SpudFiles

So I got this cannon lying around here basically 80% done and it can also fire with the necessary preparations, but it's still far away from something you can pick up, pull the trigger and shoot.

It uses a large piston valve with the hammer valve of an old fire extinguisher to vent the pilot. I can shoot it by hitting that valve with a large hammer. But i'm completely lost on an actual trigger mechanism as that would require a very tough spring.

Some ideas?

Some ideas?

If you don't want to make something from scratch, there are plenty of products that can be adapted for the job:

chop up one of these

chop up one of these

chop up one of these

chop up one of these

Don't think a toy gun's hammer would do much tho, i even doubt if the hammer mech from an actual gun would do anything as the principle of that is basically the large heavy hammer transferring all it's energy to the firing pin's tiny surface, and the return spring of a firing pin is also pretty weak. I can barely push the valve down with my thumb even without any pressure on the pilot.

I think you'd need to cut a sten gun's barrel off and mount it so that the bolt of the gun would hit the valve when you pull the trigger

The staple guns seem plausible, but i think i would need a very decent one to even attempt opening the valve. Were talking a push button type valve from a CO2 extinguisher that normally uses a hinged lever to open it, and the pilot pressure is roughly around 500/650 psi. Last time it shot a ball of kitchen paper through one side of an old fake leather shoe at point blank range.

I remember reading something on this forum about some kind of free floating valve? I probably said it totally wrong but i think they use them on PCP rifles, had something to do with the valve's stem ending in a separate chamber that was equally pressurized as the main chamber so you only needed a weak spring to keep it in place and a lot less force to open it? Hope that made some sense :p

I think you mean a "balanced spool valve".

I had made one for my Apache Fireball refurbishment.

See also Brian's original thread.

Yes that's the one i think! Bought some wire rod, washers and O-rings yesterday, gonna try and cobble something together today. If it's really that easy to push open i could probably use some kind of fast acting servo to open it, or maybe even a very small pneumatic piston leaching air from a mini air chamber operated by a blowgun...

Thanks! Is that the same type of valve the marshmallow cannon uses? :p

Alex345 wrote:Thanks! Is that the same type of valve the marshmallow cannon uses?

yes

Well i tried making one from a smooth piece of brass tube, some wire rod, washers and O rings but it's leaking from all sides so that's a shelfer... Any 40+ bars electronic valves out there that would be fast enough to act as a pilot valve?

Would it vent the pilot fast enough for a 1½" piston valve tho? If so i'm sold!

It really depends on how well the piston valve is made, ie virtually airtight piston and minimal pilot volume.

I managed to pilot a 1" piston with just a schrader valve in this pneumatic:



There were no o-rings on the piston but it was cast inside the cylinder, meaning it was an extremely tight fit, and the pilot volume was limited by keeping the travel to about 1/2".

Well it's not a home built valve, i found it in a store that sold anything from used tools to straight up junk and just paid for the brass weight. The piston has a PTFE sealing face and I've never really noticed any leaks from the valve itself.

I think that for a valve of it's size the pilot volume is quite small. The piston is in full downward position in the picture.

That's a beauty... are there o-rings around the piston, or is it a cup/parachute type seal?

jackssmirkingrevenge wrote:That's a beauty... are there o-rings around the piston, or is it a cup/parachute type seal?

The piston is basically made from two discs, the bottom disc is the PTFE one that seals off the chamber and the top disc which is the one you can see in the pic is aluminum with a plastic ring around it that has a groove in it housing a large o-ring.

The two are connected with a stainless steel rod about a cm thick. The rod also passes through 2 o-rings so the pilot area is completely sealed off from where the action takes place when it opens. I slightly reduced the diameter of the rod where it passes those 2 rings as they serve no function air cannon wise, and they really added a large amount of drag when it opened so now it opens way faster and is still leak free

It would be a slight shame that i'd always need an electric outlet nearby to use the 230v solenoid valve, but i just really want to be able to open this beast with just the push of a button... A 1/4 QEV piloted by said solenoid valve would be perfect! But i don't think the cheap ones with the plastic/rubber disc inside can take 40 bars.

24V DC 75 bar valve - it should have more than enough flow to pilot a valve like yours.

jackssmirkingrevenge wrote:24V DC 75 bar valve - it should have more than enough flow to pilot a valve like yours.

It says it has a 1mm orifice, seems a bit small? I'm no expert on solenoid valves, but the same seller also has a 70 bar 24v one with an 8mm orifice. It's A LOT more expensive tho.