PenGun Problem

[iemand]

I've recently made a pengun, using jsr's work as inspiration.
But, there's a problem: it has absolutely no power at all, even at 145 psi

Yes, I know the chamber is probably too small, but it's not even capable of pushing a 1 gram bolt out of the barrel

I think it has something to do with the schrader valve, it might release some, if not all air when I disconnect it(the piston is sealing because I can shoot small pieces of tissue)

Any help would be appreciated

[inonickname]

Without seeing the internals, it's difficult. Is it possible that you didn't have a buffer on the end of the barrel, and it cut a piece of rubber out from the seal, which gets jammed in there and limit flow? unsure..

[iemand]

Nope, I've got piece of rubber to seal the barrel, and the piston isn't jammed because I can hear it moving when I turn the pengun upside down. It's also unlikely that the rubber sealing piece is cut by the barrel, because of the way I put the barrel in there.

Like this: ((o))
o is the barrel
outer () is the chamber tube
inner() is a piece of tube which fits into the chamber and around the barrel(some kind of adaptor piece you could say)
It's all lined and grinded down so its smooth, no sharp edges.
This prevents imo the rubber from getting cut.

I however have a bit large space built in into this thing(between the piston and the filling valve), which is about 0.2 inch.


But I still think the problem is that there is to much air released when I disconnect the pump from the valve, because those schrader valves have a pin sticking out which allows filling and releasing. That pin is also pressed when I attach it to my pump, and then when I disconnect it, quite some air leaks out before the valve seals again(I guess)

[jackssmirkingrevenge]

But I still think the problem is that there is to much air released when I disconnect the pump from the valve, because those schrader valves have a pin sticking out which allows filling and releasing. That pin is also pressed when I attach it to my pump, and then when I disconnect it, quite some air leaks out before the valve seals again(I guess)

That's definitely a problem, have a look through the original pen gun thread for solutions. Also, you definitely need 300 psi plus pressures for decent power in such a small calibre, so think of investing in a shock pump assuming the construction can take it.

[Sticky_Tape]

Maybe the shrader is cut too short and the sealing thing on the inside of the shrader valve opposite of the threads is pushing on the back of the piston seriously limiting flow? There are a whole lot of problems that could be occuring maybe you should just make a new one. It won't take too long.

[jook13]

I bet the piston is too loose. If it isnt tight enough, too much air escapes around the piston.

If air is escaping as you disconnect but it doesnt fire, that tells me the piston isnt too effecient.

[iemand]

I bet the piston is too loose. If it isnt tight enough, too much air escapes around the piston.

If air is escaping as you disconnect but it doesnt fire, that tells me the piston isnt too effecient.

Hmm, that could be the problem, but the gap around the piston is less then 0,5mm

[clide]

Any noticeable gap is probably too much. You want your piston to fit as close as possible while still being able to move. If you have something like a 7 mm chamber and a 6.5 mm piston then that gap is equivalent to a 2.6 mm hole. That is going to cause problems if you are venting with a schrader valve.

[psycix]

Maybe you could epoxy a syringe tube in there, and use the syringe piston as actual piston. This grants you a very well fitting piston.

[iemand]

Maybe you could epoxy a syringe tube in there, and use the syringe piston as actual piston. This grants you a very well fitting piston.

I'll give that a try, but not on this one Maybe for my next project... Anyway, thanks for the help

[psycix]

So this pengun is ditched? You could still fix it.

Hmm, that could be the problem, but the gap around the piston is less then 0,5mm

For a piston of only a few mm diameter, thats ONE HELL OF A GAP
Maybe add a layer of duct tape or even better, an O-ring.

[iemand]

Well, for the fixing part, I used metal epoxy, so that's not gonna work unless I cut it open which would destroy it totally It's now somewhere hidden with the other "epic fail" projects.

btw isn't the syringe-tube-piston a bit too tight fitting?

[psycix]

The fitting is a perfect seal, though I do not now how well it seals on higher pressures.
You could poke a hole with a needle or something like that to let air trough.