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How to use O rings in piston valves

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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Poll ended at Sat May 09, 2009 11:55 am

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Total votes : 7
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Unread postAuthor: Technician1002 » Fri Jun 11, 2010 9:35 pm

Correct, the piston is free to slide on the rod between the nut in front and the small pin in back. I didn't get a good picture of the pin in back. Sorry for the fuzzy shot. I took the photo and looked at it later. Here the pin is through the rod, bent on both ends, and the second end is ready to be cut short and pressed flat against the rod. This makes the rod 1/8th inch larger in diameter with the folded over pin. The pin is now small enough to retract into the stop assembly behind the piston. The marshmallow cannon is exactly this way, except the rod is much smaller so the pin looks huge.

The rod is pushed in and the pin in back pushes the piston closed. The rod and nut extend out the front of the piston. To fire the rod is pulled back. The piston stays in place until the nut goes inside the piston and hits it. At this time, the reset pin is safely out of the way for the piston to be blown back at high speed by the air pressure.

Without that feature, at 100 PSI the rod would try to go through your hand with over 300 lbs of force when triggered.

Instead it flies back freely and lands safely in the bumper.
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Pin behind the piston
Last edited by Technician1002 on Fri Jun 11, 2010 11:33 pm, edited 2 times in total.
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Unread postAuthor: Ranger » Fri Jun 11, 2010 9:52 pm

Wow, I make one comment and start this thread up again :D

GunFreak, look at his marshmallow gun for more details about the piston and rod, its ingenious.

I figured that the leak could not have been much, but I didnt want to make a guess and later be wrong. I probably would have said something like 10-20 psi and that might have made Technician mad.

I think this is a great gun design, but can this valve design be put in GGDT?
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Unread postAuthor: Technician1002 » Fri Jun 11, 2010 9:58 pm

It performs much like a burst disk. Without the broken edges of a burst disk, or the extra length for union or camlock for a burst disk, it seems to have a slightly higher COF. When launching a 4 inch foam ball and clocking the acceleration, I tried to match real performance with GGDT. I had to up the COF to get a match in acceleration and muzzle velocity.

Below is the launch data for the 2 oz 4 inch foam ball. This scope trace was generated by the ball with a small magnet passing through pick up coils on the barrel spaced 1 foot apart. Feel free to take this data and match a curve in GGDT.

Final foam ball speed was in excess of 450 FPS. Not bad for a 2 inch valve feeding a 4 inch barrel.

I probably would have said something like 10-20 psi and that might have made Technician mad.


I don't get mad. I laugh my ass off.
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Attachments
60 PSI Foam ball_1.jpg
Voltage passed through zero (centerline) when the magnet went from approaching to leaving the coil. This is the 60 PSI test.
Gauge.JPG
A good view of the gauge so you can find the leakage rate in the video.
Last edited by Technician1002 on Fri Jun 11, 2010 11:35 pm, edited 3 times in total.
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Unread postAuthor: Gun Freak » Fri Jun 11, 2010 10:12 pm

Alright, the whole QDV is becoming very clear to me now. Thanks a lot Tech!! :D
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