Bearing Failure 2

[amalki]

hi

I have a problem with a bearing failure, the case is so complicated and I hope my English help me to explain it clearly

we have at our company a motor pump train that deal with kerosene with 40 hp, 2935 rpm, 3 phases, 380 volts. the history of the motor says that it undergoes repeated failures after about 1 year of continuous serviec. The motor burned out which concluded after investigation that the root cause was the DE damaged bearing (NDE bearing was Not damaged). For this motor according to the manufacturer data sheet the bearing is ball bearing with grease lubrication but for different reasons ( I think with water or somthing like that) in the past they have changed it to be shielded ball bearing from both sides and C3 internal clearance. the bearing manufacturer is KOYO and we suspect that the installed bearing was not C3 !! since the number typed on the damaged bearing is not including C3 which means that when the motor overhauled the installed bearing was not C3 if we can say so
my inquiries are:

- Did koyo write the type of internal clearance (C3) on their bearings ( not in the box, I mean on the baring itself)?
- Is the change that happened for the bearing internal clearance from C3 to CN for example cause the damage of the bearing? is it possible or will not be an issue?
- another thought that the bearing might need to be redesigned to increase its capacity, if so how can I find the reactions on the bearing if the rotor coupled with pump and there is a cooling fan at the other end of the rotor ?

Thanks a lot for any valuable help you offer and appreciate your patience to complete reading until here

 

[MikeHalloran]

I don't think internal clearance is your problem; lubrication is.

When a double SEALED bearing leaves the factory, it contains grease, and the grease will be specified or coded on the box.

When a double SHIELDED bearing leaves the factory, it contains NO GREASE.
It contains only a light flushing/ preservative oil.

The factory assumes that when you order double shielded bearings, you intend to demount one of the shields, or pressure flush some grease into the bearing, or provide continuous oil lubrication, but they certainly don't expect you to mount and use the bearing as shipped.






Mike Halloran
Pembroke Pines, FL, USA

 

[electricpete]

When a double SHIELDED bearing leaves the factory, it contains NO GREASE.
It contains only a light flushing/ preservative oil.

The factory assumes that when you order double shielded bearings, you intend to demount one of the shields, or pressure flush some grease into the bearing, or provide continuous oil lubrication, but they certainly don't expect you to mount and use the bearing as shipped.

That's news to me. I haven't seen that in any double shielded bearing I can remember. Certainly SKF and FAG come prelubricated with grease between the shields and those OEMs do NOT want you to open up the shields in the field.

What bearing OEM are you referring to?



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[electricpete]

- Is the change that happened for the bearing internal clearance from C3 to CN for example cause the damage of the bearing? is it possible or will not be an issue?

It is possible although I wouldn't think likely. CN has less internal clearance than the standard (for electric motors) C3.
Too little original internal clearance might cause bearing failure if exacerbated by other factors: excess interference between shaft and inner ring, between outer ring and shaft, also inner ring hotter than outer ring due to heat from shaft or cooling on housing (well meaning people who apply supplementary cooling to the housing can create problems).


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[amalki]

Thanks all for your appreciated response


another things I forgot to tell:
- after about half an hour of the motor tripping the temperature of the casing of the motor was 95 degree centigrade
- Fan cover found loosening resulted in touching with the plastic fan
- the DE bearing founded seized with rotor and sign of overheating in inner race

please see attached pictures for the damaged DE bearing and the NDE bearing
DE Bearing

NDE Bearing

thanks a lot

 

[MikeHalloran]

Let me rephrase my advice in a way that may be more acceptable to Pete.

We can agree that if a bearing has double CONTACT SEALS, it must have grease inside it as it comes from the factory.

If a bearing has SHIELDS, or the modern accursed LOW FRICTION LABYRINTH SEALS, presence of grease should not be assumed, unless the box specifically states that grease is present, and what kind of grease it is.
When you have unwrapped the bearing and are ready to install it, just give it a little spin. If a bearing spins easily by hand (no turbine spinning with compressed air please), consider the possibility that it was ordered without grease, and you might wish to add grease rather than assume its presence.

This might not have been an issue decades ago, when bearing houses were staffed by bearing guys, but nowadays much counter staff comprises minimum wage monkeys, and many businesses are run by by business school graduates who claim that running the business well is independent of what sort of business it is. ... which I assert is not true for distribution of technical products.

Back to Pete's point, SKF's online catalog indeed states that both sealed and shielded bearings are pre-packed with grease. ... but it gives an L10 life, for the grease (as distinct from the L10 life for the bearing), only for sealed bearings. Grease life is sort of not mentioned for shielded bearings.




Mike Halloran
Pembroke Pines, FL, USA

 

[JJPellin]

You did not mention your lubrication practices. I have often heard and believe that more motor bearings fail from over-lubrication than fail from under-lubrication. Our standards require single shielded bearings in motors. This removes all doubt about the presence of grease. And it can reduce the possibility of over-lubrication.

Johnny Pellin

 

[electricpete]

Back to Pete's point, SKF's online catalog indeed states that both sealed and shielded bearings are pre-packed with grease. ... but it gives an L10 life, for the grease (as distinct from the L10 life for the bearing), only for sealed bearings. Grease life is sort of not mentioned for shielded bearings.

I guarantee you that SKF and FAG double shielded bearings have grease inside when they come from the factory. I have talked to application engineers from both companies and neither one even wants you removing the shields because they’re afraid you might cause a problem. Same thing with F.A.G. Myself, I haven’t run accross a manufacturer that does not include grease within double shielded bearings, but I haven’t used Koyo.

Here’s what Koyo’s literature says:

http://www.koyousa.com/brochures/pdfs/techbulletin-polyrex.pdf

Koyo is pleased to announce that all of our 5200, 5300, 6000, 6200 & 6300 sealed and [double?]shielded ball bearings are now stocked with Mobil Polyrex EM grease fill.

http://www.koyousa.com/koyocatalog/technical/09/9_1_3.htm

Bearing Applications
*All electric Motors

Features and Benefits
· standard radial ball bearing with single-shielded, double shielded or unshielded
· with single shielded and unshielded bearings users can lubricate the bearing with proper grease for the operating environment and prevent grease incompatibility [no mention of the same option for double-shielded]

So, I’m skeptical that Koyo would provide double-shielded bearings without grease.
But it’s definitely worth double-checking by pulling one from the same supply to answer the question.

Still studying the photos, ideally would like a lot more info. Would like as many more photos as possible from all angles. Would like to look at the bore, the od, both sides of the shields, the races, the shaft seat, the housing seat, the adjacent cavity of the housing. Does the bearing bore show signs of spinning on the shaft? (I have a thread with case study of that... the appearance doesn't jump out unless you look closely). Looks like one ball is smaller than the others.. that would be odd... is it really smaller? Can you look closer at the small ball to see if there is evidence of how it got smaller? Are these two photos the same bearing? I’m thinking not because the upper photo seems to have discolored bore and the lower seems to have a pristine bore... so then the bottom photo is the opposite end bearing? Were the bearing fits checked? As mentioned, was lubricstion attempted at the site?

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[electricpete]

Top photo - looks like in the periphery we can see 3 shields? Which are which?

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[electricpete]

Since it is drive end bearing and you're having repeat failure, can you tell us more about the load that this motor drives and through what type of coupling or belt?





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[electricpete]

Also, these repeat failures are all the same motor (bearing changed, but not motor)?
In that case it would be really important to check those fits as they may have been destroyed in first failure.

Are there any "sister" (identical) machines and do they have the same problem?

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[Skogsgurra]

The balls shown with the damaged DE bearing have a matte appearance. That is, in my world, an indication that there may be an electrical (EDM) damage.

But, I have only one hammer and everything looks like a nail to me

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Tmoose]

1 year continuous service was about the service life of several brand new brand name motors (20 to 50 HP, unknown rpms? it was a few decades ago) direct driving some water pumps at a HVDC converter station.
The customer never greased the bearings. A six month relubrication schedule, with detailed grease quantities was specified by the OEM.

If I felt I had to use sealed or shielded bearings I'd look into ordering them with somewhat exotic grease with greater oxidation resistance.

I'd confirm if C3 bearings are called for by the motor manufacturer in my application. The rotor of most types of motors is a source of heat, so the shaft and inner ring are likely to be hotter than other types of machines( FAG used to predict 10 degrees C, I thnk). The extra half thou or so of clearance in a C3 bearing could come in real handy.

 

[amalki]

sorry for late I'll come back soon with more details

thanks a lot for your priceless help

 

[caoimhin1]

I've never heard of a bearing company supplying a non-greased shielded bearing. Even on large deep groove ball bearings which might have a removable shield. The risk of cross contamination of ungreased product with greased product would be too high.
One of the basic questions is to make sure that one bearing is floating and one is free....sounds obvious, but a common error. If both are fixed, increased temperatures and early failure will ensue.

 

[BrianE22]

I must be missing the obvious. The damaged bearing looks like it is badly corroded.

 

[BearingDave]

I deal with bearings on a daily basis.

I confirm that in my experience, NTN, SKF, FAG/INA and NSK deep groove ball bearings that are capped (either with contact seals, low contact seals or non-contact shields) come pre-lubricated with grease. Typically, either seals or shields cannot be removed from the bearing and then re-installed. Once the seal or shield is removed, the part is deformed and there is no cost-effective way of putting them back in.

I would also agree with Electricpete that we would need to know if the motor is driving a pulley/belt system or if it is direct drive with a coupling. Typically, the rule of thumb is:
Direct drive: Ball bearings should be fine
Belt drive: Put a cylindrical roller bearing instead of a ball bearing on the drive side, as greater radial pull from the belt might exceed what is normally acceptable for the ball bearing.
Obviously, if the loads are known, you can make the best selection based on the L10 life of either the deep groove ball bearing or cylindrical roller bearing for either a belt or coupling drive system.

Last comment: Just as BrianE22 pointed out, the bearings appear to be completely corroded. Whether that is due to water contamination or kerosene contamiation, there appears to be an issue with the sealing arrangement of the motor. A review of the sealing arrangement might help prevent water ingress. Also, if the bearings are capped with non-contact shields, then a chemical (water, kerosone) can make its way through the small gap and affect the lubricant.

 

[Tmoose]

generic fit and bearing clearance from FAG Publ WL 01 200 EA. "Roller Bearings in Electric Machines"

C0 (standard clearance) up 100 mm/4 inch shafts.
I'd still burrow into the OEM's tech department to get the recommended bearing.

I'll try to attach a second page showing the IP ratings of various sealing arrangements. I've never seen IP ratings assigned to anything but electrical enclosures any where else.
Simply Adding a Forscheda type V-ring boosts the IP rating nicely. Not shown are examples of details necessary to manage leakage that can sneak past a gap or labyrinth and avoid creating a wading pool for the bearing to soak in.

 

[Tmoose]

looks like the first page with fits and bearing clearances didn;t stick. Here it is again.