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School T-shirt 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: rcman50166 » Tue Sep 15, 2009 5:08 pm

Hey I need help figuring out how to make the plug behind the piston removable so I can access the valve. This means no glue. Any suggestions? I've seen bolts anchoring the plug but exactly how is it kept air tight?
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Unread postAuthor: cannon monkey » Tue Sep 15, 2009 7:50 pm

ya threads fittings
thats what sombody told me...
put thread female on your chamber and thread male on the plug
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Unread postAuthor: Technician1002 » Tue Sep 15, 2009 8:08 pm

For most as mentioned above, threaded. In my case, I used a different style valve that doesn't use a sealed pilot area. That area isn't under pressure.
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Unread postAuthor: inonickname » Tue Sep 15, 2009 10:16 pm

A female threaded fitting on the chamber and a male on the pilot setup which is larger than the i.d. of your pipe. If you use the bolts to retain it you will need an o-ring in front of the bolts to airtight it.
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Unread postAuthor: rcman50166 » Fri Sep 18, 2009 3:15 pm

Ok well here's a new update. I've been getting feedback from members on chat and have nearly finalized the gun. I redid the model in AutoCAD and it's starting to look good.

What isn't shown: A 3/4NPT QEV will port the back and that will be ported by a blowgun. The input will be the Kobalt CO2 regulator set at 100 psi.


EDIT: does anyone recommend a spring behind the piston?
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Here is the finalized design
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Unread postAuthor: Technician1002 » Sat Sep 19, 2009 4:16 pm

Sealing a bolt through the chamber and barrel will be extremely difficult.
If the pin is to prevent the shirt from impacting the valve, drilling the end of the barrel and inserting a headless bolt that does not extend the length outside the barrel and then sealing it with a coupler on the outside of the barrel work well. Then a barrel support away from the breech of the barrel will remove the support structure away from the high velocity area. The support can be entirely internal so it doesn't compromise the pressure chamber integrity.

That will take a lot of wasted air to operate. Model it in GGDT. At 100 PSI, the volume of air will expand upon release about 5X. Having a chamber several times the volume of the chamber is a waste of energy. Consider enlarging the barrel to reduce the volume of the chamber. A chamber with as little as 1/3 to 1/2 the volume of the barrel works very well.

Back to the original question, the cannon if filled from the pilot will not need a spring if the EQ between the piston and chamber is not too large.
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Unread postAuthor: rcman50166 » Sat Sep 19, 2009 8:18 pm

Technician1002 wrote:Sealing a bolt through the chamber and barrel will be extremely difficult.
If the pin is to prevent the shirt from impacting the valve, drilling the end of the barrel and inserting a headless bolt that does not extend the length outside the barrel and then sealing it with a coupler on the outside of the barrel work well. Then a barrel support away from the breech of the barrel will remove the support structure away from the high velocity area. The support can be entirely internal so it doesn't compromise the pressure chamber integrity.


Where would I find these headless bolts, and how do you fasten them without a head?
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Unread postAuthor: spudtyrrant » Sat Sep 19, 2009 8:28 pm

Technician1002 wrote:Sealing a bolt through the chamber and barrel will be extremely difficult.
it wouldn't be that hard plus it would eliminate the need for a barrel support you could do it as simple as some all-thread, lock nuts and Teflon tape. but it would also be through a single layer so i would advise doing techs idea.
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Unread postAuthor: Technician1002 » Sat Sep 19, 2009 11:22 pm

rcman50166 wrote:Where would I find these headless bolts, and how do you fasten them without a head?


You can find headless bolts anyplace you can find a sharp hacksaw. :D

They are not simply fastened in place. The bolt for the breech to keep the shirt out of the valve could be a nail, broken car antenna, welding rod or any stiff metal. It would simply be dropped into a hole drilled through the breech. The ends of the rod would then be covered with something to seal the ends. Bath caulk and tape, a coupler cut in half and glued in place, etc. If using a half coupler the other half can be threaded with 3 holes and filed out to slide onto the barrel a few inches from the breech and hold 3 bolts, rods or other arrangement to center the barrel. Using 3 it won't wobble. Glue it into place. This way the barrel supports don't compromise the barrel by making holes in it.
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Unread postAuthor: rcman50166 » Sat Sep 19, 2009 11:34 pm

New design update:

Fixed the diameter of the chamber.
Removed the centering bolt
Put in centering blocks
Put in piston block pin
Added threads
Made the end a threaded plug/cleanout cap
Added color

Like always, feedback is appreciated
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The Updated Design
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Unread postAuthor: Technician1002 » Sat Sep 19, 2009 11:50 pm

Much better. The shirt stop doesn't have to be huge. Big reduces flow. Anything you can't bend with airflow or pressure stuffing in a shirt is fine. A 16p nail is cheap.

The centering blocks can be moved rearward. A location about 6 inches from the breech provides excellent rigidity for postion.

The close ratio piston (OD is closer to the valve seat diameter) will improve the opening speed. I think you will like that.

Now check the parts store. It is unlikely you will find a plug the piston will slide into as shown. Most will have an ID smaller than the ID of the pipe.
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Unread postAuthor: rcman50166 » Sun Sep 20, 2009 12:22 am

Technician1002 wrote:Much better. The shirt stop doesn't have to be huge. Big reduces flow. Anything you can't bend with airflow or pressure stuffing in a shirt is fine. A 16p nail is cheap.

The centering blocks can be moved rearward. A location about 6 inches from the breech provides excellent rigidity for postion.

The close ratio piston (OD is closer to the valve seat diameter) will improve the opening speed. I think you will like that.

Now check the parts store. It is unlikely you will find a plug the piston will slide into as shown. Most will have an ID smaller than the ID of the pipe.


Better?
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Unread postAuthor: Technician1002 » Sun Sep 20, 2009 1:15 am

Very nice. :)
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Unread postAuthor: rcman50166 » Sun Sep 20, 2009 12:23 pm

Alright then I think I'll propose this design on Monday. Can we come to a common consensus and say that this design is safe enough to be used at a public event?
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Unread postAuthor: Technician1002 » Sun Sep 20, 2009 7:50 pm

rcman50166 wrote:Alright then I think I'll propose this design on Monday. Can we come to a common consensus and say that this design is safe enough to be used at a public event?


It depends on who your safety committie is. Some my never approve any PVC. With a proper bumper so the PVC doesn't shatter it should be OK. Test at 100 PSI and range check. You may find you can reach the back rows on less pressure. Range test and submit your proposal for operating at a lower pressure. My little cannon has a range of 200 Feet on 100 PSI. If the stands at the local stadium isn't loaded, performance is fine on 1/2 pressure.
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Range of the small launcher at 80-100 PSI.
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The little launcher in action at the stadium shooting cross the field into the crowd.
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