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A few piston questions.

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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A few piston questions.

Unread postAuthor: iisthemuffin » Sat Jul 12, 2008 8:17 pm

Ive seen a few diagrams and in them all it seems like air is supossed to be able to go around the piston so that the built up air will slam the piston back.

Im sorry if i am unclear in that meaning. Im basically saying from what ive seen there should be a tiny gap between the piston and pvc housing.

But from a few threads i have read, its seems as though there shouldnt be. It seems like people are implying that it should be air tight against the piston housing.

I would appreciate if someone were to clarify for me.
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Unread postAuthor: keep_it_real » Sat Jul 12, 2008 8:27 pm

There are two general ways to do it. One is having the very small gap between the piston and its housing. The only rule here is the pilot valve should have higher flow than around the piston.

Another way is to have a perfect seal around the piston and then have some kind of check valve that lets air pass from the pilot area (behind the piston) to the chamber. I use the first setup just because it's simple.
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Unread postAuthor: iisthemuffin » Sat Jul 12, 2008 8:29 pm

Ok makes sense. So as long as my pilot valve has better flow than the gap it will actuate?

Also is there a standard on how much of the housing my piston should occupy?
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Unread postAuthor: keep_it_real » Sat Jul 12, 2008 8:43 pm

Theoretically yes although it needs to overcome friction too. The more flow in the pilot valve the better.

The piston only needs to move back a quarter of its diameter. Anymore than that and the flow stays the same. Is that a good answer?
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Unread postAuthor: hoarp001 » Sat Jul 12, 2008 8:46 pm

I machined my piston to be a very tight fit in the valve body. With two sealing Orings and lots of grease, the air couldnt get through, it was a perfect seal.

i then drilled myself a small transfur hole, about 3mm in diameter in the top of the piston. This did the job, it allowed air through to the chamber but when I fired, quite a lot of air leaked out through the pilot before it fired properly.

To solve this I took a rectangular peice of rubber about 20x10mm and put a strip of superglue along one edge and glued it so the rubber flap covered the hole. This way it would let air through into the chamber, but the flap covered the hole and wouldnt let air back through the other way. Abit of grease over the hole and on the underside of the flap helps the seal.

It worked perfectly and now my piston opens first time, every time without fail.

The design of my piston is such that it has a seperate piston and seal. We designed it this way to reduce weight, and introduce the adjustability with the threaded rod. You can screw it longer and shorter to fine tune it.

It also allows a a large surface area on the back of the piston for the air to act upon, which is why I believe it is so successful. It also allows a large surface area for the pressure to push down on which is why it has such a good seal. This piston is qutie large, its built inside a 3.5inch tee peice with a full 3 inches of porting, but the same principal works on any size.

If you make the piston a very good seal with afew O rings, drill a hole and install a simple rubber flap valve, you cant really go wrong...

Image

Image
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Unread postAuthor: psycix » Sun Jul 13, 2008 8:22 am

For optimal results, the piston should be airtight and thus you should use o-rings.
But when filling, the piston should let air flow into the chamber, solution:
Equalisation hole: a small hole for letting air go in, oftenly drilled UNDER the sealing face flap so it acts as a chaeckvalve and will close when the gun gets pilotted.
Another way is to create a floating o-ring or, simply dont use o-rings for a non-airtight design.
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Unread postAuthor: iisthemuffin » Sun Jul 13, 2008 10:02 pm

So the equlization hole lets air in front of the piston to build up the force to push ot back? And you saying that this will work more efficiently than one with a gap?
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Unread postAuthor: psycix » Mon Jul 14, 2008 5:52 am

You fill the piston from behind, the pilot side. When using o-rings, the piston is airtight and no air can get to the chamber. Then you need a equalisation hole, a very small hole to let air flow into the chamber from the pilot.
When firing, you let air out of the pilot, and you dont want air going from chamber into the pilot. Point is, that when you have a hole in the piston, this is going to happen.

Solutions: Make the hole as small as possible and for the best results, a check valve will make sure no air goes the wrong way.
Having a rubber flap over the hole (possibly the sealing face) will act as a checkvalve. When filling, the air will blow open the flap and go into the chamber, then pilotting, the air sucks the flap into closed position and no more air can get into the pilot.

When NOT using O-rings, air can slip past the piston, so you will NOT need an equalisation hole. Bad side is that when pilotting, some air will slip past the piston, and since you want to let all air out of the pilot volume ASAP, this is not helping.
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Unread postAuthor: evw2k » Tue Jul 15, 2008 5:07 pm

Okay, I have been trying to figure this out for a few weeks now and still don't get it. I bought a tee for 3'' pvc and crafted a piston out of 4'' abs plastic by cutting a portion of it and folding it down to the diameter i need for it to fit into the tee.

the tee is like this. pilot and fill-------piston------barrel
|
--------chamber----fill valve

The problem I have is that the tee is pretty small and the piston needs to cover the entire hole from the chamber. so already the piston needs to be 3'' long, but then it doesn't have enough room to actuate and open the chamber. How do people get around this?

edit: okay they didn't let me do my diagram thing, but the chamber and fill valve come off under the piston. Its a tee.... T
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Unread postAuthor: pat123 » Tue Jul 15, 2008 5:10 pm

the piston fits in a pipe on th epilot fill side of the tee. the barrel should extend into the tee.
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Unread postAuthor: iisthemuffin » Tue Jul 15, 2008 5:11 pm

Umm im a little unclear in what you are saying but if the problem is what i think it is then you could push your barrel further into the tee and shorten the piston.
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Unread postAuthor: Pilgrimman » Tue Jul 15, 2008 5:27 pm

The barrel has to extend into the tee. To do this, you have to grind out the stop on the reducer bushing to allow the pipe to go through. This is one of the trickiest parts to get right.
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Unread postAuthor: evw2k » Tue Jul 15, 2008 7:56 pm

sorry, i guess i wasn't that clear.

I have no problem getting the barrel to meet the piston. So I threw together a crappy not to scale in any way pic.


Image

The stuff in red is what im worried about. The piston can only move back like 1'' before it hits the endcap. With so little travel, the hole to the chamber will never open all the way. Am I doing something wrong? All the tutorials I was basing this on didn't have this problem and I'm following exactly.
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Unread postAuthor: hoarp001 » Tue Jul 15, 2008 7:59 pm

The piston only needs to move back a quater a diameter of the port to acheive max flow. So if its a 1.5in port, the piston only needs to move back by 9.5mm.

In my 3.5inch piston valve, the piston only moves back about 20mm, which is about two thirds of an inch.
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Unread postAuthor: Pilgrimman » Tue Jul 15, 2008 8:24 pm

The smallest area is all you need to worry about. In this case, the smallest flow area will be your barrel. So the only worry you will have with this particular piston (From what I see in the diagram) is that the piston will sag down into the tee.
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