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A snippet from the Copperhead development studios ^_^
Not long after completing Copperhead Prime I found something interesting about the dual valves I had on the pilot line from the QEV.
Pulling the trigger while the safety valve was on would vent a small volume of compressed air from between the safety and the blowgun and the gun would of course not fire.
However resetting the trigger by opening the safety after that caused the QEV to release a blast of air out through the barrel.
I deduced that the small pressure drop in the pilot caused by the safety valve refilling the blowgun as it was opened meant that for a split second the pilot pressure was lower than the chamber pressure.
The QEV therefore vented compressed air through the barrel until the chamber pressure was equal or lower than the pilot again.
The process could be repeated to get multiple blasts of air from the chamber - I can get ~5 good power shots from Copperhead Prime in this way from a fully charged chamber. This is partly due to it's oversized chamber - a smaller chamber would have given less shots of satisfactory power.
I've since simplified the whole process and substituted single valves in the following diagrams so a single valve and a single trigger pull/release controls the whole system:
This is the basic idea:
It uses a sleeve valve with the outward port sealed by a plug to trigger the pressure difference in the QEV.
It resets, ready to fire again by sliding the sleeve back or having a spring reset it. Ammo feed has a blow forward bolt or an interrupter paddle to seal the magazine.
Lets get advanced:
This valve is a 3 port spool valve. The pressure difference on the QEV is caused by releasing a small bit of pilot air into the middle port which has been plugged.
However, instead of in the sleeve valve of the compressed air being wasted, it is being harnessed to drive a small pneumatic cylinder against a spring to reload the cannon.
Higher chamber pressures will generally give better results.
The special thing about this method is that it allows you to charge up a cannon from an air source and get multiple controlled shots without being attached to an airline or a gas cylinder.
Last edited by Hotwired on Mon Jan 21, 2013 6:49 pm, edited 2 times in total.
Alrighty! I can't wait to see how it comes out.
Here's my idea for an economical homemade replacement for your three-way valve:
<img src="http://xfb.xanga.com/39dd932439630141017843/m103981198.jpg" alt="3_way_vavle">
The rod could be a screw with nuts and washers on it to make the o-ring grove. Just be careful to keep the diameter of that thing small: 300-400psi means some serious force on that rod once the trigger has depressed it and opened the schrader valve. The trigger is going to kick back when you pull it.
I've heard it said that a blowgun valve can be modified to do the same trick, but I can't figure out how.
I'm not sure if I'd be comfortable with a piston behind the trigger getting 300psi dumped behind it though
However the 3 port valves are not cheap wherever they're bought from so a replacement for it is definitely worth thinking about.
Great work Hotwired, one of the best designs I have seen here in awhile.
I have a few things that you might want to consider about the design if you haven't already.
The ratio of barrel diameter to piston diameter will be important in the design. If your barrel is too close to the piston diameter you will need a large pilot expansion chamber (best term I could think of) to get the gun to fire, and you would be left with a large volume of low pressure air which may not be helpful for operating a loading mechanism. If the barrel diameter is too far from the piston diameter, you may encounter issues with the valve honking. I think the honking issues could probably be fixed, but it is still something that would best be addressed in the initial design.
Similarly piston weight and pilot volume could also affect the valve. Too light and/or too small of a pilot volume could also possibly cause the valve to honk. Too big of a pilot volume and a heavy piston could just be wasteful.
Why I am concerned about honking is because the pressure in the pilot will increase as the piston moves back, similarly the pressure will fall as the piston moves forward. If the piston moves forward too quick and/or the pilot pressure drops too quickly, the piston may have a tendency to bounce back open and repeat.
I suppose since you've already found a valve that this works well on these points are a bit moot, but still they are things that may need to be considered if anyone tries this with a homemade valve.
I know you designed this to fire off a tank of decreasing pressure, but have you given any thought to making the design work with a constant air source like a compressor or CO2? If you just plug it into the chamber you may have issues with the incoming air firing the piston before the pilot equalizes. The only other way I can think of is to fill from the pilot and add an extra port and complexity to the fill valve so that it fills in the forward position but shuts off in the firing position.
Looks like some really good work you've been up to. I like how the exhaust air is used to reload the cannon too.
A "safety" sear would help your worries I guess.
I also noticed that in diagram 2, "lets get advanced", you were using cartridges, or non-spherical ammo instead of balls like in the first diagram. Is there any reason for this, or just an insignificant difference?
Good luck for construction.
really good quote/phrase here
@clide: I've been working on using a similar concept to Hotwired's to intentionally get the valve to repeat (see the auto-diaphragm thread). From my experience, I don't think the valve will repeat. But you are right, smaller pilot volume is better (less air to pilot, so less waste), and the expansion chamber probably has a minimum volume that will trigger the gun properly.
Also, if you want to put, say, CO2 into the gun through the pilot area, that brings you right back to a design like your slider or gb semi-auto (which are very nice guns, btw). But I think that kinda defeats the stand-alone philosophy. What do you say, Hotwired?
Almost all bouncing valves (bouncing in the sense that I am referring to) would bounce open and closed so fast that I can't really imagine them being useful. They let out small quantities of air, close, and probably even reopen before your first projectile has left the barrel and before another can be loaded. Take a sprinkler valve actuated by a solenoid for example. It opens and closes so fast that it creates a tone. IIRC jimmy once measured the tone to see what the frequency was, but I can't remember what it was. Plus that kind of behavior is sensitive to projectile weight, change your projectile and the valve could change the bouncing behavior.
The minimum volume for the expansion chamber would be pretty easy to calculate. Just use the seat and piston diameters to determine what pressure drop is needed to open the valve and use P1V1=P2V2 for the pilot to calculate how much volume is needed to drop the pressure that low. Although you would probably want to go a bit bigger than the minimum to account for things like the equalization hole.
The benefit of this design over the Slider for a constant air supply is that this design doesn't completely vent the chamber which allows it to be much more efficient. It would work in much the same way as the GB Semi with a constant air supply. But like the GB Semi it would still work without the air supply, but with decreasing pressure each shot.
Re bouncing: I wasn't sure that bouncing was the cause of sprinkler honk (I've heard sprinklers honk, so I know what you're talking about.) I thought it was something akin to the whoppee cushion effect, or making a party balloon scream by stretching its mouth wide and releasing air. I can't imagine the entire valve oscillating fast enough to be in the audible frequency range.
Anyway, I think the whole thing is moot anyway, because we aren't dealing with a slow pilot emptying rate. The pilot psi drop is going to be fast, so there shouldn't be a problem with bouncing.
Yep, that efficiency gain is one of the big things I like about Hotwired's idea. But like you said, it's hard to integrate an constant air supply into it.
The strange thing is, Hotwired is (and I have been too, for that matter) trying to get that air efficiency with a barrel-sealer, when the whole idea of the barrel sealer is to make a valve that flies open as fast as possible and doesn't want to close again.
The diagrams are simplified but I intend to use the same style of piston QEV I have on my existing cannon and it works in a semi-semi-automatic manner (using two valves) with the ~20cm of 8mm copper tubing it has on it so I'm not too worried about that.
I'm planning to use it at 250-300psi starting pressure with over twice the chamber volume of copperhead and although the secondary pilot volume won't be very large, the high pressure should still be able to shift a small, well sealed pneumatic cylinder.
A constant air supply can't really fit into the design too easily. An airline could be arranged to let air in automatically when the trigger was not pulled back, a 5 port valve would do it. But I think the best thing would be to fire from a charged chamber and take a break to refill ammo and air.
The cartridges are there because that particular design sketch is the original sketch for my ongoing cannon design which will fire plastic slugs just to be different ^_^
Little more difficult to make things for cylindrical rather than spherical ammo but it's all the same when everything works as it should.
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