As fixed type constant velocity universal joints, there have been publicly known joints of a six-ball Rzeppa type (BJ), a six-ball undercut-free type (UJ), an eight-ball Rzeppa type (EBJ), an eight-ball undercut-free type (EUJ), and a track groove crossing type in which paired track grooves cross each other (see, for example, Patent Literature 1). Those joints are used as appropriate in accordance with purposes, required characteristics, and the like.
Referring to FIG. 14a and FIG. 14b, description is given of an example of the fixed type constant velocity universal joint of the track groove crossing type (hereinafter referred to as “crossing fixed type CVJ”). FIG. 14a is a vertical sectional view (sectional view taken along the line A-O-B in FIG. 14b) of a state in which the crossing fixed type CVJ for a propeller shaft forms an operating angle of 0°, and FIG. 14b is a front view of the crossing fixed type CVJ. The constant velocity universal joint 100 includes an outer joint member 102, an inner joint member 103, balls 104, and a cage 105. Eight arc-shaped track grooves 107 are formed in a spherical inner peripheral surface 106 of the outer joint member 102. The track grooves 107 are formed so that planes including ball raceway center lines x of the track grooves 107 are inclined with respect to a joint axial line n-n and the track grooves 107 are adjacent to each other in a circumferential direction with their inclination directions opposite to each other. Eight arc-shaped track grooves 109 are formed in a spherical outer peripheral surface 108 of the inner joint member 103. The track grooves 109 are formed so as to be mirror-image symmetrical with the paired track grooves 107 of the outer joint member 102 with respect to a plane P including a joint center O at the operating angle of 0°. That is, the inner joint member 103 is assembled to an inner periphery of the outer joint member 102 so that the paired track grooves 107 and 109 cross each other.
As illustrated in FIG. 14a, curvature centers of the track grooves 107 of the outer joint member 102 and the track grooves 109 of the inner joint member 103 are each positioned at the joint center O. The balls 104 are interposed in crossing portions between the paired track grooves 107 and 109, and are received and held in pocket portions 105a of the cage 105 arranged between the spherical inner peripheral surface 106 of the outer joint member 102 and the spherical outer peripheral surface 108 of the inner joint member 103. Curvature centers of a spherical outer peripheral surface 111 and a spherical inner peripheral surface 112 of the cage 105 are each positioned at the joint center O. In the constant velocity universal joint 100, the balls 104 are interposed in the crossing portions between the paired track grooves 107 and 109. Therefore, when the joint forms an operating angle, the balls 104 are always guided in a plane bisecting an angle formed between axial lines of the outer joint member 102 and the inner joint member 103. With this, rotational torque is transmitted at a constant velocity between the two axes.
Further, the track grooves 107 of the outer joint member 102 and the track grooves 109 of the inner joint member 103 are adjacent to each other in the circumferential direction with their inclination directions opposite to each other. Therefore, when both the joint members 102 and 103 rotate relative to each other at the operating angle of 0°, forces in the opposite directions are applied from the balls 104 to the pocket portions 105a of the cage 105 that are adjacent to each other in the circumferential direction. With this, the cage 105 is stabilized at the position of the joint center O. Thus, a contact force between the spherical outer peripheral surface 111 of the cage 105 and the spherical inner peripheral surface 106 of the outer joint member 102, and a contact force between the spherical inner peripheral surface 112 of the cage 105 and the spherical outer peripheral surface 108 of the inner joint member 103 are suppressed. As a result, it is possible to attain the constant velocity universal joint 100 that is capable of suppressing torque loss and heat generation, and is excellent in durability. Thus, with use of the constant velocity universal joint 100, it is possible to attain a propeller shaft that is suppressed in torque loss and heat generation, and is enhanced in efficiency.
Operability of fixed type constant velocity universal joints including the constant velocity universal joint 100 described above, in which rotational torque is transmitted between both the joint members through intermediation of the balls, is secured by setting wedge angles of ball tracks formed by the paired track grooves (by holding the balls with the paired track grooves). In the constant velocity universal joint 100 of the track groove crossing type, as described above, when wedge angles, which are formed so that a force of pressing the cage 105 to one axial side (for example, right side in FIG. 14a) is applied from the balls 104 at the operating angle of 0°, are defined as positive wedge angles, and wedge angles, which are formed so that a force of pressing the cage 105 to the other axial side is applied from the balls 104 at the operating angle of 0°, are defined as negative wedge angles, ball tracks that form the positive wedge angles and ball tracks that form the negative wedge angles are alternately formed in the circumferential direction. With this, desired joint performance is secured. In the constant velocity universal joint 100 described above, the curvature centers of the track grooves 107 and 109 are each positioned at the joint center O, and hence sizes and opening directions of the wedge angles are determined in accordance with angles (inclination angles) that are formed by the track grooves 107 and 109 (by planes including the ball raceway center lines thereof) with respect to the joint axial line n-n, contact angles of the balls 104 with respect to the track grooves 107 and 109, and operating angles.