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Preparation of stainless steel for Loctite threadlocker

Preparation of stainless steel for Loctite threadlocker

Preparation of stainless steel for Loctite threadlocker

(OP)
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.

RE: Preparation of stainless steel for Loctite threadlocker

bobbkr2,

   I did not read your previous post all that carefully.  The Loctite products I mentioned, work.

   5/16-32 is a weird thread.  Normal threads are 5/16-18UNC and 5/16-24UNF.  According to a spreadsheet I have prepared, you can apply 122 and 135lb.in torque respectively.  I assumed the threads were lubricated, with a torque factor of 0.15.  I assumed we were torquing to 90% of a yield stress of 55,000psi, quoted for stainless steel cap screws by a local vendor.  The resulting clamping force will vary a lot because the torque factor is hard to predict, but you are asking about torque.

   Unless your 5/16-32 threads are hollow, you can use a torque wrench, and apply well over your required 100lb.in.  Any threadlocker you apply should increase that.   

               JHG

RE: Preparation of stainless steel for Loctite threadlocker

(OP)
drawoh,

The parts are assembled and tightened to only 25 in-lb. Just enough to adequately compress the gasket between them. My problem is that I want the 2 parts permanently locked together. I'm striving for a breakaway torque of 100 in-lb.

RE: Preparation of stainless steel for Loctite threadlocker

bobbkr2,

   If you want something to stay tightened, your first strategy is to tighten it down hard.  Why are you applying only 25lb.in torque?

   How stable is your gasket around the threadlocker?

               JHG

RE: Preparation of stainless steel for Loctite threadlocker

What's your gasket material? Do you have to clamp through the gasket? Could the gasket be recessed from the bolted connection?

RE: Preparation of stainless steel for Loctite threadlocker

is it possible to stake the edges with a center punch & ball peen hammer.

thread locker should hold 100 in lbs.

RE: Preparation of stainless steel for Loctite threadlocker

I'd talk with the gasket manufacturer about how much pressure I could put on the gasket. If you're using a rubber/cork/nitrile/etc gasket, they often require a percent compression instead. You can accomplish this by recessing the gasket from the clamping face inside of a groove.

But perhaps you could just forget the gasket and weld the parts together if you need a permanent connection.

As I mentioned in your previous post, you could also lock-weld the bolts to keep them from moving. This simply means putting a small fillet between the bolt head and the substrate to keep the bolt from backing out.

There are plenty of other strategies involving locking features, but as drawoh mentioned, a proper bolted connection is usually torqued quite a lot - close to the yield strength of the bolt. The physics of the problem drive you in that direction.

RE: Preparation of stainless steel for Loctite threadlocker

If you're only using 25 in lb, that's an issue because now you're relying on the shear strength of the adhesive to accomodate the shear loads.  With sufficient shear area, any adhesive should be sufficient.  The more shear area, the higher the torque that can be accomodated.  Obviously, the amount of shear area for your situation, isn't sufficient to get the breakaway torque up to where you want it.  I see two options.
1) Increase the shear area.  I suspect this isn't a good option though.  A very large diameter head on the bolt would work, assuming you can put adhesive under the head.  I'm envisioning a bolted flange joint or similar.  Having a special bolt with a very large head might work, but you could do the calculation yourself.  Assume a shear strength of around 1000 to 2000 psi for typical epoxy adhesives.  Might will be much better than that, but I'd use that as a start.
2) Use a stronger adhesive.  I doubt you'll get much stronger than epoxy, so you might consider something like solder or silver solder.  That obviously requires heating the part considerably which could damage other parts (ex: the gasket).  If the bolt gets hot in service, that might be an issue you'll also need to consider since it will weaken the solder, but then again, heat will also damage adhesives so I assume you've already considered that.

RE: Preparation of stainless steel for Loctite threadlocker

From a CBIA pamphlet that I have:

Quote:

To enhance the strength of bonded joints, etch the stainless steel in a solution of:
Oxalic acid* 14.0 kg
Concentrated sulphuric acid* 12.2 kg (6.7 1)
(s.g. 1.82)
Water 70.0 litres
Immerse for 10 minutes at 85-90C, remove from the solution and, under clean cold running water, brush off the
black deposit with a clean stiff brush.
Dry with hot air.
Bond as soon as possible after pretreatment has been completed.

I doubt that it will double your bound strength though.

RE: Preparation of stainless steel for Loctite threadlocker

Loctite makes a primer/activator for use with their products, and the "rated" strength for stainless fasteners given in their spec. sheets for all of their threadlockers indicates that the best strength is obtained with the activator.  You asked this question once before on this forum?   

RE: Preparation of stainless steel for Loctite threadlocker

Ah, yes you did.  In there, you never mentioned if you used the primer/activator recommended for the threadlocker you used, or if you waited the prescribed interval for full curing before testing the breakaway torque.

RE: Preparation of stainless steel for Loctite threadlocker

bobbkr2,

   What sort of control do you have over the design?  If you can place your gasket in a machined pocket, the pocket controls the gasket's compression.  You can tighten the nut down has hard as the 316 stainless will allow.

   A bolted joint that includes a gasket as an elastic member is way more complicated than a joint with only metal components.

               JHG

RE: Preparation of stainless steel for Loctite threadlocker

What is the thread engagement length for the screws you are using?

Have you tried a tighter-fitting fastener?

You mentioned the OEM fasteners, indicating you are replacing the original screws...are you sure the OEM thread is not a metric 8mm x 0.8 thread?

RE: Preparation of stainless steel for Loctite threadlocker

Wups, meant 8mm x 0.75 pitch.   

RE: Preparation of stainless steel for Loctite threadlocker

Cyanoacrylate [Krazy (R)] glue comes to mind. I sometimes use that for a threadlocker when retention is more critical than a possible future dismantling.  

RE: Preparation of stainless steel for Loctite threadlocker

(OP)
Thanks to everyone for all your responses.
In reference to the initial tightening torque of 25 in-lb., this must be this low because I am clamping some very fragile components between the two stainless parts (valve bodies). I cannot have the two bodies turn (and therfore crush the components) after they are assembled and also when subsequently installed into the instrument. That is why I need to rely on the adhesive to bond the parts and provide the necessary 100 in-lb. of breakaway torque. I trust that this all makes sense.
BTW, I did look at the OEM part under a microscope and the adhesive that was used is clear-white in color. Cyanoacrylate?

RE: Preparation of stainless steel for Loctite threadlocker

Quote (btrueblood):

...You mentioned the OEM fasteners, indicating you are replacing the original screws...are you sure the OEM thread is not a metric 8mm x 0.8 thread?  

   Actually, that could be a solution to his problem.  If you can screw an M8x0.8 or M8x0.75 nut onto a 5/16-32UNEF screw, it ain't coming off!  M8x0.8 might be too close a match. smile

               JHG

RE: Preparation of stainless steel for Loctite threadlocker

Yes, drawoh, I actually thought the OEM might have deliberately made mismatched threads...and then thought the "weird thread" might be close enough to a metric size...but then, that's crazy talk.

RE: Preparation of stainless steel for Loctite threadlocker

prevailing torque
in other words interference fit on threads.
make the same thread except with a larger P.D.
on the male threads.

RE: Preparation of stainless steel for Loctite threadlocker

(OP)
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.
 

RE: Preparation of stainless steel for Loctite threadlocker

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

RE: Preparation of stainless steel for Loctite threadlocker

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.

RE: Preparation of stainless steel for Loctite threadlocker

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.  

RE: Preparation of stainless steel for Loctite threadlocker

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

RE: Preparation of stainless steel for Loctite threadlocker

What about castellated nuts and cotter pins?

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

RE: Preparation of stainless steel for Loctite threadlocker

(OP)
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!

RE: Preparation of stainless steel for Loctite threadlocker

bobbkr2,

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

               JHG

RE: Preparation of stainless steel for Loctite threadlocker

(OP)
drawoh,
Nope. Can't change a thing.

RE: Preparation of stainless steel for Loctite threadlocker

(OP)
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.

RE: Preparation of stainless steel for Loctite threadlocker

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.

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