coaxial piston cannon, pump include! piston dont work! help

[jean]

hello

sorry for my bad english
i use search but i dont understand all the topics..

yestrerday i build my first cannon all is clean and seal is good but piston dont work

when i pull blowgun the air comes out pfffffffffffff about 2 sek.

what is wrong?? please help me

[FeLeX]

We kind of need pictures to understand what could be broken.

[Hubb]

Try wrapping some tape around the piston. It seems to me that your equalization port may be too large (which, according to your diagram, is 1mm).

Also, as FeLeX suggested, try and get some pics of your actual launcher so that it can be more thoroughly evaluated.

[Matheusilla]

I agree with hubb017, the most likely reason is that the piston is too loose. When I first started making piston valves I had the same exact problem.

[Hawkeye]

Your chamber is also tiny. Why do you have the front section blocked off? The pilot volume is the same as the chamber volume. By the time the pilot is exhausted, even slightly too much air leaking from the chamber back to the pilot leaves nothing left to force the piston back.

[Hubb]

...Why do you have the front section blocked off?...

I just noticed that. Why is that? Put some holes in the front cap to make tha chamber much larger. That may fix the problem without having to wrap the piston. Oh, and what is the spring for?

[jean]

the pictures are from yesterday when i build it (spring is not mounted at this time)

the spring fix the pumpstock in position when air is press in the gun

[Matheusilla]

I think I just realized how this thing is working. "Pump included", does it work by pushing the whole assembly in from the front compressing the gas? If that is true, that's simply ingenious. If the piston fits and slides nicely, just try reducing the pilot volume, the space behind the piston like Hawkeye says. It could also be that the main chamber is not compressing any gas and that the compressed gas is only behind the piston, in the pilot space.

[ALIHISGREAT]

thats a nice clean coax you have there but i'm not sure what could be wrong other than you have a stiff piston with a too large equilization hole?
which causes the air to just exit via the blowgun

[MrCrowley]

Try upgrading to a 1/2" ball valve, if that doesn't work then your piston is too loose in the pipe or has too much friction.

So make sure your piston fits perfectly in the pipe and make sure you have some lubricant on it, then swap the blowgun for a 1/2" ball valve and it should work.

[ALIHISGREAT]

i have another thing that will vastly improve performance, you should fill the piston in, the hollow endcap is giving you a massive pilot volume which the blowgun will struggle to vent quickly.

[jean]

thanks now it woud work

i have tape the piston
change piston sealing side (see picture)
fill up thepilot room with epoxy (see picture)

but it only work when i seal the exaust with my finger an not work with blowgun
is the blowgun really to tiny?

[ALIHISGREAT]

if you can seal the exhaust with your finger i don't think you have enough pressure to work the cannon or maybe you don't have enough piston travel?

[Hubb]

Now, it looks like you have no pilot volume. What was referred to in the above post about filling the pilot was talking about putting expanding foam or something in the piston cap, not the pilot volume.

To fix this, shorten the barrel slightly to add pilot volume. And please explain why you have the front of the chamber blocked off from the back.

[jackssmirkingrevenge]

Very interesting design, reminds me vaguely of clide's mini-pump, though this is more conventional. It must take quite a bit of effort to compress the air but kudos for thinking out of the box and coming up with something different