Residual Magnetism - How much is too much?

[Zimann]

I've seen electrical discharges cause problems in ball/roller bearings, and I've seen damaged turbine babbitt bearings from electrical discharge, and I understand how this can happen.
Is magnetism any way associated with this phenomena?

What is the failure mode when the bearing housing is too magnetized? And how much is too much?

 

[Mendit]

Anything more than 3 Gauss can be a problem with high speed turbo machinery.

Damage from Electrical Discharge can ruin a bearing in less than 2 weeks of continuous operation

 

[electricpete]

how does residual magnetism lead to damage in turbo machinery? To my way of thinking, there is no problem unless there is a rate of change of flux through some loop. If magnetized part is stationary and shaft is not moving significantly (other than rotation), I don't understand it.

I do remember that I have asked SKF about possible adverse effects of rolling bearings which may be inadverantly magnetized by improper use of induction heaters (many of the bearings I inspect after removal are magnetized). They stated that it had no adverse effect.

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[sms]

Pete the problem is when the rotor is magnetized, typically due to a wet mag inspection during a overhaul that was not properly demagnetized. Then you generate some nice currents... Since the shortest path to ground is through the bearings it tears up the bearings big time.

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[sms]

One of the best resources on this subject

http://www.sohreturbo.com/articles-tech-info/

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[sms]

Sorry Pete, I didn't read the OP very well. You answered exactly right. If it ain't moving, residual magnetism should not matter.

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[Zimann]

Thank you - Since posting the question, I have read the Sohre and found a few others, including one from Alexander Leyzerovich:

http://pepei.pennnet.com/Articles/Article_Display.cfm?Article_ID=44279&pc=gls

(not sure how to make it a link)

I suppose the most important aspect is that the shaft must have some field of its own as well, then the rotational speed of the shaft inside the stationary magnetic field is relevant. I have found several sources say that greater than 3 Gauss can quickly lead to issues. - That's for ball/roller and sleeve bearings.

 

[Hollywood62]

I am in need of finding a company (consultant) who can help me in the analysis of a possible shaft current/bearing damage situation at a project my firm was involved in possibly due to shaft bearing currents, improper shaft grounding, or turbine grounding.

Does anyone have a recommendation for a consultant knowledgeable in this area?

 

[Zimann]

"sms" mentioned SohreTurbo.com. They sell the equipment that you would need to remedy those types of situations - but that being said, they have reviewed numerous similar situations and can provide an opinion (and potentially an "official" report if requested). Keep in mind that they are selling the grounding straps, etc.
Their website has quite a few interesting pictures.

 

[unclesyd]

You might want to give these people a call. The Hickham component of Sulzer did have a couple of people that were experts in the field of stray currents in rotating machinery. If they are not around Sulzer should be able to point you in the right direction,

http://www.sulzerts.com/desktopdefault.aspx/tabid-92/

 

[unclesyd]

Caught one of the turbo guys in the shop and he said check this company out.

http://www.gaussbusters.com/index.htm