Loads been applied to motor bearings 1

[Feg]

Hi

Can anyone advise on the correct way to work out the load being applied on the bearings of a motor from a centrifugal fan. This motor can run the fan no problem but every few months the NDE bearing fails. The bearings on both ends are 6209ZZ. There is no problem with dust, the fan impeler fits directly on to the motor shaft.

Thanks

 

[dV8r]

How is the balance?

 

[ozmosis]

Is it horizontal or vertical mounted?

 

[electricpete]

As far as loads, there is the weight of the motor and fan. Likely not very much. More on the DE bearing than the NDE bearing. There may be some axial load created by the fan... usually pretty small. There may be some load created by thermal expansio of the rotor if one of the bearings does not slide to accomodate it.

Certainly there can be many other factors to consider beyond bearing loading (I think bearing overloading is one of the less common cause of bearing failures).

You should take a careful look at the bearings that come out. Cut them apart and look at the lubrication condition, the races, rolling elements. You can usuallyl see the ball track on the inner race and outer race which can help you judge the type of loading that had been on the machine. There is a lot you can learn by the pattern of the defects. One common one - defects spaced exactly at ball spacing usually indicate that the damage occured while the equipment was stationary (false brinneling).

Measure shaft and housing fits.

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[Feg]

The motor is horizontal mounted, the end shield were replaced at te first failure, the fan is new and looks to be well constructed.

 

[MikeHalloran]

The ZZ suffix usually denotes two shields. Shielded bearings are usually shipped coated with a thin preservative oil that is _not_ intended to be a lubricant. You _can_ get them prelubricated from the factory, but you have to specify the lube you want, and it will be marked on a retail box if present.

Next time you replace the bearings, and you will, apply a drop of electric motor oil to the ID of the shield so it wicks in, or remove a shield and put a little grease in and replace the shield, OR, better, follow the motor manufacturer's recommendations.



Mike Halloran
Pembroke Pines, FL, USA

 

[stephenw22]

What type of fan is it? Will the fan generate thrust along the shaft when it operates?

 

[waross]

Check for a slight bend in the shaft at the problem end. If the end bell is bendable, check it also for proper bearing alignment.
respectfully

 

[electricpete]

Is there such a thing as bearings shipped in double shielded configuration without grease between the shields? I haven't heard of it.

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[MikeHalloran]

Were you wondering why you can't buy a replacement bearing that's worth a crap?

I repeat:

ALL double _shielded_ bearings are shipped WITHOUT GREASE, unless you specify otherwise when you order them from the factory. The ones you find on the shelf at the bearing distributors are 'solvent plated' with a tiny quantity of _preservative_ oil, not lubricating oil. If the distributor ordered them with grease (unlikely), the amount and type of grease will be noted on the box, because it's not the default condition.

That's all in the fine print in the catalog.

If a motor manufacturer uses -ZZ bearings, it's up to them to oil the bearings when they are installed in the motor. The replacements you buy will carry the same markings, but they have essentially zero oil in them, if you don't add any.

Contrast that with double _sealed_ bearings, which are shipped pre-greased, so you don't have to remove and replace the seal to grease them... except for bearings with _labyrinth_ seals, which are not prelubricated.




Mike Halloran
Pembroke Pines, FL, USA

 

[electricpete]

Were you wondering why you can't buy a replacement bearing that's worth a crap?

No, our bearings come with Polyrex EM grease so we don't have to pop the shields right off the bat, which is the standard way supplied from SKF.

http://www.skf.com/portal/skf/home/products?paf_dm=sharedmaincatalogue=1&lang=en&newlink=1_0_101

The information and recommendations in this section relate to bearings without integral seals or shields. SKF bearings and bearing units with integral seals and shields on both sides are supplied greased . Information about the greases used by SKF as standard for these products can be found in the relevant product sections together with a brief description of the performance data.

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[MikeHalloran]

My experience differs, but then I haven't bought bearings from SKF ... and I haven't bought OEM bearings in a long time.

My last related disaster was a lot of 12,000 stepping motors supplied with shielded instrument bearings, not lubricated with anything other than preservative oil. There were infant mortalities until we started oiling them at assembly into our stuff.




Mike Halloran
Pembroke Pines, FL, USA

 

[edison123]

I am with electricpete about the lubrication of shielded and sealed bearings. All SKF bearings of such types come with greased for life (whatever that means).

Pete has covered all possible areas of investigation. I would like to add
1. Check whether the bearing rated speed is above your operational speed.
2. Whether you use VFD's. Then, it is a different ball game.

*Nothing exists except atoms and empty space; everything else is just an opinion*

 

[petronila]

Hello Feg,

I think too Pete cover all details, but I want to add something, the best method of installing sealed ball bearings with sizes 09 or smallers is pressed on the shaft if you overheat the inner race to expand over 120 C then the internal grease will be heated and lost properties and can transform in liquid, this minds the bearing will loose the expected life and will faill due to poor lubrication, the steel properties are transformed too.

For your application this ball bearing could fail too due to mechanical problems like wrong balancing or bent shaft, did you check this ? perform a vibration analysis in order to find this synthoms.In the motor Check too the shaft and Housing Tolerances. For electric motors we use C3 clearance.

In my shop all the Time we are replacing ball and roller bearings in electric motors.Sealed or open type.

The last 3 years We only use SKF Explorer, no problems with this one are excellents, the ZZ or 2RS sealed are Always lubricated,maybe IF YOU SPECIFIED TO SOME BEARING MANUFACTURER sealed bearings with out grease thy will manufacture, this coul be a special case, but my concept is for electric motors the saled bearings are used to avoid the field lubrication.

Regards

Petronila

 

[ischgl99]

All FAG shielded and sealed bearings come prelubricated unless you specify otherwise.

Mike, your company may have ordered the bearings without lubricant planning on injecting the oil during installation. I have never heard of a company shipping shielded bearings without grease as standard, but you never know, there could be some out there doing that.

 

[MikeHalloran]

The little bearings that you buy for inline skates are shielded, but not prelubricated, because grease would drag too much, and oil would run out and make a mess of the retail package.



Mike Halloran
Pembroke Pines, FL, USA