This is a case where BPFO harmonics were clearly present in velocity spectrum, also overall acceleration in the range 2-20 kHz was very high (~13.5G). However, when bearing was dismounted no sign of wear was found in any of its components (bearing type NU 1026). The application is a stress meter roll. Speed 2300 rpm. Analyzer is CSI 2115. The question is if anyone has experienced cases where BPFO harmonics had been excited without developed wear. Could this be due to e.g. housing irregularities or inadequate bearing internal clearance in combination with wrong choice for housing tolerance? Or something else happens?

ACHENB_1.doc (46 Kb, 78 downloads)

Anytime I see harmonics of outer race defect frequencies in a velocity spectrum with the fundamental being the largest and each getting progressively smaller, I suspect an out-of-round outer race. This can show up and there be no visible wear.

Causes can include housing irregularities, metal particles trapped between the housing and race, dropped bearing, etc. I'm not sure about inadequate clearance.

I'm in agreement with Danny. Short duration impacts occurring once a relatively small defect develops in the race gives rise to harmonics in the high frequency range.

High amplitude noise visible in the TWF is not necessarily a symptom of impending failure and can be present due to lube problems or races machining.

That sure looks real.

I have been stumped a couple of times by something similar to this. Your spectrum looks very close to what mine looked like. I was certain a defect was there but nothing apparent once the outer race was cleaned up. The only thing I could figure was that a dried spot of grease was acting like a spall. The grease in these bearings were so burnt & dry, that I had to soak and scrub them in the parts washer. The rollers had brown burn rings around them.

Another option is what the old guys at the mill call a sub surface defect. I am not confident that a sub surface defect is even reality. They say a spall can be like a cavity in a tooth, developing inside until it pushes through the surface. Any comments on that statement???

Hope this helps.

Two instances...
1. The stud of a stud-mounted accel fully penetrated a fan bearing housing and pressed against the outer race of a double-row spherical roller bearing. This caused the outer race to pinch and resulted in a BPFO upon initial installation of a new bearing. I believe our instance was different than yours...in that we did not see harmonics, and High Freq Detection (HFD) was not initially elevated. Later, however, HFD increased...which prompted a maintenance inspection and the discovery of our accel mounting error. We found skidding at two places on the outer race, and the beginnings of a small crack on the outer race. My belief is that the early indications of BPFO without high HFD was simply the evidence of a pinched outer race...and the actual outer race damage occurred when we saw HFD increase. It's amusing, to me, to think that the damage would have never occurred if we did not try to mount condition monitoring equipment on the bearing. Makes me wonder if I'm really doing any good?
2. A newly installed fan bearing showed immediate evidence of 1st and 2nd order BPFO with shaft speed sidebands around the 1st order. This condition remained static for over a year. Maintenance inspection showed a paint chip between the inner ring and the shaft...with some skidding evidence on the outer race. I can't relate, in my head, the paint chip and BPFO. I would think the paint chip under the inner ring would cause BPFI with running speed sidebands...not BPFO??? But...I saw what I saw.

To give my my thoughts...I agree that housing irregularities may cause BPFO...including housing distortion if the mounting surface is not flat. I would add, through the experience I just described, that shaft irregularities may also cause something similar.

"Rolls" have a tendency to have this type signal. Not all rolls do this, but I have seen it more in rolls than in motors or pumps, Etc.

I have analyzed rolls in paper machines (for so long I can't remember how long ) and I learned early that when, (most of the time), the fundamental of the outer race is "high" and the harmonics are few and "low" that it is mostly caused by the rollers or balls not "rolling" when they enter the load zone and "skidding" or sliding when they are requirded to accelerate from a stand still to start back "rolling" in the load zone.

Now why they stop, no one has been able to tell me for sure nor have I been able to completely figure it out either. Each case may be unique in its own right but whatever stops the turning of the balls/rollers probably can't be "fixed" simply because the sure cause is not known in each case. When a new bearing is installed in these cases, the problem may not be there at first but may show up some time down the road.

One might think it is the wrong type grease, but I have tried that. But what about the ones which are on oil, hmmmm, it doesn't have to be a greased bearing, a continious oil flow or oil bath does or can do it too, if the conditions are right. That is the "key", the conditions have to right for it to happen. One end of a roll may have this and the bearing on the other end of the roll be ok.

Too much clearance when the rollers leave the load zone? Not sure. Do all balls and rollers stop rolling when they leave the load zone? Anyone know? In theory one might think they would since there is clearance between the roller/balls and the outer race when they are out of the load zone, but then shouldn't the inner race at least keep the balls turning since they are supposed to be in contact with the inner race all the time?

I observed a large roll's bearing which turned really slow,about 30 rpm, and I could see this "stopping" in real time. This bearing was on oil.

I would imagine that heavy rolls would cause a "deeper smooth hole", which I call a "SLIDE DEFECT" at the beginning of the load zone, than would a lighter rolling element.

This slide defect showing in, say a motor bearing, might well be caused from a distortated outer race or whatever other things might be wrong, but on heavier rolls, my guess is the same as I have said for years, the rollers are not turning when they enter the load zone. Maybe not in every case but from my experience it is the problem most of the time. I have seen these bearings run for years with little change in amplitude. A huge change in amplitude or a complete disapearance of the slide could warrant removal.

I used to have a pile of pictures of these slides but I have undoubtly "lost" them somewhere in my travels.

OH and By The Way, I saw this once on an inner race in a roll, something like a guide roll, where the outer race was rotating and the inner race was "held".

Sotiris,

I have never seen a "STRESS METER ROLL" and have no idea how large it is, but did you notice anything that looked like a worn spot at the beginning of the load zone of the outer race of your bearing? It will not "look" like a spald but sort of like a dull colored worn "hole". Even if it were a distortated housing or one of the other suggested causes, seems like some worn sign would be present, huh?

Of course this is only my opinion and I could be totally wrong.

It's normal to see a significant second harmonic of shaft rotating speed in an envelope spectrum if the bearing housing is oval so you might look at this if you haven't already.

It's also common to see BPFO in the envelope spectrum of bearings where the balls don't spin properly for reasons such as improper lubrication or too light load on a bearing which often occurs in overhung applications. If you inspect the bearing, you may not see any flaws as the source of the BPFO signal may be friction rather than impacts; the envelope signal can result from either. The way to distinguish between friction and impact sources in an envelope spectrum is whether there are significant higher order harmonics which is the case for impact sources.

To add to what Danny, David and George said, I have seen a hot 100hp 2-pole motor 6313 bearing with high fundamental BPFO with no other BPFO harmonics and subsequent inspection showed only the discolored grease was apparently overheated due to excess grease pack (on a somewhat challenging application for grease due to high D*N number)

My personal belief is this can come from housing irregularities, foreign material between housing and outer ring, or reduced internal clearance (which could result from excess temperature).

I have heard the same thing that rnl mentioned about subsurface race wear. This was one thing that Peakvue was supposed to be really good at detecting, the flex of the surface when the ball went across it.
Anyway, it was a good thing to say when a bearing was replaced and the mechanics wanted to know why there was no visible damage when the bearing was called out as having a defect.

Thank you all for your valuable entries.
I think that after your comments the conversation is getting more interesting since I have dug out some older data.
But first, let me provide some info about the roll. Actually this is a part of an aluminum foil roll. It is actually an instrument equipped with piezoelectric sensors and it is providing real time info about the foil profile (i.e. Light load, high speed, relatively high differential temperature application). A simple line diagram including bearing info is shown in the .doc file attached.
Back in October 2004 the self aligning ball bearing 1222 (NSK brand) was replaced after my recommendation. The spectrum which led me to this decision is shown in Figure 1, of file Roll_Spectra.doc file below. According to the literature, I was expecting a fault in the outer race, not localized but extending to a certain arc. Guess what, when the bearing was pulled out there was a dull strip on the outer race all around the periphery and this was present in both ball rows. To my understanding this signifies loss of operational clearance which leads to overload. Unfortunately I can't find the relative photo.
The difference between the spectrum in Figure 1 and the first one posted by me (ACHENB_1.doc) is both in the pattern the harmonics follow (larger 1xBPFO, significantly lower harmonics in ACHENB_1.doc) and in the acceleration TWF.
The spectrum shown in ACHENB_1.doc was taken on September 7th. Both bearings (SKF brand) were replaced by new ones, but the BPFO harmonics are still in the spectrum of bearing NU1026 (FAG) as it is shown in Figure 2 of file Roll_Spectra.doc (spectrum taken on Sept 27). Furthermore, there is a sequence of harmonics in the other bearing (FAG 1222) which is pretty close to its BPFO and this seems to have developed just after 20 days in operation! (Figure 3 in file Roll_Spectra.doc)

Ralph I hope to get a photo of the bearing, as I didn't inspect it personally.

Roll_Spectra.doc (136 Kb, 18 downloads)

Are any of these bearings on a tapered sleeve where the clearance can be "set" when installed? If so, are they being installed correctly and not being pulled down too tight? If not, are they in a high heat area?

In the bearing you saw the "too tight" fit pattern, was the balls tracking in the center of the the outer race or "walking" in and out in a misaligned pattern?

I saw a similar pattern happening on some slow turning rolls with basically the same "type" bearing, believe they were 1300 series and the same pattern was happening on the outer race. These were in a high heat area. They were changed to a spherical roller type bearing and the problem was all but done away with.

The new data you have posted makes things a little clearer than the original.

Ralph

Both bearings have cylindrical bore.
The "tight fit" pattern was perfectly centered (not misaligned)

Switching to a SRB in our case is rather difficult due to the much lower speed ratings that are implied, but

In the case that you mention, how did SRB selection cure the problem?

Do you think that selection of C4 internal clearance bearings might cure the problem?

RNL: When you refer to subsurface defect do you mean a metallurgical defect, a void or an inclusion for example?

Thank you all for your comments

quote:

In the case that you mention, how did SRB selection cure the problem?

I really don't know.

A S.W.A.G. would be maybe the rollers were "larger" and "longer" and took more heat to expand out far enough to eliminate the clearance as oppossed to a ball bearing which only has basically a "tiny" surface contact area and many more balls and expands more quickly and farther than does the roller?

If your bearings are anywhere close to what these type were, they are "loaded" down with balls and the smaller the ball the less heat it would take to increase in size when heated.

Sorry for not knowing the exact answer. Maybe someone else could shine a little light on why.

Has the journal been checked for size and compared to the bearing inner ring diameter?

Seems like a C4 fit might help but would it "hurt" the low end looseness signal?

Sotiris,

In the cases you have provided and faults found, I wonder how did the enveloped data look like?

David

Reply to Sotiris - 10/03 11:41 AM
Metallurgical defect, void or inclusion is way beyond the scope of any tech in my mill. I threw that statement out with the hope of the exact reply I got from Vibeguy2004, 30-Sept 8:33 pm. This excuse was used on me several times when I was on the crew. But Vibeguy2004's statement "the flex of the surface when the ball went across it." Does this really happen? I do not want to portray myself as something I am not, I am new to this profession, and my learning curve is slow. I will attempt to repeat what was explained to me. If you look for the black on the outer circumference of the outer race, this is where the outer race will shimmy and cause a cavity to start inside the metal. At some point it will "pop" and become a spall. I have since discounted this theory by talking with SKF rep's who say that the black means lack of contact between race and housing.

Ralph, you are obviously well versed in this subject. I want to thank you for your postings, I get a tremendous amount out of them.