silicon weather sealant around dc motor brushes/commutators 9

[electricpete]

I'd like to revisit this topic yet again.

Previous review:
thread237-9155
thread237-178054
thread237-224692
thread237-32685

http://www.morganamt.com/us/files/Morgan Carbon US/MaintenanceConcerns.pdf

DO NOT USE SILICONE SEALANT TO MAKE THE MOTOR WATERTIGHT

http://www.geindustrial.com/publibr...wal Parts|DCMotors|generic&wwparam=1367260697

Silicone, RTV, or Bathtub Seal/Caulking is not recommended for sealing DC motors

QUESTION 1 - Any further good references about this? Our repair shop said they wouldn't hesitate to use silicon sealant on a dc motor cover plate as long as it is not near Hydrogen sulfide like paper mmill. What is standard repair industry practice?

QUESTION 2 - What are alternative weather proof sealants? Something with thick consistency for example to fill a 1 / 16" gap?

I found the following alternatives, but don't know much about them.

http://www.morganamt.com/us/files/Morgan Carbon US/MaintenanceConcerns.pdf

“Urethanes, polyurethanes or elastomeric adhesive sealants are recommended for use around brush type motors.”

http://www.geindustrial.com/publibrary/checkout/DCMotors?TNR=Renewal Parts|DCMotors|generic

Titeseal
Titeseal is the preferred sealant used on waterproof motors to seal fits and bolts. It is non–hardening,
non–silicone. GE#8064200000.”

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[Skogsgurra]

Standard practice is to avoid anything with silicon. You need to be a very clever chemical scientist to know what you can use and what you cannot use.

I have seen microscope pictures of a commutator surface that looked lika an emeraud paper from some cemical union between copper and silicon. Very abrasive. That commutator consumed brushes in a week.

Gunnar Englund

http://www.gke.org

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

 

[ScottyUK]

We have used a few things - urethane is a bad choice because it is a damned good adhesive. Cover removal is slightly awkward if you install it with a urethane based adhesive, shall we say. Wonder if any of my former colleagues remember that...?

A non-hardening building mastic of the type used for expansion joints was fairly effective. I have no idea of the chemical makeup, but it wasn't a silicone based material. A quick Google of building mastic suggests it may be an oil-based material, but it is a good few years ago and the exact brand is long forgotten.

 

[rhatcher]

The cover plate should be sized to cover the opening with a minimum of 1/4-1/2" overlap outside of the bolt holes on all sides and a cork, rubber, or dense foam rubber gasket should be used to seal the cover plate. The thickness and type of gasket is determined by the fit (flatness) of the cover. A properly selected gasket and properly fitted cover will not require additional sealing even in marine environments. Also, the cover plate can be steel, aluminum, or plexiglass. In my opinion, plexiglass is preferred on at least one side (the side that is most visible to passing maintenance personnel) to allow easy viewing of the commutator to confirm proper (non-sparking) commutation and, in some cases where possible, to allow judgement of brush wear.

Silicon sealants should never be used on any part of a DC motor nor on any equipment in proximity of a DC motor

 

[mikekilroy]

It is worth stating clearly that silicone sealant itself is not the problem; it is the outgassing from it as it cures. So as I understand it from my dc brush motor days, silicone is fine to use - as long as it is used in a place that does not let the outgassing DURING CURING reach the brushes. BTW, all of our high performance brush servo motors used silicone insulated wire for the internal leads - good to like 600F temp. Only once did I see an issue with this: have a short in a motor that caused hi enough current to make the silicone insulation DISAPPEAR! Motor looked new inside other than the totally bare lead wires! I found out that day that silicone insulation turns to gas and disappears rather than melts....

 

[electricpete]

Thanks for all the input.

As Gunnar pointed out, there seems a widespread practice/belief that silicon simply shouldn't be used anywhere near DC motors. I was surprised to hear comments from our shop that they don't view it that way.. .there are simply some conditions when silicon can get you in trouble and as long as we pay attention to those we're ok (they say).... seems somewhat similar to mikekilroy's comments.

Those are somewhat contradicting views. I will point out that while I've heard it said (on the phone by our shop and on forum by mikekilroy) that silicon can be used safely with proper considerations, I haven't seen it in writing from authoritative source. I'm not questioning anyone's knowledge but in general comments from individual does not hold the same weight as comments from brush manufacturer (National Elec Carbon link above). On the other hand, we know manufacturers sometimes fail conservative with respect to anything to do with their product (SKF recommendation to replace all grease after 3rd relubrication interval comes to mind). On the third (!) and final hand, in face of ambiguity we usually can't go wrong choosing the conservative alternative when practiceable.

It sounds like there are some reasonable sealing alternatives. Good point Scotty about avoiding a sealant that will act like glue... would make brush inspection a headache. Neoprene gasket (as Ray suggested) is what our local shop recommended for customers like paper mills that want to avoid silicon. Here's what they steered us to:

http://www.armacell.com/

http://www/armacell/ACwwwAttach.nsf...001-NANA.pdf/$File/Ensolite-IR42-001-NANA.pdf

Are these type gaskets re-usable (removing and reinstalling covers several tiems) as long as inspection shows them good?


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[electricpete]

Does similar concern for silicon apply around brushes used on slip rings (wound rotor motors and sync machines) ?

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[electricpete]

Question 3 - What do you think of this configuration/approach: Joint has neoprene gasket and is assembled/tightened fully and THEN silicon applied outside. No chance for silicon fumes at time of assembly. By the time the joint is next disassembled, the silicon is fully cured, no fumes.

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[IRstuff]

puleez... it's silicone and not silicon ;-)

This might be a useful read:

http://www.dowcorning.com/content/publishedlit/The_Use_of_Silicone_Materials_in_Close_Proximity.pdf

TTFN
faq731-376

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[ScottyUK]

Interesting paper - I remember silicone contamination from when I worked in thick film hybrids a long time ago, but understand the contamination mechanism a little better now. Thanks.

 

[Compositepro]

Great link, IRstuff. I learned very valuable knowledge today. I was afraid that this topic was going to be left at a possible myth level.

 

[mikekilroy]

fwiw, we use rubber o-rings between motor endbells and stator - for IP65 sealing.... to get better IP67 sealing we simply change the rubber (neoprene? not sure if same) o rings to viton. Viton holds seal better than rubber, between hot/cold cycles; allows significantly better sealing rating....

 

[rhatcher]

Pete,
I am not smart enough to evaluate the data for the rubber sheet that you are considering and to answer your question about whether it is suitable for this application or whether it can be re-used. However, I will say that in a general case that rubber gaskets are reusable if not damaged during assembly or disassembly.

Damage during assembly is primarily caused by overtightening that squeezes the gasket out of the joint and can cause splitting at the bolt holes. In this case, the gasket no longer seals the motor and can cause a problem if not detected early on.

Damage during disassembly can occur if the gasket is stuck and is torn when the cover is removed. In either case, rubber gasket material is relatively cheap and, if replacement is required, would be, at most, a fractional cost increase compared to the total cost of reconditioning or repairing a DC motor.

Next question; In my experience silicone is only a concern only for DC motors. This is because of the arcing caused by commutation. Even if the arcing is not visible, the induction of the coils being commutated, by definition, results in an arc. This arc, even if microscopic, is apparently enough to cause a reaction with the silicone over time. Slip ring motors, whether synchronous or wound rotor, do not have arcing as a 'normal' condition of operation. This being the case, apparently silicone is not a problem for slip rings.

Final question: Do not use silicone...period. I have experience with dock (pier) cranes for ports and shipyards that indicates that a proper rubber gasket is all that is needed to keep liquid water out. However, I will add that in these conditions (high humidity environment) that some heat is recommended when the motor is not running to prevent condensation. Field economy circuits are great since they require no additional equipment but, if this option is not available, then separate heaters are recommended.

I hope this helps.

 

[mikekilroy]

Next question; In my experience silicone is only a concern only for DC motors. This is because of the arcing caused by commutation. Even if the arcing is not visible, the induction of the coils being commutated, by definition, results in an arc. This arc, even if microscopic, is apparently enough to cause a reaction with the silicone over time. Slip ring motors, whether synchronous or wound rotor, do not have arcing as a 'normal' condition of operation. This being the case, apparently silicone is not a problem for slip rings.

 

[mikekilroy]

guess I found submit button before writing anything.... no edit button....Does someone know something new about cured silicone? If I am behind the times, please correct me. but as I understand it, CURING silicone goop outsources gases that go into brushes and causes them to soften, thereby wearing very quickly. But once cured, it has NO outsourcing of gasses and thus is NOT harmful to motors whether arcing or not! Please correct me if somehow silicone spends the rest of its cured life outsourcing gases and thus shrinking smaller and smaller. Otherwise, it should be clear that CURED silicone has NO negative effects to brushes, commutators, or arcing.

 

[electricpete]

Thanks IRStuff. Not silicon as in chips but Silicone. I will have to try to implant that in my brain.

Looks like a good detailed article from Dow Corning, I will study that.

but as I understand it, CURING silicone goop outsources gases that go into brushes and causes them to soften, thereby wearing very quickly. But once cured, it has NO outsourcing of gasses and thus is NOT harmful to motors whether arcing or not!

Good question. From the Dow Corning article:

This tool performs a calculation based on the perfect gas law from the vapor pressure of the volatile species present in the materials after cure

So it seems there is still a hazard after cure if I'm reading it right.

Revisiting the paper some more - they calculate expected concentration and the threshhold for danger is 10-30ppm. The basis seems to be the vapor pressure shown in Figure 2 which varies as a function of type of silicon and temperature.

The thing I don't quite understand yet - looking at "Case 1" and "Case 2" of the Dow article, they calculate the steady state concentration for a totally enclosed container. The variables of interest seem to include the volume of the sealed container and the area and depth of the silicon. But if it is a simple equilibrium concentration calculation, it should depend only on the vapor pressure, the volume of container and amount of silicon wouldn't matter (they would matter for time to reach equilibrium but not equilibrium). I'll have to study some more to see if I'm missing something (unless anyone can offer an explanation?)

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[electricpete]

The thing I don't quite understand yet - looking at "Case 1" and "Case 2" of the Dow article, they calculate the steady state concentration for a totally enclosed container. The variables of interest seem to include the volume of the sealed container and the area and depth of the silicon. But if it is a simple equilibrium concentration calculation, it should depend only on the vapor pressure, the volume of container and amount of silicon wouldn't matter (they would matter for time to reach equilibrium but not equilibrium). I'll have to study some more to see if I'm missing something (unless anyone can offer an explanation?)

The reason I bring this up... resolving that question may shed light on mikekilroy's comment/question. It would make sense to consider volume of the container and volume of silicon if there is a certain amount (mass) of silicon vapor released per volume of silicon which is released during cure. I doubt I will be able to solve the model quantitatively but I'd like to try to understand the basic thought process they're using for this calculation.

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[mikekilroy]

thank you electricpete for your research! if indeed there is some kind of outgassing that continues in small amount after curing, this will be really good to know. I wonder how silicone sealer mfgrs can guarantee their products 20, 30, 50 yrs if it continues to outgas after curing... thanks!

 

[Compositepro]

I'm fairly familiar with silcone chemistry, so here is some background and a summary of my understanding from reading the article, above.

Most silicone RTV (Room Temperature Vulcanizing) sealants release acetic acid (vinegar) during cure. This is corrosive, so electrical grades will be designed to release methanol instead, which is non-corrosive. There are also silicones that are platinum catalyzed that do not release anything during cure. These are heat cured and if they are designed to cure at room temperature they will come in two parts that you mix just before use.

It is volatile, low-molecular weight, silane impurities in the silicone polymer that causes problems with commutators in motors. Silanes are the monomers that silicone polymers are made from. These silane molecules in the air will become silica (silcon dioxide, quartz) in an arc. Silica particles are very abrasive. I would also surmise, based on how they are formed and the problem that is caused, that these particles tend to get in-bedded into the commutator or brushes.

The silane vapors could only build to problem levels in a sealed space.

 

[rhatcher]

compositepro, that was a great (lps) answer!