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barrel sealing piston valve vs QEV

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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barrel sealing piston valve vs QEV

Unread postAuthor: cannon monkey » Mon Jul 20, 2009 8:46 pm

whats the differnce between the two
basicly
is a piston free moving and QEV the same thing but a spring pushing it up to seal?
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Unread postAuthor: daberno123 » Mon Jul 20, 2009 9:16 pm

They are both the same.

The term QEV refers to a professionally manufactured piston valve, usually used to actuate pneumatic cylinders. Technically you can't build your own QEV, if you did it should be referred to as a piston valve.
The term piston valve is reserved for a homemade valve.

Its a really minor difference, but that's generally what people refer to when they use one of those terms.
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Unread postAuthor: cannon monkey » Mon Jul 20, 2009 9:18 pm

nother thing my piston now 3/8's of an inch nipple filled with hot glue then seal face on it

is that good or would it be better to replace with a wood dowle ( wooden stick) add seal face to that then 2 groves for o-rings

if orings are better how much better?

and can i add orings to the hotglue piston?
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Unread postAuthor: theBOOM » Mon Jul 20, 2009 11:16 pm

Hm you can use either.. but from what I've heard its better to use a non brittle piston... I know that wood isnt TOO brittle but id rather use hotglue seeing as it isnt brittle at all... And yes you can add orings to the hotglue piston if especiall if you want a piston that actuates better... Be sure to lube the piston up with some sort of grease to help the actuating...
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Unread postAuthor: Technician1002 » Mon Jul 20, 2009 11:49 pm

daberno123 wrote:They are both the same.

The term QEV refers to a professionally manufactured piston valve, usually used to actuate pneumatic cylinders. Technically you can't build your own QEV, if you did it should be referred to as a piston valve.
The term piston valve is reserved for a homemade valve.

Its a really minor difference, but that's generally what people refer to when they use one of those terms.


The above is mostly true. It is true for piston valves with a close ratio of area of the valve seat to OD.

It becomes a QEV when it is designed to snap open inspite of a slow or large pilot. To achieve this, the initial opening force is low from the chamber by using a limited exposed area. The Pilot pressure then must drop considerably before the piston forces equalize to where the piston comes off the seat. When it does come off the seat, the new larger area exposed to chamber pressure snaps the valve open even if the pilot pressure no longer drops.

A large diameter piston sealing a barrel less than 1/2 the diameter does not have the properties needed to snap open without a very fast pilot vent and small pilot volume.
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Unread postAuthor: cannon monkey » Tue Jul 21, 2009 8:46 am

alright whats good for a bumper i been noticing the bottom of my piston is flaring out from slaming into the side of the tee fitting i only got some rubber from a iner tube left over from cutting the seal face

i take pictures when i find the camera i need help to make this better anyways
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