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Piston Qn.

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: SNDM » Tue Mar 04, 2008 4:13 pm

I donbt quite get this equalization hole business. Surely you lose valuable power by drilling a hole through your prevuiously airtight piston.

Why not, say, a one-way valve?

Like a ballbearing in a cone with a spring behind it:

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Unread postAuthor: clide » Tue Mar 04, 2008 9:58 pm

Yep, it will work fine. I put one way valves on pretty much all of my pistons. Just be very careful to avoid leaks in the pilot or make sure that the piston/check valve don't quite seal perfect or you might have accidental firings.
Mine are usually made of a couple of brass fittings a ball bearing and a spring. The ball-bearing on brass doesn't quite seal perfect, but it still seals enough to fire using a schrader valve.
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Unread postAuthor: Carlman » Tue Mar 04, 2008 11:15 pm

or you could just place a check valve in there to save the hassle of making sure it seals, but then again wheres the fun in that :wink:
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Unread postAuthor: jackssmirkingrevenge » Tue Mar 04, 2008 11:49 pm

SNDM wrote:I donbt quite get this equalization hole business. Surely you lose valuable power by drilling a hole through your prevuiously airtight piston.


Very few pistons are actually 100% airtight, unless you're using o-rings or some other type of rubber seal that guarantees there are no leaks whatsoever, equalisation holes are unnecessary.
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Unread postAuthor: lockmanslammin » Wed Mar 05, 2008 1:50 am

I used a 1/4 28 taper thread grease zerk in the back of my 3/4 inch piston. I was afraid that the cracking pressure would be too high so I yanked the spring out of it and used a tig welder at work to add a teeny amount of metal to the back to keep the ball from falling out...
works like a charm, and its tiny.
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 05, 2008 4:16 am

Back when I used to use syringe plungers as pistons, the seal would be 100% so an equalisation hole would be needed. I made my own check valves using brass tubing and a little rubber section from a disposable cigarette lighter.

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From the archived thread:

For the one way valve, first you need some brass/copper/aluminium tubing with an I/D of 1mm. You then need to drill out the first 3mm with a 1.5mm dia drill bit. The rubber seal is from a (empty!) disposable lighter - you have to remove the plastic from the top and screw out the nylon bit that supports the valve. If you pull the brass away from the nylon, inside you'll find this component at the base of the valve.
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Unread postAuthor: lockmanslammin » Wed Mar 05, 2008 4:43 am

crafty there.... I was concentrating on keeping the check valve tiny so I could put it right in the piston, while you ignored that idea and bypassed the piston completely, keeping piston weight to a minimum, and moving mass as simple as possible. Never even dawned on me.
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 05, 2008 4:51 am

Lock, your piston isn't airtight so you didn't even need a check valve ;)
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Unread postAuthor: SNDM » Wed Mar 05, 2008 9:33 am

Thx people, sorry for drifting off from original topic.
I somehow doubt I'll need equi holes on my pistons.
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Unread postAuthor: Jared Haehnel » Wed Mar 05, 2008 12:51 pm

Perhaps I missed it but...

What about flow?

If you move the piston back a further distance from the seal won't it allow a greater amount of air to flow through the barrel?
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 05, 2008 12:56 pm

Jared Haehnel wrote:If you move the piston back a further distance from the seal won't it allow a greater amount of air to flow through the barrel?


That's the whole point of this discussion - at 1/4 of the calibre travel, the piston provides the maximum flow that can fit through the barrel of a given calibre. Moving the piston back further does increase the potential flow, but since it's restricted by the barrel diameter it will be irrelevant, you'll still get the same flow rate.
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Unread postAuthor: Jared Haehnel » Wed Mar 05, 2008 1:04 pm

Man, I've got to stop missing the points...thanks Jack
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Unread postAuthor: lockmanslammin » Wed Mar 05, 2008 1:11 pm

I tried out some calculations for myself to see if I would come to the same conclusion, and I did. Perhaps this will help people understand it.

Imagine that you take a given pipe size and cut a length of that pipe that is 1/4 of the diameter. then you cut that little piece of pipe and flatten it into a long rectangle. if you then multiply the length of the rectangle by the width you will find that it is of equal area of the pipe cross-sectionally.

LOL never mind, I thought that would sound more simple than it does.
Maybe it will help someone,.. who knows.
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 05, 2008 1:20 pm

If I could figure it out after a night's hard liquor, it can't be that hard to understand ;)
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Unread postAuthor: The_Kangsta » Wed Mar 12, 2008 10:08 pm

The 1/4 caliber = ideal piston travel distance is correct, as Jack both stated and proved.


Does this mean if I made a co axial gun to shoot 4 mm bbs, the piston travel should be 1 mm? sounds strange, it seems a bit small......
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