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Actuating a piston with a blowgun?

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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Actuating a piston with a blowgun?

Unread postAuthor: niglch » Sun Apr 27, 2008 9:52 pm

I just finished making my first piston valve. The piston housing is a 2" tee with a 1" barrel porting. The piston itself is a 1 1/4" end cap filled with epoxy with a 1/2" thick rubber disk set into the end cap and held in by the epoxy. I also wrapped some tape around the piston to make it fit a little more snuggly inside the tee. It seals great even just with a bike pump, and I was able to pressurize the gun to 70psi with no signs of leakage at all (I was very happy I was able to get this far on my first try).
My problem is that I cannot get the piston to actuate when I vent the pilot volume. I've done some searching and I'm fairly sure the reason is because the blowgun valve I am using is not providing sufficient flow to suck the piston back faster than the air can leak around it. I bought it on McMaster (6852K11). The air just comes out of the blowgun and the piston does not move until most of the pressure has been let out of the chamber. I get little to no air coming out of the barrel. I'm going to try piloting with a ball valve to see if it works (once I buy another), but I was really hoping to use a blowgun for obvious ergonomical and aesthetic reasons. I have been designing my gun to resemble a rifle (with a stock and breech loader) and was hoping for something more trigger-like than a ball valve for the final product. Is there anything I can do to get the piston to actuate without entirely changing the valve? I have heard that a piston can be actuated with as little as a Schrader valve if made correctly, so I'm guessing it is possible.

Here are some pictures of the valve:
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Note: the rubber disks which make up the bumper have holes through the center in order to allow plenty of airflow.

Thanks very much!
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Unread postAuthor: Mitchza89 » Sun Apr 27, 2008 10:16 pm

The blowgun is the problem mate. You have a pretty darn large pilot area, and a low flow release. By the sounds of it, your piston isn't a tight enough seal to make the blowgun effective (the gap between the piston and the housing is just big enough to match the flow of the blowgun so your piston isn't going to move).

Get yaself a ball valve and try something like what I did with my piston guns. Nothing better then a nice big ball valve to dump that pilot and piss off that golf ball :D
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Unread postAuthor: elitesniper » Sun Apr 27, 2008 10:18 pm

get a sprinkler valve to pilot that baby you'll me pleased with the results :)
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Unread postAuthor: Mitchza89 » Sun Apr 27, 2008 10:19 pm

Yeah that would be better then a ball valve trigger.... I'm used to most people on here using a rediculas amount of psi :D
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Unread postAuthor: Gippeto » Sun Apr 27, 2008 10:50 pm

I think Mitch nailed it.

You need to clean things up.

Mount your schrader on the 1/4" tee with the blowgun. This will eliminate some pilot volume.

Improve the fit of your piston to the tee (more tape or cast one from epoxy).

http://www.spudfiles.com/forums/making- ... t8919.html

I know that if you pay attention to the details, it can be made to work. :wink:


http://www.spudfiles.com/forums/my-firs ... 12796.html
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Unread postAuthor: Mitchza89 » Sun Apr 27, 2008 10:57 pm

I saw this tape at my local auto shop. It was this type of cloth tape. It was very sticky stuff, but it was really unabrasive so it would be perfect for using on a piston to reduce the gap between the piston and the housing.

I also saw some automotive epoxy putty that was 10 bucks a tube. It has 3000psi tensile strength. I reckon this is B.S but it says on the packet, "The best alternative to welding or braising!". Being a curious guy, I must test this stuff out :D.

Sorry for going off topic.
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Unread postAuthor: JDP12 » Mon Apr 28, 2008 9:02 am

better yet than a ball valve or sprinkler valve, go with a QEV... now that's some nice actuating power.
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