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Permanently Locking Threads 3

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paulcook

paulcook

Mechanical
Oct 2, 2023
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I have two parts that need to be permanently threaded together.

One is cast aluminum with a 1/2-14 NPSM female thread.

The other is galvanized steel 1/2-14 NPSM nipple.

We currently use Loctite (the glue stick ones, primer green + high strength red) to keep these parts from rotating relative to each other.

Is there another way that we can thread these together so they never come apart/rotate relative to each other?
 
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@drawoh

We're using 268. It's been generally fine, but I had one fail. Maybe I didn't use enough.

Would love a mechanical solution, but obviously hard for ya'll to suggest without seeing the product.

@JohnRBaker

Any specific type you'd recommend? Easy application would be critical factor. Also must seal the threads (which the loctite does) and cure relatively quickly (<12 hrs to 75%).
 
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I suspect that any high quality epoxy will work. Around the house, I've been using Clear Weld Professional Grade Quick-Setting Epoxy from J-B Weld. It's always worked for what I needed it for. Granted, I can't recall where I had to immobilize some threaded pipe fittings, but I suspect that it would have been more than adequate.

John R. Baker, P.E. (ret)
Irvine, CA
Siemens PLM:

The secret of life is not finding someone to live with
It's finding someone you can't live without
 
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There are some prevailing torque thread designs that will not unthread.
But you will be limited by the strength of the Al.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
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I spec Loctite Red all the time... it's my 'go to' glue...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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@EdStainless

Any specific design? Distorted thread?

@JohnRWalker

Doubt we'll get more performance out of JB weld, but worth a shot! I've been looking at MMA epoxies. Maybe we can try a few.
 
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Acrylic adhesives tend to have stronger bonds than epoxies. Loctite 268 is an acrylic adhesive.

The bond strength varies a lot based on material. The cured strength on zinc chromate is very low. Consider acid etching the threads of the steel component to remove the zinc chromate coating.

Screenshot_20240306-114830_yotxxe.png
 
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I would bet the joint that failed did so from using insufficient material (unlikely as most people use far too much) or the threads were contaminated with oil or grease. It could be someone used a shop rag to wipe down a thread to "clean" it.

You can try epoxy, but if locking compound isn't working I don't see that epoxy will be better. The options to screw up epoxy are larger.

The main difference is epoxy will generally cure all on its own while anaerobic adhesives need to be removed from oxygen and so won't cure outside a joint and won't cure without primer on non-reactive metals.

I think there are primers that brush on and then are let to dry and those should give a better result than a solid stick. Use in conjunction with Locktite 271. Be aware that 271 removal instructions include a blowtorch while installation instructions include clean and dry surfaces and shaking the bottle.
 
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Thanks @TugboatEng - The part will see rain, so not keen on stripping the coating... maybe a material change is in order but I don't know what to switch to. FWIW we are using the primer.

@3DDave - thanks for the tips. We are using the primer. I think I just didn't use enough (has this "less is more" refrain from the vendor playing in my head...). We tried liquid with spray primer initially - the sticks worked better for some reason.
 
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I did state to strip the coating specifically on the threads. You can also mask the threads during the initial coating process.

Consider substituting electroless nickel for zinc plating. Thread lockers work better with nickel than zinc. It will also provide better corrosion resistance.

Is your aluminum anodized? Thread lockers don't cure well with anodized aluminum.
 
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A few drops of wicking Loctite 290 does a REAL good job creeping all the way thru a well prepared joint.
290 is not as strong as some of the retaining compounds, but if there is plenty of thread engagement it will most likely get a good grip on them all.
I don't think that will be the case for any epoxy, and maybe even for thicker or solid thread locking compounds.
 
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Consider Loctite 554 thread sealant with SF-7649 primer.

Aluminum and zinc are both difficult materials to bond. Aluminum is subject to hydration failures at the bond line, making long term reliability questionable unless extraordinary measures are taken to prepare the surface.

I don't know about zinc.
 
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It sounds like these materials are not ideal candidates for threadlocker. And yes, the aluminum part is anodized (thanks @TugboatEng - can't believe I didn't think to mention).

We really have tried a range of threadlockers, and this combo worked better than anything.

@dvd can't tell if you're serious lol - I think the tolerances would be too demanding.
 
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Anodizing mostly stops the cure where zinc chromate results in a poor bond . There are accelerators you can use to improve cure time with anodized parts.

However, you can also mask the anodizing so that the threads don't get treated.

You don't need corrosion resistance in the interfaces because the thread adhesive seals them.

The interference for mentioned above also seals the interface but is extremely sensitive to surface damage. A clearance fit and retaining compound is superior but we run into the same strength issues with your surface treatments.

AND, aluminum can also be treated with electroless nickel. If you simply use electroless nickel to treat both parts you should be able to achieve a stronger lock between the parts using standard thread adhesives.

I do admit that I don't think very highly of electroless nickel due to a limited amount of bad experiences but it's "supposed" to be the best corrosion resistant treatment for either metal. My environment is salt spray all of the time and I am very aware that some vendors are better at applying the coating than others.
 
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