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MrCrowleys GB cannon, Built by me, Update: Fixeded

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: MrCrowley » Tue Sep 15, 2009 3:54 pm

Yeah, wow. You really need to have a decent bumper for heavy pistons. I've done a similar thing before because I used a pretty weak bumper and a glue piston.

You'll have to find a way to get the female coupler off...sometimes you can cut it in to 3 pieces, but make sure you don't cut down any further than the 2" pipe, then get a screw driver and try to pry the pieces off. Worked for me before, just wear safety glasses because the PVC will schrapnel and fly off pretty damn quick.
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Unread postAuthor: psycix » Tue Sep 15, 2009 4:08 pm

Next time dont cast your piston as one big solid mass, make it hollow on the inside, and fill that with foamy stuff.
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Unread postAuthor: dewey-1 » Tue Sep 15, 2009 4:18 pm

Northir wrote:yea it weighs about twice as much as the wood one


The epoxy piston, if its about 2" diam x 5" long is probably about 10 to 12 times heavier than the golfball. (45.93gm)
Assuming 2.09 gm/cm^3 for epoxy.
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Unread postAuthor: Northir » Tue Sep 15, 2009 4:24 pm

got the broken coupler off, i used my grinder with a sanding disk, i did like the epoxy one it seemed to have a better fit, and the bumper mats i used was insulating foam tubing i figured it would be enough, guess not, im thinking of a PVC tube with caps not sure if they make PVC in a size thats snug in 2" SCH 40 though
maybe a 2" wooden dowel my hole saw was just to big and i dont have a smaller one
maybe melted plastic one
on the note of a hollow epoxy piston how thick should the walls be?
trying the 1 1/4 coupler piston hope it doesnt shatter inside
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Unread postAuthor: Technician1002 » Tue Sep 15, 2009 9:55 pm

I hate to beat a dead horse, but PVC doesn't like repeated impacts. One of my early cannons used a PVC pipe cap for a piston. It's lifetime could be predicted not by the pressure of the shot, but just by the number of shots. Lowering the pressure extended the life but did not stop failures. This is the reason I stopped using PVC for the valve. I have had very good luck with HDPE.

On using bumpers, I try to use a large and flat of a bumper as possible so the impact spreads the stress instead of having a stress point.

Due to the high impact forces and the wear and tear on bumpers in my cannons, when I go higher pressure, I'll be trying a layered bumper patterned off the Newton's cradle, but instead of designed to transfer energy down the line, each layer mass would be designed to absorb energy in a multi layer approach to keep peak impact forces to a minimum. I'll do a theory write up later showing velocities, impact forces and deceleration zone.

I'm using wear of bumpers as a guide as I know my current design won't take the energy in a 200 PSI + design. My bumpers do take a beating and show it.

The energy you get is directly related to mass and the square of the velocity. Reduced mass and velocity are important. Safely reducing velocity near the end of travel when you want high speed is the challenge.

This is one of the reasons I'm a fan of smaller pilot valves with a moderate pilot volume. I like to use the pilot area compression to decelerate the piston some like a built in dashpot.

I think piston bumper design can be a topic on it's own.
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Unread postAuthor: jackssmirkingrevenge » Wed Sep 16, 2009 1:02 am

Epoxy Piston How-To

As pointed out above, it doesn't have to be a solid piston.

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Unread postAuthor: Northir » Wed Sep 16, 2009 1:24 am

Dont know if it matters but i dont use normal Epoxy i use Simpson Strong tie set 22 its for holding all thread in concrete its a lot thicker than JB and marine epoxy
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Unread postAuthor: jackssmirkingrevenge » Wed Sep 16, 2009 1:38 am

It's probably denser than what I'm using and therefore you should make an even greater effort to remove some of the volume and make it lighter.

For starters, does it need to be that long?
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Unread postAuthor: Northir » Wed Sep 16, 2009 1:48 am

honestly dont know, i was just going by what MrCrowley suggested, ill tape it and find my min length real quick

Edit: could probably get away with half as long maybe a little more
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Unread postAuthor: jackssmirkingrevenge » Wed Sep 16, 2009 1:56 am

The piston seals against the breech of the barrel, right? If it's tight enough in the tube it's sliding in - pretty much assured if you're casting it in the same tube - then the piston really doesn't need to be longer than its diameter in order to be stable.

As you can see in the examples above, I typically use pistons that are 75% or less as long as they are wide and they work beautifully.
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Unread postAuthor: Northir » Wed Sep 16, 2009 2:02 am

well the way i figure theres about a inch and a half of dead space where the piston could fall through so id want a piston at least 2 inches long? and my width on this cannon is also 2 inches, so what im thinking is i could use a peice of 1 inch PVC as my core and then just put something like a big washer over the top and bottom then add my bumper and gasket

Edit: BAH now i have a leak in my coupler where the old one was, any fixes for that?
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Unread postAuthor: Technician1002 » Wed Sep 16, 2009 2:07 am

jackssmirkingrevenge wrote:The piston seals against the breech of the barrel, right? If it's tight enough in the tube it's sliding in - pretty much assured if you're casting it in the same tube - then the piston really doesn't need to be longer than its diameter in order to be stable.

As you can see in the examples above, I typically use pistons that are 75% or less as long as they are wide and they work beautifully.


I've used pistons even shorter than the diameter with good results.
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Unread postAuthor: jackssmirkingrevenge » Wed Sep 16, 2009 2:08 am

Northir wrote:well the way i figure theres about a inch and a half of dead space where the piston could fall through


Fall through?

You can extend the barrel into the tee so the piston is behind the hole.
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Unread postAuthor: Northir » Wed Sep 16, 2009 2:17 am

unfortunately i didn't quite think it through, and my seat is now fixed in the center on my housing im almost thinking of removing the whole valve and starting over but it was working really well untill i tried the epoxy piston
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Unread postAuthor: jackssmirkingrevenge » Wed Sep 16, 2009 2:28 am

Technician1002 wrote:
jackssmirkingrevenge wrote:As you can see in the examples above, I typically use pistons that are 75% or less as long as they are wide and they work beautifully.


I've used pistons even shorter than the diameter with good results.


;)
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