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Diaphragm questions

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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Diaphragm questions

Unread postAuthor: spud yeti » Wed Sep 12, 2007 10:22 am

As this topic's name suggests, its is about diaphragms (and questions). I have built piston valves and all the rest, but never a succesful diaphragm valve. This is about to change though...
I am building a .22 cal. pellet gun and am therfore using 22mm copper pipe as the outer pipe to create a coaxial/Tee diaphragm valve and 6mm pipe for the barrel.

The questions:
1) Should I put the diaphragm in a union, or is there an easier way to install it?

2)What thickness rubber is the most effective for this small size at +- 110 PSI? Is just a bike inner tube too thin?

3) How far should the diaphragm be from the barrel when non-pressurized for this size gun?
4) Would a piston just be easier? (this is not a real question, as I am adiment to create a diaphragm :D )

(sorry if you found any of the above answers by searching, because then you are a freak; as I couldnt! :wink: )
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Unread postAuthor: Fnord » Wed Sep 12, 2007 11:27 am

A union would probably be the easiest way to go.

If a normal bike inner tube gives you problems, just keep adding layers. It probably won't but its easy to fix at least.

It doesn't need to be very far away at all. You can even have it right against barrel, but for better flow you should put it the standard 1/4 barrel diameter back. You can probably adjust the position after the gun is made by putting rings of something in the union.

A piston would probably be easier if you have the right pipe fittings avalible.
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Unread postAuthor: spud yeti » Wed Sep 12, 2007 12:32 pm

Thank you very much for that, it clears up all my problems. I think I am still going to stick with a diaphragm because:
a) challenge! :D

b) faster actuation, with sufficient flow because of small barrel

c) more compact for this design

d) I dont actually have something suitable for a piston :wink:

Thanks again
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Unread postAuthor: meatballs » Wed Sep 12, 2007 3:09 pm

yeah for that size a diaphragm will give much better performance. and you might want to go with something thicker than bike inner tube, it cant hold much pressure, cause its so thin. on a bike its the tire that holds the pressure, the inner tube just makes the whole tire and rim assembly airtight. use gasket material from a hardware store, or some other sort of sheet rubber.
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Unread postAuthor: noname » Wed Sep 12, 2007 4:49 pm

I think I can give reasonable good answers, seeing as I have made at least 5 diaphragm valves, so here goes.

1) Unions are 100% UNNECESSARY. They just add cost, bulk, and ugliness to your launcher. A female adapter and a male plug work perfectly, and leaves less pilot volume (I can actuate a 2" diaphragm with a low flow blowgun).

2) I'd go for about 1/16" thick, but watch out. Metal barrels (mainly thin walled or brakeline) are very good at slicing through rubber, especially when the rubber is pressed up against it. Hot glue some small diameter pipe that's thicker over the barrel so the diaphragm doesn't get sliced.

3) I keep the diaphragm pressed against the barrel, even when the gun isn't pressurized. I have a "special technique" that I use to do this, and after troubleshooting my first diaphragm gun and coming out with the technique, I've used it on all my diaphragm guns and haven't had a single problem.

4) NO!
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Unread postAuthor: iknowmy3tables » Wed Sep 12, 2007 7:18 pm

unions are just if you don't know what thickness to use

hey noname could you expand on your special technique
I really like the concept of diaphragms but I always have trouble with them
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Unread postAuthor: Hubb » Wed Sep 12, 2007 7:28 pm

iknowmy3tables wrote:hey noname could you expand on your special technique


Yeah, you seem to be good with making diaphragms.
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Unread postAuthor: mopherman » Wed Sep 12, 2007 7:58 pm

guys, do you think the space inside a 1 inch threaded tee for diaphram flex?
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Unread postAuthor: noname » Wed Sep 12, 2007 9:12 pm

I was planning on writing a How-To at some point, but I'll give a smaller version here.

Trace the outside of your female adapter onto the diaphragm material with a Sharpie. Cut out the circle.
Cement the gun together, so that the barrel passes through about half of the threads on the female adapter.
Push the circle into the adapter. Push in small amounts at a time*, and evenly, so the circle doesn't flip sideways. If there's absolutely no way it's going to fit in, trim off tiny amounts from the sides until it can fit. When it's pushed all the way up against the barrel, you're done. Screw in the fill/pilot assembly and you're ready to shoot.

*The middle of the circle should be the furthest away from you, with the edges closer.
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Unread postAuthor: spud yeti » Thu Sep 13, 2007 11:58 am

OK, thank you everyone for the help so far...

I have bought a female adaptor today, which didnt fit the copper pipe too great, but thats what teflon tapes for :D

The morons didnt have any 4mm or 6mm tube, so I will try a auto shop tomorrow for some brakeline.

I will post a diagram of the gun in a few minutes
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