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Auto diaphragm testing

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: Pilgrimman » Fri Aug 10, 2007 1:47 pm

@Skywalker

The chamber repressurization you mentioned earlier is one of the methods that helps clide's HEAR valve on his GB Semi work. Yours, however, doesn't use a piston. You might try using clide's gun as a template for a new high efficiency semi automatic or automatic design! As for me, I'm off to build it!
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Unread postAuthor: Skywalker » Fri Aug 10, 2007 1:51 pm

spud yeti wrote:Yeah, I think the best thing to do is just sit and think about it, and the idea always evolves and transforms. We (the forum) just aid you in your thought process Wink


My philosophy exactly!

As to your design, I'm not sure I follow exactly. Why do you want the chamber area to vent when you pull the body of the thing forward? Wouldn't it be better to not have the 'vice versa' part?

Here's what I thought up, I call it the Air Hammer Gun (T):

<img src="http://x81.xanga.com/92083041380b9141001512/m103968457.jpg" alt="untitled">

The principle is that you pressurize the pilot to your desired operating pressure, then set the needle valve to get the flow rate/RPS, and when you pull the trigger, the pressure rises in the chamber until it is greater than the pilot. Then the phragm fires, the chamber psi drops, the phragm reseats, the pressure rises again, and the cycle repeats. So it's basically automatic; although there's probably an upper limit to the flow rate you can set the needle valve for. Too much flow, and all you get is a big fart out of the gun.

Edit: @ Pilrimman: I need to type faster! Yeah, I started to think about clide's gun. I'm not sure exactly how it might play into all this, but it's certainly something to think about.
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Unread postAuthor: spud yeti » Fri Aug 10, 2007 1:57 pm

Too much flow, and all you get is a big fart out of the gun.

Isnt everything in life like that :wink:

I put it in so instead of the schrader piloting, the whole does, and the hole in the chamber is so the chamber doesnt overpressurize.

I actually prefer your design though, because of its "automaticness" :D
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really good quote/phrase here
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Unread postAuthor: Hotwired » Fri Aug 10, 2007 3:39 pm

I've stuffed all the info on my semi-auto method in a new topic now if you'd like to have a rummage about in it :)
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Unread postAuthor: Skywalker » Sat Aug 11, 2007 1:43 pm

Ok, I tried that automatic method, using my humble bike pump as the source for the bursts of pressure into the chamber. It would get maybe three shots before things just leveled out and as soon as I pumped air into the chamber, it would just hiss out through the valve w/o triggering it. I think part of that is b/c I tore up the diaphragm some by putting ~150psi on the pilot side to prime the gun b4 I had time to pressurize the chamber side any. (To put air straight into the chamber, I just added another schrader valve there.)

I've also discovered that the leak rate needed for firing seems to change with pressure. If I set the schrader to open only a little, at high pressure the gun doesn't fire, the air just leaks out. After a while, the chamber pressure gets down to where the leak rate is matched to it properly, and the gun starts firing. If I set the leak rate high, the gun shoots very rapidly at high pressure, and as the pressure gets lower, the shots get weak and run together.

I also tried tapping the schrader valve with a hammer, at various settings on the leak rate. It seems like I could only get 4 or 5 shots out of a tank that way, though. And again, if the rate was too low the gun wouldn't fire at high pressure. Too high, and it shot the whole tank in one or two firings.

Overall, it looks like the fire rate control isn't as nice as I initially thought., and the shot per tank is pretty limited. So I guess, if you wanted a gun that got a handful of shots in automatic style between reloads, this might be a decent concept. The main trouble with it is that you can't get more shots per tank by just upping the chamber volume, like you could with a hammer valve. I'm disappointed :evil: Oh well, at least I learned some stuff along the way!
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Unread postAuthor: clide » Sat Aug 11, 2007 5:05 pm

I had the same idea when building the GB Semi. Initially I thought I was stupid and built something way too complicated with the GB Semi when this would do the same thing, but when I thought about it more there are a few downsides to this type of arrangement.

Ideally you would want a constant air supply with probably 20 psi more pressure output than your desired firing pressure.

One thing that I'm not fond of is that the firing pressure doesn't scale with the pressure of your air supply. If you set the pilot pressure so that the valve will open at 100 psi, then you must be able to consistently supply 100 psi or you will have to stop and readjust the pilot pressure to change the firing pressure.

Another thing that bothered me is that you can't really control what the pressure in the chamber is when you let go of the trigger. This means that it could be very close to the firing pressure. This could easily result in an accidental discharge of the gun.

Things I do like about the design is that it is simple to build and should be pretty reliable with the proper setup.

Edit (well not really since I haven't posted yet, but I don't want to go back and rewrite my post): I think you may be doing something slightly different than what I had in mind. You shouldn't need to change the pilot pressure at all to get something like this to operate. You can set the pilot pressure and then suppling air to the chamber will fire the gun in an automatic fashion. If anybody is interested I could probably write a long winded post about design parameters for something like this, but if not I won't waste my time.
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Unread postAuthor: Skywalker » Sat Aug 11, 2007 7:43 pm

I think we are talking about the same thing; using the air supply to increase in chamber psi to open the valve, rather than dropping the pilot pressure. Those problems you identified are the killers for that design. That, and the fact that you need something like an extra ball valve hooked to the air supply to fill the pilot with when you first get ready to use the gun, which is awkward if you want to be able to just fill and fire. But I guess that kinda goes with the second problem you identified.

The killers on the simpler, original version I've already explained in way too much detail. :roll: But I may still have to build a little toy gun on that principle anyway.
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Unread postAuthor: Hotwired » Sun Aug 12, 2007 5:19 pm

I think fire control with tapping the schrader could be fairly reliable if it was set up in a suitable way.

Bring on ye mspaint animations:

Image

At a more realistc speed :wink: :

Image

That would give a single burst fire of fully automatic but if the trigger catch was set up to relock after each shot, perhaps with another lever, theres no reason why it couldn't be a semi.
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Unread postAuthor: Skywalker » Sun Aug 12, 2007 6:34 pm

Hmm. I like it. (Did you look at my auto-hammer idea? ) Perhaps since the hammer in this case acts to give a bit of that volume-metered effect you have on your Copperhead, then the gun will be more consistent about triggering than just tapping it with a real hammer, or relying on the leak-rate adjustment like I've been doing.

I'll have to do some more tests with this gun. I'm not sure how I'll go about making that hammer and the cylinder for it. It'll need to be a pretty small diameter set-up to operate on the limited air from that schrader.
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