In the case of a restrained type, flanged ring set, Baldwin Steam Locomotive Standards calls for one complete cast iron ring to be backed by a complete bronze ring. When SP2479's pistons were removed, piston rings were found to alternate bronze, cast iron within one ring segment. There were a couple of exceptions. Any thoughts as to why the change, or advantage, to the alternating pattern?

Thanks in advance - John Ezovski

Sounds like a mistaken premise to me.

Alternating segments would almost guarantee steam leaks near the edges, as the outside radius of the 'ring'
as a whole would either have to be machined to be circular at fitting temperature, or mean operating temperature, and due to the different coefficient of expansion of the two materials the segment radii would be wrong at other temperatures.

I had thought one point of the bronze was to wear to a working seal at higher temperatures (with the iron doing most of the load-bearing). That's defeated if the segments of either material don't form complete rings. Likewise, there would be no particular gain in tribology by alternating the segments -- at least, not one I can see.

About the only thing I can figure out would be if you wanted a ring with intermediate expansion characteristics between bronze and steel.

Looking forward to hearing what the explanation is.

S.P. 2472 had the same alternating bronze / cast iron / bronze / cast iron sectional rings, "as received" from S.P. service. GGRM made new segments from bronze and cast-iron cylindrical stock, and has plenty of material left to make more. These were made on a horizontal boring mill.
- Doug Debs

Alternating piston rings of iron and bronze I believe was pretty standard practice. I think the confusion may be in the description in the first post that says "alternating segments", as if you could have a single piston ring that was somehow made of alternating radial sections of iron and bronze. I can't imagine how (or why) you'd construct such a ring, so I'm guessing he meant that complete 360 degree piston rings of bronze and cast iron were somehow "joined" into a single piston ring assembly.

The point of alternating bronze and cast iron is that the softer bronze wears off onto the surface of the cast iron cylinder liner, reducing friction and wear on the cast iron rings.