Piston-Crank Connection

[FinFisher]

Hi! new member here!

I'm making a lamina flow engine (also known as a thermoacoustic stirling engine) for a university class. I am making the cylinder out of 306 brass and the piston out of .0004 grain graphite. I was wondering what the best way to connect the crank arm to the piston in the cylinder is? Should I use flanged bearings in the back side of the piston? Should I just use a press fit rod going through a hole in the crank arm? What is the best way to connect the two systems reliably and without inducing too much friction? I am worried about any holes through the piston being a source of friction as well.

Let me know what you all think!

Thanks
Fin

[Tom Booth]

All the model engines I've ever purchased, that have graphite pistons have the piston drilled and threaded.

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I'd hesitate to say that is the best or only option, but it's never given me any problems.

[FinFisher]

Interesting! The threads in the graphite seem strong? I have never tapped graphite before and I'm worried how the threads will hold up if i tried.

[Tom Booth]

Interesting! The threads in the graphite seem strong?....

The ones I've had experience with, supplied with kits. I did not make them myself, but I have sometimes unscrewed the piston and screwed it back on (gently, finger tight) and they seem to hold secure.

These are, of course, just Low temperature, low RPM engines, used only occasionally.

If I were going to construct larger ones for bigger higher RPM engines, I think I might drill all the way through the center of the piston to hold the piston more securely with a washer head or joint connector type bolt of some sort.

[FinFisher]

Thanks for the advice! I bought a 6" graphite rod so I think I'll use the first inch or two experimenting with tapping and see how it goes. The idea of using a joint connector is a great idea! If the tapping doesnt work i'll just use a through hole and use a joint connector.

[FinFisher]

I tapped the graphite and it works with 10-24 threads. I am now making the connector from the piston to the crank arm. I am making the connector out of brass. What hardware does the example you sent use to connect the metal to the crank arm?

[Tom Booth]

...What hardware does the example you sent use to connect the metal to the crank arm?

These small models just use slotted spring pins. Probably not the best possible option.

[FinFisher]

Ah what would you recommend that might be better in this application?

[Tom Booth]

Ah what would you recommend that might be better in this application?

The main problem with a slotted spring pin IMO might be friction. It is tightly fixed in place. I also find them nearly impossible to remove without potentially damaging something. They grip tight and don't punch out easily.

A pretty universal standard is a wrist pin with snap rings. The pin is free to move reducing friction and it makes disassembly without damage, when necessary easier.

A DIY option that would not involve as much difficult machining would be a simple nut and bolt, but to prevent the nut from turning the nut and bolt can be drilled and secured with a cotter pin. (standard castle nut).

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Or an unthreaded Clevis yoke

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