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Piston Qn.

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: jackssmirkingrevenge » Fri Jul 27, 2007 6:20 am

Hmmmm makes sense but I still that a bit of extra travel would be beneficial....


Why?

or rather, por que?

Beyond the magical caliber/4 figure, you are getting absolutely no extra advantage in flow - simply increasing pilot volume, and also expanding the size of the air reservoir which in turn lowers the pressure.
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Unread postAuthor: Gepard » Fri Jul 27, 2007 6:23 am

'Cos it saves the gas going right around and then in,
Chances are you can't get exactly d/4 and so it's best to err on the side of performance.

Only talking about an extra 5-10mm not lots....

Michael
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Unread postAuthor: jackssmirkingrevenge » Fri Jul 27, 2007 8:07 am

Gepard wrote:'Cos it saves the gas going right around and then in


Sorry for being such an efficiency nazi but I can't stand waste. In practical terms, a few mm won't make *that* much of a difference, but since air is a limited power source compared to solid propellants, it makes sense (for me at least) to want to extract every available ft/lb from a specific launcher.

No matter how much the piston moves back the air in a coaxial is still going to have to change direction to exit through the barrel, I don't see how it will improve performance to have the piston move back further.
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Unread postAuthor: Gepard » Fri Jul 27, 2007 9:09 am

We talking about a co-axial? I thought we were talking about piston valves in general......

Michael
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Unread postAuthor: Modderxtrordanare » Fri Jul 27, 2007 10:14 am

Gepard wrote:We talking about a co-axial? I thought we were talking about piston valves in general......

Michael


Does it matter? The air still has to turn around 180°.
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Unread postAuthor: Gepard » Fri Jul 27, 2007 10:18 am

I don't see the need to carry on this thread....

Where does it go round 180 degrees in a 'normal' piston?

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Unread postAuthor: Modderxtrordanare » Fri Jul 27, 2007 10:21 am

Gepard wrote:I don't see the need to carry on this thread....

Where does it go round 180 degrees in a 'normal' piston?

Michael


If you have an over under set up like the majority of the barrel sealing piston valves that have been made, the air goes through two 90° bends. It's not an immediate 180° turn. But some of the air, such as in the pilot area outside of the barrel, does have to make a 180.

Edit: I just noticed Jack's avatar being animated, maybe I'm just retarded though... :?
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Unread postAuthor: Gepard » Fri Jul 27, 2007 10:24 am

Ah - true, but I was refering to the flow of the gas from the chamber to the barrel through the piston.

Michael
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Unread postAuthor: dongfang » Fri Jul 27, 2007 4:08 pm

Hi jack.....

for 1/4 of the caliber of travel, you get an available area of 2*pi*r*h, in this case 2*3.142*0.5*0.25, which equals 0.7855 in 2


You have a hangover all right.

What's pi doing in the formula for the area of a rectangle? Your valve passage is really pi times smaller than the barrel cross section.

Gepard:
Where does the 90 come into it?

I must ask you that? You were the one who started with a 90 somewhere :lol: :lol:

I guess it is our valve seat diameter.

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Unread postAuthor: Gepard » Fri Jul 27, 2007 4:13 pm

No - he's right, he's talking about a co-axial where gap the piston leave is a cylinder.

You mentioned 90 in your post. I even quoted you! :D

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Unread postAuthor: dongfang » Fri Jul 27, 2007 4:58 pm

Ok, coaxial. Maybe I should read what they write :lol:

Hey, that is sure in favor of the coaxial in efficiency.

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Unread postAuthor: jackssmirkingrevenge » Sun Jul 29, 2007 12:03 am

dongfang wrote:You have a hangover all right.

What's pi doing in the formula for the area of a rectangle? Your valve passage is really pi times smaller than the barrel cross section.


I'm the one who should be asking how the hell did you get a rectangle of flow into an area that is a cylinder ;p didn't drink *that* much y'know :)
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Unread postAuthor: flamerz14 » Sun Jul 29, 2007 3:05 am

:shock: :shock: :shock: ARRGGHHH. :evil: :evil: I hate maths.and i'm only 14. joo makes me miserable..so simple conclusion:

1.the smaller the pilot volume the better
2.light pistons are good
3.equilisation holes are for airtight pistons and should be <5mm

last one not complete... 1/4 or 3/4?
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Unread postAuthor: Modderxtrordanare » Sun Jul 29, 2007 5:37 am

flamerz14 wrote:1.the smaller the pilot volume the better
2.light pistons are good
3.equilisation holes are for airtight pistons and should be <5mm


Equalization holes should actually be just small enough as to have a smaller flow rate than that of your filling valve. (i.e. Schrader)

The 1/4 caliber = ideal piston travel distance is correct, as Jack both stated and proved. (Albeit with his nasty hang-over.)
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Unread postAuthor: jackssmirkingrevenge » Sun Jul 29, 2007 6:05 am

(Albeit with his nasty hang-over.)


Feeling a lot better now, cheers :D good to be back home and on my own PC, instead of having to share an internet cafe with drooling perverts browsing porn and death-mutes having animated webcam conversations that look like their trying to defend themselves from a particularly visciois swarm of wasps.

and flamerz, yeah, what Modder said ;)
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