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Sticky Piston

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: jrrdw » Sun May 18, 2008 9:41 am

spudfarm wrote:mabe you can use some springs that pulls the piston back like 1/100" and then the pressure can work on the whole sealing face


You would have to rebuild the valve to make this idea work. Spring hooks would have to be added, tension would have to be adjusted, tension adjusters would have to be added. The tee would need to be acomendateing to the springs. There's probly more I'm missing but thats just off the top of my head.
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Unread postAuthor: SpudFarm » Sun May 18, 2008 9:58 am

or you can just screw a small hook in the back of the piston and one in the pilot side and hook up a short and tight spring.
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Unread postAuthor: jrrdw » Sun May 18, 2008 10:19 am

If there is constent pull, pulling the piston away from the sealing face, how will he get the piston to seal to fill the storage chamber? Keeping easy use in mind.
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Unread postAuthor: SpudFarm » Sun May 18, 2008 10:45 am

the pressure behind the piston pushes it.. when the pressure get lower then the spring stregth it will open releasing all the air from the full chamber
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Unread postAuthor: daberno123 » Sun May 18, 2008 12:15 pm

psycix wrote:Ahh I see!
You are trying to seal a 1,5" porting with an 1,5" piston?
That means youve got NO frontal area for the pressure to work out force on.


Actually I have 1.5" porting and a 2" piston. My problem is what brogdenlaxmiddie said, and that is that my sealing face is being pushed into the barrel since there is no washer behind it. I'm putting in an order to McMaster soon and I added some 2" washers and o-rings for the new piston.

I think adding a spring behind the piston will just make it harder to seal.
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Unread postAuthor: SpudFarm » Sun May 18, 2008 12:21 pm

there was many people using springs a few years ago when they tought it was needed to pull the piston back
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Unread postAuthor: daberno123 » Sun May 18, 2008 1:33 pm

Ok, here's the diagram of the new piston I'm going to be making. Anyone see any problems with it? Its basically the same as the previous one, with the addition of o-rings and a washer behind the sealing face.
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Unread postAuthor: elitesniper » Sun May 18, 2008 1:38 pm

Try to fit a small check valve in it, more efficient.
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Unread postAuthor: daberno123 » Sun May 18, 2008 2:45 pm

Good idea elitesniper, just added one to my McMaster order.

Part #7768K26

Edit: I'm also changing the servicability of the back from bolts to a threaded reducing bushing. Hopefully it will make it alot easier (and faster) to take apart. The bolts always leaked anyways. Has anyone had any trouble with threaded pvc parts not sealing? They never seem to screw in all the way.
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Unread postAuthor: jrrdw » Sun May 18, 2008 4:48 pm

When he said check valve I think he ment drilling a small hole in your piston, not ordering a seprete one from McMasters, I'm not sure but thats what I think.
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Unread postAuthor: psycix » Sun May 18, 2008 5:10 pm

Drilling a small hole under the flap of your sealing face can act as a check valve.
It just flaps open to let air pass into the chamber, but when pillotted, the flap is sucked against the hole and seals it.
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Unread postAuthor: daberno123 » Sun May 18, 2008 6:52 pm

So I would have to drill through the washer and the cap behind the sealing face?

I was going to install one from McMaster inside the piston but this method seems much easier. As long as I can drill through the washer...

EDIT: OK, I'm pretty sure I'll be able to find a drill bit to drill through metal. Where will the holes in the washer need to be? Where it presses up against the walls of the 1.5" porting? Or inside the porting?
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Last edited by daberno123 on Sun May 18, 2008 7:12 pm, edited 1 time in total.
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Unread postAuthor: jrrdw » Sun May 18, 2008 7:10 pm

That little hole has to be located outside of the barrel's inner diameter. Thought that would be importent to point out.
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Unread postAuthor: daberno123 » Sun May 18, 2008 7:17 pm

Wow jrrdw, you seem to read minds.

I don't think I'll be able to drill outside the walls of the porting, there will only be about .05" around there. If I drill a little further in, like right where the washer presses against the wall, will I still be okay?

Otherwise I could add the check from Mcmaster inside the valve and drill holes around the circumference of the piston before the first o-ring groove. It would work in the same basic way, except it would be heavier.
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Unread postAuthor: psycix » Mon May 19, 2008 5:19 am

You will have to.
If you are drilling it on the walls of the porting (or even worse, inside the porting), the chance exists that it will not be able to let air pass onto the chamber since the piston presses the flap against the hole.
When its outside of the porting, the sealing face can flap open somewhat.
It doesnt need to be a large hole

I don't think I'll be able to drill outside the walls of the porting, there will only be about .05" around there.

In that case, your piston may still be too small for the porting.
If you had a larger one (say, 2,5"), you would not only be able to install the check valve hole easier, but it would also perform better.

You COULD make the porting pipe have tapered edges (like a spud-cutter, but then not sharp) this makes not only the sealing face seal better, it also improves flow AND because the porting walls that press against the sealing face are thinner it will be easier to install a check valve-hole.
It also increases initial frontal area somewhat, and since yours is small, it is relatively a large difference.
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