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General Safety Questions

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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General Safety Questions

Unread postAuthor: webgeek » Sun May 31, 2009 10:15 am

Hi all; I created my first pneumatic gun a week ago and had so much fun I created another using a modified sprinkler valve and a blowgun. It was even more fun so I ended up giving it to my father as an early birthday present.

Now I've got some friends interested in paying me a bit to make them some pneumatic guns as well. This made me start thinking about safety. I want people to have fun with what I make but more importantly, I NEVER want anyone hurt with anything I build. Surfing around the web looking for info, I ran across this site and felt this is the perfect place to ask my question.

The design I'm using for my guns is a 2" chamber that's ~40 inches long with a basic tire stem valve in a cap on one end and a 2"-1" reducer bushing on the other. A pair of 1" 90 degree elbows are used to make this into an over/under design. I then go into a 1" Orbit sprinkler with a 1/4" brass fitting threaded (manually - can't find a tap for this) into the top of it. I use some opaque white hard plastic tubing to go down to a blow gun I've cut in half. I then make interchangable barrels via threaded couplings - a 1/2" and a 1" currently. I drill PVC ball valves to make the barrels breech-loading capable. Every item is pressure rated - the blow gun is rated to 90 PSI, the white tube is rated to 130 IIRC. The PVC is all standard Schedule 40 and rated for pressure as are all fittings.

The only threaded fittings are the blowgun, 1/4" brass fitting in the top of the sprinkler valve, and the inlet/outlet of the sprinkler. All other joints are compression and I use 2-step solvent/glue - first the purple cleaner stuff, then the glue itself. I wait 24 hours prior to testing anything. The white hose between the sprinkler and blowgun is connected via hose barbs and secured via a 3/4" hose clamp on both ends. I plug the solenoid hole with epoxy putty I work into the tiny hole and fill the rest up into the threads for strength. I use a little epoxy putty on the stem valve inside the cap for strength as well so it would be VERY hard to remove it later. Finally, I use teflon tape on all the screw fittings.

Sorry for the huge description, but I wanted to make sure I didn't leave anything out.

When shooting the gun, what is a safe pressure to use? I've never exceeded 100 PSI and that seemed a good value. It's more than powerful enough at that point. Is this too high already? I see a lot of discussion on different values so I'm curious if this is a rule of thumb here.

How can I make it safer? I REALLY don't like the manually threaded 1/4". Anyone know the specific tap and drill bit needed to do that properly? Link preferred if possible.

It seems adding a pressure gauge would be nice, or even better, one of those "pop out pressure valve" things that air compressors have to keep it from over-pressurizing. Do you experts typically use those or know where to get good ones?

Any other suggestions?

Thanks very much in advance!

-- Mike
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Unread postAuthor: cannon monkey » Sun May 31, 2009 11:14 am

dont mind me i just skimming around to make them more safe is to use pressure rated parts guys on here tell u more bout them
maybe a pressure gage and or a pop off vlave so they cant be over filled

there some where on the vavle is a swich manual open it... very small feel 10 psi flip it if it fire's thats it well gule or tape that closed ( i hand a problem with that when it hit my leg first day i finished mine)
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Unread postAuthor: daberno123 » Sun May 31, 2009 11:14 am

It sounds like you've got everything covered, but a picture of one of your completed guns would be nice, just to make sure.

I'd feel comfortable using PVC, but some might suggest using all metal pipes and fittings.

Does your gun have any type of barrel support? When firing heavier projectiles (such as a barrel full of water) out of an over-under design, those elbows take a lot of force from the recoil. There have been a few instances where the elbows have just snapped.

One thing that would be a good addition would be a pop-safety valve threaded into the chamber so one of your buddies doesn't decide to pump as much air into it as he can (a heavy guy using a decent bike pump could probably pump it to ~200 psi if he put his mind to it). You can buy them in the air tools section of any hardware store. Most are set to 125 psi, but different pressure ranges can be custom ordered off sites like http://www.mcmaster.com

As to your question about a suitable pressure to use, 100 psi will be fine, its the pressure I use for all my PVC guns. If you want to go higher, use Sch 80 or steel pipe.

As long as these spudguns are treated with the same respect as a firearm, I don't see any major safety issues with them.
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Unread postAuthor: webgeek » Sun May 31, 2009 11:55 am

there some where on the vavle is a swich manual open it... very small feel 10 psi flip it if it fire's thats it well gule or tape that closed ( i hand a problem with that when it hit my leg first day i finished mine)
Hmmm, I like that idea - I'll see how to work that into the next guns.

but a picture of one of your completed guns would be nice, just to make sure.

Good idea - here is a little YouTube video of it.
[youtube]http://www.youtube.com/watch?v=7QXmGzi6XTU[/youtube]

One thing that would be a good addition would be a pop-safety valve threaded into the chamber...
McMaster-Carr to the rescue! The 4-in-1 valve at the bottom looks perfect. The $9.57 one:
http://www.mcmaster.com/#5867t11/=244s5m -- #5867T11 even replaces the need for a separate fill valve!

Does your gun have any type of barrel support? When firing heavier projectiles (such as a barrel full of water) out of an over-under design, those elbows take a lot of force from the recoil. There have been a few instances where the elbows have just snapped.
No, and that's a great point. My new designs will incorporate something for that.

As to your question about a suitable pressure to use, 100 psi will be fine, its the pressure I use for all my PVC guns. If you want to go higher, use Sch 80 or steel pipe.
Great! 100 PSI will do for me :)
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Unread postAuthor: cannon monkey » Sun May 31, 2009 12:01 pm

just add barrel suport pressure gage/ pop off valve so u know if it loaded and u cant over fill it

and that swich on the valve get that so it cant be fired when not noticing

and maybe a small ball valve so u cant use the blow gun to fire it kind of like a safty like on real guns
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Unread postAuthor: Carlman » Sun May 31, 2009 12:31 pm

Sounds to me like you followed 'safe cannons 101' to a T! Great post and even more commendable work with safety. Onya mate 8)
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Unread postAuthor: psycix » Mon Jun 01, 2009 1:16 pm

Very wise of you to ask first before doing things you are unsure of!

-A pressure gauge is always good, so that people can see how much pressure is inside.
-A popoff valve will prevent them from over-filling and blowing the cannon up.
-A barrel support is necessary for over/under constructions. Use a block of wood between the barrel and the chamber and then strap them together. This will prevent the gun from snapping at the elbow due to recoil forces.
-100psi seems fine
-When threading the shrader valve through the PVC (or anything else, like the popoff) make sure to thread through TWO layers of PVC. This would be where a fitting joins with a piece of pipe.
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Unread postAuthor: webgeek » Mon Jun 01, 2009 3:12 pm

Thanks for all the advice and comments. I'll work all this into my next gun design.

One question though:
-When threading the shrader valve through the PVC (or anything else, like the popoff) make sure to thread through TWO layers of PVC. This would be where a fitting joins with a piece of pipe.
I follow what you are describing but I rarely see it done. I assume this is because it puts the valve in a weird location (off a side as opposed to an end). Is there a way to do this with the end caps?. Thanks!

--Mike
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Unread postAuthor: psycix » Tue Jun 02, 2009 11:05 am

but I rarely see it done

Look better, because its done that way on almost all well-built cannons and is always recommended.
Its safer, more leak-proof and stronger.
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Unread postAuthor: webgeek » Sun Jun 07, 2009 12:43 am

Look better, because its done that way on almost all well-built cannons and is always recommended.
Its safer, more leak-proof and stronger.
Sorry, I didn't mean to sound like I was arguing - just that I'd not seen it very often before. I have some plans from http://www.myaircannons.com/ and none of them run the schrader valve through a joint. It does make more sense to do it that way though and I'll be sure to do it in my future designs.

--Mike
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