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[Mege66]

This is a question for the model engineers among us:

I’m planning to use O-rings as piston rings in a new steam engine design (instead of PTFE piston rings as used so far), and I’m trying to figure out the ideal amount of compression for the O-rings.

The technical reference manuals of the manufacturers recommend at least 6% compression for dynamic applications (like sealing a piston rod or a piston ring). For example, a 1.5 mm cross-section O-ring would be compressed to about 1.41 mm. In my experience, though, 6% is way too much for small model engines. When I turn the engine by hand, there’s noticeable friction, and the engine only starts moving smoothly at fairly high steam pressures. Getting a smooth, slow operation seems almost impossible.

Some people suggest that a piston O-ring might not need any compression at all, and that the seal created by the steam pressure pressing the O-ring against the surfaces is enough. Without pressure, there’s barely any sealing—but then, there’s no steam pressure anyway, so maybe that’s fine.So, is the “right” compression somewhere between 0% and 6%?

What compression do you use in your own designs, or have you measured in commercial engines during maintenance?

[1967Brutus]

I am not THAT kind of engineer that I know required ring compression under different condition, but I DO know this: If a ring is NOT compressed, it won't seal, and it also won't build a seal under pressure. It will start to flutter, and "steam cuts"...

Personally, I would stick to the Teflon piston rings. I know that O-rings can work, several brands apply that tech, but it would not be my choice.