Seal ring

1. A seal ring, comprising: a center axis being a rotation center;

an outer peripheral surface being an outward cylindrical surface whose center is positioned at the central axis;

an inner peripheral surface being an inward cylindrical surface disposed between the center axis and the outer peripheral surface whose center is positioned at the central axis;

a side surface connecting the outer peripheral surface and the inner peripheral surface; a groove formed in a dented groove from the side surface, extending along a radial direction spaced from the outer peripheral surface, having a first end connected to the inner peripheral surface and a second end at an opposite side of the first end, extending in an arc shape between the first end and the second end, and returning oil entered from the first end to an inner peripheral surface side via the second end; and

an inner peripheral wall separating the groove from the inner peripheral surface between the first end and the second end,

wherein the groove comprises an inner side surface, an outer side surface, and a bottom surface between and connected to the inner side surface and the outer side surface,

wherein the inner side surface of the groove is connected to the inner peripheral wall and extends in the arc shape with respect to the inner peripheral surface between the first end and the second end, and

wherein the outer side surface of the groove is opposite to the inner side surface of the groove and extends in the arc shape with respect to the inner peripheral surface between the first end and the second end;

wherein the groove has a pocket extending between the first end and the second end, and an undercut extending from the first end in a direction opposite to the pocket;

wherein a plurality of the grooves are disposed successively along the radial direction;

wherein the undercut of the groove extends to the pocket of a groove of an adjacent groove.

2. The seal ring according to claim 1 , wherein

the second end is connected to the inner peripheral surface.

3. The seal ring according to claim 2 , wherein

the groove has a top with a shallowest depth from the side surface.

4. The seal ring according to claim 3 , wherein

a depth of the groove from the side surface becomes successively shallow from the first end to the top.

5. The seal ring according to claim 3 , wherein

the width of the groove is successively narrowed down from the first end to the top.

6. The seal ring according to any one of claim 2 , wherein

the groove is configured of a first groove and a second groove,

the first groove and the second groove share the first end, and

the second ends are disposed at opposite sides across the first end.

7. The seal ring according to any one of claim 2 , wherein

the groove has a symmetry shape in a radial direction of the inner peripheral surface.

8. The seal ring according to claim 2 , wherein

a plurality of the grooves is disposed to have a rotational symmetry as to the center axis.

9. The seal ring according to claim 1 , wherein a width of the pocket is shallowest at the second end, and a width of the undercut is equivalent to a width of the second end.

10. The seal ring according to claim 1 , wherein a depth of the pocket is shallowest at the second end, and a depth of the undercut is equivalent to the second end.

11. The seal ring according to claim 1 , wherein the undercut extends to the inner peripheral surface.

12. The seal ring according to claim 11 , wherein

the undercut pierces through the inner peripheral surface.

13. The seal ring according to claim 11 , wherein

the undercut is disposed spaced from the inner peripheral surface.

14. The seal ring according to claim 1 , wherein the undercut pierces through the pocket of the adjacent groove.

15. The seal ring according to claim 1 , wherein the undercut is disposed spaced from the pocket of the adjacent groove.