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I started this thread on O Rings because I didn't see too much info on using them in air cannons. I found one thread that mentioned using a floating ring, but not much was said.
I am using "Floating" O Rings in my quick dump valve cannons. In building the larger size cannons we learned tight rings lead to stuck valves, so on a whim from a bit of knowledge from my automotive class, I decided to use O rings as piston rings where they do not seat in the bottom of the groove.
To prevent loss of seal in less than perfect valve cylinders, I went slightly wide on the grooves so the ring couldn't stick down in the groove. Using air pressure to force the ring against the side of the cylinder wall and the side of the piston ring groove stops leaks. Here is a short video showing how well this fixes stuck and leaky pistons.
Here is a video of a preschooler pulling the valve at 100 PSI.
Tell me what you think. Did I miss anything other than what lube to use and not use?
***Edit***Instead of starting a new thread for the valve body, I am including it here
I just made a video to show the making of the valve body for the quick dump valve. You don't have a machine shop? Neither do I.
If you don't weld, this valve can be made and assembled inside a large T with epoxy or JB weld as an external valve. You lose some efficiency with the extra plumbing.
We took the effort to drill out the tanks and braze the valve bodies inside the tank for a really short tank to barrel path and a fully balanced coaxial design for recoil control.
Last edited by Technician1002 on Thu Apr 09, 2009 5:42 pm, edited 2 times in total.
hmmm nice and pretty informative... thx for contributing
Children are the future
unless we stop them now
Mabey post a video of you using a Drill press to machine your piston?
Aka what tools did you use?
Very Informative. I dont think Ill be making a Quick dump valve but an O-ringed piston is in my reaches.
I didn't make a video and it would take too long when a couple simple pictures do the job.
I cut squares from the plastic and drilled a hole into, but not through it.
Then I screwed in a bolt with the head cut off and put it in the drill press chuck.
I used a wood chisel clamped in a vise as the tool to cut it round by lowering the chuck past the tool.
After it is round, a ground down chisel was used to cut into the sides to make the 2 grooves. Sorry I don't have a photo of cutting the grooves.
Tools used include;
A skill saw with fence to cut strips and then cut squares
A drill press
A Drill Press Vise
A set of wood chisels
A grinder to make a chisel very narrow
Digital Calipers to check everything for size.
Hey tech, I've been doing some research since my pistons didn't seat this morning and it seems like in order for the ring to seal, the OD of the ring has to in fact be larger than the ID of the chamber bore. In technical designs, there is even a "Concentricity" measurement, being "a Total Indicator Reading between the piston O.D. and the piston groove diameter" that represents the amount the ring compresses outward against the bore of the housing.
This is fine and dandy, but it seems to me that in your design when the piston slides across the ports the o-ring would expand into the open space and catch on the lip of the vent ports--like mine keeps doing. Is it just the sanding that you've done that prevents this, or am I missing something? Wouldn't it further decrease the breakout/operating friction and prolong o-ring life if this could be eliminated?
BTW, some great links on o-ring design:
http://www.rlhudson.com/O-Ring%20Book/d ... namic.html
http://www.allsealsinc.com/parker/dynam ... ealing.pdf
Without stick on sandpaper I would have never been able to enter the summer contest. No joke. Nice video, goog stuff for sure here.
This was a concern and I was worried about that very issue. My first QDV did not have floating rings, so it tended to be hard to pull. (see ABS Cellular Core Test Cannon thread ) The 2 inch cannon built for the competition (not here) was a design experiment to make the cannon easy to pull. To keep the rings from expanding out too far, rings were selected to just touch with very little compression. This design was so the rounded edge of the ring would compress it only slightly to re-enter the bore of the pipe without any problems. The shake to open the Marshmallow in the o ring video shows this pressure is minimal. To assist this and to reduce the cuts from a sharp edge, the edges are rounded inside and out. The rounded edges reduce turbulence and improves flow in addition to protecting rings from getting pinched and cut So far I have had to replace no rings due to damage ever. The bumper is showing much more sign of wear and abuse. I bought spare rings, but didn't need them.
To reduce air pressure from trying to ride the rings out of the groove and onto the face of the piston, the edges of the grooves are cut at almost a 90 degree angle so there is no slope for them to ride up. The diameter of the piston is very close to the ID of the pipe it rides inside so most of the ring is recessed inside the groove with only the rounded edge protruding to make contact with the wall of the pipe.
For a pic of the rounded edges, see the 2.5 QDV build thread. I hope this answers your questions.
Normally floating rings are used in applications below 200 PSI so they don't blow out of the groove. My current build is to test this near that limit before I design and build the 4 inch 8 gallon cannon for up to 200 PSI.
Been a while since someone has posted on here, but I just wanted to point something out. If you let the first video load completley, then drag the scroll (? the grey and black thing) button all the way across. You will see the pressure gauge on the T shirt cannon slowly go down. Not a perfect seal, but a really good one
Bahaha Ranger you're right, but, depending on the increments, it was only about 5-10 psi and plus it was pressurized about 5 minutes during the video and most likely a few minutes before shooting the video, so who cares? Haha still a good catch
It seals better at high pressure. For the demo indoors I used low pressure. Most of the time there is little reason to ever leave it charged for that long. For a normal load and shoot, the pressure drop is minimal.
It depends on how long ago I last took it apart and wiped the dust and dirt out of the o rings. Part of regular maintenance is to pull the piston, pull off the o rings, wipe down the piston and o rings, re grease everything and put it back together. It has been so long since I made the video, I have no idea how long ago the last clean was before I made the video. I did not grease it just before the video because I did not want to shoot out excess grease. The first few shots tend to remove any loose excess grease. I've been using it this year and did not bother to clean it yet this year. When it bleeds down too fast, I'll take it apart and clean it.
It gets stuff in the grease. Everything from tennis ball fuzz to paper towel fuzz collects in the grease. I added a photo of the gauge so you can see the pressure drop. Holding pressure for 5 minutes is not bad for a home made valve.
Last edited by Technician1002 on Fri Jun 11, 2010 11:24 pm, edited 1 time in total.
Wow the pressure drop was only about 5 psi, that's pretty good for a dirty piston on low pressure. To remove it, do you take off the back cap and pull out the internals or remove the handle and push it out the front?
The nut unscrews off the front of the rod and the piston pulls out the front.
Ah, thanks for adding the picture! I presume that nut also serves as the "pin" that pulls the piston back too? And there is another sort of "pin" behind the piston that pushes the piston back forward to seal?
You are correct. The nut pulls the piston open. There is a welding rod pin behind the piston. Since little force is required to close the valve, it uses the same 1/16th inch rod as the Marshmallow cannon. It is pushed through the hole and the ends are folded over so it can't fall out.
This makes the pin small enough to retract back out of the way when the piston is fired. The piston can't come back far enough to hit the pin when fired.
Let me get this straight... the pull rod, or whatever you call it, is not tighly fiteed though the piston right? It slides relatively easily though the piston, so when you pull the valve open, the nut on the end leaves contact with the piston until you pull it back further?
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