RCA of bearings 2

[Muhammad Mazher]

Hi everyone! I am seeking advice from experts regarding the root cause analysis for an incident at our petrochemical plant. Specifically, one of the critical blower fan's bearings on the coupling side seized, despite satisfactory oil level and condition. While one bearing remained operational, the incident has raised concerns about our maintenance practices. I am keen to understand potential issues with oil distribution and the effectiveness of lubrication within the bearing housing. Any insights or recommendations from experienced professionals would be greatly appreciated. I am attaching pictures for clear observation.

 

[BridgeSmith]

The proper and more effective place to post this would be in the Bearing Design, Manufacture and Maintenance subforum.

 

[electricpete]

Diagnosing a wrecked bearing can be difficult (there's lots of damage, so what came first). I would gather more evidence and inspect further. Mark the asfound orientation, check the fit dimensions where practical, cut in half both races and photograph the races and fit surfaces, along with all the rollers. Review maintenance records including bearing last replacement and fits at the time. Of all of that posting the photos is probably the big one.

Is there a reason you are focusing on lubrication? To my thinking loss of lubricant event typically results in overheating which will show at least brown stains. I don't see any of that evidence of overheating so I'm not inclined to blame this directly on loss of lubrication. I do see a lot of evidence of mechanical trauma.

I would be interested to know the history of vibration monitoring on these bearings (how often, when was the last check before this failure and what did it show). Lube analysis may be of interest too, but not as much as vibration analysis which is more reliable for early detection of rolling bearing defects imo.

Failures that originate as spalling on the races or defects on the ball are common and can be detected early on vibration. But if left unchecked they can spread to something like this.

Cage failures are the one type of failure that often can result in a wreck like this without giving much warning on vibration.

Based soley on above logic, if you have no vibration monitoring then at this point (awaiting further info) I'd be inclined to guess that the initial damage was a bearing roller or race defect as an initiator which went undetected as it progressed to cause more and more damage. If you have frequent vibration monitoriong not showing then I'd be inclined to guess that the initial damage was likely cage failure. Of course knowing initial damage is not the same as knowing the cause. IF you have no bearing condition monitoring on a critical machine, then in my book that is a big contributor (regardless of exact cause of initial defect on balls and races, these types of defects are common and what is generally expected for critical machinery is they will be detected in time to be fixed on a non-urgent basis).

 

[3DDave]

I'm leaning to cage failure. One roller is allowed to go a little crooked and then the damage progresses quickly.

Examine the other cage and rollers and bearing surfaces, assuming it was installed at the same time, same manufacturer, made at around the same time as the failed one.

Looking at all the destruction may not tell much. The rollers are melted from skidding, so any initial failure has been removed.

How old is this plant?

Adding vibration and thermal monitors at all the bearings may be the only defense against unexpected shutdowns or worse.

 

[electricpete]

I dunno. The photo below looks like severe outer race spalling which could be the initial damage that released a lot of debris which could lead to everything else seen. I'm not saying that's what happened but I wouldn't rule it out without seeing more

 

[geesaman.d]

Is there any thrust load in this system? If not, are the bearings set in such a way to keep all rows of rollers in firm rolling contact? Was there a disruption in the loading before the failure?

While SRBs are touted to support thrust load, the bearing people do not warn you about the dangers of having one complete set of rollers not in firm rolling contact. The unloaded rollers can skid and fail suddenly if they are forced to go from coasting/not rotating into rolling motion. A thrust loaded SRB can fail instantly if the unloaded race is suddenly engaged by the appearance of a sufficiently large radial load. The other issue with an SRB in thrust (or preloaded to create thrust) is the thrust loaded row is physically unstable and rely on the cage to stay properly aligned. So if enough wear occurs in the cage those rollers will flip over and jam.

SRBs are really good at rejecting grease from the rolling surfaces but I think they are pretty happy in oil. So unless the oil viscosity is significantly high or speeds are very high, that's not a red flag item to me.

The requested dissected bearing information will be helpful too. It can raise questions due to the multiple failure stages that are often observed, but at least it can rule out some others.

 

[3DDave]

Not saying not spalling, just that there is so much damage that welding and tearing loose and skidding of melted rollers is going to make looking at that bearing and determining a cause difficult.

That's why a tear-down of the other bearing is likely more important - they should have run for similar amounts of time in similar conditions. If there is spalling there or cage wear that would be a guide. If it is skidding from one not being loaded ... not sure what to see about that.

I await information on how old this coupling is and how old the plant is.

 

[TugboatEng]

There are 3 normal modes of rolling element bearing failure. The first is outside race rotation. This is the most common and the one you should expect if your bearings are not pinned. The next is cage failure. This is the one you don't want as it is catastrophic and sudden. The third is spalling which indicates the bearing has reached the end of its fatigue life. This is the most predictable and you should not see it if you have a reasonable maintenance interval.

Please provide bearing number, shaft rpm, oil viscosity, and ambient temperature. I ask these questions because improper oil or grease viscosity for bearings of this diameter at 1800ish rpm are almost universal conditions in my industry at least.

 

[Tmoose]

Overhung fan?
If the drive end bearing radial load is too light, rollers can skid and cause surface damage that can cascade into big trouble.
Specific and fairly unusual kinds of vibration monitoring can sort-of detect roller skidding pretty early.

3DDave said "Adding vibration and thermal monitors at all the bearings may be the only defense against unexpected shutdowns ....."
You need them. Temp sensors touching the outer race are way better for early problem detection than temp sensors in the oil sump or even the housing outer surface.
I'd expect that kind of monitoring with fast acting real time displays up in the control room would likely have set off alerts and alarms long before the problem was terminal.

"despite satisfactory oil level and condition. "
How is satisfactory oil level determined ?
If oil condition means oil analysis, oil analysis would have to be done very frequently to catch the beginning of this problem.
Like maybe every day.
Is oil pumped to each bearing?
Or is the large sump in the common housing filled up so the oil level reaches the lowest rollers?
Or some other completely different design?

https://res.cloudinary.com/engineer..._Image_2024-05-19_at_11.35.54_PM_1_dsanus.jpg

Looks to me like that bearing got mighty hot.

Looks like the inner race cylindrical bores are clamp against shaft shoulders.
Axial expansion must be considered, and allowed. In simpler systems one bearing's outer race would be axially "fixed" and the other would have axial clearance between the outer race faces and the housing shoulders or cover pilot. The large housing that has both outer bearing seats and the cover pilots need features controlled to provide proper axial location and allow thermal expansion.

The lower housing half appears to have a shoulder on the inboard edge of the failed (drive end) bearing.

https://res.cloudinary.com/engineer...pp_Image_2024-05-19_at_11.35.51_PM_wpey5j.jpg

 

[electricpete]

https://res.cloudinary.com/engineer..._Image_2024-05-19_at_11.35.54_PM_1_dsanus.jpg

Looks to me like that bearing got mighty hot.

I agree there may be indication of overheating of the shaft near the bearing, and that seems important to understand. But there seems to be an absence of brown varnish or other evidence of associated widespread overheating in the bearing itself (at least the parts we can see). I have a hard time reconciling those things. Do others have comments / ideas on that?

...The one thing it brings to my mind is maybe the bearing inner ring spun on the shaft. We can see locknut with lockwasher engaged but that's not a guarantee bearing didn't spin on the shaft. Maybe further disassembly photos and any available fit measurements will shed light.

 

[TugboatEng]

Once the cage fails all of the rollers bunch up and stop rotating. The inner race may have spun in the stationry rollers causing it to heat up. You can see the flat spots on the rollers in pic#9.

There are a few cage material options. Consider if the cage was of the correct material? Stamped steel and machined brass are the most common. I have never had to specify a cage so I can't explain the differences.

 

[Muhammad Mazher]

@ electricpete, 3DDave , geesaman.d, Tmoose , TugboatEng
Unfortunately we have no Vibration readings for this equipment.
I took some new pictures after cutting the bearing. I saw some interesting findings.
These pictures shows like there is some surface fatigue or spalling. Please look and share your thoughts on this.

In above picture, this circular groove or indentation mark on upper race confusing me.

​

Please look these rollers picture, There is indentation mark on almost same area of rollers. May be this is an indication of some extra load or unbalance load. Share your experience.
If you have any guide or website regarding this, Please do share.

 

[Muhammad Mazher]

@All
I am attaching some more pictures for your better understanding.
I think in this case, there is a possibility of surface fatigue which accelerated due to low oil level. Total oil capacity is 20 liters but the oil we took from bearing block was 17.5 liters but oil color was satisfactory.

If you have any guide or website regarding this, Please do share.

 

[Muhammad Mazher]

Also please look below picture of inner race. We tried a lot but it is stuck with shaft. Due to unavailability of proper tools, we decided to cut it.

 

[Tmoose]

I believe those final pictures show cage, roller, race and shaft damage typical of a catastrophically failed bearing.

Like the honorable EPete said, "there's lots of damage, so what came first".

https://i.imgflip.com/ox2ln.jpg

- Is filtered oil pumped to each bearing?

- Or is the common housing just a 20 liter sump, that, when correctly filled, has the lowest rollers rolling through oil ?

-----------.
Although I see no signs of either:
- Maybe there is supposed to be a simple oversized ring dangling on the shaft close to each bearing?

https://en.wikipedia.org/wiki/Ring_oiler

- A flinger or two mounted to the shaft to fling oil from the sump towards the bearings or channels in the housing cover that lead to the bearings.

------------.

- Maybe some other completely different design?

https://mes2005.tripod.com/Lubrication.pdf

https://oaktrust.library.tamu.edu/b...d=A4FC4C8A779409B5B3C3EDD68A8D52ED?sequence=1

 

[Tmoose]

https://res.cloudinary.com/engineer...pp_Image_2024-05-21_at_10.58.02_PM_kp8kpf.jpg

Is that the 17.5 liters of oil being drained from the housing that just suffered that failed bearing?

 

[Tmoose]

When was this fan commissioned and put into service?

How many hours did the failed bearing have in service.

What types of bearings are in the drive motor?

What make and model of coupling ?
Is the coupling the same make and model provided by the OEM ?


Was any motor>fan shaft alignment done recently?

Was the coupling replaced recently?

Was the fan oil replaced recently?

Is there a "sight glass" installed on the bearing housing showing the actual oil level ? There should be.

https://media.noria.com/sites/magazine_images/articleimages/SightGlassHeader.jpg

 

[TugboatEng]

Next time you need to remove a bearing race, stand the shaft on end so it is vertical and take your largest torch and heat the race RAPIDLY until it falls by gravity.

 

[Muhammad Mazher]

@Tmoose
Yes, 17.5 liters of oil being drained from the housing.

Common housing for bearings.

One more thing to note that it was a locating side bearing while the other one was non locating side.

 

[Muhammad Mazher]

@Tmoose
Fan was installed in 2010 and bearing were recently replaced 1 year ago.
Coupling was according to OEM recommendation.
No alignment done recently.
No oil Change activity since last 10 months.