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Piston Problem

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 Problem

Unread postAuthor: no-limit » Tue May 06, 2008 12:55 am

Well I finally made my first piston valve. When I fill the thing up it works perfect. The problem is when I try and fire it. My piston is virtical as I shoot rockets. The piston won't drop until all the pressure is out, it then slowly drops. All the air comes out really fast so I think my exhaust is good. This is a 2" tee with 2-1 1/4" end caps for the piston. Barrel goes about 1/8" past the bushing. Anyone know what it is or do you need more info.
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Unread postAuthor: MrCrowley » Tue May 06, 2008 1:07 am

What's your exahust valve? How good of a fit is your piston?


There are probably 10-20 topics exactly like this, search and go through them.
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Unread postAuthor: no-limit » Tue May 06, 2008 1:25 am

I went though way more topics than that before I even tried to make one. The exhaust valve is off a 1/4" brass tee going thru a blow gun, everything exhaust really well like I said before. The 2 1 1/4" caps fit pretty good in the 2" pvc. It does get a little big after the pvc pipe ends in the tee, don't know how to get around that.
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Unread postAuthor: psycix » Tue May 06, 2008 1:30 am

I cant see any pilot valve in the pic you posted.
You need to let air out behind the piston.
This way the pressure behind it will drop and because there is still pressure at the front, it will be pushed back.

Tap in a blowgun or ballvalve at the back.
How do you fill it up? I cant see any schrader/quick disconnect.
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Unread postAuthor: MrCrowley » Tue May 06, 2008 1:31 am

What is the exhaust valve? Is the blowgun the exhaust valve? If so that is your problem, 1 1/4" caps aren't a good enough fit to pilot with a blowgun.

I meant topics like this one, asking for help because of a problem exactly the same to this one. 80% of those people solved their problem and yours is the same. So obviously you didn't go through them, did you?


Just because the air exahusts fast doesn't mean its any good. Your problem is a mix of the exhaust valve being too small and your piston not being a good enough fit in the tee.
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Re: Piston Problem

Unread postAuthor: Carlman » Tue May 06, 2008 1:32 am

no-limit wrote:Barrel goes about 1/8" past the bushing


theres your problem
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Unread postAuthor: MrCrowley » Tue May 06, 2008 1:35 am

Why's that the problem? The air should still be able to push the piston back...if there is any area for it too. :idea:

How much area of the sealing face is exposed when it's sealed against the barrel?

Also your piston doesn't look like a good fit in the picture. It must be pretty long otherwise it will not have anything to keep it stable at the front.
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Unread postAuthor: no-limit » Tue May 06, 2008 1:39 am

What is the exhaust valve? Is the blowgun the exhaust valve? If so that is your problem, 1 1/4" caps aren't a good enough fit to pilot with a blowgun.


What would I use?because the blowgun is the exhaust valve and all the air comes out pretty fast, maybe not fast enough though, and yes I went over them and over them and over them, this is really had for me to catch on to for some reason.

So from what Carlman says I need more air in the front of the piston.
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Unread postAuthor: MrCrowley » Tue May 06, 2008 1:44 am

Technically you don't, but it will hinder performance.

What do you mean you went over and over them, what did you go over?

I bet if you stuck a 1/2" ball valve on there it would work, but not to it's full potential. Try making o-ring grooves for the piston and extend the barrel further inside the tee.
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Unread postAuthor: no-limit » Tue May 06, 2008 1:46 am

I tried to make it something like this one.
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Unread postAuthor: MrCrowley » Tue May 06, 2008 1:51 am

Haha I made that last year. To be honest I made it for the instructables competition hoping people would fail at building it. I didn't want a bunch of 14 year olds running around with things this dangerous.

If you read my posts you should know what to do.

Since your not paying much attention....


TRY AND MAKE THE PISTON A BETTER FIT IN THE TEE, IT OBVIOUSLY ISN'T.

TRY AND BRING THE BARREL FURTHER INTO THE TEE, IT WILL HINDER PERFORMANCE AS IS.

INCREASE THE PILOT VALVE (EXHAUST VALVE)


That is all you need to do and i'll guarantee it will work.
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Unread postAuthor: no-limit » Tue May 06, 2008 2:06 am

Ah but I am paying attention and was going to do the above mentioned. Making the pistion fit better is the hard part. I also have this one for a model too but couldn't figure some of it out.
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Unread postAuthor: MrCrowley » Tue May 06, 2008 2:10 am

Try using a deodorant can like I did, it's been going for almost a year without any reapirs.

http://www.spudfiles.com/forums/how-to- ... 10381.html
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Unread postAuthor: no-limit » Tue May 06, 2008 2:29 am

haha follow that one again, remember I have been paying attention. That one is for 14 yr olds. hahahaha
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Unread postAuthor: MrCrowley » Tue May 06, 2008 3:02 am

Yeah but it gives an idea of how my deodorant piston is setup.
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