Piston Valve Questions

theanswerguy · Sat Mar 17, 2007 6:54 pm

Hi everyone. I'm a rather new spud gunner with beginner combustion and pneumatic cannons under my belt. I have a good idea about the physics and ratios and all that, but I do have a few Q's about the pneumatic type and I'm hoping that someone will be helpful enough to answer them or at least point me in the right direction...rather than being 100% unhelpful and telling me to look for myself. Believe me, I've looked. And every topic seems to get the same answer: It's not that hard, you're stupid, and figure it out.

I watched a short video on building a piston valve where the professor made the piston out of hot glue sticks molded in a 2" pipe. He then glued a rubber gasket of sorts to it. It's spring loaded from the plug up against the 1" barrel. He then drilled two .5" holes in the cap -- one for the blow gun and one for a ball valve. This is all in a 2" T fitting. I understand how it seals off the barrel, but I don't understand how it actually fires. I get the idea of how it works from the diagram in the Wiki files, but his particularly doesn't sound like it will work because the mold of the 2" pipe won't seal on the inside diameter of the 2.375" T fitting. That leaves a .375" gap between mold and fitting. Here's the question: If the blow gun was opened to "release pressure behind the piston," will that be enough to pull the piston back to allow air to get into the barrel even with that gap?

Looking at the diagram in the Wiki files, how the heck is that built? Let's say it's a 1" barrel and a 2" chamber. That would be a 2" cap on the back, but what kind of cap (the piston) is on the inside of the chamber and how is it made to fit so perfectly?

Any answers to these questions would be very much appreciated. Thank you in advance.

-Jim

VH_man · Sat Mar 17, 2007 7:11 pm

the piston is just anything that fits nice in the chamber. try PVC rod from MCmaster. or look around for things that work. if you cant find anything, mold the piston out of hot gule. it works quite nicely........ by the way, for 3 inch pipe, Pringles cans (reinforced) work great as pistons. for 1 1/4 inch chambers 3/4 inch endcaps work perfectly, and for 4 inch chambers 3 inch endcaps fit quite nicely. hope i helped........ Nice to know someone used the Wiki

theanswerguy · Sat Mar 17, 2007 8:14 pm

You did help out, actually. However, when you say "anything that fits nice in the chamber" do you mean it has to have almost a tight seal as seen in the diagram above? Will something without a seal allow the proper depressurization behind the piston?

This, for example, has a small equalization port that slowly fills the chamber and the space behind the piston. I video-tutorial I watched never included the equalization port. Isn't this a needed feature in the piston?

Will the threaded nipple thing stay nice and tight even under 100psi? Just hard to believe, I guess.

Last question for this post: What's the purpose of the ball valve if I use a tire (shrader?) valve to fill it, and the other port connected to a blow gun to trigger it?

Thanks again.

chaos · Sat Mar 17, 2007 8:27 pm

the diagram you used theanswerguy actually seals perfectly, there is no gap and there are o-rings shown in that diagram you posted.

meh and the ball valve releases the pressure creating a vacuumed effect behind the piston sucking the piston back and opening the barrel port, you need both ball valve and shrader valve.

peace chaos

VH_man · Sat Mar 17, 2007 8:29 pm

For the valve you posted, you do need O rings (chamber sealer). the barrel sealer, on the other hand, does not need O rings, as both sides of the piston (minus the diameter of the barrel on one side) are under the same pressure. you just need to remove most of all the air behind the piston, make a relative vacuum, and BAM the valve snaps open.

chaos · Sat Mar 17, 2007 8:32 pm

don't you need o-rings for a coaxial?

theanswerguy · Sat Mar 17, 2007 8:47 pm

Yeah, I got it, I think. Thanks for all the help guys. I'm going to try to build one tomorrow.

PS - Can the ball valve (to release pressure behind the piston) be replaced with any other type of valve, like the blow gun? (assuming a blow gun is the blower nozzle thingie on most air compressors)

PSS - A copper fitting can be threaded/tapped directly into a PVC fitting with not worries of a blow-out?

Thanks for the excellent advice!

chaos · Sat Mar 17, 2007 8:57 pm

Yes you can use a blow gun and it looks better to.

i dont know about the pvc.... i never use pvc... all my guns are stainless steel.

peace
chaos

LGM · Sat Mar 17, 2007 9:11 pm

Yes, copper can be threaded into PVC. If the valve will be in a 1 1/4" housing or above, a blowgun wil not work, at least not well. The ball valve will work much better.

frankrede · Sat Mar 17, 2007 11:43 pm

By copper you mean brass? right? If so then you should pick your self up a 1/8" npt tap or a 1/4" npt tap. Or just thread into a hole slightly smaller than your threads and ass Teflon.

willarddaniels · Sun Mar 18, 2007 12:48 am

just thread into a hole slightly smaller than your threads and ass

That is pretty darn funny! Especially when you take it out of context.
Welcome to the forums theanswerguy. We are glad to have you aboard.

EDIT: yes, I know that "d" and "s" are right next to each other on the keyboard... but some typos are just great!