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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: Jimmy K » Sat Sep 11, 2010 3:53 pm

Some guy today gave me a 20 foot long 4" SCH 40 pipe. Naturally I'd like to use all of this as my barrel. Is this feasible with my valves/chamber? Or should I give up hope on having a 20' barrel?
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Unread postAuthor: Gun Freak » Sat Sep 11, 2010 3:56 pm

You are using two of these 2" valves right? It would work, but the performance wouldn't be as good as, say, a 4" piston valve.
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Unread postAuthor: Jimmy K » Sat Sep 11, 2010 4:00 pm

I'm using those two 3" valves I got off of eBay. So still a lack in performance?
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Unread postAuthor: Ragnarok » Sat Sep 11, 2010 8:04 pm

Jimmy K wrote:Is this feasible with my valves/chamber?

Yes. While an overly long barrel can be a problem on some cannons, something like this shouldn't suffer for a barrel of that length.

I'd say things were getting less ideal as far as chamber to barrel ratio (assuming the 11 gallon chamber), but while perhaps you won't be getting the absolute maximum power from the barrel, it will still work fine - and you won't be lumbered with the issues a larger chamber will bring.
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Unread postAuthor: Technician1002 » Sat Sep 11, 2010 11:27 pm

If you start with a very long barrel, you could follow my example to trim the barrel to the optimum length for your desired projectile, pressure, and valve. Here is a link to show what my results were with a shirt launcher starting with 10 feet and working back to peak performance. The chamber is 3 gallon and a home built 2 inch piston valve is used.

You should be able to trim the barrel for maximum performance. Note that the acceleration pretty much stopped and the added length reduced the terminal velocity. Cutting the barrel shorter increased the terminal velocity. The optimum length varies on valve, projectile and pressure.
http://inteltrailblazerschallenge.wikispaces.com/Barrel+length+trim+method
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Unread postAuthor: Jimmy K » Sun Sep 12, 2010 8:31 am

Ragnarok wrote:...assuming the 11 gallon chamber...


Got a 20 gallon tank and compressor. Mounting it onto the trailer soon with the pan/tilt mechanism.

For the connection of the 4" barrel to the two 3" valves, I was just going to use a 4" tee and put in 3" bushings. There would then be two 90 degree turns for the air to go around. Is there any better way to do this? I'm concerned this might be too restrictive. I looked for a wye, but I could not find any that large.
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Unread postAuthor: Technician1002 » Sun Sep 12, 2010 11:11 am

I'm working on a design of a 4 inch QDV in a 4 inch T. I'll get a sketch together later this week and send it to you. Do you have a drill press or wood lathe?
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Unread postAuthor: dewey-1 » Sun Sep 12, 2010 12:43 pm

Technician1002 wrote:I'm working on a design of a 4 inch QDV in a 4 inch T. I'll get a sketch together later this week and send it to you. Do you have a drill press or wood lathe?


Tech, do you want a 2D/3D CAD drawing of it?
Are you using a metal or PVC tee? SCH 40 or SCH 80?
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Unread postAuthor: Jimmy K » Sun Sep 12, 2010 2:08 pm

Technician1002 wrote:I'm working on a design of a 4 inch QDV in a 4 inch T. I'll get a sketch together later this week and send it to you. Do you have a drill press or wood lathe?


I have a drill press. Sold my lathe a few months ago.
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Unread postAuthor: Technician1002 » Sun Sep 12, 2010 4:45 pm

@dewey-1, It will be all ABS construction. I have a few details to work out before spending the time in a CAD program. The design goal is higher performance from a low pressure cannon. This is the reason for the large valve and ABS construction.

The basic outside shape is a 10 foot barrel (may be trimmed later), A 4 inch T will house the QDV piston, and a short pilot area for the bumper. Off each side will be a pair of 4 inch street elbows with a pair of 5 foot long 4 inch chambers. Design pressure will be 20-50 PSI for water balloons.

Piston will be composed of 2 3/4 inch thick HDPE pucks separated by a length of 3 inch ABS pipe. The control rod will extend into the space between the pucks so the rod and nut can both push the valve closed and pull it open. This should be lighter than my solid pistons in my earlier designs and work well at low pressure. A set of through bolts to hold the sandwich together will complete the piston. The bolts will be just inside the 3 inch spacer to keep it centered. The heads of the bolts and nuts will be recessed. Caulking will seal the spacer to the ends. The pucks will have regular floating o rings.

Some filler will be placed inside the T between the ports to retain the o rings through the piston travel. After I pick up a T, I'll look into ways to address this detail. Most likely a strip of 4 inch pipe will be cut to fit and glued into place between the barrel and pilot, then sanded to the proper height.

@Jimmy K, This should be able to be built on a larger drill press. I think mine is too underpowered to turn the 4 inch components. A neighbor has a larger one I can borrow. I'll be experimenting with smaller cutting bits, light cuts, and such to see if it can be done. The only parts that will need to be made on the drill press are the piston end plates with the groove for the o rings.
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Unread postAuthor: Jimmy K » Thu Sep 16, 2010 5:47 pm

So I have this huge 20 foot pipe laying in my barn right now waiting to be mounted. I have a few questions before I mount this thing. With the rapid expansion of air at great volumes, the temperature would drop drastically, correct? So since this long barrel would be under some tension from a cable support to the end, and since PVC is very fragile under extreme cold temperatures, would I run the risk of cracking or shattering the barrel? Would this be suited for winter use (sub freezing temps)?
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Unread postAuthor: Technician1002 » Thu Sep 16, 2010 5:57 pm

The chill is simply cold air with a small volume and low thermal mass. This chilling is much like reaching into your deep freeze to grab a package in the bottom. Do you put on gloves to prevent frostbite? Not an issue.

In the winter in sub freezing weather the risk of shattered PVC is greater. It is not recommended to use in freezing weather.
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Unread postAuthor: Jimmy K » Thu Sep 16, 2010 8:11 pm

Does PVC have a tendency to crack/shatter from gradual fatigue? As in if I have the barrel suspended by a cable (for compensating barrel sag), would it be possible to go out one day and find that it cracked in several spots?
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Unread postAuthor: Technician1002 » Thu Sep 16, 2010 8:41 pm

Yes, PVC fatigues over time and flexing. I broke several pistons in my Mouse Musket. They broke at about 1 year of age most of the time. I replaced the piston with HDPE, but the Musket developed another crack.

My Marshmallow cannon had a PVC end cap that broke after about a year and a half. I replaced that with steel.

For my experience with PVC, I started this thread.

http://www.spudfiles.com/forums/pvc-is-fragile-t21334.html
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Unread postAuthor: c11man » Thu Sep 16, 2010 9:27 pm

use a good bumper, build it correctly, dont drop it, and glue it correctly. it shouldnt fail. pvc does have it limitations, design the launcher correctly and you can overcome those limitations.

the expanding air will not cuase the barrel to break from the cold, flexing will. i use a 10ft THIN wall barrel with the a solid support about 4ft down and a suspension type support on the end, it holds up just fine. so if you build the barrel supports sturdy enough your 20ft barrel will not break
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