Bearing brinelling 4

[xj25]

Hi everybody,

We have had a requirement from a motor supplier (induction, 200kW range), about the need of inmobilising the rotor in longitudinal direction, to avoid brinelling problems in the bearings. This is to apply once the motor is mounted on the machinery we build.

We have had lots of motors transported before without that requirement without any problem, but we have had an incident with some motors recently, where some bearings were damaged.
Additionally, that specific case used ceramic bearings (ceramic rollers), that is a relatively new solution for us, recomended to get a better behaviour with VF drives in terms of bearing life.

The task of inmovilising is not so easy, due to access constraits etc. so it is a problem for us.

My question goes about:
- Anybody knows if it has any relation the fact of using ceramic bearings to the risk of "false brinelling"?
- makes sense the requirement of rotor locking in long. direction for any type of bearings, if our experience says that is not? (for regular steel bearings I mean)

False Brinelling link

Thanks for reading

 

[Skogsgurra]

Trying to keep the rotor still during transport is the exact opposite to what you should do. It could even be worth-while to install a small DC motor with a worm gear and add a timer that activates the motor in intervalls. Say once per day and turn the rotor a few degrees each time. The arrangement is, of course, temporary and should be removed before starting the machine. Needless to say. But still...

That practice - but manual rotation - is applied on spare motors that may be affected by small vibrations on the storage place. All to help getting some grease between balls or rollers and raceways. If you can arrange that the transporter gan get at the motor shaft and rotate it manually, it will also help. The intervall can be considerably longer than 1 day. Even once/month can be sufficient. But a small nudge once a day keeps Brinelling away.

I don't know if ceramic (hybrid) bearings are more affected. It could well be so since the ceramic is smoother (less rough) and that reduces the apparent Herzian contact area so that forces concentrate on that small area. Ceramic is brittle and the locally high force may, even if I haven't see it, result in cracking.

Why ceramic bearings? They are expensive and not a perfect solution to EDM (the PWM residuals usually find their ways through shaft and coupling and may kill other innocent bearings in the machinery. Seen that quite often. Ceramic bearings are also quite expensive. Have you tried common-mode filters? They are usually a good solution in that power range. For smaller motors, they do not work so well. But definitely at 200 kW.


Gunnar Englund

http://www.gke.org

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

 

[jraef]

Although I'm in total agreement with Gunnar on the issue of brinelling, FALSE brinnelling is different than what is being discussed here. Brinelling is the deformation of bearing surfaces from pressure, ie standing still with lots of weight on the bearings causing indentations. FALSE brinelling is similar looking damage, but is the result of vibration while standing still for long periods (or corrosion, but that's not germane here). Gunnar's points about prophylactic rotation are to prevent brinneling and can also help prevent false brinneling by evening out the wear when vibrating in place for long beriods and keeping the lubricant from being displaced by the vibration. Immobilizing to stop false brinneling is likely a pipe dream because you cannot totally immobilize anything, and immobizing to try to stop true brinelling would mean having to totally suspend the weight of the rotor so that it doesn't put pressure on the bearings, also impractical because of material tolerances. I'm not sure why they would say to immobilize longitudsnally however, unless they are concerned for thrust bearings or these vertical motors.

The ceramic bearing issue is a new one on me though, I'd never though about the brittleness of the ceramics. But I'm also in agreement with Gunnar on that, ceramic bearings are rarely the right solution. Isolating the motor bearings just moves the problem to somewhere else. I'm in the eliminate the issue by using filters and/or shaft grounding techniques.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Skogsgurra]

Yes, Jeff. As a sake of coincidence and weirdness, I live very close to Brinell Road in Fagersta. Named after the guy who introduced the Brinell Press to test softer steel for hardness. It is used instead of the Vickers method, which is better suited for hardened steel.
The Brinell Press uses a steel ball that is pressed into the specimen under test, the impression's diameter is measured and the name Brinelling (false, to be correct) comes from the similarity with the indentations in raceways and the mark made by the Brinell Press.
He was a Swede, of course

Gunnar Englund

http://www.gke.org

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

 

[jraef]

Yes, the "Brinell Scale" of material hardness is named after him as well.

I predict that some day in the near future, a system to measure AC induction motor bearing EDM damage caused by inverter operation will be called the Englund Scale... I use your paper on it in a lot of seminars now, I link people to your site as well. The only thing is, you used the word "stochastically" and without fail, I have to define that to someone in the room!


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Skogsgurra]

Jeff, I wonder which paper you use? There is a new article in the Electronic Environment Magazine #2 this year. It starts on page 28 and contains a lot of new findings and field measurements. I can send you a 'class room copy" in pdf if you want it.

Gunnar Englund

http://www.gke.org

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

 

[xj25]

I will come back if I got more detailed info. The issue is FALSE brinelling, allegedly caused by vibration in transport, and the supplier tells us locking for transport, but refers specially about longitudinal locking. We have 2 screws for that in the NDE.

Mr. Englud, I would appreciate if you can send that "class room copy", I found it online but no so easy to read.

Thanks

 

[electricpete]

The issue is FALSE brinelling, allegedly caused by vibration in transport, and the supplier tells us locking for transport, but refers specially about longitudinal locking.

That is correct - the standard practice called out in many motor repair specs is to "block" the shaft axially for shipment.
If you push/pull axially on an unblocked motor, you will get movement at least as much as the axial internal clearance of the fixed bearing.
It may be a few thousandths of an inch axial movement for a 6313 deep groove ball bearing.
If you think about what the ball is doing in a deep groove bearing, it is not sliding by a few thousandths, but rather primarily rolling such that contact shifts from one side of center to the other side of center ... this distance is a distance rolled is a few tenths of an inch (ball diameter is only slightly samller than race twice the bearing radius of curvature... a phenomenon known as osculation... the closer the ball diameter is to twice the radius of curvature the further the contact point will travel on the race to accommodate given axial displacement... until at the extreme endpoint where they are equal it takes zero axial motion to move he contact point because you no longer have point contact but line contact).
I have frequently see this damage pattern (few tenths inch axial marks spaced at ball spacing, perhaps several families of this with random spacing between) on machines which have been stationary while vibrating in the plant. The damage pattern roughly matches the axial path that the ball would roll back and forth across (although there may be more complicated description of the damage mechanism at micro level). The vibration shows bearing fault, we pull it and see that pattern referred to as false brinneling. I assume it is the same thing that we worry about during transport although I've never dissected a bearing damaged in transport.

We have 2 screws for that in the NDE.

It's not clear what you are describing. If you are describing bolts for an inner cap that clamps the outer/stationary race against the endbell, that is not blocking the rotor. Any blocking of the rotor would have to involve some axial restraint of shaft or inner/rotating race (not outer race). Typically that requires "something attached" to the end of the shaft extension, then you install a solid piece wedged between the frame and the something attached. If you are lucky enough to have a coupling hub installed, just wedge a piece of wood between hub and frame. If not, sometimes there is a threaded hole on the endface of shaft, you can use that to affix a bar to the end of the shaft for "something attached". Otherwise you may have to clamp onto the shaft (the clamp is something attached, wedge piece between clamp and frame) but be careful not to damage the surface where the coupling will end up.


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(2B)+(2B)' ?

 

[electricpete]

https://www.easa.com/sites/files/resource_library_public/EASA_AR100-2010_1010.pdf

EASA AR 100 recommended repair practice for electric motors.
See 1.11 regarding packaging and transportation.... "Blocking of the shaft is recommended"



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(2B)+(2B)' ?

 

[romke]

there may be another option to prevent false brinelling occurring. the method described above means that you temporarily try to make the rotating and stationary parts of the motor into one solid unit so that relative motion due to vibration originating from the outside world cannot take place. another option would be to decouple the motor more or less from the outside world by packing it in such a way that vibrations are not guided into the motor, but absorbed by the material that it is covered in. one way to do that might be to support the motor during transport on a "bean bag" (a bag filed with granular material that easily moves over each other thus absorbing the energy). that method is frequently used in photography to support long lenses, but might also work in modified form in this case.

 

[waross]

Bear in mind that a 200 kW motor may weigh from 3000 lbs to over 6000 lbs depending on the speed and enclosure.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Tmoose]

Have the damaged bearings been evaluated by the bearing manufacturer, and by your team with a bright light and magnification to determine the true nature of the damage?

 

[xj25]

We have an independent analysis that confirms hypothesis of false brinelling.
The point is that we have not ever blocked longitudinally the motors once assemmbled to transmission gear,etc., and we can not trace clearly if the problem occured while transport of the assembly, or while the transport of the motor.
Bearing manufacturer seems not to associate it with the fact that ceramic rollers are used.
It could be a specific one-time problem of the transport, or situation that we will never know or repeat. But now our design team is studying how to block the axle once everything assembled, and that is not so easy. In RAM terms I would say the probabilty the fault repeats is low (based on experience), but the consequences are too big to ignore...


Link ref. data

 

[Tmoose]

For all the things that hybrid ceramic bearings do very, very well, there are a few circumstances where steel balls and rollers may be "better."
The circumstances I can think of are related to the higher stiffness of the HC ball/roller material. For identical geometry, that increased stiffness means the ceramic ball/roller will not conform as well to the steel race, and kind of "sink in," like riding a narrow tired road bicycle on across a sand dune.

The attached image shows the catalog values of some HC ( hybrid ceramic) cylindrical precision spindle bearings.
Note the static load rating ( defined as when small permanent deformations form in rollers or races) are significantly lower for bearings using HC rollers.

For true brinelling ( denting ) the HC roller bearing specs declare them to be more vulnerable.

But, in addition, if false brinelling results when the roller quivers and shivers down into, and through, the lubrication film and exposes the steel race, then maybe the HC bearing would be more susceptible to that as well, even though the HC roller itself is tribologically magnificent.

So there is my hypothesis, pulled straight out of my hat.

 

[electricpete]

Good insights to consider from Tmoose as usual.

I don't have experience to say whether ceramic rolling elements make the machine more susceptible, but I want to repeat that in my experience it is standard industry practice to block rotors axially for shipment in virtually all the motor repair specifications I have seen. It may be this practice is ignored without problem many times, simply because the motor has a soft ride without excessive vibration. The axial blocking makes the situation more robust so on the occasion when you do have a rough ride the motor is protected. I'm just proposing an alternate theory to consider. It may certainly be a completely wrong theory, but if so (if the problem is only cermaic bearings), then I'm not sure why standards and specifications would require blocking the rotor without any mention of type of rolling element.

I would say the damage pattern from a distance seems like it could be consistent with axial micro-sliding of rolling bearings in axial direction. If such were the case, then even if it were limited to ceramic bearings, I'd think axial blocking would help.

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(2B)+(2B)' ?

 

[oldsparkie]

Bearing Brinelling damage can occur during transport and also when the motor is installed.

"MOTOR DESIGN"
Smaller motors are usually fitted with a ball race type bearing at each end. Clearance are small enough to minimize any movement whilst enough to cater for thermal expansion.
Larger motors are usually a ball race NDE with a roller at DE. The roller provides more support for the rotor and also for tangential load whilst at the same time catering for thermal expansion both longitudinally and vertically.

"TRANSPORT LOCKS"
Depending on physical size (number of poles) some smaller KW motors should be fitted with transport locks simply to support the rotor weight and avoid damaging the bearing surface.

Larger motors typically fitted with a DE roller require a transport lock. Vibrations induced into the rotor whilst being transported will damage the outer race of the bearing without this.

I acknowledge other responses saying no transport locks. My experience approaching 50 years says the exact opposite.

I suggest there is a considerable degree of probability of transport damage based on the location factor and transport path conditions involved. These are best mitigated with transport locks.

VSD's can cause fine etching (different to the mechanical brinelling chop marks that resemble being hit with a blunt chisel).
The most effective solution is using a reputable high end market VSD and fitting earthing brush (sometimes 2) to the DE bearing. Also making sure the earth loop impedance between VSD & Motor is as low as possible.