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Custom end caps for ally pipe?

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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Unread postAuthor: Ragnarok » Thu Nov 18, 2010 11:22 am

Technician1002 wrote:It only takes a few minutes per end so shop charges should be reasonable.

Depends. If you're looking for someone actually certified to weld pressure vessels, they can charge pretty much what they want to per hour.
(At least in my experience.)
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Unread postAuthor: Technician1002 » Thu Nov 18, 2010 11:32 am

For hobby cannons not needing certified welds, a shop that makes windows is fine. Do your own leak testing and as with any home built chamber, hydrostatic test it.

Shop x rays or hydrostatic testing/certification is extra.
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Unread postAuthor: spudamine » Thu Nov 18, 2010 3:37 pm

Hmm, after reading through this thread I guess this doesn't look like the best idea for a chamber:
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I know square isn't a great idea for pressure vessels but there are some very good reasons for this. Anyway Rag's order of magnitude comment has got me slightly worried. On the plus side this is aluminium with a 5mm wall and will be in a combustion so only holding low pressures for fractions of a second. Size is 120mm x 120mm x 250mm long, endcaps will be bolted on with threaded rod outside the chamber.
So what do you guys think, epic fail in the making?
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Unread postAuthor: Ragnarok » Thu Nov 18, 2010 4:14 pm

spudamine wrote:Anyway Rag's order of magnitude comment has got me slightly worried.

I'll admit, that was a guess. I didn't actually bother to look up/work out the figures - instinct just said it was probably in that kind of range.
(It's the kind of thing I should know though - perhaps I do, but forgot.)

The thing is, I wouldn't be worried about re-pressurising a round chamber over and over. In a square chamber, there is a major risk of metal fatigue at the corners.

It may hold, it may not - it's not a gamble I'd personally take.
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Unread postAuthor: velocity3x » Thu Nov 18, 2010 4:27 pm

spudamine wrote:So what do you guys think, epic fail in the making?


It'll only be square until it's fired. No telling what it's shape (or yours) will be after that.
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Unread postAuthor: chinnerz » Thu Nov 18, 2010 7:35 pm

So other than epoxy there is nothing else to use?

i rang up my local hardware shops today, one of them sells epoxy in a tub plus hardener for it but the guy i was talking to had no idea what it was for. I'll take a trip down there tomorrow when i have access to a car and take a picture of it. hopefully its the right stuff.
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Unread postAuthor: Moonbogg » Thu Nov 18, 2010 7:43 pm

You could hold it on with bolts and use o-rings. Just round the edges smooth.

EDIT: On a side note. My venom cannon has a flat back end on the chamber and it hasn't broken yet. But for the whole chamber to be square...hmm, doesn't sound too good.
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Unread postAuthor: chinnerz » Thu Nov 18, 2010 7:54 pm

Hmm that might just work... if i can find the right sized bolts.

something like this right?
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Unread postAuthor: Ragnarok » Thu Nov 18, 2010 8:22 pm

Moonbogg wrote:My venom cannon has a flat back end on the chamber and it hasn't broken yet.

That's a bit different. Axial stresses in a pressure vessel are half those of circumferential stresses and there's also no hard corner for cracks to propagate from (unless you got the weld on the internal side of the join ground down to a sharp angle).
Still nowhere near as strong as the rest of it, but not as much of a problem as using your typical aluminium square tube stock as a pressure vessel.

Of course, in really high pressure vessels, you'll get no flat faces at all, and get hemi-spherical ends instead.
In the interests of a little extra safety, I occasionally modify flat endcaps that are going to be used at higher pressures so that the internal surface is hemispherical and better distributes the load. (Usually with the judicious application of epoxy putty.)
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Unread postAuthor: Crna Legija » Fri Nov 19, 2010 1:12 am

you can go to safeway/woolworths and ask someone they have 100ml tins of araldite epoxy its expensive tho like 30 bucks or get the 25ml syringes type for like $8-10
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Unread postAuthor: spudamine » Fri Nov 19, 2010 2:36 am

I Just found a paper on non circular pressure vessels:
http://www.gowelding.com/pv/square.pdf
haven't had time to read it yet buy fig.20 near the bottom is quite interesting, comparing square and cylindrical sections with the same wall to section ratio. my square section comes out with a pressure of around 75psi compared to around 2000psi if it was cylindrical. I've no idea what failure pressure this is though or what the material is, further research required I think.
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 19, 2010 3:03 am

Ragnarok wrote:In the interests of a little extra safety, I occasionally modify flat endcaps that are going to be used at higher pressures so that the internal surface is hemispherical and better distributes the load. (Usually with the judicious application of epoxy putty.)


If poured internally (ie the top surface of the poured epoxy will be the one exposed to pressure) I find that epoxy tends to naturally form a curved surface with the chamber walls.
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Unread postAuthor: Ragnarok » Fri Nov 19, 2010 7:54 am

spudamine wrote:I Just found a paper on non circular pressure vessels

Blimey. Looks like "order of magnitude" is generous about the capacities of square ducting. For likely ratios, we're starting there and going up.

Also, 5000th post.
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Unread postAuthor: Technician1002 » Fri Nov 19, 2010 12:51 pm

Make a proper round chamber small enough to fit inside and slip on the square tube as a decoration and call it good. Now you have a proper pressure chamber and a nice look. Fill the voids between the square tube and chamber inside with caulk, foam insulation or anything handy to prevent it from looking loose.
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Unread postAuthor: spudamine » Fri Nov 19, 2010 3:25 pm

Make a proper round chamber small enough to fit inside and slip on the square tube as a decoration and call it good. Now you have a proper pressure chamber and a nice look. Fill the voids between the square tube and chamber inside with caulk, foam insulation or anything handy to prevent it from looking loose.


Thats what I was thinking but it's not quite that simple, there was a reason for the square tube, I need to attach a 2" T partway down the chamber and was planning on bolting a flange fitting into the chamber, you can't bolt a flange onto a tube :lol:
So i've found some round tube that will just fit into the square, the flange can bolt onto the square and a few fittings and a whole load of epoxy will fill the gap. The bolts holding the endcaps on can be nicely hidden between the two tubes and everyones a winner, apart from my gratuitous C:B ratio :cry:
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