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Sheet rubber for pistons?

Post about things you have launched or thought about launching. Also post about various materials used for building cannons. No posts about explosive projectiles!
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Unread postAuthor: ramses » Mon Mar 28, 2011 5:50 pm

Good idea. I guess I didn't really give bumper design a second thought.

Do you think it would be better to reverse the piston, fill the cap with plywood disks, and attach the sealing face to that? Obviously the OD of the valve seat would be reduced to fit loosely inside the cap.
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Unread postAuthor: Technician1002 » Mon Mar 28, 2011 7:38 pm

I would probably recommend replacing the piston entirely with another material. Repeated high forces on PVC cause it to eventually fail. My only remaining piston made of PVC is in my ABS cannon. It uses a bumper that is longer than the piston.

The Mouse Musket and The Dragon used PVC pipe caps for pistons. The Mouse Musket went through a piston about once a year. The Dragon didn't last more than a couple low pressure shots and was never rebuilt.

The ABS test cannon has a double layer on the piston as it is pipe caps inside pipe.

A 3 inch piston glued on a section of 3 inch pipe and then closed off to support a large plywood disk and bumper may survive longer.
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Unread postAuthor: tony48 » Mon Mar 28, 2011 8:37 pm

I ordered some 1/8" high strength silicone sheet from mcmaster. I think this will work well but any comments?

Shown here:
http://www.mcmaster.com/#silicone-rubber-sheets/=bmx6dl
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Unread postAuthor: mark.f » Mon Mar 28, 2011 9:46 pm

At roughly half the tensile strength of Buna-N, I could see room for improvement. However, with a 3" piston on a 2" SCH-40 sealing face you will see around 430 PSI between PVC and rubber. Use a properly sized washer and you should have no problems.

EDIT: and yield strength seems to be about 700 PSI on the low end of silicone rubbers, so I'd say you'll be fine, again, with a properly sized washer.

EDIT II: calculation mistakes.

EDIT III: yet more calculation errors. In all three cases you're still good though. :D
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Last edited by mark.f on Mon Mar 28, 2011 10:30 pm, edited 1 time in total.
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Unread postAuthor: tony48 » Mon Mar 28, 2011 10:13 pm

What would be a properly sized washer?
And thanks very much for the help
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Unread postAuthor: mark.f » Mon Mar 28, 2011 10:28 pm

A properly sized washer would be one that matched both the OD of the bolt that held the sealing face on and the ID of the barrel port as closely as possible without jamming.
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Unread postAuthor: ramses » Tue Mar 29, 2011 10:34 am

Technician1002 wrote:I would probably recommend replacing the piston entirely with another material.


poop. time to design a toolie valve.
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