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New piston valve idea and a couple questions

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 piston valve idea and a couple questions

Unread postAuthor: noob of noobs » Tue Aug 21, 2007 7:32 am

I’m building a piston valve, and I have what I think might be a new design. Here: http://img258.imageshack.us/my.php?image=newpistonbh9.png

Basically, it’s a regular piston valve made of galvanized steel, with a smaller piston rear, designed to push and pull the piston in and out of the nipple faster. The idea of shrinking the piston’s rear down should in theory allow lower pilot volume. Also, take this for an example: if you suck water through a larger straw, it’s harder than a thinner straw, because A) there’s less air between your mouth and liquid that needs to be depressurized, and B) because there’s less water in the straw, so less weight pushing the liquid down. Other benefits include that the T allows for easier attachment of ¼” Schrader and hose barbs without gluing, the piston could be made lighter, and less lubricant would be needed (which could be easily applied by the way.)

Got a couple questions though.
A) Would this design work well first of all?
B) How do you make a sealing lip on galvanized steel? In PVC, you can just dremmel out a bushing and shove some pipe through, but threads only go one way on steel and since everything is threaded, that doesn’t seem possible. I’ve heard you can use copper and epoxy, but I’d prefer not to use epoxy if possible.
C) I already tried to make a valve like this out of PVC, but I ran into a problem. Firstly, due to sucky craftsmanship, it leaked like mad out of the service screws and end cap, and secondly, the piston would move forward, but I couldn’t get it to unseal. I was using a ½” ballvalve as a pilot, but the dang thing didn’t move. I used Crisco as a lubricant. Any tips?

Thanks for your time.
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Unread postAuthor: jackssmirkingrevenge » Tue Aug 21, 2007 7:40 am

I was using a ½” ballvalve as a pilot, but the dang thing didn’t move.


I'm not surprised, that piston will never fire - in order for an piston valve to work, the diameter of the piston must be bigger than the inner diameter of the barrel.

You should revise your notes :wink:
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Unread postAuthor: noob of noobs » Tue Aug 21, 2007 8:24 am

I don't quite get it. By the diameter of the piston, do you mean the part which provides suction in pushing and pulling the piston? If so, I've never heard that before... :oops:
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Unread postAuthor: Intel Xeon » Tue Aug 21, 2007 8:36 am

The force pushing the piston against the barrel is stronger than the force on the pilot side of the piston. Thus, it will never fire.
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Unread postAuthor: jackssmirkingrevenge » Tue Aug 21, 2007 8:46 am

Look at it this way:

when the light blue area is pressurised, the yellow arrows show the force which the air pressure is putting on the piston. Since the barrel end has a larger area, there is more space for the air to exert pressure therefore a greater force.

This means that even if your pilot area is totally empty, the force towards the barrel will always be greater than the force towards the pilot, therefore as long as the chamber is pressurised the piston will never move.
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Unread postAuthor: DYI » Tue Aug 21, 2007 8:50 am

That valve will not open. There is significantly more force holding it against the barrel than pushing it back.
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Unread postAuthor: jackssmirkingrevenge » Tue Aug 21, 2007 9:02 am

The only way it might work is if you make it like this, with a long piston travel and two-part piston:

Image

That way, the rear part of the piston will have time to accelerate and thus build up a force that is greater than the air keeping the barrel sealed.
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Unread postAuthor: dongfang » Tue Aug 21, 2007 3:20 pm

Jack,

It that the DSTV on the rear on the first diagram?

Regards
Soren
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Unread postAuthor: jackssmirkingrevenge » Tue Aug 21, 2007 8:52 pm

dongfang wrote:It that the DSTV on the rear on the first diagram?


*blink*

Forgive me but I don't see the DSTV anywhere, this is an exhaust valve, not a mechanically actuated one like your design.
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Unread postAuthor: noob of noobs » Tue Aug 21, 2007 9:01 pm

Ohh..... I get it! :idea: Wait... that's bad... :oops: Ah well, back to the old drawing board. :cry:
Thanks for the help. I think I'll just try a regular piston for now...
In light of this though, does anyone know any other way to put a sealing lip into a metal T?
Once again, thanks for the help! :D
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