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how to attach high-stress fittings

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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how to attach high-stress fittings

Unread postAuthor: FishBoy » Sun Mar 22, 2009 6:23 pm

I want to attach a fitting to the 3/16" steel (Al?) rigid tubing which is the output on my fridge compressor so that it will have a 1/4" threaded end. It will most likely be used up to around 500 psi and I am not sure as to the best way to attach the fitting.

Ideas-

1. Score the tubing and JB-Weld the fitting on; I do not how strong JB-Weld actually is.

2. Weld it, pretty straightforward.

3. [Submit your possibly better suggestion here]
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Unread postAuthor: jook13 » Sun Mar 22, 2009 7:21 pm

Man, my fridge had 3/16 or 1/8 copper output, which 1/4 inch copper just so conveniently fit nicley over, so I sodered it.
JB Weld (or other appropriate epoxy, I like marine epoxy) would work fine, just make sure to clean and roughen up the surfaces well and give it 24 hours to set up.
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Unread postAuthor: psycix » Sun Mar 22, 2009 7:21 pm

I used a compression fitting.
But instead of the metal ring, a piece of rubber bike pump hose.
Holds 30 bar without any problems when tightened down firmly.

I believe you can use a 1/4" threaded fitting with an endcap, but then with an hole drilled through the endcap.
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Unread postAuthor: Gippeto » Sun Mar 22, 2009 9:50 pm

I'll second the recommendation for a compression fitting.

But;

Why would you replace the ferrule (olive to some) with a piece of rubber?

Compression fittings were designed to work with a ferrule, and they do. I routinely use compression fittings to thousands of psi.

Have a little faith in those engineers that were paid to design the thing.
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Unread postAuthor: psycix » Mon Mar 23, 2009 5:52 am

Because the ferrule only works on a perfectly round pipe with the right OD.
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Unread postAuthor: POLAND_SPUD » Mon Mar 23, 2009 10:43 am

@ psycix I agree with psycix that using rubber is a good idea..
on my fridge compressor I use a homemade compression fitting made from: two identical 1/2" male-female fitings with several rubber washers squeezed in between them...
washers had a small hole so I put the tube through the washers and then tighthened the fittings as much as I could... seems pretty simple doesn't it ?? suprisingly it has never leaked... not even at 500psi
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Unread postAuthor: FishBoy » Mon Mar 23, 2009 2:53 pm

I am still unsure as to how a compression fitting works; how is it tightened?
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Unread postAuthor: jimmy101 » Mon Mar 23, 2009 3:41 pm

They are just assembled with a pair of wrenches. For one manufacturer's instructions see http://www.bulksrl.it/pdf/tube/ing/general_info_ing.pdf

Usually, compression fittings are rated to a higher pressure than is the pipe (or tube) it is designed to be used with.

IIRC, standard compression fittings for standard 1/4" copper pipe is rated to several thousand PSI.
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