mark.f wrote:Couldn't you just have one piston on the pipe in one side, and one on the other side, with the large gap working as the port itself?
I abandoned that design long ago as a floating ring is expanded under pressure to make a tight seal. When it comes out of the end of the pipe, the ring expands enough to be removed from the groove. My early coaxial was built that way and o rings were lost when used above 45 PSI. I considered it a failure. My later designs fixed that problem. My ABS cannon is the first properly working QDV I built.
For the ring getting stuck, it indicates the ring is too big. The wide part of the ring should remain down in the groove at all times with only the edge bulge sticking out to make a seal against the pipe.
Photo showing the fat part of the ring always remains in the groove. This is the piston in my Marshmallow Cannon.
The front ring as a floating ring has sagged to the bottom so the top side shows no protrusion. Even off center the fat part hanging out the bottom has not extended outside the groove.
This photo has rings that are too large. I use this to make it clear the rings are loose and floating in the groove. The rings used on the working cannons are only slightly larger than the piston and just large enough to contact the cylinder walls.
To add the "bars" between the ports to properly retain the ring, you don't want the ports too wide. Making them less than 1/3 the pipe circumference works for me in the smaller sizes.
Another recommendation is to have very little gap between the piston and cylinder wall. I machine my pistons for very little gap. They are just small enough to slide freely without sticking or dragging.
This way you can use the smallest possible o ring without the need for it to extend too far above the surface to work.
I would recommend building it in a larger T and use a pair of reducers to hold the barrel/valve cylinder and use 3 ports instead of one. A 2 inch T would work nicely for this.
I have future plans to build a 2 inch iron QDV in a 3 inch T so the chamber can be changed for testing. Large for low pressure for fruit and small high pressure for insane pop can launches.
