Thoughts on conductive loctite 3888 for threads with current passing

[USAeng]

This is on a sparking rod. The current travels along a steel rod that is .093" that is threaded at the end with #3-56 threads.... then that rod is threaded into a little brass button.

The company was using loctite 262 (red loctite) on the threads between those 2 items... seemed to work ok but 262 is an insulator. Loctite 3888 is a conductive threadlocker. Do you think the conductive loctite is necessary? The threads are about 3/8" in length so I figured that the metal on metal contact would provide enough surface area as the .093" rod and the loctite 262 would seal the area and protect from corrosion....

I definately think the 3888 would be more suited for this application, but my REAL question is: do you think it would make any difference using the 3888?

I already tried Hinkel(loctite) tech service but the tech guy did not seem to know the answer to this unfortunately.

Thanks for any ideas.

 

[MikeHalloran]

If there's any axial load on the thread while the Loctite is setting, i.e. if there's a jam nut or a shoulder, conductive Loctite is unnecessary because one thread flank will be in intimate contact, and the Loctite will just fill the gap to the other thread flank.

The issue is covered in a Loctite FAQ somewhere, so I'm surprised the Henkel guy didn't know about it. Maybe he's new. Ask an _old_ Loctite guy for confirmation.





Mike Halloran
Pembroke Pines, FL, USA

 

[USAeng]

There is no axial load... the pieces are put together so the only load would just be from the very very small weight of the brass piece.... but I still imagine enough thread contact... what do you think about that?

 

[IRstuff]

A more germane piece of information might be the actual voltages and currents involved. Loctite does not seem to have a dielectric strength value for 262, but other Loctite products have DS up to 15kV/mm.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[MikeHalloran]

I think assembly and (undisturbed) cure in a gravity field would assure some contact between the metallic parts when cure is reached, even without a preload other than the parts' weight.





Mike Halloran
Pembroke Pines, FL, USA

 

[VEBill]

The fact that conductive Loctite 3888 exists indicates that there is probably a requirement for it, at least in certain cases.

Grab a DVOM and see if there is continuity.

 

[OperaHouse]

It might be fine if you never have to adjust or remove it. If it is disturbed there will be barrier sections left in the threads. This could become an issue with higher currents. Conductive would minimize that risk.

 

[btrueblood]

"Sparker rod" = part of an electric igniter, say for a gas burner?

Typical spark voltages in air are in the 10's of kVolts. As IRstuff noted, the dielectric strength of the loctites are in the 10's of kV per mm. So, unless you have a gap in your threaded assembly bigger than 1mm (and I'd bet you have minimum gaps measurable in nanometers or less, as Mike says), you can likely pass the required voltage thru any films of loctite that might get made accidentaly - they break down on the first or second try of the ignition coil, carbonize, and conduct just fine from then on.

 

[itsmoked]

That would be my expectation also.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[OperaHouse]

I never assume anything. I TIG weld with electrodes thinner than that and pump a lot of amps. General questions get a general answer. I think we should all think of the next guy to touch something before we glop stuff on a connection.

 

[USAeng]

Its funny you say that because the wire is a tig wire... just used for a different purpose.

We have been using the loctite 262 without any noticable complications... the loctite 3888 is multiple times more expensive because of the silver, so I wanted to make sure there was nothing I was missing by using the 262... seems like the 262 will be fine. Thank you

 

[btrueblood]

"I TIG weld with electrodes thinner than that and pump a lot of amps."

Right, no loctite on 89-amp cables/connectors is likely a good idea. Typical welding voltage is in the single or low double digits, and without the voltage to punch holes in any films, you will not be able to pass current effectively.

An ignition coil or sparker passes milliamps of current (ok, maybe 10ths of amps, depends), but at high voltages. Different end of the lever.