Preparation of stainless steel for Loctite threadlocker

[bobbkr2]

Does anyone know of a preparation done to stainless steel to increase the breakaway strength? Perhaps to make the material more active? The threads are 5/16-32 316 stainless steel. Any other suggestions other than Loctite? I would like to reach a breakaway torque of 100 in-lb.

 

[bobbkr2]

The threads on the OEM part is indeed 5/16-32 UNEF (class 2). The threads on my new parts are duplicates of the OEM parts.

 

[drawoh]

bobbkr2,

So, since these things are valves, we can assume there are holes through them, reducing the stress area and the allowable force and torque?

JHG

 

[plasgears]

We fastened sub-fractional motor shafts to rotors with a heat cure loctite product and subjected the assy to induction heat. Our initial torque was nil, but it cured to sufficiently high torq. It was on an automated turn table. Cycle time for each was about 30 sec. There was little or no initial lube, just slight residual stamping lube on the rotor laminations.

 

[TheRocketScientist]

Interference fit on SS fasteners is asking for a LOT of galling issues. SS galling leads to siezure and permanent mating [welding] of the nut and screw, and it must then be twisted off and discarded.

That would be great IF it happened after your desired torque was attained, but typically it happens midway during the securing process.

 

[drawoh]

bobbkr2,

If this is stuff you are designing, there might be another strategy.

Your stainless steel parts have shoulders that clamp together. The O-ring sits in a groove which controls the compression. Now, you can tighten everything down hard.

JHG

 

[ChrisDuncan]

What about castellated nuts and cotter pins?

As already mentioned there are several grades of loctite, 3 actually, 271 is the strongest.

 

[bobbkr2]

I am almost positive that on the OEM assembly (that I am trying to duplicate) the threads are bonded together with a cyanoacrylate glue. I tried some cyanoacrylate glue with a shear strength of 2500 psi and achieved a breakaway torque of 80 in-lb. I am waiting for some samples of a higher strength glue to test, hoping to reach 100 in-lb. Getting closer!

 

[drawoh]

bobbkr2,

You are 80% of the way there. Could you increase the thread engagement by 25%?

JHG

 

[bobbkr2]

drawoh,
Nope. Can't change a thing.

 

[bobbkr2]

Just thought I would let everyone know that I found a solution to my problem.
I was able to bond the two threaded parts together using a cyanoacrylate glue with a high shear strength, that is made for bonding metal to metal. It is Cyenergy 6103 available through Ellsworth Adhesives. I was able to obtain a breakaway torque of 110 in-lb.
Thanks again for all the help.

 

[mrpi]

I'd be nervous about using adhesive to bond threads to get a minimum break away torque for anything I'd be selling.

If this is for personal use, its probably not a big deal.

I think surface preparation of the two parts will be critical to achieving consistent results. It may have worked once, but can you guarantee that it will work on the 100th try???

Loctite makes primers that are designed for materials with low surface energy, they've been very helpful when I've called them.

Also, have a look at the data sheet of the product. For Loctite products, the cured strength of the test parts they use is often 1/2 (or less?) of the shear strength of the base adhesive??? Something like that.

How about milling a slot in the male threads, then peening the nut into the slot, like on some automotive spindle nuts (vw comes to mind).

Or how about a clamping nut? Where its split axially with a pinch bolt, so after you tighten it, you tighten up the pinch bolt for more radial force on the threads? I've seen that on VW spindle nuts too.

Or another idea that comes to mind is a shoulder on the bolt and a wave-washer to get the axial pre-load on the seals?

Sounds like you've about arrived a solution that will work for you. Just be careful if you're using the cyanoacrylate as a thread retainer if its not specifically designed for it.

Beat to fit, paint to match.