Shaft damages under oil seals due to shaft currents? 6

[edison123]

A 11 KV, 2.5 MW cage motor has white metal sleeve bearings on both sides where the bearing shells are insulated from the pedestal and the top cover. There are two fiberglass split oil seals for each bearing on either side. The split halves are held in place by an external circular spring.

After two years of running, the motor was opened up for an inspection and the shaft was found to be scored under these seals. The photos of the damages is attached in a PDF file. The scoring is deep to the extent of 4 to 5 mm and they are exactly under the seals. You need to pull the seal axially to see the scoring damages.

The shaft material has formed a hard sludge into the oil seal grooves, which is hard to remove. Strangely, the oil seals themselves do not show any sign of overheating despite the shaft material deposit in them.

The drive end bearing bottom half was found to have a small cable connecting it to the ground. No idea why the chinese OEM would insulate both the bearings and then ground one of them.

A different motor of 3.5 MW capacity from the same chinese OEM at the same plant also has these shaft wear-outs under the non-metal seals.

What could be the reason for this shaft wear-out under the seals, especially when the seal is a non-metal? Did the grounding of one bearing inflict some kind of shaft current damages to these seal areas ? If yes, why under the seals and not anywhere else?

Hoping Gunnar would weigh in. Thanks in advance to all.

PS: The white metal bearings and the shaft journals themselves are clean and do not show any pitting marks due to shaft currents.


Muthu
www.edison.co.in

 

[LionelHutz]

My guess would be the seals wearing down the improperly hardened surface on the shaft.

 

[EdStainless]

I am with LH, it looks like the seals are cutting into a soft shaft.

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P.E. Metallurgy, Plymouth Tube

 

[Skogsgurra]

Hello Muthu
I do not work much with plain bearings. So I don't have a well-founded opinion about this. But I need to ask about the pittings. They look like capacitive discharges. Not that I understand what the opposite electrode would be - if it isn't a thin deposit of metal on the seal's inner side. Is that something that you can see? Or is the seal's inside not accessible for inspection?

OTOH, fiberglass doesn't seem to be the best choice of material if you slide it against steel for some time. My thinking is that fiberglass is a bit like sandpaper.

DOL motors? Or VSD?

Gunnar Englund

http://www.gke.org

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

 

[edison123]

Thanks LionelHutz and EdStainless.

Can a non-metal cut into a metal like that? I have not heard of shaft seal areas being specially hardened. Is that a norm?

Hi Gunnar. Thanks for chipping in. The motor is DOL. The seals do not have any metal parts, except for the outer circumferential spring, which is not in contact with any metal part of the motor. I removed the seals and could see that the shaft material has actually congealed into the ID of the seals at the split planes and got fused with the seal at those points. But the rest of the ID looks clean with no rubbing marks or shaft material deposits in the annular oil grooves. I have seen many motors with such fiberglass rubbing seals and none had such shaft machining like this.

Yesterday, they opened up another identical 2.5 MW motor and found the same problem in that motor too.

This power plant is very close to the sea. Will a humid saline atmosphere cause such an issue? Though I don't see how.

Muthu

http://www.edison.co.in

 

[waross]

My friend didn't realize that a fabric cargo strap was hanging down between the cab and the box and was rubbing on the aluminum drive shaft. It eventually cut his drive shaft to the point of complete failure.

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

 

[krisys]

Muthu,

Throw some light on whether one bearing was insulated. Whether the insulation was intact when open the motor?

If the metal corrosion is happening without the fiber glass, it directs towards the
a) Electrolytic corrosion
b) Corrosion due to the circulating current jumping and intermittent sparking, as a result pitting takes place

A small "forensic investigation" could help you to identify the root cause.

 

[electricpete]

I vote along Lionel, Ed and Bill in attributing this to mechanical damage rather than electrical.

I have twice seen shaft mechanical damage from a soft rubber-like seal. The two cases I saw happen to have been greased bearings or seals. I don't fully understand the mechanism but I don't have reason to believe it is limited to grease bearings or seals.

The first case is in file VaporExtractorGreaseSealShaftDamage.pdf. The fan is mounted directly to the motor shaft. The fan also sits within a housing which has a seal the motor shaft must pass through. Slide 1 and 2 show the seal (grease is added to intermediate chambers of the seal for sealing purposes. Slide 3 shows the motor shaft damage…. not a great picture but there was definitely a lot of metal damage.

The second is in file PumpLipSealRub.pdf. The file only include visual and thermal images on the outside of the machine when we were investigating high temperature. Slide 1 labels the components. There is a lip seal where the shaft exits the housing and we noted very high temperature in this area (there are also photos of the coupling, just heat conducted from this area by the shaft). I don't have photos, but I was present for inspection after the pump bearing housing was disassembled. There grease cavity had a lip seal which was a rubber component that had a spring compressing it (similar to your seal) and there was a lot of metal removed on pump shaft where the seal had been riding. The cavity was also full of grease. As you see from the last slide, the temperature of this seal was different from the identical sister units. And the temperature profile does not particularly match what we expect if heat is originating in bearing due to excess grease. Also this shaft damage had not previously seen on these pumps. So so there may have been something unique on this pump leading to both the temperature difference and the damage. Maybe when the cavity got full with grease it changed the way the seal behaved... or maybe there was some debris that created an abrasive environment. Unsolved mysteries.

edit - deleted my comments about jumper which were off-base.

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(2B)+(2B)' ?

 

[electricpete]

I can't figure out how to attach two files to one post. Here is the 2nd attachment.

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(2B)+(2B)' ?

 

[edison123]

Thanks pete. That 'trainwheel' is a big reveal.

I am also of the view now that this is not a shaft current issue and that nonmetals can eat into the metals.

Now how to solve this? I thought of nitriding the seal area alone but we do not have any good company around here that can do this insitu. I am looking at a 5 to 6 mm shrink fit metal sleeve, which has a nitrided outer surface done outside and then fitted onto the seal areas. But then I have to machine the shaft areas before the seals seating area to get this done.

Or a Teflon seal?

Muthu

http://www.edison.co.in

 

[EdStainless]

Fitting a seal may be your best bet.
You have to machine the shaft anyway, or replace it.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[edison123]

Thanks Ed. I am leaning that way. Will be in talks with the client soon.

Muthu

http://www.edison.co.in

 

[Tmoose]

Speedi sleeves - " The standard size range covers sleeves for
shaft diameters from 11,99 to 203,33 mm (0.472 to 8 in.). "

 

[edison123]

Thanks Tmoose.

Muthu

http://www.edison.co.in

 

[romke]

It could be a lubrication problem. The fact that the seals are there to keep the oil within the bearing does not mean that the seals should not need some form of lubrication, all be it in minute quantities. In this particular case it looks like there was some oil but that there was insufficient flow between seal and shaft, so that in fact the oil got trapped underneath the seal and eventually oxidized to the hard stuff that was capable to cut into the shaft, which if sufficiently compressed can be harder then the shaft material. The cause for it could well be that the seals fit somewhat to tight, either due to their geometric dimensions or the pressure applied through the springs that keep them in place. For the seals to function well there needs to be a very small amount of "bleeding" of oil to the outside of the bearing thus forming a small layer of lubricant between shaft and seal surface.

 

[edison123]

Thanks romke. One thing I saw was that two halves of the seal, when removed from its seating, became oval at the split planes with the ends expanding away from the bore. I guess when they are slid through the grooves in the housing, they do take a round shape. This could possibly exert excess pressure in the seal ID and squeeze out any lubrication film?

Are the seals supposed to be exactly round before they are fitted onto the shaft and housing?

Muthu

http://www.edison.co.in

 

[romke]

Yes. the seals, just like the bearing halves, are supposed to be exactly round when fitted and thus the right amount of force is needed to get that effect. They also should have a "loose fit", meaning that the shaft should turn without undue contact within bearings and seals - a few microns play is necessary, somewhere in the order of 10-50*10[sup]-3[/sup] mm, thus also making it possible for some lubricant to get between bearings and shaft to form a separating and friction reducing layer. Even elastomer lipseals need a small amount of lubricant to prevent undue wear, although the lip will rub continuously against a shaft.

The smoother the surface of the materials used, the smaller amount of play is needed - the lubricant film should have at least the thickness of the sum of the average roughness of the materials involved.

 

[TugboatEng]

Fiberglass seems like an odd choice for seal material. Even glass reinforced Teflon seals require very hard running surfaces to prevent grooving. Perhaps this seal is not supposed to slide on the shaft but rotate with it? Think a labyrinth seal or something similar.

 

[DAVIDSTECKER]

Looking at the surface finish I estimate it to be about a 500rms.
Most shaft riding and sealing surfaces are generally a 32rms for plain bearings.
If that is true then the reduced contact area may be creating high contact pressures on the peaks of the grooves causing wear.

 

[mike gergen]

Read this. Your shafts look like a shaft fretting issue, this site explains it.

http://www.eninepump.com/zhichi/technical_14.html