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Piston problem again

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: MrCrowley » Wed Feb 06, 2008 11:52 pm

Okay, diagram time.

Bolt and Washer
Sealing Face
Metal Disk To Support Sealing Face*
End Cap


*Turns out this doesn't have grey, so the grey in the picture is subsituted for the brown in this post. Grey = Metal Disk. :)
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Unread postAuthor: LikimysCrotchus5 » Thu Feb 07, 2008 12:19 am

Yea i just thought of that right in the post before, i had it edited already. Sorry to bother you :oops:

Just took me a little more time :roll:

wouldnt the metal scratch the pipe though?
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Unread postAuthor: MrCrowley » Thu Feb 07, 2008 12:48 am

I'm here to help. :)

Do you mean the end cap or the pipe the piston is housed in?

If you mean the latter, just find a disk smaller then the I.D of the pipe but has a larger diameter then the sealing face.

If you mean the end cap, just stick a washer between the disk and the end cap.
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Unread postAuthor: nabu92 » Sun Feb 10, 2008 2:36 pm

Edit: I made some videos to make it a little more clear if you didnt understand the problem before.
http://www.youtube.com/watch?v=-PLYb3MwSZ8
http://www.youtube.com/watch?v=_wQwv8L97T4
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Unread postAuthor: frogy » Sun Feb 10, 2008 2:56 pm

If it's not "pulling back"... Your piston isn't moving backwards, it is because of a bad pilot or a bad sealing piston... It's not rocket scientist... When a vacuum doesn't pull an object, it's not a strong enough vacuum...

I have no idea why you spent $40 on a sprinkler valve, when you can buy one for less than $20... I can get them for $10... You can even make a small barrel sealing tee and use it as the pilot.
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Unread postAuthor: nabu92 » Sun Feb 10, 2008 7:38 pm

It wasnt just the sprinkler valve ($14), I had to buy pvc and copper fittings $12, a blowgun $7, and more pvc glue $6 because I ran out. It all adds up.
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Unread postAuthor: nabu92 » Sun Feb 10, 2008 7:46 pm

frogy wrote:If it's not "pulling back"... Your piston isn't moving backwards, it is because of a bad pilot or a bad sealing piston... It's not rocket scientist... When a vacuum doesn't pull an object, it's not a strong enough vacuum...

I have no idea why you spent $40 on a sprinkler valve, when you can buy one for less than $20... I can get them for $10... You can even make a small barrel sealing tee and use it as the pilot.


So how can I fix this? My pilot cant be bad because I tried 2 different ones already, if the barrel didnt seal, I couldnt pump it up to 90 psi without it leaking and my piston is lubed and I can pull it back very easily with my hand.

I'll try to get pics later today
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Unread postAuthor: frogy » Sun Feb 10, 2008 9:59 pm

A bad rear seal on the piston is pretty hard to describe... If your barrel isn't leaking and the gun fills with air fast, your problem is your rear of your piston not making a good seal with the tee...

The rubber on the back of the piston should be straight and stiff, so that it contacts the sides of the tee pretty tightly... You'd think that this would make the gun not fill, but this is not the case... Explaining it simply, if air wants to get through, it will find a way... Your gun's chamber may fill slightly slower with air...

But, when you pilot the valve, all the air will be pulled out from behind the piston, and since there's less of a gap for air to just slip around the piston, the air will actually push the piston backwards and escape out the barrel...

If you can't cut your rear gasket very round, it's probably best you oversize it, or invest in o-rings so your piston seals better...
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Unread postAuthor: nabu92 » Sun Feb 10, 2008 10:13 pm

OMG I THINK YOU'RE RIGHT!
There is no rubber on the back of my piston lol, just the end of my deoderant can. I was going to try to describe how the air was getting straight from the chamber to the valve, but decided it was too complicated. I'm, going to modify my piston tomorrow, Thanks for the help.
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Unread postAuthor: fretmelter » Mon Feb 11, 2008 8:47 am

wow that makes a lot of sense i never uinderstood it like that
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