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Piston design with a built in check valve

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: FighterAce » Fri Nov 26, 2010 9:24 am

I know what pump you're using... I had it too. And due to its volume its no surprise you dont have to wait for it to equalize.

Right now I'm using a stirrup pump that has about 3 times the volume of the little shock pump. But I wont be using it all the time. Since I use high pressure and low volume I can afford myself to use pre-pressurized tanks and fill it within a couple of seconds.
If your piston can take that, I highly doubt it would work with a low flow pilot valve and if it did there would be a clear difference when compared to a 100% sealing piston.
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Unread postAuthor: saefroch » Fri Nov 26, 2010 6:19 pm

Today, I completed my check-valved piston and tested it. Performance benefits are obvious. The muzzle report is much more a concussive blast than a whoosh... and I thought it was pretty concussive before. :twisted:

Good luck with your piston, I guarantee you'll like it.
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Unread postAuthor: Selador » Sun Nov 28, 2010 4:45 pm

The way I built my 1-1/4" piston valve, the same seal that seals the barrel, also seals the piston chamber, when the piston is blown back.

Here is a pic to show the piston and it's seal in the two positions.

(Piston travel, etc, not accurate to any degree, of course. This is just for illustration purposes.)

By the way, this also seems to eliminate piston bounce.
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Unread postAuthor: mlz3000 » Sun Nov 28, 2010 4:49 pm

That's interesting, what do you use as a sealing face? My neoprene washer just folds a little and slides inside of the pilot.
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Unread postAuthor: Selador » Sun Nov 28, 2010 4:56 pm

mlz3000 wrote:That's interesting, what do you use as a sealing face? My neoprene washer just folds a little and slides inside of the pilot.


The seal is made of the red rubber you can get in small sheets at Lowes.

It has a fender washer on the piston side, to press it against the barrel. That washer is just less than the ID of the piston chamber. And smaller than, but close to the OD of the barrel.

It has a smaller washer on the barrel side. That washer is smaller than the ID of the barrel.
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Unread postAuthor: Gun Freak » Sun Nov 28, 2010 5:14 pm

How do you service the piston if the sealing face stops it from going back?
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Unread postAuthor: Selador » Sun Nov 28, 2010 5:30 pm

Gun Freak wrote:How do you service the piston if the sealing face stops it from going back?


The rubber flexes enough, that the piston can be shoved out of the piston bore, from the barrel end.

The washer on the very nose of the seal is small enough to allow that backward flex, when needed.
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Unread postAuthor: FighterAce » Sun Nov 28, 2010 5:41 pm

Oxbreath wrote:The way I built my 1-1/4" piston valve, the same seal that seals the barrel, also seals the piston chamber, when the piston is blown back.

Here is a pic to show the piston and it's seal in the two positions.

(Piston travel, etc, not accurate to any degree, of course. This is just for illustration purposes.)

By the way, this also seems to eliminate piston bounce.


Your going in the same off topic direction as JSR. My goal is to get air from the pilot to the chamber while sealing 100%. Your idea applies if you dont want air leaking into the pilot area once the valve opens. I thnk JSR's idea is far more durable... thats why I've been using it way before I even started this topic.

Gun Freak wrote:How do you service the piston if the sealing face stops it from going back?


If it cant go back then it must go forward :D

EDIT
Didnt even see your post...

If the rubber flexes so you can get it out, what keeps the air pressure from doing the same?
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Unread postAuthor: Selador » Sun Nov 28, 2010 6:13 pm

FighterAce wrote:Your going in the same off topic direction as JSR. My goal is to get air from the pilot to the chamber while sealing 100%. Your idea applies if you dont want air leaking into the pilot area once the valve opens. I thnk JSR's idea is far more durable... thats why I've been using it way before I even started this topic.


Sorry about that. You are correct. It was in fact, JSR's post that prompted me to post what I did.

I didn't mean to hijack. I was'nt paying enough attention.

FighterAce wrote:If the rubber flexes so you can get it out, what keeps the air pressure from doing the same?


The bumper.
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Unread postAuthor: saefroch » Sun Nov 28, 2010 11:18 pm

Just thought I'd post something that might have something of significant relevance to the author of this thread. Normal 1/16" thickness o-rings placed around UHMWPE will be forced off by a combination of o-ring deformation and stretching and extrusion of the UMHWPE. I'm not sure about PE-1000 but it probably has properties similar to UHMWPE. To compensate for these issues, I did this:
Image
I chucked a 3/8" button head bolt into a drill press and got the head down to just under .5" in diameter, almost the same diameter as my o-ring. It is threaded (I actually tapped the female threads) into the rest of the piston, and also serves as the check valve seat.

Let me know when you start work on your piston, FighterAce. I feel I'm nearing the end of development (dangerous words, I know) and would like to see somebody else try to put a check valve in a piston.
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Unread postAuthor: Selador » Mon Nov 29, 2010 5:59 pm

Has anyone built a chamber-sealer with a schraeder valve cast into the center of the piston ?
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Unread postAuthor: Gun Freak » Mon Nov 29, 2010 6:19 pm

Oxbreath wrote:Has anyone built a chamber-sealer with a schraeder valve cast into the center of the piston ?

Probably not, but it would be the same as any check valve in the center of the piston except schraders with springs don't crack open until 60-70 psi.
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Unread postAuthor: FighterAce » Mon Nov 29, 2010 7:08 pm

saefroch wrote:Let me know when you start work on your piston, FighterAce. I feel I'm nearing the end of development (dangerous words, I know) and would like to see somebody else try to put a check valve in a piston.


There are 4 weeks left of this semester and as you can imagine, there are many exams. Thats why I cant tell you exactly when I will have time to attempt this project. It may even be next year.

Couldnt this problem of deformation be solved with a redesign? Also, why isnt your piston surface polished? Something tells me you didnt machine this to a very high tolerance so you put the O ring in there to fill in the gap. Not to mention the offset check valve and the front sealing face also seems a little offset ...
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Unread postAuthor: saefroch » Mon Nov 29, 2010 7:44 pm

Deformation was solved with a redesign. :wink: That bolt at the back wasn't originally part of the design at all. It's crappily machined because I have no machining experience other than this project and one other small project. I didn't put the o-ring in there to fill the gap, that was originally part of the design. It is partially necessary because the brass nipple the piston seals into isn't machined very well :roll: so I needed a seal that would work in a slightly greater or slightly lesser diameter pipe. If you have access to good machining facilities and skills, yours will certainly be better than mine. The original prototype:
Image

Yup, it looks very offset. Yup, the tolerances are terrible, which admittedly, I could have fixed. But it does work. I can even hear a little hiss inside my Trident at the end of every stroke on my pump. Very satisfying. :lol:
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Unread postAuthor: FighterAce » Tue Nov 30, 2010 1:18 pm

Hmm... is it just me or there is not enough "meat" between the bottom of the O ring groove and the threads on the inside. I think thats why the original broke.

If you have a lathe, you're way ahead of me :D I still have to ask my friend to machine stuff for me. An old friend... hes been machining stuff for more then 40 years.
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