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PostPosted: Mon Dec 29, 2014 7:44 am 
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Hi,

I'd like to get an estimate of what pressure a threaded connector will fail at. (I would like to be able connect a pressure relief valve into a 1377 valve.) I did some searching on the internet and there seems to be a formula that gives pullout force of a bolt assuming adequate thread engagement, and some rules of thumb for what constitutes adequate engagement.

Pullout stregth is described as shear area * shear strength, and a calculator for shear area is available here: http://www.engineersedge.com/calculator ... 0a_pop.htm

Assuming both components are made of aluminium with 30k shear strength and an 8-32 thread, using pitch diameter of .1437 the calculator gives the shear area as 0.01007. Multiplying by the 30k shear strength for 6061 gives a failure force of 302.1 pounds.

Now, I'm assuming that the force in this case is the pressure inside the valve acting on the surface area of the the bolt. To be conservative, let's use the major diameter to calculate the area being acted on: pi*(0.164/2)^2 gives 0.0211 square inches.

So, if I divide the failure force of 302.1 by the area being acted upon of .0211 square inches, I should get the pressure that would cause failure? That comes out to around 14ksi which seems huge - did I get it right?

I found a couple of indications that 1.5 to 2 x the nominal diameter should provide for sufficient thread depth, so I will check the wall thickness against the diameter that I plan to use and recheck the estimate if necessary.

I was thinking that I would need to remake the valve in brass or steel, but so far the calculations suggest that aluminium should be ok, as long as I execute the threads well. I don't actually know what pressure a 1377 runs at, but I'm guessing that a simple pump like this probably struggles to get over about 1500 psi.

Many thanks for any advice you can give,
Jim


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PostPosted: Mon Dec 29, 2014 12:45 pm 
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I don't think that is the proper calculator.... as I don't see the thread engagement anywhere.... I think that calculator is to determine the area for calculating the tensile strength of the fastener.... I use half the area of the cylinder represented by the thread engagement.... because half the thread is in the hole, half on the bolt....

Pitch diameter = 0.1437" gives a circumference of PI times that or 0.45".... With an engagement of 0.10", and the 1/2 factor you get 0.0226 sq.in., which at 30K shear strength is 676 lbs. to failure.... More thread engagement would give a higher value....

The area of the end of the screw subject to pressure you calculated correctly at 0.0211 sq.in.... At 3000 psi, that load would be 63 lbs.... so even with only 0.1" thread engagement you have at least a 10:1 safety margin.... certainly not a problem....

Pressure inside a 1377 valve with FTP can reach 1500 psi or higher.... It takes about that pressure for the valve to start retaining air with stock springs....

Bob

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Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
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PostPosted: Mon Dec 29, 2014 2:17 pm 
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Many thanks Bob. I got concerned when I discovered my wall thickness wasn't enough for the recommended 1.5 to 2 times the major diameter of any useful screw size, so I started working with #4-40 as the smallest I could make. It will be far easier and physically stronger now that I can safely work on the original 8-32 idea.

Jim


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PostPosted: Mon Dec 29, 2014 2:55 pm 
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What is your wall thickness?.... ie how much thread engagement?....

Bob

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Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Mon Dec 29, 2014 3:37 pm 
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I have .133" of wall to work with. Less a little for the flat to seal against. Should still be more than the .1 in your example.


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PostPosted: Mon Dec 29, 2014 4:01 pm 
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The load is so small (63 lbs.) it's not a problem.... How are you going to seal it?....

Bob

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Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Mon Dec 29, 2014 5:37 pm 
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Funny you should say that. My plan was to trap an o-ring between flats on the outside of the valve and whatever was being screwed in to the port. When it was threaded for #4-40 I did a quick test using a hex cup-head screw and it worked perfectly. But now, with the real thing I'm having trouble getting a seal. I'm going to try making a washer out of ptfe or delrin and see if that will do the trick. There's a small chance the leak is coming from elsewhere, so I may have to do some bubble testing to double check.

So close!

Jim


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PostPosted: Mon Dec 29, 2014 6:19 pm 
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If the O-ring isn't captive on all sides, it is likely expanding and leaking....

Bob

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Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Mon Dec 29, 2014 10:06 pm 
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The worst leak turned out to be from the 1/8 npt connection to the pressure gauge (I thought it would be interesting to be able to see what pressures were being generated, so the first thing I made to plug into the port is an adapter for a 1" gauge). There's still one or more slow leaks that I'll have to chase down eventually, but at least I can start to get the feel of the thing now.


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PostPosted: Mon Dec 29, 2014 11:32 pm 
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Great dialogue between the two of you. Great mixture of scientific innovator and front-line tester -- and you both have balancing skills in both. Would love to see more discussion involving the both of you. As a frequent follower of both posters, Collaborative/Comparative posts on an agreed-upon subject and conclusion would be amazing.


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PostPosted: Tue Dec 30, 2014 8:08 am 
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Heh, glad you're enjoying it. It's kind of like the master and the apprentice. "Patience grasshopper, you drill your holes in the wrong places!". Bob has done so many amazing builds, I'm like a kid in a candy shop trying to work out what I can follow along with :)


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PostPosted: Wed Dec 31, 2014 11:44 am 
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rsterne wrote:


Pressure inside a 1377 valve with FTP can reach 1500 psi or higher.... It takes about that pressure for the valve to start retaining air with stock springs....

Bob


13-XX stem return springs vary greatly from the factory. I've seen some retain air at as little as 3 pumps on the ftp...

There are other factors than the spring that affect air retention.
Dunno if there's any 13-77's left in the supply chain with the restrictor plate under the transfer port de-tune- but they have been around.

Regards,

Doc Sharptail

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