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Piston vs. Piston [Update: video comparison]

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: jeepkahn » Tue Feb 24, 2009 11:18 am

I can obviously see that either a. no one reads my posts, or b. no one understands my posts...

several of you have shown with diagrams(ali and mrr) what I was explaining... re-read my post and you'll understand(at least one of the reasons) why the solid piston isn't working as well as the washer setup...
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Unread postAuthor: jackssmirkingrevenge » Tue Feb 24, 2009 11:28 am

This is what your diagram should have looked like, as has been pointed out already the red forces cancel out.

I would think that the performance advantage you're experiencing is much more a case of reduced frictional drag than anything else.
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Unread postAuthor: john bunsenburner » Tue Feb 24, 2009 12:04 pm

I think the followign is the most likely think happening here:
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Last edited by john bunsenburner on Tue Feb 24, 2009 2:26 pm, edited 2 times in total.
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Unread postAuthor: MRR » Tue Feb 24, 2009 12:06 pm

@jsr If I understand your post right the piston is "ONLY" actuated by the negative pressure when piloted ?
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Unread postAuthor: jeepkahn » Tue Feb 24, 2009 12:19 pm

jackssmirkingrevenge wrote:This is what your diagram should have looked like, as has been pointed out already the red forces cancel out.

I would think that the performance advantage you're experiencing is much more a case of reduced frictional drag than anything else.


They cancel each other when you look at it from a static standpoint,but in reality they don't EXACTLY cancel each other out... you have to take into consideration the wall thickness of the barrel, because it's the inner diameter of the barrel that controls the sealing force and it's the surface area of the outer diameter of the barrel that you have to subtract from the piston surface area when calculating for the unseating force, also you have to account for the fact that with a solid piston there is going to be a drop in pressure around the sealing face that will try to suck the piston back towards the barrel, and the washer setup will help combat that because the chamber pressure will be acting fully on the piston to continue the opening action... the smaller the bore of the gun as well as barrel wall thickness play into account of these dynamics and are effected greatly if you do the math to calculate the chamber i.d. versus barrel i.d. minus the barrel o.d. ...

but on larger bore guns these factors can play a huge role in piston speed when you factor the relatively thick walls of sch40 pvc and if you are running a relatively small diameter chamber bore to barrel diameter ratio...
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Unread postAuthor: sniper hero » Tue Feb 24, 2009 12:41 pm

I didn't read the hole thread but I got the same blowgun and modded for better results you could do the same although you have to turn the blowgun around.
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Unread postAuthor: jackssmirkingrevenge » Tue Feb 24, 2009 12:53 pm

MRR wrote:@jsr If I understand your post right the piston is "ONLY" actuated by the negative pressure when piloted ?


That is my understanding. If it weren't true, then you could build a piston of that design that is almost identical in diameter to the barrel diameter, and it would work beautifully, but I'm 100% sure this won't work in practice. Anyone who argues otherwise is free to build one and prove me wrong ;)

with a solid piston there is going to be a drop in pressure around the sealing face that will try to suck the piston back towards the barrel


True, but nothing like the pressure drop happening in the pilot chamber ;) and in any case, the "spool" piston will have no effect compared to a solid one, because for all intents and purposes as far as pressure is concerned, what's acting on one side is acting on the other so the force gained or lost is zero.
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Unread postAuthor: john bunsenburner » Tue Feb 24, 2009 2:28 pm

So jsr, are you sure what I suggested isn't happening? After all the piston is evidently opening faster and there has to a reason for that, the above picture is the only way i can explain that.
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Unread postAuthor: jeepkahn » Tue Feb 24, 2009 2:38 pm

Let me start by saying that I hope nobody takes anything I say as argueing, I like to be involved in intelligent discussions because everybody wins if it's done without anyone taking offense...

JSR, you are correct about cancelling out of one force by the other, but only in a static situation, the dynamics will effect the piston once the seal is broken with the barrel.... with the "spool" piston,once the seal is broken, it changes the pressure from acting on the piston less the surface area hidden by the barrel, to the pressure is acting on the entire piston surface area, with a solid piston, because of dynamics, once the seal is broken you are dealing with less force acting to force the piston back, because of the pressure drop across the barrel rim causing the piston to move slower due the effect of lower pressure caused by the exiting gases acting in oppostion to the lower pressure in the pilot chamber...

Remember, we're talking about why one opens faster, not why one opens and the other doesn't... nobodies debating how they work, we're discussing why one works faster....
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Unread postAuthor: Ragnarok » Tue Feb 24, 2009 2:48 pm

john bunsenburner wrote:Damn it guys you really dont get it do you? I drew something that should clear it up:

You clearly don't either. There are no ways in which one can boost piston force by adding surface area(s), because the maths behind it means that the extra forces must ALWAYS cancel out.
That is not to say that they might not affect the opening time in other manners, but those will not be improvements to surface area.

To counter your picture with the tiny seat diameter, one actually gains opening force from having a piston where the seat diameter is closer (but obviously not equal to) to the piston diameter - as long as the piloting and blow-by can sustain such a thing.

Simple fact of the matter, to speed up a piston, you can only get so much opening force, which hits it's theoretical maximum at chamber pressure times piston cross sectional area (you can't of course actually achieve said force though, because the piston wouldn't actually actuate) - so the next best thing is weight, making it as spartan as possible.
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Unread postAuthor: john bunsenburner » Tue Feb 24, 2009 2:56 pm

SO then you try justifing his second piston opening faster, that was my best guess, as it made sense to me. Mind explaining why exctly the forces have to cancel out and why oen can benifit from a seat diameter closer, but not equal to, the piston diameter? All I see from my diagram is that the forces dont cancel out and so the piston open faster, please explain why i a wrong.
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Unread postAuthor: ALIHISGREAT » Tue Feb 24, 2009 3:20 pm

john bunsenburner wrote:SO then you try justifing his second piston opening faster, that was my best guess, as it made sense to me. Mind explaining why exctly the forces have to cancel out and why oen can benifit from a seat diameter closer, but not equal to, the piston diameter? All I see from my diagram is that the forces dont cancel out and so the piston open faster, please explain why i a wrong.


OMG do i have to expalin in words of no more than two syllables? :D :wink:

It is basically the same as the front of the solid piston (actually a little less because of the bolt...)

Pressure acts in all directions so if the gun is filled to 100psi... then the back of the sealing face will be pushed forward by this force...

This pressure acting forward cancels out the extra pressure acting backward on the front of the sealing face....

This means that its only really the front of the main piston body that is being pushed back....

and the front of the main piston body is the same area as the front of the solid piston... so there is no gain....


Now... having said that, it could be the fact that more surface area can be affected quicker with the bolt piston that makes it perform better... the solid piston has to open for the whole surface area to be taken advantage of, but the bolt piston has a higher immediate effective surface area?

but then the cancelling forces thing comes in to play again?!?!
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Unread postAuthor: Ragnarok » Tue Feb 24, 2009 3:24 pm

Is it a n00b? Is it a wolf?

No, it's Explanation Man to the rescue! (I should get a shirt made that says that...)

john bunsenburner wrote:SO then you try justifing his second piston opening faster.

I believe I had. It clearly takes longer to fill with the second piston, so I suspect a tighter fit. It's also probable the friction is lower.

Mind explaining why exctly the forces have to cancel out and why oen can benifit from a seat diameter closer, but not equal to, the piston diameter?

Point 1 (Version 1): If the forces didn't cancel out, you'd be getting something for nothing, and perpetual motion would be possible. The laws of thermodynamics prevent perpetual motion, so hence, it cannot be possible.

Point 1 (Version 2): Simple geometry states that for any shape, the angles must add up to 360 degrees. Given pressure always acts perpendicular to any surface, this means that the forces on concave parts of a shape must cancel. It is possible to get forces from pressure to be unequal on convex parts of a shapes and that's what why piston valves work at all.

Point 1 (Version 4... no, wait... 3): You are claiming extra opening force but no increase in a closing force to counter it - if that were true, why would the piston not just naturally pop open?

Point 1 (Version 5): Don't let me eat pears. I hate pears. You know humans, they do stupid things, and I might get it into my head that it's a good idea to eat a pear. I don't want to wake up and have to taste that.

Point 2: A seat diameter closer to the piston diameter generates a lower "backwards" force when the piston is sealed, as less surface area is exposed to pressure.
That requires the pilot pressure to drop lower before actuation - hence when the piston finally moves, and exposes the full front of the sealing face, the net force is higher, because the forces from on the pilot side are lower.

All I see from my diagram is that the forces dont cancel out and so the piston open faster, please explain why i a wrong.

If the explanations above don't placate you, this a simple analogy. In fact, you can feel welcome to skip the rest...

...oh, sorry.

For a thought experiment, take the piston design. Put it in an isobaric environment. We'll use the atmosphere, it's close enough.
Does it have a tendency to start moving on its own?

It doesn't? Well then, that must mean all the pressure forces are cancelled, mustn't it? If there's not mysterious movement when it's just sitting in plain isobaric air, then there is no "free force". One is not going to suddenly appear when you put it into a launcher.
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Unread postAuthor: jeepkahn » Tue Feb 24, 2009 3:39 pm

john bunsenburner wrote:SO then you try justifing his second piston opening faster, that was my best guess, as it made sense to me. Mind explaining why exctly the forces have to cancel out and why oen can benifit from a seat diameter closer, but not equal to, the piston diameter? All I see from my diagram is that the forces dont cancel out and so the piston open faster, please explain why i a wrong.


The easiest way to "get it" is turn the gun "inside out" so to speak...

Take any pneumatic and attach equal vacuum cleaners to the pilot and the barrel, with a solid piston the barrel side vacuum cleaner is gonna try to hold the piston forward, the pilot vacuum is gonna try to pull it backwards, and even after the barrel is unseated the vacuum is still gonna be trying to pull the piston forward but the larger side(the pilot side) is gonna have more pull, so the piston will still move towards the pilot side just not as fast.

Now, with a spool valve, once the seal is broken, the extra airspace behind the barrel sealing face is going to allow air to escape from what would normally be dead space taken up by a solid piston, allowing the piston to slide all the way back before the barrelside vacuum has a chance to cancel out any of the pilot side vacuum...

that's why a piston with equal seat and piston won't work...
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Unread postAuthor: sputnick » Tue Feb 24, 2009 5:54 pm

ALIH-

MRR has made the diagram correctly, if you really look, every part of it has an opposite. The parts above the green line are irrelevant because there is the same surface area on either side.

So both of your diagrams are correct, just MRR's is a little less easy to understand.
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