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Barrel Sealing Coax Valve Problems

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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Barrel Sealing Coax Valve Problems

Unread postAuthor: webgeek » Thu Jun 21, 2012 8:33 pm

Hi all; I'm desperate for a quick fix. I'm exhibiting something kinda like this at the Kansas City Maker Fair this year:
[youtube]http://www.youtube.com/watch?v=K73s4MRJSzc#![/youtube]

I did this last year using my modded sprinkler valve gun and it worked great. I was about to get fireballs about 20 or so feet in the air - pretty killer actually and quite popular. My gun is a lot more powerful than what he is using in the video.

So this year I made a 4" chamber, 2" barrel gun using this exact design just doubled:
[youtube]http://www.youtube.com/watch?v=9oARDyn4WwE&feature=my_liked_videos&list=LLtd3l-5qUNE66WyEIU5gVKA
[/youtube]
I had to create my own piston using plywood and gasket rubber on the face. I had some trouble getting the piston to seal to the barrel but solved this with some epoxy and rubber o-ring on the barrel. Now it seals perfectly against the barrel every time. I trigger the cannon via a modded 1" sprinkler valve which is quite snappy. The whole design is custom made just for this fireball purpose with a propane ignition ring and everything!

Other than it not working, there is one big problem with the cannon. I have far too much piston travel (probably 6") I know this is bad and stupid of me, it was a dumb mistake when gluing and I can't really fix it in time for the fair.

Now the problem. When I fire the cannon, all the air goes out the sprinkler valve and none of it goes down the barrel. I mean none at all. I've put some leaves in the barrel and they actually get sucked back onto the piston when it fires. I'm at a loss to fix this. The piston fits reasonably tight to the chamber, air gets around it fine, but it's not really badly leaking.

My thinking is that the combined mediocre piston fit combined with the far travel allow too much air to evacuate through the very effective sprinkler valve while the piston is moving backwards. I suspect if I increased the PSI up to something more like 90-100 it might work OK due to the increased pressure but at say 40 PSI I get a loud air pop sound from the exhaust and nill from the barrel.

So, what do you experts suggest? Can I make a long piston to minimize the travel? Am I off base on the problem? Anyways, I'd love any advice as this is driving me crazy and I'm really out of time to fix it.

Thanks!

-Mike
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Unread postAuthor: mattyzip77 » Thu Jun 21, 2012 9:20 pm

You want to keep piston travel to a. minimum so you could try to make a bigger piston but bigger piston means it will be heavier and you will need a proper bumper. Are you sure the sprinkler valve is goin the right way? try upping your pressure 10 psi at a time up to 70 you may need more pressure to actuate it. Without pictures of your actual cannon its hard for us to help you. Are you filling through your pilot valve?
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Unread postAuthor: MrCrowley » Thu Jun 21, 2012 10:00 pm

Can you access the pilot area? If so, just fill the extra room up with bumper material. You could cut some more 'spacer' disks from plywood and drill 1" holes through the middle of them.

Apart from that, the piston fit is probably the culprit. With those big piston valves you do tend to have that problem. To remedy this, you could probably coat the piston in a super glue or thin epoxy; anything that will leave an even coat should do it. If it's too big, it wont take 5 minutes to sand the coat down to fit. Make sure you use lubricant as well. Not only does it allow you to get a tighter fitting piston, but it should help reduce air flow around the piston.
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Unread postAuthor: webgeek » Thu Jun 21, 2012 10:26 pm

Are you sure the sprinkler valve is goin the right way?
Yes.
try upping your pressure 10 psi at a time up to 70 you may need more pressure to actuate it.
It actuates fine. The air rushes out of the valve as expected, just not out the barrel.
Are you filling through your pilot valve?
No, I plumbed in a manual valve and a air line connection so its trivial to fill for a second shot. Basically flip the manual valve open, cannon auto-fills, flip it closed. Ready to fire.

Can you access the pilot area?
Yup, one of the nice things about that specific design is the pilot area is easily accessible.
You could cut some more 'spacer' disks from plywood and drill 1" holes through the middle of them.
This is what I was thinking of doing but I wanted to get forum feedback first.
To remedy this, you could probably coat the piston in a super glue or thin epoxy; anything that will leave an even coat should do it.
Ah, such a simple idea. I'll go do this now so I can sand it in the morning. Thanks for all the feedback everyone, it's greatly appreciated! Hopefully I can get this working and post some big fireballs!

-Mike
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Unread postAuthor: jackssmirkingrevenge » Thu Jun 21, 2012 11:47 pm

MrCrowley wrote:Not only does it allow you to get a tighter fitting piston, but it should help reduce air flow around the piston.


... which is essentially why you want a tighter fitting piston :)

As opposed to adhesive fillers, I would try wrapping the piston in masking tape or duct tape, so that if you get something wrong at least it's easily reversible.

As was suggested, using spacers to reduce your pilot volume and piston travel to 1" or so will also be very beneficial.
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Unread postAuthor: MrCrowley » Fri Jun 22, 2012 12:04 am

jackssmirkingrevenge wrote:
MrCrowley wrote:Not only does it allow you to get a tighter fitting piston, but it should help reduce air flow around the piston.


... which is essentially why you want a tighter fitting piston :)

I was implying that the lubricant is surprisingly good at getting pistons to fit that wouldn't fit normally if dry AND that lubricant also reduces air flow as it fills the gaps around the piston.

I realised that my wording could make it sound like I was talking about the same thing but I thought I'd leave it to see if anyone would pick it up :wink:
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Unread postAuthor: jackssmirkingrevenge » Fri Jun 22, 2012 12:32 am

MrCrowley wrote:I was implying that the lubricant is surprisingly good at getting pistons to fit that wouldn't fit normally if dry


As with many things in life ;)
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Unread postAuthor: MrCrowley » Fri Jun 22, 2012 12:34 am

Dammit, I gotta start proofreading more often :D
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Unread postAuthor: Crna Legija » Fri Jun 22, 2012 1:35 am

Is the piston bigger then the barrel seal?.
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Unread postAuthor: webgeek » Fri Jun 22, 2012 8:10 am

As opposed to adhesive fillers, I would try wrapping the piston in masking tape or duct tape, so that if you get something wrong at least it's easily reversible.
Before I posted here for help I tried this. One layer of duct tape wasn't enough and two was too much. I'll be sanding the piston this morning to see how it goes as it's already been filled. I'm also going to use some axle grease on it too.

As was suggested, using spacers to reduce your pilot volume and piston travel to 1" or so will also be very beneficial.
Yup, this is next.

Is the piston bigger then the barrel seal?.
Yes, the piston fits the ID of 4" PVC. The barrel is 2" PVC. The sealing face covers the entire 4" for convenience. The barrel seals perfectly every time - one of the few things that has worked great thus far.

-Mike
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Unread postAuthor: MrCrowley » Fri Jun 22, 2012 1:47 pm

webgeek wrote:Before I posted here for help I tried this. One layer of duct tape wasn't enough and two was too much.
Ah, no wonder you're having problems. If you can fit a layer of duct tape between the piston and piston housing, the gap is definitely too big.
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Unread postAuthor: webgeek » Fri Jun 22, 2012 1:49 pm

It's fixed now. Wow, what a monster. It's shooting flour 2x the height of my house! Thanks much for the help, the combination of tighter piston and less travel/volume behind it totally fixed the problem. Anyone in the KC area should head to the Maker Faire at Union Station and you can fire it off :)

-Mike
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Unread postAuthor: mattyzip77 » Fri Jun 22, 2012 2:17 pm

I would be carful about letting anyone fire your cannon unless you have them sign a waiver or disclosure.
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Unread postAuthor: webgeek » Fri Jun 22, 2012 2:30 pm

It's actually fired remote trigger and it's mounted vertically with me handling both the loading and the charging. I also have smaller pneumatic guns with electric valves that the kids shoot but we keep the pressure low on em and they are far heavier built for kid-friendly-ness.

-Mike
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