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Piston Valves Explained Visually

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: LikimysCrotchus5 » Sat Jan 12, 2008 9:00 pm

Plz look throught the forum. Also check the wiki. Ask reasonable questions plz. This is not the place to ask. Do research first then ask what you cant find answers to after doing extensive research.
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4SPC, My 4" piston 3" porting cannon
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Fix up copper cannon
Fix up 4SPC
Start Stirrup pump
Start Toolies piston bazooka
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Unread postAuthor: Ragnarok » Sat Jan 12, 2008 9:18 pm

anthony5640 wrote:how do i make a piston?

Use your ingenunity. You can make pistons out of lots of different things. Mine was made out of a length of bolt with washers on it (but that's only useful at low calibres), some people use PVC endcaps, some people lathe them from PVC rod... making the piston just requires a little lateral thinking.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: schmanman » Sat Jan 12, 2008 9:47 pm

I once made a piston out of:

part of the front of one of these badmittion thingies

a piece of surgical tubing

a piece of 1/4 inch air hose

a piece of bike innertube

a piece of a pencil

a piece of a marker cap

a small piece of duct tape

....And some gorilla glue



It actually worked great for about 70 shots, then the barrel cut through the badmittion thingy, and it died.

then I used it as ammo....
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Persistence is a measure of faith in yourself
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Unread postAuthor: MrCrowley » Sat Jan 12, 2008 9:53 pm

schmanman wrote:It actually worked great for about 70 shots, then the barrel cut through the badmittion thingy, and it died.

then I used it as ammo....


The circle of life :D
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Unread postAuthor: anthony5640 » Wed Jan 16, 2008 1:11 am

how do u make these animations?
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Unread postAuthor: DVLCHLD » Sun Feb 03, 2008 4:40 pm

Sweet. Now I finally know how a piston valve works. It's kind of like how a sprinkler valve works but on a grander scale with the piston taking the place of the diaphram. Thanks for those animated graphs.
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Unread postAuthor: fireman565 » Tue Feb 19, 2008 1:33 am

One thing I missed trying to build my own piston was 'frontal pressure' applied to the front of the piston when pressurized.

In Phase Three where it's stated, "Once the force pressing on the back of the piston falls below the force acting on the front of the piston, it begins to slide back. Suddenly there is more surface area exposed on the front of the piston, and the jump in force slams the piston back leaving an opening for air to flow into the barrel to accelerate the projectile. " I didn't quite pick up on this part of building a piston until after I had built one. Of course common sense says the piston front face has to be larger than the port to which it seals. From what I've discovered this frontal pressure is the 'pressure' that's on the residual area of the seal around the port.

Now, my piston still opened even though I built it with no area for frontal pressure. It wasn't performing at its best, even though it would fire a spud 150 yds, it was trying to suck the seal through the barrel nearly tearing it on every fire. I ended up painstakingly adding a sleeve that protruded into the chamber to give it this frontal pressure needed.

Image

Forgive me Clide, I snatched part of your FLASH.
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Unread postAuthor: Hubb » Sat Mar 08, 2008 1:38 am

I know on a coaxial and barrel sealing piston, the piston has to be considerably larger than the exhaust in order for the forces to make it work, however, I wouldn't think that would apply to a chamber sealing piston valve. Is my assumptions correct?
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Unread postAuthor: clide » Sat Mar 08, 2008 4:23 pm

hubb017 wrote:I know on a coaxial and barrel sealing piston, the piston has to be considerably larger than the exhaust in order for the forces to make it work, however, I wouldn't think that would apply to a chamber sealing piston valve. Is my assumptions correct?


Yes, but working and performing well are different things. If you make a chamber sealing valve with the seat the same size as the piston then you'll have relatively crappy opening times and piston bouncing issues unless you have a monster pilot valve...I wouldn't suggest it.
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Unread postAuthor: Hubb » Sat Mar 08, 2008 6:08 pm

Thanks. I was considering a chamber sealing valve on my next project but I got it it worked out to go with a barrel sealer (which was what I preferred anyway).
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Unread postAuthor: Antonio » Sun Mar 09, 2008 6:50 am

Hey clide thnx for the animations, they helped allot.
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Unread postAuthor: taytayswims » Sun May 04, 2008 4:21 pm

great info. i understand them now but i don't think i can make one just yet. ill experiment for a while until i really know what i am doing
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Unread postAuthor: SpudFarm » Sun May 04, 2008 4:22 pm

that is the real smart man! ^^
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Unread postAuthor: Rob Thornton » Wed May 28, 2008 12:46 am

Pete Zaria wrote:
octane89 wrote:Can someone answer this for me-
Is it possibly to have too much pilot volume? I have a 3" Tee, and the pilot volume is about the size of a 3" cap.


Yes, too much pilot volume is a common problem. A larger pilot valve does help, though it's best to try to reduce this volume somehow, either by re-designing the valve or filling the volume with something (foam anyone?).

A picture would be helpful...

Peace,
Pete Zaria.


Although reducing the pilot volume will be beneficial, as Pete said: your pilot valve is a more important factor.
By using a QEV (quick exhaust valve) as your pilot valve, you'll be able to dump that pilot chamber's air faster than you can say 'cat in a hat'.

In my opinion, with a QEV as your pilot valve, you won't have to reduce the pilot volume in the first place...

ps You can find QEV's at pneumatic supplies shops (the guys that sell compressors and stuff) otherwise google it!

Cheers
Rob

EDIT: I've found agreat QEV manufacturer - although they're based in the EU, they have international distribution and are by far the cheapest I've been able to find.
.http://www.eurofit.it/prodotto_dettagli ... rod_id=117
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Unread postAuthor: vovka351 » Mon Jun 30, 2008 4:23 pm

so, i am going to be building a 2" piston.
two questions:
1) what is a good piston material for 2"?
2)why does a chamber sealing gun need an O-ring?
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