Constant velocity joint and method of manufacturing the same

1. A constant velocity joint comprising:

a bottomed cylindrical outer joint member having an opening at one side in an axial direction and having a concave spherical inner circumferential surface provided with a plurality of outer ball grooves;

an inner joint member located inside the outer joint member and having a convex spherical outer circumferential surface provided with a plurality of inner ball grooves;

a plurality of balls that roll in the plurality of outer ball grooves and the plurality of inner ball grooves; and

a cage located between the inner circumferential surface of the outer joint member and the outer circumferential surface of the inner joint member to accommodate the plurality of balls, wherein

each of the plurality of outer ball grooves includes a first ball groove and a second ball groove, the first ball groove allowing a corresponding one of the plurality of balls to roll therein when the constant velocity joint operates, the second ball groove located closer to the opening than the first ball groove and having a ball groove radius larger than a ball groove radius of the first ball groove,

the second ball groove has a curved line and a connecting line in an axial cross section passing through a bottom of a corresponding one of the plurality of outer ball grooves, the curved line connected to a groove bottom end of the first ball groove and curved radially outward in a concave manner, the groove bottom end being an end of a bottom of the first ball groove located closer to the opening than to a bottom of the outer joint member, the connecting line connecting the curved line and the opening, and

the curved line is curved with a center of curvature that is located closer to the opening than the groove bottom end of the first ball groove and that is located radially inside the outer joint member.

2. The constant velocity joint according to claim 1 , wherein

in the axial cross section passing through the bottom of the corresponding one of the plurality of outer ball grooves, a distance from the center of curvature to the curved line is smaller than a distance from a rotation axis of the outer joint member to the groove bottom end.

3. The constant velocity joint according to claim 1 , wherein

the curved line is shaped to follow a single circular arc with the center of curvature.

4. The constant velocity joint according to claim 1 , wherein

a distance from the connecting line to a rotation axis of the outer joint member increases in a direction from the bottom of the outer joint member to the opening of the outer joint member, and

in an area of the second ball groove where the connecting line is located, the ball groove radius of the second ball groove gradually increases in the direction from the bottom of the outer joint member to the opening of the outer joint member.

5. The constant velocity joint according to claim 1 , wherein

the connecting line is parallel to a rotation axis of the outer joint member in a direction from the bottom of the outer joint member to the opening of the outer joint member, and

in an area of the second ball groove where the connecting line is located, the ball groove radius of the second ball groove is constant.

6. The constant velocity joint according to claim 1 , wherein

the first ball groove has a ground surface, and

the second ball groove has a plastically worked surface.

7. A method of manufacturing the constant velocity joint according to claim 1 , the method comprising:

forming the first ball groove by grinding; and

forming the second ball groove by plastic working.