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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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Piston vs. Piston [Update: video comparison]

Unread postAuthor: MRR » Sat Feb 21, 2009 6:55 am

Well, I am new at this forum but I visit this side for quite a while on the search for new ideas and designs.

I read a lot about piston valves and the importance of surface in this issue. So one day I asked myself; if surface is so important than why not providing surface to a piston?

In the beginning I started with "Full Body Pistons" made of hot glue. They are easy to make, solid and reliable. The Problem is that you need a certain amount of negative pressure (if the barrel is not significant smaller than the piston) to make the piston hit back.

Image

So I came up with the idea to build a piston that provides "Multi Surface Sections".

Image

The results are astonishing:
The hot glue piston need a 1/2" ball valve to work.
The metal piston works with a simple blow gun (with better performance).

The "Multi Surface Piston" hits back faster than the air is able to exhaust from the pilot blow gun (it's like an air bumper).

I don't claim that this concept works the way I described (because I don't have a real scientific evidence) but the results speak for themselves.

[UPDATE]
[youtube]http://www.youtube.com/watch?v=JPfLwk7W0Gc[/youtube]
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Last edited by MRR on Sun Feb 22, 2009 10:55 am, edited 1 time in total.

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Unread postAuthor: littlebro05 » Sat Feb 21, 2009 7:20 am

Wow... That is awesome! You gotta film piston vs. piston... it'll be even more convincing... I might actually copy your idea if you don't mind XD.... I'll start with hot glue first though since well... never built a piston in my life lol.

Edit: Oops wait now I get it! More volume for and SA what ever you want to call it so more pressure to push back :D... But imagine using a 1/2" ball valve against that new piston you created... imagine da powa!!
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Last edited by littlebro05 on Sat Feb 21, 2009 7:24 am, edited 1 time in total.
It's Nerf or Nothing.

Don't start crying when I play this as my hobby, Paintball, Airsoft and Nerf are equally good hobbies, except you can play Nerf in public. Good day! Hmph!
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Unread postAuthor: Ragnarok » Sat Feb 21, 2009 7:20 am

Um... afraid not. Bear in mind, pressure acts on all surfaces equally - the force on the front surface of those washers is countered by the pressure on the other side. Only the pressure differences between the front surface and rear surface can possibly count.

If it opens any more efficiently, the extra surfaces have nothing to do with it. It is however possible that you have lightened the piston, which might account for it's improvement (although given that's mostly steel you've replaced it with, I doubt it)- or possibly simply created one that fits better.

It's nothing to do with the extra surfaces.

EDIT: We've actually had something similar around for a while, anyway - generally known as "bolt pistons" - here's one I was half way through making.
It cuts weight over a solid piston, and is pretty simple to make - it also allows adding of bumper springs and rubber washers which can be configured like a check valve. Not uncommon on small-ish piston valves.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: ALIHISGREAT » Sat Feb 21, 2009 7:31 am

damn Rag beat me to the physiscs lesson :roll:
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Unread postAuthor: littlebro05 » Sat Feb 21, 2009 7:51 am

Hey! Wait a minute... Aren't you McRedRooster on Youtube? Because I just realised that you piston is very similar to that of on youtube.

If you are I'm planning to make one of your piston guns :D.
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It's Nerf or Nothing.

Don't start crying when I play this as my hobby, Paintball, Airsoft and Nerf are equally good hobbies, except you can play Nerf in public. Good day! Hmph!
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Unread postAuthor: MRR » Sat Feb 21, 2009 8:25 am

Of course you are right. When I said "one day I had the idea" I meant a really long time ago when I was dumber about this subject (reading educates :wink:).
The metal piston is actually heavier than the hot glue piston and the fit is compareable.

Hypothesis:
The hot glue part at the end has a bigger diameter than the washers (piston housing: 21mm I.D. Washers: 20mm). My suggestion is that the air, when you open the pilot, interfere between the washers and "grab" the piston.

Image
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Unread postAuthor: littlebro05 » Sat Feb 21, 2009 8:39 am

Have you tested both pistons under the same conditions where the variable is the different piston. When I watched you video I finally knew the meaning of "pilot valve". So did you used the same barrel, same ammunition and same pilot valve of 1/2" piston? Because I'd assume that the ball valve would work better than the blow gun... But just a suggestion... I've never really built an advanced air gun before... I'm only starting off a ball valve actuated, then to the advanced ball valve actuated. But after that I'm going work on your inspiration :).
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Don't start crying when I play this as my hobby, Paintball, Airsoft and Nerf are equally good hobbies, except you can play Nerf in public. Good day! Hmph!
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Unread postAuthor: Ragnarok » Sat Feb 21, 2009 8:40 am

MRR wrote:My suggestion is that the air, when you open the pilot, interfere between the washers and "grab" the piston.

Although to a minor extent an effect like that might be exhibited (indeed, that is much what the wind does, converting flow speed to pressure), the air flow required to make a significant difference would be so much that you'd be wasting most of your air on opening the piston - nor would a blowgun be up to providing those flows.

Indeed, in some rare circumstances, it might make a small difference to opening time to allow air flow in such manner, but would actually ruin efficiency to do it, and it would require a much larger pilot valve.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: MRR » Sat Feb 21, 2009 8:58 am

I tested both pistons on the same setup with very different results.

Hot glue piston doesn't move with blow gun pilot.
Metal piston does.

Hot glue piston works fine with ball valve.
Metal piston works "great" with ball valve.

Is it a real good design for pistons, just a coincidence or does hot glue pistons simply suck? I have no idea! All I can say is that the performance of the metal piston is way better.
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Unread postAuthor: Sticky_Tape » Sat Feb 21, 2009 9:27 am

I was thinking here and, I don't think The would work because of what rag said. I thought of that before I saw rag's post. But I think The OD of the barrel compared to the diameter of the piston has some kind of effect like popoffs. Think, the gun is fully charged and the olny force pushing the piston onto the barrel is airpressure duh but the pressure is olny working on the cross sectional surface area of the barrel but on the back of the piston the other surfaces cancel each other out hard to imagine. But when the gun is piloted the pressure has to drop enough to make the piston come away from the barrel due to the negative pressure but what happens when the piston comes away from the barrel? The equal ammount of surface area the end of the barrel has is revealed on the front of the piston sort of like a pop off maybe that is why piston valves open so fast. Like pop offs. BTW what's with the large equilization holes?
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You can tell how awesome a cannon is by the pressure used.
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Unread postAuthor: MRR » Sat Feb 21, 2009 9:53 am

When the pressure between the washers is equalized on a pressurized gun and you open the pilot, isn't it possible that the gaps give an advantage when the piston is pulled into the direction of the lowest resistance?

I'm going to reassemble my gun and film a comparison.
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Unread postAuthor: jeepkahn » Sat Feb 21, 2009 10:42 am

If you want an extremely light, excellently sealing piston, take a look at how I make mine... a cup, so to speak...
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Attachments
coax piston.jpg
notice the rubber doorknob bumper cushions the slam backwards and acts as a washer to keep the rubber sealing disc from flexing towards the pilot side...
inside of piston.jpg
notice the rubber inside the cup seals the barrel, and the shape of the cup keeps everything in alignment with the barrel and chamer... extremely light, so light that the only reason I use a bumber is to keep the cup from wearing where it hits the endcap...

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Unread postAuthor: MRR » Sat Feb 21, 2009 12:02 pm

This piston design doesn't work on my pneumatic. The piston is in a 3/4" tee, sealing a hose barb. This piston looks good for a coax.
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Unread postAuthor: jeepkahn » Sat Feb 21, 2009 12:25 pm

that would explain why the stacked washer setup beats the solid piston, with the solid piston you are dealing with not enough surface area being acted upon by the chamber pressure to unseat the piston from the barrel...

copied from my other post...

edit: I know most on here know this but for any noobs I'll post it. This is for any solid/non-toolie piston valve.

the force sealing the barrel is pressure times the surface area equal to what is exposed to atmosphere via the barrel.

the force (with pilot opened) that is trying to move the piston backwards/open is the pressure times the total surface area minus the surface area hidden from the chamber pressure by the barrel outer diameter.

if the first number is higher than the second, the piston will not move, or if they are very close numerically the piston will not move quickly...

I think people forget this sometimes...
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interact.jpg
goes back to my dissertation on surface areas and calculating opposing forces in another thread..

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Unread postAuthor: FishBoy » Sat Feb 21, 2009 5:31 pm

I have a question about your tests. Were either of the pistons lubricated druing the test? If not, that would explain the speed difference since hot glue tends to "grab" surfaces in my experience.
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