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piston set up q

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: clide » Sun Apr 22, 2007 4:18 pm

Yeah, that was me, and yeah, you are pretty much right about the stuff you said rmich. Although chamber sealing valves don't always actuate right when you start venting, they are subject to the same kind of relation. If you had a 3" piston with a 2" sealing port, the chamber sealing setup would be faster because it allows the pilot pressure to decrease more before firing than a barrel sealing valve would.

The downside to making the piston diameter really close to the outlet diameter is that it requires a better piston and/or a better pilot valve. Otherwise you might be venting so long that you drop a significant amount of your chamber pressure before the gun fires.

You could fire a gun that had the same piston and sealing face diameters with a vacuum, but I would really, really say it is not worth the effort. Which brings me to my next point. In a typical gun, there is going to be hardly any performance difference between a valve that opens very fast, very very fast, or extremely very very fast. The only time you are going to have any difference is if your valve is bouncing back or not actuating fully, and this would take something like a chamber sealing valve with the seat very close to the piston diameter and a small exhaust valve. (Or so I thought, the bouncing piston thread has me stumped, but I suspect it has something to do with too much blow by around the piston because I have never seen something like that)

So I would say that people shouldn't bother making their piston diameter very close to the sealing port diameter, because unless you have a piston with o-rings and a check valve, the effect of venting off more chamber pressure will outweigh any opening speed gains they might have.
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Unread postAuthor: rad14701 » Sun Apr 22, 2007 6:08 pm


Yes, reducing the pilot area volume should improve piloting performance...


Using a vacuum would be bulkier than it would be worth... The problem is that a vacuum chamber is only going to be effective until the vacuum chamber reaches atmospheric pressure, after which it will equalize with the pilot pressure... You will have much more pressurized volume than vacuum volume, unless your vacuum chamber is on the large side... Novel concept, but not practical... There are easier ways to effectively pilot a pneumatic...


Your logic has some merits... The main thing to differentiate is whether you consider barrel seal diameter or actual piston diameter... Different piston physical configurations can have drastically different levels of overall performance... Oh, and your ramblings related to discussion related to promoting spudguns/pneumatics are always welcomed... Keep exercising the gray matter...
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Unread postAuthor: deusXmachina » Sun Apr 22, 2007 6:58 pm

You can calculate the pressure (of the pilot) that is required to actuate the valve by finding the area that is sealed (i.e. the barrel sealing surface) and the area of the pilot area minus the area of the OD of the barrel pipe. You divide the first number by the second and multiply by 100, this gives you the percentage of the pressure required. The closer to 100 the easier it will actuate (if it is too close to 100 it won't seal to begin with)

Say you have a 3" barrel in a 4" pipe. 100%*(pi2^2-pi1.75^2)/(pi1.5^2) = 23%. The pressure of the pilot would have to be less than 23% of the chamber (at that instant, not the original pressure) to even start moving. If you have a leak around the piston, you might never reach the required ratio.
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Unread postAuthor: Velocity » Sun Apr 22, 2007 8:30 pm

Alright, I am glad to see that I am on the right track here. Assuming that a piston valve was made with o-rings and a check valve, I think that it would be able to have a full 2" porting valve in a 2" tee. I have talked about this concept before. The trick is, you use 2" SCH 80 for the sealing face, and 2" SCH 40 for the piston track. If this will make a barrel sealing piston valve actuate faster, then I am definitely incorporating it in my next design (which will appear in a few weeks)
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