Please explain why sometimes a shim is also installed in the groove of a piston ring, in a piston.
Unless you are talking about a multi-piece oil control ring, I don't know what you mean.
No its not a piston ring. This is basically a very thin ring shaped strip of metal inserted into the inner top portion of the first groove of the piston during its manufacture. (The compression ring is assembled below it). I cant seem to find any related pictures or links of it. here's a rough drawing.
Where have you seen this in use?
Is this for an ICE, a compressor, liquid pump, hydraulic cylinder, etc... ???
What materials are the parts made out of?
Does the shim seal on the top of the compression ring?
Does the shim wear on the OD where it contacts the cylinder?
What kind of pressure and temperature do the parts see in operation?
I've seen a similar arrangement on the oil control ring in an internal combustion engine, but this is to retain a corregated oil control ring and provide a scraping effect.
OP - where have you seen this?
I've never heard of such a thing, and I've seen a lot of pistons. As others have mentioned, it resembles a component of a 3-piece oil control ring, but those aren't a pre-installed part of the piston, and don't protrude farther into the skirt than any other ring.
I have the answer:
This was used prior to the introduction of the alfin ring (cast iron ring) which is now used in certain heavy vehicle diesel engines.
Separate names with a comma.