Hello guys and girls!

Im sitting here thinking about a theory i have in mind.
If you have read my earlier posts, i had and still have some problems with dominant 3,1 peak and high vibrations.
One of the motors were sent in for overhaul, and they could not find any damage on the bearing.
Now to my thoughts:

On a horizontal mounted motor, the gravitation will always cause a force on the bearing because of the shaft pushing down on it. I would assume that this force will continue to stay there during rotation of the shaft in some way.
What i have seen mentioned and read, is that the outer ring of the bearing will be the one receiving most of the force/energy within a bearing and therefore usually is the first to break.
At this point i would like to call this force a vibration, because of a peak in the spectrum or a wave in the waveform.
Could this vibration be seen as a BPFO frequency, because of the balls striking the outer ring as they come into the loadzone of the bearing?
Could that be possible in any way?

Certainly i'm aware of your point.

But is it possible to get this frequency if there isn't a defect, but because of the increased load of the outer ring during this transition into the load zone.
To explain some of my cases.

I have 3 motors with a dominant 3,1x in the spectrum.
It shows no sign of impacting in the waveform, except the pure 3,1x sinus and the 1x sinus.
There seems to be a harmonic of the 3,1x at 6,2x.
But no clear sidebands, no clear ball resonances and no clear impacts.
How can the 3,1x be such a clear sinusoid?

I believe a pinched outer ring may create the BPOR evidence you describe.
We experienced one case history in which a BPOR frequency peak was apparent immediately after bearing replacement. It was a double-row sperical roller fan bearing. I wouldn't call it a bearing defect, because the condition remained static over several months...it did not degrade exponentially as much of the "four-stages" of bearing life literature describes. Upon disassembly, we discovered that shim material had been used to take-up excessive clearance between the outer ring and housing across the top half of the bearing. There was a "skidded" area at one place on the outer race. This skidded area was not severe enough to affect surface relief as sensed by your fingers, but was visible. We saw no evidence of spalling.
I believe this condition was caused by a pinched outer ring, creating a loaded area in the roller path, resulting in a "skidded" area where the rollers transition from sliding to rolling friction in their path around the bearing race. All this evidenced by a BPOR peak in the velocity spectra.

Hello George.
That is indeed interesting for these cases.
This would be in many ways the same phenomena that i was trying to explain. Especially this sliding then rolling action.

There is an opinion that OR pinching causes a little bump to appear on the outer race. IMHO, pinching causes rather egg-shape outer race which is more pron to the effect of transition from skidding to rolling in the load zone. This phenomenon occurs at BPFO frequency as George pointed out. But it is not an impact and therfore is not causing multiple harmonics, just few in the low frequency range.

An impact, on another hand, due to a spall results in multiple harmonics. Again, as it has been mentioned by others, no bearing defect - no BPFO. Any other abnormal vibration condition of a rotor is irrelevant.

There is a phenomena where I have seen that the balls are not rolling when they enter the load zone and then accelerate to or close to the speed they should be turning. The area where they strike the load eventually becomes "marked" with a worn area. Some say the bearing are too lightly loaded, but sometimes I wonder if this is the real cause. I have seen this more on roller type bearing than on ball bearings, and they were not necessarily on thick grease, but oil also.

For your three motors to be showing the same signal, it would seem strange to me that all would have a pinched outer race. All might could have the "not turning when entering the load zone" problem though. Did I not see where you said this was a new plant and none of the motors were actually in operation yet? If so, then this non load setup might be giving the "no turn" problem. I guess all three motors are of the same size and manufacturer, huh?

2x rpm in an envelope spectrum is the primary symptom of a pinched race.

David:
That would explain exactly my cases. No impact and no multiple harmocis. I will DEFINATLY take this theory further on for investigation.

Ralph:
All motors,same manufacturer, and about the same size. Rather small ones.
I think 2 of them ran loaded and 1 unloaded, but they all gave the same 3,1x peak, with no clear sign of impacting.

Duncan:
Never seen any symptoms mentioned for pinched race. Can you give a further explanation for this 2x?

Svein, see:

http://vibrotek.com/article.php?article=articles/new94vi/index.htm

Vibbase:
If i could pinpoint eny error in my collection i would be eager to do so. Only thing i can think of right now, is that the freq band could have been broader to be able to see the resonance of the bearing balls.I can't remember the details, as these motor where run a good while ago.But i see from some of the plots, that it ends at 1500 hz, and i guess that would be to low.Motor running at 3000 rpm, i would guess that resonance of the balls would show up between 4000 and 6000 hz or even up to 10 or 15 khz ,depending of the design and size of the bearing. Outside source is not likely, as mentioned in another post.
This motor was run coupled with a fan, no other equipment running i vicinity.

Duncan:
Been looking only briefly on the link you gave, haven't had time for anything more. But it looks like a good one. Will study it later. Thanks.

Duncan:
Not sure i really understood those values listed. "The main freq of modulation" i am not sure i understand.
What is meant with this?
Could you explain this a little further?
Is this the main peak, sideband....?

Bearings generate broadband signals when loaded while the inner or outer races are rotated. This signals are modulated by friction and by impact sources. Specific defects and loads modulate these broadband signals at frequencies mainly determined by the physical parameters of the bearing elements and by the speed of rotation.