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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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Piston problem again

Unread postAuthor: nabu92 » Sun Feb 03, 2008 1:54 pm

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Alright, so I'm looking in my garage for some junk for my cannon and I see this box full of old plumbing supplies. I look in it and find this thick rubber toilet sealing thing covered with brown stuff lol, so I cleaned it up and put it on my piston and it works perfectly.
Here's the problem, I brought it outside and started filling it up . It took a few tries, but it finally filled up to 60 psi almost instantly. I removed the compressor hose from my gun, and when I open the valve, all of the air rushed through it instead of the barrel! The valve that I'm using is here:

http://i98.photobucket.com/albums/l250/ ... G_0804.jpg

It's 1/4 and I know you guys said at least 1/2 but I was at least expecting something to happen. Does anyone know whats going on? Thanks for your time.

Also: A little bit of air leaks through the bolts that I use to seal the end cap on, I used teflon tape but it still doesnt seal, Should I just seal them with epoxy after I fix the gun? I can probably find weak epoxy that I can break easily just incase I have to open it up again.
What else can I use to seal the bolts?

HELP!


EDIT: Ok now I'm really mad. 80% said the problem was the pilot valve, so Instead of the 1/4 ball, I added a sprinkler valve. I pressureizd it to 60 psi, and pull the blowgun and all the air comes out of the sprinkler valve, not even a little came out of the barrel. $40 later and no difference.
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Last edited by nabu92 on Sun Feb 10, 2008 2:50 pm, edited 7 times in total.

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Unread postAuthor: MrCrowley » Mon Feb 04, 2008 12:40 am

You will need at least a 1/2" ball valve, preferably a 1" sprinkler valve.
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Unread postAuthor: jackssmirkingrevenge » Mon Feb 04, 2008 12:44 am

MrCrowley wrote:You will need at least a 1/2" ball valve, preferably a 1" sprinkler valve.


If the pilot chamber is small enough and the piston isn't too leaky, a blowgun will be more than adequate for it to work - with my d/4 1" piston, even a schrader is enough. For a 1.5" barrel, maximum piston travel should be around 1 cm, so you're good in that department.
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Unread postAuthor: MrCrowley » Mon Feb 04, 2008 12:50 am

jackssmirkingrevenge wrote:
MrCrowley wrote:You will need at least a 1/2" ball valve, preferably a 1" sprinkler valve.


If the pilot chamber is small enough and the piston isn't too leaky, a blowgun will be more than adequate for it to work - with my d/4 1" piston, even a schrader is enough. For a 1.5" barrel, maximum piston travel should be around 1 cm, so you're good in that department.


A 1/2" ball valve should give you far better performance and it's not hurting anything :)

If you want a 'trigger setup' go for a modded sprinkler valve.
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Unread postAuthor: jackssmirkingrevenge » Mon Feb 04, 2008 1:43 am

Naturally, smaller pilot volume = better efficiency but the better your pilot valve, the more performance you're going to get. So to summarise, a blowgun would likely be enough as a pilot, but with a large ball valve or sprinkler valve you'll get better performance ;)
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Unread postAuthor: MrCrowley » Mon Feb 04, 2008 1:57 am

Agreed.
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Unread postAuthor: nabu92 » Mon Feb 04, 2008 7:10 am

Alright thanks guys, I'll have pics up tomorrow.
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Unread postAuthor: Novacastrian » Mon Feb 04, 2008 7:15 am

Ball valves work very well for piloting, their weakness is accuracy, unless you go for a spring loaded ball valve.
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Unread postAuthor: nabu92 » Mon Feb 04, 2008 5:44 pm

Sorry, deleted this.
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Last edited by nabu92 on Tue Feb 05, 2008 4:34 pm, edited 1 time in total.

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Unread postAuthor: nabu92 » Mon Feb 04, 2008 9:48 pm

Can the piston being too tight be the problem?
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Unread postAuthor: jackssmirkingrevenge » Mon Feb 04, 2008 11:06 pm

nabu92 wrote:Can the piston being too tight be the problem?


The piston should be loose enough for you to be able to push it easily with one finger. On the other hand, if it's not tight enough, it will let air rush past it and never actuate, especially with a small pilot.
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Unread postAuthor: fireman565 » Tue Feb 05, 2008 1:22 am

jackssmirkingrevenge wrote:The piston should be loose enough for you to be able to push it easily with one finger. On the other hand, if it's not tight enough, it will let air rush past it and never actuate, especially with a small pilot.


How loose? I'm building a 3" piston with a 2-1/2" coupling as a sleeve inside the TEE for the piston to slide. The piston is made out of a 2" coupling with plugs in the end sanded to fit flush. I have about an 1/8" space around piston. Moves great. But how much clearance is needed?

I'm basically trying to replicate unisonmind's 'new 3" piston'. Clean design, interchangable barrel and chamber.
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Unread postAuthor: jackssmirkingrevenge » Tue Feb 05, 2008 1:40 am

I have about an 1/8" space around piston.


That sounds like a lot.

Here's the theory, say you have a 0.9" piston in a 1" tube. This means that there's an area of (area of tube - area of piston) 0.15 in<sup>2</sup> for the air to flow through. If your flow through your pilot valve is less than this area, then the piston will never actuate, as when you open the valve the pilot area will fill from the chamber at a faster rate than it is emptying, so all the air will simply go through your pilot valve while the barrel remains sealed.

Unisonmind used a massive pilot valve in the launcher you linked to, so 1/8" of clearance wouldn't be an issue, but the tighter it is, the better your performance will be for a given pilot valve. My pistons usually have barely any clearance as they are cast from epoxy within the tube itself, meaning even a schrader is enough to actuate a 1" piston.
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Unread postAuthor: fireman565 » Tue Feb 05, 2008 1:56 am

So basically, more area through the valve than area around the piston.... click
Thanks jack. Man this stuff's addicting.
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Unread postAuthor: jackssmirkingrevenge » Tue Feb 05, 2008 1:59 am

It's all a matter of balancing flow, once you understand the concept it's pretty logical.
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