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Old 11-03-2017, 03:22 PM   #1 (permalink)
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help me design a pneumatic pulse valve

I've been making pneumags for years, and so far the only problem I haven't been able to fix is short stroking. Walking an e-marker is easy because the board tells the solenoid to deliver a pulse of air or juice or whatever for a few milliseconds, and then the board won't allow you to shoot again for another few milliseconds until everything resets.

All the mechanical valves we use for pneumags (Fabco MSVs, MAC-33s, etc), however, are either "on" or "off." There's no in between. If your finger is off the trigger, everything is at rest. If your finger is on the trigger, then the valve opens, air flows through to push the piston, which in turn pushes against the sear and fires your marker.

To walk the trigger reliably, however, you need to hold the trigger down long enough to let enough air through to fire the trigger. Then you have to let go completely in order to let the system fully reset. When you're not careful, though, you often hit the trigger again with your second finger before your first finger has completely let go, and this causes issues.

So what if the valve itself was redesigned? This is what I'm thinking.



With a pulse valve like this, air would fill a reservoir when everything's at rest. At the halfway point of pulling the trigger, the middle o-ring shuts off extra air coming in, sealing the reservoir. At the end of the trigger stroke, the last o-ring on the right moves past the piston hole, pushing the piston out to hit the sear. Then all the air from the reservoir is released out a vent. The sear will push back on the piston, resetting that part, and a spring will push back on the trigger rod, resetting that part. Once everything is reset, the reservoir fills up and it starts all over again.

I've been fiddling/designing this thing for a few months now, and I finally had time this week to work up a prototype. I even built it out of Plexiglas so I could be sure to drill the air channels in the right place.

pulsevalve.JPG

It does work....sort of. But I'm running into two problems.

The first problem is that the trigger stroke is hard. I think this is because I'm using 1/4 inch OD o-rings, which have a lot more surface area/drag than smaller o-rings, like 1/8 inch. But that was what I had on hand.

The second problem is that the o-rings aren't sealing well against the Plexiglas. The pulse valve does work at low psi, but if I crank it up to 60 or 80 (which is the pressure I need to fire an automag), then the air starts leaking past the o-rings.

So does anyone have any ideas what kinds of different materials I could use? A stronger kind of clear acrylic? Different o-rings with thinner walls? Any input would be helpful.



(By the way, the piston on the prototype is pointed upward so it will go into a Spyder. I will be putting this into Automags when I get the design right, but an Automag will fire reliably whether the pulse is short or not. A Spyder won't. That's why I'm building the prototype to go into a Spyder. If the pulse isn't short enough, the Spyder with sputter and not recock correctly.)

Last edited by rawbutter; 11-03-2017 at 03:30 PM.
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Old 11-03-2017, 04:05 PM   #2 (permalink)
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A thought or two...

Do you need acrylic/Plexiglass for a particular reason, or is it simply easy to work with?
If not, I would suggest switching to brass. It is easy to obtain a good surface finish, and work with, compared to other metals.

If acrylic/Plexiglas is required...

Have you polished the inside surface of the bores in the acrylic/Plexiglass?

Have you looked at the Parker Oring Handbook or anything similar?

Ty
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Old 11-03-2017, 05:22 PM   #3 (permalink)
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My thought (with very limited knowledge of machining and valves) would be to try delrin. It's self lubricating, has similar properties to the acrylic, and has been able to handle pressures from paintball markers in the past, it might kill 2 birds with 1 stone. If it has to be transparent, I would look into some of the polymers used in making knife scales. I'm not sure how they deal with air pressure, but I have seen several blades with scales that have a semi transparent finish that can stand up to a lot. As i said before, take all this with a grain of salt, I am not well versed on material tolerances with regards to air pressure and ease of machining.

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Old 11-03-2017, 11:55 PM   #4 (permalink)
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Great ideas. Thanks guys.

I'm not set on Plexiglas. I was planning to use brass eventually. But I wanted to start with Plexiglas so I could see where the air channel holes were going. Otherwise I'm shooting blind when I move onto more expensive materials.
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Old 11-04-2017, 06:47 PM   #5 (permalink)
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If it works fine at lower pressure, then why not increase the size of the piston instead of trying to make it work with higher pressure?

And just looking at the picture, you can see your spool o-rings are crushed a significant amount. Try using the next size down o-ring stretched over the spool, you might be able to get it to still seal (there is a smaller gap in sizes between 005 and 006 than there is in larger o-rings like 014 and 015).
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Old 11-04-2017, 06:55 PM   #6 (permalink)
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Check out the video by member Stemmy or DannyDs design for a full mech Ion,seems there's a valve similar to what you're looking for on your project

https://youtu.be/uhEoZZ7kEuI
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Old 11-05-2017, 04:54 PM   #7 (permalink)
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Quote:
Originally Posted by tymcneer View Post
Have you polished the inside surface of the bores in the acrylic/Plexiglass?
By the way, how do I do this? I've already rolled up some 600 grit sandpaper into a tube and stuck it down the hole to twist and go back and forth. Is there a better method?
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Old 11-05-2017, 05:49 PM   #8 (permalink)
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Quote:
Originally Posted by tymcneer View Post
Have you looked at the Parker Oring Handbook or anything similar?

Ty
Parker handbooks is where its at! All the dims you need for all sorts of sealing surfaces. If I had to take a guess:
a) your hole is slightly undersized
b) your shaft is slightly oversized (where the oring sits)
c) the surface finish of the bore isnt fine enough
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Old 11-05-2017, 11:12 PM   #9 (permalink)
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Rawbutter, sounds like you need a machinist to take this to the next level. Plexi was fine for your proof of concept, but you'll want to move to brass. You're never shooting blind if you've done your measurements right, brass will hold higher pressures, machines nicer, holds better tolerances. Passages should be drilled undersized then reamed to spec to ensure accuracy and surface finish. Designing for proper tolerances and oring crush will give your pistons smoother travel with minimal friction. All easier said than done, but that's why these guys above me are the pros
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Last edited by freedummy; 11-05-2017 at 11:16 PM.
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Old 11-06-2017, 02:45 AM   #10 (permalink)
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Quote:
Originally Posted by rawbutter View Post
By the way, how do I do this? I've already rolled up some 600 grit sandpaper into a tube and stuck it down the hole to twist and go back and forth. Is there a better method?
Plexiglass can be chemically polished with acetone. Use it on a spare piece to perfect your technique.

Ty
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