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pistons

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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pistons

Unread postAuthor: qwert123 » Sat Feb 26, 2011 4:35 pm

hello spud files, i have my hot glue piston hardening next to me on my desk, and i was wondering how big of a pilot if a pilot at all should i have for a 1" piston. thanks for the input.
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Unread postAuthor: Technician1002 » Sat Feb 26, 2011 4:56 pm

Are you talking about the size of the pilot valve or the pilot volume?

A small pilot volume slightly smaller than the piston should work fine with a proper bumper in it so the piston moves into the pilot volume and the valve will open at least 1/4 the valve seat distance back.

Pilot valve size will vary depending on the amount of air that can leak past the piston when the pilot valve is opened. The lower the leakage (often called equalization) the smaller the valve needs to be. An 1/8th or 1/4 inch ball valve works wonders as a pilot valve. A blow gun works fine on lower leakage pistons.
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Unread postAuthor: jrrdw » Sat Feb 26, 2011 5:01 pm

Or EQ hole???
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Unread postAuthor: Technician1002 » Sat Feb 26, 2011 5:10 pm

Sometimes a hole is made. Sometimes a piston without a seal simply uses leakage for the same thing. It doesn't have to be a hole. Some hot glue pistons use a small groove on the side instead of a hole.
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Unread postAuthor: jrrdw » Sat Feb 26, 2011 5:35 pm

qwert123 wrote:if a pilot at all should i have



I think qwert123 is refering to asking about a EQ hole rather then a pilot chamber and or valve.
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Unread postAuthor: qwert123 » Sat Feb 26, 2011 6:43 pm

Ya, I did mean eq hole. Sorry
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Unread postAuthor: jrrdw » Sat Feb 26, 2011 7:19 pm

qwert123 wrote:Ya, I did mean eq hole. Sorry


In that case it depends on the piston fit. Clearance between the piston and cylinder wall. If air gets by it and it still opens nicely then no, you wont need a EQ hole.

I have a 2" o-ringed piston and my EQ hole is only 3/32nds, with the sealing face gasket acting as a check valve.
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Unread postAuthor: MrCrowley » Sat Feb 26, 2011 11:53 pm

If it doesn't have o-ring seals and the piston can move inside the piston housing without substantial force, you wont need an Eq hole.
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Unread postAuthor: mark.f » Sun Feb 27, 2011 12:23 am

For a 1" piston, I would suggest making a piston out of a bolt, some 1" fender washers, jam nuts, and some 1/8" gasketing. Sandwich some disks of the rubber, slightly oversized, between the fender washers in two locations (two spots that sit in the track pipe), trim to slightly larger than the washers with scissors, and then chuck it in a drill and sand the rubber disks to the same diameter as the fender washers. Then, tighten the fender washers to compress the rubber and expand it until it fits snugly in the track pipe with some lubrication.

Then, my friend, you will need an equalization hole, or simply handle equalization separately with a check valve and some tubing between your pilot and the chamber.

I still don't understand why some members make pistons out of hot glue. With all the documented methods and materials, I just can't see the reason. It's heavy, weak, and problematic.
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Unread postAuthor: Secret Squirrel » Sun Feb 27, 2011 1:17 am

After trying several times to make hot glue pistons, I have given up on the concept. They either were hopelessly stuck in the mold, torn apart upon removal of the mold, or quickly damaged during use.

I turned to epoxy resin for the same purpose and had much better luck. It's lighter, stronger, and much easier to cast than hot glue.

For an easy piston you could always try an appropriately sized end cap. Bolt on a rubber washer for a seal, and add a turn or two of electrical tape for a tighter fit if necessary. That's how I constructed the piston in my first cannon, and I never had a problem with it.

I did however have a problem with piston impact. After many shots the repeated impact of the piston on the back of the pilot cracked the back end of the launcher. Include a bumper in your design; I cannot stress this enough. It will save you headaches rebuilding later.
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Unread postAuthor: Crna Legija » Sun Feb 27, 2011 1:33 am

my frst piston was made of hot glue and i never had a problem with it up to 200psi then the bolt holding my sealing face started to get riped out.
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