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Advice on Piston

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: psycix » Fri Mar 13, 2009 3:37 pm

Maybe you are simply not pumping fast enough. Many cannons do not seal until like 20 psi.
Try to pump like on crack (or use an compressor) If you can get it up to 50 psi, and it still leaks, then the problem is significantly more.

Besides your sealing face, some pistons leak through the bolt which holds the sealing face. Check that out.
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Unread postAuthor: jrrdw » Fri Mar 13, 2009 4:18 pm

Besides your sealing face, some pistons leak through the bolt which holds the sealing face. Check that out.


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Unread postAuthor: limbeh » Sat Mar 14, 2009 2:51 am

I haven't found out whether it was filling or venting fast enough lol. The problem of getting the piston to seal properly must be solved first before I can find out about the fill / vent problem.

For those recommending O-rings, would you recommend WD-40 as lubricant between PVC tee & neoprene o-rings? Teflon o-rings are a big hassle to get here, plus this is a trial big-piston cannon.

Thanks.

As the pictures of the inside of the tee in question in my computer have disappeared, I'll upload them on Monday when I return to my school workshop.

The picture of the entire cannon is here though. [/img]
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Attachments
Cannon.JPG
The cannon. It is pictured with some sort of elbow to attach the ball valve, but this has been changed subsequently to an air hose
Cannon.JPG (22.7 KiB) Viewed 323 times
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Unread postAuthor: psycix » Sat Mar 14, 2009 6:44 pm

jrrdw wrote:
Besides your sealing face, some pistons leak through the bolt which holds the sealing face. Check that out.


Hello hello hello hello

There's a echo in here. :P

I must have missed your post. I always open all unread topics in tabs and then read them one by one, so you probably posted within that time.
So this proves that great minds think alike right? :)
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Unread postAuthor: limbeh » Sun Mar 15, 2009 4:53 am

Ah...........I see. Time to epoxy bolt?
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Unread postAuthor: limbeh » Sun Mar 15, 2009 9:07 am

Ah. I'll report the results on Monday.
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Unread postAuthor: limbeh » Sat Mar 21, 2009 12:05 am

Sorry for the delay. Oil spill in my workshop while I was sanding, had to clean up the mess before the oil reached my stuff.

Update on piston:

Peel and stick sandpaper works fine. Not complete yet, but the same plastic bag now takes 5 pumps to drop off instead of 1 to fly off the last time.

As of now I'm not too familiar with using a compression spring (though I intend to get one at first opportunity). Just to check, it should be mounted on the end with the pilot & schrader to push the piston up to seal right?

What about using foam rubber as a seal? Would it be advisable at a pressure of around 160 psi (max pressure + safety margin of the cannon)?

Here are the pictures of the piston and the inside of the cannon in question:
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Attachments
Inside of cannon.JPG
The inside of the tee. Note that the sanding is still not finished yet.
Pistons.JPG
The pistons. The one on the right is the actual one that I intend to use, the other one is where I'm mounting the peel & stick sandpaper on.
Pistons 2.JPG
Another view of the pistons. The peel & stick sandpaper on the "fake" piston is now clearly shown.
Inside of cannon2.JPG
Another view of the inside of the tee. Might be a bit disorienting at first glance though.
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Unread postAuthor: inonickname » Sat Mar 21, 2009 12:58 am

Ooh, a plexiglass piston.. make sure you make a good bumber and don't let it get too hot, or you'll know about it.
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Unread postAuthor: limbeh » Sat Mar 21, 2009 1:11 am

Too hot? From repeated use or what? I've never operated pistons before, or rather nobody in my country has, so I'm not sure how it can get that way.

My max pressure including 10psi safety margin is 150 psi. This is a pneumatic, not a combustion or hybrid.
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Unread postAuthor: psycix » Sat Mar 21, 2009 1:51 pm

Pistons dont get hot from use unless there's some insane friction going on.
Those pistons look nice!
A spring isnt needed when you fill from the back. A bumper, however is VERY important. You could smash your nice plexiglas piston in one shot, or even worse, have it hammer through the back of the T.
Anything soft and squishy will do as bumper.

I'm looking forward to seeing it in use. :)
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Unread postAuthor: limbeh » Sun Mar 22, 2009 3:22 am

Oh ok. Thanks psycix.

Actually the 6mm plexiglass is spare, left over from another project. I have enough such that just about every subsequent piston coming out from my workshop would be incorporating plexiglass in it.

I'm thinking of incorporating an o-ring into the piston, but I haven't got the o-ring yet. Will update.
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Unread postAuthor: inonickname » Sun Mar 22, 2009 4:31 am

Well, plexiglass tends to be insanely brittle. It will be showing no signs of stress, then right after fail catastrophically. Any increase in heat only makes this much, much worse. You may be using a better grade perspex than me..but oh well.

Ideal gas law also states that as a gas gets smaller it will get hotter, and as it expands it will cool.

That's the least of your worries, don't fire it without a bumper! One shot without one will destroy the plexiglass for sure.
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Unread postAuthor: limbeh » Sun Mar 22, 2009 5:14 am

Hmmm. I haven't done any destructive testing of plexiglass, but then and again I'm not gonna risk any explosions, particularly not with a cop for a brother (this is Singapore, even the Aussies for all they complain have it good compared to us).

My bad, typing in Mandarin.
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Unread postAuthor: covey12 » Thu Mar 26, 2009 8:08 pm

i dont think ive ever seen a piston bigger than 3 inches, this should be good when its done.
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