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piston valve issues?

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 issues?

Unread postAuthor: rightgard » Thu Jun 09, 2011 1:46 am

Ok im sick of tring to find the forum for this topic!I built a 2'' piston valve, the piston is machined out of solid teflon and is hollowed out to save weight. The tolerances between piston and pilot chamber are very good about .025''. My piston seat is 1-1/2'' pvc, the pilot area is as small as i can make it and exhuasts out a 1'' rainbird sprinkler vavle that has been turbocharged! OK so my problem is the valve seems to re-seat after firing? I can dry fire it with the berrel off and see that its re-seated, and can push it back with a ramrod. when i do this i can hear some air hiss by.
my piston is alowed to slide back about 1-1/2''. i tried to post pictures but i couldnt figure out how to do that on this website. i have a video on youtube under { piston valve air cannon test fire } about all i can think of is to put o-rings in my piston to reduce the piston seat diameter? i read something about large ratio and small ratio valve and dont know if im to close to the 1:1 ratio.? any help on this would be awsome...Thanks
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Unread postAuthor: Crna Legija » Thu Jun 09, 2011 2:58 am

half a mm is pretty lose for a piston that big. if you have a lathe might as well put a O-ring on it not that hard

your pilot volume can be smaller, because your piston travel should be around 1/4 the barrel port dia.

do you have a spring as bumper?, it could also be that your shutting your pilot valve early.
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Unread postAuthor: Technician1002 » Thu Jun 09, 2011 10:41 am

In a nutshell, the rapid pressure drop in the chamber and the slower pressure drop in the sprinkler valve is allowing enough pilot pressure to remain to close the piston.

The sprinkler valve has a spring that closes it when the pressure drops. This can retain some residual pilot valve pressure on the main piston.

Try changing out the sprinkler valve with small ball valve.

The other option is to leave it as is. The main piston doesn't close until the projectile is long gone anyway, so the cannon resealed is a bonus.
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Unread postAuthor: rightgard » Thu Jun 09, 2011 12:44 pm

I have foam for my bumper. and yah i kinda thought that i needed an o-ring on my piston. i've seen some real rough looking pistons that worked well without them though? should i use some wood to take up some of the pilot volume? also has anyone tried removing the spring in those sprinkler valves? and how do you down load pictures to this forum? It tells me i tried to upload a emty file? well i'll try a few more things and post my results...thanks!
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Unread postAuthor: Lockednloaded » Thu Jun 09, 2011 1:14 pm

You can remove the spring from the sprinkler valve if you fill it from the pilot side, and this may solve your problem
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Unread postAuthor: velocity3x » Thu Jun 09, 2011 1:22 pm

I have a 1.75" dia piston x 1.50" valve seat x 1.80" retract and a piston to cylinder to wall clearance of 0.002". The pilot valve is a 0.50" ball valve. I think the others nailed it. The problem must be in your sprinkler valve.
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Unread postAuthor: rightgard » Thu Jun 09, 2011 2:07 pm

You guys rock!!! I took the spring out of my sprinkler valve and had much more power and the piston does not reseat! i can charge the cannon and hear that piston seat and the pilot valve doesn't screech like it did before...now i'm going to try to get all i can out of it to see how far i can huck tennis balls "for my dogs" and keep the thing hand held!!! THANKS TO ALL THAT HELPED!!!
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Unread postAuthor: Labtecpower » Thu Jun 09, 2011 2:21 pm

For MOAR POWAH! you can add O-rings, as you stated before. A good sealing piston really makes a difference! :)
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Unread postAuthor: ramses » Thu Jun 09, 2011 3:01 pm

If you add an O-ring, you will need to either make it a floating o-ring to act as a check valve, or add an equalization hole.
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Unread postAuthor: rightgard » Thu Jun 09, 2011 11:25 pm

i don't have the right size o-rings for a floating setup...i do have some bulk o-ring material that i got from work that i could use, but what works better? floating or reg. with an equalization hole? oh and i think the reason i cant post pitures is because they are too big? anybody have that problem?
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Unread postAuthor: Labtecpower » Fri Jun 10, 2011 12:24 am

floating O-ring setup is the best, because of the reduced friction.
Regular will work just fine, but you indeed need to put a check valve in the piston.

I have posted 6 mp photos, so it shouldn't give any problems.
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Unread postAuthor: Technician1002 » Fri Jun 10, 2011 11:19 am

This page has some info on floating o rings at the bottom.
http://www.spudfiles.com/spud_wiki/index.php?title=Piston_valve
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