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new pneumatic design, what do you think?

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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new pneumatic design, what do you think?

Unread postAuthor: rockeurfoo » Fri Aug 21, 2009 11:49 am

hi everybody!

I am new on this forum but I read it a long time ago.
First, sorry for my english, I live in france and there is not as good forums as this...
I've already made some basic pvc pneumatic cannon witch have good results at 3 bar (53 psi?) and I would like to go up with pressure fittings (up to 230 psi or 16 bar)

So I plan to build a pvc 16 bar pneumatic cannon, with a piston valve and a modified sprinkler to pilot the launcher.

Here is a pic of the cannon I would build, and I have some questions.
It's a chamber sealing cannon.

Image

The picture is not scaled!

The chamber will be 90mm (3.5") diameter pvc with 2 reducers to use a 50mm (2")tube (on witch the piston will push on). I will use a coupler between the tee and the chamber for opening the cannon.
The piston will be 63mm (2.5") diameter and will be made of two 2"caps (one entire with a bit of tube, the second will have the head cut down and will be glued a little far to put an O ring between)

On the picture I write "Volume". I wonder if too much air can stay here instead of pushing the potato...
What do you think of it? maybe I have to fill this volume with something to reduce it?

many thanks !
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Unread postAuthor: Hubb » Fri Aug 21, 2009 12:12 pm

I think you should completely eliminate the "volume". That is dead space. And your design is giving you a lot more than usual. As the volume goes up on a launcher chamber, the pressure goes down. Dead space adds unnecessary volume, which reduces the power output. You design is putting a lot of unnecessary dead space. And just looking at your drawing (I know you said it was not to scale) I would probably estimate more than a 20% drop in pressure.

Of course, I could be all wrong.....

But I do have to ask why you want a choke like that between your chamber and barrel. Choking will increase the velocity of the airflow, but also cuts the pressure. Going from 3.5" to 2" probably won't make much of a difference, but I'm just curious.

Oh, and welcome to Spudfiles. I like the thought of a new member coming in strong like this.
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Unread postAuthor: rockeurfoo » Fri Aug 21, 2009 12:25 pm

Thanks for your help!

In fact i shearch something witch could fit in the tee and reduce the diameter of the chamber, To build my piston I want to use a 63mm tube because 50mm caps have an outer diemeter of 63. I don't think that a 75mm tube can fit into the 90mm tee (after the gluing surface the diameter is very reduced).

Image

But maybe I made it wrong but I don't mind how I could make a 90mm piston in a tee like this, with existing parts (i love the fact that caps have inner and outer diameters that can fit into couplers)
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Unread postAuthor: ramses » Fri Aug 21, 2009 2:43 pm

welcome to spudfiles. The valve that you describe is a chamber sealing piston valve. first picture.

I hate to shoot you down, but I have had a number of ideas that have already been developed on this site, without my knowledge.
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Re: new pneumatic design, what do you think?

Unread postAuthor: twizi » Fri Aug 21, 2009 3:12 pm

rockeurfoo wrote:hi everybody!

So I plan to build a pvc 16 bar pneumatic cannon, with a piston valve and a modified sprinkler to pilot the launcher.


i would not use a sprinkler unless u want shrapnel in your leg also pvc at 16 bar not the best idea either use steel and a ball valve to pilot the the piston valve.
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Unread postAuthor: MrCrowley » Fri Aug 21, 2009 4:21 pm

Yeah I actually agree with twizi on this one. PVC at 230PSI is not a good idea even if it is rated for 260PSI.
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Unread postAuthor: rockeurfoo » Sat Aug 22, 2009 6:52 am

no i'm ok, my compressor goes to 110 psi only so i dont mind to use it with the max pressure!

and the sprinkler is intended to be used as a pilot valve so i don't understand how it could explode? oh yes I understand! the sprinkler is 10 bar max that's why you say that!

On the first post i write pvc 16 because this the name that we give to it. pvc 16 (16 bar max)

I will use it at 8 bar only!

ramses : i know that the cannon i want to build is a chamber sealing piston valve, posting here is just about this dead space between my chamber and my barrel. or is there anything that i did not understand?

I want to use existing parts to build my launcher and this is why i think a design as this but I have to reduce this dead space...
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