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.
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Preparation of stainless steel for Loctite threadlocker
- Thread starter bobbkr2
- Start date
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
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.
- Thread starter
- #3
Why a breakaway torque of 100lb?
What is the real loading condition that you are concerned about? Maybe there's another way to look at it.
What is the real loading condition that you are concerned about? Maybe there's another way to look at it.
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mfgenggear
Aerospace
is it possible to stake the edges with a center punch & ball peen hammer.
thread locker should hold 100 in lbs.
thread locker should hold 100 in lbs.
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.
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.
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.
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.
btrueblood
Mechanical
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?
btrueblood
Mechanical
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.
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
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.
btrueblood
Mechanical
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?
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?
TheRocketScientist
Mechanical
Cyanoacrylate [Krazy (R)] glue comes to mind. I sometimes use that for a threadlocker when retention is more critical than a possible future dismantling.
- Thread starter
- #17
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?
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?
![[smile]](/data/assets/smilies/smile.gif "[smile] [smile]")
btrueblood
Mechanical
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.
mfgenggear
Aerospace
prevailing torque
in other words interference fit on threads.
make the same thread except with a larger P.D.
on the male threads.
in other words interference fit on threads.
make the same thread except with a larger P.D.
on the male threads.
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