A fixed constant velocity universal joint is a kind of constant velocity universal joint that is, for example, used as a means for transmitting rotational force from an engine to wheels of an automobile at a constant speed. The fixed constant velocity universal joint has a configuration in which two axes on a driving side and a driven side are connected, and rotational torque can be transmitted at a constant speed even when the two axes are at an operating angle. In general, a Birfield-type (BJ) and an undercut-free type (UJ) are widely known as the fixed constant velocity universal joint described above.
For example, the UJ-type fixed constant velocity universal joint includes an outer ring 3 serving as an outer member, an inner ring 6 serving as an inner member, a plurality of balls 7, and a cage 8, as shown in FIG. 41 and FIG. 42. A plurality of track grooves 2 are formed on an inner spherical surface 1 of the outer ring 3, running along an axial direction and evenly spaced in a circumferential direction. A plurality of track grooves 5 are formed on an outer spherical surface of 4 of the inner ring 6, running along an axial direction and evenly spaced in a circumferential direction. The track grooves 5 form pairs with the track grooves 2 on the outer ring 3. The balls 7 are interposed between the track grooves 2 on the outer ring 3 and the track grooves 5 on the inner ring 6 and transmit torque. The cage 8 is interposed between the inner spherical surface 1 of the outer ring 3 and the outer spherical surface 4 of the inner ring 6 and holds the balls 7. A plurality of pockets 9 housing the balls 7 are disposed in the cage 8 along the circumferential direction.
A track groove 2 on the outer ring 3 is composed of an inner side track groove 2a and an opening side track groove 2b. The inner side track groove 2a has a track groove bottom that is an arc section. The opening side track groove 2b has a track groove bottom that is a straight section parallel with an outer ring axial line. A center of curvature O1 of the inner side track groove 2a is shifted in an axial direction, from a joint center O towards an opening side of the outer ring 3. A track groove 5 on the inner ring 6 is composed of an inner side track groove 5a and an opening side track groove 5b. The inner side track groove 5a has a track groove bottom that is a straight section parallel to an inner ring axial line. The opening side track groove 5b has a track groove bottom that is an arc section. A center of curvature O2 of the opening side track groove 5b is provided separated from the joint center in an axial direction by an equal distance k in an inward direction on a side opposite of the center of curvature O1 of the inner side track groove 2a on the outer ring 3.
A curvature radius O3 of a cage outer spherical surface 8a is shifted in an axial direction, from the joint center O to an opening side of the cage 8. A center of curvature O4 of a cage inner spherical surface 8b is provided separated from the joint center O in the axial direction by an equal distance k1 in an inward direction on a side opposite of the center of curvature O3 of the cage outer spherical surface 8a. Conventionally, in this type of constant velocity universal joint, track offset amounts of the inner and outer rings are large, and an offset amount of the cage is small. A pitch angle of two adjacent balls 7 is 60 degrees, as shown in FIG. 42. In other words, six balls 7 are disposed along the circumferential direction at a pitch of 60 degrees.
In recent years, demands are being made for a fixed velocity universal joint that is more compact and has increased torque load capacity. As a method of reducing the size and increasing the torque load capacity of the fixed constant velocity universal gear having six balls, balls that are as large as possible being disposed on a pitch circle diameter (PCD) that is as small as possible can be considered. However, when large balls are used, columns (window columns) between the pockets in the cage become narrow. Rigidity of the cage deteriorates. Torsional torque load at a high angle particularly contributes to deterioration of strength of the constant velocity universal joint because damage to the cage occurs more easily.
In particular, as shown in FIG. 41, when the track offset amounts of the inner and outer rings are large and the offset amount of the cage is small, a track groove depth on an outer ring inner side becomes shallow, and the torque load capacity at a high angle decreases. In other words, the ball easily runs over a track edge in relation to the torque load at a high angle, generating excessive stress in an edge section. Therefore, damage caused by a chip in the edge section and a locking phenomenon with the cage caused by plastic deformation occur. Such damage and locking phenomenon contributes to cage damage by degrading workability and reducing durability life. Moreover, a track depth on an inner side becomes shallow in the inner ring as well, and disadvantages similar to those of the outer ring occur. Therefore, improvement of joint strength and durability at a high angle range has been an issue since the past.
Conventionally, durability of an overall constant velocity universal joint is achieved through securement of cage durability being achieved by an increase in a circumferential direction length dimension of a column section present between pockets adjacent in a circumferential direction (Patent Document 1). In other words, in an invention described in Patent Document 1, space between balls housed within a same pocket is reduced by a plurality of balls being housed in a single pocket. As a result, the circumferential direction length dimension of the column section present between the pockets adjacent in the circumferential direction is increased. The number of balls is seven or more.
Conventionally, there is a fixed constant velocity universal joint in which two balls are housed in each pocket in the cage, and periodic change in moment during rotation is controlled (Patent Document 2). In other words, in an invention described in Patent Document 2, a circumferential direction length of a single window column is increased with all pockets in the cage serving as a long window with wide circumferential direction spacing between the pockets.    Patent Document 1: Japanese Patent Laid-open Publication No. Heisei 11-303882    Patent Document 2: United Kingdom Patent No. 1537067