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Need a simpler solution to piston valve problem

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: PotatoNick » Wed Sep 17, 2008 1:03 am

jackssmirkingrevenge wrote:
PotatoNick wrote:Where I disagree with you is the reason for having lift of up to .50d(or any value greater than .25d) . I don't believe that much, if any, piston bounce occurs during the time a projectile is in the barrel because you have so much force constantly being applied from the pressure pushing the piston backwards I would think that it would more than cancel out any opposing force from the very low spring rate of PVC.


I used to agree religiously with that view, however my change of heart is explained in reasonable detail here.


Since this thread has now become people arguing about using DWV fittings on a pneumatic, I don't mind being the propagator of a slight thread jack.

Ah, I see what's happening. I don't think you've quite explained it properly though. Have you ever had a sealing face get sucked off the piston and shot out the barrel during a valve test? I believe that the same effect is occurring here, and that "piston bounce" is still a very minimal factor. It would just kind of cause the effect to induce movement of the piston a little sooner.

This is pretty advanced to actually try to solve because it is outside the range of standard fluid dynamics equations since we're using a compressible fluid that's traveling at such a great speed. I think fluid dynamics equations start to significantly lose their accuracy for compressible fluids at about Mach 0.3 .

What I believe we're seeing is some form of venturi effect.

Oh and even with all this said, I still don't believe in the d/4 rule
:D because of boundary layers and other characteristics of fluid flow.
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Unread postAuthor: McThunderThighs » Wed Sep 17, 2008 7:57 am

There is a very easy way to test where the piston should fall (even though it isn't that easy).

If we allow the same pressure before and behind the piston, and then start to move it back mechanically, the point where the piston stops resisting us is the critical point. Essentially what you want is the point at which the piston is far enough away from it's sealing face to allow enough air to move into the area of the barrel to equalize the surface area on both sides of the piston.

I think you would find it is considerably farther than you think. At least when we do turning tests for the occasional dynamics lab, you need quite a bit of space to allow the air to create it's own smooth channels through which to flow.

Someone should test that out.
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Unread postAuthor: psycix » Wed Sep 17, 2008 4:17 pm

Its not the piston itself that fails, it is the pipe* that takes the stress from the 2" pistons that might snap, just because of the pistons smacking into the bumper like a hammer.
It also
*Marked by circle no. 2 by Crowley.

You seem not to accept some of our advises, you think this is because we are wrong, but this really is because you seem not to understand everything enough.

Also, its just a waste of building two 4" pistons and two 2" pilot pistons AND buying two sprinklers while even only one 4" valve with a sprinkler could outperform this all if you just used better pipe and fittings (and thus would be using higher pressures)

Oh and on the coaxial subject, if you build your piston/diaphragm valve inside a tee it is NOT coaxial.
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Unread postAuthor: McThunderThighs » Wed Sep 17, 2008 10:58 pm

Ok. I'm calling in backup on this one.

From OED (Oxford English Dictionary): Definition 1 of "coaxial":

Having a common axis.



In the valves on our cannons, in the Ts, there is a pipe, smaller than the t that extends though the T and then meets and seals against a piston.

The exhaust (this smaller pipe) and the chamber (the bigger pipe around the smaller pipe) SHARE AN AXIS, specifically the y axis.

I'm sorry, but I must be illiterate if I can't read the dictionary correctly.

The sprinkler doesn't work, it just doesn't move enough air. Case dismissed. Sorry, our cannon just doesn't perform like your's does.

If you read the thread, you would learn that the 2" valves and the majority of the fittings around them are being replaced.

/rage
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Unread postAuthor: frankrede » Wed Sep 17, 2008 11:21 pm

McThunderThighs wrote:Ok. I'm calling in backup on this one.

From OED (Oxford English Dictionary): Definition 1 of "coaxial":

Having a common axis.



In the valves on our cannons, in the Ts, there is a pipe, smaller than the t that extends though the T and then meets and seals against a piston.

The exhaust (this smaller pipe) and the chamber (the bigger pipe around the smaller pipe) SHARE AN AXIS, specifically the y axis.

I'm sorry, but I must be illiterate if I can't read the dictionary correctly.

The sprinkler doesn't work, it just doesn't move enough air. Case dismissed. Sorry, our cannon just doesn't perform like your's does.

If you read the thread, you would learn that the 2" valves and the majority of the fittings around them are being replaced.

/rage




ON THIS WEBSITE WE USE THE TERM COAXIAL TO DESCRIBE A CERTAIN CANNON LAYOUT. WE HAVE OUR ACCEPTED DEFINITION AND WE USE IT.
YOUR CANNON DOESN'T WORK AS INTENDED BECAUSE YOU SUCK AT BUILDING.
WE HAVE GIVEN YOU OUR KNOWLEDGE ON IT, ACCEPT IT.


God, I am sick and tired of this guy and I really don't care if I am flaming, there's no getting through with this guy.
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Unread postAuthor: jackssmirkingrevenge » Thu Sep 18, 2008 12:38 am

McThunderThighs wrote:In the valves on our cannons, in the Ts, there is a pipe, smaller than the t that extends though the T and then meets and seals against a piston.

The exhaust (this smaller pipe) and the chamber (the bigger pipe around the smaller pipe) SHARE AN AXIS, specifically the y axis.


As frank so eloquently put it, while you're technically correct, the term "coaxial" here is normally used to refer to a launcher which has a single chamber with the barrel at its centre. So really, you're all correct. Now put those handbags away and concentrate on launching potatoes ;)
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Unread postAuthor: MrCrowley » Thu Sep 18, 2008 12:54 am

As JSR said, everyone is right. For example, think of a hybrid.

Here we think of a hybrid as a spudgun that is charged with a compressed gas, which when mixed with the correct ratio of another compressed gas creates an explosive mix that can be detonated by an igniter.

On the other hand you may think a Hybrid is one of those cars that left-wing people buy to help the enviroment while you laugh at them in your big SUV. :P

Same thing here, we think of a coaxial as a launcher with the barrel completely or mostly surrounded by the chamber with the valve housed inside the back of the chamber. Think of a coaxial cable, which is a cable consisting of an inner conductor, surrounded by a tubular insulating layer.

Kind of similar to that.

What we call your valve configuration is a barrel sealer, because it seals against the barrel but in a different configuartion to a 'coaxial' launcher.
To stop confusion, it's best if you refer to your valve as a barrel sealer.
http://www.spudfiles.com/forums/piston- ... t8157.html
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Unread postAuthor: McThunderThighs » Sat Sep 20, 2008 11:18 pm

Ok, one more question:

I am looking for a relatively soft material that will act as a good seal (cushy stuff). What have you guys used?

We've been having trouble getting the valves to seal consistently, and seeing as we just annihilated one of the pistons, we're going to start from scratch again (with the pistons, not the whole thing).

My plan is to use 3" endcaps again, but to cut grooves in them for o-rings (I can hear frank screaming already). The other problem we are dealing with is sealing against the barrel. Neither the barrel nor the endcaps are perfectly regular (despite a large amount of sanding) so we were using mousepad behind the gasket to give the gasket some room to maneuver. However, this has not always worked perfectly, and has lead to many gasket failures (air seeps through the mousepad).

So what soft materials seal well?
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Unread postAuthor: MrCrowley » Sat Sep 20, 2008 11:44 pm

McThunderThighs wrote:but to cut grooves in them for o-rings (I can hear frank screaming already)

Well as long as you have an equalization hole he wont be.
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Unread postAuthor: starman » Sun Sep 21, 2008 1:09 am

McThunderThighs wrote:Ok, one more question:

I am looking for a relatively soft material that will act as a good seal (cushy stuff). What have you guys used?

So what soft materials seal well?


Neoprene rubber, you can get it from Mcmaster Pn 9013K272.

You may also find some neoprene rubber washers at a hardware store but probably not the size you need. Just get your self a sheet of it to have on hand.
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 21, 2008 2:35 am

Truck tire inner tube also works well ;)
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Unread postAuthor: McThunderThighs » Thu Sep 25, 2008 6:16 pm

Hey all! Thanks for the suggestions, we're using the red gasket stuff on top of foam rubber. Looks like it will work nicely.

Looking for some approval on the new valves:
Image

Everything says PW on it. Action shots this weekend hopefully (waiting for the 1.25" pistons to cure)!
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Unread postAuthor: jackssmirkingrevenge » Fri Sep 26, 2008 12:39 am

Is that sprinkler actuating piston actuating even bigger piston? Can ytou say Rube Goldberg? :D should work well, looking forward to seeing the results :)
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Unread postAuthor: McThunderThighs » Sat Sep 27, 2008 9:53 am

Shes a working!!

Everything is going great now that the valves are working.

Here was a pic of our testing setup for one valve:
Image

We put together the second valve, pumped her up to 30psi and shoved a tennis ball down the barrel:
Image

This was the damage to our carpet backstop from both valves at 30psi even though it had held up find at 80psi on one valve:
Image
The ball went straight through, and there was a little tab of carpet left hanging on, and carpet bits all over the place. We didn't do any more tests.

Thanks all! I really appreciate all of your help from the last few weeks!
-MTT
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Unread postAuthor: starman » Sat Sep 27, 2008 10:37 am

jackssmirkingrevenge wrote:Can ytou say Rube Goldberg? :D


Yes I can...Rube Goldberg... 8) Don't think I've ever seen so many pistons and sprinkler valves actuated at the same time, on the same gun.
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