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2" piston valve or dual sprinkler valve?

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: jackssmirkingrevenge » Tue Jan 29, 2013 2:10 pm

If the coupling/endplug fits exactly in the chamber tube (by exactly I mean less than 0.02 inches difference in diameter) then yes.

This assumes you're making a coaxial type piston by the way.
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Unread postAuthor: Nick481 » Tue Jan 29, 2013 2:20 pm

Alright. If it doesn't fit good enough I suppose I can just do the full hub design. Wouldn't be much harder.
Edit: not sure what a coaxial piston is. Mine is going to be a barrel sealing piston housed inside a 2" PVC tee.
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 29, 2013 3:12 pm

Nick481 wrote:not sure what a coaxial piston is. Mine is going to be a barrel sealing piston housed inside a 2" PVC tee.


By coaxial I meant barrel-sealing, carry on :)
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Unread postAuthor: MrCrowley » Tue Jan 29, 2013 4:07 pm

Nick481 wrote:Alright. If it doesn't fit good enough I suppose I can just do the full hub design. Wouldn't be much harder.
Edit: not sure what a coaxial piston is. Mine is going to be a barrel sealing piston housed inside a 2" PVC tee.
I hear people have also had success with using sanding drums. I haven't seen them where I live so have never tried them, but they look like they could make a good piston (albeit a heavy one). If you have a drill press and hole saw of appropriate size, you can also make a piston from multiple disks of wood. I did this with a 2" diameter plywood piston and machined an o-ring groove on a lathe although it could be done with a drill press if you're careful.
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Unread postAuthor: Nick481 » Tue Jan 29, 2013 5:58 pm

I know dwv fittings should never be used in pressure situations, but I want to make a full coaxial cannon with the barrel running inside the tank. I would have to use a 3 inch tank though. The only problem is the only way to get from 3 to 2 inch pvc is with a dwv bushing like this one http://www.lowes.com/pd_23311-1814-PVC+ ... facetInfo= I'm just wondering what are your guys thoughts on maybe filling in that gap with some runny epoxy and making sure the pieces are glued on very well with primer and cement. I would be very tentative to do it and I'm not an idiot about to blow myself up with dwv fittings so I wanted to know what you guys think about it. The gun will probably be maxed at 100 psi. If not I guess I could do a 2 inch tank in fact though.
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Unread postAuthor: MrCrowley » Tue Jan 29, 2013 6:02 pm

You can get those bushings in pressure rated form. If you can't find any locally, buy online from McMaster or similar.
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Unread postAuthor: Nick481 » Tue Jan 29, 2013 6:11 pm

Yeah I figured they're probably available online. I think I will be able to do a 2 inch tank and still make the valve and chamber coaxial with the barrel. Just have to angle the tank off in to different shapes but nbd. I would make a drawing on paint but my computer doesn't work and I'm on an iPad.
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Unread postAuthor: MrCrowley » Tue Jan 29, 2013 6:15 pm

A 2" chamber for a 1.5" porting piston valve in a tee should be fine. You could always model your proposed cannon designs in GGDT or ask someone else to do them for you (since your computer doesn't work) to see the difference in performance between a 2" and a 3" chamber.
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Unread postAuthor: Nick481 » Tue Jan 29, 2013 6:29 pm

That's not a bad idea. Also, would using sdr 21 1.5 inch pvc for the seat of the valve have reduced sealing ability to sch 40? If so I can coupler it to some sch 40 but I would prefer not to.
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Unread postAuthor: MrCrowley » Tue Jan 29, 2013 6:39 pm

My only worry would be how thin it is. If you use some pipe to sleeve over it, you're decreasing the flow around the barrel and lowering the ratio of the piston diameter to the seat diameter (which may not be a bad thing, a few members prefer a low ratio piston). Personally, I'd go for a Sch 40 NSF-PW 1.5" PVC seat. Worrying about whether a part may or may not fail takes all the fun out of these cannons :wink:
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