Tripod type constant velocity joint

Abstract

To improve strength of a root part of a leg shaft of a tripod member.SOLUTION: In a tripod type constant velocity universal joint 1 of a double roller type, a carbon content in a core of a tripod member 3 is 0.23 to 0.44%, and a hardened layer 16 of a carburized layer is provided on a surface of a leg shaft 32. An intermediate part 33 provided between a trunk part 31 and the leg shaft 32 of the tripod member 3, is provided with: a first region P having a radius of curvature Ra in a cross section in a direction perpendicular to an axial direction of the joint, including an axial line of the leg shaft 32; and a second region Q having a radius of curvature Rb in a cross-section in the axial direction of the joint, including the axial line of the leg shaft 32, where Ra>Rb is established.SELECTED DRAWING: Figure 10

Description

[Technical field]

[0001] The present invention relates to a tripod-type constant velocity universal joint used for power transmission in automobiles and various industrial machines. [Background technology]

[0002] In drive shafts used in the power transmission system of automobiles, a sliding type constant velocity universal joint is often connected to the inboard side (the center side in the vehicle width direction) of the intermediate shaft, and a fixed type constant velocity universal joint is connected to the outboard side (the end side in the vehicle width direction). The sliding type constant velocity universal joint here allows both angular displacement and relative axial movement between the two shafts, while the fixed type constant velocity universal joint allows angular displacement between the two shafts but does not allow relative axial movement between the two shafts.

[0003] A tripod-type constant velocity universal joint is known as a sliding type constant velocity universal joint. There are single roller type and double roller type tripod-type constant velocity universal joints. The single roller type has a roller inserted into the track groove of the outer joint member, which is rotatably attached to the truss of the tripod member via a plurality of needle rollers. The double roller type has a roller inserted into the track groove of the outer joint member, and an inner ring that is fitted onto the truss of the tripod member to rotatably support the roller. The double roller type has the advantage that the induced thrust (axial force induced by friction between parts inside the joint) and the sliding resistance can be reduced compared to the single roller type because the roller can be oscillated relative to the truss.

[0004] Patent Document 1 below discloses an example of a double-roller type tripod constant velocity universal joint. In such a double-roller type tripod constant velocity universal joint, the outer peripheral surface of the trunnion of the tripod member and the inner peripheral surface of the inner ring are in point contact or near-point contact on the torque load side. Therefore, in this type of tripod constant velocity universal joint, the surface pressure at the contact portion between the outer peripheral surface of each shaft and the inner peripheral surface of the inner ring becomes high, especially when a high load torque is applied. This may affect the durability of the contact portion of the outer peripheral surface of the trunnion.

[0005] To solve this problem, Patent Document 1 listed below discloses a double-roller type tripod constant velocity joint in which a hardened layer is formed on the leg shaft by carburizing, quenching and tempering, the tripod members are made of steel with a carbon content of 0.23 to 0.44%, and the effective hardened layer has a limit hardness of 600 Hv. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2020-106087 A Summary of the Invention [Problem to be solved by the invention]

[0007] The double-roller type tripod constant velocity universal joint described in Patent Document 1 is obtained, for example, by carburizing and quenching chromium-molybdenum steel with a carbon content of 0.34%, followed by high-temperature tempering. With this configuration, the carbon content in the steel can be increased more than in the past, so that even if the contact surface pressure at the contact area between the outer circumferential surface of the truss and the inner ring becomes high due to an excessive torque load, the durability of the truss at the contact area can be improved.

[0008] On the other hand, the inventors of the present application further studied the tripod-type constant velocity universal joint described in Patent Document 1 and found that while durability at the contact portion of the outer circumferential surface of the trunnion under torque load could be ensured, there was a problem with the strength of the trunnion's base portion. A tensile load is repeatedly applied to the trunnion's base portion as torque is transmitted, but the fatigue strength of the base portion decreases, resulting in insufficient torsional strength of the trunnion's base portion.

[0009] SUMMARY OF THE PRESENT EMBODIMENTS An object of the present invention is to improve the strength of the base portions of the leg shafts of a tripod member. [Means for solving the problem]

[0010] The present invention, which has been made based on the above findings, provides an outer joint member having track grooves extending in the joint axial direction at three locations in the circumferential direction, with a pair of roller guideways in which each track groove is disposed opposite to each other in the joint circumferential direction, a tripod member made of steel, the tripod member having a body portion having a central hole, three trunnions protruding in the radial direction of the body portion, and an intermediate portion located between the body portion and the trunnions and having a curved cross section including the axis of the trunnion, rollers attached to each of the trunnions, and an inner ring fitted around the trunnions and supporting the rollers rotatably, the rollers being rotated forward along the roller guideways, The tripod type constant velocity universal joint is movable in the axial direction of the outer joint member, the roller and the inner ring form a roller unit that can swing relative to the trunnion, the carbon content in the core of the tripod member is 0.23 to 0.44%, and a carburized hardened layer is provided on the surface of the trunnion, characterized in that a first region having a radius of curvature Ra in a cross section perpendicular to the joint axial direction and including the axis of the trunnion, and a second region having a radius of curvature Rb in a cross section in the joint axial direction and including the axis of the trunnion, are provided in an intermediate portion of the tripod member, and Ra>Rb.

[0011] This configuration reduces stress concentration in the first region where torque mainly acts. Therefore, it is possible to increase the torsional strength at the base of the leg shaft where tensile load acts repeatedly with torque transmission. As a result, by performing carburizing and quenching on steel with a carbon content of 0.23% or more, the internal hardness of the tripod member is increased and the depth of the hardened layer on the surface is deepened, while the durability of the tripod member can be increased even in a situation where a decrease in the torsional strength of the tripod member is concerned due to a decrease in toughness caused by an increase in internal hardness.

[0012] It is preferable to provide a connection region S between the first region and the second region, the connection region S smoothly connecting both regions.

[0013] It is preferable that Ra / PCD≧0.0850, where PCD is the pitch circle diameter of the roller guideway of the outer joint member. By making Ra / PCD≧0.0850 in this way, the thickness of the first region, i.e., the minimum distance t between the large diameter portion of the spline and the first region, can be increased. By increasing the thickness of the first region in this way, the strength of the base portion of the truss shaft, particularly the fatigue strength, can be increased even if the depth of the hardened layer is increased and the toughness of the tripod member is reduced.

[0014] In this case, it is preferable that t / PCD≧0.145, where t is the minimum distance from the large diameter portion of the spline formed on the inner circumferential surface of the trunk portion of the tripod member to the first region.

[0015] The surface hardness of the truss of the tripod member is preferably 653 HV or more, which can increase the durability of the outer circumferential surface of the truss under high torque load, particularly the durability of the contact portion with the inner circumferential surface of the inner ring.

[0016] The internal hardness of the tripod member is preferably 513 HV or more. By making the internal hardness 513 HV or more, the effective hardened layer depth required for the tripod member can be obtained. Effect of the Invention

[0017] According to the present invention, it is possible to improve the durability of the base portions of the leg shafts of the tripod members. [Brief description of the drawings]

[0018] [Figure 1] FIG. 2 is a cross-sectional view in the joint axial direction showing a double-roller type tripod constant velocity universal joint. [Diagram 2] FIG. 2 is a cross-sectional view taken along line KK in FIG. [Diagram 3] FIG. 2 is a cross-sectional view taken along line LL in FIG. [Figure 4] 2 is a cross-sectional view showing a state in which the tripod type constant velocity universal joint of FIG. 1 has an operating angle. [Diagram 5] FIG. 4 is a cross-sectional view showing a hardened layer formed on a tripod member. [Figure 6] FIG. 13 is a diagram showing the hardness distribution in the leg shaft of a conventional product. [Figure 7] FIG. 13 is a diagram showing the hardness distribution at the leg shaft of the improved product. [Figure 8] FIG. 4 is a front view of the tripod member as viewed from the joint axial direction. [Figure 9] FIG. 2 is a side view (partial cross-sectional view) of a tripod member. [Figure 10] 9 is a cross-sectional view taken along line MM in FIG. 8. [Figure 11] FIG. 1(a) is an enlarged cross-sectional view of a first region, and FIG. 1(b) is an enlarged cross-sectional view of a second region. [Figure 12] 3 is an enlarged cross-sectional view of the first region and its vicinity of the tripod member in FIG. 2. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] An embodiment of a tripod type constant velocity universal joint according to the present invention will be described with reference to Figs.

[0020] The tripod type constant velocity universal joint 1 of this embodiment shown in Figs. 1 to 4 is of a double roller type. Fig. 1 is an axial cross-sectional view of the double roller type tripod type constant velocity universal joint, and Fig. 2 is a cross-sectional view taken along line KK in Fig. 1. Fig. 3 is a cross-sectional view taken along line LL in Fig. 1, and Fig. 4 is an axial cross-sectional view showing the tripod type constant velocity universal joint when an operating angle is taken. In the following description, the joint axial direction and the joint circumferential direction respectively refer to the axial direction and the circumferential direction of the tripod type constant velocity universal joint when the operating angle is set to 0°.

[0021] As shown in Figures 1 and 2, the tripod type constant velocity universal joint 1 is mainly composed of an outer joint member 2, a tripod member 3 as an inner joint member, and a roller unit 4 as a torque transmission member. The outer joint member 2 is cup-shaped with one end open, and three linear track grooves 5 extending in the joint axial direction are formed on the inner peripheral surface at equal intervals in the joint circumferential direction. Each track groove 5 is formed with a roller guide surface 6 that is disposed opposite to the joint circumferential direction of the outer joint member 2 and extends in the joint axial direction. The tripod member 3 and the roller unit 4 are housed inside the outer joint member 2.

[0022] The tripod member 3 integrally comprises a body 31 (trunnion body) having a central hole 30, three leg shafts 32 (trunnion journals) protruding in the radial direction from trisecting positions in the joint circumferential direction on the outer peripheral surface of the body 31, and an intermediate portion 33 connecting the outer peripheral surface of the body 31 and the outer peripheral surface of the leg shafts 32. The tripod member 3 is coupled to the shaft 8 so as to be capable of transmitting torque by fitting a male spline 81 formed on the shaft 8 as a shaft into a female spline 34 formed in the central hole 30 of the trunnion body 31. The tripod member 3 is fixed to the shaft 8 in the joint axial direction by engaging an end face on one side of the tripod member 3 in the joint axial direction with a shoulder portion 82 provided on the shaft 8 and engaging a retaining ring 10 attached to the tip of the shaft 8 with an end face on the other side of the tripod member 3 in the joint axial direction.

[0023] The roller unit 4 is mainly composed of an outer ring 11 which is an annular roller centered on the axis of the leg axle 32, an annular inner ring 12 which is disposed on the inner diameter side of the outer ring 11 and fitted onto the leg axle 32, and a large number of needle rollers 13 interposed between the outer ring 11 and the inner ring 12. The roller unit 4 is housed in the track groove 5 of the outer joint member 2. The roller unit 4, which is composed of the outer ring 11, the inner ring 12, and the needle rollers 13, is structured so that they cannot be separated by washers 14, 15.

[0024] In this embodiment, the outer peripheral surface 11a (see FIG. 2) of the outer ring 11 is a convex curved surface whose generating line is a circular arc having a center of curvature on the axis of the leg shaft 32. The outer peripheral surface 11a of the outer ring 11 is in angular contact with the roller guideway 6.

[0025] The needle rollers 13 are disposed rollably between the cylindrical inner peripheral surface of the outer ring 11 as an outer raceway surface and the cylindrical outer peripheral surface of the inner ring 12 as an inner raceway surface.

[0026] The outer circumferential surface of each trunnion 32 of the tripod member 3 has a straight shape in the axial direction of the trunnion 32 in a cross section in any direction including the axis of the trunnion 32. Also, as shown in Fig. 3, the outer circumferential surface of the trunnion 32 has a substantially elliptical shape in a cross section perpendicular to the axis of the trunnion 32. The outer circumferential surface of the trunnion 32 contacts the inner circumferential surface 12a of the inner ring 12 in a direction perpendicular to the joint axial direction, i.e., in the direction of the major axis a. A gap m is formed between the outer circumferential surface of the trunnion 32 and the inner circumferential surface 12a of the inner ring 12 in the joint axial direction, i.e., in the direction of the minor axis b.

[0027] As shown in FIGS. 1 and 2, an intermediate portion 33 between the body portion 31 and the leg shaft 32 of the tripod member 3 is formed so as to describe a concave curve in any cross section including the axis of the leg shaft 32.

[0028] The inner peripheral surface 12a of the inner ring 12 forms a convex arc shape in any cross section including the axis of the inner ring 12. In addition, because the cross section of the truss 32 is substantially elliptical as described above, and a predetermined gap m is provided between the truss 32 and the inner ring 12, the inner ring 12 can swing relative to the truss 32. As described above, the inner ring 12 and the outer ring 11 are assembled via the needle rollers 13 so as to be relatively rotatable, and therefore the outer ring 11 can swing integrally with the inner ring 12 relative to the truss 32. In other words, the axes of the outer ring 11 and the inner ring 12 can tilt relative to the axis of the truss 32 within a plane including the axis of the truss 32 (see FIG. 4).

[0029] As shown in Fig. 4, when the tripod type constant velocity universal joint 1 rotates with an operating angle, the axis of the tripod member 3 is inclined with respect to the axis of the outer joint member 2, but since the roller unit 4 is swingable, it is possible to prevent the outer ring 11 from intersecting the roller guideway 6 at an angle. As a result, the outer ring 11 rolls horizontally with respect to the roller guideway 6, so that it is possible to reduce induced thrust and sliding resistance, and it is possible to realize low vibration of the tripod type constant velocity universal joint 1.

[0030] As already mentioned, the cross section (horizontal section) of the trunnion 32 is substantially elliptical and the cross section (longitudinal section) of the inner peripheral surface 12a of the inner ring 12 is an arc-shaped convex cross section, so that the outer peripheral surface of the trunnion 32 on the torque load side and the inner peripheral surface 12a of the inner ring 12 come into point contact or contact over a small area close to point contact. This reduces the force tending to tilt the roller unit 4, improving the stability of the posture of the outer ring 11.

[0031] The tripod member 3 described above is manufactured from a steel material through the following main processes: forging (cold forging) → machining (turning) → broaching the spline 34 → heat treatment → grinding the outer circumferential surface of the leg shaft 32. The outer circumferential surface of the leg shaft 32 can be finished by cutting hardened steel instead of the grinding process. In addition, a spheroidizing annealing process and a bonderizing process can be added before cold forging. If there is no problem with the hammering forgeability during cold forging due to circumstances such as the use of a material with a low carbon content, the spheroidizing annealing process can be omitted. Carburizing, quenching and tempering are performed as heat treatment.

[0032] FIG. 5 is a cross-sectional view showing the hardened layer 16 formed by heat treatment of the tripod member 3. The hardened layer 16 is formed by hardening the carburized layer by quenching. The hardened layer 16 is formed on the entire surface of the tripod member 3, including the outer circumferential surface of the leg axle 32, the outer circumferential surface of the body 31, the surface of the intermediate portion 33, and the surface of the female spline 34. In the completed tripod member 3, the outer circumferential surface of the leg axle 32 is finished by grinding (or hardened steel cutting), so the depth of the hardened layer 16 on the outer circumferential surface of the leg axle 32 is shallower than other regions by the amount of the machining allowance due to grinding, etc. Note that this machining allowance is usually small, about 0.1 mm, so in FIG. 5, the thickness of the hardened layer 16 is drawn uniformly on the entire surface.

[0033] As already mentioned, in a double roller type tripod constant velocity universal joint, the outer peripheral surface of the trunnion 32 and the inner peripheral surface 12a of the inner ring 12 make point contact or near-point contact in the region X on the torque load side as shown in Fig. 3, so there is a problem that the surface pressure of the contact part becomes high when a high torque load is applied. If the surface pressure is excessive, it may affect the durability of the trunnion 32 at the contact part X.

[0034] In order to solve this problem, the present inventors carried out the following verification.

[0035] Generally, the tripod member 3 is forged from chromium-molybdenum steel, which is a type of case-hardened steel, and then carburized, quenched, and tempered as a heat treatment to form a hardened layer 16 on the surface. FIG. 6 shows the hardness distribution from the surface to the core of the leg shaft 32 when a conventional tripod member 3 material (e.g., chromium-molybdenum steel of JIS G4052, equivalent material with a carbon content of less than about 0.23%) is used and carburized, quenched, and tempered (quenching temperature 860°C, tempering temperature 180°C). In this case, as is clear from FIG. 6, the hardness of the surface exceeds 513HV, but the hardness falls below 513HV in a very shallow area from the surface. Therefore, when an excessive torque is applied, the durability of the leg shaft 32 at the contact portion is affected. Therefore, in order to solve the above problem, it is necessary to form the hardened layer 16 as deep as possible.

[0036] The effective hardened layer depth means the distance from the surface of the steel material to the position of the limit hardness. According to JIS G0557, the limit hardness of the effective hardened layer is 550 HV, but it is also stipulated that "if the hardness at a position three times the distance from the surface of the hardened layer exceeds 450 HV Vickers hardness, a limit hardness exceeding 550 HV may be used by agreement between the parties concerned." In this embodiment, as described later, the internal hardness (hardness of the unhardened area) of the tripod member 3 is 513 HV or more, so in this embodiment, the limit hardness of the effective hardened layer depth is stipulated as 600 HV in consideration of the above exception. Note that the harder the hardness of the hardened layer 16, the more preferable it is in terms of durability of the leg shaft 32, so it is preferable to stipulate the limit hardness of the effective hardened layer depth as 653 HV or more.

[0037] The easiest way to deepen the hardened layer 16 is to increase the depth of the carburized layer, but forming a deep carburized layer requires a huge amount of carburizing time, which leads to high manufacturing costs. It is also possible to use a steel material with a high carbon content, such as carbon steel for machine construction such as S50C to S55C, as the material, and change the heat treatment method to induction hardening, which allows deeper hardening than carburized hardening, but in this case, the material becomes harder as the carbon content increases, which increases the processing load when forging the tripod member 3, and leads to problems such as an increase in the size of the forging equipment.

[0038] After considering the above, the inventors verified the effectiveness of using case-hardened steel with a higher carbon content than conventional steels while keeping the carburizing and quenching / tempering conditions the same as conventional steels. Figure 7 shows the hardness distribution when carburizing, quenching, and tempering were performed using chromium-molybdenum steel with a carbon content of approximately 0.34% as the material. The quenching temperature was 850°C, and the tempering temperature was 180°C. The horizontal axis in Figure 7 (depth from the surface) is shown on the same scale as in Figure 6.

[0039] As is clear from the results in FIG. 7, it was found that the depth of the hardened layer 16 can be increased as intended by increasing the carbon content of the case-hardened steel. It can also be seen that the increased depth of the hardened layer 16 results in an internal hardness of 513 HV or more. On the other hand, since the hardness (internal hardness) of the core after carburizing, quenching and tempering reaches about 550 HV, there is a risk that the toughness of the leg shaft 32 will decrease, and the repeated fatigue strength of the tripod member 3 will decrease. Measures to address this issue will be described later.

[0040] In the above description, the case where a material equivalent to a carbon content of about 0.34% is used as the material of the tripod member 3 is exemplified, but the type of material that can be used is not limited. For example, in the case of chromium-molybdenum steel, in addition to SCM435, SCM440, etc. can be used. Also, so-called H-steel (e.g., SCM435H, SCM440H, etc.: specified in JIS G4052) with guaranteed hardenability can be used. In the case of case-hardened steel, other types of steel can be used, and for example, chromium steel (e.g., SCr435, SCr440, etc.) specified in JIS G4053 can be used as the material. As for chromium steel, for example, H-steel such as SCr435H, SCr440H, etc. can be used. Not limited to case-hardened steel such as chromium-molybdenum steel and chromium steel, carbon steel for machine structure such as S10C to S35C (JIS G40 51) may also be used as material.

[0041] Considering formability during cold forging of the tripod member 3, it is preferable to use steel with a carbon content of 0.44% or less. However, in cases where formability during forging is not an issue, such as in the case of hot forging, steel with a carbon content exceeding 0.44% can also be used. Case-hardened steel with a carbon content of 1% or less does not cause any particular problems during hot forging.

[0042] As already mentioned, it was found that the improved product described above has a problem with the strength of the base part (middle part 33) of the leg shaft 32. The cause of this is presumably that the toughness of the tripod member 3 is reduced by increasing the hardness of the entire tripod member 3 from the surface to the deep region, which results in a reduction in the fatigue strength of the tripod member 3 in the middle part 33 where tensile loads are repeatedly applied with torque transmission, affecting the strength of the middle part 33. If an attempt is made to solve this problem by reviewing the material aspect or heat treatment method, this may reduce the durability of the contact part X of the leg shaft 32, and therefore it is desirable to solve the problem from a different perspective.

[0043] Based on the above verification, in the present invention, it was decided to review the shape of the tripod member 3 in order to improve the strength of the base portion of the leg shaft 32.

[0044] As shown in Figures 8 to 10, the intermediate portion 33 is formed so that a cross section including the axis of the trunnion 32 is a concave curve over the entire circumference of the trunnion 32. As shown in Figure 11, the intermediate portion 33 is formed in an arc shape with a radius of curvature Ra in a cross section including the axis of the trunnion 32 and perpendicular to the joint axial direction (the major axis direction of the ellipse formed by the cross section of the trunnion 32), and is formed in an arc shape with a radius of curvature Rb in a cross section including the axis of the trunnion 32 and in the joint axial direction (the minor axis direction of the ellipse formed by the cross section of the trunnion 32). The radius of curvature Ra is larger than the radius of curvature Rb (Ra>Rb).

[0045] 10, the first region P having a radius of curvature Ra and the second region Q having a radius of curvature Rb in the intermediate portion 33 are each formed with a width in the circumferential direction of the truss 32. For example, as shown in FIG. 10, the first region P can be formed in a partial circumferential region of the truss 32 centered on a plane perpendicular to the joint axial direction and including the axis of the truss 32. The second region Q can be formed in a partial circumferential region of the truss 32 centered on a plane in the joint axial direction and including the axis of the truss 32. In the circumferential direction of the truss 32, two first regions P are disposed opposite each other in the direction perpendicular to the joint axial direction, and two second regions Q are disposed opposite each other in the joint axial direction.

[0046] 10, a connection region S is formed in the middle portion 33 between the first region P and the second region Q adjacent to each other in the circumferential direction of the truss 32, and the connection region S is smoothly connected to the first region P and the second region Q. In the connection region S, the radius of curvature gradually changes in the circumferential direction of the truss 32.

[0047] In this way, the intermediate portion 33 is formed of any one of the first region P, the second region Q, and the connection region S. Each of these regions P, Q, and S can be formed of a single arc in a vertical cross section including the axis of the truss axle 32, or can be formed of multiple arcs with different radii of curvature. In the latter case, the shape of the intermediate portion 33 is determined so that the minimum radius of curvature in the cross section including the axis of the truss axle 32 in the first region P is Ra and the maximum radius of curvature in the cross section including the axis of the truss axle 32 in the second region P is Rb, and Ra>Rb. It is also possible to form one or two of the regions P, Q, and S as a single arc, and the remaining regions as multiple arcs.

[0048] In this way, by making the radius of curvature Ra of the first region P larger than the radius of curvature Rb of the second region Q, it is possible to reduce stress concentration in the first region P where torque mainly acts. Therefore, it is possible to increase the torsional strength at the base of the leg shaft 32 where tensile load acts repeatedly with torque transmission. As a result, by performing carburizing and quenching on case-hardened steel with a carbon content of 0.23% or more, it is possible to increase the internal hardness of the tripod-member 3 and deepen the depth of the hardened layer on the surface, while also increasing the durability of the tripod-member 3 even under conditions where a decrease in the torsional strength of the tripod-member is concerned due to a decrease in toughness caused by an increase in internal hardness.

[0049] Moreover, the second region Q is a portion that contributes less to torque transmission. Therefore, by making the radius of curvature Rb of the second region Q smaller, more material is removed during processing, and the tripod member 3 can be made lighter while avoiding a decrease in strength. This effect can be more prominently obtained by making the angle β of the range in which the second region Q is formed about the axis of the truss 32 larger than the angle α of the range in which the first region P is formed about the axis of the truss 32, as shown in FIG.

[0050] In the double-roller type tripod member 3 of this embodiment, the needle rollers 13 do not roll on the outer circumferential surface of the truss 32, so there is no need to provide a reduced-weight portion (recessed portion) in the region from the outer circumferential surface of the truss 32 to the surface of the intermediate portion 33 to avoid interference with the needle rollers 13. Therefore, no edge is formed due to the recessed portion in the region from the surface of the intermediate portion 33 to the outer circumferential surface of the truss 32, and the region from the intermediate portion 33 to the outer circumferential surface of the truss 32 is smoothly continuous. Therefore, the effect of alleviating stress concentration by providing a difference between the radii of curvature Ra and Rb can be sufficiently obtained.

[0051] The radius of curvature Ra of the first region P of the intermediate portion 33 is preferably set to Ra / PCD≧0.0850, where PCD (see FIG. 2) is the pitch circle diameter of the roller guideway 6 of the outer joint member 2. Also, t (see FIG. 5) is the minimum distance from the large diameter portion 34a of the spline 34 formed on the inner circumferential surface of the trunk portion 31 of the tripod member 3 to the first region P, and t / PCD≧0.145. Note that PCD, Ra, and t are all expressed in the same unit (mm).

[0052] The reasons for determining these numerical ranges are as follows.

[0053] FIG. 12 is an enlarged cross-sectional view of the middle part 33 (first region P) of the tripod member 3. As shown by the solid line in FIG. 12, the inner diameter side of the first region P is smoothly connected to the outer circumferential surface of the trunk part 31 by drawing a tangent line. On the other hand, the outer diameter side of the first region P is connected to the outer circumferential surface of the truss 32 via a minute step Z. This step Z is caused by the outer circumferential surface of the truss 32 receding by the grinding allowance when grinding the outer circumferential surface of the truss 32 after cold forging of the tripod member 3. As shown by the two-dot chain line, when the radius of curvature of the middle part 33' (first region) is increased, the arc-shaped middle part 33' reaches the grinding target region G before grinding on the outer diameter side of the middle part 33', and the grinding allowance Y becomes large. The increase in the grinding allowance Y has an adverse effect on the grinding accuracy. From the viewpoint of preventing deterioration of grinding accuracy, the conventional product has R / PCD<0.0850. Note that the "conventional product" here means a product in which the middle portion 33 has a uniform radius of curvature R over its entire circumference.

[0054] In the present invention, since Ra / PCD≧0.0850, the thickness of the first region P, i.e., the minimum distance t (thickness in FIG. 12) between the large diameter portion 34a of the spline 34 (see FIG. 5) and the first region P, can be increased. Specifically, t / PCD≧0.145 can be achieved. By increasing the thickness of the first region P in this way, even if the depth of the hardened layer 16 is increased and the toughness of the tripod member 3 is reduced, the strength, particularly the fatigue strength, of the first region P of the intermediate portion 33 of the truss 32 can be increased. Therefore, it is possible to increase the torsional strength of the truss 32 and improve the design freedom of the tripod member 3.

[0055] Increasing the radius of curvature Ra of the first region P in this way increases the grinding allowance Y on the outer diameter side of the first region P. However, through verification by the present inventor, it was confirmed that as long as Ra / PCD is in the range of 0.20, there is no adverse effect on the grinding accuracy when grinding the outer circumferential surface of the truss 32. Therefore, the upper limit of Ra / PCD is preferably 0.20. In other words, it is preferable to set it to 0.0850≦Ra / PCD≦0.20. In addition, if the value of t / PCD is too large, the tripod member 3 will become unnecessarily large and will increase in weight, so the upper limit of the value of t / PCD is preferably 0.20 (t / PCD≦0.20).

[0056] In addition, the radius of curvature Rb of the second region Q is preferably in the range of 0.0550≦Rb / PCD≦0.0820. As described above, when Ra>Rb, more material is removed during processing, but when Ra and Rb are close to each other, it is difficult to obtain a clear material reduction effect. Therefore, it is preferable that Rb / PCD≦0.0820. Furthermore, if Rb / PCD is too small, when the second region Q is formed by forging, the formability may deteriorate and it may not be possible to form it into a predetermined shape. Therefore, it is preferable that 0.0550≦Rb / PCD.

[0057] The above-described embodiment of the present invention can also be applied to double-roller type tripod constant velocity universal joints having other configurations.

[0058] For example, the outer peripheral surface of the trunnion 32 can be formed into a convex curved surface (e.g., a convex arc-shaped cross section), and the inner peripheral surface 12a of the inner ring 12 can be formed into a cylindrical surface. Also, the outer peripheral surface of the trunnion 32 can be formed into a convex curved surface (e.g., a convex arc-shaped cross section), and the inner peripheral surface 12a of the inner ring 12 can be formed into a concave spherical surface that fits with the outer peripheral surface of the trunnion. In this case, by providing flanges on both ends of the inner diameter of the outer ring, the washers 14, 15 can be made unnecessary.

[0059] The above-described tripod type constant velocity universal joint 1 can be used not only in drive shafts of automobiles, but also in a wide range of power transmission paths of automobiles, industrial equipment, and the like. [Explanation of symbols]

[0060] 1 Tripod type constant velocity joint 2 Outer joint member 3 Tripod member 4 Roller unit 5 Track groove 6 Roller guideway 8 shafts 11 Lola (Outering) 12 Inner Ring 13 Needle roller 16 Hardened layer 30 center hole 31 Torso 32 Leg axis 33 Middle Section 34 Female spline P 1st area Q 2nd area

Claims

1. an outer joint member provided with track grooves extending in a joint axial direction at three locations in a circumferential direction, each track groove having a pair of roller guide surfaces disposed opposite to each other in the joint circumferential direction; a tripod member made of steel, the tripod member including a body having a central hole, three leg shafts protruding in a radial direction of the body, and an intermediate portion located between the body and the leg shafts and having a curved cross section including the axis of the leg shaft; A roller attached to each of the leg shafts; an inner ring that is fitted onto the trunnion and rotatably supports the roller; the rollers are movable in the axial direction of the outer joint member along the roller guideway, the roller and the inner ring constitute a roller unit that is swingable relative to the trunnion; The carbon content in the core of the tripod member is 0.23 to 0.44%, In a tripod type constant velocity universal joint in which a carburized hardened layer is provided on the surface of the leg shaft, a first region having a radius of curvature Ra in a cross section perpendicular to a joint axial direction and including an axis of the trunnion, and a second region having a radius of curvature Rb in a cross section in the joint axial direction and including the axis of the trunnion, the second region satisfying Ra>Rb; a step Z is provided between the first region of the intermediate portion and the trunnion, the step Z being reduced in diameter from the first region in the trunnion radial direction and connected to an outer circumferential surface of the trunnion; A tripod-type constant velocity universal joint, wherein a pitch circle diameter PCD of a roller guideway of the outer joint member satisfies 0.085≦Ra / PCD≦0.

20.

2. 2. The tripod type constant velocity universal joint according to claim 1, further comprising a connection region S between said first region and said second region, said connection region S being smoothly connected to both regions.

3. 2. The tripod type constant velocity universal joint according to claim 1, wherein t is a minimum distance from a large diameter portion of a spline formed on an inner peripheral surface of a body portion of the tripod member to the first region, and t / PCD is greater than or equal to 0.

145.

4. 4. The tripod type constant velocity universal joint according to claim 1, wherein the surface hardness of the trunnions of the tripod members is 653 HV or more.

5. 5. The tripod type constant velocity universal joint according to claim 1, wherein the internal hardness of said tripod members is 513 HV or more.

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