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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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Piston cannon problems.... yet again :P

Unread postAuthor: Gun Freak » Sat Jun 12, 2010 4:59 pm

This isn't really my cannon but I was testing it for the first time today and there are a few problems. The piston leaks and air comes out the barrel port, but I know the cause and solution for this. My real problem is something I cannot wrap my head around though... the valve just blows out anywhere from 20-40 psi. First few were 40 (on the bike pump gauge, but in reality it's about 30) and the last shot was at about 20, or 10 in reality. It is a 2" piston valve 1.5" porting 1.25" coupling piston built in check valve identicle to Superna just with a 1/2 BV pilot. I have a video here:
[youtube]http://www.youtube.com/watch?v=645a0l80CCk[/youtube]
You can barely hear it in the video, but in person it's pretty loud.
Ideas?
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Unread postAuthor: Ranger » Sat Jun 12, 2010 5:24 pm

I'm just a newbie but I think I know what the problem is. When you pressurize it the pressure builds up in the pilot area until it can open the built in check vavle. When this happens all the pressure rushes to the main chamber and the pilot area has a lower pressure so the piston is pushed open. So I think the check valve is too hard to open.

I dont know :?: Its just a thought
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Unread postAuthor: geardog32 » Sat Jun 12, 2010 5:41 pm

i think your sealing face is sliding and coming out the barrel. whatever you have holding your sealing face onto your piston needs to secure it better. in mine i used a washer that just barley fit inside the sealing face with the sealing face behind it.

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Unread postAuthor: Gun Freak » Sat Jun 12, 2010 5:59 pm

The washer holding it on is as big as it can be, that cannot be the problem.
@Ranger, that is impossible because the check valve can be opened with the pressure of my mouth :shock: good guess though buddy :)
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Unread postAuthor: geardog32 » Sat Jun 12, 2010 6:08 pm

i still think your sealing face is blowing out. thats what i would mess around with if i were you.
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Unread postAuthor: c9d » Sat Jun 12, 2010 10:32 pm

could the pressure be leaking through your piston? i had this problem with my first piston design... a little silicone in the bolt passage between the bolt/washer, washer/seal, and seal/piston fixed it.
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Unread postAuthor: Technician1002 » Sat Jun 12, 2010 11:08 pm

If you think about how a piston valve works, air will hold the piston against the barrel. It fires when the pilot area is vented.

A leak in the pump due to a leaky quick connect, or bad check valve in the pump (my pump failed this way) and the pilot could drop in pressure between pump strokes and fire the cannon.

Pump slowly and watch the pressure gauge between strokes. When you reach the bottom of a stroke, hold for a moment and note the reading. Does is drop as you raise the pump handle?

My hand pump will do this to the same type of cannon. I fill mine into the chamber for this reason. A pump fault can't fire the cannon.

Note in the video it did pop right after the end of a pump stroke, not during a stroke.

On my pump, the lever on the pump is raised to disconnect the pump and tipped to the side to clamp it. Are you sure the pump is attached correctly?

If you can borrow a compressor, does it still fail while filling?

Edit: A leak through the piston bolt will do this. It is a leak from the pilot to the barrel.
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Unread postAuthor: BeaverRat » Sun Jun 13, 2010 1:07 am

When building mine, this same exact problem occured. I would hand pump it and it would blow out every time at about 20 PSI. The first thing I tried was to get a 1.5 inch fender washer that barely fit in the barrel seal. I was always planning on doing this, but I wanted to try it out and all I had was flat washers. This didn't fix the problem. The real problem was a leak through the piston (a 1-1/4 coupling) from the pilot area to the barrel, bypassing the chamber because I had a good o-ring seal. I fixed it by sealing the bolt with a lock nut and a small rubber washer in the fender washer to nut interface. It has never popped out once since and I brought it up to 100 PSI.
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Unread postAuthor: deathbyDWV » Sun Jun 13, 2010 1:46 am

I think tech said it but, you might not be getting enough flow with your bike pump to seal...
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Unread postAuthor: BeaverRat » Sun Jun 13, 2010 11:29 am

I think if it's sealing at 20 PSI, it should continue to seal at greater PSI's. Mine seals at any PSI I put it at while using my bike pump. I give it several sharp bursts of air to push seal the piston first before I start pumping. I think my tight o-ring seal and checkvalve are making a bike pump possible.
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Unread postAuthor: Gun Freak » Sun Jun 13, 2010 1:22 pm

Well after filling and blowing out over and over again, I took out the piston and the sealing face is blowing out. In the same place, over and over again. Another problem was that the front o-ring was coming out of the 2" pipe it travels in, and required too much force to get pushed back to expose the barrel. I took off the front o-ring and it fires like a charm, at 20 psi though. Once I get to higher pressure it still blows out. Tech, I will run some tests with my compressor to see if that is the problem. My pump does leak when you move the handle wrong. But that can't be the problem. As some of you said, it is leaking though the face, it isn't, because it is all epoxied. The seat is crooked because when modding the front bushing, too much lip was taken off on one side making the seat go in crooked. Maybe the compressor will solve the problem...
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Unread postAuthor: clemsonguy1125 » Sun Jun 13, 2010 2:40 pm

I had the exact problem on my 2 inch valve, from what I could tell pressure was entering the coupler from the back of the piston and blowing out the sealing face. Try finding a bolt that fit snuggly in the washer and make sure the bolt is tightend down. You and piston valves really dont get along do you :D
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Unread postAuthor: Gun Freak » Sun Jun 13, 2010 2:43 pm

Lol no, piston valves don't like me :? :D
Everything is really tight, the washer is big enough and there is no air leaking around the sealing face so I don't know what to do.
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Unread postAuthor: clemsonguy1125 » Sun Jun 13, 2010 2:47 pm

It might be the washers middle hole is too big and the bolts too loose in it, also try use 2 pairs of vice grips and tightening down the bolt really good
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Unread postAuthor: Gun Freak » Sun Jun 13, 2010 2:48 pm

I used a vise and vise grips, trust me it's tight, and any more tightness will break the check valve.
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