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Piston Valves Explained Visually

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: spudtyrrant » Tue Jul 28, 2009 5:09 pm

bazooka joe wrote:Ok, I've been having some issues with my first piston valve not sealing correctly. I made a coaxial with a 2 inch barrel and a 3 inch chamber, and used a longboard wheel with a couple sheets of cut rubber for the sealing face as a piston, but it doesn't seal. Whenever I put air into my launcher it all just leaks out the barrel at just a slightly slower rate then my compressor filling it. I can get it to 40 psi but its gone within 10 seconds or less.

Can anyone help me with this? I figured since this was a discussion about pistons that I could post this here. I can post pictures if needed.
Thanks in advance

post pictures and pour soapy water onto the joint to see where its leaking from
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Unread postAuthor: bazooka joe » Tue Jul 28, 2009 5:25 pm

It's not a leak in any of the joints, the air is coming straight out of the barrel, like the piston isn't properly sealing.

I'll go take some pictures now, be back shortly
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Unread postAuthor: Mr.Sandman » Tue Jul 28, 2009 5:32 pm

Try using a piston that has a flat sealing face. Not one with a giant hole through it. :wink:
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Unread postAuthor: bazooka joe » Tue Jul 28, 2009 5:56 pm

Mr.Sandman wrote:Try using a piston that has a flat sealing face. Not one with a giant hole through it. :wink:


i made it have a flat sealing face, I cut approx. 5 pieces of rubber and bolted them onto the wheel.

here's the pictures, btw

Image

Image

Image

Image

Image

If you need a specific picture I can go take more. I really hope someone can figure out whats wrong, because this is my first one and I have no idea whats going on with it :(

Also, keep in mind I still need to finish the launcher and add things to it, such as bumpers for the piston, and is not completed yet. Any advice on improvements would be greatly appreciated.

EDIT: That hose in the first picture leads to a blowgun, which is the pilot valve, in case that wasn't obvious :D
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Unread postAuthor: spot » Tue Jul 28, 2009 6:11 pm

Your pilot volume is huge... That's probably your problem.
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Unread postAuthor: bazooka joe » Tue Jul 28, 2009 6:23 pm

Is it really? damn, everything's already glued :(

how can I make my pilot volume smaller?
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Unread postAuthor: Mr.Sandman » Tue Jul 28, 2009 7:04 pm

I meant that behind the rubber there is a hole which means the air probably leaks through the middle of your sealing gace where the bolt is. Like previously stated,your pilot volume is indeed to large.
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Unread postAuthor: bazooka joe » Tue Jul 28, 2009 7:14 pm

Ah I see, that makes more sense. I'll figure out some way to seal it eventually, but any tips on doing it would help me finish this cannon a lot faster :D

So as for the pilot volume being to large, is there any way I can fix that? And about how big should it be?
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Unread postAuthor: jonnyboy » Tue Jul 28, 2009 7:21 pm

bazooka joe wrote:Ah I see, that makes more sense. I'll figure out some way to seal it eventually, but any tips on doing it would help me finish this cannon a lot faster :D

So as for the pilot volume being to large, is there any way I can fix that? And about how big should it be?


Put a big piece of wood in the back of the pilot with a large bumber, a long piston also helps but make sure it's not too chunky.
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mobile chernobyl wrote:I can shoot a Canuter Valve off my '82 Chevy Ram F150 AT LEAST 3/4 Mile with 'ma cannon made of soup cans duct taped together, then I just squirt some bacardi 151 in the chamber and hold up my cigarrete lighta and WHOOSH! That thing flies at least 3/4 mile
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Unread postAuthor: Mr.Sandman » Tue Jul 28, 2009 7:22 pm

Now that everything is glued its too late. However making a lighter longer solid piston would boost performance.
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Unread postAuthor: Technician1002 » Tue Jul 28, 2009 7:24 pm

It is leaking out the barrel. That indicates a flaw in the sealing surface. Ignore the pilot.. Sheesh. Start with the leak..

This is like talking to the drunk under the sreetlight. He wanted help finding his keys. After an hour there was no sign of them. I asked where he lost them.. "Over there" he said and pointed cross the street.

"If you dropped them there, why are we looking here?" I asked.

" The light is better here" :D

Start with the obvious. Where is the leak.. Front of the piston. End of the barrel.

If the end of the barrel has been cut with a saw, saw marks may provide leakage paths. I always glue a piece of sandpaper (300-600 grit) to the end of a dowel the diameter of my piston (or use the piston) and rotate against the valve seat to polish all scratches out of it.

Another possible leak it through the threads of the bolt.. Was this path sealed with pipe goop or bathroom caulk? I hope this helps.

After the leak is fixed, we can address the pilot size.. It might not need fixed.
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Unread postAuthor: bazooka joe » Tue Jul 28, 2009 7:26 pm

If i put a big piece of wood in the pilot area behind the piston, wouldn't that stop the air from flowing? or would the air just push it forward with the piston?

and for a bumper could i just use part of that hose in the first picture?
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Unread postAuthor: bazooka joe » Tue Jul 28, 2009 7:32 pm

double post, i know (stupid computer wont let me edit for some reason)

Thanks Technician for pointing that out, I should definitely fix the piston itself first. I'll try using bathroom caulking to fill the threads of the bolt and see if that helps. And i should've checked the barrel first, i'll try and figure out some way to sand it and make the seal better, if that is part of the problem

I'll get on that today and I'll let you know if it worked
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Unread postAuthor: Mr.Sandman » Tue Jul 28, 2009 7:57 pm

Technician1002 wrote:It is leaking out the barrel. That indicates a flaw in the sealing surface. Ignore the pilot.. Sheesh. Start with the leak.. .

Another possible leak it through the threads of the bolt.. Was this path sealed with pipe goop or bathr

After the leak is fixed, we can address the pilot size.. It might not need fixed.
Dont know if you saw this but i pretty much was focused on the leak then i suggested after that he decrease the volume :wink: :roll:
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Unread postAuthor: MrCrowley » Tue Jul 28, 2009 11:06 pm

I say Technician has the right idea, the bolt might need sealing. I had problems with a bolt on my first piston. Other thing might be that the piston is a bad fit in the pipe, it looks uneven, like it could lean to one side. This can cause jamming issues and an uneven sealing face which will cause it to leak.

Improvement:
1: Epoxy the bolt head on the sealing face
2: Take of the duct tape and make a solid piston :)
3: You'll need much more than a blowgun to pilot that. Try a 1/2" ball valve, a 3/4" or 1" sprinkler valve is always better. Bigger the valve, the better the performance. A blowgun wont do anything.

Oh and it looks like you are using DWV. As far as i'm aware all end caps that look like your one are DWV. Meaning they're not pressure rated.

Make sure all your pipe and fttings are pressure rated, those caps can blow out as low was 50PSI.

Read this:
http://www.spudfiles.com/spud_wiki/inde ... _Rated_PVC
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