something) caused the grease to become black
or
B lub problem caused the overheating.

I think B because we know it is doubleshielded bearing not replaced since 1989. No sign of fresh red grease anywhere inside (the outboard bearing shows more red grease).

Ball path appears centered indicating no abnormal thrust load. No signs of spinning on inner race or movement within housing. One ball and section of cage was cleaned. Ball and cage in very good condition.

Outer race has a series of shiny marks as is from axial movement and inner race has just a few. The outer race marks are deep enough to feel with your fingernail. Two scenario's come to mind:

A - False brinneling
OR
B - Damage during dismount and subsequent handling.

The pattern of more marks around one half of outer race than others may suggest false brinneling. However the spacing does not show clear ball spacing. In some place there is a regular spacing of approx 1/8" between series of marks, not consistent with false brinneling. Also marks are very very shiny would seem to indicate they are fresh.

Vibration shows no prominent fault frequency patterns. That seems to indicate the outer ring marks were not present when we took the vib.

Low frequency range: http://reliability-magazine.com/pub/gcSlide7.JPG

Higher Fmax: http://reliability-magazine.com/pub/gc22Slide11.JPG

There are large random spaced twf spikes apparently due to lube problem

acceleration time waveform: http://reliability-magazine.com/pub/gcSlide9.JPG

spike energy time waveform: http://reliability-magazine.com/pub/gc22Slide10.JPG

What do you think? Lub just never got it? Marks occurred during disassembly and handling after bearing removed?

[This message has been edited by electricpete (edited 04-20-2004).]

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Ron Ensing
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posted 04-20-2004 10:13 AM     Click Here to See the Profile for Ron Ensing   Click Here to Email Ron Ensing     Edit/Delete Message   Reply w/Quote
Electrical fluting??

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Jon Chandler
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posted 04-20-2004 01:12 PM     Click Here to See the Profile for Jon Chandler   Click Here to Email Jon Chandler     Edit/Delete Message   Reply w/Quote
I have to agree with Ron....from the picture, it looks like the early stages of fluting. Is this a VFD unit?

Jon
Spintelligent Labs

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electricpete
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posted 04-20-2004 01:19 PM     Click Here to See the Profile for electricpete   Click Here to Email electricpete     Edit/Delete Message   Reply w/Quote
I agree it looks like fluting however it is not powered from a vfd. Not even a large motor. Standard induction motor. 75 hp, 3600 rpm.

One detail: The motor was idle for approx 6 months during the period between vib data taken in September 03 until motor noted rough by hand during pump seal April 04. If there were false brinneling happened during that time period it would explain why we didn't see any bpor pattern during last vib check last September. (also if damage occurred during removal that explains it as well).

I have a question: Is there any reason to suspect that poorly lubricated bearing is more susceptible to damage from false brinneling? One might think that with better lubrication the ball is more likely to roll rather than scrape the race. With poor lubrication ball is constrained by cage from rolling. Does that sound reasonable? If not, then IF we think false brinneling occurred it would be completely independent/coincidental to the finding of poor lub?

[This message has been edited by electricpete (edited 04-20-2004).]

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JPeterson
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posted 04-20-2004 02:01 PM     Click Here to See the Profile for JPeterson   Click Here to Email JPeterson     Edit/Delete Message   Reply w/Quote
We have usually only run into brinelling problems that were caused by improper installation, dropping or by the simple storage of bearings on a shaft. As I'm sure all of you know, in the storage of a simple large pulley system w/bearings mounted there is metal to metal contact at rest. The trouble is... there is really no "rest". At our mine there is blasting every day. The ground shakes and brinelling happens, even to the bearings in the warehouse on the shelf! We've seen large bearings display outer race defects immediately upon entry of service from this effect.
Also, I've heard of another cause of BPFO in newly refurbished or new larger motors with antifriction bearings. If run uncoupled the load zone of the bearing is on the bottom due to the weight of the shaft. That is the only place in which the cage is driven and it is being driven by the ball/roller at that site which impacts it. After leaving that site the balls/rollers are being driven by the cage until they reach the load zone again. Then... whack... the friction of being driven by the inner and outer race (in the load zone) drives the ball/roller into the cage again. This all takes place at the frequency of BPFO because the location does not change and the rollers passing through the load zone and hitting the cage at that specific site cause the frequency. It mimmicks BPFO. This is one cause of BPFO in a new/refurbished motor when in fact there is nothing wrong. Just thought I'd throw this one out there.
Have a good day.
JP

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Eben
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posted 04-20-2004 03:40 PM     Click Here to See the Profile for Eben   Click Here to Email Eben     Edit/Delete Message   Reply w/Quote
I looks like eltrical fluting to me.

The marks along with the feel you described.

I one just like this recently no VFD small size but it was more advanced when it was changed.

Just my two cents worth.

Good Luck,
Eben

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electricpete
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posted 04-20-2004 05:09 PM     Click Here to See the Profile for electricpete   Click Here to Email electricpete     Edit/Delete Message   Reply w/Quote
Thanks for the good comments.

Regarding fluting vs false brinneling:

SKF Publication 401 (Bearing Failures and their Causes) says on page 12 “The fluting resulting from vibration s sometimes closely resembles the fluting produced by the passage of electric current. However, in the latter case the bottom of the depression is dark in color, not bright or corroded. The damage caused by electric current is also distinguishable by the fact that the rolling elements are marked as well as the raceways. “

Therefore it appears my damage was caused by false brinneling rather than electric current since the marks are shiny and the balls are undamaged.

I was under the impression that fluting is a concern for vfd's or large motors or possibly machines which generate static. This motor is attached to a centrifugal water pump.

Regarding effect of lubrication on false brinnelling.

SKF Publication 401 (Bearing Failures and their Causes) says on page 12
Roller barings have proved to be more suscptible to [false brinneling] damage than ball bearings. This is considered to be because the balls can roll in every direction. Rollers on the other hand can roll in only one direction; movement in the rmeaning direcitons takes the form of sliding.”.

“An oil bath, in which all rolling elements in the load zone are immersed in the oil, has also proved to provide satisfactory protection [against false brinneling].”

Both of these suggest to me that lack of lubrication makes it more likely that ball will slide vs roll in response to vibration due to cage/ball friction.

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Ber van Loon
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posted 04-20-2004 05:25 PM     Click Here to See the Profile for Ber van Loon   Click Here to Email Ber van Loon     Edit/Delete Message   Reply w/Quote
El Pete spoke about shiny spots. Under a microscope EDM shows a typical frosted surface which can be diagnosed with a strong magnifying glass or a microscope.

The surface pattern of the EDM spots is formed by randomwize stapled flattened droplets of steel. These droplets are flattened (by the rolling elements) as soon as they are formed by the spark.
If you like I could post an EDM picture taken with an electronic microscope. I'd have to find the CD first.....

[This message has been edited by Ber van Loon (edited 04-20-2004).]

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dana_payton
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posted 04-20-2004 08:53 PM     Click Here to See the Profile for dana_payton   Click Here to Email dana_payton     Edit/Delete Message   Reply w/Quote
Pete,

I agree with Ron & Jon, it looks a lot like electrical fluting, maybe intermittent. False Brinnelling is usually more regular in appearance and not as dark. It is typically spaced equal to the roller spacing-exactly. In the first photo there are some spots that "look" like they could be brinnelling but it is not clear enough to see. Could someone have been welding nearby and grounded to a connected structure or even the machine base? That could cause intermittent marking like that.

The brown/gold streaks are typical of overheated grease lubrication. They usually show just adjacent to the roller/ball path. In rollers they will even show on the roller surface and end adjacent to the loaded path on the roller.

This is somewhat sideways...why the gSE TWF? We've found the gSE FFT brings out peaks much more clearly than TWF's especially when using a slight increase in resolution over typical[if you call 1600-3200 lor slight]. In fact, we have to be careful not to call stuff before it is actually visible to the naked eye. The maintenance people get miffed if the bearings don't fall apart in their hands or have giant spalls showing when they pull them out.

What are you collecting data with? The velocity FFT looks like it is at 400 LOR. We routinely collect 1600 lines without much ado. The Datapac is fast enough to do this without a real "significant" impact on rate of collection. Of course if you are using a "FasssssT" Track this is all moot. BUT, If you want to see clear non-synchronous stuff in velocity without loosing it in fat bottomed synchronous peaks you really need the higher res. The bearing defect frequencies in the velocity FFT you uploaded look like they might be hiding in the runspeed harmonics. Did you notice that the 3x & 5xRS are slightly larger than the others. For an NTN 6311, BPFO is 3.072 and BPFI is 4.982. That's just about too close to separate them from a natural harmonic at 400 or even 800 LOR in a velocity FFT.

...if I can help or confuse further [one of our tech's is a former SKF-Peru bearing Maintenance Trainer]

Dana

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DragonT
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posted 04-21-2004 12:53 AM     Click Here to See the Profile for DragonT   Click Here to Email DragonT     Edit/Delete Message   Reply w/Quote
What is greasing practice for this motor?
For two side shield bearings, grease doesn't get in or out, only oil from new grease suppose to come in bearing. So in order to add oil, you should open plug relief plug, or pipe, pump grease until you see fresh grease show up. Purpose is to have grease against shield.
I have noticed this is one of the most not followed manufactures recommendations. I prefer, one side open bearing, but…

In your picture, there is grease, but probably never refreshed with oil, and that would be explanation for overheating.
Regards.
Dragan
P.S.
I’m in process of evaluating our greasing practice, bought ultrasound and listen when is enough, same time watching acceleration enveloping spectrum, so far good results, but it will take to finish evaluation.

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electricpete
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posted 04-21-2004 07:31 AM     Click Here to See the Profile for electricpete   Click Here to Email electricpete     Edit/Delete Message   Reply w/Quote
Dragon
I am in agreement.

Our greasing practice for double-shielded bearing is roughly the same as for single-shielded/open. We add grease (based on bearing vendor recommendations except we use 1/2 the quantity for double-shielded bearings) with machine shut down and drain port open. Then run for an hour with drain port open before replacing drain plug.

I think what we have here is proof positive that greasing of a double-shielded bearing was ineffective in some cases. Interestingly enough, the other bearing in the same motor did have fair quantity of fresh red mobility grease inside. Also the other bearing in same machine did now show any signs of overheating and showed dramatatically less of the marks that I believe are false brinneling. That leads me to believe the lack of lubrication was both a cause of both the overheating and possibly a contributor to the false brinneling.

Although greasing appeared ineffective for this particular double-shielded bearing, it also did not cause any damage in my opinion. The cavity was not full and the shields were not at all bent as I have seen in other cases of greasing double-shielded bearing.

Another point on the difference between the bearings. The overheated one was NTN. The other one was "SNR". Both 6311 double-shielded.

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electricpete
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posted 04-21-2004 12:16 PM     Click Here to See the Profile for electricpete   Click Here to Email electricpete     Edit/Delete Message   Reply w/Quote
Thanks Dana for your comments on the point setup. We do in fact have spike energy spectrum stored in May 2003 and it does show harmonics of BPOR with fundamental BPOR being highest peak at 0.4 g's spike energy spectrum pk/pk scale.
http://reliability-magazine.com/pub/gcSlide14.JPG

We don't normally make any calls on spike energy without seeing it in normal acceleration or velocity spectrum and I don't see it in normal spectrum here, especially slide 11 high fmax log scale… usually the bearing defect harmonic series BPOR jumps out that kind of plot.

The first spectrum is 800 lines, Fmax is 54000 cpm, giving bin width of 67.5 cpm. I’m not sure correction for Hanning window but the bin width equates to approx 0.02 orders. I agree higher resolution might help distinguish the fault peaks.

Spike energy magnitude is up and down on this machine without any apparent correlation to lube or other activities and in general we have found similar erratic behavior of spike energy magnitude across most of our machines ever since we changed to quick-lock accelerometer mounts. We are considering monitoring/trending acceleration time waveform peak instead of spike energy as early-warning of bearing problem for our anti-friction bearing machines.

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deepak
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posted 04-21-2004 01:04 PM     Click Here to See the Profile for deepak   Click Here to Email deepak     Edit/Delete Message   Reply w/Quote
Electricpete,
I will have to disagree with the majority here. Those marks appear to be too fresh to have been caused by fluiting.
To me it appears like some damage cased during removal. Definitely not false brinnelling but maybe not electric arcing too...
Also regarding greasing of shielded bearings, there was an artcile in a Hydrocarbon Processing which had talked about merits of greasing shielded bearings. I have observed that the quantity of grease that goes in through the shield is often too small to really help teh bearing. Wherever possible am replacing ZZ bearings to normal ones...

Deepak

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Mike Drake
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posted 04-23-2004 11:02 AM     Click Here to See the Profile for Mike Drake   Click Here to Email Mike Drake     Edit/Delete Message   Reply w/Quote
Hey, Pete -
I have had to deal with more fluting in the past 6 years than in the previous 10 and have a good share of false brinneling experience. The marks are not from fluting. The pattern would not be offset, it would be impossible to get the original shine on the balls and you would have either grayish etched looking bottoms in the marks from High Frequency fluting or dark, burnt looking bottoms from EDM. DRAKE

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Reliability Analyst
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posted 04-23-2004 11:33 AM     Click Here to See the Profile for Reliability Analyst     Edit/Delete Message   Reply w/Quote
Pete,
One condition that I've seen in electrically fluted bearings is that they are sometimes magnetized. I can't confirm whether or not the magnetism was the result of an induction heater but see if a paper clip will hold to the races.

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in2vibe
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posted 04-23-2004 11:44 AM     Click Here to See the Profile for in2vibe   Click Here to Email in2vibe     Edit/Delete Message   Reply w/Quote
I as well have seen way more than my fair share of electrical flutting and this is not it. I am really kinda amazed that is the gereral opinion here.
Regards,
James

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DragonT
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posted 04-23-2004 01:23 PM     Click Here to See the Profile for DragonT   Click Here to Email DragonT     Edit/Delete Message   Reply w/Quote
Fluting looks like this: http://reliability-magazine.com/pub/Fluting.jpg http://reliability-magazine.com/pub/Outrace.jpg
Have a great weekend.
Dragan

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Ber van Loon
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posted 04-24-2004 04:37 PM     Click Here to See the Profile for Ber van Loon   Click Here to Email Ber van Loon     Edit/Delete Message   Reply w/Quote
.. and if you look at EDM through a microscope it looks like this: http://www.conditionmonitoring.nl/edm.pdf

comments are welcome.

[This message has been edited by Ber van Loon (edited 04-24-2004).]

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in2vibe
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posted 04-26-2004 10:53 AM     Click Here to See the Profile for in2vibe   Click Here to Email in2vibe     Edit/Delete Message   Reply w/Quote
Here are some examples of flutting.
http://reliability-magazine.com/pub/flutting1.jpg
http://reliability-magazine.com/pub/flutting2.jpg

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MH
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posted 05-05-2004 10:30 AM     Click Here to See the Profile for MH   Click Here to Email MH     Edit/Delete Message   Reply w/Quote
I wonder if those outer race faults could have started as heat checking and developed into their present state as a result of maintained heat levels and metal fatigue at each point? Just a thought, but resonable considering the discoloration of the race, and discription of lube condition. What do ya think?

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electricpete
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posted 05-05-2004 11:24 AM     Click Here to See the Profile for electricpete   Click Here to Email electricpete     Edit/Delete Message   Reply w/Quote
Hmmm. I don't know what heat checking is. Does anyone have more info on that.

Here is another piece of important info that I didn't mention. This is a TEFC motor, so the air blows across the outboard endbell, across the top of the motor and keeps going (does not blow across inboard endbell). Feeling the temperature profile on top of the motor (fins), it is coolest at outboard and gradually hotter as move toward inboard. Then when your round the corner onto the inboard end-bell it is even hotter. 60-70C across the entire inboard endbell for both running motors. It does not seem hotter as I go toward the shaft/bearing. So the temperature pattern would seem to me mostly determined by heat produced within the motor winding (not bearing) and the vent pattern.

So instead of my preferred scenario (lack of lub penetration leads to overheating of bearings and discoloration of grease), is it possible I should consider instead the scenario that hot environment (~65C end bell) combined with high speed (3600rpm) relatively large greased bearing just cooks the grease. This would explain why a problem on inboard not outboard (although we had 2 different style double shielded bearings which could also explain). Note the thermally-based speed rating for this bearing under grease lubrication is listed at 5500rpm.

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