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Piston won't seal.

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Piston won't seal.

Unread postAuthor: JET47 » Tue Mar 18, 2008 9:38 pm

Hey guys, I found this site a couple weeks ago and it's really been helping me out, but i can't figure out why my piston (third cannon, first pison attempt) won't seal on the barrel.

it's sortve a coaxial, the barrel is mounted inside the fill chamber.
i'm using a schrader valve to fill, so i stuck a spring on the piston to help it start sealed. but no matter how much i compress the spring to start (by extending the surface it mounts on against the back of the pilot area) it won't seal. i did manage to get it to seal once by pushing really hard on it with my hand, and air couldn't be blown through the barrel.

my best guess is that there's some imperfection on the sealing end of the barrel that is preventing a seal. im using neoprene washers from home depot, but theyre pretty thin. someone a couple pages back had thin neoprene too but his was working.

im planning on building a larger version once i get this working, with 4inch pvc for the fill chamber and 2 or 1.5 inch inside for the barrel. this one is 1.25 and .5, just cause thats all i had laying around

thanks.

EDIT: ahhh how did this get in combustion?? i swear i clicked pnuematic >.<
and i can't find where to move it... if thats possible by an admin it'd be greatly appreciated
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Cannon.swf [ 1.88 KiB | Viewed 576 times ]

IMG_0497[1].jpg
piston
IMG_0500[1].jpg
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Unread postAuthor: Novacastrian » Tue Mar 18, 2008 9:48 pm

For a start get rid of the spring, you don't need it. Your guess with the end of the barrel not being perfectly square to the sealing face would have to be my best guess as well, it is going to be difficult to rectify the problem if you can't get to the barrel end.
It should also have a convex end and not be flat, this helps to compress the neoprene and form a good seal.
Is your piston hollow? The pressure may be expanding it and stopping air getting past into your chamber. If this is the case you may want to fill it with epoxy or similar.


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Unread postAuthor: hi » Tue Mar 18, 2008 9:51 pm

it could be an uneven pipe cut or the sealing face could be getting sucked into the barrel.

what pressure are you using it at? if its below 30 psi, then you may just need more pressure.

i like to epoxy my pistons to the sealing face so it doesnt get sucked into the barrel.
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Unread postAuthor: JET47 » Tue Mar 18, 2008 9:56 pm

i used the spring because i was worried that since i'm filling with a schrader valve the volvume of air coming in wouldn't be greater than the total area of the space between the piston and the walls, someone was talking about that in another topic.

so i should sand down the outside edges of my barrel a little? i wasnt thinking hard enough when i bought the fittings for this one, shouldve picked up one of those threaded separating peices, forget what theyre called.

the piston is some type of sleeve, exact same size as 1 inch pvc which i thought was kindve weird. its got a .5 inch piece of pvc inside, the space between is filled with hot glue, and theres a quarter sitting on the edge inside that supports the spring, front half is filled with hot glue too.

and the air is coming straight out the barrel on the first pump. no pressure ever gets build up, i havent even epoxied the barrel/chamber and fill parts together, which is y i can still get to the piston. but it leaks out the barrel before it has a chance to push through those joints.
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Unread postAuthor: bigbob12345 » Tue Mar 18, 2008 10:53 pm

As nova said I think your barrel is uneven.
I find the best way to slove this is to stick a coupling on the end. I do it on all my pitons to make sure they are perfectly sqaure.
I reilize you cant reach into it to solvent weld it together so just smother some epoxy on the inside of the coupling and stick it on :P
it would make the piston travel distance shorter so before gluing it just stick it in there dry fit and test it. If everything works alright go ahead and stick it on.
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Unread postAuthor: JET47 » Tue Mar 18, 2008 10:57 pm

cool thanks that sounds promising. ive got a spare end cap on hand thatll fit the barrel, ill saw the tip off and slide it on unodified-part-out, see if that helps
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Unread postAuthor: hi » Tue Mar 18, 2008 11:01 pm

if you look up my diaphragm cannon there is a coupler in the middle because i measured it wrong and the diaphragm wouldnt seal, so i had to cut it and fix it, then put a coupling on to seal it.
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Unread postAuthor: Hubb » Tue Mar 18, 2008 11:05 pm

A while back, I posted a theory to solve this problem. I haven't tested it yet, but it has potential.

http://www.spudfiles.com/forums/piston-valve-improvement-t10720.html
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Unread postAuthor: psycix » Wed Mar 19, 2008 8:42 am

I think using the coupler method might constrict flow...
But it should work to get it square. :)

I always use uncut pipe as sealing barrel end.
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Unread postAuthor: bigbob12345 » Wed Mar 19, 2008 8:47 am

You have got me confused on how using a coupler would concrict flow.
please explain.
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Unread postAuthor: bigbob12345 » Wed Mar 19, 2008 8:47 am

You have got me confused on how using a coupler would concrict flow.
please explain.

Edit: sorry for this, the delete function isnt working.
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Unread postAuthor: Gippeto » Wed Mar 19, 2008 9:30 am

If you suspect the sealing face is "not square", you could replace your gasket material with peel and stick sandpaper.(available in the autobody section of most stores)

Rotate your piston against the barrel and sand to perfection.

The coupler MIGHT impede airflow. Sanding to perfection will not.
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Unread postAuthor: sniper hero » Wed Mar 19, 2008 10:13 am

just buy a sand disk for your drill

_______ < sandpaper facing upwards
.....|.....
.....|..... < stick that goes into your drilling machine
.....|..... ignore the dots

and then make sure you sand it straight
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Unread postAuthor: Procrastonator » Wed Mar 19, 2008 10:20 am

im guessing you are using a bike pump to blow it up... if you have an air compressor use it and it will probably have the pressure to seal it. my gun doesnt seal right untill i get 40 psi in it...

kindof a bummer i know...
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Unread postAuthor: SpudFarm » Wed Mar 19, 2008 11:02 am

Gippeto's ideà is the best way to go... then you get your seat to be just like the piston.. use it! :)
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