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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.

[Geoff]

I cant give you an answer but i was told that there needs to be just a little room for the O ring to roll just a little on both the up and down strokes. I would also recommend silicon O rings as some "rubber" O rings can react to heat and steam oil causing them to expand and jam. I have experienced this but once i switched to silicon it was pefect. Good luck with your build.


Cheers Geoff

[ooyah/2]

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?

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%?



    Hi Mege66

Many years ago when Silicon "O" rings became available there was no technical dimensions to which the groove and depth  could be made, so there was al sorts of ideas as to what they should be machine to.
I do remember that some people stated that the rings should be allowed to float up and down as the as the piston moved from T.D.C to B.D.C.

Fortunately some of the suppliers started to supply dimensions for groove width and depth and the main supplier was Blackgates Engineering and they still do, over the past 40 years I have been using these dimensions  during the building and machining of many engines mainly the Stuart Turner range which can be found on this Forum.

I would suggest that a copy of Blackgates Cat should be purchased  and the dimensions are all there. they also do a wide range of fittings.

Be aware that only Silicon rings are suitable for steam, the black ones melt and will cause an engine to seize.  Black rings are most suitable for water pumps and rod glands..

The rings are dimensioned in order to allow the rings to expand and fill completely the groove and make a perfect seal when steam is applied.
Note that Black rings are not suitable for steam applications but can be used for water pumps and glands

George.

[ooyah/2]

[/quote

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%?



    Hi
Many years ago when Silicon "O" rings became available there were no technical dimensions to which the groove and depth  could be made, so there was al sorts of ideas as to what they should be machine to.
I do remember that some people stated that the rings should be allowed to float up and down as the as the piston moved from T.D.C to B.D.C.

Fortunately some of the suppliers started to supply dimensions for groove width and depth and the main supplier was Blackgates Engineering and they still do, over the past 40 years I have been using these dimensions  during the building and machining of many engines mainly the Stuart Turner range which can be found on this Forum.

I would suggest that a copy of Blackgates Cat should be purchased  and the dimensions are all there. they also do a wide range of fittings.

Be aware that only Silicon rings are suitable for steam, the black ones melt and will cause an engine to seize.  Black rings are most suitable for water pumps and rod glands..

The rings are dimensioned in order to allow the rings to expand and fill completely the groove and make a perfect seal when steam is applied.

George