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piston help

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 help

Unread postAuthor: boyce123 » Wed Jun 27, 2012 11:24 pm

I made a 2" piston valve pneumatic this morning and have some issues with piston return. The valve is just a basic 2" T design, with a sealing guide made out of a 2" piece of pvc. The piston is made out of a piece of wood that i turned on my wook lathe (so it didn't turn out exactly perfect, but it fits loosely in there) I don't have my sprinkler valve yet, so i've been trying to dump the pilot with a ball valve, the ball valve is reduced to a 1/4" pig tail that i've been using for filling/leak testing. The pig tail set-up has alot of resistance, so i'm thinking the piston failure is either a lack of a seal because of the imperfect piston, or simply a failure to dump the pilot volume fast enough. I've only been testing at 50 psi ( because i cut the sealing rubber to small in one spot :oops: , so above 50 psi pops the seal... i will make a new face tomarrow for higher pressure testing.) what do you guys think
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Unread postAuthor: Crna Legija » Thu Jun 28, 2012 1:29 am

i'm guessing its the loose fit, should be well under 0.5mm if you can wrap a layer or two of tape around it do that.
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Re: piston help

Unread postAuthor: jackssmirkingrevenge » Thu Jun 28, 2012 1:40 am

boyce123 wrote:(so it didn't turn out exactly perfect, but it fits loosely in there)


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Unread postAuthor: jrrdw » Thu Jun 28, 2012 6:45 am

You have a "wook lathe"? At any rate with any lathe you should be able to make a round anything that fits.

Fix the all ready known issues then try again.
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Unread postAuthor: boyce123 » Thu Jun 28, 2012 10:13 am

well... when turning wood, any imperfections in the grain end up causing the piece to be slightly off round, hence the problem with a wood piston. i guess what i really was asking was, after i fix the known issues (beyond the wait for a sprinkler valve in the mail) will the ball valve (with all the restrictions in the piping) be capable of making the piston function... I meant a "loose" fit for the piston as the piston having the ability to slide, yet there might be some some issues with to much of an air gap.
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Unread postAuthor: jsefcik » Thu Jun 28, 2012 10:15 am

for an awesome piston my friend, you can use a 2 inch sanding drum, it works beatifully, just take off the sand paper, put a bolt through the middle of the drum and add a rubber sealing face...try this if you continue to have problems with your wood piston
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Unread postAuthor: Technician1002 » Thu Jun 28, 2012 11:33 am

Is a Wook Lathe something a Wookie would use?

Seriously, a piston must have very tight tolerance for the pilot to be effective. Excessive chamber to pilot leak rate is the main problem with piston valve builds. Can you groove it and add a floating o ring?
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Unread postAuthor: mattyzip77 » Thu Jun 28, 2012 11:50 am

use a 1 1/4 coupler plugged at both ends and add a washer and a sealing face, and a few o ring grooves. The piston in question is most likely to small. Try adding a few layers of duct tape to make it tighter. Also, if you plan on keeping this piston if it ends up working, be sure to seal it with water seal or somthing like that.
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Unread postAuthor: boyce123 » Thu Jun 28, 2012 1:15 pm

i took the valve apart, it is a bit looser than i thought, but was tight enough that a layer of tape prevented it from going in. I never thought this would be a permanent piston, i only made this in an attempt to get the cannon done for the fourth on time (without having something machined). The coupler is a good idea, i think i'll take a couple 1 1/4 caps and put them end to end if they fit well enough. but i'll try cutting an o ring groove in the piston i have first.
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Unread postAuthor: mattyzip77 » Thu Jun 28, 2012 1:26 pm

End caps dont work good cuz they are dome shaped. Go with the coupler, you dont need to plug it, you could always put a washer at either end and or even fill it with hot glue or epoxy.
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Unread postAuthor: ramses » Thu Jun 28, 2012 1:32 pm

mattyzip77 wrote:End caps dont work good cuz they are dome shaped. Go with the coupler, you dont need to plug it, you could always put a washer at either end and or even fill it with hot glue or epoxy.


I actually use a 3" endcap in my 4" tee valve. I filled the cap with cut plywood disks, added a steel plate ("washer") and rubber disk as a sealing face. The round part faces the pilot.

You must make sure that there is no room for the piston to tilt, or you will have trouble getting the valve to seat initially. The advantage is that it is "guaranteed" to not leak through the piston between the pilot and barrel/chamber. Hot glue is also quite heavy.
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Unread postAuthor: boyce123 » Thu Jun 28, 2012 5:58 pm

the 1 1/4 end caps at my hardware store are squared off at the ends. i'll be working on the cannon tomorrow, i'll see if i can get some pics of what i come up with.

EDIT: I just went out and cut an O ring groove in the wood... the seal ended up being to good (not letting any air into the chamber), so i drilled a small leak hole through the middle. After that, i hooked it up to my air tank, standing vertically, and tried a dry fire. It worked great, accept the part where i was inside, and i hadn't anticipated how loud it would be. The 130 psi dry fire lifted it into the air about a foot, and temporarily deafened my right ear (still ringing). I think i will stick to the wood piston for as long as it works, and eventually i will step up and get a piston machined.

Thanks for the help and suggestions guys, i'll get it posted with damage pics when my sprinkler valve arrives.
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Unread postAuthor: Crna Legija » Fri Jun 29, 2012 1:00 am

you can machine hdpe on a wood lathe just take it slow, great you got it working and yeah they are really loud.
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Unread postAuthor: boyce123 » Fri Jun 29, 2012 10:27 am

i don't have a decent chuck to center it with, and i know someone who might turn it for me, just need to shell out for the raw material. I got it out today, and attatched a barrel for range testing. With the same length barrel, its launching similar, even farther than the big advanced combustion that i have.
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Unread postAuthor: sgort87 » Sat Jun 30, 2012 1:56 pm

Crna Legija wrote:you can machine hdpe on a wood lathe


I gave up that fight a LONG time ago. That stuff has a mind of its own on a wood lathe. Did you know HDPE rod stock has wings and teeth? ...well it does. Always wear a helmet.
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