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Trouble shooting a coaxial 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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Trouble shooting a coaxial valve

Unread postAuthor: Teflon » Wed Mar 27, 2013 12:27 am

Hi Spudder's

So ive been working on a coaxial piston valve, but i am having troble getting it to fire..

the design is simple, as seen below.

Its made from Pvc and epoxy mostley, the piston is a cast epoxy piston (Thanks JSR) the cheack valve and pilot valve are both made from small bolts with o rings, the piston face seals agains a piece of black rubber hoseing epoxied to the barrel. the general shape of the thing is shaped pvc cast into epoxy.

so, sucking on the barrel with the pilot valve open makes the piston pull forwerd and seal perfectly, blowing into the barrel with the pilot valve open makes the piston snap back. the gun does not leak at all when pressurised, however when i pump it up and press the pilot valve, air rushes out the pilot and the damn piston just dosent move...

any ideas what might be wrong?

The diagram is drawn roughly to scale.

i might be able to get some pictures of it later, but the piston is perminantly cast into the gun...

Regards
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My piston valve
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Unread postAuthor: Labtecpower » Wed Mar 27, 2013 12:39 am

As it is cast, it could be that your piston is too airtight.
If that is the case, try making a very shallow lengtwise groove with a hacksaw or so :)
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Unread postAuthor: Teflon » Wed Mar 27, 2013 12:46 am

Ah i knew i forgot to mention somthing.

Blowing into the barrel with the pilot valve open, i can continue to blow air through the system... so that cant be the problem... :(

any other thoughts?
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Unread postAuthor: Labtecpower » Wed Mar 27, 2013 1:50 am

In that case your piston isn't tight enough :D
Clean it, and wrap a piece of tape around it to fill the gap.
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Unread postAuthor: Teflon » Wed Mar 27, 2013 3:13 am

Heh yeah that cant be it either,

Tape... no way any tape could ever fit.

When I say *I can continue to blow air through the system*, I mean cheek puffing face exploding blowing will consistently allow a small trickle of air past the piston.

so im quite sure that air is leaking past it when im pumping
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Unread postAuthor: Labtecpower » Wed Mar 27, 2013 4:03 am

Ahh ok :D
How fast does your pilot valve release the pilot air? If it doesn't have a large enough flow the piston won't open. Especially if your piston diameter is close to the diameter of the barrel.
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Unread postAuthor: Teflon » Wed Mar 27, 2013 5:29 am

hmm I was thinking that may be the problem, the piston diameter is 19mm, the barrel is 6mm ID and 10mm OD for the rubber tubeing.

the pilot valve is a 3mm bolt with the shaft ground down half way laterally, has an O-ring sealing it and is in a 3mm hole, i would have thought half a 3mm hole would be pleanty, but atm im just pushing it in by hand, maybe if i have it hammer actuated it may speed thing up enough... though the piston is a pretty good fit and only required the lightest blowing to move..

i dont know, i might just wait till next month when i get my SIEG C2 :D
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Unread postAuthor: RBurke83 » Wed Mar 27, 2013 7:34 am

I had a similar problem the first few times I tried making a piston gun. Even though I thought my tolerances were extremely close, I used a sub -standard epoxy which I believe shrunk slightly after curing. The cure i found was to use a hacksaw blade to cut a groove all the way around the piston for an o-ring to sit. If you get the groove the perfect size, great - if not, then like me you can rotate the piston with the oring on some fine grit sandpaper and very slowly remove oring material until the fit and slide is perfect and air can still get around the piston into your chamber. Good luck, dont give up!
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Unread postAuthor: RBurke83 » Wed Mar 27, 2013 7:38 am

BTW, post some pictures of your set-up, it looks like a fine design.
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Unread postAuthor: Labtecpower » Wed Mar 27, 2013 10:22 am

3 mm bolt in a 3 mm hole is WAY too small to actuate it properly!
Try mounting a blowgun on it, i'm sure it'll work :)
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