Patent ID: 8820750

Claim:
An internal combustion engine oil ring, comprising: a substantially ring-shaped oil ring main body with a radially inward concave recess on an inner peripheral side and an outer peripheral surface side that slides on a circular cylinder inner wall; and a coil expander arranged in the recess on the inner peripheral side of the oil ring main body, the coil expander for pressing the oil ring main body against the circular cylinder inner wall, wherein the oil ring main body includes a i) web comprising an upper end and a lower end, ii) an upper, first rail, and iii) a lower, second rail, the web having a plurality of oil return holes to allow oil scraped from a surface of the cylinder inner wall to flow down to an underside of a piston, the oil return holes penetrating through the web in a radial direction, the oil return holes arranged in a circumferential direction of the web, the upper end of the web being connected to a lower surface of the first rail located on an upper side of the web, the lower end of the web being connected to an upper surface of the second rail, wherein the first rail, the web, and the second rail are integral with each other, the first rail, second rail, and the web together defining, as seen in cross-section of the oil ring main body, a radially outwardly concave outer peripheral groove, the first and second rails providing a plurality of recessed stepped portions, a first one of the recessed stepped portions being formed at a corner of a sliding surface of an outer peripheral sliding projection in sliding contact with the cylinder inner wall surface, of the first rail, a second one of the recessed stepped portions being formed at a corner of a sliding surface of an outer peripheral sliding projection in sliding contact with the cylinder inner wall surface, of the second rail, in vertical cross-section of the first rail and the second rail, each of the first and second recessed stepped portions reducing a thickness of the respective sliding surface, a wall surface of each of the first and second recessed stepped portions has a concave arcuate surface extending toward an outer diameter of the ring and having a radius of curvature of 0.02 mm to 0.08 mm, the concave arcuate surface of the first rail and the concave arcuate surface of the second rail face each other, and the outer peripheral sliding projection of the first rail is symmetrical to the outer peripheral sliding projection of the second rail with respect to a radial axis of the web, wherein the wall surface of each of the recessed stepped portions is continuously formed of the concave arcuate surface and a curved surface without a planar surface therebetween, the curved surface being successionally formed between the concave arcuate surface and an inner side wall of the outer peripheral sliding projection, the inner side wall of the outer peripheral sliding projection of the first rail and the inner side wall of the outer peripheral sliding projection of the second rail facing each other.