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How does my piston valve look?

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: SpudBlaster15 » Fri Aug 24, 2007 11:39 pm

Building a piston valve out of a steel T is relatively difficult, but it can be done. Preferably with the use of some epoxy. :D
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Unread postAuthor: frankrede » Sat Aug 25, 2007 12:30 am

SpudBlaster15 wrote:Building a piston valve out of a steel T is relatively difficult, but it can be done. Preferably with the use of some epoxy. :D
I hate to hijack this topic but, I want to make a 200-300 psi all steel cannon, and I want to use a piston valve, what would be my best approach for making a piston valve for those pressure?
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Unread postAuthor: Novacastrian » Sat Aug 25, 2007 2:12 am

I think you may get less power, but it should still work i think :?
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Unread postAuthor: chaos » Sat Aug 25, 2007 2:15 am

frankrede wrote:
SpudBlaster15 wrote:Building a piston valve out of a steel T is relatively difficult, but it can be done. Preferably with the use of some epoxy. :D
I hate to hijack this topic but, I want to make a 200-300 psi all steel cannon, and I want to use a piston valve, what would be my best approach for making a piston valve for those pressure?


perhaps cast polyurethane that eliminates the need for a sealing face, which is what i am going to use for my s400's next piston. though one problem i see is the end of the piston flattening and flaring which will eventually jam the piston.
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Unread postAuthor: jackssmirkingrevenge » Sat Aug 25, 2007 2:28 am

frankrede wrote:I want to make a 200-300 psi all steel cannon, and I want to use a piston valve, what would be my best approach for making a piston valve for those pressure?


ta-daaa! :D
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Unread postAuthor: gwoloshyn » Sat Aug 25, 2007 4:39 pm

**UPDATE**

I constructed a new valve after understanding now, pictures are below. That 1" barrel you see there is just a scrap peice and will not be used. Only thing I'm worried about is the pilot travel, its only about a half inch. Will that be enough?
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Unread postAuthor: Gepard » Sat Aug 25, 2007 4:46 pm

1/2" is fine - you suppose to have 1/4 of the barrel's diameter but on a small valve like that 1/2" isn't a problem.

If you have too long a travel then the piston can just build up speed but 1/4" won't make that much difference and shouldn't cause you any problems.

When's the test firings?
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Unread postAuthor: MrCrowley » Sat Aug 25, 2007 4:50 pm

Gepard wrote:1/2" is fine - you suppose to have 1/4 of the barrel's diameter but on a small valve like that 1/2" isn't a problem.
If you have too long a travel then the piston can just build up speed but 1/4" won't make that much difference and shouldn't cause you any problems.

When's the test firings?



Note that rule is used in co-axials only really, Clide thinks barrel sealers could do with a bit more room.
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Unread postAuthor: Gepard » Sat Aug 25, 2007 6:07 pm

Ah ok sorry....so how much would account for it being in a T then? Or isn't there really a specific amount and it's just 'abit over d/4'.
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Unread postAuthor: jackssmirkingrevenge » Sat Aug 25, 2007 8:13 pm

Gepard wrote:Ah ok sorry....so how much would account for it being in a T then? Or isn't there really a specific amount and it's just 'abit over d/4'.


It can be calculated, you have a final figure - the surface area of your barrel which is (0.5*caliber)<sup>2</sup>*pi - to which the area available for the air to flow through the vertical "T" component must be at least identical to.

I constructed a new valve after understanding now, pictures are below. That 1" barrel you see there is just a scrap peice and will not be used. Only thing I'm worried about is the pilot travel, its only about a half inch. Will that be enough?


Much better now, glad you grasped the concept, and yes, half an inch of travel should suffice.
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Unread postAuthor: Marco321 » Sat Aug 25, 2007 8:23 pm

Looks very nice, clean and well made. Cant wait to see it in a gun :D
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Unread postAuthor: gwoloshyn » Sat Aug 25, 2007 9:15 pm

Thanks for the comments. This valve will be used on my next launcher in production, I plan to use concrete slugs for ammo molded out of 1" pipe. :)

Question: The end plug on the valve is only held in place by some screws so I can have access to the piston for maintenance if needed.. without epoxy I won't get an airtight seal will I? It seems air will leak out of the fitting if I it's only held in place by those screws. A little confused about that :/
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Unread postAuthor: noob of noobs » Sat Aug 25, 2007 9:43 pm

If you cut off part of the back plug (which it looks like you did) you can take the part you cut off, and glue it into the T. Take an appropriate sized gasket and sandwich it between your back plug and the piece you glued in. You might have to shove the plug on pretty hard when putting in the screws, but it should provide a better seal, and act as a bumper. I can post a picture if you want me to.

EDIT: I've heard silicone sealant works well too, but I don't know much about it except for it's easy to take on and off.
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Last edited by noob of noobs on Sun Aug 26, 2007 11:32 am, edited 1 time in total.
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Unread postAuthor: Marco321 » Sat Aug 25, 2007 11:33 pm

For my gun i had the same problem, so i put some tape around where the cap fits on, then i put a little lube on it, mine creates an airtight seal now
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