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Problematic Piston - Fires multiple times

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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Problematic Piston - Fires multiple times

Unread postAuthor: pyro6314 » Thu Apr 19, 2007 10:07 pm

Any one ever have problems with a barrel sealing piston valve firing multiple times. I usually get one decent release then two or three wimpy ones after. I have a 3/32 pilot hole and minimal leakage around the rest of my piston...Any Ideas?
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Unread postAuthor: Pete Zaria » Thu Apr 19, 2007 10:14 pm

Is there a spring behind the piston, or does it seal by air pressure alone?

How big is your pilot valve?
Are you keeping the pilot valve open, or only opening it for a moment?

Give us more info.

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Pete Zaria.
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Unread postAuthor: hi » Thu Apr 19, 2007 10:14 pm

what, you mean that you exaust it and it opens and closes a couple times all by its self?
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Unread postAuthor: pyro6314 » Thu Apr 19, 2007 10:19 pm

No spring, fairly hard piece of rubber hose as a bumper and yes with a 1/2 inch ball valve (wouldnt fire over 40 psi with a 1/4 and quick disconnect) it will exhaust once then refire a few whimpy times with the 1/2" valve still open...crazy hey?

Other than that...my new (and first) golfballer is amazing. I cant even see the ball go - its just gone!
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Unread postAuthor: scatdawg » Thu Apr 19, 2007 10:19 pm

so your chamber or barrel has to be re-sealing before all air is exhausted. do you have a spring behind your piston?. also a rubber stopper can bounce the piston back hard enough to reseal the piston
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Unread postAuthor: scatdawg » Thu Apr 19, 2007 10:20 pm

how much space behind the piston to the bumper when sealed?
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Unread postAuthor: pyro6314 » Thu Apr 19, 2007 10:37 pm

there is a total of just over 1/2 in of travel to the bumper. Im thinking maybe my equalization hole is a little too big. I was going to put the hole behind the rubber on the front to make a check valve but I already made a hole before I thought of that.
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Unread postAuthor: frankrede » Thu Apr 19, 2007 10:48 pm

Hmm, if your gun still has air even after exhausting with a piston valve then your gun is missing out performance wise. If you fix your piston you can expect an increase in performance.
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Unread postAuthor: pyro6314 » Thu Apr 19, 2007 10:54 pm

I have a 2:1 volume ratio, but yes I agree. I think it pilots like crap, I was expecting it to be satisfied by a 1/4" ball valve and a quick disconnect.
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Unread postAuthor: keep_it_real » Fri Apr 20, 2007 12:17 am

Switching to a modded sprinkler valve for a pilot might help. It could get all the air out the barrel before the piston bounces back. If it doesn't help the piston problem, it will help performance.
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Unread postAuthor: jrrdw » Fri Apr 20, 2007 6:20 am

Sounds like a flow restriction leaving air in the storage chamber/s. I think the questions should be, How is your piston valve configured? Where is the air flow exit, (sealing end of barrel) as opposed to the chamber exit? Is the barrel its self restricting the flow? Does the piston have enough room to open sufficiently? If your equalization hole was to big the air would just seep out the eshaust/pilot valve, it wouldn't repeat fire.

Look at it this way, you just accesidently built the first fully automatic spudgun.
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Unread postAuthor: scatdawg » Fri Apr 20, 2007 11:58 am

i think its a pilot problem. its not getting all the air out from behind the piston. when the pressure equals or becomes slightly less in the chamber, the piston would seal again. if it doesnt seal well enough it could exhaust again.
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Unread postAuthor: pyro6314 » Fri Apr 20, 2007 2:14 pm

Thats what I thought to but I mean there is a 1/2" ball valve wide open venting an Oringed piston that has minimal leakage and one 3/32 equalization hole.

Could be my design too. My barrel side seat has only 1/2" gap before my piston sleeve is there coming about 1/2" in covering the reservoir side of the Tee. Could be some weird turbulence causing the piston to come back and seal again.
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Unread postAuthor: rad14701 » Fri Apr 20, 2007 5:55 pm

What are the dimensions of the major components of your gun...??? This sure sounds like an odd problem... Are you able to change the bumper...??? Is it possible that the piston is hitting something before reaching the bumper...???
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Unread postAuthor: pyro6314 » Fri Apr 20, 2007 11:09 pm

All housed in a 3" tee...piston is about say 2.75" running in a machined track with no obstructions. bumper is changeable but I only have 1/4" of working room for a bumper
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