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Piston length ?

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 length ?

Unread postAuthor: c9d » Thu May 13, 2010 11:29 pm

I am making a 3" piston for a coaxial setup and was wondering if the length of the piston mattered? My pistons overall length currently is 2" from front to back.
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Unread postAuthor: deathbyDWV » Thu May 13, 2010 11:43 pm

I would suggest having the piston at least as long as the diamater of the pipe it's sliding in to make sure it doesn't get jammed. Otherwise, it will could slip crooked when pressurized or piloted and jam... Although, as long as it's lubricated good and fits correctly it will probably still work...
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Unread postAuthor: Gippeto » Thu May 13, 2010 11:49 pm

Could be a huge topic to delve into details, this vs that, etc. ...It's sufficient to say "It'll work fine at 2" length." :)


Edit: If the piston is a sloppy enough fit that jamming becomes a real issue...it's FAR too sloppy of a fit to work anyway. :wink:
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Unread postAuthor: c9d » Fri May 14, 2010 12:07 am

@Gipetto I tried to go with your sealing piston idea you suggested to me before. I will try to get some pics up when I finish it.
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Unread postAuthor: Gippeto » Fri May 14, 2010 12:44 am

The u-cup? That'll work nicely I'm thinking. :D
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Unread postAuthor: c9d » Fri May 14, 2010 12:49 am

Its not exactly a "u cup seal", but as close to it I can get using stuff from the hardware store.
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Unread postAuthor: mark.f » Fri May 14, 2010 7:25 am

Well, here's a simple way to do it...

Measure the distance from the back of your barrel to the front of your plug/cap/bushing/etc. that closes off the back of the launcher.

Take that distance, subtract your bumper length (when compressed if it is foamy) and your desired opening distance. This will give you the overall length your piston should be.

Extremely simple.

Now, if your gun isn't built yet, this gives you the opportunity to adjust your piston length to reduce weight. Like deathbyDWV said, make it long enough so that it doesn't jam, then add your bumper length and opening distance to the piston, and adjust how far your barrel goes back into the launcher accordingly.
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Unread postAuthor: Technician1002 » Fri May 14, 2010 11:43 am

The shorter the length the less mass. Shorter is better with that yardstick.

Longer gives more area to align it and more area to provide a seal. Longer is better with that yardstick.

A good balance is somewhere in between. Short to be light and long enough to keep alignment and seal in the sleeve it retracts into. There is no law stating the piston must be longer than it's diameter.

A traditional piston makes a barrel seal on it's face, where a QDV (spool valve) makes seals on the side. As such a QDV will use a longer heavier piston than is necessary for a traditional piston valve.

Many commercial piston valves have the piston length well below 1/2 the diameter. This includes the Martin Blaster valves (commercial hopper blasters) and QEV's.

For ease of building with common home tools, a longer pistion is often built to overcome the precision requirements of short pistons. There is nothing wrong with this trade-off. Build it so it works is rule number one.
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Unread postAuthor: john bunsenburner » Fri May 14, 2010 2:00 pm

It is funny no one mentioned it but: For a given pilot volume(without the piston) a larger piston will decrease the amount of air vented, and also decreases the piston travel. Thus if you have a gun built already a longer piston may just be preferable to a shorter one.

(The above is a hypothesis and is not based on any sound evidence, if my reasoning is faulty please tell me so)
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Unread postAuthor: c9d » Sat May 15, 2010 10:27 pm

What would you guys recommend to lubricate this piston? Something that doesnt affect PVC or destroy rubber
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Unread postAuthor: Gun Freak » Sat May 15, 2010 10:54 pm

I use vaseline.
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Unread postAuthor: Gippeto » Sat May 15, 2010 11:56 pm

I've been using MolyKote 55 on my builds. It's silicone based, and pretty much non reactive. It does swell buna o-rings slightly, but hasn't caused any problems thus far.

Other silicone based lubes are available at auto parts stores...dielectric grease being one....a bit on the thick side, but a "dab 'l do ya". :)

Pretty sure the disc brake caliper lube is also silicone based.
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Unread postAuthor: Technician1002 » Sun May 16, 2010 1:28 am

With the plastics an plumbing o rings I use, I use either a plumbing grease or white lithium grease.
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Unread postAuthor: Slauma » Sun May 16, 2010 11:44 pm

I use lithium grease as well. From what I've read and seen it plays nicely with all of the most common construction materials used in spudding; pvc, cpvc, abs, metals, o-rings of various material
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Unread postAuthor: TheLostOtter » Mon May 17, 2010 1:47 am

I'm using a 3" ABS endcap inside a 4" PVC chamber for my coaxial cannon. Very reliable and the piston is pretty much a chode lol, maybe 2.5" long and 3.5" diameter. I used a few coats of black rubberized spray paint to get it to seal better. It's still wasn't a perfect fit but the thing shoots so hard I'll just assume it can't get much better.

And I'm using vegetable oil to lube the thing. Didn't want to use anything that would react with the PVC so I figured that would be my safest bet using something I already had at home.
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