SpudFiles

KamranGo wrote: I've bought some plumbing lubricant actually, it seemed to be what i was looking for, but i haven't tried it out yet. Do you find that you need to reapply it very often? and does it have a low friction coefficient?

I've used it on a 2" piston for over 30 shots in half a day, it only seized up when I filled the barrel with water, which leaked into the valve and took all the lube away.

I apply about a fingers worth on a 2" piston 8cm long and after that I can push it through 2" pipe with ease.

Btw, I say go for it. You may as well, you'll only learn from your mistakes.

I suggest you look at a pneumatic cannon called SWAT, it's on this website somewhere.

Just make sure you know what you're doing first.

KamranGo wrote: However, i'm still a little bit hazy as to where you experience a lot of structural failure? or where exactly are points to be wary of? I want to still pursue building this valve and i was a little bit confused where you had experienced problems before. Though, i think this topic is starting to digress.

Two places most common with structural failure:

1) Piston housing, you NEED a really good bumper for a piston this size.
hell I need a decent bumper in my 2" piston.

You would be surprised how powerful they are.

I suggest you build a smaller piston first, as a test setup.

But with a piston this big, there is going to be huge forces involved.

2) Chamber-Tee elbow. If you have a over-under cannon, the elbow that connects the tee to the chamber is under a lot of stress.

A cannon this size will need a lot of support. I'd recommend a barrel-chamber support right next to the tee, one half way up the barrel and one at the end.

Haha thanks for the support, i'll definitely check that gun out.

And yes, it will be an over-under and i definitely will be including much support between the barrel and the chamber, that was clearly going to be necessary with a cannon this size.

Good good, just make sure you go way over the top with a bumper. These parts aren't exactly cheap, and neither is surgery

Seems like you already got the lubricant worked out, but I second the suggestion for the plumbing lubricant (silicon grease) or white lithium grease also works well and is easy to find.

Yes, the piston will snap open with a lot of force on a valve that big. While were are on the topic of that elbow. If you are planning a removable barrel I would suggest not firing the gun without a barrel unless you are sure the valve is secure because the kick of just the air coming out of a valve that size is enough to break that elbow or even throw the whole gun back violently.

Wow, i didn't know that this could possibly have that much recoil actually. I'll definitely be sure to reinforce the back of the gun before i do any sort of test shot, even empty.

I just thought i'd let all of you know i've come up with a pretty sweet mechanism to absorb the recoil of the piston. I'm really hoping this thing is a success.

Though this topic has really become more of a "help me" with this valve, i have another questions regarding my piston valve construction.

Because i want to be able to access the piston in case i need to service it after i have built the valve, I designed the pilot chamber to be capped with a threaded end-cap (like this: http://www.hardwarestore.com/media/prod ... ont200.jpg). However, i've noticed that most people have opted for using hex nuts to secure whatever caps the end, rather than this threaded ending thing. So i wanted to know if it is because these threaded plugs have a tendency to leak? or are they just a pain in the ass compared to using hex nuts?

I'm aiming for 125 PSI and i know two different valves i've seen with the hex-nut configuration were not really intended for such high pressure (more like 100 psi, really). So, what should i do?

That's a cleanout cap, not pressure rated. They blow at low as 40psi. One guy even died from the cause of one blowing out.

I say go for it.

Edit: Maybe use one of <a href="http://www.knightsandarmor.com/4thofjuly4.jpg">these</a>

Novacastrian wrote:I say go for it.

You serious?

Usually using bolts to hold the end cap on is usually done to make the valve compact. Personally I am not a big fan of the method and I think it can be dangerous.

That fitting you posted is a clean-out cap and is not made for pressure. Using a 6" clean-out with a piston slamming into it is not a good idea and I would say it is more likely than not to break.
Make sure that all the fittings for this valve have NSF-pw printed on them otherwise it is not very safe to use them with pressure

After looking around at a few sites I can't seem to find a pressure rated 6" threaded plug, so you may have to go with the bolt-in method. Like I said I'm not a big fan, but I think you are safer with that than a clean-out plug. Just make sure that nobody is ever standing behind it while the gun is charged.

Oh, well then, that explains a few things. That's really unfortunate as well.

Thanks Novacastrian, useful as ever.

Clide, i'm definitely not worried about this valve being compact, however i agree that it does not seem to be a very safe method of securing the pilot chamber. Unfortunately the only other alternative i can think of is securing using a test plug (like so: http://www.hand-tite.com/images/titan.jpg) inside of the pvc tube, and then securing the endcap over the back with the screws to ensure the test plug does not fire out the back... though i do not know what pressures these test plugs can withstand, and how they would often fail (given they could not fire out)

McCrowley, I actually just went and read your How-To on the 2" piston valve. I'm somewhat skeptical that simply screwing some screws into the endcap and pilot chamber is enough to make it airtight. Would perhaps lubricating the inside of the endcap help ensure a perfect seal? or maybe some rubber on the end of the PVC pipe, before the cap?
I also noticed you used only 4 screws, while others have used far more than this. I worry that either 4 screws will not be strong enough to hold as much pressure as i desire, but having too many screws will weaken the PVC pipe too much and possibly result in structural failure.

MrCrowley wrote:

Novacastrian wrote:I say go for it.

You serious?

Yeah, why not. I forgot to add that i enjoy reading the Darwin Awards Website!

Seriously though at the right pressures i can't see why it can't be done, minus the end cap. (Sorry i didn't notice that).

I did have some leak issues, I've changed the setup now, but you can seal it off with an o-ring if you can make an oring groove.

Also 4 screws are more then enough for 2" pipe, too many will weaken the fitting and pipe.

For your setup you will probably need to make an o-ring groove and have around 12 screws.

Also can't you use a threaded adapter and threaded end cap?
<A HREF="http://www.hamuniverse.com/KC0YNRpic4.jpg">Like This</a>
Or can't you get them in 6" size?

What about a 6" plug?

@Nova

Yeah there is no way that clean out cap will survive, other then that he should be fine.

Sure, that's basically the idea i was going for. I was under the impression that a threaded adapter and threaded endcap were actually the cleanout plug and threaded female adapter i was using... oh well. I'm not sure if they make them at 4" but i'll look around. So these threaded adapters and endcaps can withstand 125+ psi?

Though, back to the bolt-on method, instead of making an O ring groove could i simply place a circular sheet of rubber inside the back of the endcap and then push it all the way against the tube, then bolt it on? Would that achieve the same effect?

Yeah it should, I tried that but couldn't really find the correct sized rubber sheet and gave up.

And yes the threaded end cap and adapter should widthstand those pressures as long as you have a decent bumper.

Hmmm, ok. My bumper is dope. I'm using a combination of 4 6" springs, plus a 3" rubber end cap upon which a 2" rubber endcap is going to be glued, to absorb the shock. Not sure what Hooke's constant on these springs are, but they're really good.

Just in case you were interested... I actually found some rubber sheets (exactly 4" across, about 1/4" thick, coincidentally) at home depot in the sink repair section/area i believe. I suppose those large rubber flabby things they sell to put over sink or bathtub drains would probably work as well, they run pretty large.