Had a very clear outer race fault indication come through on the drive end of a motor. When we disassembled the motor and pulled the bearing off (or, more precisely, it fell off), we found the inner ring had cracked into 4 pieces. There was a significant area of damage on the outer raceway.

We had taken data on this bearing a month ago (both velocity and peakvue), and nothing was indicated.

The motor has been in service for about 8 years. Its 440kW (590HP), 750RPM motor (usually runs about 746RPM), the bearing is a Koyo N226 cylindrical roller running on a 130mm (5.1 inch) shaft.

I'm can't picture how the inner ring could get broken up like that. Each bit of the inner ring had an additional two cracks (close to, but not quite at roller pitch) in them.

Any ideas?

Thanks,

Ian

Inner_ring_cracked_into_4_pieces.doc (274 Kb, 130 downloads)

Ians,
Did this Equipment ever set idle?

KellyB,

No, it pretty much runs 24/7.

It drives a vacuum pump on a paper machine, and gets shut down when we shut the machine for maintenacne (usually about once a month, for 8-12 hours)

There was no evidence of brinelling or false brinelling,(none that I can see, at least) on either of the raceways. All the previous data showed no indication of any problems. The only substantail peak in the data was at 100Hz, which is 2xLF, and I had assumed was electrical related

Ian

Reason I asked is because of a bearing failure we had on a Paper machine vacuum pump. The inrace was not cracked however we did have the spalled areas and brinnelling at the approx roller spacing.

It happened the same way for me no indcation one month and hte next month disaster. I had often wondered about that.

I will try to attach the failure report.

13272_East_Vacuum_Pump.doc (4,672 Kb, 93 downloads) Word Doc Failure Report

I would say that the cause of this bearing failure is a loose fit on the shaft, race cracking of this nature is almost always a fit issue. in the first photo it appears that the bearing was spinning on the shaft was it before or after the failure unknown. outer race damage is a result of the inner race failing

Ian,

How about posting some data plots for this bearing both from last month and the most recent? A bearing progressing to this stage of failure without early warning is a good topic of discussion. Thanks,

Ian
Was this motor direct drive or belt driven? We had trouble with roller bearings in direct drive applications. It was determind that there wasn't enough radial load a the rollers would slide generating heat.

Europe and Rest of World except NA:

LF=50Hz
2XLF=100Hz

Ian:

Are you sending this bearing out for failure analysis? The pieces look pretty symetrical... Could this be caused by sudden shock? If you find out why this happened, please update this interesting thread.

I wonder........

I have had a few bearings fail in the past, and questioned the heat treatment process, as could it have been off standard just enough to contribute to early failure.

Most bearings for us, that are sent out for failure analysis, are of all things sent to what else... the bearing manufacturer.

Heat treatment process would involve heating the alloy metal to a known temp for the particular metal, then quenching, after that heating back up to a tempering temp and allowed to cool over an "X" amount of time. This process in the end should produce a part, bearing, etc at certain Rockwell "C" heat treatment number.

I am sure there is quality inspection that checks parts, and throws out any not in spec, but would it be impossible for one not in spec to slip through?

I wonder if real failure analysis can be done without an indepth analysis of metal partical formation, that would be able to determine if improper heat treating was a contributor?

just a thought...............

ps... looks like the bearing above was spinning on the race, inner race fit looks worn, with circular grooves.. BUT.. did this occur before or after the cracks?



Mike

Does this sound feasible?

The inner race was installed "tight" to the journal, but not quite tight enough. Later an outer race spald developed, not from the inner race being loose, but probably from "water marks" developing during the "shutdowns".
During the 8-12 hour shutdowns, condensate was causing "water marks" corrision to develop on the inner race, causing it to become weaker in this corrision areas. The outer race defect, which probably was caused by the same type "water marks" was constantly "hammering" against the rollers with energy transferring to the inner race, causing it to "become loose" on the journal and beginning to allow the journal to spin inside the inner race, slowly "tearing" metal loose from the journal and inner race, causing a "build up" of metal between the journal and the inner race which in turn seized the inner race back to the journal but with too much interference clearance. The excessive stress on the inner race from this interference clearance inturn caused the inner race to "explode" at the weaker corrision areas during one of the "shutdowns" as it cooled down faster than the shaft.
Assumming this to be a belt driven drive, based on the type bearing on the inboard end, the extra forces of the belt tension added to the problem, if indeed it is a belt drive.

Only my opinion and I could be totally wrong.

I had a suction pressure roll inner race crack axially like that, but a single crack due to a bad fit (most likely too tight). This is typical since one weak spot cracks to relieve the stress. What is so interesting about this bearing, is 4 cracks equally spaced. Being in service for so long (assuming the bearing was in service for those full 8 years), could mechanical and thermal fatigue come into play here?

Ralph,

That sounds like a logical chain of events to me.

The damage to the outer race looks like either a manufacturing defect or the result of the process you described. Either way, it sure doesn't look like normal wear.

Good Luck,

quote:

Originally posted by MarkD:
could mechanical and thermal fatigue come into play here?

I wonder if inspecting the end view at each crack, if they look like they all occured during or close to the same time frame, or if some were darker than others in color indicating maybe age differences in cracks.

Lots of maybe's ... one crack could have loosened bearing on fit, then continued running allowed impacting to cause more cracks to develop.. just too many maybe's without full analysis of the bearing.

The inner race crack could have occurred instantly (although it is hard to say as to when) but the outer race pit probably took some time to develop.

Ian, I wonder if any signs of outer race defect were present before the last reading was taken?

All,

Thanks for the replies.

The answer to some of the questions:

Yes, it is a belt drive.

No, I couldn't see any indications of faults prior to the rather dramatic appearance of the outer race frequencies. I will post some data when I get back to work.

The inner ring had most certainly spun on the shaft. In fact, the shaft has lost about 3mm (1/8 inch) off the diameter, however I believe that this happened after the inner ring had failed. (we were very lucky that this motor was stopped. It was very close to pole-ing out, which would have been a "world of hurt")

There were a series of other cracks in the inner ring that had not yet penetrated right through.

Yes, I will get the bearing analysed, and I'll also post some photos of the fracture surfaces.

MikeH, I'm no metalurgist, but the fracture surface does look to me to be similar to a typical fatigue crack progression.

I'll post more info after Easter.

Again, thanks to all for the replies.

Ian

Here is some additional data. The attached doc shows some plots of velocity and peakvue data (note that the units of velocity is mm/sec, and 1mm = 0.04inches).

There is also a picture of one of the fracture faces and one of the cracks that had not propogated right through.

I'm beginning to form the opinion that the outer race fault happened first, and the subsequent "hammering" has caused the inner ring to fatigue.

I have seen a few dryer can bearings split the inner ring due to thermal stress, but there is usually only the one crack, not a fragmented inner ring as in this case.

I'll send the bits off to a metalurgist, and post the results.



Ian

cracked_inner_ring.doc (210 Kb, 70 downloads)

Is there any relation of the 2xLF and the outer race damage? For example, could stray currents be finding there way to the areas to start a defect but then the subsequent damage destroys that defect and replaces it with a spall or crack? Is it possible that this thing was spinning continuously to "distribute" the fatiguing?

This is a great case study and good photo's. With such an advanced failure it's a real challenge for you to sort out which comes first.

My gut feel is similar to the others that either cracking or spinning occurred first (which one I'm not sure) and everything else followed from there as others have postulated. I have a hard time imagining this began as an outer race fault hammering the inner ring to failure....I would think we'd see more fatigue spalling on the inner race surface where the rollers ride if that were the case.

Dear Ians,

I need to get data of this files(*.RBM) because It's easy for me to analy.Maybe i can give some comment to you.

Peera

The plot thickens....

It turns out that the belt tension was adjusted a couple of months ago, and was (in the opinion of the fitters who did the adjusting) way too tight. The tension was calculated and set using guidelines that came with a belt tensioning device. I suspect that an error was made when calculating the tension.

The tension pulley bearings failed shortly after the adjustment was made, and the tension was set to more "reasonable" levels when the bearings were replaced. (i.e. test with the fingers to "that feels about right")

The other issue that has emerged is that new guarding was added to the belt drive a while ago, which restricted access to the front bearing. The vibe readings were being taken from out near the edge of the front plate of the motor. This won't have helped to see inner race faults, where there is some attenuation to begin with....

There are some rather strange markings on the bore of the inner race. There are what appear to be a series of lines theat follow the shape of the crack. You can't feel the lines, however they are quite plain to see (but are not easy to photograph)

Thanks to all for the replies.


Ian

cracked_inner_ring1.doc (126 Kb, 40 downloads)