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Coaxial trouble

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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Coaxial trouble

Unread postAuthor: Howzer » Mon May 03, 2010 5:22 pm

I've made a coaxial launcher to launch nerf footballs for airsoft/paintball scenario games, but for some reason my piston won't release when i pull the trigger and all of the air escapes through the pop-off valve.
So far i've tested the gun at about 40 psi. I've only checked the pressure using a tire gauge, bu i'm thinking of adding a pressure gauge to the launcher.
Let me describe the gun

the chamber is approx 14" long 2" sch40 pvc
the barrel(inside the chamber) is about 11" long 1" sch40 pvc
the piston is an 1 1/4" sch40 pvc cap with a piece of rubber that fits inside the 2" pvc with just enough room to slide freely.
the piston is assembled 1/4" machine SS(stainless steel) machine screw, SS washer, rubber washer, PVC cap, rubber washer(fits snug inside the cap), SS washer, SS lock washer, SS lock nut.
At the back of the chamber is a 2" rubber tip toe washer (i think thats what it was called) for a bumper
The pilot valve is a 1/4" NPT 140 psi pop-off/safety relief valve, attached center on the butt cap of the chamber
the fill valve is an 1/8" NPT schrader valve

I believe the piston has about 1/2"-3/4" to slide inside the chamber.

The piston can be heard sliding freely when the launcher is shaken vertically.
I've tried spraying silicon oil into the chamber by removing the schrader valve and spraying around the piston, then shaking the launcher to slide the piston and spread the oil.
I haven't had any issues with the schrader valve releasing the piston, but neither will the pilot valve when the chamber is pressurized.
If i can get the piston to work, and the schrader valve does release the piston when finished filling, i plan on adding an 1/8" ball valve in an alternative fill assembly.

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Unread postAuthor: Hotwired » Mon May 03, 2010 5:46 pm

It's possible your pilot valve is too small and that backflow around the piston is equalling the flow out from the pop-off or at least is close enough to mean the force on the pilot side stays higher than on the chamber side.

The pilot side has more surface area so it will stay closed even when the pilot pressure is below chamber pressure. Pilot pressure has to be very much lower for the piston to trigger so you can see it won't take much leakage round the piston to counter the small pop-off valve trying to pilot it.

So my vote goes to considering a larger pilot valve.


Nice layout by the way.
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Unread postAuthor: Howzer » Mon May 03, 2010 5:53 pm

yeah, i'm thinking the chamber is going to have to be 3" with a 1" inner barrel...

or what if i use 1/2" pvc for the inner barrel to add more surface area to the main pressure side of the chamber?

you really think the pop-off doesn't have enough dump? i guess it does make more of a pfft instead of a pop, but i thought it had pretty good flow. I dunno, this is my first launcher...
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Unread postAuthor: chinnerz » Mon May 03, 2010 6:13 pm

yeah, thats what i was going to say...

try getting a >1/4'' ball valve and opening it by hand as quick as you can... that should give you the answer.
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Unread postAuthor: Howzer » Mon May 03, 2010 6:57 pm

I guess i can do that to test it, but i want to keep my trigger setup...

sooo... do you think using a 1/2" barrel will work? i'd kinda like to keep this thing 2", when it's finished i want to make it look similar to a RPG7...
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Unread postAuthor: chinnerz » Mon May 03, 2010 7:08 pm

dont know man. the only thing which may help is making your pilot volume smaller... or making some kind of valve with a higher flow
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Unread postAuthor: Howzer » Mon May 03, 2010 7:16 pm

yeah, i'm looking at some 1/2" relief valves right now, i can really only find this one

http://www.amazon.com/Midwest-Control-S ... B0025Q2UAK

atleast thats the only one in the style i want... i wonder where i might find it locally...

how small can i make my pilot volume while still keeping enough room to slide the piston and have a decent dump gap..

errr... how big of a gap do i want from the inner barrel to the piston when triggered?

i'm thinking i can cut down the cap i'm using for a piston to slim down the pilot space...
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Unread postAuthor: Gaderelguitarist » Mon May 03, 2010 7:42 pm

I love the trigger setup! very nice thinking.

Why not try to optimise the piston? Add o-rings and a check valve. You could use a simple flap of material as a check valve. I would draw a picture, but its a pretty simple concept.

how big of a gap do i want from the inner barrel to the piston when triggered?


Last Time I checked, it's 3/4 the measurement of the inner diameter of the pipe against which the piston seals.
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Unread postAuthor: MrCrowley » Mon May 03, 2010 7:50 pm

I think a pop-off valve might be able to pilot a well made 2" piston, so I guess your piston isn't a good enough fit. As the others said, try going up to a bigger pilot valve like a 1/2" ball valve. If that doesn't work, it's your piston.
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Unread postAuthor: Howzer » Mon May 03, 2010 7:57 pm

Yeah, i'll have to rebuild the chamber... Can you use threaded pvc on a fill chamber? teflon tape/thread sealant should work right?

also, how should i attach o-rings to the piston? right now it is just a pvc cap w/ rubber washer, maybe glue a plug in it with a gap for the o-ring?
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Unread postAuthor: MrCrowley » Mon May 03, 2010 8:05 pm

Yes you can. You can see here how I used threaded fittings so I can service the piston valve in case it doesn't work or becomes faulty.
I don't need to use teflon tape on the threads to get it to seal though.

Yup you could do that with the o-ring or you could just cut a groove with a hacksaw and file it down. On one of my first piston valve cannons I couldn't get it to fire with a 1/4" ball valve so I used a 1/2" ball valve, although it fired it didn't have great performance. So even if you used a 1/2" ball valve and it fires, if the performance isn't great, you need to rebuild the piston.
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Unread postAuthor: Lockednloaded » Mon May 03, 2010 8:54 pm

i would reccomend a new "style" for pisto to decrese pilot space and raise performance. You will nee to rebuild the chamber, but it will be cheap.
All you need is a 2" female adaptor, 2" male adaptor and a 1.5" plug. (this is only for the back portion. The 1.5" plug ftis perfectly into the make adaptor for a piston, and to decrese the pilot volume even more, fill the cap with pakcing peanuts and hot glue
If you don't get what I'm talikg about check out Mini Piston Gun v2
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Unread postAuthor: Howzer » Mon May 03, 2010 10:30 pm

yeah, i just went to lowe's about an hour and a half ago, I'm going to try a 1/2" inner barrel this time with the 2" chamber. I picked up another 1 1/4" cap and a 1 1/4" plug. I'm gonna cut them down this time to make it shorter, i think i might cut a groove in the cap for the o ring because the plug has a hex end which isn't quite as big around as the cap. I think i also might find a smaller rubber washer for the barrel seal so i have room for some equalization holes. I'm also thinking of filling the plug with something and drilling out some holes to put in a spring, bb, and tiny o-ring for a check valve.
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Unread postAuthor: Gaderelguitarist » Mon May 03, 2010 10:51 pm

Tack thin pieces of material to the chamber side of those equalization holes and you'll have some cheap home made check valves that will really optimize your piston.
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Unread postAuthor: Howzer » Mon May 03, 2010 11:49 pm

here's an image of my new piston design, i'm not sure what i want to use for the fill material since i need to drill a hole in it for the home made bb check valve...

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edit: on second thought i may end up filling it with hot glue and using sections of a bic pen as spaces for the check valve... i dunno it's in the works...
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