Pressure rating for Coaxial cannons

[Luke31]

I was contemplating about making a coaxial type air cannon design but I had a thought about the pressure rating. We know that certain pipes has been factory tested to withstand a certain amount of pressure inside of it. But what if that pressure is being applied outside of it? That's basically what's happening to the barrel inside of a Coaxial cannon chamber. Its receiving a vacuum like/crushing pressure from the chamber because its literally inside of the chamber. Does the same pressure rating still apply? Because I don't think pipes were pressure tested in that sort of way.

Maybe I'm not understanding physics all the way. If someone could shed some light that would help.

[jrrdw]

That is a really good question. I think it would hold / withstand greater pressure do to the fact there's no where for the pipe to go but into it's self. Sure it can collapse into the inside diameter of the pipe but getting the pipe wall to fold in like that...

[Luke31]

If the barrel were to shatter at least it would be in an enclosed space. Could the shrapnel penetrate the chamber and go flying out? Dunno.

[mrfoo]

Good question. The force being applied to the barrel will be compression, rather than tensile, and PVC (like many materials) is stronger in that respect. Tensile strength of rigid PVC, 34 … 62 MPa, Compressive strength 55 … 89. So in that respect, you're probably OK.

However.

These figures are significantly lower than those for metals.

PVC's fatigue strength is very low. So although a piece of rigid PVC has a bending strength of ~80MPa, repeating a load of 17MPa just 10 times can cause it to break. And Impact strength is awful, one of the worst of all plastics for that. Worth considering when you have a piston hammering down a tube towards your face.

[Moonbogg]

See link with external pressure ratings for PVC. They call it "critical collapse" pressure ratings. You can compare the critical collapse rating with the internal pressure rating.

Critical collapse ratings:
https://www.usplastic.com/knowledgebase ... entkey=777

Compare those figures to the internal pressure rating:
https://www.usplastic.com/knowledgebase ... entkey=875

For example, 2" SCH40 PVC has an internal pressure rating of 280psi while the critical collapse rating of the same pipe is 316psi. So, as expected, it's stronger from the outside. Additionally, any coaxial cannon will have a larger pipe taking internal pressure loads while a smaller pipe is taking the same pressure externally, so the smaller pipe is always stronger anyway, assuming it's the same material and schedule. This doesn't hold true if you use SCH40 PCV for the chamber and a bunch of toilet paper rolls taped together for the barrel though.

Also, it's been mentioned that PVC is fragile. Piston PVC cannons have been known to break. I've seen people post about their cannon breaking after a year of use while others seemed to break right away. That said, PVC piston cannons have been used for decades and most of them probably didn't break, but eventually they will all break because PVC tends to age like milk.

[farcticox1]

yes, PVC starts to decay as soon as it is made I read, UV is a real killer.

[Luke31]

Thanks for the info. I'm going to upgrade to steel pipe anyway for my later builds. I'm mostly concerned about raising the outer diameter of the chamber since the chamber diameter has to be larger than the barrel for it to fit around it, and the larger the pipe diameter the lower the PSI rating.

I assume having the barrel run through the chamber will decrease the volume it can carry, making someone inclined to raise the diameter of the chamber even more. You can see how this might be of a safety concern.

Is there any point I should just buy an air tank and punch holes through it instead of buying a high diameter pipe? The air cannons I'm building are big (3-5 inch barrels, 10 ft lenth).

I am interested in building a coaxial gun for space and aesthetic reasons. But I'm not sure if a coaxial air cannon of this size has been made by anyone else yet.

[Moonbogg]

What do you mean by buying an air tank and punching holes through it? Making a PVC cannon this large makes me especially uncomfortable.

[farcticox1]

What do you mean by buying an air tank and punching holes through it? Making a PVC cannon this large makes me especially uncomfortable.

The air tank is for the build ?

[Moonbogg]

Yes. I assume he's cutting holes in the front and back for the barrel to slide through, but I'm not sure if that's what he meant.

[Luke31]

What do you mean by buying an air tank and punching holes through it? Making a PVC cannon this large makes me especially uncomfortable.

I'm building it out of steel pipe. No more pvc.

Yes I meant punch holes through an air tank like this one to fit the barrel through https://www.summitracing.com/parts/air-12955

[Moonbogg]

That would be pretty sweet. Keep us updated.

[Anatine Duo]

has anyone here tried a 10' muzzleloader? I whine about loading a 3'

Don't get me wrong, I love the elegance of the coaxial, but after I am finished my works-in-progress I think I will confine them to short barrel applications.