Cage for constant velocity universal joint, fixed type constant velocity universal joint incorporating same, and drive shaft incorporating said fixed type constant velocity universal joint

Abstract

Provided is a cage (5, 65, 95) for a constant velocity universal joint, which is formed into a ring shape with a substantially uniform thickness, including a plurality of pockets (20, 80, 110) formed in a circumferential direction of the cage (5, 65, 95), for receiving torque transmitting balls, respectively, the cage (5, 65, 95) being formed of carbon steel including 0.41 to 0.51 mass % of C, 0.10 to 0.35 mass % of Si, 0.60 to 0.90 mass % of Mn, 0.005 to 0.030 mass % of P, and 0.002 to 0.035 mass % of S, with the balance being Fe and an element inevitably remaining at the time of steelmaking and refining, the cage (5, 65, 95) being subjected to carburizing, quenching, and tempering as heat treatment, each of the plurality of pockets (20, 80, 110) having a side surface (23, 83, 113) finished after the heat treatment.

Claims

1. A cage for a constant velocity universal joint, which is formed into a ring shape with a substantially uniform thickness, comprising a plurality of pockets formed in a circumferential direction of the cage, for receiving torque transmitting balls, respectively, the cage being formed of carbon steel comprising 0.41 to 0.51 mass % of C, 0.10 to 0.35 mass % of Si, 0.60 to 0.90 mass % of Mn, 0.005 to 0.030 mass % of P, and 0.002 to 0.035 mass % of S, with the balance being Fe and an element inevitably remaining at the time of steelmaking and refining, the cage being subjected to carburizing, quenching, and tempering as heat treatment, each of the plurality of pockets having a side surface finished after the heat treatment, wherein the cage has a surface layer which is formed by the heat treatment and a core, the surface layer having a surface hardness of 58 HRC or more and a carbon concentration of 0.55 to 0.75 mass %, and the core having a core hardness of from 56 to 59 HRC.

2. The cage for a constant velocity universal joint according to claim 1, wherein the cage has a spherical outer peripheral surface and a spherical inner peripheral surface, and wherein the cage has the substantially uniform thickness so that an axial offset amount between a curvature center of the spherical outer peripheral surface and a curvature center of the spherical inner peripheral surface is less than 1 mm.

3. The cage for a constant velocity universal joint according to claim 1, wherein the carbon steel for forming the cage comprises 0.42 to 0.48 mass % of C.

4. The cage for a constant velocity universal joint according to claim 1, wherein a total case depth of the surface layer is set to 0.25 to 0.55 mm.

5. A constant velocity universal joint, which incorporates the cage for a constant velocity universal joint according to claim 1.

6. A drive shaft, which incorporates the constant velocity universal joint according to claim 5.

7. The cage for a constant velocity universal joint according to claim 2, wherein the carbon steel for forming the cage comprises 0.42 to 0.48 mass % of C.

8. The cage for a constant velocity universal joint according to claim 2, wherein a total case depth of the surface layer is set to 0.25 to 0.55 mm.

9. A constant velocity universal joint, which incorporates the cage for a constant velocity universal joint according to claim 2.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a longitudinal sectional view of a fixed type constant velocity universal joint incorporating a cage for a constant velocity universal joint according to a first embodiment of the present invention.

(2) FIG. 2 is a longitudinal sectional view illustrating a state in which the fixed type constant velocity universal joint of FIG. 1 forms a maximum operating angle.

(3) FIG. 3 is a longitudinal sectional view of the cage of FIG. 1.

(4) FIG. 4 is a lateral sectional view of the cage of FIG. 1.

(5) FIG. 5 is a longitudinal sectional view of an automotive drive shaft to which the fixed type constant velocity universal joint of FIG. 1 is applied.

(6) FIG. 6 is a longitudinal sectional view of a fixed type constant velocity universal joint incorporating a cage for a constant velocity universal joint according to a second embodiment of the present invention.

(7) FIG. 7 is a longitudinal sectional view of the cage of FIG. 6.

(8) FIG. 8 is a longitudinal sectional view of a plunging type constant velocity universal joint incorporating a cage for a constant velocity universal joint according to a third embodiment of the present invention.

(9) FIG. 9 is a schematic view illustrating a state of track grooves and the cage of the plunging type constant velocity universal joint of FIG. 8.

(10) FIG. 10 is a longitudinal sectional view of the cage of FIG. 8.

(11) FIG. 11 is a schematic view illustrating carburizing conditions of Example and Comparative Example.

(12) FIG. 12 is a graph showing measurement results of hardness of each of columnar portions of cages of Example and Comparative Example.

(13) FIG. 13 is a longitudinal sectional view of a plunging type constant velocity universal joint, for illustrating findings in a verification process to arrive at the present invention.

(14) FIG. 14 is a longitudinal sectional view of a cage of the constant velocity universal joint of FIG. 13.

(15) FIG. 15 is a longitudinal sectional view illustrating a related-art fixed type constant velocity universal joint.

EMBODIMENTS OF THE INVENTION

(16) Now, embodiments of the present invention are described with reference to the drawings.

(17) A cage for a constant velocity universal joint according to a first embodiment of the present invention is described with reference to FIGS. 1 to 5. FIG. 1 is a longitudinal sectional view of a fixed type constant velocity universal joint incorporating the cage of this embodiment. As compared to a related-art six-ball constant velocity universal joint, a constant velocity universal joint 1 of this embodiment, which is an eight-ball type Rzeppa constant velocity universal joint, has a smaller track offset amount, a larger number of balls, and has a smaller diameter. Thus, it is possible to achieve a highly efficient constant velocity universal joint that is lightweight and compact, and is suppressed in torque loss. The constant velocity universal joint 1 mainly comprises an outer joint member 2, an inner joint member 3, balls 4, and a cage 5.

(18) In a spherical inner peripheral surface 8 of the outer joint member 2, eight track grooves 6 are formed equiangularly so as to extend along an axial direction. In a spherical outer peripheral surface 9 of the inner joint member 3, track grooves 7 opposed to the track grooves 6 of the outer joint member 2 are formed equiangularly so as to extend along the axial direction. Each of the eight balls 4 for transmitting torque is incorporated between the track groove 6 of the outer joint member 2 and the track groove 7 of the inner joint member 3. The cage 5 for holding the balls 4 is arranged between the spherical inner peripheral surface 8 of the outer joint member 2 and the spherical outer peripheral surface 9 of the inner joint member 3. A spline 17 is formed in an inner peripheral surface 16 of the inner joint member 3, and a spline 19 of a shaft 12 is fitted into the spline 17 and fixed in the axial direction with a retaining ring 18. An outer periphery of the outer joint member 2 and an outer periphery of the shaft 12 coupled to the inner joint member 3 are covered with a boot 13, and the boot 13 is fixed by fastening with boot bands 14 and 15. Grease is sealed inside the joint as a lubricant.

(19) The cage 5 has a spherical outer peripheral surface 10 fitted to the spherical inner peripheral surface 8 of the outer joint member 2, and a spherical inner peripheral surface 11 fitted to the spherical outer peripheral surface 9 of the inner joint member 3. The spherical outer peripheral surface 10 and the spherical inner peripheral surface 11 each have a curvature center formed at a joint center O. On the other hand, a curvature center A of the track groove 6 of the outer joint member 2 and a curvature center B of the track groove 7 of the inner joint member 3 are offset in the axial direction by equal distances with respect to the joint center O. Thus, when the joint forms an operating angle, the balls 4 are always guided in a plane bisecting an angle formed between axial lines of the outer joint member 2 and the inner joint member 3 (operating angle). As a result, rotational torque is transmitted at a constant velocity between the two axes.

(20) Due to the configuration that the curvature center A of the track groove 6 of the outer joint member 2 and the curvature center B of the track groove 7 of the inner joint member 3 are offset in the axial direction by equal distances with respect to the joint center O, the opposing track grooves 6 and 7 of the outer joint member 2 and the inner joint member 3 form a wedge shape expanding from an interior side toward an opening side of the outer joint member 2. Each ball 4 is received in the track grooves 6 and 7 of the wedge shape, to thereby transmit torque between the outer joint member 2 and the inner joint member 3. The cage 5 is incorporated so as to hold all the balls 4 in the plane bisecting the operating angle. The track grooves 6 and 7 are formed into an elliptical shape or a Gothic arch shape in lateral cross section, and the track grooves 6 and 7 are held in so-called angular contact with each ball 4 at a contact angle (approximately from 30° to 45°). Thus, the ball 4 is held in contact with the track grooves 6 and 7 on their side surface sides, which are slightly spaced apart from groove bottoms of the track grooves 6 and 7.

(21) FIG. 2 is a longitudinal sectional view illustrating a state in which the constant velocity universal joint 1 forms a maximum operating angle θ.sub.max. The maximum operating angle θ.sub.max of the constant velocity universal joint 1 is approximately 47°. When the torque is transmitted under the state in which the constant velocity universal joint 1 forms an operating angle, a push-out force is applied to the ball 4, with the result that a pocket load caused by the ball 4 is applied to a side surface 23 (see FIG. 3) of a pocket 20 of the cage 5 in a direction of expansion of the space between the track grooves 6 and 7 of the wedge shape. Further, due to the pocket load, the cage 5 is pressed against the spherical inner peripheral surface 8 of the outer joint member 2 and the spherical outer peripheral surface 9 of the inner joint member 3. The wedge angle formed between the track grooves 6 and 7 of the wedge shape becomes higher as the operating angle becomes higher. Therefore, the push-out force applied to the ball 4 becomes greater as the torque and the operating angle become higher. Thus, in order to form a high operating angle while transmitting the torque, the cage 5 needs to have sufficient strength. Further, the spherical outer peripheral surface 10 and the spherical inner peripheral surface 11 of the cage 5 are slid in contact with the spherical inner peripheral surface 8 of the outer joint member 2 and the spherical outer peripheral surface 9 of the inner joint member 3, and hence the cage 5 needs to have sufficient abrasion resistance.

(22) Further, in order to reliably hold the ball 4 in the bisecting plane and prevent abnormal noise, the ball 4 is often incorporated into the pocket 20 of the cage 5 with a negative clearance. That is, the diameter of the ball 4 is slightly larger than a window dimension K (see FIG. 3) between the side surfaces 23 and 23 of the pocket 20 of the cage 5, which face each other in the axial direction. Further, when the joint forms an operating angle, the ball 4 is moved inside the pocket 20 of the cage 5 in a radial direction and a circumferential direction. The ball 4 is moved in the radial direction and the circumferential direction under the state in which the ball 4 is incorporated into the pocket 20 with the negative clearance as described above, and hence sufficient strength and abrasion resistance are necessary.

(23) FIGS. 3 and 4 illustrate the cage of this embodiment. FIG. 3 is a longitudinal sectional view taken along the line C-C of FIG. 4, and FIG. 4 is a lateral sectional view taken along the line D-D of FIG. 3. As illustrated in FIG. 3, the cage 5 is formed into a ring shape having the spherical outer peripheral surface 10 and the spherical inner peripheral surface 11. The curvature center of the spherical outer peripheral surface 10 and the curvature center of the spherical inner peripheral surface 11 are positioned at the joint center O with no axial offset (offset amount: 0 mm). The cage 5 is formed so as to have a small and uniform thickness. A mating portion 21 for incorporating the inner joint member 3 is formed at an end portion of the spherical inner peripheral surface 11 of the cage 5, which is positioned on the opening side of the outer joint member 2. This portion is thinned.

(24) It is preferred that the cage 5 have a substantially uniform thickness so that the axial offset amount between the curvature center of the spherical outer peripheral surface 10 and the curvature center of the spherical inner peripheral surface 11 is less than 1 mm. In a case of a cage having a large and non-uniform thickness, cooling is difficult to carry out at the time of carburizing and quenching. Thus, it has proved that a slack-quenched structure is liable to be generated. Further, when a thickness difference is increased, the window formation pressability is reduced. For those reasons, the axial offset amount between the curvature center of the spherical outer peripheral surface and the curvature center of the spherical inner peripheral surface is set to less than 1 mm, preferably less than 0.7 mm.

(25) As illustrated in FIGS. 3 and 4, the plurality of (eight) pockets 20 for receiving the balls 4 are formed in the cage 5 in the circumferential direction, and a columnar portion 22 is formed between the pockets 20 and 20. As described above, when the joint transmits the torque, the push-out force is applied to the ball 4, with the result that the pocket load caused by the ball 4 is applied to the side surfaces 23 of the pocket 20 of the cage 5, which face each other in the axial direction. Further, due to the pocket load, the spherical outer peripheral surface 10 and the spherical inner peripheral surface 11 of the cage 5 are pressed against the spherical inner peripheral surface 8 of the outer joint member 2 and the spherical outer peripheral surface 9 of the inner joint member 3, respectively.

(26) The material for the cage 4 is carbon steel, which contains, as components thereof, 0.41 to 0.51 mass % of C, 0.10 to 0.35 mass % of Si, 0.60 to 0.90 mass % of Mn, 0.005 to 0.030 mass % of P, and 0.002 to 0.035 mass % of S, with the balance being iron (Fe) and an element inevitably remaining at the time of steelmaking and refining. The cage 5 is subjected to carburizing, quenching, and tempering as heat treatment. After the heat treatment, the spherical outer peripheral surface 10 and the spherical inner peripheral surface 11 of the cage 5 are finished by grinding or cutting, and the side surface 23 of the pocket 20 is also finished by cutting. The cage 5 provides an advantage achieved by using low-cost carbon steel that is not actively doped with expensive chromium (Cr), molybdenum (Mo), or boron (B) in an increased amount. In addition, the carbon content of carbon steel is higher than that of carburizing steel, thereby being capable of greatly shortening the carburizing time. As a result, it is possible to achieve a cage having high strength, which is capable of achieving reduction in heat treatment cost and improvement in productivity, and is increased in core hardness. Further, the material is not a special material, thereby being capable of achieving global procurement of the material. Still further, the heat treatment involves the carburizing, quenching, and tempering, and hence the abrasion resistance is excellent as well as the strength characteristics.

(27) Further, it is preferred that carbon steel for forming the cage 5 contain 0.42 to 0.48 mass % of C. With this setting, a surface hardness of 58 HRC or more and a core hardness of from 56 to 59 HRC are obtained. In this case, the strength is most stable, thereby being capable of achieving a cage having high strength.

(28) It is preferred that a total case depth of the cage 5 be set to 0.25 to 0.55 mm. Higher surface hardness is appropriate after the carburizing, quenching, and tempering from the viewpoint of abrasion resistance, but lower surface hardness is appropriate from the viewpoint of strength. Therefore, it is preferred that a minimum hardness of 58 HRC, which does not cause generation of the slack-quenched structure, be set to a lower limit. As the case depth, an optimum depth is determined from the viewpoint of carburizing time and rolling life. Smaller case depth is desired from the viewpoint of carburizing time, but the rolling life becomes shorter when the case depth is small. Further, the spherical outer peripheral surface 10 and the spherical inner peripheral surface 11 of the cage 5 and the side surface 23 of the pocket 20 are finished after the carburizing, quenching, and tempering, and hence a machining allowance of 0.2 mm at a maximum is considered for the spherical outer peripheral surface 10 and the spherical inner peripheral surface 11, whereas a machining allowance of 0.1 mm at a maximum is considered for the side surface 23 of the pocket 20. In addition, from the viewpoint of rolling life, a thickness of 0.05 mm at a minimum needs to be secured for a carburized layer remaining after the final finishing. From those factors, it is preferred that the lower limit of the total case depth after the carburizing, quenching, and tempering be 0.25 mm. On the other hand, it is preferred that the upper limit be 0.55 mm in consideration of fluctuation in components of the material and fluctuation in carburizing conditions.

(29) Further, it is preferred that a carbon concentration in a surface layer of the cage 5 be set to 0.55 to 0.75 mass %. When the concentration in the surface resulting from the carburizing exceeds 0.75 mass %, pro-eutectoid cementite is liable to be deposited on an acute portion at a grain boundary after the carburizing, quenching, and tempering so that the strength is reduced significantly. For this reason, it is preferred that the upper limit be 0.75 mass %. When the concentration is less than 0.55 mass %, on the other hand, the hardness and the softening resistance characteristics are reduced so that the abrasion is increased significantly. For this reason, it is preferred that the lower limit be 0.55 mass %. Through the limitation of the carbon concentration, the total case depth after the carburizing, quenching, and tempering can further be increased, with the result that the strength is not reduced in a range of up to 0.75 mm.

(30) FIG. 5 illustrates an automotive front drive shaft 50, to which the fixed type Rzeppa constant velocity universal joint 1 incorporating the cage 5 for a constant velocity universal joint according to the first embodiment is applied. The Rzeppa constant velocity universal joint 1 mainly comprises the outer joint member 2, the inner joint member 3, the balls 4, and the cage 5 according to the first embodiment. The inner joint member 3 is spline-coupled to one end of the shaft 12. An inner joint member 33 of a plunging type double offset constant velocity universal joint 31 is spline-coupled to another end of the shaft 12. The double offset constant velocity universal joint 31 mainly comprises an outer joint member 32, the inner joint member 33, balls 34, and a cage 35. At positions between an outer peripheral surface of the Rzeppa constant velocity universal joint 1 and an outer peripheral surface of the shaft 12 and between an outer peripheral surface of the double offset constant velocity universal joint 31 and the outer peripheral surface of the shaft 12, bellows boots 13 and 42 are fixed by fastening with boot bands 14, 15, 45, and 46, respectively. Grease is sealed inside the joint as a lubricant.

(31) When the cage 5 of the first embodiment is applied to the fixed type constant velocity universal joint that is required to have the strength at high operating angles, it is possible to achieve low cost, high strength, and abrasion resistance comparable to that of the related-art cage made of carburizing steel (for example, SCr415 or SCM415), and by extension, to secure low cost, high strength, and abrasion resistance of the fixed type constant velocity universal joint and the drive shaft.

(32) Next, a cage for a constant velocity universal joint according to a second embodiment of the present invention is described with reference to FIGS. 6 and 7. FIG. 6 is a longitudinal sectional view of a fixed type constant velocity universal joint incorporating the cage of this embodiment, and FIG. 7 is a longitudinal sectional view of the above-mentioned cage. A constant velocity universal joint 61 of this embodiment is a six-ball type undercut-free constant velocity universal joint. The constant velocity universal joint 61 mainly comprises an outer joint member 62, an inner joint member 63, balls 64, and a cage 65.

(33) In a spherical inner peripheral surface 68 of the outer joint member 62, six track grooves 66 are formed equiangularly so as to extend along the axial direction. In a spherical outer peripheral surface 69 of the inner joint member 63, six track grooves 67 opposed to the track grooves 66 of the outer joint member 62 are formed equiangularly so as to extend along the axial direction. Each of the six balls 64 for transmitting torque is incorporated between the track groove 66 of the outer joint member 62 and the track groove 67 of the inner joint member 63. The cage 65 for holding the balls 64 is arranged between the spherical inner peripheral surface 68 of the outer joint member 62 and the spherical outer peripheral surface 69 of the inner joint member 63. A spline 77 is formed in an inner peripheral surface 76 of the inner joint member 63, and although illustration is omitted, a spline of a shaft is fitted into the spline 77 and fixed in the axial direction with a retaining ring. An outer periphery of the outer joint member 62 and an outer periphery of the shaft coupled to the inner joint member 63 are covered with a boot, and grease is sealed inside the joint as a lubricant.

(34) The cage 65 of this embodiment has a spherical outer peripheral surface 70 fitted to the spherical inner peripheral surface 68 of the outer joint member 62, and a spherical inner peripheral surface 71 fitted to the spherical outer peripheral surface 69 of the inner joint member 63. A curvature center E of the spherical outer peripheral surface 70 and a curvature center F of the spherical inner peripheral surface 71 are slightly offset in the axial direction by equal distances with respect to the joint center O. An offset amount f3 therebetween is 1 mm or less. The track groove 66 of the outer joint member 62 comprises an arc-shaped track groove portion 66a formed on the interior side, and a linear track groove portion 66b formed on the opening side. The arc-shaped track groove portion 66a has a curvature center G, and the linear track groove portion 66b is formed in parallel to a joint axial line X. The track groove 67 of the inner joint member 63 comprises an arc-shaped track groove portion 67a formed on the opening side, and a linear track groove portion 67b formed on the interior side. The arc-shaped track groove portion 67a has a curvature center H, and the linear track groove portion 67b is formed in parallel to the joint axial line X. The curvature center G of the arc-shaped track groove portion 66a of the outer joint member 62 and the curvature center H of the arc-shaped track groove portion 67a of the inner joint member 63 are offset in the axial direction by equal distances with respect to the joint center O. When the joint forms an operating angle, the balls 64 are always guided in a plane bisecting an angle formed between axial lines of the outer joint member 62 and the inner joint member 63 (operating angle). As a result, rotational torque is transmitted at a constant velocity between the two axes.

(35) Also in the constant velocity universal joint 61, the opposing track grooves 66 and 67 of the outer joint member 62 and the inner joint member 63 form a wedge shape expanding from the interior side toward the opening side of the outer joint member 62. Therefore, similarly to the cage 5 of the above-mentioned first embodiment, a pocket load is applied to the cage 65, and along with this, spherical contact forces are applied between the spherical outer peripheral surfaces 69 and 70 and the spherical inner peripheral surfaces 71 and 68, respectively. Further, in the constant velocity universal joint 61, the track grooves 66 and 67 of the outer joint member 62 and the inner joint member 63 comprise the linear track groove portions 66b and 67b, respectively. Thus, it is possible to form an operating angle of, for example, approximately 50° higher than that of the constant velocity universal joint 1 described above in the first embodiment. Further, with the linear track groove portion 66b and 67b, the wedge angle becomes even higher. Therefore, the cage 65 needs to have sufficient strength and abrasion resistance.

(36) FIG. 7 illustrates the cage of this embodiment. FIG. 7 is a longitudinal sectional view taken at a center of a columnar portion of the cage. The cage 65 is formed into a ring shape having the spherical outer peripheral surface 70 and the spherical inner peripheral surface 71. Six pockets 80 for receiving the balls 64 are formed in the circumferential direction, and a columnar portion 82 is formed between the pockets 80 and 80. The curvature center E of the spherical outer peripheral surface 70 and the curvature center F of the spherical inner peripheral surface 71 of the cage 65 are slightly offset in the axial direction by equal distances with respect to the joint center O. The offset amount f3 therebetween is 1 mm or less, and the cage 65 has a substantially uniform thickness.

(37) As in the first embodiment, the material for the cage 65 of this embodiment is carbon steel, which contains, as components thereof, 0.41 to 0.51 mass % of C, 0.10 to 0.35 mass % of Si, 0.60 to 0.90 mass % of Mn, 0.005 to 0.030 mass % of P, and 0.002 to 0.035 mass % of S, with the balance being Fe and an element inevitably remaining at the time of steelmaking and refining. The cage 65 is subjected to carburizing, quenching, and tempering as heat treatment. A side surface 83 of the pocket 80 is finished after the heat treatment.

(38) Also in the cage 65 of this embodiment, similarly to the cage 5 of the above-mentioned first embodiment, it is preferred that carbon steel for forming the cage contain 0.42 to 0.48 mass % of C, and have a surface hardness of 58 HRC or more and a core hardness of from 56 to 59 HRC, that a total case depth of the cage be set to 0.25 to 0.55 mm, and that a carbon concentration in a surface layer of the cage be set to 0.55 to 0.75 mass %. Therefore, redundant description thereof is omitted herein.

(39) A cage for a constant velocity universal joint according to a third embodiment of the present invention is described with reference to FIGS. 8 to 10. The constant velocity universal joint incorporating the cage of this embodiment is a plunging type cross groove constant velocity universal joint. A constant velocity universal joint 91 of this embodiment mainly comprises an outer joint member 92, an inner joint member 93, balls 94, and a cage 95. Six track grooves 96 are formed in a cylindrical inner peripheral surface 98 of the outer joint member 92. Six track grooves 97 opposed to the track grooves 96 of the outer joint member 92 are formed in a projecting outer peripheral surface 99 of the inner joint member 93. The cage 95 for holding the balls 94 is arranged between the cylindrical inner peripheral surface 98 of the outer joint member 92 and the projecting outer peripheral surface 99 of the inner joint member 93. The paired track grooves 96 and 97 of the outer joint member 92 and the inner joint member 93 are inclined opposite to each other in the circumferential direction, and each of the balls is incorporated at a crossing portion of both the track grooves 96 and 97 (see FIG. 9). With this structure, a backlash between the ball 94 and each of the track grooves 96 and 97 can be suppressed, and hence this constant velocity universal joint is widely used particularly for an automotive drive shaft and an automotive propeller shaft, which are averse to the backlash.

(40) As illustrated in FIG. 9, ball grooves 96a and 96b are formed on an inner periphery of the outer joint member 92, and the adjacent ball grooves 96a and 96b are inclined opposite to each other in the circumferential direction. Ball grooves 97a and 97b are formed on an outer periphery of the inner joint member 93, and the adjacent ball grooves 97a and 97b are also inclined opposite to each other in the circumferential direction. The paired track grooves 96a and 97a or the paired track grooves 96b and 97b of the outer joint member 92 and the inner joint member 93 are also inclined opposite to each other, and each of the balls 94 is incorporated between the paired track grooves. With this structure, when the joint forms an operating angle, the balls 94 are always guided in a plane bisecting an angle formed between axial lines of the outer joint member 92 and the inner joint member 93 (operating angle). As a result, rotational torque is transmitted at a constant velocity between the two axes.

(41) A spline is formed in an inner peripheral surface of the inner joint member 93, and a spline of a shaft 102 is fitted into the spline and fixed in the axial direction with a retaining ring. To prevent leakage of lubricating grease and entry of foreign matter, a boot 103 is fixed to the outer joint member 92 and the shaft 102, and an end plate 104 is fixed to an opposite end surface of the outer joint member 92.

(42) As illustrated in FIGS. 9 and 10, the cage 95 has a plurality of pockets 110 arranged at predetermined intervals in the circumferential direction. A curvature center of a spherical outer peripheral surface 100 and a curvature center of a spherical inner peripheral surface 101 of the cage 95 coincide with a center of the width of the cage 95 with no axial offset.

(43) As in the first embodiment, the material for the cage 95 of this embodiment is also carbon steel, which contains, as components thereof, 0.41 to 0.51 mass % of C, 0.10 to 0.35 mass % of Si, 0.60 to 0.90 mass % of Mn, 0.005 to 0.030 mass % of P, and 0.002 to 0.035 mass % of S, with the balance being Fe and an element inevitably remaining at the time of steelmaking and refining. The cage 95 is subjected to carburizing, quenching, and tempering as heat treatment. A side surface 113 of the pocket 110 is finished after the heat treatment.

(44) Also in the cage 95 of this embodiment, similarly to the cage 5 of the above-mentioned first embodiment, it is preferred that carbon steel for forming the cage contain 0.42 to 0.48 mass % of C, and have a surface hardness of 58 HRC or more and a core hardness of from 56 to 59 HRC, that a total case depth of the cage be set to 0.25 to 0.55 mm, and that a carbon concentration in a surface layer of the cage be set to 0.55 to 0.75 mass %. Therefore, redundant description thereof is omitted herein.

EXAMPLES

(45) Examples of the present invention and Comparative Example are described below. First, an overview of a processing method for a cage is described. Several types of processing method for a cage are given as described below. (1) Steel pipe.fwdarw.Cutting.fwdarw.Upsetting.fwdarw.Turning.fwdarw.Window (pocket) formation pressing.fwdarw.Window finishing.fwdarw.Carburizing, quenching, and tempering.fwdarw.Final finishing (2) Steel pipe.fwdarw.Cutting.fwdarw.Turning.fwdarw.Rolling.fwdarw.Turning.fwdarw.Window formation pressing.fwdarw.Window finishing.fwdarw.Carburizing, quenching, and tempering.fwdarw.Final finishing (3) Steel bar.fwdarw.Cutting.fwdarw.Hot forging.fwdarw.Turning.fwdarw.Window formation pressing.fwdarw.Window finishing.fwdarw.Carburizing, quenching, and tempering.fwdarw.Final finishing

(46) The respective steps of the above-mentioned processing methods are typical examples, and any appropriate modification and addition may be made as necessary. For example, heat treatment may be added so as to soften a workpiece that is work-hardened after the upsetting or rolling, or the window finishing before the heat treatment may be omitted. The present invention is not limited to the processing method.

(47) In Examples and Comparative Example, the processing was carried out based on the steps of the above-mentioned method (1). Components of a material for a steel pipe made of carbon steel, which was used in Example 1, are described below.

(48) [Components of Material for Steel Pipe Used in Example 1]

(49) The material contains 0.45 mass % of C, 0.24 mass % of Si, 0.76 mass % of Mn, 0.014 mass % of P, 0.012 mass % of S, 0.02 mass % of Cu, 0.16 mass % of Cr, 0.03 mass % of Ni, 0.01 mass % of Mo, 0.001 mass % of Ti, and 0.0001 mass % of B, with the balance being Fe and an element inevitably remaining at the time of steelmaking and refining. The hardness of the steel pipe was 185 HV after annealing.

(50) Components of a material for a steel pipe made of low-alloy steel (SCr415 as specified in JIS G 4052), which was used in Comparative Example, are described below.

(51) [Components of Material for Steel Pipe Used in Comparative Example]

(52) The material contains 0.15 mass % of C, 0.29 mass % of Si, 0.73 mass % of Mn, 0.015 mass % of P, 0.018 mass % of S, 0.02 mass % of Cu, 1.00 mass % of Cr, 0.02 mass % of Ni, 0.01 mass % of Mo, 0.001 mass % of Ti, and 0.0001 mass % of B, with the balance being Fe and an element inevitably remaining at the time of steelmaking and refining. The hardness of the steel pipe was 150 HV after annealing.

(53) As the cages of Examples and Comparative Example, cages of an eight-ball fixed type constant velocity universal joint (allowable maximum operating angle: 47°) adaptable to a nominal size of 25.4 of a constant velocity universal joint in Table 3 of page 3 of the Japanese Automobile Standards (JASO C 304-89: “Constant velocity universal joints for halfshaft of automobiles,” established on Mar. 31, 1989, published by the Society of Automotive Engineers of Japan) were used for processing. A curvature center of a spherical outer peripheral surface and a curvature center of a spherical inner peripheral surface of the cage are not offset in the axial direction, and the cage has a uniform thickness.

(54) At the time of processing, the steel pipe made of carbon steel, which was used in Example 1, had an increased carbon content, and hence the hardness of the steel pipe was increased. An optimum cutting tool was selected from among commercially available cutting tools, and an optimum material was also selected from among commercially available materials as a material for a window (pocket) formation punch. Thus, the life of each of the cutting tool and the punch used for the processing of Example 1 was at a level equal to that of Comparative Example using the steel pipe made of low-alloy steel, and the processing was able to be carried out easily.

(55) Table 1 shows conditions for the carburizing, quenching, and tempering. Characteristic values after the carburizing, quenching, and tempering in Table 1 are measurement results of hardness of a cross section taken at a carburized case-hardened part of the columnar portion 22 of the cage 5 illustrated in FIGS. 3 and 4. In Table 1, quenching oil of JIS Class 1, No. 1 and quenching oil of JIS Class 2, No. 2 are defined in conformity with JIS K 2242. Tempering conditions are 180° C. and 120 minutes. FIG. 11 schematically illustrates carburizing conditions of Comparative Example and Example 2. Further, FIG. 12 illustrates measurement results of hardness of a cross section taken at a carburized case-hardened part of each of the columnar portions of the cages of Comparative Example and Example 3. In Example 1 and Example 2, the same material was used and the same carbon potential (hereinafter referred to as “CP”) of carburizing was set. Under those conditions, cages having different total case depths were manufactured as Example 1 and Example 2 with their carburizing times set different from each other. In Example 3, the same material as that of Example 1 was used and the CP at the time of carburizing was increased (CP: 0.75 mass %). Under those conditions, a cage increased in carbon concentration in the surface after the carburizing was manufactured as Example 3.

(56) TABLE-US-00001 TABLE 1 Total case depth Hardness (HRC) Carburizing conditions (effective Surface Carburizing Quenching case depth: (Carburizing CP oil 50 HRC) amount: Soaking (mass %) Quenching Temperature (mm) mass %) Core Example 1 860° C. × 860° C. × — JIS Class 1, 0.25 58 56 20 minutes 70 minutes 0.75 No. 1 80° C. (0.58) Example 2 860° C. × 860° C. × — JIS Class 1, 0.55 59 56 20 minutes 240 minutes 0.75 No. 1 80° C. (0.65) Comparative 930° C. × 930° C. × 860° C. × JIS Class 2, (0.70) 61 42 Example 20 minutes 240 minutes 1.0 30 minutes No. 2 150° C. (0.85)

(57) The cage of each of Example 1, Example 2, Example 3 and Comparative Example was matched with balls, an outer joint member, and an inner joint member, and other components were assembled thereto, to thereby assemble a fixed type constant velocity universal joint. Using the constant velocity universal joint, a strength test and a life test were conducted. In each of Examples and Comparative Example, the tests were conducted on four samples. Table 2 shows results of comparative evaluation based on average values.

(58) TABLE-US-00002 TABLE 2 Rolling life test Operating angle: 8°, torque: 1,000 Nm, number of revolutions: 200 rpm Abrasion amount (depth) Quasi-static torsional test of side surface portion Operating angle: 44°, of pocket of cage number of revolutions: 4 rpm after operation for Fracture torque 500 hours Example 1 Increased by 15% Comparable Example 2 Increased by 6% Comparable Example 3 Increased by 3% Comparable Comparative Reference Reference Example

(59) [Strength Test (Quasi-Static Torsional Test)]

(60) As a result of the test, assuming the strength of Comparative Example to be a reference, the strength was increased by 15% in Example 1, 6% in Example 2, and 3% in Example 3. The reason why the strength was increased is conceivably because the cage of each of Example 1, Example 2, and Example 3 had a lower carbon concentration in the surface layer than the cage of Comparative Example so that the core hardness was increased.

(61) [Rolling Life Test]

(62) As a result of the test, assuming the rolling life of Comparative Example to be a reference, the results of Example 1, 2, and 3 were comparable to that of Comparative Example. The reason why the rolling life was comparable to that of Comparative Example is conceivably because the core hardness was increased so as to achieve reinforcement by an amount corresponding to the reduction in surface hardness.

(63) As understood from the carburizing conditions of Table 1 and FIG. 11, in Example 1 and Example 2 using carbon steel, the carburizing temperature is lower than that of Comparative Example, and the period of time required for heating and cooling is shorter than that of Comparative Example. Further, the carburizing time can be shortened as compared to Comparative Example using carburizing steel. Thus, it was confirmed that the heat treatment cost was reduced and the productivity was improved.

(64) In FIG. 12, the total case depth is such a depth that the hardness is increased from the core hardness of 615 HV. In Example 3, in which the CP at the time of carburizing was increased (CP: 0.75 mass %) so as to increase the carbon concentration in the surface after the carburizing, the carbon concentration in the surface layer is lower than that of Comparative Example, and hence the toughness of the surface is increased. Thus, even when the case depth is increased, the strength is not reduced significantly. Therefore, the total case depth can be increased so that the upper limit can be raised to 0.75 mm.

(65) In the above-mentioned embodiments, the Rzeppa constant velocity universal joint and the undercut-free constant velocity universal joint are described as the fixed type constant velocity universal joints incorporating the cage, and the cross groove constant velocity universal joint is described as the plunging type constant velocity universal joint, but the present invention is not limited thereto. In addition to the above-mentioned constant velocity universal joints, the present invention is also applicable as appropriate to a cross-groove type constant velocity universal joint, a counter-track type constant velocity universal joint, and the like as the fixed type constant velocity universal joints. Further, the constant velocity universal joint comprising six or eight balls is described, but the present invention is not limited thereto, and may also be carried out in a case of three to five, eight, ten balls or more.

(66) In addition, in the above-mentioned embodiments, the track grooves and the balls are held in angular contact at a contact angle, but the present invention is not limited thereto. The track grooves and the balls may be held in circular contact by forming the track grooves into a circular shape in horizontal cross section.

(67) Further, the present invention is not limited to the above-mentioned embodiments. As a matter of course, the present invention may be carried out in various modes without departing from the spirit of the present invention. The scope of the present invention is defined in claims, and encompasses equivalents described in claims and all changes within the scope of claims.

DESCRIPTION OF REFERENCE SIGNS

(68) 1, 61, 91 constant velocity universal joint

(69) 2, 62, 92 outer joint member

(70) 3, 63, 93 inner joint member

(71) 4, 64, 94 torque transmitting ball

(72) 5, 65, 95 cage

(73) 6, 66, 96 track groove

(74) 7, 67, 97 track groove

(75) 10, 70, 100 spherical outer peripheral surface

(76) 11, 71, 101 spherical inner peripheral surface

(77) 12, 102 shaft

(78) 20, 80, 110 pocket

(79) 23, 83, 113 side surface

(80) A curvature center

(81) B curvature center

(82) E curvature center

(83) F curvature center

(84) G curvature center

(85) H curvature center

(86) K window dimension

(87) O joint center

(88) X joint axial line

(89) f1 offset amount

(90) f2 offset amount

(91) f3 offset amount