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This 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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This piston

Unread postAuthor: Gun Freak » Tue Apr 20, 2010 5:15 pm

Is this piston's sizes the right way around or should the back be bigger than the front? I'm thinking this could have been the reason for my piston valve fail.
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Unread postAuthor: Marffy » Tue Apr 20, 2010 5:32 pm

looks good to me but you do need to have something that will hold the sealing face and some pictures of where the piston is going would be great.


:D FIRST POST!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! :D
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Unread postAuthor: MrCrowley » Tue Apr 20, 2010 5:57 pm

Generally a piston is the same diameter all the way around. I think I know what you're doing though but i'm not sure what the point is. Even if you increase the area of the sealing face, like you have done, the force would still be the same as the pressure can also act on the back of the sealing face (assuming the smaller diameter of your piston rides in a pipe, so when the pilot is exhaust, only (well mostly) the air behind the back of the piston is exhausted).

edit: Depending on the size of the seat, you might need a bigger washer and i'm pretty sure you're going to have to epoxy the front bolt and washer so it doesn't leak.
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Unread postAuthor: Gun Freak » Tue Apr 20, 2010 6:13 pm

Marffy wrote:looks good to me but you do need to have something that will hold the sealing face and some pictures of where the piston is going would be great.


:D FIRST POST!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! :D


The sealing face is on it :)

Depending on the size of the seat, you might need a bigger washer and i'm pretty sure you're going to have to epoxy the front bolt and washer so it doesn't leak.


Yeah I already know that, I just need to get some next time I got to the hardware store or home depot, I will need some for my 2" Piston valve 1.5" porting.
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Unread postAuthor: Technician1002 » Tue Apr 20, 2010 6:28 pm

The piston will need to seal against the walls of the cylinder it rides inside. The rubber in the photo looks to lack a precise round shape. If large amounts of air can blow past the piston, it is unlikely to be pushed open when an attempt is made to fire the cannon.

If you have a drill press, the bolt could be placed in the chuck and the rubber then be gently worn down to the correct shape and size so the fit is very close to airtight.

The larger the piston, the more important a good fit becomes simply because the area that can leak is much larger, gaps can be wider, etc so the area of the leak can become huge quickly.

Either a really large pilot valve needs to be used or another method of providing a good seal is required such as o rings on larger pistons.
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Unread postAuthor: Gun Freak » Tue Apr 20, 2010 6:44 pm

Technician1002 wrote:The piston will need to seal against the walls of the cylinder it rides inside. The rubber in the photo looks to lack a precise round shape. If large amounts of air can blow past the piston, it is unlikely to be pushed open when an attempt is made to fire the cannon.

If you have a drill press, the bolt could be placed in the chuck and the rubber then be gently worn down to the correct shape and size so the fit is very close to airtight.

The larger the piston, the more important a good fit becomes simply because the area that can leak is much larger, gaps can be wider, etc so the area of the leak can become huge quickly.

Either a really large pilot valve needs to be used or another method of providing a good seal is required such as o rings on larger pistons.


I used a bench grinder to wear down the outside of the washers because they didn't have the next size down.

I may end up rebuilding it.

If I ever get around to finishing my little sniper.
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Unread postAuthor: Marffy » Tue Apr 20, 2010 9:29 pm

Gun Freak wrote:
The sealing face is on it :)


Yes but i meant like a washer or something to hold it in a flat position :)
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Unread postAuthor: psycix » Wed Apr 21, 2010 1:20 pm

The back has to be a larger diameter than the diameter of the porting it seals on.

This causes a net force backwards when the pilot volume is vented, opening the valve.
What people here mean, is that you will probably need a metal washer behind the rubber washer (sealing face).
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Unread postAuthor: Gun Freak » Wed Apr 21, 2010 3:32 pm

There is a metal washer behind the sealing face already :D
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Unread postAuthor: dewey-1 » Wed Apr 21, 2010 3:39 pm

Gun Freak wrote:There is a metal washer behind the sealing face already :D


Maybe if you took a picture from back side, would help clarify things.
Add the picture to your original posting.

The concern is to know the backing washer size.
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Unread postAuthor: Mr.Sandman » Wed Apr 21, 2010 3:41 pm

Perhaps you should try an epoxy piston because it seems like you have done quite a few things wrong here on this piston, Maybe you need new washers and need to cut them more circular.
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Unread postAuthor: Gun Freak » Wed Apr 21, 2010 3:45 pm

I was actually thinking of using an epoxy or hot glue piston next. Bolt is heavy and I don't like it.
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Unread postAuthor: Marffy » Wed Apr 21, 2010 5:54 pm

Gun Freak wrote:There is a metal washer behind the sealing face already :D


oh, oops, didn't see it. oh well, then it looks good but maybe another angle of the piston would be nice :)
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Unread postAuthor: psycix » Thu Apr 22, 2010 3:13 pm

Gun Freak wrote:There is a metal washer behind the sealing face already :D

Is it large enough? Larger than the porting, so that the rubber doesn't flap.

And then the back of the piston should still be larger than the porting.
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Unread postAuthor: Gun Freak » Thu Apr 22, 2010 6:43 pm

Yes the washer is pretty big. Almost as big as the neoprene.
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