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Two Piston design options.

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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Two Piston design options.

Unread postAuthor: Panzerfaust » Fri Jun 29, 2007 5:04 pm

While after the poor performance of my Tee Piston (see http://www.spudfiles.com/forums/piston-performance-t8695.html) I have decided to rebuild it putting male/female adapter that allows piston access in front of the piston instead of behind it. As dead space was determined to be the factor lowering my performance, i am working to decrease that. The valve resides within a 2" tee, using 2" porting in 1" porting out and 1" porting fill/fire.
Option 1:
Use no 2" pipe to fit piston in, piston resides inside reducer bushing. This would create tiny dead space but also tiny piston movement. However, in my last topic Jacksmirkingrevenge stated that my piston would only have to move 0.3 inches for full flow. This design leaves no room for a bumper.
Image
Piston in fully open position
Image
Piston resides inside this bushing
Image
Option 2:
Uses coupler containing 2" pipe making a cylinder for the piston to reside in. Allows 1"+ of piston movement and room for bumper. However, dead space is substantially larger than Option 1.
Image
Option 3 (just thought of)
I guess i could move the face (pipe end) the the piston seals against farther back (using option 2) in the piston's cylinder. this would move the piston's overall position back and thus decrease it's dead space. However, when the air out hole is in the middle of the tee i would think it would have better flow than when it is farther back inside 2" pipe. But the loss of dead space might overweigh this possible restriction of airflow.

I am worried about the first version, the pipe that the pistons sits againt would have to be perfectly adjusted (and that section is now removable to allow piston acces) and i think it would Jam up if it came out of its bushing. Second option however still has substantial dead space. The only way to decrease that space and still use some 2" pipe to give the piston a cylinder to move in would be to use and endcap with holes drilled into it for fill/fire. I really do not like the idea of drilling my own holes, they would have to be of much smaller diameter and i feel as if they would compromise integrety of the endcap.
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Unread postAuthor: hi » Fri Jun 29, 2007 5:49 pm

you could just use a diaphragm. then you dont need a bumper.
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Unread postAuthor: jrrdw » Fri Jun 29, 2007 5:53 pm

Are you calling the pilot chamber dead space? If thats the case, dead space is not your preformance problem. My cannon has more the option 2, and has great preformance. If it where me fixing your valve i would put the piston in a single piece of pipe. Give it at least 1" opening room, at the very least.
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Unread postAuthor: hi » Fri Jun 29, 2007 5:56 pm

1'' gives that piston alot of time to speed up and slam into the tee, so be careful. use a good bumper.
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"physics, gravity, and law enforcement are the only things that prevent me from operating at my full potential" - not sure, but i like the quote

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Unread postAuthor: Panzerfaust » Fri Jun 29, 2007 6:23 pm

Yes i have a bumper. but if dead space (pilot volume) wasent my problem last time then what was? i am building this new valve to improve because the last one dident work very well. Reference to the my last thread posted above to see what i am talking about. The last valve had the piston in 2" pipe and more than 1" actuation movement and its perofrmance was barely better than a sprinkler. The entire purpose of building a new valve was to improve over the old one. i thought the consensus about my last valve was that it's pilot volume was unecissarily large. Perhaps that was not truly the problem. If so then i need to figure what was wrong with the old one before i build the new one... any suggestions on that topic? I have to go somewhere now, when i get back il take a picture of the old valve to post.
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Unread postAuthor: jrrdw » Fri Jun 29, 2007 6:31 pm

Does the piston slide nicly in the chamber? Is it to tight? To loose?
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Unread postAuthor: hi » Fri Jun 29, 2007 6:33 pm

it could be that to much air is getting around it and it doesnt slide back fast enough because when you release the pilot volume, then air from the chamber goes into the pilot.
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Unread postAuthor: Panzerfaust » Sat Jun 30, 2007 10:26 am

Piston slid easily and could be operated by mouth pressure. in fact i was always impressed/amazed that i could actually presurize the gun by mouth presure, that alone was enough to actuate piston and make seal. I will need to go take some pictures but i see no problem with the old valve other than perhaps its large pilot volume that would have made it's performance so bad. It had at least 4-5 inches of room behind the piston. I would think the evacuation of that pilot volume alone would drop overall resivoir pressure some...
Image]
as you can see, old valve had about 7 inches of room behind sealing surface, caused by male/female adapters mostly. New design put these adapters infront of the piston, reducing room behind piston to about 5 inches, with a fair amount of that room being taken up by the bumper. Though my piston actuates smoothly and with little airflow, when i put it in a piece of pipe it actually does have a fair amout of room around the edges. Il add some more tape layers and see if it helps.
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Unread postAuthor: jrrdw » Sat Jun 30, 2007 2:54 pm

What did you make your piston out of? Tape will porbly come loose and get the inside of the travel sticky and hang your piston up. You didn't want to use o-rings?
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Unread postAuthor: rednecktatertosser » Sat Jun 30, 2007 3:30 pm

Could the problem be that the 1" barrel poring is restricting flow too much? if so you could switch to 1.5" barrel porting keeping the same piston design to fix your problem.
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Unread postAuthor: Panzerfaust » Mon Jul 02, 2007 10:34 am

The piston has some kind of wierd red tape on it that is very very slippery. I would use o-rings but i dont have the neccesary tools (lathe) to make grooves for them. I did think alot about using 1.5" porting, but 1.5" fits very tightly in a 2" tee, and i was worried it would either restrict flow due to lack of room around the pipe or that it would leave so little air infront of the piston that it would not actuate/actuate ineficiently. If these worries dont make sense to you, think about making a 2' coaxial gun with 1.5" barrel. the barrel would take up so much of internal space that there would be little room for air and airflow.
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Unread postAuthor: rednecktatertosser » Mon Jul 02, 2007 12:51 pm

True but also consider the vaccum effect behind the piston assisting the opening, as for the orings you can use a drill or drill press and a hack saw to make oring grooves, i did that to a good effect. just put a bolt through the center hole and stick that in the drill chuck and spin and cut grooves.
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