TECHNICAL FIELD 
     The present invention relates to a fixed type constant velocity universal joint, and more specifically, to a fixed type constant velocity universal joint which is used in a power transmission system for automobiles and various industrial machines and which allows only angular displacement between two shafts on a driving side and a driven side. 
     BACKGROUND ART 
     For example, a fixed type constant velocity universal joint can be taken as an example of a constant velocity universal joint used as means for transmitting a rotational force from an engine to wheels of an automobile at a constant velocity. The fixed type constant velocity universal joint has a structure in which two shafts on a driving side and a driven side are coupled to each other and rotational torque can be transmitted at a constant velocity even when the two shafts form an operating angle. Generally, a Birfield type (BJ) constant velocity universal joint and an undercut-free type (UJ) constant velocity universal joint have been widely known as the above-mentioned fixed type constant velocity universal joint. 
     Further, as illustrated in  FIG. 6 , the fixed type constant velocity universal joint of the Birfield type (BJ) includes: an outer race  3  having an inner surface  1  in which a plurality of track grooves  2  are equiangularly formed along an axial direction and serving as an outer joint member; an inner race  6  having an outer surface  4  in which a plurality of track grooves  5  are equiangularly formed in pairs with the track grooves  2  of the outer race  3  along the axial direction and serving as an inner joint member; a plurality of balls  7  interposed between the track grooves  2  of the outer race  3  and the track grooves  5  of the inner race  6 , for transmitting torque; and a cage  8  interposed between the inner surface  1  of the outer race  3  and the outer surface  4  of the inner race  6 , for retaining the balls  7 . In the cage  8 , a plurality of window portions  9  for housing the balls  7  are arranged along a circumferential direction. 
     On opening edges (side edges) of each of the track grooves  2  of the outer race  3  and opening edges (side edges) of each of the track grooves  5  of the inner race  6 , in order to avoid stress concentration on both the side edges thereof, chamfers  10 ,  10 ,  11 , and  11  are provided as illustrated in  FIGS. 7 and 8 . 
     In some conventional cases, the chamfers are finished so as to have a round shape (Patent Literatures 1 to 3). By finishing of each of the chamfers into a round shape as just described, stress concentration upon application of high torque (upon input of excessive torque from a vehicle) is easily reduced. Further, the round-shaped chamfers are designed to prevent the edges from being chipped when the balls are pressed against the track grooves and climb onto track edges (track-groove side edges) upon the application of high torque. As a result, shortening of a service life is prevented. 
     Incidentally, as illustrated in  FIG. 7 , on an opening side of the outer race  3 , there is provided an inlet tapered portion  12  functioning as an angle-limitation stopper so that a shaft does not form more than a certain angle when forming an angle. Normally, a track-groove corresponding edge portion  12   a  on the inlet tapered portion  12  (edge portion on an axial end portion of each of the track grooves) is formed as a sharp edge. However, in order to reduce stress concentration at a high angle, the track-groove corresponding edge portion  12   a  is chamfered by a machining process in some cases. Further, as illustrated in  FIG. 8 , an axial edge  13  of each of the track grooves  5  of the inner race  6  is formed in a shape of a sharp edge portion. 
     CITATION LIST 
     
         
         Patent Literature 1: Japanese Utility Model Application Laid-open No. Hei 06-24237 
         Patent Literature 2: Japanese Utility Model Examined Publication No. Hei 07-25458 
         Patent Literature 3: Japanese Patent Application Laid-open No. 2008-2625 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     When the constant velocity universal joint is exposed to high torque (input of excessive torque from a vehicle), there occurs a phenomenon that the balls  7  climb onto the track-side edge portions of the track grooves  2  and  5 , with the result that the balls  7  reach the chamfers  10  and  11  on both the side edges of the tracks. Under the circumstance, conventionally, each of the chamfers  10  and  11  have been formed in a round shape so as to reduce stress concentration, and thus the edge portions of the chamfers  10  and  11  have been prevented from being chipped. 
     Meanwhile, at the time of an unexpected high-angle operation, in particular, when an angle expected during use of a constant velocity universal joint is exceeded for some reason, the ball  7  moves to the track-groove corresponding edge portion  12   a  on the inlet tapered portion  12  of the track grooves  2  of the outer race  3  or to the axial edge (edge portion)  13  of each of the track grooves  5  of the inner race  6 . As a result, the ball  7  comes into contact with the track-groove corresponding edge portion  12   a  and the axial edge  13 . When high torque is applied in this state, the ball  7  bites into the track-groove corresponding edge portion  12   a  and the like, with the result that the track-groove corresponding edge portion  12   a  and the like are chipped. Once an excessively high angle is formed and the track-groove corresponding edge portion  12   a  and the like are chipped, damage develops from the chipped portions, with the result that a durability life of the joint as a whole is shortened. 
     In view of the above-mentioned problems, the present invention has been made to provide a fixed type constant velocity universal joint which is capable of achieving the following even at the time of an unexpected high-angle operation, in particular, even when an angle expected during use of a constant velocity universal joint is exceeded: reduction of stress generated when the balls and the edge portions (edge portions on the axial end portions of the track grooves) interfere with each other, suppression of chipping of the edge portions, and prolongation of a service life of the joint as a whole. 
     Solution to Problem 
     A first fixed type constant velocity universal joint according to the present invention includes: an outer joint member having an inner surface in which a plurality of track grooves are formed; an inner joint member having an outer surface in which a plurality of track grooves are formed; a plurality of balls interposed between the plurality of track grooves of the outer joint member and the plurality of track grooves of the inner joint member, for transmitting torque; and a retainer for retaining the plurality of balls, in which a cutout round portion is provided at least at a ball-contact-point corresponding part on a track inlet end of each of the plurality of track grooves of the outer joint member. 
     A second fixed type constant velocity universal joint according to the present invention includes: an outer joint member having an inner surface in which a plurality of track grooves are formed; an inner joint member having an outer surface in which a plurality of track grooves are formed; a plurality of balls interposed between the plurality of track grooves of the outer joint member and the plurality of track grooves of the inner joint member, for transmitting torque; and a retainer for retaining the plurality of balls, in which a cutout round portion is provided at least at a ball-contact-point corresponding part on a track inlet end of each of the plurality of track grooves of the inner joint member. 
     A third fixed type constant velocity universal joint according to the present invention includes: an outer joint member having an inner surface in which a plurality of track grooves are formed; an inner joint member having an outer surface in which a plurality of track grooves are formed; a plurality of balls interposed between the plurality of track grooves of the outer joint member and the plurality of track grooves of the inner joint member, for transmitting torque; and a retainer for retaining the plurality of balls, in which: a cutout round portion is provided at least at a ball-contact corresponding part on a track inlet end of each of the plurality of track grooves of the outer joint member; and a cutout round portion is provided at least at a ball-contact-point corresponding part on a track inlet end of each of the plurality of track grooves of the inner joint member. 
     According to the present invention, even at the time of an unexpected high-angle operation, in particular, even when an angle expected during use is exceeded for some reasons and the balls are positioned at axial end portions of the track grooves of the outer joint member and/or the inner joint member, it is possible that the cutout round portion prevents each of the balls from biting into the axial end portions. 
     The cutout round portion may be finished by cold forging formation, and an entire of each of the plurality of track grooves may be finished by cold forging formation. Further, a tapered portion expanding from an interior side to an inlet side may be provided at an inlet end portion of the outer joint member, the tapered portion being finished by the cold forging formation, and the cutout round portion may be finished by a cutting process. Still further, machining allowance may be provided with respect to a grinding process of the plurality of track grooves, and the cutout round portion may be secured as a cold-forging finished portion even after the grinding process of the plurality of track grooves. 
     It is preferred that a PCD clearance representing a difference between a pitch circle diameter of each of the plurality of track grooves of the outer joint member and a pitch circle diameter of each of the plurality of track grooves of the inner joint member be set to range from −0.02 mm to +0.3 mm. With this setting, backlash between components including the outer joint member, the inner joint member, the balls, and the retainer (cage) can be suppressed to the minimum. Note that, when the PCD clearance is less than −0.02 mm, it is difficult to secure operability of the constant velocity universal joint. In contrast, when the PCD clearance is more than +0.3 mm, the backlash between the components becomes larger. 
     Advantageous Effects of Invention 
     According to the constant velocity universal joint of the present invention, even when the balls are positioned at the axial end portions of the track grooves of the outer joint member and/or the inner joint member at the time of an unexpected high-angle operation and the like, it is possible that the cutout round portion prevents each of the balls from biting into the axial end portions. That is, even in such a case, it is possible to reduce stress generated when the ball and the edge portions (edge portions on the axial end portions) of the track grooves interfere with each other, to thereby reduce a chipping risk of the edge portions. As a result, a service life of the constant velocity universal joint as a whole can be prolonged. 
     The cutout round portion can be finished by cold forging formation, a cutting process, or the like, and hence formation thereof does not involve complication. In particular, when the track grooves, the cutout round portion, and the inlet tapered portion are finished simultaneously by cold forging, post-processes (turning or ground-finishing after thermal treatment) can be omitted. Therefore, it is possible to achieve reduction of a formation time period and cost reduction. 
     Further, when the track grooves are finished by a grinding process, it is preferred that machining allowance be provided with respect to the grinding process of the track grooves and the cutout round portion be secured as a cold-forging finished portion even after the grinding process of the track grooves. With this method, the cutout round portion can be reliably formed. 
     By setting of the PCD clearance to range from −0.02 to +0.3 mm, the backlash between the components can be suppressed to the minimum, and generation of rattling noise can be suppressed at the time of attachment of the constant velocity universal joint to a vehicle. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  A sectional view of a fixed type constant velocity universal joint according to an embodiment of the present invention. 
         FIG. 2A  A perspective view of a main portion, illustrating cutout round portions formed in an outer race of the fixed type constant velocity universal joint, the cutout round portions being provided at a ball-contact-point corresponding part. 
         FIG. 2B  A perspective view of a main portion, illustrating the cutout round portion formed in the outer race of the fixed type constant velocity universal joint, the cutout round portion being provided over the entire of an axial end portion. 
         FIG. 3A  A perspective view of a main portion, illustrating cutout round portions formed in an inner race of the fixed type constant velocity universal joint, the cutout round portions being provided at the ball-contact-point corresponding part. 
         FIG. 3B  A perspective view of a main portion, illustrating the cutout round portion formed in the inner race of the fixed type constant velocity universal joint, the cutout round portion being provided over the entire of an axial end portion. 
         FIG. 4  A sectional view illustrating shapes of track grooves of the fixed type constant velocity universal joint. 
         FIG. 5A  An enlarged sectional view of a main portion of a finished product, illustrating a forming method for the outer race of the fixed type constant velocity universal joint. 
         FIG. 5B  An enlarged sectional view of a main portion in a state in which machining allowance is provided, illustrating the forming method for the outer race of the fixed type constant velocity universal joint. 
         FIG. 6  A sectional view of a conventional fixed type constant velocity universal joint. 
         FIG. 7  A schematic perspective view of an outer race of the conventional fixed type constant velocity universal joint. 
         FIG. 8  A schematic perspective view of an inner race of the conventional fixed type constant velocity universal joint. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following, description is made of the embodiment of the present invention with reference to  FIGS. 1 to 5 . 
     A fixed type constant velocity universal joint according to the present invention includes, as illustrated in  FIG. 1 , an outer race  23  having an inner surface  21  in which a plurality of track grooves  22  are formed along an axial direction and serving as an outer joint member, and an inner race  26  having an outer surface  24  in which a plurality of track grooves  25  are formed along the axial direction and serving as an inner joint member. The track grooves  22  of the outer race  23  and the track grooves  25  of the inner race  26  are provided in pairs, and balls  27  for transmitting torque are interposed between the track grooves  22  of the outer race  23  and the track grooves  25  of the inner race  26 . A cage (retainer)  28  is interposed between the inner surface  21  of the outer race  23  and the outer surface  24  of the inner race  26 , and the balls  27  are retained in a plurality of window portions (pockets)  29  arranged at a predetermined pitch along a circumferential direction of the retainer  28 . 
     The track grooves  22  of the outer race  23  and the track grooves  25  of the inner race  26  have a Gothic-arch shape obtained by only a forging process, or by a cutting process after the forging process, or the like. As illustrated in  FIG. 4 , by adoption of the Gothic-arch shape, the track grooves  22  and  25  and the ball  27  are held in angular contact with each other. That is, the ball  27  is held in contact with the track groove  22  of the outer race  23  at two points C 11  and C 12 , and in contact with the track groove  25  of the inner race  26  at two points C 21  and C 22 . Angles formed between a center O 1  of the ball  27  and each of the contact points C 11 , C 12 , C 21 , and C 22  of the track grooves  22  and  25  are contact angles α. 
     Each of the track grooves  22  of the outer race  23  has chamfers (chamfered portions)  30  and  30  provided on both side edges (groove opening edges) thereof, and each of the track grooves  25  of the inner race  26  has chamfers (chamfered portions)  31  and  31  provided on both side edges (groove opening edges) thereof. Further, an inlet tapered portion  35  expanding from an interior side to an inlet side is provided at an opening end of the outer race  23 . The inlet tapered portion  35  functions as an angle-limitation stopper. 
     As illustrated in  FIG. 2A , cutout round portions  32  and  32  are provided at a ball-contact-point corresponding part on a track inlet end  22   a  of each of the track grooves  22  of the outer race  23 . Further, as illustrated in  FIG. 3A , cutout round portions  33  and  33  are provided at the ball-contact-point corresponding part on a track inlet end  25   a  of each of the track grooves  25  of the inner race  26 . 
     As illustrated in  FIG. 2B , the cutout round portion  32  of the outer race  23  may be provided over the entire of the track inlet end  22   a . Further, as illustrated in  FIG. 3B , the cutout round portion  33  of the inner race  26  may be provided over the entire of the track inlet end  25   a  as well. 
     Incidentally, the cutout round portion  32  of the outer race  23  and the cutout round portion  33  of the inner race  26  can be formed by forging simultaneously with other portions at the time of forging. Further, when the track grooves  22  and  25  are formed by only a forging process, or by a cutting process after the forging process, or the like, the cutout round portions  32  and  33  may be formed by processes such as cutting and grinding after the forging. 
     When the track grooves  22  and  25  are finished by a grinding process after finishing of the cutout round portions  32  and  33  by cold forging, it is preferred to set machining allowance in track-groove grinding portions so that the cutout round portions finished by cold forging after grinding are reliably secured. 
     For example, in a case of the outer race  23  as illustrated in  FIG. 5A , when machining allowance  36  is set on the track groove  22  and the machining allowance  36  is removed by a grinding process of the track groove  22  as illustrated in  FIG. 5B , the cutout round portion  32  finished by cold forging is not influenced by the grinding process of the track groove  22 . As a result, the cutout round portion  32  is capable of maintaining a shape after being finished by the cold forging. Note that, although not shown, on the inner race  26  as well, the machining allowance  36  may be secured in a grinding process of the track groove  25 . 
     Incidentally, in the constant velocity universal joint, a PCD clearance is set to range from −0.02 mm to +0.3 mm. The PCD clearance represents a difference between a pitch circle diameter of each of the track grooves  22  of the outer race  23  and a pitch circle diameter of each of the track grooves  25  of the inner race  26 , that is, a difference between a pitch circle diameter of the balls  27  (outer race PCD) in a state in which the balls  27  are held in contact with the track grooves  22  of the outer race  23  and a pitch circle diameter of the balls  27  (inner race PCD) in a state in which the balls  27  are held in contact with the track grooves  25  of the inner race  26 . Setting of the PCD clearance to zero or a negative value means closing of the PCD clearance. 
     Although the cutout round portions  32  and  33  are provided to the outer race  23  and the inner race  26  as described above in this embodiment, as another embodiment, it is possible to use a constant velocity universal joint in which the cutout round portion  32  is provided only to the outer race  23 , or possible to use a constant velocity universal joint in which the cutout round portion  33  is provided only to the inner race  26 . 
     Further, although the case where each of the balls  27  and the track grooves  22  and  25  are held in angular contact with each other is described above in this embodiment, in some constant velocity universal joints, each of the balls  27  and the track grooves  22  and  25  are held in circular contact with each other. In the case where such circular contact is made, each of the balls is held in contact at one point with each of the inner race track and the outer race track, and the one contact point moves over the entire of cross-section of each of the track grooves. Thus, as illustrated, for example, in  FIGS. 2B and 3B , the movement at the one contact point can be coped with by the cutout round portions  32  and  33  formed over the entire of the track inlet ends  22   a  and  25   a.    
     In the present invention, at the time of a high-angle operation, when the balls  27  are positioned at axial end portions of the track grooves  22  and  25  of the outer race  23  and/or the inner race  26 , the balls  27  are prevented from biting into the axial end portions. That is, it is possible to reduce stress generated when the balls  27  and edge portions (edge portions on the axial end portions) of the track grooves  22  and  25  interfere with each other, to thereby reduce a chipping risk of the edge portions. As a result, a service life of the constant velocity universal joint as a whole can be prolonged. 
     The cutout round portions  32  and  33  can be finished by cold forging formation, a cutting process, or the like, and hence formation thereof does not involve complication. In particular, when the track grooves  22  and  25 , the cutout round portions  32  and  33 , and the inlet tapered portion  35  are finished simultaneously by cold forging, post-processes (turning or ground-finishing after thermal treatment) can be omitted. Therefore, it is possible to achieve reduction of a formation time period and cost reduction. 
     When the track grooves  22  and  25  are finished by a grinding process after finishing of the cutout round portions  32  and  33  by cold forging, it is preferred to set machining allowance in the track-groove grinding portions. When the machining allowance is removed by a grinding process of the track grooves  22  and  25 , the cutout round portions  32  and  33  finished by the cold forging are not influenced by the grinding process of the track grooves  22  and  25 . As a result, each of the cutout round portions  32  and  33  is capable of maintaining a shape after being finished by the cold forging, and hence the cutout round portions can be formed at low cost. 
     By setting of the PCD clearance to range from −0.02 to +0.3 mm, backlash between components can be suppressed to the minimum, and generation of rattling noise can be suppressed at the time of attachment of the constant velocity universal joint to a vehicle. That is, by setting the PCD clearance to be small as just described, a phase region free from a load on the ball  27  can be reduced or eliminated. As a result, behavior of the ball  27  can be stabilized until the ball  27  is re-accommodated into the track groove  22  of the outer race  23  after once dropping off the track groove  22 . In addition, the behavior of the ball  27  can be stabilized also by reduction or elimination of the phase region free from the load on the ball  27 . As a result, it is possible to suppress generation of vibration or abnormal noise. 
     Hereinabove, although description has been made of the embodiment according to the present invention, the present invention is not limited to the above-mentioned embodiment, and various modification can be made thereto. For example, a size, a curvature radius, and the like of each of the cutout round portions  32  and  33  to be formed can be variously changed as long as problems do not occur, for example, in the following cases: the balls are less liable to bite into the axial end portions, the balls roll, and operating angles are formed. Further, a center curvature of each of the track grooves  22  of the outer race  23  and a center curvature of each of the track grooves  25  of the inner race  26  may be offset in a radial direction (radial offset) relative to a joint axis. Still further, arrangement pitches of the track grooves  22  and  25  in a peripheral direction may be equal pitches or unequal pitches, and the number of the balls, in other words, the number of the track grooves  22  and  25  may be arbitrarily increased and reduced. 
     INDUSTRIAL APPLICABILITY 
     As the constant velocity universal joint, one of an undercut-free type may be used, in which track groove bottoms are each provided with a circular-arc portion and a straight portion, or another constant velocity universal joint may be used, which has a shape in which portions corresponding to linear portions of the undercut-free type exhibit tapered shapes. Alternatively, still another constant velocity universal joint may be used, in which track groove bottoms are provided with a plurality of circular-arc portions having curvature radii different from each other. 
     REFERENCE SIGNS LIST 
     
         
         
           
               21  inner surface 
               22 ,  25  track groove 
               22   a  track inlet end 
               24  outer surface 
               25   a  track inlet end 
               27  ball 
               28  retainer 
               32 ,  33  cutout round portion 
               36  machining allowance
A fixed type constant velocity universal joint has cutout round portions ( 32) provided at two portions of a ball-contact-point corresponding part on a track inlet end ( 22 a) of each of the track grooves ( 22) of the outer joint member for reduce biting of balls. The fixed type constant velocity universal joint is capable of achieving reduction of stress generated when balls and edge portions interfere with each other, suppression of chipping of the edge portions, and prolongation of a service life of the joint as a whole. These advantages are achieved even at a time of an unexpected high-angle operation, and in particular even when an angle expected during use of the constant velocity universal joint is exceeded.