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Problematic Piston - Fires multiple times

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: rad14701 » Sat Apr 21, 2007 9:23 am

You haven't mentioned the size of your barrel...

Your problem is quite possibly related to what I just covered in another forum thread the other day... Using calculations, which I'm sure yours will fail, you need at least as much, if not more, piston area extending beyond the OD of your barrel than that of the ID of your barrel... I don't think you have enough area to push the piston back efficiently...

This is roughly how you make this calculation...

Variable key

cID = Chamber Inside Diameter
bID = Barrel Inside Diameter
bOD = Barrel Outside Diameter

cIA = Chamber Inside Area
bIA = Barrel Inside Area
bOA = Barrel Outside Area

PE = Piston Efficiency

The calculations

cIA = ((cID/2)^2)*3.1416
bIA = ((bID/2)^2)*3.1416
bOA = ((bOD/2)^2)*3.1416

PE = (cIA - bOA) / bIA

if (PE LESS THAN 1) You will most likely have piloting problems
if (PE GREATER THAN OR EQUAL TO 1) You will have better piloting efficiency

Now, this is just my own theory which is based on years of hands-on experience and not based on slide rule physics... I go by what works in the shop, not by what looks good on paper, but this calculation should be a good starting point to work from...
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Unread postAuthor: Solo » Sat Apr 21, 2007 10:50 pm

I've experienced that same problem using a 1.25" T (3/4" porting) and triggering with a blowgun. I had a spring in mine rather than a bumper, but my friend's gun (2" tee, 3/4" porting) had no bumper or spring and still bounced if he swung the ball valve fast enough.

I think I ended up making the piston more airtight, and then putting a check valve into it. (Easy check valve: just drill a hole through the piston face underneath the sealing material.)

(Edit: yeah, I'm thinking there may be a way to make an automatic out of this business. I've been suspecting it for a while, but more and more people keep mentioning this bouncing piston phenomenon.)
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Unread postAuthor: jrrdw » Sun Apr 22, 2007 6:38 am

pyro6314 wrote:Thats what I thought to but I mean there is a 1/2" ball valve wide open venting an Oringed piston that has minimal leakage and one 3/32 equalization hole.

Could be my design too. My barrel side seat has only 1/2" gap before my piston sleeve is there coming about 1/2" in covering the reservoir side of the Tee. Could be some weird turbulence causing the piston to come back and seal again.



I'm now wondering if the air flow is sucking the piston back to sealing posistion? Maby you should increase the piston travel in order to allow the clearance needed to exhaust the storage chamber/s fully. You said your leaving the ball valve open? Maby put you hand over the pilot exhaust port, that way when you fire your hand can be covering the port to stop air from returning back into the pilot chamber. Stopping all return air will stop/slow down the return of the piston.
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Unread postAuthor: pyro6314 » Sun Apr 22, 2007 1:29 pm

rad14701 wrote:Using calculations, which I'm sure yours will fail...


First off, where the hell do you get off telling me I dont know what Im talking about...obviously your work "on paper" is incorrect; let alone your whole theory on the forces involved that even make a piston valve work.

How do you suppose a Mauler valve works then? There is even less area difference between the barrel and the outer piston than in my setup.

So why dont you take up your argument with sgort instead? See what he has to say...
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Unread postAuthor: rad14701 » Sun Apr 22, 2007 5:29 pm

@pyro6314

I may used the wrong choice of words, but you really didn't need to be quite as huffy with your reply... I don't know who you are, or who you think you are, but I can assure you that I am not just some snot-nosed kid that you are talking to... If you have better calculations, feel free to share them... It sure doesn't reflect well on you to ask for help and then offend those who are willing to offer up some help... For what it's worth, I've built a lot more different mechanical contraptions than just a simple pneumatic cannon or two so I know what I know... For now all I know about you is that you're whiner... If you spoke out like that in my forum, to anyone, you'd be gone...
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