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| judgment_arms |
 Posted: 10/20/2006 12:58 PM Post subject: plywood piston |
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The Judge
Joined: 17 Oct 2006
Posts: 1243
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Greetings all,
I have come here in order to tap into the vast and semi-limitless knowledge of Spud-files!
Ok, now on more serious note. Do you guys know if plywood will work for a piston?
I’m planning on making a coaxial air cannon soon and was trying to figure out what to use for the piston. In addition, I would also like to know if inner-tube rubber will work instead of neoprene for seals. |
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| joannaardway |
| Posted: 10/20/2006 13:35 PM Post subject: |
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Brigadier General
Joined: 09 Oct 2006
Posts: 961
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There might be problems with using plywood - wood is slightly porous. Plywood even more so. Try something else.
Inner tube rubber might work - I've used it before, but it is a little too thin to really seal anything but a nearly perfect fusion of piston and sealing face. Using multiple layers isn't very good either. |
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| judgment_arms |
| Posted: 10/20/2006 13:40 PM Post subject: |
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The Judge
Joined: 17 Oct 2006
Posts: 1243
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Yes it’s porous but don’t you drill bleed holes in it anyway?
Plus if you paint it, it ceases to be porous. |
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| joannaardway |
| Posted: 10/20/2006 13:51 PM Post subject: |
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Brigadier General
Joined: 09 Oct 2006
Posts: 961
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| Plywood is also very brittle - it won't stand up to the massive forces in a piston valve. |
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| schmanman |
| Posted: 10/20/2006 13:55 PM Post subject: |
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blabbering idiot
Joined: 25 Mar 2006
Posts: 1667
1006.17 Spud Bux
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| jrrdw did it without a problem. |
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| judgment_arms |
| Posted: 10/20/2006 14:05 PM Post subject: |
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The Judge
Joined: 17 Oct 2006
Posts: 1243
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| Plywood is used in sailplanes, boats, housing, and a plethora of other applications requiring lots of strength. Plywood is by no means brittle. I mean no offence to you my friend, you my have bean mistaken or, perhaps you thought I was talking about OSB or fiber board. |
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| MrCrowley |
| Posted: 10/20/2006 14:08 PM Post subject: |
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Blizzard of Ozz
Joined: 23 Jun 2006
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| how thick is the plywood? |
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| judgment_arms |
| Posted: 10/20/2006 14:13 PM Post subject: |
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The Judge
Joined: 17 Oct 2006
Posts: 1243
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It all depends on what I can get but it shall be at lest two inches thick.
Is that enough or do you believe it should be thicker? |
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| djt |
| Posted: 10/20/2006 16:19 PM Post subject: |
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Major
Joined: 11 Mar 2006
Posts: 387
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| two inches sounds plenty. and about the porous thing, you have to put a piece of neoprene or something on the face of the piston anyway and the rubber wont be porous |
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| joannaardway |
| Posted: 10/21/2006 12:18 PM Post subject: |
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Brigadier General
Joined: 09 Oct 2006
Posts: 961
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Ok, this is a simple question I and my twin use to help decide if a material is suitable for a piston:
"Would you trust (whatever) to be part of your chamber wall?"
If not, then don't use it - it's either not air tight enough, or too brittle. Would you use plywood as part of your chamber wall? I would assume not.
Believe me, the forces in a piston valve are huge. Even a small 20mm seated one has about 85 pounds of force on it at a typical working pressure. That boils down to an equivalent "pressure" on the surface of the piston (where the seat touches it) of about 923 psi.
Which is a helluva lot. |
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| judgment_arms |
| Posted: 10/21/2006 12:29 PM Post subject: |
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The Judge
Joined: 17 Oct 2006
Posts: 1243
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Um… the utopia guy used hot glue. And about the not being air tight once again don’t you drill bleed holes in it plus there is a rubber seal between the piston and the barrel.
923 PSI!?! Who taught you math. I’m only running the thing at a 100PSI. |
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| djt |
| Posted: 10/21/2006 12:52 PM Post subject: |
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Major
Joined: 11 Mar 2006
Posts: 387
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| i think hes trying to say 923 pounds of force because it would be 100 pounds PER square inch. but i doubt you would have 9 square inches |
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| Brian the brain |
| Posted: 10/21/2006 13:53 PM Post subject: |
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Dutch Spud Clan Leader
Joined: 05 Dec 2005
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The Chappy cannon used MDF for it's piston.
The set up was like a washer and bolt kind of piston.We used the MDF as washers.
It worked but gave out at about 20 shots.It broke beyond repair, so now it has 8 mm aluminium "washers" and it works like a charm now.
Of course, the thickness was onlu 1/2 inch so 2 inch should work. |
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| Infernal2 |
| Posted: 10/21/2006 13:55 PM Post subject: |
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Staff Sergeant
Joined: 11 Sep 2006
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Wood works fine for pistons. Sure, there is unbelievable pressure placed on wood but remember, wood is the most commonly used building material on the earth for a reason. Wood is both plentiful and durable.
For example, a 5/8 piece of oak, painted with an oil based acrylic and then lubricated is a very effective piston. The acrylic does readily seal the wood.
That said, plywood is an awful composite for a piston. Ply wood is (depending on width) two to three layers of slat over OSB composite (dried pulp). It is not very strong and should not be used.
You can use it but I personally wouldn't. If it does shatter you will be cleaning wood out of every working part in your gun. Think, a possibly punctured diaphragm, sealing face....etc...
Stick with solid hardwoods. |
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| judgment_arms |
| Posted: 10/21/2006 14:39 PM Post subject: |
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The Judge
Joined: 17 Oct 2006
Posts: 1243
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Never seen anything like you described all the plywood I’ve got is thin sheets of wood layered together. I think I’ll take my risk and go with plywood I’d trust that a little bit more the hot glue or some of the other things people have used.
Thanks all |
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