Patent ID: 7036823

Claim:
A side rail for use in an oil control ring, said side rail being formed into a ring shape having its generally rectangular section that comprises a pair of side surfaces in parallel to each other, an outer surface formed between two outer peripheries of said side surfaces for contact to an inner surface of a cylinder, and an inner surface formed between two inner peripheries of said side surfaces for contact to a spacer expander of said oil control ring, said outer surface comprising a pair of edge surfaces connected to said adjacent side surfaces, an intermediate surface formed between said edge surfaces and a boundary portion between each of said edge surfaces and said intermediate surface, each of said edge surfaces being formed with a radius of curvature equal to from one fifth to eight fifth of said side rail width to easily ride an oil film on a cylinder in the inclined condition of the side rail in operation and allow the side rail to perform smooth sliding movement on the inner surface of said cylinder, said intermediate surface being formed by a barrel face formed with a constant radius of curvature greater than radius of curvature for said edge surfaces, said barrel face forming a convex surface slightly outwardly protruding from said boundary portion between said intermediate surface and edge surfaces by a height equal to or less than about 0.01 mm, the radius of curvature for forming the intermediate surface providing a larger contact area to the inner surface of the cylinder, and axial length of said intermediate surface being in a range between about 30% and about 70% of the side rail width.