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Piston+Return Spring performance?

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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Piston+Return Spring performance?

Unread postAuthor: RBurke83 » Mon May 06, 2013 11:59 am

I am designing a new launcher based on my experiences with my first piston launcher, and my design necessitates a chamber-side fill port with a multishot capacity, ideally 3 200+psi shots. I'm just wondering if anyone who has developed a piston with a return spring has noticed any notable performance drop offs having that spring pressure reseal the valve? Thanks in advance
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Unread postAuthor: jackssmirkingrevenge » Mon May 06, 2013 1:46 pm

Doing some math will give you an idea of the forces involved.

Let's say you have a 0.75" piston and a 0.25" barrel.

At 200 psi, there is a force of 88lbs from the pilot keeping the piston shut, with 78lbs trying to push it open from the chamber, meaning a net force of 10lbs is keeping it shut.

Let's say opening the pilot valve drops the pilot pressure to 100 psi. There is now 44 lbs trying to keep the piston closed, meaning there is now 34 lbs net trying to open the piston.

Since you only need a spring strong enough to fight the almost negligible friction of the piston, do you really think it will make that much of a difference?
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Unread postAuthor: RBurke83 » Mon May 06, 2013 3:33 pm

Well, honestly I wasn't sure. I was thinking about a spring which could act as a piston sealing aide and also as a bumper, which I guess at 2-500 psi doesn't have to be extremely stiff, I'm just trying to find a happy medium between reliable valve sealing and not closing prematurely.
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Unread postAuthor: jackssmirkingrevenge » Tue May 07, 2013 12:04 am

I'm not a fan of bumpers :)
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