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Old 05-16-2013, 10:06 PM   #1 (permalink)
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Electro to mech conversion (paging the pneumatically minded and theory inclined)

I'm building a mechanical Bushmaster B2K. I've got some things figured out, but some things I need some help ironing out.

First, what I do know: I'm thinking I'm probably going to run a Fabco msv2 as the trigger. It will be fed by it's own lpr so it can be pressurized as low as possible as it will be a pilot for the valve replacing the 'noid.

Which leads me to what I don't know: What valve to use to replace the 'noid. I've found some piloted 4-way valves to directly replace the 'noid (clippard mma-41nas for example) but their size are prohibitive. The clippard is 3 1/4 inches, it almost fills the whole frame!

So I have to find another 4-way valve or an entirely different alternative.

One idea for an alternative (and this is where pneu-theory comes in) is to possibly use a 3-way with qevs to replace the 4-way. The reason I think that would work is the 4-way has 4 functions: forward, reverse, exhaust forward, exhaust reverse. While it is in reverse it is exhausting forward and vice-versa.

If a 3-way (I believe that is forward, reverse) had qevs at the connection to the ram, then it should vent as soon as the pressure is switched to the opposite side right?

If the 3-way were to work I'd have to find a piloted 3-way, but I believe they come significantly smaller.

Input is appreciated. Thanks in advance for your help.
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Old 05-17-2013, 08:49 PM   #2 (permalink)
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You can't use a 3-way with QEVs to replace a 4-way, sadly doesn't work.

Not a whole lot of options for a pilot actuated 4-way that fits into a frame either sadly. What I've done recently is actually drill a hole into the back cap of a MAC 43, and actuate the spool manually. Super light, super short. You won't have dwell control, and the bolt will stay forward as long as the trigger is held down, but unless you really mess with a lot of other things, the poppet should still close after the initial burst.
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Old 05-17-2013, 11:11 PM   #3 (permalink)
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Hey, that's an idea. And it would be actually possible to fit it in the frame at that point. Nice!
Do you use straight air to cycle the valve, or a short stroke ram?
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Old 05-18-2013, 12:40 AM   #4 (permalink)
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I did it directly with a pin, but a short stroke ram could work, though you are adding additional complication at that point.
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Old 05-18-2013, 12:56 AM   #5 (permalink)
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So the trigger actuates the valve itself? How is the pull weight on that?
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Old 05-18-2013, 10:55 AM   #6 (permalink)
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Almost microswitch light. I can actuate the trigger with a piece of tubing.
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Old 05-18-2013, 01:00 PM   #7 (permalink)
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Hmmm... I wonder if I could do the same with the stock 'noid. What do you use to make it return to battery? A spring? Or am I missing something obvious?
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Old 05-18-2013, 03:18 PM   #8 (permalink)
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They have an internal spring to return, the solenoid coil is only capable of moving the spool in one direction, so it needs a mechanical means of returning normally anyway.

I've only ever tried it to a MAC 43 before, but you should be able to do it to most solenoid valves. Just put a tiny hole in the body behind the armature, insert a pin into that hole, and use that to manually move the armature, which pushes the spool.
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Old 05-18-2013, 05:21 PM   #9 (permalink)
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Ok. I will try that.
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