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2" Piston Valve Pilot

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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2" Piston Valve Pilot

Unread postAuthor: shardbearer » Tue Sep 11, 2012 6:15 pm

I finished a 2" Tee Piston Valve a couple weeks ago, pretty standard stuff, 1 1/2" Porting, 1 1/4" Coupler for a piston, 2" Chamber/Stock and barrel, shoots golfballs (wadded with paper towels) through 5/8" OSB at 45 PSI.

My main problem is the pilot. Currently I'm using a 1/2" ball valve, but the handle keeps falling off and it's annoying to have such a long trigger pull. Also, the cannon at 60 psi (as high as i've brought it), even without a barrel, sounds like a very loud thud instead of a proper crack, meaning I need a better pilot.

I searched all my hardware stores for a sprinkler valve, but to no avail. I can't decide whether to replace it with a QE-04 1/2" QEV or a 1" sprinkler ordered online. Both would be piloted by a modded blowgun, and the sprinkler is actually more expensive. Seems to me that the sprinkler would have better flow but the QEV would be faster and the flow good enough, so that's what i'm leaning towards. What do you think?
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Unread postAuthor: iknowmy3tables » Tue Sep 11, 2012 11:36 pm

either seem like fine choices, but I'll say go with the QEV because they have nice pilot fill rates where as from my last experience with a sprinkler valve filled via pilot it was a little slow.
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Unread postAuthor: mattyzip77 » Wed Sep 12, 2012 8:26 am

Go with the qev but if you can get a 3/4 inch one from mcmasters. Dont forget your bumper or the valve will not last very long...
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Unread postAuthor: shardbearer » Thu Sep 13, 2012 6:46 pm

Ok, I'll order a QEV, but the 3/4" ones from Mcmaster are too expensive. And I'll get an extra one, since they're just so useful.

What should I use as a bumper? Anything I can think of will block air flow between the piston and pilot, except if I attach the bumper to the back of the piston or put inside the back (it's shaped like a cup). I tried a pool noodle, but it pulled out of the piston and stopped it from opening more than halfway after a couple shots until I fixed it. Also, the area on the bushing that the piston hits is very small, a ring on the edge less than 1/8" wide.

After about 20 shots it seems to be working fine, no signs of cracking. I think my fairly light piston could be helping, and I'm going to remake the main tee (it leaks) and maybe the piston (rubber is off center) anyway. But you're right, more pressure and a better pilot means a faster moving piston, and I should stay safe.
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Unread postAuthor: mattyzip77 » Thu Sep 13, 2012 7:24 pm

No matter what you use as a piston you need a bumper. Dont matter how light your piston is. I use a half of a tennis ball with the center cut out.
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Unread postAuthor: shardbearer » Thu Sep 13, 2012 7:49 pm

Tried that, it didn't fit, and when I cut a slit through the side I finally got it in, but it kept going sidways in the pipe and it didn't stick far enough out to actually do anything.

EDIT: Here's a quick diagram of my setup, horribly not to scale.
Image
The female threaded adapter usually breaks first, right? Or the pipe between that and the tee?
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Unread postAuthor: mattyzip77 » Thu Sep 13, 2012 8:48 pm

Yea the female adapter will most likely break first. I use the tennis ball and check it after every 10 shots. If your piston is as light as you think maybe try a thick washer.... after cutting the slit in the tennis ball try cutting out a small chunk for a better fit. 1 1/4 inch couplers are heavy. If you have acess try a 2 inch rubber sanding drum for a piston. Its the best piston I have come across and needs no modification.
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Unread postAuthor: Technician1002 » Fri Sep 14, 2012 12:46 am

Due to the physics of F=MA, a piston hitting a sudden stop with no bumper provides a hammer blow to whatever it hit. This results in broken parts unless a soft gentle deceleration zone is provided for the piston movement.
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