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About piston valves

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: boyntonstu » Thu Apr 15, 2010 3:14 pm

I have been thinking about sealing the rear of the piston after it has opened.

Think of corking a bottle and you understand.

The Trom-Boyn is pressurized through the pilot vent and hence the piston will uncork an re-seat itself at the breech for the next shot.

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Unread postAuthor: velocity3x » Thu Apr 15, 2010 3:34 pm

boyntonstu wrote:The Trom-Boyn is pressurized through the pilot vent and hence the piston will uncork an re-seat itself at the breech for the next shot.


Excellent idea but, far from unique.
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Unread postAuthor: boyntonstu » Thu Apr 15, 2010 3:35 pm

velocity3x wrote:
boyntonstu wrote:The Trom-Boyn is pressurized through the pilot vent and hence the piston will uncork an re-seat itself at the breech for the next shot.


Excellent idea but, far from unique.


Great!

Llink to someone who has made it please.
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Unread postAuthor: velocity3x » Thu Apr 15, 2010 3:44 pm

boyntonstu wrote:
velocity3x wrote:
boyntonstu wrote:The Trom-Boyn is pressurized through the pilot vent and hence the piston will uncork an re-seat itself at the breech for the next shot.


Excellent idea but, far from unique.


Great!

Llink to someone who has made it please.


Well....I did it as I'm sure many others have.
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Unread postAuthor: psycix » Sun Apr 18, 2010 10:15 am

jackssmirkingrevenge wrote:
iknowmy3tables wrote:but it's a sealing face and not a plug, therefore it needs to have a certain amount of pressure in the pilot to push the sealing face and make it seal


Regardless, if the piston rides in a tube that is the same diameter or smaller than the barrel, it will never open. Take a look at your design in detail and check the pressure vectors, it balances out so no net force on the piston. The most that can happen is that the rear o-ring flexes and pressure in the chamber leaks through the pilot.

Many spudguns do not seal under 5-30 psi, but seal just fine above that. This is due the sealing face not being able to seal without getting pressed against the porting.

Apply the same logic to this topic.

No net force -> sealing face does not get pressed against the porting
No force on sealing face -> sealing face loses its ability to seal

This causes a leak into the barrel, raising pressure there, causing a net force, causing the piston to start moving.

Your prediction works in theory, mine works in practice.
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Unread postAuthor: jackssmirkingrevenge » Sun Apr 18, 2010 10:26 am

psycix wrote:Your prediction works in theory, mine works in practice.


I agree on this point.

In the diagram, with the pilot chamber full there is a net force keeping the piston shut, because the pressurised air behind the piston is stronger than the atmospheric pressure pusing on the part of the sealing face exposed inside the barrel. In theory, assuming a 100% airtight piston, once the pilot chamber is completely empty the seal will begin to leak and yes, assuming the projectile is tight enough and has enough inertia to allow pressure to build up between its base and the piston face, then the piston will open. How well this works in practice is a matter of experiment though, few people seem to go for airtight pistons anyway and the benefits of lower piston weight might not overcome the lost face surface area when it comes to valve opening speed.
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Piston locking

Unread postAuthor: boyntonstu » Sun Apr 18, 2010 11:04 am

[/quote]Well....I did it as I'm sure many others have.[/quote]

How does your piston lock open?

Has anyone built a locking piston in a copper cannon without using a lathe or milling machine?

(I did it today)
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Piston locking

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Unread postAuthor: Technician1002 » Sun Apr 18, 2010 11:19 am

The difficulty on getting flat face valves to seal initially so the chamber could be filled is one of the reasons I looked for an alternative.

The O rings were much easier to get a seal on the QDV cannons than it was for the Mouse Musket or the Dragon. The QDV cannons can easily be hand pumped directly into the chamber. The Piston cannons either required a shot in the pilot to seat and seal the valve before filling in the chamber, or the barrel needed to be plugged and suddenly released to seat the valve.

For valve performance, since the pilot area is never under pressure, when fired, the piston is driven all the way open and held there until the chamber pressure is gone. No piston latch is required to hold it open.

The closer the valve has the OD to the seat, the lower the pilot pressure can be before the valve opens, With low to no pilot area pressure, the chamber pressure holds these valves open longer as the chamber pressure has to drop to a much lower value before the pilot area pressure forces the valve back closed.

I used to fill the Mouse Musket by sealing the barrel with my thumb, adding a couple strokes of air and then letting my thumb pop out. If I didn't do that, it often would not make an initial seal.
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