FIG. 14 is an illustration of a fixed type constant velocity universal joint. The fixed type constant velocity universal joint is of an undercut-free type, and includes an outer joint member 3 including a cup section 10 having a plurality of track grooves 2 formed in a radially inner surface 1 thereof, an inner joint member 6 having a plurality of track grooves 5 formed in a radially outer surface 4 thereof and paired with the track grooves 2 of the outer joint member 3, a plurality of balls 7, which are interposed between the track grooves 2 of the outer joint member 3 and the track grooves 5 of the inner joint member 6, and are configured to transmit torque therebetween, and a cage 8, which is interposed between the radially inner surface 1 of the outer joint member 3 and the radially outer surface 4 of the inner joint member 6, and is configured to retain the balls 7. In the cage 8, a plurality of window portions 9 configured to accommodate the balls 7 therein are formed along a circumferential direction.
In the fixed type constant velocity universal joint, a groove bottom of each of the track grooves 2 of the outer joint member 3 includes a straight portion 2a (linear portion parallel to an axial direction of the outer joint member) on an opening side, and a circular-arc portion 2b on a deep side. A groove bottom of each of the track grooves 5 of the inner joint member 6 includes a circular-arc portion 5a on an opening portion side, and a straight portion 5b (linear portion parallel to an axial direction of the inner joint member 6) on the deep side. In this case, a center O1 of the track grooves 2 of the outer joint member 3 and a center O2 of the track grooves 5 of the inner joint member 6 are axially offset to opposite sides with respect to a joint center O by equal distances f and f.
In general, the outer joint member of the constant velocity universal joint is manufactured by the method as follows. First, a columnar billet is formed by hot forging, warm forging, or cold forging into a schematic shape of the outer joint member, and then subjected to a turning process into an arbitrary shape. After that, the processed product is subjected to heat treatment, and the inner spherical surface and the track grooves are subjected to a finishing process such as grinding and quenched-steel cutting.
The finishing process performed on the track grooves after forging, turning, and heat treatment as described above involves increase in cost of equipment and tools required for the finishing process on the track grooves. Moreover, time periods are required for the finishing process, thereby causing inconvenience that a material yield is lowered. Accordingly, in the related-art methods, the track grooves of the outer joint member are formed by cold-forging finishing (Patent Literature 1 and Patent Literature 2). Further, for example, the track grooves, radially inner spherical surface portions, a cup-inlet chamfer, track chamfers, and track-inlet chamfers of the outer joint member may be finished by cold forging (Patent Literature 3).
The track grooves of the outer joint member are formed by cold-forging finishing, thereby being capable of omitting various types of machining such as cutting work and grinding that are performed after cold forging in the related art. As a result, the yield is increased, thereby being capable of reducing cost of the constant velocity universal joint.
Incidentally, for the cost reduction, a weight of a product may be reduced by downsizing the constant velocity universal joint. However, when assembling components (particularly when incorporating the cage into the outer joint member), the cage has been incorporated under a condition that an inlet diameter of the cup section of the outer joint member is set larger than an outermost diameter of a hole portion of a cage window.
For the downsizing of the constant velocity universal joint, it is also necessary to downsize interior components (such as the inner joint member, the cage, and the balls) arranged inside the cup section of the outer joint member. However, it is necessary to keep torque bearing capacity.
Inside the constant velocity universal joint, load is transmitted in the order of the inner joint member, the balls, and the outer joint member, and in the order of the outer joint member, the balls, and the inner joint member. However, when a ball pitch circle diameter (PCD) of the constant velocity universal joint is reduced along with downsizing thereof, the load on the inner joint member, the outer joint member, and the balls is increased as compared to that on the related-art product. Accordingly, the constant velocity universal joint is designed, for example, in such a manner that a ball diameter is increased in order to disperse the load, thereby equalizing a contact surface pressure between the balls and the inner joint member or the outer joint member to that on the related-art product. Further, the same consideration is also given to a shape of the cage configured to accommodate the balls therein.
However, even when the shape is optimized by the design, depending on the ball diameter, the ball PCD, and the cage shape, as illustrated in FIG. 15 to FIG. 17, the cage 8 or the like may not be easily incorporated into the outer joint member 3.
That is, when the cage 8 is incorporated into the outer joint member 3, first, as illustrated in FIG. 16 and FIG. 17, the cage 8 is inserted into the radially inner surface 1 of the outer joint member 3 under a state in which the cage 8 is turned by 90° about a Y axis. Then, after the cage 8 is pushed into a bottom portion of the cup section 10 of the outer joint member 3, the cage 8 is turned by 90° about the Y axis, to thereby be arranged in a normal posture. Here, the Y axis refers to an axis orthogonal to an X axis corresponding to a center axis of the outer joint member 3.
However, when the cage 8 is inserted into the outer joint member 3 as illustrated in FIG. 16 and FIG. 17, an edge 12 (see FIG. 15) of a cage axial end portion (side surface portion) of each pocket (window portion) of the cage 8 may interfere with a radially inner opening portion (track chamfer) 13 of each of the track grooves 2 as illustrated in FIG. 17.
In the related art, a chamfered portion is formed at a boundary portion between a radially inner spherical surface portion of the outer joint member and a radially inner surface of a mouth opening portion. In this manner, incorporation of the cage into the outer joint member is facilitated (Patent Literature 4). In this case, the chamfered portion prevents a burr and the like from being generated on the boundary portion, thereby preventing the boundary portion from being formed into an angular shape. Further, in order to increase workability of incorporation of the balls, a cut portion has been formed in an end portion of each of the track grooves that are open to an inlet tapered portion of the cup section (Patent Literature 5 and Patent Literature 6).