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Piston Valve Sealing, but not Filling?

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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Piston Valve Sealing, but not Filling?

Unread postAuthor: vakimo » Sat Nov 05, 2011 10:57 pm

I built a small spud gun to test out the piston valve concept, but im having some problems. The gun fills very nicely from 20-200psi, but when the valve is vented all the pressure is just released, instead of going out the barrel.
It uses 1/4" valves, which i thought might be venting to much air, even though the piston seals the vent when reaching its full 1.5" backstroke.
Grainger has a nice 1/8" 3/2way solenoid valve for $35 that i was thinking of using instead. Would that work?

Any help would be appreciated!
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Unread postAuthor: Gun Freak » Sat Nov 05, 2011 11:12 pm

AAHHHHH!!! Flimsy back plug, DWV fittings, 200 psi in PVC?!?!?! What are you thinking! You're gone blow this thing up.

Pilot valve can never let too much air out. Only too little. Is there an equalization hole in your (hot glue?) piston? And is there even a sealing face?
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Last edited by Gun Freak on Sat Nov 05, 2011 11:14 pm, edited 1 time in total.
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Unread postAuthor: pneumaticcannons » Sat Nov 05, 2011 11:13 pm

you can never vent too much air. Is you're piston A tight fit in the pipe? plus, you're coupler is dwv, but I see that you have inserted screw's to keep it secure so i guess It should be fine.
*Edit*
As gun freak said, the back plug is flimsy and NEVAR PUT 200 PSI IN THAT THING AGAIN. seriously, use some judgment...
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Last edited by pneumaticcannons on Sat Nov 05, 2011 11:18 pm, edited 1 time in total.
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Unread postAuthor: Gun Freak » Sat Nov 05, 2011 11:16 pm

Ninja'd, PC :D

In my mind, screws weaken the joints. The solvent weld (if done correctly) is sufficient on it's own.
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Unread postAuthor: MrCrowley » Sat Nov 05, 2011 11:16 pm

Screws would probably make things worse. I'm surprised the fitting didn't crack and blow apart around those screwed at 200PSI.

Scrap the chamber and barrel, save the pilot setup and the piston. Build a new chamber and barrel. The problem is probably with your piston, it must not be a good enough fit inside the piston housing.

Grainger has a nice 1/8" 3/2way solenoid valve for $35 that i was thinking of using instead. Would that work?

Most likely not. Is that a 1.5" chamber and piston? You'll get bad enough performance piloting that with 1/4" valves unless you put o-rings on your piston.
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Unread postAuthor: pneumaticcannons » Sat Nov 05, 2011 11:21 pm

Ninja'd, PC

LEAVEMEALONE! I'm a slow typer!! :D :D :D
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Unread postAuthor: vakimo » Sat Nov 05, 2011 11:53 pm

It may look thrown together, but its pretty thought out. the rear plug is solid brass (not to flimsy), and the piston is turned nylon with a silicon filled pocket for the sealing face. there is less than a 1/32" gap on either side of the piston while in the chamber. It doesnt have orings because the air has to seep around the piston to fill the chamber. The screws were calculated to blow out at 250psi, and were wrapped in gorilla tape just in case. The operating pressure is only 125, and the 200 was tested behind a plexiglass shield. Sorry if i didnt give enough background information, hope that clears a few things up. I was thinking that Orings combined with a built-in check valve on the piston might help, but id rather avoid that complication.
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Last edited by vakimo on Sat Nov 05, 2011 11:57 pm, edited 1 time in total.
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Unread postAuthor: Mr.Tallahassee » Sat Nov 05, 2011 11:56 pm

Is it just me or does that back plug look like metal wrapped in teflon tape? As Mr.C said, keep the plug and pilot valve and scrap everything else. Then put O-rings on the piston. Maybe even try floating O-rings in a one-way setup? Since you are using 1/4'' ball valves you should probably minimize the amount of air getting back from the chamber into the pilot as much as possible. Make sure that you use "nsf-pw" pressure rated pipe and couplings with full length sockets.

PC, I'm a fast typer. I just take forever to figure out what I'm going to say. :lol:

EDIT: I said I'm slow at thinking about what I'm going to type. :lol:
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Unread postAuthor: Crna Legija » Sun Nov 06, 2011 12:05 am

vakimo wrote:It may look thrown together, but its pretty thought out. the rear plug is solid brass (not to flimsy)


but there aren't many threads id say it will blow out or crack the threads.

vakimo wrote:the piston is turned nylon with a silicon filled pocket for the sealing face. there is less than a 1/32" gap on either side of the piston while in the chamber


thats still way to much make it. 0.3mm or less, just add a layer or 2 of tape to fill it up.
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Unread postAuthor: MrCrowley » Sun Nov 06, 2011 12:14 am

vakimo wrote:there is less than a 1/32" gap on either side of the piston while in the chamber.

That's still too big. The cross sectional area of the gap between piston and chamber is 0.95cm^2 (based on 1.5" piston diameter). The cross sectional area of a 6mm ball valve is about 0.3cm^2, so it's not surprising your piston doesn't work properly.

vakimo wrote:It doesnt have orings because the air has to seep around the piston to fill the chamber.

Then drill an equalisation hole. Either you'll have to machine a new piston (or wrap it in some thin tape) or add a floating o-ring or a normal o-ring but with an equalisation hole. Is your piston lubed up as well by the way?

vakimo wrote:The screws were calculated to blow out at 250psi, and were wrapped in gorilla tape just in case.

When you say blow out do you mean the pressure at which the screws will be forced out of the hole? That's not the issue about drilling screws in to chambers, weakening the pipe is the issue. It has been shown on several cannons that a cannon will fail because of holes drilled in the chamber. Granted, you've drilled through two layers of PVC, but I would still be wary. If done correctly, PVC cement and primer is much better than PVC cement and primer + screws.
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Unread postAuthor: jackssmirkingrevenge » Sun Nov 06, 2011 12:19 am

MrCrowley wrote:
vakimo wrote:there is less than a 1/32" gap on either side of the piston while in the chamber.

That's still too big. The cross sectional area of the gap between piston and chamber is 0.95cm^2 (based on 1.5" piston diameter). The cross sectional area of a 6mm ball valve is about 0.3cm^2, so it's not surprising your piston doesn't work properly


Always a good idea todo the math ;)
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Unread postAuthor: MrCrowley » Sun Nov 06, 2011 12:22 am

Was looking for that lovely diagram you used to throw around, couldn't find it so out came the calculator :D
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Last edited by MrCrowley on Sun Nov 06, 2011 12:31 am, edited 1 time in total.
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Unread postAuthor: jackssmirkingrevenge » Sun Nov 06, 2011 12:27 am

MrCrowley wrote:Was looking for that lovely diagram you used to throw around, couldn't fit it so out came the calculator :D


By an elaborate process of deduction I can confidently postulate that you've been drinking :D
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Unread postAuthor: MrCrowley » Sun Nov 06, 2011 12:35 am

Dammit, did spelling "find" completely and utterly wrong give me away? :wink:

How the hell do you remember stuff like that? I followed that link where you referred to an earlier post of yours which referred to an even earlier post of yours :shock:


For those wondering, no PUI. It's Sunday and I have a final exam on Wednesday. Keep an eye out for spelling mistakes on the 14th though, I finish the semester then.
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Unread postAuthor: vakimo » Sun Nov 06, 2011 12:57 am

Correction: the gap is smaller than that, but i cant fit calipers in there. to give you an idea, it will barely slide with 2 dollars wrapped around it .Ill go ahead a make another piston with closer tolerance just in case though. I think oil/grease would be good, any recommendations on what type? like gun oil maybe?

again the screws hold up to the pressure at 200psi, well above the working pressure at 125. my only concern is the affect of repetitive piston impact on the rear set of screws, even though thats cushioned by a half inch of rubber and silicon.
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