Piston valves explained visually [Update 3]

[syco kilaa]

ok clide, what he is basically saying is that he wants to get a 4" tee, cap one of the legs, then machine down a peice of pvc pipe that will slide inside of the tee, then he will machine a hole in the side of the pvc, he does this so that he could make a piston for it but there would be obstructions such as the little lips inside the tee that would normally stop a regular piston. then he would use jb weld to hold the pvc pipe inside the tee.

[pbmann123]

kinda but instead of a pvc tbe down the middle i would use an aluminium one with a bearing finish and then cast and machine the piston out of aluminium. this gets rid of those nast obstuctions and will be easier to work with than pvc. it is just my personal opinion that aluminium is easier to work than pvc

-changed my contradiction. thx for the heads up draculon

[draculon]

EDit: No problem pbman.
I also think that aluminum is easier to work with because it has less surface impurities.

[clide]

I understood that part. The main thing I was not understanding was the piston. Plus I didn't want to clutter this thread too much. But I guess I'll just explain both possible scenarios.
It basically sounds like you want to make a tee valve with the port that the piston sealing against being the same size as the pipe it slides in.

In the case of a chamber sealing. It would work to an extent, being that it would let air out of the chamber. But how fast it would do that would be a different thing. The piston will start moving as soon as you start to vent since the surface area on each side is the same, but once the piston cracks open, the pressure on the chamber side will quickly fall, and often the pressure differnential will not be enough to continue to move the piston. I have tried a valve like this http://forums.spudtech.com/topic.asp?AR ... IC_ID=1400 and it simply cracked open a little and let all the pressure out. Notice on the animations that right when it starts to move back the force acting on the piston makes a jump, but you don't have that jump on this piston design so it will tend to have some problems.

The other thing it could be is a barrel sealing configuration, pretty much what MonkeyDoo was trying to describe earlier in the thread. In this one you don't even need a pilot simply because pressure back there will do nothing except maybe shoot the piston out the barrel if you don't have a stop for it. But you do need something to pull the piston back. In theory this will work because you do have that force jump. I don't think I have seen a gun that relies on pressure to do the work once the initial seal is broken, but the Big Red cannon does use this type of piston with a pneumatic ram type device to pull it back. And it seems to work quite well. The main thing wrong with this design is that it is a safety hazard. Some pressure leaking into the barrel side or a slight bump could fire the gun accidently.

[pbmann123]

i cant get my pic to work so if you want i will send it to you. but any way the valve has a smaller face area when the chamber poressure is on it in the sealed postion but when opened the valve slides back exposing more face area and more surface to be pushed upon. the valve i s pioleted with a modded rain bird 1' dv 100 diaphram valve. the aluminium that it slides in is machined in multiple pieces that thread together but a steel sleeve sits inside of it to make a smooth surface, really smooth, bearing smooth. i hope you understand, the pic will help.

[clide]

Yeah, a pic would help, you can send it to the email in my profile.

[pbmann123]

i just didnt want to send you somethin because you might jnot approve. if you wish to send me anything you can send it to my gmail northofthe49@gmail.com

[Krazy_K]

Great help, thanks a lot.