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Piston Help

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 Help

Unread postAuthor: shardbearer » Tue Nov 13, 2012 8:28 pm

I have a 2" golfball cannon, pretty standard PVC barrel sealer. I just broke my first piston which was a 1 1/4" PVC coupler, with a 3/8" thick epoxy plug on one end, drilled through for the bolt. I am looking into building a new piston, and have a few ideas.

UHMWPE looks like the best material, for its high impact strength and slipperyness. Unless it's too soft? Maybe Delrin?

Different designs:

Solid piston—Heavy, easy to make, easy to attach bumper to
Drill out solid piston with forstner bit, making a cup shape—Light, weaker, hard to make, hard to attach bumper to
Epoxy in tube, make a cup—Epoxy might not stick to UHMWPE, weaker, easy to make, hard to attach bumper to

Any opinions or suggestions? Is the solid piston too heavy, or should I just go with it and use a good bumper?
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 13, 2012 11:30 pm

UHMWPE looks like the best material, for its high impact strength and slipperyness. Unless it's too soft? Maybe Delrin?


I would go for the latter, much stronger.

Drill out solid piston with forstner bit, making a cup shape—Light, weaker, hard to make, hard to attach bumper to


Sounds like the best option. Prevention is better than cure, better to remove stress than find ways of dealing with it. I would also not put the bumper on the piston, rather on the pilot endcap, it avoids adding weight to the moving part.

Another important aspect is to not let your piston travel too far, doesn't need to be more than 50% or so of barrel diameter.
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Limit forces

Unread postAuthor: Technician1002 » Wed Nov 14, 2012 12:50 am

In the bumper department, both limiting the movement so it does not pick up excessive velocity, and providing a proper decelleration zone go a long way to protect pistons.

A car analogy is limiting damage to a car by slowing down to a stop before you hit a brick wall, and limiting your speed as you hit a brick wall.

Lack of a bumper is the same as hitting the brick wall with the pedal floored and a good distance to pick up speed. Bumper slows you down safely, and short distances limit piston velocity at the end of the run.

For piston material, HDPE and UMH-HDPE have been tested and work well in my cannons without a failure in several years of use.

On my 2 inch QDV, the bumper is engauged when the piston has opened about 1/2 way so the other half of the travel is slowing it back down before it runs out of room.
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Unread postAuthor: mattyzip77 » Fri Nov 16, 2012 8:18 am

If your looking for a fast fix and a awesome piston I highly reccomend a 2 inch rubber sanding drum. you gan pick them up at any harbor freight for like 6 bucks. Just add your sealing face, bolt, and washers and your done. You can even adjust it by tightening or loosening the bolt. I have tried all other pistons and this is by far my favorite. Good luck!!! :D
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Unread postAuthor: shardbearer » Fri Nov 16, 2012 10:17 am

Thanks guys, I found some 2" Nylon rod lying around that I'll machine into a cup. Unfortunately I don't have a Harbor Freight nearby. Would Lowes or Home Depot carry 2" sanding drums? And what are they made out of?
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