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Aluminium piston?

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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Aluminium piston?

Unread postAuthor: saefroch » Fri Jun 11, 2010 10:16 am

I am feeling pretty satisfied with http://www.spudfiles.com/forums/pneumatic-rifle-thingy-finished-for-now-t20961.html , but the sheer size is becoming somewhat of an issue. So... I was thinking of making a half-size version of if that would be far more portable, and with a much smaller chamber, would be much easier to pump up to very high pressures. I also then began thinking that I might be able to make a piston with the same basic shape as before, with a front sealing face and a valve on the back, but this time out of a solid piece of aluminum. My question here is thus: Will aluminum withstand the impacts produced by operation at 250+ psi, assuming there is no bumper to slow down the impact (into the male side of a bushing)? I plan to make the piston fit into a 3/8" NPT fittings, so it would have a maximum diameter of .495", but at some parts would have to probably be as small as .3" or .2" even.
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Unread postAuthor: Technician1002 » Fri Jun 11, 2010 10:32 am

Short answer, No. Long answer, If the pilot is designed to act as a dashpot to dissipate the energy before impact, maybe.
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Unread postAuthor: jhalek90 » Fri Jun 11, 2010 5:27 pm

MY aluminum piston was smashed to bits at about 100 psi.

.25'' porting mind you.

I made several pistons.... and tried them at 100, 200, 300, 400, 500, and 600 psi.

The 600 psi piston in stuck in the pilot section of the gun, and can not be removed.
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Unread postAuthor: Hotwired » Fri Jun 11, 2010 6:01 pm

A tough plastic such as nylon is preferable to aluminium. Especially with no bumper.
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Unread postAuthor: saefroch » Fri Jun 11, 2010 9:05 pm

Hmmmm... I'll certainly take that into consideration. I switched around the internals of my gun so I'm not stopping the piston on a 3/4"-1/2" bushing, not a corresponding female-female adapter, so now I've got a place to seat a bumper for a piston, so one of those might be able to survive.

I don't know so as I'd like to try using a nylon piston... that might be too fragile or weak.

The idea of the change was to make a much less rickety piston that's all one piece of metal with just the seals at either end. I considered making one out of a solid cylinder of steel, but I figured that the inertia of such a piston would be considerable, so I'd end up making the middle of it rather skinny to compensate, but after reading your comments I'm thinking steel might offer a better strength:weight ratio.

The piston itself is housed in a 3/4" tee, but it has 1/2" porting.
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Unread postAuthor: Technician1002 » Fri Jun 11, 2010 9:55 pm

Check the specific weight of steel and compare it to the specific weight of UHMWPE. :D The steel piston will have to be much smaller to have the same mass.

The lighter the piston, the less time it is exposed to acceleration. The less time it is exposed to acceleration, at the same force, the less momentum it has. The less the momentum, the less energy the bumper will need to absorb.

The faster lighter piston is less likely to break things and is faster.
Iron has a specific density of 7.87 gm/cm3. UMWHDPE has a density of 0.94. It floats in water.
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Unread postAuthor: saefroch » Fri Jun 11, 2010 10:17 pm

So.... you're suggesting I can crash UHMWPE into brass, and because of the very low mass it will accelerate better, but won't break on impact? So for a piston, UHMWPE is better than steel?
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Unread postAuthor: ramses » Fri Jun 11, 2010 10:24 pm

unless you are subjecting it to many thousands of PSI, extreme heat, or (assuming stainless steel) corrosive chemicals, yes!
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Unread postAuthor: velocity3x » Fri Jun 11, 2010 10:56 pm

Give Acetal (aka Delrin) a try. It's stronger than UHMWPE and has a high self lubricity.
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Unread postAuthor: spudtyrrant » Fri Jun 11, 2010 11:08 pm

saefroch wrote:So.... you're suggesting I can crash UHMWPE into brass, and because of the very low mass it will accelerate better, but won't break on impact? So for a piston, UHMWPE is better than steel?
sir i have used hot glue pistons at 400psi with no bumper, and they have not sustained any damage at all, i'm sure you will be just fine with a high density plastic. :wink:
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Unread postAuthor: saefroch » Fri Jun 11, 2010 11:35 pm

Sir, with all due respect, my gun shattered a hot glue piston at 150 psi.
But I will, however try a high-density plastic. I'll search the local hardware stores on the next run, see if I can get a better price than on McMaster for a 1.08+" diameter rod. If not, well the current piston does work... it's just rickety.
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Unread postAuthor: Technician1002 » Fri Jun 11, 2010 11:42 pm

My Mouse Musket did not use a bumper due to the recess in the back of the piston and in the plug in the breech. It broke PVC pistons on a regular basis, but the HDPE has been very reliable in all the cannons I use it in. ABS is used as the back stop cylinder in my 2 inch QDV and has never given any trouble. The bushing on the end is HDPE along with the piston. I made a spare piston but never needed it. I do have a bumper in that cannon, but it's mostly flattened now.
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Last edited by Technician1002 on Sat Jun 12, 2010 9:01 am, edited 1 time in total.
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Unread postAuthor: c19o » Sat Jun 12, 2010 3:42 am

Would the Type II PVC on McMaster serve the purpose as well as Delrin?
I'm having a 2" porting 500+ PSI stainless steel piston valve machined soon. Should I tell the shop to use a different material? A neoprene rubber bumper will be used.

@saefroch: I would really use a bumper, even if it is minimal.
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Unread postAuthor: saefroch » Sat Jun 12, 2010 7:03 am

Oh yes, I am on the lookout for bumper materials now that I can actually get one situated. I'll look into different types of plastic for making a solid piston... maybe even do a comparison and operate at high pressures, see which begins to fracture or bend first. Even if you can tell me that already, I'm sure what with all the complexities of my piston, it'll be a good exercise.
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Unread postAuthor: chinnerz » Sat Jun 12, 2010 7:26 am

saefroch wrote:Sir, with all due respect, my gun shattered a hot glue piston at 150 psi.


Hot glue isnt brittle, so how did it shatter??
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