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4 inch 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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4 inch piston

Unread postAuthor: Raybateman » Tue Apr 24, 2012 4:01 pm

Does anybody have an idea of how i can make a 4 inch piston for a chamber sealing piston valve?
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Unread postAuthor: MrCrowley » Tue Apr 24, 2012 4:59 pm

Just like any other chamber sealing piston valve; except larger. The concept is the same, scale it up.

Check out the SpudWiki, the SpudTech forum archives, the Pneumatic Cannon Showcase section and the advanced search tool for more information.
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Unread postAuthor: Raybateman » Tue Apr 24, 2012 6:38 pm

Thank you.
I understand the basic design and operation of a chamber sealing piston valve. Do you know what the best easily available material is to make the piston out of assuming I only have basic hand tools? What about a bumper?
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Unread postAuthor: MrCrowley » Tue Apr 24, 2012 6:40 pm

Not sure. Have you searched the SpudTech Archive or the Pneumatic Cannon Showcase to see what other people have used?
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Unread postAuthor: Raybateman » Tue Apr 24, 2012 7:26 pm

yes and i haven't found anything for a chamber sealing valve. Most valves made are barrel sealing.
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Unread postAuthor: MrCrowley » Tue Apr 24, 2012 7:28 pm

Well you can still use the same materials for the piston; the main difference is that you need to add two o-rings to your piston. A chamber sealing valve is pretty much an upside down barrel sealing valve that seals the chamber, has two o-rings and an equalisation hole.
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Unread postAuthor: Raybateman » Tue Apr 24, 2012 7:49 pm

This valve is becoming more complicated then i originally planned. I would like to have threads on the part of the tee that hooks up to the chamber and on the part that the barrel attaches to while keeping the pilot area slip; however, i can't seem to find a 4 inch tee with that configuration. the reason i would like to have that configuration is that i would like to have the smallest chamber potential possible because my chamber will be sectioned off with ball valves to create ideal chamber to barrel ratios for many barrel sizes. any ideas on a design? I think my piston will be made out of wood. could i properly make a seal if i was to use something like 3/16 inch wood bolted together with one circle slightly smaller then the rest to make an o-ring groove? i could paint the surface in polyurethane or epoxy to make it smooth and seal better.
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Unread postAuthor: MrCrowley » Tue Apr 24, 2012 7:57 pm

The best you can do is buy female threaded fittings that slip inside the ports of the tee, but this would only be recommended for your barrel port (assuming your barrel is 3" in diameter or less) as having on one the chamber port would restrict your chamber flow to 3" or less.

The other option is buying the male threaded version that also slips inside the tee port but gives you 4" male threads to attach to your chamber and barrel. This has a disadvantage for the barrel though as it will increase the distance between chamber and barrel and would increase the risk of your barrel being snapped off at this junction due to the forces.

In my opinion the best option is to glue the chamber as normal in to the tee. Glue a 90 degree elbow on the barrel port (to make the barrel inline with the chamber) and then attach a threaded fitting on the other end of the elbow so you can change barrels.

Scrap the ball valve idea. Download GGDT (search google) and figure out the best C:B ratio for your multiple arrels and chamber. Realistically, you'll probably only have three barrels (3", 2.5" and 2") for such a cannon. It's not difficult to make a chamber large enough that it performs well for all three barrels but isn't so large that it wastes a lot of air for some of the barrels.

I'd recommend going with a much thicker wood, like 3/4", so you don't have to cut out so many disks and bolt them together to make a piston long enough. Your idea would work, but it would be much easier using bigger sections of wood and then cutting an o-ring groove with a hacksaw and file. Painting it in some sort of waterproof finish would be recommended; be sure to take this in to account when cutting your piston as it will increase the piston diameter.
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Unread postAuthor: Fnord » Tue Apr 24, 2012 8:16 pm

I'd recommend using a piston with the sealing face recessed back inside to save on weight. This is better to use on a barrel sealing design. You'll have to increase the travel a bit to ensure it doesn't block the barrel port with a chamber sealer.

I would NOT use wood on anything that needs to be air-tight. Don't even *try.*
(I found that out the hard way with that hybrid-block-of-wood I made for the ghetto contest) :D
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Unread postAuthor: Raybateman » Tue Apr 24, 2012 8:20 pm

What harm can restricting the chamber to 3 inches do if the barrel is only 3 inches? the reason i wanted it threaded right out of the tee is for simplicity in the pilot area. It would be a lot easier to make if i didn't need a way to get the piston out of it. the piston could be removed thru where the chamber screws on. My plan was to have the gun highly versatile and able to shoot projectiles from 1/2 inch (hence the design with small chamber) to 3 inches and 4 inch projectiles (with a lower power level, of course).
Could i just make it out of 3/4 inch wood with two layers being 3/16 so i don't have to cut grooves?
Edit: that is an interesting piston design but how would that work in a chamber sealing valve? i was planning on making a piston of the following design to save weight and it is near opposite of what you propose:
l----lllllll
If not wood then what?
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Unread postAuthor: MrCrowley » Tue Apr 24, 2012 8:24 pm

What harm can restricting the chamber to 3 inches do if the barrel is only 3 inches?
Well that is probably fine, I was more getting at the point of having your 4" chamber restricted at a 2" joint; which some members have done in the past.

It would be a lot easier to make if i didn't need a way to get the piston out of it
You will need to get the piston out. It is a lot easier to get the piston out of its own port rather than through a chamber or barrel port. I would highly recommend a threaded port for the piston. I can't see how you could remove the piston through the chamber port when the piston has to seal against that same port. It could be done with a lot, and I mean a lot, of faffing about* but it is much easier to make the piston have its own threaded port.

Don't bother about shooting projectiles as small as half an inch from a 3" porting piston valved cannon.

As Fnord said, wood is probably a bad idea. When I used it, I didn't need it to seal 100%. You will.


*The chamber would thread in to 3" fittings on the tee but the seat the piston seals against is actually attached to the chamber and is only 2.5" in diameter. The piston housing is then sleeved inside with 3" pipe so the piston is housed in a 3" diameter pipe and it seals against the 2.5" diameter seat on the chamber. If everything is done right, the piston might fit through the 3" threaded fitting. It could be done, but it'd be a waste of money and time and performance (scaling down the port from 3" to 2.5").
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Unread postAuthor: Raybateman » Tue Apr 24, 2012 8:29 pm

The plan was to have the adaptor screw in to the full 4 inch tee so when removed i could pull the piston out of the 4 inch part.
The reason for this cannon is to build one that can shoot almost everything

The valve will be housed in a 4 inch t with a 3 inch seat and no 2.5 inch pipe involved


edit by Moderator: Don't double post.
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Unread postAuthor: MrCrowley » Tue Apr 24, 2012 8:37 pm

Okay, let me explain again. It is possible to have the piston removed through the chamber port but it is more complicated and expensive to implement than a simple threaded port for the piston itself. It will also reduce the capability of the cannon.

Shooting projectiles smaller than 1" from a 3" porting piston valved cannon wouldn't be very good. Not only would it look ridiculous, but there is so much wasted performance and wasted air that you may as well just build a second, smaller cannon for such ammunition.

I agree that this cannon should have several barrels, but think in the range of 1.5" to 4". You're not going to be shooting a single paintball with this cannon; you're going to be consumed with shooting all the canned foods, tennis balls, golf balls, nerf footballs and soda cans.

As for the threaded chambers: regulate the pressure and not the volume. You could find a chamber that would suit every barrel size from 1.5" to 4" at the same pressure without losing too much performance/wasting too much air (depending on the barrel used), but you could also regulate the chamber pressure instead of the chamber volume if your barrels are long enough (at least 6' I imagine).

Even if you wanted to do this ball valve idea, large ball valves are expensive and adapting down to 2" ball valves for each chamber section will severely limit flow.

You will find that by trying to make a cannon that does everything, it ends up doing everything not very well.

The valve will be housed in a 4 inch t with a 3 inch seat and no 2.5 inch pipe involved
Did you read what I said? I said that the easiest, conceivable way that I can see for removing the piston from the chamber port is using a 4" tee with a 2.5" seat and a 3" piston. IF YOUR PISTON IS THE SAME SIZE AS THE SEAT, YOU CAN'T REMOVE IT FROM THE CHAMBER/OPERATE THE PISTON.

If your seat is 3", your piston will have to be bigger. To make the seat 3", you will probably use a 4"-3" adapter in the tee that will thread the chamber on. If your piston is larger than the inside diameter of this adapter, which it will be, you cannot remove it through that adapter.

Perhaps there is another way but you'll have to enlighten me as between writing about this design of yours and writing an essay on some boring research paper I have to do for university, I cannot see think of a better way.


edit:

The space you will save having the piston removed through the chamber port instead of the pilot port will be about 4". You still have to have a threaded adapter on the pilot port for the pilot valve to be attached to. Instead of having a 4"x1" slip fitting to female threaded adapter, you'll have a 4" socket fitting, joined to the tee by some 4" PVC, with 4" threads that the 4" piston (that seals against the 3" seat) can be removed through.
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Unread postAuthor: jackssmirkingrevenge » Wed Apr 25, 2012 12:05 am

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Unread postAuthor: jhalek90 » Wed Apr 25, 2012 3:32 pm

also, a 3'' dome end cap fits in 4'' schedule 40 PVC perfectly, and i have used one with some success.

Just bolt a big steel washer, and a rubber gasket into the inside of the dome, and you have a perfect piston for a 3'' porting cannon made in a 4'' T.

just be sure to use an really good bumper. i used coiled, and slotted rubber hose.
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