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QDV Orings Mcmaster

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: Technician1002 » Mon Feb 28, 2011 8:28 pm

If the ring is in contact with the bottom of the gland (groove) then the ring is no longer considered floating. The disadvantage is higher moving friction as the ring gets compressed between the cylinder wall and the bottom of the groove. It works, but with higher friction will operate slower, have higher wear, and may require much higher forces to move the valve.

My ABS cannon is much harder to pull because it does not nave floating rings. When freshly greased, it is not too difficult to pull, but when stored for a while and the grease is squished off the face of the ring, sometime it requires a hammer to dislodge the piston and get it lubed and moving again. Sometimes I store the piston out of the cannon for the winter due to this sticking.

To ensure the gland is deep enough I cut it just deep enough so the ring when pushed to the bottom of the gland, does not have any protruding above the surface of the piston. This way it does not hit the bottom of the gland when it is in the cylinder. The ring is sized so it is loose on the piston and is just large enough to touch the cylinder wall all the way around.

The T shirt cannon and marshmallow cannon do not have a sticking problem at all. They are both easy to pull even when stored for a long time.

O rings are chosen to fit nicely in the pipe to provide a seal against the pipe. The piston is then made so the ring does not get squished between the cylinder wall and the bottom of the gland.
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Unread postAuthor: clemsonguy1125 » Mon Feb 28, 2011 8:49 pm

Since I only have one port will this cause the piston to be hard to pull?
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Unread postAuthor: Gun Freak » Mon Feb 28, 2011 9:12 pm

One port, meaning one hole cut in your cylinder? If so, negligibly.
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Unread postAuthor: Technician1002 » Mon Feb 28, 2011 9:38 pm

The number of ports doesn't make a difference. The pull force is when the rings are sealed on solid pipe. Once moving, and the port opens, the air will assist the opening and it will finish opening much faster than you can possibly pull it.
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Unread postAuthor: velocity3x » Mon Feb 28, 2011 11:10 pm

@ Tech
What's the highest psi you've used in this particular valve design?
What is the piston dia?
Does it have a tendency to blow off the forward o-ring upon firing (especially using high pressures)?
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Unread postAuthor: Technician1002 » Tue Mar 01, 2011 12:50 am

I had an early design without bars as it was built more like a traditional barrel sealer piston and it blew the ring off when used above about 45 PSI. This is why all my later designs have ports with bars between them.

I've fired the 2 inch t shirt cannon and ABS cannon at a max pressure of 100 PSI with no issues except some pretty massive recoil. The 1 inch marshmallow cannon has been fired at 115 PSI with no issues.

My 2.5 inch is a slow work in progress and has not been fired yet. With the heavier steel tank, it is intended for higher pressure. The photo of the o ring test fit above is the 2.5 cannon. It's slow build is in this thread.
http://www.spudfiles.com/forums/2-5-inch-qdv-build-t19456.html
Bad weather (winter) and a full schedule has this progressign very slowly.

Using the 3 ports with the metal between them has done a nice job retaining the o rings.

Here is an AVI video of the 2 inch valve in operation at 100 PSI. The 3 inch barrel is launching a full Gatoraid. The recoil is massive. I got a bruise from this shot. I am supporting the breech with my thigh. Watch the breech against my leg when it is fired. It jump back a couple of inches.
https://inteltrailblazerschallenge.wikispaces.com/file/view/3+inch+Gateraid+shot.avi

This shot I believe is also 100 PSI, but I didn't record the official test pressure. This time it is a 4 inch barrel with a foam ball. The recoil is much lower.
https://inteltrailblazerschallenge.wikispaces.com/file/view/4+inch+foam.avi The camera operator didn't know how to shut it off.. :roll:
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Unread postAuthor: Labtecpower » Tue Mar 01, 2011 9:30 am

Some info about my QDV gun:

ID of the valve housing: 42,3 mm

O-ring size: 43 mm OD

Ports: 3x4 cm


It started as a floating O-ring design, but it leaked too much. I filled up the groove to compress te ring against the valve tube.
It seals very good now, and isn't hard to move.


I dont know what material you are going to use, but do yourself a favor, and DONT pick anything that needs a lot of sanding. It is horrible. For my valve, I worked for about 30 hrs to get the weld seam away.
I have a lot of patience, but i'm not sure if you do.
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Unread postAuthor: Technician1002 » Tue Mar 01, 2011 11:09 am

It is true, when using iron pipe with a seam, you will need lots of time. I started with 80 grit and worked up to 300 after the seam was removed. The ABS cannon did not need any sanding except to round the edges of the ports. :D A small valve like the Marshmallow cannon is not bad, but the larger sizes take lots of time.
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Unread postAuthor: clemsonguy1125 » Tue Mar 01, 2011 6:10 pm

I'm using PVC, so no sanding. I have little patience on thing like that, but I was afraid all the upward pressure from the port at the bottom will make it hard to trigger but I guess not, thanks, the piston is from standard pve fittings.
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Unread postAuthor: Gun Freak » Tue Mar 01, 2011 6:43 pm

clemsonguy1125 wrote:I was afraid all the upward pressure from the port at the bottom will make it hard to trigger but I guess not

No because air is also going to get of the other side of the piston, making it a ballanced force.
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Unread postAuthor: clemsonguy1125 » Tue Mar 01, 2011 7:05 pm

But it would be the force required to start the piston moving. But it shouldnt be an issue.
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Unread postAuthor: Gun Freak » Tue Mar 01, 2011 7:42 pm

No it won't :D Air pressure is pushing equally all around the piston. The only resistance to opening is the o-ring compression against the wall of the cylinder.
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Unread postAuthor: clemsonguy1125 » Tue Mar 01, 2011 9:01 pm

Thats good, but the only open part will be at the bottom of the tee where the port is. I need to make a sketch later.
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Unread postAuthor: Gun Freak » Tue Mar 01, 2011 9:22 pm

I understand that, but no matter where the opening is, as long as air gets around your piston, it will still be ballanced, as I said before.
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Unread postAuthor: clemsonguy1125 » Wed Mar 02, 2011 4:24 pm

That what I was missing, air leaking around the piston thanks.
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