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How far your piston should travel

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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How far your piston should travel

Unread postAuthor: Velocity » Sat Jan 19, 2008 11:46 am

I was not really sure where to put this post, but if a moderator feels that there is a better section that it should be in, feel free to move it.

A lot of people new to pistons always ask how far a piston should be able to travel back, and the response from most members who are experienced with piston valves is the diameter of the barrel divided by 4 (d/4). This number seems way too small to many, who believe the piston should need to travel back much more in order to achieve full flow. Using basic physics/math, here is an explanation as to why the d/4 rule is correct.

Consider the amount of air which can flow through the barrel. The amount of air which can flow through is equal to the cross-sectional area of the barrel. In our case, that cross-sectional area is circular, meaning the cross-sectional area would be equal to (pi)(r^2).

When the piston is fully open, consider the amount of air which can flow through the gap created by the piston. Image this gap is a cylinder of air. The amount of air which can flow through that cylinder would be equal to the surface area of the cylinder, minus the top and bottom. Therefore, the cross-sectional area would be equal to 2(pi)(r)(h), where h is the amount of piston travel, effectively the height of the cylinder.

To achieve maximum flow and maximum efficiency, you want the amount of air which can flow through the gap created by the piston to equal the amount of air that can flow through the barrel. Therefore:

(pi)(r^2) = 2(pi)(r)(h); divide by pi
r^2 = 2rh; divide by r
r = 2h; solve for h
h = r/2; replace r with d/2 (radius = 1/2 diameter)
h = (d/2)/2
h = d/4

And there you have it. The piston travel should equal the diameter of the barrel divided by four. If for some reason I did something wrong, please correct me. I just was thinking about why that rule made sense last night before I fell asleep, and this is what occurred to me. I thought this would be a helpful post for those who are just getting into pistons.
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Unread postAuthor: jackssmirkingrevenge » Sat Jan 19, 2008 11:49 am

I thought we had already established the math? Nice of you to put it in a dedicated post though :)
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Unread postAuthor: pizlo » Sat Jan 19, 2008 11:58 am

when will there be a huge compiled piston thread with the animations, and lots of FAQ's?
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Unread postAuthor: Velocity » Sat Jan 19, 2008 12:25 pm

jackssmirkingrevenge wrote:I thought we had already established the math? Nice of you to put it in a dedicated post though :)


Yeah, I was sure that the math had been established somewhere... but I thought a single post would help people find it with the search function. And maybe a dedicated post might deserve a sticky :lol: .

I don't think a single thread with all types of piston stuff would be too good, because it would be too long... however, a thread with links to all of the piston information (clide's piston diagrams, this piston travel thread, how to build certain types, etc) would certainly be beneficial.
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Unread postAuthor: jackssmirkingrevenge » Sat Jan 19, 2008 12:27 pm

After experimenting with burst disks I had even qualified it as a rule that was not absolute...
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Unread postAuthor: jrrdw » Sat Jan 19, 2008 1:13 pm

So anything past that is a waste/overkill/bad for proformance?
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Unread postAuthor: jackssmirkingrevenge » Sat Jan 19, 2008 1:17 pm

jrrdw wrote:So anything past that is a waste/overkill/bad for proformance?


as long as the piston doesn't bounce, yes.
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Unread postAuthor: bigbob12345 » Sat Jan 19, 2008 1:26 pm

wow I didnt know this I always assumed the farther it travels back the more flow and power I guess I will go and shorten the distince they travel back.
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Unread postAuthor: Hotwired » Sat Jan 19, 2008 1:30 pm

Of course for chamber sealing valves it's quite different and at a minimum would need travel equal to the diameter of the chamber/barrel (whichever is smallest) :)
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Unread postAuthor: ALIHISGREAT » Sat Jan 19, 2008 1:31 pm

bigbob12345 wrote:wow I didnt know this I always assumed the farther it travels back the more flow and power I guess I will go and shorten the distince they travel back.


i always assumed that too :roll: even though when you think about it there is a maximum amount of air that can enter the barrel at once although does the 1/4" rule take into account velocity of the air because maybe more piston travel would allow the air to enter the barrel more freely :?:
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Unread postAuthor: SpudBlaster15 » Sat Jan 19, 2008 1:35 pm

For barrel sealing "T" valves, the orifice through which the air flows towards the barrel port really should be modeled as a rectangle rather than the lateral area of a cylinder. In this case;

pi*r<sub>barrel</sub><sup>2</sup> = l<sub>port</sub>*w<sub>port</sub>

Solving for the length of the port (piston travel), we get;

l<sub>port</sub> = pi*r<sub>barrel</sub><sup>2</sup>/w<sub>port</sub>

For coaxial launchers, the d/4 calculation is accurate, but for launcher with "T" valves, using such a design would probably limit flow quite a bit.
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Unread postAuthor: jrrdw » Sat Jan 19, 2008 2:50 pm

My valve is built into a cross. Would you figure that as a "t" or a coaxle? I guess i'm the one in the crowd this go around! :P
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Unread postAuthor: SpudBlaster15 » Sat Jan 19, 2008 7:25 pm

Jrrdw, could you provide a more thorough explanation of your valve's design?
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Unread postAuthor: ALIHISGREAT » Sat Jan 19, 2008 7:33 pm

SpudBlaster15 wrote:Jrrdw, could you provide a more thorough explanation of your valve's design?


well it would make sense that the chambers are either side of the cross, the barrel is at the front and the pilot ant the back :)
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Unread postAuthor: jrrdw » Sun Jan 20, 2008 9:15 am

ALIHISGREAT wrote:
SpudBlaster15 wrote:Jrrdw, could you provide a more thorough explanation of your valve's design?


well it would make sense that the chambers are either side of the cross, the barrel is at the front and the pilot ant the back :)


Bingo! The chambers are 5' of 3" diameter. The barrel is 8' of 1" diameter. 2" piston diameter, cylinder lenth/pilot chamber 8"/10"s long. Piston travle approx 4"/6"s. Piloted with 1" modded sprinkler valve bought from bcarms.com.
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