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QDV problems

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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QDV problems

Unread postAuthor: Ranger » Tue Jun 22, 2010 6:15 pm

OK I made a QDV cannon, it has a 3" chamber and 1 1/2" SDR-21 golfball barrel. The piston has floating O rings and is 2" long, its made from a plastic cutting board (HDPE). The barrel has been sanded and honed to make a perfecty smooth surface.

The piston seals, but I always have to tinker with it to get it to seal. I apply air (air compressor) and then fiddle with the piston and then it seals, I want it to seal the first time and not have to play with it to get it to work.

Technician: Any advice? I know you have the most experience with these valves, have you had this problem before?
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Unread postAuthor: Technician1002 » Tue Jun 22, 2010 6:27 pm

Cut the grooves in the piston so they are not much wider then the o ring width. Narrow enough to blow into place on the first shot of air, but not wide enough to let lots of air blow around the ring without moving it to a sealing postion. The Marshmallow Cannon and 2 inch T Shirt cannon seals on the first stroke of a hand pump.

Try to keep your o ring grooves relatively narrow. The rings should not bind or stick in the grooves at all.

A small amount of grease helps the ring move into position and permits less air blow by.

Calibrating the rod to precisely position the piston helps. Mine is simple to operate. Push in the rod and add air. No fiddling needed.

The Arlington team used a long threaded rod for the shaft and a pair of nuts to make the stop position adjustable. I operated mine several times to find the positon and marked it with a sharpie, then added the knob. I pinned it into positon so the calibration won't change.

Congratulations on your first QDV. Post video.

Here is a pic of me signing their launcher at the competition. You can see their rod is long between the tank and golf ball. A pair of nuts makes an adjustable stop. Sorry I don't have a close up of that detail on their launcher.
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Last edited by Technician1002 on Tue Jun 22, 2010 7:27 pm, edited 1 time in total.
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Unread postAuthor: Ranger » Tue Jun 22, 2010 6:54 pm

Ahhh. My O ring grooves are pretty wide, I cut them with the intention of using a 3/16" thick O ring but I could not find any so I had to use a 1/8" thick O ring.

I will post pics and video when its done.

Looks like I need to make another piston :x :(
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Unread postAuthor: deathbyDWV » Tue Jun 22, 2010 6:57 pm

Congrats on the qdv. I tried making a ghetto one once but failed...



Technician1002 wrote: A pair of nuts on the rod adjust the distance the rod can go in.

Hmm, I didn't know that's how it works... :P :roll:
(I need to stop taking things out context...)
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Last edited by deathbyDWV on Tue Jun 22, 2010 7:15 pm, edited 1 time in total.
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Unread postAuthor: Technician1002 » Tue Jun 22, 2010 7:07 pm

I bought the rings to fit the pipe and then cut the piston to fit the rings. A little elbow grease sanding the inside of the pipe helps it seal.
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