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Design on a Coaxial Piston Cannon

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: jackssmirkingrevenge » Fri Aug 26, 2011 1:26 am

Gippeto wrote:As a suggestion, I would make the piston hollow (soup can), and put the seal on the inside...think Jack may have suggested it recently.

Reduces the weight of the piston and makes life easier on the bumper


Indeed, the lighter you can make it the better for your launcher. You also get better performance to boot, a lighter piston will accelerate faster.

Just to give you an idea of the weight saving possible, assuming these were made out of aluminium (and I can assure you the cup piston will be just as durable as the solid one):
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Unread postAuthor: Moonbogg » Fri Aug 26, 2011 1:36 am

Regarding lighter pistons vs heavier ones, will a lighter piston accelerate faster and impact with more speed and as a result still hit with as much force as the heavier one? A heavier one weighs more but will move slower. I'm just wondering if it might balance out.
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Unread postAuthor: jackssmirkingrevenge » Fri Aug 26, 2011 1:48 am

Moonbogg wrote:Regarding lighter pistons vs heavier ones, will a lighter piston accelerate faster and impact with more speed and as a result still hit with as much force as the heavier one? A heavier one weighs more but will move slower.


Fair point, a lighter piston gains some extra momentum by accelerating to a faster velocity, however in practice it's easier to stop a fast light piston than a relatively slow heavy one.

When something is heavy it doesn't have to be moving very fast to have a lot of momentum, look at trains ;)

Commercial quick exhaust valve pistons are a perfect example of a light piston made of a relatively soft material that can take the impact and with limited travel to provide good performance without overstressing the system.

Image
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Unread postAuthor: Atattack » Fri Aug 26, 2011 7:41 am

I have a roll of 1/2" neoprene sheet. Was going to place a 3.5" piece of the sheet on both ends of bumper. Then JB Weld a piece of the sheet on the end cap where the piston will be slamming against. Figured that would protect the UHMW piston enough, having neoprene hitting neoprene? Do you guys think that would tolerate the high forces?

If I hollow out the inside of the piston, how thin should I make the walls while still having them be durable? Also JSR, do you think I should hollow it out as deep as your image? Thanks guys.
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Unread postAuthor: jackssmirkingrevenge » Fri Aug 26, 2011 7:53 am

I would be perfectly comfortable with a UHMW piston made to the dimensions illustrated for a 2 inch barrel at 300 psi - excluding the seal it would weigh just 225 grams which is extremely good for a 4 inch piston.

I would not put any rubber on the piston, you're just adding extra weight that will reduce performance and increase stress on the launcher - it would be better to layer rubber on the endcap where the piston will strike it.

Make sure you have a nice flat edge where the piston will hit the rubber, to spread the imapct as evenly as possible.
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Unread postAuthor: Atattack » Fri Aug 26, 2011 7:59 am

How thick would you make the walls of the bore out portion of the piston? 1/2"? The reason I was leaning towards a flat piston is I was trying to stop the piston limiting the diameter at the base of the barrel where the air will be exhausting into the barrel. I figured if I had a 2" barrel and a 4" chamber. A 1/2" piston would cut the diameter in half but I may be able to get away with a thinner piston.
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Unread postAuthor: jackssmirkingrevenge » Fri Aug 26, 2011 8:03 am

If you look at my diagram, the cavity has a diameter of 3.5 inches, meaning the "cup" has 1/4" walls. Assuming your barrel has an outer diameter of 2.25", this gives an area of 5.65 in<sup>2</sup> for the air to flow through. The cross section area of your barrel is 3.14in<sup>2</sup>, so no performance loss.
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Unread postAuthor: Atattack » Fri Aug 26, 2011 8:06 am

Thanks JSR, missed that 3.5" measurement you had on your diagram. I will make those changes today. Great advice.
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Unread postAuthor: jackssmirkingrevenge » Fri Aug 26, 2011 8:18 am

One question, is the endcap going to be removable? It would be a good idea to have a servicable piston in case the rubber wears out over time.

Goes without saying but make sure the breech end of the barrel is at a perfect 90º cut, smooth and it's a good idea to bevel the edge slightly, inside and out. This not only helps smoothen the airflow but also reduces wear on the sealing rubber. I would also bevel the edges of the cup walls to smoothen flow.
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Unread postAuthor: Atattack » Fri Aug 26, 2011 8:22 am

I have a 4" galv steel end cap that threads on so it will be serviceable. I never thought of beveling the piston or barrel but good idea. When you say bevel are you saying put a 45 degree angle on each side?
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Unread postAuthor: jackssmirkingrevenge » Fri Aug 26, 2011 8:36 am

Atattack wrote:I never thought of beveling the piston or barrel but good idea. When you say bevel are you saying put a 45 degree angle on each side?


Pretty much, it should look something like this:
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Unread postAuthor: Atattack » Fri Aug 26, 2011 9:02 am

Is it hard to thread a bolt into UHMW? Bolt I'm using is 1/4". Just drill a small pilot hole I'm guessing.
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Unread postAuthor: jackssmirkingrevenge » Fri Aug 26, 2011 9:06 am

If you can get someone with a proper tap set to do it for you it's better, but you can use self tapping screws if you want.
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Unread postAuthor: Gippeto » Fri Aug 26, 2011 9:36 am

Couple passing thoughts:

You could probably get away with 1/8" wall on the bored out piston. It's not absorbing any substantial force anyway.

Might want to check with your machinist about using mechanical tubing if the seam in the pipe is an issue.

Decreasing the mass of the piston will not increase it's momentum....it WILL increase it's kinetic energy. Momentum is the killer of things.
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Unread postAuthor: Atattack » Fri Aug 26, 2011 9:40 am

Can you explain by using mechanical tubing?
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