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Distance the piston needs to move?

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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Distance the piston needs to move?

Unread postAuthor: Metalcookie » Wed Mar 10, 2010 6:33 pm

Hello all, I'm new to the forums. Some might recognize me from the benheck forums.

Anyways, my attempts at building a piston valve failed, but I'm willing to try again. I don't give up easily. I was wondering, how far does the piston need to move back in order for good airflow? I've heard from Klugeboy that your piston needs to move 1/4 the diameter of the port. So, with a 1 inch port, would it need to move 1/4 of an inch to get good airflow?
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Unread postAuthor: jrrdw » Wed Mar 10, 2010 6:38 pm

Metalcookie wrote:So, with a 1 inch port, would it need to move 1/4 of an inch to get good airflow?


Yes, it's called the D4 rule.

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Re: Distance the piston needs to move?

Unread postAuthor: velocity3x » Wed Mar 10, 2010 7:09 pm

Metalcookie wrote:how far does the piston need to move back in order for good airflow?


Depends on the kind of performance you hope to achieve. I shoot high pressure (1000psi) in high volume. I make my valves according to the "D1" rule! D2 or D4 will launch a projectile but, D1 gives serious performance.
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Unread postAuthor: Metalcookie » Wed Mar 10, 2010 8:12 pm

Which rule would you recommend for a noob like myself?
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Unread postAuthor: clemsonguy1125 » Wed Mar 10, 2010 8:24 pm

Well the less room you give your piston to move back the smaller the pilot volume so 1/4 inch sounds good but I wouldnt suggest more than 3/8 inch if you want more space to move back
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Unread postAuthor: MrCrowley » Thu Mar 11, 2010 12:02 am

The 'D4' rule is mainly for coaxials. I like to give a bit more for barrel sealing pistons in a tee configuration. As long as your pilot valve is decent and your pilot volume isn't huge, you should be fine giving your piston a little bit extra room to move back.
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Unread postAuthor: jackssmirkingrevenge » Thu Mar 11, 2010 1:32 am

Have a read through this post.
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Unread postAuthor: grock » Thu Mar 11, 2010 2:05 am

well, on mine i used some math. if you have a 3 inch (pretend its ACTUALLY 3.0") barrel, you can figure the surface area of the port is ~7 square inches. the circumfrence of the pipe is about 9.5" therefore, (7/9.5) the piston would only need to move back .75 of an inch, to get (IMO) max air flow. it worked pretty well for me, threw some 4 lb projectiles pretty damn hard, the gun's recoil tilted my stand (which happened to be a forklift)
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Unread postAuthor: jackssmirkingrevenge » Thu Mar 11, 2010 2:10 am

grock wrote:well, on mine i used some math. if you have a 3 inch (pretend its ACTUALLY 3.0") barrel, you can figure the surface area of the port is ~7 square inches. the circumfrence of the pipe is about 9.5" therefore, (7/9.5) the piston would only need to move back .75 of an inch, to get (IMO) max air flow.


That is pretty much where the D/4 rule came from, 0.75" is 1/4 of 3" ;)
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Unread postAuthor: psycix » Thu Mar 11, 2010 7:19 am

For coaxials:
D/4 + a little
D/4 origins from the math grock posted. However, always give it a little bit more to avoid restriction and to make up for things like piston bounce, sealing face flapping, air friction etc.

For T-valves, porting a bit smaller than the piston OD and T ID.
D/2 + a little
The air can now only come from one side (180*) instead of from all sides (360*), like in a coax.
D/2 should do it, and then throw in a little extra to avoid restrictions.
D/3 or larger recommended, especially on larger portings.

For T-valves, porting close to T ID:
D/1 + a little
When the chamber side approaches the porting (such as porting = 90% of piston dia) you will need to open it completely for full flow, and then a bit more to avoid restrictions.
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Unread postAuthor: grock » Thu Mar 11, 2010 1:16 pm

jackssmirkingrevenge wrote:
grock wrote:well, on mine i used some math. if you have a 3 inch (pretend its ACTUALLY 3.0") barrel, you can figure the surface area of the port is ~7 square inches. the circumfrence of the pipe is about 9.5" therefore, (7/9.5) the piston would only need to move back .75 of an inch, to get (IMO) max air flow.


That is pretty much where the D/4 rule came from, 0.75" is 1/4 of 3" ;)


thought that was just coincidence. didn't know it usually worked. funny though
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Unread postAuthor: jackssmirkingrevenge » Thu Mar 11, 2010 1:22 pm

Yep, I had illustrated the math here.
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Unread postAuthor: Metalcookie » Thu Mar 11, 2010 10:03 pm

Thanks for the input guys :D

I have but one more question: what would be a good material to make the piston out of? I was thinking either a 1 1/4 inch coupler or a piece of molded hot glue.
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Unread postAuthor: jackssmirkingrevenge » Fri Mar 12, 2010 12:13 am

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Unread postAuthor: Metalcookie » Sat Mar 13, 2010 9:21 pm

Guys thanks so much for your help! I built a working piston valve! :D I used spare parts from mah other cannon for most of the parts, and tested it at around 75 psi. I can't really estimate what kind of performance it has, because It's really dark out, and I didn't have proper spuds. I'm going out of town all this week, but will be back friday. I'll post pics and what not next weekend.
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