Pneumatic problems

[paddo]

heres the story. i finished my new pneumatic cannon yesterday and today i tried to test fire it. it is a barrel sealing coaxial. the piston is made from hot glue and it seals perfectly onto the barrel NO air leaks out.
the piston also slides inside the chamber easily.

i put 70 psi into it and turned the exhaust valve to fire the gun but it didnt work. the exhaust valve is i think 3/8 inch or 1/4 im aussie so i normally use metric. it opens very easily and fast.

what happens is the air just comes out of the ex. valve. now i think either the piston isn't letting air through into the front of the chamber, this i don't think is likely because the piston doesn't seal perfectly against the side of the chamber

OR

the piston doesn't seal well enough against the side of the chamber and the air finds it easier to rush past the piston and escape instead of taking the piston for a ride aswell.

if anyone has had similar problems please help because id really like to make it work soon. im fairly sure its the second scenario but if anyone has any other ideas id like to hear them.
thanks very much for any ideas or replys..

[hi]

so let me get this strait, you open the exhaust and all the air goes out through there right?

thats nothing new. your piston may look like its a good fit, but its not. you need to go around it once or twice with painter tape or duct tape, either one works. also, put some oil on it, like gun oil. whats happening is its easier for the air to go around the piston than for it to push it back. its like if you were one on side of a big rock that you could move it, but had to use all the energy that you possibly had to move it, wouldn't you rather just go around it? thats whats happening with the air and piston. so like i said, just go around with tape and it will be fine, it should at least.

what kind of rubber did you use as the sealing face? I'm just curious, it has nothing to do with your problem.

[paddo]

i used 2 layers of motorbike inner tube. sealed really well the first time and kept doing it. i used 2 layers because on the first there was a line where it had been joined as a tube and i thought that could make it leak so i chucked another on. and on the end of the barrel i put a strip of it around there aswell with about 1 mm overhanging that hits the piston.

i used some lube called INKS on the inside of the chamber for the piston. it doesn't eat any rubber or plastic away and all this other stuff it says on the label. it was $AU 15 a can so i thought it would definitely be better than a $4 can of DW-40.

ok ill try making it seal against the chamber wall better. yeh i thought that would be the problem (aka the second scenario i named)
to do this ill have to add a 50mm coupling because of the stupid end cap assembly that was the only type i could get at 4 different shops. so it mite take a bit longer because i don't want to just use a dwv since iv spent more bucks on the rest of the fittings and pipe.

just one more qu. how much of a gap should there be between the chamber walls and the piston surface ? touching but enough to still freely slide ?
thanks heaps hi. ill fix it asap and put up some reports on how it goes.

[jrrdw]

Your piston could very easily fall under either 1 or 2. My self i'm a fan of equalization holes because you can o-ring the piston and get a instant results on piston ativation. The air has instant accsess to main chamber/s through eq holes. Building it this way eliminates the 1st scenario all together and partly fixes the 2nd scenario.

"just one more qu. how much of a gap should there be between the chamber walls and the piston surface ? touching but enough to still freely slide ?"

Keep the friction to a slight drag, just like using a feeler gauge. Me i fitted mine so that it took just a little push to get it moving, but once moving, slides nice and smooth.

[rad14701]

Another possibility is that your pilot valve is restricting the ability of your pilot chamber to evacuate rapidly enough for the main chamber to force the piston off the barrel seat... If the air is bypassing the piston faster than the pilot side can evacuate then you're fighting an uphill battle... Increasing the exhaust capability might be an easier fix than tinkering with your piston... Just my two cents...

[paddo]

im not sure about hat because it exhausts fairly fast. but i mite try that before the piston changes just in case. thanks for the replys guys.