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Use of autococker valve for a pilot, weird GGDT valve data

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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Use of autococker valve for a pilot, weird GGDT valve data

Unread postAuthor: scianiac » Mon Dec 21, 2009 1:02 am

After looking on ebay for a valve for my airsoft sniper rifle I found this. In function it seems perfect to function in a semi-auto manner but it's seemingly very small orifice might cause problems. The gun's chamber diameter is only 1.5 in and the piston stroke is very small. I think that the piston speed is not too important considering the air can't enter the barrel until the blow-forward slide is all the way forward and has cleared the rod that blocks airflow through it until the projectile is fully sealed in the barrel. I have a similar valve but of a larger size that is currently installed on a unfinished shotgun that I've lost interest in that I also can use. Using a valve like this is much easier as apposed to the two valves linked together in terms of construction.

The problem comes when I enter small values into GGDT for the vent orifice. When any value below .2 in. is used the graph of valve position turns into a escalating sine wave. I assume the oscillation is caused by the air leaving the chamber faster than the vent and lowering the pressure low enough to reverse the piston's movement until the chamber flow is lowered enough to reverse it back.

Does anyone have any experience with these valves and can advise on there flow rates or orifice sizes? And has anyone ever actually experienced what GGDT said would happen. Also would having the rod that is attached to the piston block the flow through the blow-forward until the last .25 in. or so limit the airflow too much?

The blow-forward stroke is about 3.5 in. for complicated reasons and I've attached a GGDT config file.

Any questions are welcomed and I can post a picture if need be (my explanations can be fuzzy at best)

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Unread postAuthor: psycix » Mon Dec 21, 2009 9:07 am

If your pilot valve is too small you will also get trouble in practice, not just in GGDT.
Pistons can "oscillate" or bounce back and forth, an overkill pilot valve and a little extra piston travel helps a lot to prevent it from (nearly) closing up during firing.
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