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Piston problems

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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Piston problems

Unread postAuthor: frankrede » Wed Jul 02, 2008 9:25 pm

Ok guys I am having a major problem
My cannon is nearly complete, but for some reason it doesn't fire correctly, my piston seals and doesn't leak, it is actuated by a 3/4" sprinkler valve but when I pilot the cannon most of the time except for once all of the air went out my pilot.
The piston is a wooden dowel with a few layers of tape around it and it is a really nice fit and slides smoothly.
Is my pilot too strong? or what?
I really am horrible when it comes to piston cannons...
heres a picture of the pilot assembly
if more pictures are needed of the internals or piston I can get those
<a href="http://s30.photobucket.com/albums/c325/redensekjf/?action=view&current=1215044445.jpg" target="_blank"><img src="http://i30.photobucket.com/albums/c325/redensekjf/1215044445.jpg" border="0" alt="Photobucket"></a>
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Unread postAuthor: Launch-Tek » Wed Jul 02, 2008 10:26 pm

More details would be good.
If I'm not misunderstanding your design, it sounds like the hole in the piston is too big and it is just venting the air through it instead of moving the piston.

If your sprinkler valve is not opening your hole in the diaphragm may be too big.

Hope that helps.
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Unread postAuthor: frankrede » Wed Jul 02, 2008 10:33 pm

The sprinkler opens fine, its just that most of the air goes through the sprinkler

its a coaxial

The piston seals almost perfectly actually, maybe its sealing to good.
I don't have a equalization hole.
I will probably be creating a new piston soon, maybe it will work better
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Unread postAuthor: brother361 » Wed Jul 02, 2008 10:45 pm

i a new piston will probly be your best bet i dont know alot about this but it seems like it might be getting stuck try making a hot glue piston there very easy to make and very cheap
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Unread postAuthor: Launch-Tek » Wed Jul 02, 2008 10:49 pm

I must be missing something because I would think you would need an equalization hole so the pressure could get into the chamber.
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Unread postAuthor: MrCrowley » Wed Jul 02, 2008 10:53 pm

Is the tape on the piston evenly spread over the dowel?

Is the piston lubed up? How easy is it to push the piston inside the pipe?

Does the piston ever get jammed a little when pushing it in?


99% of the time this happens because your piston isn't a good enough fit, it may seem that way but it probably isn't. If the tape doesn't add too much thickness you could always take it off and expand the dowel by coating it in a waterproof wood wax, it also keeps the piston lubricated.

I do this to my wooden pistons to waterproof them and then turn them down a tiny bit on the lathe to make a good fit, but since yours is already too small, the wax may expand it enough.

On second thought, try drowning the piston is varnish (literally), and leave to dry, then coat with wax. That will seal the varnish inside, which will expand the wood a fair amount, keep the piston waterproof and lubricate it.

@Launch-Tek,
I o-ringed a piston once and it still didn't require an equalization hole. If the piston moves fairly easily in the housing, I doubt he'll need one. Though it could be a possibility, may as well check it out to make sure, frank.
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 02, 2008 11:06 pm

Also check if the piston is too short and therefore can skew from side to side, what about C:B ratios and pressure?
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Unread postAuthor: Launch-Tek » Wed Jul 02, 2008 11:17 pm

I think I understand now. The air bleeds past the piston seal to fill the chamber. In that case It sounds like "your piston isn't a good enough fit" like MrCrowley said and too much air bleeds past the piston when you vent it.

Do you get any air out your barrel when you fire it?
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Unread postAuthor: frankrede » Thu Jul 03, 2008 1:11 am

Launch-Tek wrote:I think I understand now. The air bleeds past the piston seal to fill the chamber. In that case It sounds like "your piston isn't a good enough fit" like MrCrowley said and too much air bleeds past the piston when you vent it.

Do you get any air out your barrel when you fire it?

Actually a lot of the time almost no air comes out, just enough to send the marble 1ft
<a href="http://s30.photobucket.com/albums/c325/redensekjf/?action=view&current=1215065561.jpg" target="_blank"><img src="http://i30.photobucket.com/albums/c325/redensekjf/1215065561.jpg" border="0" alt="Photobucket"></a>
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Unread postAuthor: MrCrowley » Thu Jul 03, 2008 1:17 am

I think the piston is getting jammed on one side and on the other the air just goes straight past it.

It doesn't look very even or neat at all, so the tape is your issue, i'm sure of it.
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Unread postAuthor: frankrede » Thu Jul 03, 2008 1:26 am

MrCrowley wrote:I think the piston is getting jammed on one side and on the other the air just goes straight past it.

It doesn't look very even or neat at all, so the tape is your issue, i'm sure of it.
ok, I'll tell you guys how it goes with a new piston
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Unread postAuthor: starman » Thu Jul 03, 2008 1:56 am

The piston needs to fit very well, just ever so slightly less enough to still run smoothly in the cylinder. When quickly moving the piston back and forth in the cylinder, you should feel some air resistance. If it doesn't fit well, air will just slip around it and out the pilot.

I had the same issue with my piston. Finally built up the exact needed size with multiple layers of paint coatings. Was tedious and took some time but it worked out great.
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Unread postAuthor: frankrede » Thu Jul 03, 2008 3:47 am

Does anyone know any good things to use as pistons in 1.5" sch 40 pipe?
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Unread postAuthor: MrCrowley » Thu Jul 03, 2008 4:33 am

Wood?

If you have any holesaws you can make a piston from some thick wood you may have laying around.
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Unread postAuthor: jackssmirkingrevenge » Thu Jul 03, 2008 6:12 am

MrCrowley wrote:If you have any holesaws you can make a piston from some thick wood you may have laying around.


*cough*epoxycastingwouldgiveyouaperfectfittingpiston*cough*

:D
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