Idea for a pneumatic gun

tromic said:

Back to this (Valentins idea), I would make a tabular valve something like this:

Click to expand...

This is the counter-boring that I referred to before (post #32). Going back to the earlier design a possibility for the elimination of damage on the rear tubular valve "O" ring would be to widen the bore of the inner barrel to a size where the rear "O" ring just loses contact around its periphery just before it enters the windowed section of the barrel and only returns to the original inner barrel ID after the rear edge of the window is passed.

tromic said:

Well, Pete, you are right. That would be demanding to make. To make that boring after the thread maybe lathe could be used too. But this drawing is just an basic idea of possible configuration, not blueprint for manufacturing. On a link I attached that valve is made much more simple.





Mushroom like part corresponds to my dark blue valve. Piston like part corresponds to my light blue part.
Mushroom like part might seal against piece of Polyurethane tubing. This design seems to me to be quite reliable and easy to make.

Design like this is very good choice for barrels up to 11 mm ID. For barrels 13 mm ID it won't be (maybe?) the best choice. In that case tabular valve (Valentins) or just a piston with locking possibility (as I draw before) would be better choice.

Click to expand...

Here a pin and a slot is used to control the front valve's stroke (dark blue valve equivalent). The slot is rotated away in the photo, you can just see the edge of it. Cutting slots in a slim metal cylinder is not easy, probably drilled out a few holes and then filed the opening to final size. The slot could be milled out, but then you need a milling attachment or a dedicated milling machine.

Re: pneumatic gun, hydraulic trigger mechanism

Here is a challenge for speargun designers. Design a pneumatic speargun that uses a hydraulic trigger mechanism, either the hydraulic trigger circuit controls a releasing valve (inner barrel plug or diaphragm equivalent) or it controls a sear tooth acting as a piston catch or a ball type of sear fulfilling the same function.

The first option has already been done in hydropneumatic spearguns where it is a relatively simple arrangement and the water inside the gun is both the working fluid that pushes the spear out and is also the hydraulic circuit operating fluid. The SOS "Ringo" was a hydro-elastic speargun that used a diaphragm as the releasing valve rather than a piston acting as an inner barrel rear plug (it has been discussed here previously).

Safety aspects will ideally need to account for a hydraulic circuit failure so that the gun is restrained from shooting if this occurs. Hydraulic circuit can use either an independent oil or hydraulic fluid supply, or the ambient water and can be pressurized to reverse hydraulic fluid flow. Complexity of the gun is not a limitation, nor does it have to float after spear discharge, although this would be an advantage.

The cupboard of speargun ideas has been fully ransacked over the past decades, so such a gun may have already been created, but I do not recall seeing anything on this topic (for which there may be very good reasons, safety being one of them!).

Not sure just what advantages this design brings. Same sealing problem as before (see superimposed diagram of transverse view of barrel port and side valve plug), plus "O" ring acting as a return spring is not very reliable when "O" ring breaks. Air flow and pressure differential to create shooting action not very obvious either. Two dimensional diagrams showing longitudinal cross-sections, where everything looks square, often hide problems as many components have circular cross-sections.

I see that this latest version has a pilot valve on the side (red) that in turn causes the main valve (yellow) to open for the actual shot. When would you reset the yellow valve by pushing it forwards? Ideally you want the orange piston to be pushed back with the yellow valve still rearwards and then for the latter to come forwards to close off the port at the rear open end of the inner barrel once the piston is in, or near, the "ready to shoot" position. After the shot the orange piston will be at the muzzle end and the inner barrel will be full of air at 30 bar which will keep the yellow valve pushed back on the rear bulkhead at the termination of the inner barrel. So how to time the operation of and control the position of the yellow valve? The pilot/servo valve is an interesting idea used in the "Aquatech", but water used on either side of the main valve stays at a constant volume regardless of pressure changes, so the valve does not respond to pressure changes until a pilot valve opens to let water out.

Without a "before" and "after" some ideas look plausible on paper, especially when viewed in isolation as a "snapshot". I think that this is one of them, unless there is a realistic sequence of movements and an interaction of the various components to reach this "snapshot" condition then it is just not possible to engineer a gun that way.

The problem with the design is that any air inside the gun is at 25 bar with the spear at its fullest insertion distance into the gun, even the air that is enclosed inside tiny gaps between and surrounding the moving parts due to running clearances. Once the loading effort is removed from the spear that trapped air will expand. The tiny crescent shaped gap that you encircled would be at loaded gun pressure (30 bar for that particular example) and would open up, forcing the yellow valve and piston apart before you even pull the trigger. If the gap there was say 0.5 mm wide then the air expansion to ambient pressure would take it to 30 x 0.5 = 15 mm, or if the gap was 0.1 mm wide then it would be 3 mm, maybe still enough to shoot the gun.

I understand what you are trying to do, i.e. the loading action on the spear squeezes all the air out of the inner barrel so that either end of the orange piston/yellow valve combination is at ambient pressure, but with no air left behind the yellow valve, so any ambient pressure effect at that end is transmitted via by the metal cylinder projecting out of the rear of the gun. The strong spring at the rear then pushes everything forwards (spear, orange piston and yellow valve) and creates a low pressure behind the yellow valve, as in loading a "vacuum barrel" and creating a partial vacuum. But in a vacuum barrel the start pressure is ambient as the piston (here the yellow valve) moves back, but the start pressure for the yellow valve is 25 bar in any gap, not ambient. I don't think it is possible to squeeze all the air out no matter how small the gaps are, especially when you are pushing it out into high pressure areas and expecting to leave zero pressure in the final gaps. Air stops flowing from a hand pump, or any telescoping volume when being closed up, when the pressure inside and outside are equal to each other. So even the tiniest gap and channel inside the gun will be at cocked air pressure or 25 bar. The rear spring, which will be opposed by internal pressure, can only come forwards if the pressure facing it is low and in the last two versions you try to achieve that by having virtually no air gaps. Thus the changes in the last two diagrams where the orange piston and yellow valve are now sealing into each other with an extra "O" ring, but now air cannot get between them when you pull the trigger, unlike before when you had the gap between them (unless you are relying on the 1 mm annular ring 11 mm ID, 13 mm OD being exposed to pressure).

The higher the gun's operating pressure then the more of a problem trapped air becomes and there is a practical limit to how small the gaps can be made even though on paper they can look miniscule, that cannot be achieved in reality and as their diameter is constant then the gaps grow in length pushing items apart. That action will fire the gun prematurely.