| 摘要 |
According to an embodiment of the present invention, a ring groove 11 for a two-piece oil ring 10, of a piston 20, is provided that has a ring body 5 equipped with a coil expander 6, where the ring body 5 has an upper outer surface and a lower outer surface, both orthogonal to the axial direction of the piston 20. The ring groove 11 has an upper inner surface 13 and a lower inner surface 14, both orthogonal to the axial direction of the piston 20; a radially inward-most, bottom surface 12 connecting the upper and lower inner surfaces 13, 14; and oil drain holes 18 each having an opening 19 thereof located on the bottom surface 12. The ring groove 11 has an axial width slightly greater than the width of the two-piece oil ring 10. The ring groove 11 has the radially inward ends 15, 16 of the upper and lower inner surfaces 13, 14 of the ring groove 11 respectively located radially inwardly apart by a first predetermined distance from the radially inward ends of the upper and lower outer surfaces of the ring body 5 when the oil ring 10 is mounted within the ring groove 11 so that the outer circumferential surface of the ring body 5 is in contact with the inner circumferential surface of a cylinder 7 engaged with the piston 20. According to an aspect of the present invention, the first predetermined distance is preferably not smaller than 1/300, and not greater than 1/100 of the inner diameter of the cylinder 7. The bottom surface 12 of the ring groove 11, according to the embodiment, comprises: a concave surface formed by a generating line 17, 21 drawn on a cross-sectional plane of the ring groove 11 along the centerline of the piston 20, the generating line 17, 21 having the upper and lower ends thereof located coinciding with the radially inward ends 15, 16 of the upper and lower inner surfaces 13, 14 of the ring groove 11, the generating line 17, 21 surrounding the coil expander 6 projecting from the radially inward end of the ring body 5 so that the generating line 17 is apart from the surface of the projecting portion of the coil expander 6 by a certain predetermined distance. According to an aspect of the present invention, the certain predetermined distance is preferably around a range of 1/300 to 1/100 of the inner diameter of the cylinder 7. With the arrangement described above, with the decreased cavity space behind the oil ring 10 in the ring groove 11, according to the present invent
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