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Piston Problem

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: hi » Tue May 06, 2008 8:42 am

i dont think your chamber is big enough. i made a small "test" chamber like that and my valve wouldnt open, but with a bigger chamber, it worked perfectly.
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Unread postAuthor: Pilgrimman » Tue May 06, 2008 9:10 am

Ditto. You need more air so that the pilot doesn't exhaust it all. Also, the piston needs to be a good fit, as has been mentioned. Both of these problems plagued my piston until I figured them out! BTW, it's nice to see that you did some real research beforehand!
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Unread postAuthor: no-limit » Tue May 06, 2008 10:27 am

I can't figure out how to make the pistion fit better. The piston fit pretty good in the pilot area because of the 2" pipe inside the tee and cap that I used to put the end cap on with. The pipe then stops when it hits the part it butts up againest. I have not yet figured out how to make it fit better there. Air will leak around the piston to the exhaust so maybe it has to be a tight fit. It would be nice if I could machine the tee out to fit the 2" pipe all the way though it to the sleeve at the front.
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Unread postAuthor: MrCrowley » Tue May 06, 2008 3:11 pm

As long as there is part of the piston still in the 2" pipe, it wont matter if it is a bad fit further up front where the 'lip' is.

Try putting an o-ring on the very back of the piston, also bring the barrel forward an inch and a bit, or what ever works for you.

Ditto. You need more air so that the pilot doesn't exhaust it all.

Shouldn't be a problem if the piston has a good seal inside the tee, I.E a good fit. You should be able to pilot it off a little chamber.

The perfect piston will only exhaust the pilot volume, not any from the chamber. So there shouldn't be a problem with a small chamber, though because he doesn't have o-rings or a perfect fit on this piston I doubt it will only exhaust the pilot volume.


@no-limit,

Here's what you do, put an o-ring on the back end of your piston or make one out of a deodorant can.

Make sure you use lubricant too.

Increase the pilot valve to 1/2", you could possibly do it with a blow gun, but it will hinder performance.

Extend the barrel further into the middle of the tee.


Do that and it will work.
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Unread postAuthor: no-limit » Wed May 07, 2008 11:24 pm

I think I have most of it worked out on my new one. I was just wondering about a to good of a seal, and the air can't get to the chamber to fill it.

What kind of lub. is the best for this, my new piston fits real snug, had to sand and sand for it to slide at all. I will post a picture when it is all assembled. I don't know if it needs an o-ring for sure or not.

I am using a 1" sprinkler for the pilot valve, not taking any chances.
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Last edited by no-limit on Thu May 08, 2008 12:03 am, edited 1 time in total.
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Unread postAuthor: MrCrowley » Wed May 07, 2008 11:51 pm

Just ask for plumbers lube at the hardware store.
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Unread postAuthor: dongfang » Thu May 08, 2008 3:46 am

Hi,

You could just try to wrap some duct tape around the piston, until it seems to seal pretty weel. Maybe 2 bands, one at the front and one at the rear. It it get too tight, you could use some silicone oil or KY jelly (yes seriously) to lube it.

Then, test again.

If it becomes much better, then you know the problem. Then you can think of how to make a permanent solution.

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Unread postAuthor: psycix » Thu May 08, 2008 7:31 am

Personal lubricants like "KY jelly" or "Durex play" work perfectly.
Just make sure they are on waterbase, not on oil.
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Unread postAuthor: dongfang » Thu May 08, 2008 9:49 am

Hi,

Silicone oil is Ok, I should think. Disagree?

But never mineral oil!

Regards
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Unread postAuthor: no-limit » Fri May 09, 2008 12:13 am

Here are a few pics. of the piston and works, just need to make the chamber. Think it will work???
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Unread postAuthor: dongfang » Fri May 09, 2008 4:07 am

Hi,

Wow, you sure work cleanly with that primer.

It looks well designed. Some people would probably want to get rid of that tee in the pilot section, to reduce dead space. (You could always fill it from a Schrader, installed directly into the bushing behind the tee.)

It is difficult to get a good impression of how well your piston seals against the tee / busing / pipe inside tee and bushing wall.

PS: Does it fire OK when you just open that ball valve?

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Unread postAuthor: watto » Fri May 09, 2008 5:01 am

That faucet adaptor is pretty long and would have a lot of pilot volume I'm guessing, notice in my picture how small my faucet adaptor thing is, It slots straight into the tee without any pipe. Your barrel also seems too far back, meaning your piston wont have much of itself in the housing unless its real long (which is an option) The piston doesnt need to move back much, mines about 1/3 of the barrel diameter, which I think is a bit on the small side for a tee valve but works well. Does the piston wobble when its up against the barrel? If so put tape around the back of the piston, the part thats in the housing, that'll last for a week or 2, but sounds like you need to make a new piston eventually, and put a proper chamber on it.

Image
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Unread postAuthor: no-limit » Fri May 09, 2008 12:52 pm

I would really like to know how you can make such a short piston. How do you get the piston to stay in the tee without it falling into the chamber. I tried the faucet idea like you did but had the above problem and that is why I added the sleeve at the end. Did you machine the tee out? My piston goes back half way through the sleeve right up to the chamber opening.

If you look at the pics of my first piston you could see, from what others were telling me, is that it needs to go back further. But now you say yours works further up? The last piston I made probably seals as good as this one, havn't tried it yet but the old one sealed real good.

I'm not using the ball valve to exhaust the pilot, that's a safty valve. The sprinkler valve is the pilot valve. On the other side of the safty valve is the quick connect to fill with, hook it straight to my C02.
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Unread postAuthor: JDP12 » Fri May 09, 2008 12:58 pm

you make such a short piston by extending the barrel back into the tee further so that the piston is further back, and have the barrel go over the bottom part of the tee so that the piston doesn't even move that far forward.
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Unread postAuthor: no-limit » Fri May 09, 2008 1:07 pm

So I have one who says it's to far back and one that says it's to far forward?
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