Fixed type constant-velocity universal joint

1. A fixed type constant velocity universal joint, comprising:

an outer joint member having a spherical inner peripheral surface in which a plurality of track grooves are formed so as to extend in an axial direction of the outer joint member,

the outer joint member having an opening side and an interior side spaced apart from each other in the axial direction;

an inner joint member having a spherical outer peripheral surface in which a plurality of track grooves are formed so as to be paired with the plurality of track grooves of the outer joint member;

a plurality of balls for transmitting torque,

the plurality of balls being interposed between the plurality of track grooves of the outer joint member and the plurality of track grooves of the inner joint member; and

a cage for holding the plurality of balls,

the cage having a spherical outer peripheral surface and a spherical inner peripheral surface, which are fitted to the spherical inner peripheral surface of the outer joint member and the spherical outer peripheral surface of the inner joint member, respectively,

wherein the plurality of track grooves of the outer joint member comprise:

first track groove portions positioned on the interior side; and

second track groove portions positioned on the opening side,

wherein each of the first track groove portions has a ball raceway center line that is formed into an arc shape and has a curvature center at a position offset to the opening side with respect to a joint center,

wherein planes including at least the ball raceway center line and the joint center are inclined in a peripheral direction of the outer joint member with respect to a joint axial line with their inclination directions opposite to each other in the first track groove portions adjacent to each other in the peripheral direction,

wherein each of the second track groove portions has a ball raceway center line comprising a linear part so as to increase an effective track length at a maximum operating angle,

wherein the ball raceway center line of the each of the second track groove portions is connected to the ball raceway center line of the each of the first track groove portions at a position on the opening side with respect to the joint center, and

wherein the ball raceway center line of each of the plurality of track grooves of the inner joint member is formed so as to be mirror-image symmetrical with the ball raceway center line of corresponding one of the plurality of paired track grooves of the outer joint member with respect to a joint center plane at an operating angle of 0°.

2. The fixed type constant velocity universal joint according to claim 1 , wherein an angle (β) formed by a straight line connecting the joint center and a connection point between the ball raceway center line of the each of the first track groove portions and the ball raceway center line of the each of the second track groove portions with respect to the joint center plane at the operating angle of 0° is set to 3° to 10°.

3. The fixed type constant velocity universal joint according to claim 1 , wherein the curvature center of the ball raceway center line of the each of the first track groove portions is arranged on an inclined axis, which is inclined in the peripheral direction with respect to the joint axial line.

4. The fixed type constant velocity universal joint according to claim 1 , wherein the curvature center of the ball raceway center line of the each of the first track groove portions is arranged at a position offset in a radial direction of the outer joint member with respect to an inclined axis, which is inclined in the peripheral direction with respect to the joint axial line.

5. The fixed type constant velocity universal joint according to claim 1 , wherein a curvature center of the spherical outer peripheral surface of the cage and a curvature center of the spherical inner peripheral surface of the cage are arranged at positions offset respectively to the opening side and the interior side with respect to the joint center.

6. The fixed type constant velocity universal joint according to claim 1 , wherein the entire ball raceway center line of the each of the second track groove portions comprises the linear part, and is formed as a tangent with respect to the ball raceway center line of the each of the first track groove portions.

7. The fixed type constant velocity universal joint according to claim 1 ,

wherein the ball raceway center line of the each of the second track groove portions further comprises an arc part having a curvature radius that is different from a curvature radius of the ball raceway center line of the each of the first track groove portions, and

wherein the arc part is connected to the ball raceway center line of the each of the first track groove portions.

8. The fixed type constant velocity universal joint according to claim 1 , wherein the linear part of the ball raceway center line of the each of the second track groove portions is formed parallel to the joint axial line.

9. The fixed type constant velocity universal joint according to claim 1 , wherein the ball raceway center line of the each of the second track groove portions is inclined in the peripheral direction with respect to the joint axial line with its inclination angle being gradually decreased toward the opening side.

10. The fixed type constant velocity universal joint according to claim 1 , wherein a number of the plurality of balls is set to eight, ten, or twelve.