loctite adhering to aluminum 1

[Bob_3]

Were having issues with steel to aluminum loctited assemblies coming loose. Normally they'll withstand 40 ft-lbs. of torque, but when they fail, it's only at 5 ft-lbs.
We clean and rinse parts, and use primer and #638 retaining compound for strength. When we take the failed assemblies apart, it seems half the loctite is gone, and only residue on the trailing half of the threads remains. Is the loctite being wiped away by the threads?
Maybe the retaining compound is to viscous for threads, but when it works, it work very well. Has anyone had this type of consistancy problems?

Thanks...

 

If the Loctite is failing at a fraction of the normal torque, it is probably not doing its intended job at all.
I suggest you contact Loctite, or at least seek information and recommendations I am sure can be found at their web page.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[hydtools]

Is this a threaded assembly? Use a Loctite threadlocker.

Ted

 

[drawoh]

Bob_3,

Loctite 242 has always worked for me. What Loctite are you using, and what exactly is your material?

--
JHG

 

[Tmoose]

I'd discuss this with Loctite tech - What are you rinsing the parts with, and how are they being dried?

As others have suggested, one of the high strength thread lockers may be more appropriate.

The stated gap filling capability of the 638 product "approaches .25 mm/ .010" ".
The internal clearance of your 1-1/4-7 threads (*) can easily exceed that.

* When information is not provided in OP, I provide my best guess. Yesterday I was working with a dainty 1.25-7 thread, so what else could I assume ??


====.

As far as how/where/when to apply Loctite to the components being assembled, Loctite has something to say -

https://tdsna.henkel.com/NA/UT/HNAUTTDS.nsf/web/FB168836AB931A10882571870000D859/$File/638-EN.pdf

"For Slip Fitted Assemblies, apply adhesive around the leading edge of the pin and the inside of the collar and
use a rotating motion during assembly to ensure good coverage."

I'd think assembling a threaded part would be similar to to twisting together a slip fitted assembly.

 

[BridgeSmith]

I would guess that the aluminum likely has a smoother finish than the steel. There's likely no chemical bond, so thus, very little adhesion. If you want higher strength, you'll likely have to roughen the aluminum surfaces. Run a wire brush across the threads and you'll likely see a much different result.

 

[EdStainless]

Some of the primers have an activation stage, they require humidity, or time, or a min temperature in order to work.
Your may not be triggering all of the time.
Talk to your supplier.

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

 

[kingnero]

Loctite 638 isn't a thread locking compound.
Follow the advice given above.

 

[Bob_3]

Thank you all for your comments, so here's an update to what we've been able to find out.

It seems our overseas supplier is applying a corrosion inhibitor to the aluminum parts since they sit on the water for a month, so the loctite won't cure properly in the presence of nitrates. The short term solution is to wash the aluminum parts, and long term is change the type of inhibitor.

Thanks again...

 

[hydtools]

Have the supplier use sealed packaging for parts instead of applying an inhibitor to the parts.

Ted

 

[kingnero]

What did Loctite say about the choice of product for this application?

 

[Bob_3]

These are threaded assemblies with no abutting shoulder to induce tension, so we rely on loctite only. As stated earlier, #638 was chosen for being far and above the highest strength. Yes, it is more viscous, and the gap filling capabilities rate at .010 in, but that is the maximum gap, not minimum, so #638 is fine for this application.

The inhibitor was not only blocking the metal ions and affecting curing, but also changing the surface tension and wetting ability of the adhesive on the metal.

...Learn something new every day.

 

[Sparweb]

Can you get the supplier to stop applying the inhibitor?
Or are you removing it yourself, now?

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[Tmoose]

"The short term solution is to wash the aluminum parts, ...."

But the original post said - " We clean and rinse parts, ..."

What are the details of the apparently unsuccessful "cleaning ?"