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Piston Face Question

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 Face Question

Unread postAuthor: phatjohnsn » Wed Sep 02, 2009 12:11 pm

Hey guys,

I've been a "lurker" for awhile and this is the first time I'm posting as I've finally begun building myself a gun! Hopefully you won't mind helping a n00b. I've searched for my question a few times but may not have done due diligence in that department so if its been answered you can slap me around and tell me to move on.

My set up right now is pretty close to completion (read: glued and all), I have a 3 inch tee to house my piston with a 2 inch barrel sticking almost completely through the tee (there is an 1-1.5 inch that the piston goes over the tee outlet, if that makes sense).

My piston is made of a resin (I believe marine resin but I'm at work now so I'm not positive what the can says) which was molded using a piece of 3 inch pipe as an attempt to replicate the inside diameter of the tee.

My question: is it possible that the face of the piston does not overlap the barrel enough to push my piston back when the pilot valve is released? My piston is just under 3 inches in diameter and it is sealing the barrel which is 2 inch (inside diameter) PVC. My fear is that the piston face doesn't have enough surface area sticking out around the barrel it is sealing to be pushed by the air pressure back into it's seat.

The gun, so far, pressurizes fine but the problem I'm running into is that when I release my 1/4" ball valve behind my piston, I'm losing all the air out of the ball valve instead of the barrel. I definitely think the problem is my piston not being snug enough right now (especially after seeing the diagram about 1mm spacing area) but was still concerned about the question I posted above. If anyone has a good plan as to increasing my piston diameter, I'm all ears (and yes I've seen the other posts about that topic). I'm thinking about using duct tape to start as that is a nice temporary fix.

Any help would be really appreciated! Thanks.
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Unread postAuthor: Hubb » Wed Sep 02, 2009 12:19 pm

Try the duct tape. If that don't work, you could always rebuild your piston and add an o-ring and equalization hole. A link in my sig leads to how I made a 2" piston, but you seem to be smart enough to be able to adapt some of the techniques to make a bigger one.

Oh, and welcome to Spudfiles.
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Last edited by Hubb on Wed Sep 02, 2009 12:29 pm, edited 1 time in total.
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Unread postAuthor: Technician1002 » Wed Sep 02, 2009 12:27 pm

I think your summation of the problem is entirely correct. Fix the large leak past the piston and everything should work.

As for the force that will open the piston, some simple math to find the area of the piston facing the pilot, the barrel and the chamber will provide all the forces and pressures needed to make the piston move.

The entire front of the piston is exactly the same diameter as the back of the piston, so with area = pi X radius squared, this area is easily found. (3 inch diameter) The smaller area that is exposed to the barrel can also be calculated. (2 inch diameter) The 2 inch area subtracted from the 3 inch area provides the area exposed to the chamber.

The piston has two sides. The back has it all exposed to the pilot pressure. Pressure X area = force on the back.

The front has pressure only on the area exposed to the chamber. Again area times pressure = force.

At some point the force on the back of the piston needs to be less than the force on the front of the piston so it will move back.

If your leak is too big, you can't vent the pilot fast enough to lower the pilot pressure far enough to move the piston.

Fix the leak so the pilot can lower the pressure low enough in the pilot without also emptying the chamber in the process. For a 3 inch piston, I highly recommend an o ring.

Since you are a noob. here is a link to a guide in using an o ring on a piston in an air cannon.
http://www.youtube.com/watch?v=BbDiZiRVrnk
Most of the veterans have already seen my video on the subject.
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Unread postAuthor: phatjohnsn » Wed Sep 02, 2009 12:35 pm

welcome to Spudfiles.


thanks for that and the quick response.

My next step is def to make my piston fit tighter, I was more so curious about the piston face than actually thinking that was the problem. My seal with the barrel is great so far, I've pressurized the whole thing up to about 80psi and not lost much in the way of pressure.

I was concerned with the face, because I was nervous it was a flawed design and dreaded having to actually purchase materials to make my cannon. Lets just say someone's work site "over ordered" material for this project.
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Unread postAuthor: Hubb » Wed Sep 02, 2009 12:39 pm

A while back, I proposed a theory on making the seal work better in a piston valve. It was to place a piece of felt (or other soft cloth I suppose) in between the rubber and the actual piston. The theory stated that the felt would give a little bit along with the rubber to help even out any imperfections in the barrel.

This worked. I have used it in every one of my piston valves and, while I do try to make the barrel as square as possible, I have never had a problem. In fact, I tested this and only lost about 3psi in a launcher (2" x 20" chamber) over the course of about 12 hours.

I can't find the link right now on the topic but I touched on it a little bit in my how-to.
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Unread postAuthor: phatjohnsn » Wed Sep 02, 2009 1:36 pm

Thanks again Hubb and Technician!
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