This is a pillow block on an HVAC fan.
This bearing was not monitored (vibration)
Failure progression (noise) was observed fast
Any comments to the cause.

IMG_1330.JPG (109 KB, 218 downloads)

Rennie, this bearing appears to be an inclusion spall. That means there was an bad inclusion, typically an aluminum oxide inclusion stringer just under the raceway surface.

Examination of the spall in a scanning electron microscope would verify the presence of an inclusion in the middle of the spall, assuming the inclusion hasn't spalled away from advanced damage.

This can even happen to bearings with the cleanest of steel, because no bearing steel is perfectly clean. But if you run into a rash of such failures, then the bearing steel quality is suspect.

Kestas,

Could you explain a little further exactly what you mean by 'inclusion spall'? I've seen a lot of similar damage that we typically blamed on lubrication and/or contamination.

I will defer to Kestas' comments on inclusion spall. That is something I haven't heard of. Are there distinguishing features of the appearance that leads towards this conclusion?

To me it looks like fatigue spalling or flaking. That is a pretty broad category and one of the hardest to narrow down.

As you know, fatigue damage is the basis for L10 life. If we exceed the endurance loading level, fatigue failure can occur based on combination of operating time, loading level, and statistical luck. Also L10 life is adjusted for lub conditions by some OEM's.

Loading could be simple radial loading due to weight of machine, belt tension etc.

It could be axial loading (perhaps from preload or thermal expansion). In the case of excess axial loading we would expect to see evidence in the bearing ball tracks... offset from center for ball bearing.

It could be misalignment or cocked bearing in which case again there should be distinctive clues in the ball tracks.

It can be loading created from housing irregularities. Check housing for fit and ovality by checking three different directions.

It could be a variety of other conditions that lead to surface damage at one small location, and that initial defect grew by fatigue so that the original defect was no longer visible. For example:

• For example we might have had false brinning or installation trauma cause small initial defect. look for very small defects at roller spacing on other parts of the race (It's easy to miss very small defects unless you look carefully).
  
• poor lub conditions contribute toward initial defect. Look for glazing and frosting as one indication of lub conditions... also look at the lub itslef if you have it.
  
• It might have originated form particle denting from contamination. In this case we might expect to see particle denting other places along both races. Try to separate out the effects of particle denting from debris from this spall which would not be uniformly distributed around the races, more downstream of the defect than upstream.

It can be age. Fatigue is a cumulative process and so it becomes more likely in older bearings subject to a given loading level over decades than new bearings subject to the same loading level over a few years.

I'm sure there are a few causes I've forgot. Fatigue spalling can be the final result of many different initiators.

This message has been edited. Last edited by: electricpete,

My basis for this presumption is the fact that this spall looks "deep and tight" versus many other types of spalling that are often surface initiated. That, and the fact that I see "(((|)))" character to the spall leads me to this educated guess. After seeing and analyzing in detail literally hundreds of spalled bearings during my tenure at my company, some things become second nature and intuitive, and I can categorically say that this spall is likely from a harmful oxide inclusion.

Humbleness aside, even the senior bearing engineers at my company turn to me for final word on this type of spalling.

But like I said, this spall needs a SEM/EDS examination to verify this claim, and a full laboratory examination to rule out any other possibilities.

If I find time, I'll try to dig out a magnification series of something similar.

Thanks for your response Kestas.

By the way, I have no doubt about your substantial bearing expertise from your posts on the forums.

There is something in your response that doesn't make sense to me. I think you are saying that the defect origination site is at the center of the damage pattern. But isn't it typical for the spalling damage to propogate in the direction of ball rolling? (debris released from the spall rolls downstream and causes stress on the raceway section immediately downstream). So if we were to look for the origination of the defect it would be near the leading edge of the damage pattern, wouldn't it?

That's what the textbooks tell you. In reality, as a subsurface-initiated single spall progresses, it not only breaks up the metal in the trailing direction, but it also breaks up the spall edge on the leading adge as well.

Thanks again for your response. I'd like to discuss it further. Perhaps I can learn some more along the way.

Attached is a view from FAG Publ. No. WL 82 102/2 ED, entitled "Rolling Bearing Damage Recognition of damage and bearing inspection" (free download somewhere on their webiste). It shows a given bearing which was periodically disassembled to monitor the progress of spalling. There seems no doubt that in this particular bearing, the spall grows on only one end (which I presume to be the trailing edge). I put a red arrow pointing to a distinctive feature in each spall... an oval shaped lump near the upper right hand corner (which I presume to be the leading edge). That feature remains virtually unchanged in the views from 2.5% all the way through 25%, even though the spall has grown dramatically at the other end = bottom end (which I presume to be the trailing end). Also on the bottom row on the 25% view through the end I have used a green circle to point out two straight lines that form an acute angle approx 20 degrees, with vertex "pointing" to the left. It remains in the same position with respect to the top of the picture even as the fault grows on the bottom end (which again I presume to be the trailing end).

So on this one bearing photo'd in my attachment, the spall certainly grows predominantly on the trailing edge (the growth is not symmetrical at all). The publication doesn't distinguish whether it is surface-initiated or subsurface initiated... my guess from the apparent depth of this spall (my attachment) even in the early stages is that it represents sub-surface initiated fatigue... would you agree?

How many others act like that? I doubt many people disassemble bearings for inspection, find spalling, and them put them back in service for experiment, so tough to gather more data.

I will certainly admit there may well be more than one method for spall propogation. There should be no doubt that debris coming out of the spall and depositing downstream where the dents cause more spalling is one method ... and this of course propagates assymetrically toward the trailing edge as previouisly discussed. I can guess there may be other methods such as the stress concentrations which occur when the ball is near the edge of the spall... in my simple thinking similar to placing a heavy weight near the edge of a crumbly cliff... it can cause the cliff to crumble away and the concept can apply equally on either end of the valley... could perhaps cause the defect to propogate symmetrically both directions. I have never seen direct evidence of symmetric propagation (such as the pictures above provide for the assymetric propagation) and as you note it seems ignored from the bearing inspection literature while there is plenty mention of the effects of debris extending the spall in the downstream direction.

I wonder what would be the conditions that inhibit the known-possible assymetric growth pattern from debris such that we end up with purely symmetric growth.... good flushing action from the lubricant sweeps the debris completely clear of the spall ?

FatigueProceedingAtTrailingEdge.ppt (402 KB, 119 downloads)

Interesting series of photographs. There are a few comments I can make.

This looks like a taper roller outer ring. I don't have a lot of experience with inclusion spalls on taper applications. Either I haven't seen that many, or taper roller spalls don't exhibit the "tight and deep" character I mentioned.

At a glance, that spall appears to be surface initiated.

On many deep groove ball bearings I've seen the onset of inclusion spalling where the material had cracked, but not yet spalled off. I've been able to pick off the material to find an inclusion in the magic zone of heaviest shear stresses. In all of those cases, subsurface cracking originated at the spall and progressed evenly in the trailing and leading directions before the cracks erupted to the surface.

The figure in my attachment is an inner ring of an angle contact ball bearing (see heading at top of the figure).

I can believe the idea that cracks progress symmetrically in both directions before getting to the surface. I would think that once chunks start coming out, the asymmetric effects of debris release will start showing up

Not discounting anything said in earlier posts and with all respect to both Kestas and Electricpete - a crusty old failure analysis guy (read: someone who knew a lot more than me) once told me that "most" spalled bearings can be traced back to:
1. LOAD,
2. LUBE, or
3. ALIGNMENT

His data showed - across the board for all machine parts - that only 4% of failures were "warrantable" (ie defect in material or workmanship).

So, it is easy to blame the manufacturer and the bearing steel but it might be worth eliminating these three causes - perhaps before the bearing manufacturer suggests you do so

Good luck.

If the inclusion is deep in all liklihood if load, lube and alignment (internal or external) are good the inclusion will do little to L10 life. If it's near the surface rapid deterioration will take place like a chug-hole in pavement and progressive deterioration will assume those lines of decay - or, I could be wrong. One does not expect to see 'spot' degradation with exclusion of water, acid, false brinneling, etc...

A pet peave; preloading should never happen. Such preloading I'm referring to is interference of rolling elements and raceways. Clearance at zero - maybe the C-60 isotobe development by Exxon will usher in longitivity in the bearing world. One can only hope.

quote:

maybe the C-60 isotobe development by Exxon will usher in longitivity in the bearing world. One can only hope.

What is C-60 isotobe?

just by the look of it, its subsurface......

The defect shown is undoubtedly spalling. Spalling may be due to several reasons

1.Due to high preload.
2.Misalignment.
3.Bearing sitting skew on the shaft.
4.Inclusion of rust.
5.Fretting corrosion.
6.Fluting
7.Oval housing or shaft seating.
Some measures to think over.
Regards.
Irshad Akhtar

Hello,

I thought this issue was over but I was wrong.

One month after having the bearing replaced, the new one started failing.

We had to replace this bearing again. Attached are photos of the failed new bearing.

We observed the same failure mode (spalling) this time on the inner ring.
We were surprised to discover that the inner ring was cracked.
We also noted corrosion on the inner ring. The shaft was greased before assembling the new bearing and this had been running for only one month, so we did not expect this.

This time we slightly reduced the belt tension, 'cos we could see nothing wrong.

Any suggestions to the reason for the cracked inner ring and again the inner ring spalling.

Many Thanks

Hello,

I thought this was over but I was wrong.

After one month the new bearing started failure.

We replaced this bearing again. Attached are photos of the condition.

We found out that the same failure mode (spalling) on the inner ring this time.
We were surprised to discover that the inner ring was cracked.
We also noted corrosion on the inner ring. This was greased before sliding the bearing on it. we did not expect this after only one month.

Any suggestions of the reasons for these failures, please. Thanks

I thought this was over but I was wrong.

After one month this bearing started to fail again.

we replaced it and found the following:

Same failure mode (spalling) occurred on the inner ring this time (instead of the outer ring as in the previous failure)

we were surprised to find the inner ring cracked.

the shaft was greased before sliding the bearing on it. we found sign of corrosion on the inner ring.

any suggestions as to the reasons for these failures.

Now we had slightly slacked the belt tension to reduce the load on the bearing.

many thanks

I am very sorry but I cannot upload the attachment. It is giving me an error. This is a word document.

As you can see the message is being posted but not the attachment. I did not see this happening ...that's why the message is repeated.

any suggestions.

Could the doubled messages be deleted please.

Rennie

Did you witness/quality control the replacement of the latest bearing that failed? cracked inner ring????? was the shaft greased to assist fitting because of heavy interference? was the bearing heated prior to fitting? if so what method was used? or was the inner ring bashed on with a hammer. Was the work carried out in a clean enviroment?

Just some thoughts

Jonesy.