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Safety First - How to make a cannon Failsafe

A place to ask general spud cannon related questions.
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Safety First - How to make a cannon Failsafe

Unread postAuthor: katypotaty » Sun Nov 14, 2010 8:51 am

Hi,

I've just built a fairly powerful pneumatic cannon out of pressure rated schedule 40 pvc with schedule 80 fittings. The barrel is 6' of 1.5" and the chamber is 4' of 3". It's a spudzooka / diaphram based design with modifications. I believe it is well built, but I'm also particularly concerned that should it ever fail things could be real bad.

I want to make it failsafe. For me, safety is much more important than the weight of the gun. I am considering four options:

- wrapping it with kevlar, carbon fiber, or some such material
- building a wooden box around it
- building an aluminum box around it
- putting the whole cannon in a 6" ABS pipe with endcap

I would be most appreciative of any thoughts on the matter.

Thanks.
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Unread postAuthor: metalmeltr » Sun Nov 14, 2010 9:13 am

What pressures do you plan to put into this gun?
Schedule 40 pipe with even schedule 40 fittings will take as much pressure as the pipe is rated for at the temperature the pipe was rated at. Schedule 80 fittings would withstand even more pressure so I do not feel as though it is nessasary to go though extra measure to contain the cannon in the off chance of an explosion.
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Unread postAuthor: katypotaty » Sun Nov 14, 2010 9:21 am

I plan to take it to 100 or 125 psi. I know the rating for the pipe is much higher (approx. 300psi) and the risk of catastrophic failure is low. And I agree it's probably very safe. Nonetheless, catastrophic failure cannot be ruled out and I absolutely need the gun to be failsafe - ie., should the worst happen and the thing explode, it has to do so safely for the operator.
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Unread postAuthor: metalmeltr » Sun Nov 14, 2010 9:29 am

From what you describe you have no need to worry at even 150psi.
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Unread postAuthor: jackssmirkingrevenge » Sun Nov 14, 2010 9:44 am

If properly constructed from pressure rated pipe, then you have a much greater chance of accidentally shooting something you didn't intend to than having the pipe fail on you.

If you're that concerned about failure though and not too bothered about weight, why not just go ahead and make the whole cannon out of metal piping instead of building awkard boxes around it?
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Unread postAuthor: katypotaty » Sun Nov 14, 2010 9:47 am

I don't mean to be rude, and I trust you're just trying to be helpful, but I'm really not interested in your opinion about whether the gun is safe at my operating PSI.

My question concerns making the gun failsafe.
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Unread postAuthor: motorfixer1 » Sun Nov 14, 2010 9:48 am

If you absolutely wanted to reinforce your pressurized components wrapping them would be the lightest option. I am not sure where you would obtain kevlar fabric in any quantity as we use insulation made with kevlar at work and it is very expensive and hard to find.
As for sleeving the whole works in ABS, that is a quick bit of insurance and would be the fastest option with the most available materials. Usually 3" couplers fit right inside 4" pipe as for sch40 pvc with little or no modification.
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Unread postAuthor: jackssmirkingrevenge » Sun Nov 14, 2010 10:08 am

katypotaty wrote:My question concerns making the gun failsafe.


I reiterate that if properly constructed and operated within the parameters guaranteed by the manufacturer of the materials, there is absolutely no reason not to consider a PVC spudgun "failsafe", in that without subjecting your chamber to physical abuse like vigorous hammering, a lifetime of pressurisation cycles will not be sufficient to cause it to fail, catastrophically or otherwise.

My point was that given the above, the weak link in the "fail" cycle is the operator.

Coating your cannon in all manner of carbon fibre or mesh will greatly reduce the chance of injury in the event of failure, but that eventuality is extremely unlikely. Notice that no user on this forum approaches their launchers wearing a bomb suit, specifically because well built (and even most badly built) launchers do not shatter into a cloud of razor sharp fragments on a regular basis.

Again, if you want to increase the factor of that unlikelyhood, go for all-metal construction.

A pop-off valve to prevent pressurisation beyond what you consider safe would also be beneficial in this respect.
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Unread postAuthor: jimmy101 » Sun Nov 14, 2010 11:21 am

katypotaty wrote:I don't mean to be rude, and I trust you're just trying to be helpful, but I'm really not interested in your opinion about whether the gun is safe at my operating PSI.

My question concerns making the gun failsafe.

Don't be such an ass.

It is not possible to make anything "failsafe", all you can do is reduce the dangers.

Like others have said if you limit the gun to less than 150 PSI you are as safe as you are going to get. Assuming you weren't an idiot when you assembled the gun. A pop-off just in case some idiot tries to take it well beyond spec is a reasonable thing to do.

Wrapping the gun in Kevlar won't do squat. And figuring the cost, is a stupid idea. Save the Kevlar cost and use a stronger material to begin with.

Encasing the gun in metal or wood is just plain stupid. Go back and build the gun out'a a much stronger material instead of trying to retrofit dumb ass safety features onto an already adequate build.

If you are that paranoid about non-existent dangers then take up crocheting.
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Unread postAuthor: jackssmirkingrevenge » Sun Nov 14, 2010 11:43 am

Thank you jimmy for making my attempts at passive aggressive eloquence seem hollow and futile :)

jimmy101 wrote:If you are that paranoid about non-existent dangers then take up crocheting.


Nah, there's needles involved! But it makes the point that shooting stuff is inherently dangerous, and if you look through the now defunct share your cannon accidents thread (or a more recent version), the vast majority of incidents are caused by shooting/ shooting at objects unintentionally or injuries incurred during fabrication, and there are no instances I can detect of a properly put together launcher failing.

katypotaty, don't take it personally, your quest for safety is to be applauded and I wish most members, including myself, were so safety conscious, but if you're safe by a factor of 100 rather than 1000, you're still pretty damn safe - if you're concerned beyond that, perhaps spudding isn't the most recommendable of hobbies.
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Unread postAuthor: Technician1002 » Sun Nov 14, 2010 12:02 pm

To work on failsafe, you need to know the failure modes and the causes. From there you can address each to mitigate the failure.

#1 failure is from PVC flexing due to handling or being dropped which starts a crack which propagates into a catastrophic failure.

The solution is to use an exoskeleton to take all impact forces and distribute them so point impact pressure points do not cause a stress crack.

In the event of a failure containment of the fragments is desired while safely dissipating the pressure charge without breaking the secondary containment.

I would recommend casing it in an ABS sleeve that is drilled with many vent holes. The launcher would be suspended inside on shock absorbing mounts so the cannon is padded inside the containment sleeve.

The containment would look somewhat like this launcher where the perforated sleeve protects the barrel from impacts. Instead of a solid connection as shown, a rubber or other shock absorbing mount can be used.

The containment would enclose both the chamber and barrel.

Me personally, I used a steel tank that won't propagate a tear in the metal at normal operating pressure. You can safely use a hammer and chisel and punch a hole in it without an explosive failure. Youtube is full of videos of people shooting full propane tanks. It makes holes and a big fire, but rarely exploding tanks. Some thin disposable tanks already overheated and thus at high pressure to fail explosively. Keep your pressure reasonable. For safety, use a mailable steel tank below the pressure that would cause a tear to propagate.

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For your enjoyment, here is a tank heated in a fire so the pressure is higher than normal, then shot. It made a hole in the tank. The tank jetted, but did not explode. Don't try this at home. These idiots already did.
[youtube]http://www.youtube.com/watch?v=QFPedGsqtrs[/youtube]
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Unread postAuthor: Fnord » Sun Nov 14, 2010 1:21 pm

$10 worth of fiberglass cloth will be enough to wrap the chamber several times. Carbon fiber is stronger by weight but when it comes to volume there isn't much difference.
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Unread postAuthor: Selador » Sun Nov 14, 2010 1:58 pm

Technician1002 wrote:To work on failsafe, you need to know the failure modes and the causes. From there you can address each to mitigate the failure.


Great post, Tech !

:D


Katy,

If this is going to be used by or around students, then nothing you can do will make it completely failsafe.

If it works at all, and students have or can gain access to it, without supervision... I guarantee that some student is going to find a way to cause a fail.

That said...

Wrestling mats.

Or the kind of rubber that is used either for kneeling pads, or for standing on, when having to stand/work on concrete.

Also, horse stall mats. (Fairly cheap, actually.)

You have two options. You can use one or both.

1. Do as others have already suggeted here. Encase your entire gun in ABS. Do so with pipe that has enough capacity to encase both your gun, and a wrapping of one of the rubber products above.

If you are going to wrap, use the kneeling pads. Then ABS large enough to encase all of it.

2. Make a 'loose' box, (or two), of the rubber mats around the gun. About a foot away, is good.

With the loose mats just hanging there, they will absorb much more of the impact, than if they were laying directly on the gun, or wrapped around it tightly.

If this is at a school, then the wrestling mats should be readily available. Hang them high enough that anything going over the top of them, will not have any chance of hitting anyone.

Let the 'tail' end, drape out across the floor. This will almost eliminate anything getting out under the bottom.



Either option increases the safety factor by a magnitude or two.

Put them both together, and you'll have as close to failsafe as possible. Short of just rebuilding the entire thing out of metal pipe, and adding a couple of safety pop-offs to limit inflation pressure. (At least one of the pop-offs, hidden from the students.)



The only way to make this truly failsafe, is to not use it at all.
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Unread postAuthor: MrCrowley » Sun Nov 14, 2010 3:02 pm

Another option would be to build a larger cannon and use it at much lower pressures.
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Unread postAuthor: Moonbogg » Sun Nov 14, 2010 6:26 pm

Ditto on what everyone else already said, especially tech. But still, when using any spud gun, there is a level of risk that must be accepted if you are to use any cannon at all. Risk can be reduced but not eliminated. Think about what you would say when standing before a judge. "Well your honor, this kid lost an eye using my cannon, but it shouldn't have happened because I wrapped it in cloth and then covered the thing in ABS pipe. Sorry"
Would that excuse suffice? I doubt it. Accept the risk or don't use the cannon.
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