Constant-velocity joint

1. A constant-velocity joint comprising:

an outer joint member in a shape of a bottomed tube that is open on one side in an axial direction, the outer joint member having first outer ball grooves and second outer ball grooves provided in an inner peripheral surface of the outer joint member;

an inner joint member disposed inside the outer joint member, the inner joint member having first inner ball grooves and second inner ball grooves provided in an outer peripheral surface of the inner joint member;

balls disposed between the first outer ball grooves and the first inner ball grooves and between the second outer ball grooves and the second inner ball grooves; and

a cage disposed between the outer joint member and the inner joint member, the cage having cage pockets configured to store the balls, wherein

each of the first inner ball grooves has a first inner large-diameter portion configured such that the corresponding one of the balls rolls on the first inner large-diameter portion,

each of the second inner ball grooves has a second inner large-diameter portion and as inner extension portion, the second inner large-diameter portion being configured such that the corresponding one of the balls rolls on the second inner large-diameter portion, and the inner extension portion extending from the second inner large-diameter portion toward a bottom of the outer joint member,

each of the first outer ball grooves has a first outer large-diameter portion configured such that the corresponding one of the balls is rollably supported between the first outer large-diameter portion and the corresponding one of the first inner ball grooves,

each of the second outer ball grooves has a second outer large-diameter portion and an outer extension portion, the second outer large-diameter portion being configured such that the corresponding one of the balls is rollably supported between the second outer large-diameter portion and the corresponding one of the second inner ball grooves, the outer extension portion extending from the second outer large-diameter portion toward an opening of the outer joint member, the outer extension portion being configured such that the corresponding one of the balls is supported so as to be rollable between the outer extension portion and the inner extension portion, and the outer extension portion constituting a part of an opening-side end portion of the outer joint member,

a center locus of the corresponding one of the balls rolling on the first outer large-diameter portion has an arc shape having an arc center at a position that is on an opposite side of a central axis of the outer joint member from the first outer large-diameter portion, and that is closer to the opening of the outer joint member than a joint center is,

a center locus of the corresponding one of the balls rolling on the second outer large-diameter portion has an are shape having an arc center at a position that is on an opposite side of the central axis of the outer joint member from the second outer large-diameter portion, and that is closer to the bottom of the outer joint member than the joint center is,

a center locus of the corresponding one of the balls rolling on the outer extension portion is positioned, throughout a length of the center locus, in a region including a tangent line to an opening-side end portion of the center locus of the corresponding one of the balls rolling on the second outer large-diameter portion, the opening-side end portion being on an opening side of the outer joint member, and the region being positioned outward of the tangent line in a radial direction of the outer joint member, and

an opening-side end portion of the center locus of the corresponding one of the balls rolling on the outer extension portion, the opening-side end portion of the center locus being on the opening side of the outer joint member, is positioned outward of the tangent line in the radial direction of the outer joint member.

2. The constant-velocity joint according to claim 1 , wherein a distance from a center of the corresponding one of the balls disposed at an opening-side end portion of the outer extension portion, the opening-side end portion being on the opening side of the outer joint member, to the central axis of the outer joint member is equal to or longer than a distance from the center of the corresponding one of the balls disposed at a bottom-side end portion of the outer extension portion, the bottom-side end portion being on a bottom side of the outer joint member, to the central axis of the outer joint member.

3. The constant-velocity joint according to claim 2 , wherein

the inner extension portion has an inner uniform-diameter portion extending from the second inner large-diameter portion toward the bottom of the outer joint member,

the outer extension portion has an outer uniform-diameter portion extending from the second outer large-diameter portion toward the opening of the outer joint member, and

a center locus of the corresponding one of the balls rolling on the inner uniform-diameter portion and the outer uniform-diameter portion is linear and a distance from the central axis of the outer joint member to the center locus is constant.

4. The constant-velocity joint according to claim 3 , wherein

the inner extension portion has an inner diameter-increasing portion extending from the inner uniform-diameter portion toward the bottom of the outer joint member,

the outer extension portion has an outer diameter-increasing portion extending from the outer uniform-diameter portion toward the opening of the outer joint member, and

a distance between a center locus of the corresponding one of the balls rolling on the outer diameter-increasing portion and the central axis of the outer joint member increases toward the opening of the outer joint member.

5. The constant-velocity joint according to claim 4 , wherein the center locus of the corresponding one of the balls rolling on the outer diameter-increasing portion has an arc shape having an arc center at a position outward of the center of the corresponding one of the balls rolling on the outer diameter-increasing portion, in the radial direction of the outer joint member.

6. The constant-velocity joint according to claim 1 , wherein the center locus of the corresponding one of the balls rolling on she outer extension portion has an arc shape having an arc center at a position outward of a center of the corresponding one of the balls rolling on the outer extension portion, in the radial direction of the outer joint member.

7. The constant-velocity joint according to claim 1 , wherein

the inner extension portion has

an inner diameter-decreasing portion extending from the second inner large-diameter portion toward the bottom of the outer joint member,

an inner curved portion extending from the inner diameter-decreasing portion toward the bottom of the outer joint member, and

an inner diameter-increasing portion extending from the inner curved portion toward the bottom of the outer joint member,

the order extension portion has

an outer diameter-decreasing portion extending from the second outer large-diameter portion toward the opening of the outer joint member,

an outer curved portion extending from the outer diameter-decreasing portion toward the opening of the outer joint member, and

an outer diameter-increasing portion extending from the outer curved portion toward the opening of the outer joins member,

a center locus of the corresponding one of the balls rolling on the outer diameter-decreasing portion is linear and a distance from the central axis of the outer joint member to the center locus decreases toward the opening of the outer joint member,

a center locus of the corresponding one of the balls rolling on the outer curved portion has an arc shape having an arc center at a position outward of a center of the corresponding one of the balls rolling on the outer curved portion, in the radial direction of the outer joint member, and

a distance between a center locus of the corresponding one of the balls rolling on the outer diameter-increasing portion and the central axis of the outer joint member increases toward the opening of the outer joint member.