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Piston cannon problems.... yet again :P

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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Unread postAuthor: sgort87 » Sun Jun 13, 2010 2:54 pm

These blowouts were a huge problem with my first Mauler designs. I tried all kinds of washers before I realized that it was actually the RUBBER!

Yes, a wide washer is good a good idea, which is why I machine my own washers to the maximum diameter to ensure the best hold on the gasket (and with a conical shape to ensure perfect alignment into the barrel port every time), but thick soft rubber - which I thought would be better for sealing - has this tendency to mush up in the weakest spot and keep popping out. I use 3/32" hard rubber sheet for my front seals. If you're using something thick and/or soft, try using some thin hard stuff and see how it goes.

P.S. I'm on the computer at work and can't see your video, so I have no idea if it shows the seal at all and voids my assumption about your choice of rubber. I'm just kinda goin' out on a limb here.
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Unread postAuthor: Gun Freak » Sun Jun 13, 2010 2:58 pm

The vid doesn't shot the seal, you can just hear the blowout. The rubber I am using is a thin hard neoprene washer from Ace. It isn't thick and soft, it's thin and hard, and yes, that's what she said :D
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Unread postAuthor: sgort87 » Sun Jun 13, 2010 3:17 pm

Hah!

Okay, then onto my next thought. When I put my front assemblies together, I use a thin layer of silicone to hold everything and to ensure no leakage out the barrel from the chamber or pilot. Do you have silicone on both sides of the gasket? The bursting may be from chamber pressure getting behind the gasket as the pilot and chamber equalize.

Oh, and about that torque... your front assembly bolt should not need to be vise-grip tight. You'll ruin PVC threads if you go more than a couple pounds of torque. I usually put some Teflon tape on the bolt and just snug it all up with a nut driver (fingers only - no palm gripping).
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Unread postAuthor: clemsonguy1125 » Sun Jun 13, 2010 5:24 pm

Oh, I just remembered you used hubbs piston, I was talking about Jorde2es(I forget his username) piston with 2 washers on the coupling, forget what I said, If you have a lowes or possibly a home depot check for packageing rubber, the stuff I use is red and works great, It comes on 2 thinknesses and both work great
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Unread postAuthor: BeaverRat » Sun Jun 13, 2010 8:11 pm

Yes, the plumbers pack red rubber works great!
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Unread postAuthor: Gun Freak » Sun Jun 13, 2010 8:31 pm

Sgort it isn't that tight, I didn't want to do it too tight because it would start to pull the check valve up and then it would be pointless. And I think the bursting problem is getting better, I was able to fire it at about 50 today the it bursted again at about 40. Dammmmm. I epoxied the nut and washer on the front, but not behind it. I HIGHLY doubt that it the problem though, my piston valve is identical to this one and it never blew out or anything!
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Unread postAuthor: BeaverRat » Sun Jun 13, 2010 10:57 pm

Replace the rubber sealing face... It could have some sort of imperfection.
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Unread postAuthor: Technician1002 » Mon Jun 14, 2010 1:28 am

Just a dumb question, the check valve in the piston, is the port outside the sealing surface (against the barrel) or inside the diameter of the barrel? Is it possible that while trying to fill the chamber the pilot pressure is blowing a bubble behind the seal and blowing it into the barrel? The check valve port should be outside the circle of the barrel seal so pressure goes into the chamber.
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Unread postAuthor: Gun Freak » Mon Jun 14, 2010 10:59 am

The holes in the piston where the check valve air come out are around the piston in the chamber part. The air does not go behind the sealing face.
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