Outer joint member for constant velocity universal joint

Abstract

An outer joint member 11 includes a cup part 12 in which a track groove with which a roller 19 is engaged is formed on an inner periphery, and a shaft part 13 joined to the cup part 12. The cup part 12 has a bottomed cylindrical shape with one end opened, and includes a cylindrical part, a bottom part, and a solid short shaft part 31 protruding from the bottom part. A solid part 32, which is solid in form, is formed at one end of the shaft part 13. The short shaft part 31 of the cup part 12 and the solid part of the shaft part 13 are joined by friction welding. The core part hardness of the joint part between the short shaft part 31 of the cup part 12 and the solid part of the shaft part 13 is Hv 350 or less at an average value and Hv 390 or less at the maximum value.

Description

Outer joint member of constant velocity universal joint

[0001] The present invention relates to an outer joint member of a constant velocity universal joint.

[0002] Constant velocity universal joints that make up the power transmission systems of automobiles and various industrial machines connect two shafts, one on a driving side and one on a driven side, so that torque can be transmitted, and can transmit rotational torque at a constant velocity regardless of the operating angle of the two shafts. Constant velocity universal joints are broadly divided into fixed-type constant velocity universal joints that allow only angular displacement, and sliding-type constant velocity universal joints that allow both angular displacement and axial displacement. For example, in a drive shaft that transmits power from an automobile engine or motor to the drive wheels, a sliding-type constant velocity universal joint is used on the differential side (inboard side), and a fixed-type constant velocity universal joint is used on the drive wheel side (outboard side).

[0003] Whether of the sliding type or the fixed type, a constant velocity universal joint comprises, as its main constituent parts, an outer joint member having a cup portion whose inner peripheral surface is formed with track grooves with which a torque transmission element engages, and a shaft portion extending in the axial direction from the bottom of the cup portion. This outer joint member is often formed by integrally forming the cup portion and the shaft portion from a solid rod-shaped material by subjecting the material to cold plastic processing such as forging or ironing, or machining such as cutting or grinding.

[0004] Depending on the layout of an automobile's engine and motor, the gearbox may be positioned off-center in the vehicle's width direction. In such cases, an outer joint member with a long shaft portion (long stem) may be used as the sliding-type constant velocity universal joint on the inboard side of either the left or right drive shaft. When the inboard outer joint member of one drive shaft is equipped with a long stem, the long stem is rotationally supported by a rolling bearing. The length of the long stem portion varies depending on the vehicle model, but is generally approximately 300 to 400 mm. Because of the long shaft portion of this outer joint member, it is difficult to precisely mold the cup portion and the shaft portion integrally. For this reason, some outer joint members are constructed of two members: a cup member forming the cup portion and a shaft member forming the shaft portion, and the two members are joined by friction welding. Examples of such friction-welded joint members include a solid-stem type described in Patent Document 1 and a hollow-stem type described in Patent Document 2.

[0005] JP 61-132284 A JP 2006-64060 A

[0006] When the shank of the long stem is joined to the cup portion by friction welding, the joint is subjected to thermal effects due to the friction welding. During friction welding, the joint is rapidly heated and then rapidly cooled, and depending on the processing conditions, some of the structure of the joint, for example, the axial core portion of the joint, may become martensite. In this case, an imbalance in the degree of hardening occurs within the joint, resulting in unstable quality and strength of the outer joint member after friction welding. Furthermore, when the outer joint member after friction welding is subjected to heat treatment, cracks (quench cracks) may occur in the portions that have already been hardened by friction welding.

[0007] Therefore, an object of the present invention is to stabilize the quality and strength of an outer joint member in which a shaft portion and a cup portion are joined by friction welding.

[0008] In order to achieve the above-mentioned object, the present invention provides an outer joint member of a constant velocity universal joint comprising: a cup portion having track grooves formed on its inner circumference with which a torque transmission element engages; and a shank portion joined to the cup portion, wherein the cup portion is cylindrical with one end open and has a cylindrical portion, a bottom, and a solid short shank portion protruding from the bottom, wherein a solid portion is formed at one end of the shank, and the short shank portion of the cup portion and the solid portion of the shank are joined by friction welding, wherein the core hardness of the joint between the short shank portion of the cup portion and the solid portion of the shank is an average value of Hv 350 or less and a maximum value of Hv 390 or less.

[0009] By specifying the core hardness of the joint in this manner, it is possible to prevent the core from becoming martensite due to friction welding, reduce variations in the degree of hardening in the joint, and stabilize the quality and strength of the outer joint member. In addition, cracks are less likely to occur in the joint during heat treatment after friction welding.

[0010] At the joint between the short shaft portion of the cup portion and the solid portion of the shaft portion, when the carbon content of the shaft core is C and the carbon content of a portion away from the shaft core by a distance of 1 / 4 of the diameter of the joint is C0, it is preferable that C / C0 be 1.07 or less.

[0011] This suppresses central segregation at the joint, thereby avoiding localized hardening during friction welding, reducing the difference in hardness between the axial core and peripheral areas, stabilizing quality and strength, and preventing cracks from occurring during heat treatment after friction welding.

[0012] The core hardness can be measured within a circular region of the joint having a diameter of 10 mm and centered on the axis.

[0013] The joint between the short shank of the cup and the solid portion of the shank may be provided on a bearing mounting surface.

[0014] According to the present invention, it is possible to stabilize the quality and strength of an outer joint member in which a shaft portion and a cup portion are joined by friction welding.

[0015] FIG. 1 is a cross-sectional view showing the overall structure of a drive shaft 1. FIG. 2 is a side view of an outer joint member. FIG. 3 is a cross-sectional view showing a cup member and a shaft member before friction welding. FIG. 4 is a cross-sectional view showing a cup portion and a shaft portion after friction welding. FIG. 5 is a front view showing an end face of a raw material. FIG. 6 is a front view showing an end face of a raw material. FIG. 7 is a front view showing a cut surface when the outer joint member is cut at a joint. FIG. 8 is a diagram showing measurement results of hardness at a joint in this embodiment and a comparative example. FIG. 9 is a diagram showing test results evaluating the presence or absence of cracks during heat treatment when the C / C0 value of the raw material is changed.

[0016] An embodiment of the present invention will be described below with reference to FIGS.

[0017] 1 is a diagram showing the overall structure of a drive shaft 1 that uses an outer joint member 11 of a constant velocity universal joint 10 of this embodiment. The drive shaft 1 is mainly composed of a sliding type constant velocity universal joint 10 arranged on the differential gear side (right side in the drawing: hereinafter also referred to as the inboard side), a fixed type constant velocity universal joint 20 arranged on the drive wheel side (left side in the drawing: hereinafter also referred to as the outboard side), and an intermediate shaft 2 that connects both constant velocity universal joints 10, 20 so as to be able to transmit torque.

[0018] The sliding type constant velocity universal joint 10 shown in Fig. 1 is a so-called tripod type constant velocity universal joint (TJ), and includes an outer joint member 11 having a cup portion 12 and a long shaft portion (long stem portion) 13 extending axially from the bottom of the cup portion 12, an inner joint member 16 housed on the inner periphery of the cup portion 12 of the outer joint member 11, and rollers 19 as torque transmission elements arranged between the outer joint member 11 and the inner joint member 16. The inner joint member 16 is provided with three trunnions 18, each with a roller 19 rotatably fitted thereon, at equal intervals in the circumferential direction. The inner joint member 16 of the tripod type constant velocity universal joint is also called a tripod member.

[0019] Three axially extending track grooves are formed at equal intervals in the circumferential direction on the inner peripheral surface of the outer joint member 11. Rollers 19 fitted onto the outer surfaces of the trunnions 18 are engaged with the track grooves in the circumferential direction, thereby transmitting torque between the outer joint member 11 and the inner joint member 16. As the sliding type constant velocity universal joint 10, in addition to a tripod type constant velocity universal joint, other sliding type constant velocity universal joints, for example, double offset type constant velocity universal joints, can also be used.

[0020] An inner ring of a support bearing 6 is fixed to the outer peripheral surface of the shaft portion 13, and an outer ring of this support bearing 6 is fixed to the vehicle body via a bracket (not shown). The outer joint member 11 is rotatably supported with respect to the vehicle body by the support bearing 6. The provision of the support bearing 6 minimizes runout of the outer joint member 11 during operation, etc. The outer joint member 11 is provided with a cylindrical bearing mounting surface 30 (see FIG. 2) for fixing the inner ring of the support bearing 6.

[0021] 1 is a so-called Rzeppa-type constant velocity universal joint, and includes an outer joint member 21 having a bottomed cylindrical cup portion 21a and a shaft portion 21b extending axially from the bottom of the cup portion 21a, an inner joint member 22 housed on the inner periphery of the cup portion 21a of the outer joint member 21, balls 23 as torque transmission elements arranged between the cup portion 21a of the outer joint member 21 and the inner joint member 22, and a cage 24 arranged between the inner circumferential surface of the cup portion 21a of the outer joint member 21 and the outer circumferential surface of the inner joint member 22 to hold the balls 23 at equal intervals in the circumferential direction. Note that an undercut-free type constant velocity universal joint may also be used as the fixed constant velocity universal joint 20.

[0022] The intermediate shaft 2 has splines (including serrations; the same applies hereinafter) 3, 3 for transmitting torque on the outer diameter of both ends thereof. The inboard spline 3 is spline-fitted with a hole in the inner joint member 16 of the sliding type constant velocity universal joint 10, thereby connecting the intermediate shaft 2 and the inner joint member 16 of the sliding type constant velocity universal joint 10 so that torque can be transmitted therebetween. The outboard spline 3 is spline-fitted with a hole in the inner joint member 22 of the fixed type constant velocity universal joint 20, thereby connecting the intermediate shaft 2 and the inner joint member 22 of the fixed type constant velocity universal joint 20 so that torque can be transmitted therebetween. Although a solid type is shown as the intermediate shaft 2, a hollow type can also be used.

[0023] Grease as a lubricant is sealed inside both constant velocity universal joints 10, 20. To prevent the grease from leaking out and foreign matter from entering the joints from outside, cylindrical boots 4, 5 are fitted between the outer joint member 11 and the intermediate shaft 2 of the sliding type constant velocity universal joint 10, and between the outer joint member 21 and the intermediate shaft 2 of the fixed type constant velocity universal joint 20, respectively.

[0024] Next, the structure of the outer joint member 11 of the sliding-type constant velocity universal joint 10 having a shaft portion 13 will be described. As shown in FIG. 2 , the outer joint member 11 comprises a cylindrical cup portion 12 having one open end and track grooves on its inner peripheral surface, along which rollers 19 (see FIG. 1 ) roll, formed at three equal circumferential positions, and a shaft portion 13 extending in the axial direction and having a spline Sp as a torque transmission connecting portion formed on the outer diameter of the end opposite the cup portion 12 (inboard side). The cup portion 12 integrally comprises a cylindrical portion 12a, a bottom portion 12b, and a solid short shaft portion 31 protruding from the bottom portion 12b. A solid portion 32 is formed at one end of the shaft portion 13. In this embodiment, the shaft portion 13 is entirely solid, but a partial axial region of the shaft portion 13 may also be hollow. Even though the shaft portion 13 is partially hollow in this manner, a solid portion 32 is formed at the end of the shaft portion 13 on the cup portion 12 side.

[0025] The short shaft portion 31 of the cup portion 12 and the solid portion 32 of the shaft portion 13 are joined at the position of dashed line A shown in Figure 2. The joint between the cup portion 12 and the shaft portion 13 exists at a bearing mounting surface 30. The bearing mounting surface 30 is formed across the outer peripheral surface of the short shaft portion 31 of the cup portion 12 and the outer peripheral surface of the solid portion 32 of the shaft portion 13.

[0026] 3 shows the state before the cup portion 12 and the shaft portion 13 of the outer joint member 11 are joined. The outer joint member 11 is manufactured from two members: a cup member 12' that forms the cup portion 12, and a long shaft member 13' that forms the shaft portion 13. The cup member 12' has a short shaft portion 31, and the shaft member 13' has a solid portion 32 at at least one end. An end face 34 of the short shaft portion 31 of the cup member 12' and an end face 35 of the solid portion 32 of the shaft member 13' are both formed as flat surfaces extending in the radial direction without any irregularities.

[0027] The cup member 12' and the shaft member 13' are formed of medium-carbon steel with a carbon content of 0.43 mass% or more and 0.66 mass% or less. A carbon content of less than 0.43 mass% is undesirable because the required strength and durability cannot be obtained. Furthermore, a carbon content of more than 0.66 mass% is undesirable because it reduces forgeability and machinability and significantly increases hardness during air quenching after joining. By making the carbon content of the cup member 12' and the shaft member 13' different, productivity during friction welding can be improved. For example, the cup member 12' can be formed of carbon steel for machine structures S53C specified in JIS G4051, and the shaft member 13' can be formed of the same S45C.

[0028] The cup member 12′ shown in FIG. 3 is manufactured through a forging process and a machining process. Forging involves heating a billet obtained by cutting a steel bar, placing the billet in a forging die, and applying pressure to the billet in the die with a punch. Forging processes such as upsetting, extrusion, and ironing can be selected as appropriate. Forging can also be performed in multiple stages. During forging, the region that will become the opening of the cup member 12′ can be expanded in diameter, and then the region that will become the bottom, including the short shaft portion 31, can be reduced in diameter. The inner peripheral surface of the cup portion 12, including the track grooves, is formed by forging. After forging, the end surface 34 of the short shaft portion 31 is finished by machining, such as turning, and then a boot mounting groove, a retaining ring groove, and the like are formed by machining, such as turning, to obtain the cup member 12′ shown in FIG. 3.

[0029] The shaft member 13' is manufactured through a forging process and a machining process. A billet obtained by cutting a steel bar is formed into a rough shape by upset forging or the like, and then the end face of the billet is machined by turning or the like to form the end face 35. In addition, the outer circumferential surface of the billet is machined by turning or the like to form the bearing mounting surface 30 and the retaining ring groove 36. The end of the shaft member 13', where the spline Sp is to be formed, is also machined by turning or the like to form a predetermined spline lower diameter. Then, the spline Sp is formed at the end of the shaft member 13' by rolling (see FIG. 2).

[0030] The cup member 12' and the shaft member 13' manufactured through the above-described procedure are joined by friction welding the end face 34 of the short shaft portion 31 and the end face 35 of the solid portion 32 together.

[0031] Friction welding is performed through a frictional heating process and a pressure process. In the frictional heating process, the end face 34 of the cup member 12' and the end face 35 of the shaft member 13' are pressed against each other in the axial direction, and one of the cup member 12' and the shaft member 13' is rotated at high speed to rub against each other, and the frictional heat generated during this process softens the butted portions. In the pressure process, the rotation of one of the members is stopped while maintaining the butted state, and then an axial pressure is applied to the cup member 12' and the shaft member 13' and maintained for a certain period of time. As a result, the cup member 12' and the shaft member 13' are solid-state bonded by mutual atomic diffusion under high temperature and high pressure, and the two are integrated together as shown in FIG. 4.

[0032] After joining the cup member 12' and the shaft member 13', the bearing mounting surface 30 formed on the outer peripheral surface of the short shaft portion 31 and the outer peripheral surface of the solid portion 32 is machined by turning or the like to remove burrs and the like produced by friction welding, thereby obtaining the outer joint member 11 shown in Fig. 2. The outer joint member 11 obtained in this manner is subjected to a predetermined heat treatment. The heat treatment is performed, for example, by induction hardening the surface layer of the track grooves on the inner circumference of the cup portion 12 and the outer peripheral surface (including the splines Sp) of the shaft portion 13.

[0033] In the outer joint member 11 described above, the steel structure at the joint between the short shaft portion 31 of the cup member 12′ and the solid portion 32 of the shaft member 13′ may be transformed into martensite due to the rapid heating and cooling that accompanies friction welding. The core, where heat from the surroundings is concentrated, is particularly prone to high temperatures, and is therefore prone to transforming into martensite. If the core of the joint transforms into martensite in this way, the hardening degree at the joint varies greatly, resulting in greater variations in the quality and strength of the outer joint member. Furthermore, cracks are more likely to occur at the joint during heat treatment after friction welding.

[0034] To solve the above problems, the core hardness of the joint of the outer joint member 11 is set to an average value of Hv 350 or less and a maximum value of Hv 390 or less. To keep the upper limit of the core hardness within the above range, it is possible to adopt techniques such as reducing the pressing force between the two members 12' and 13' or the rotational speed of one of the members during the friction heating process, or reducing the axial pressure during the pressure application process. By specifying the core hardness of the joint in this way, it is possible to avoid the transformation of the core into martensite that occurs during friction welding, reduce variations in the degree of hardening at the joint, and stabilize the quality and strength of the outer joint member. Furthermore, cracking is less likely to occur at the joint during heat treatment after friction welding.

[0035] Friction welding can be performed using the brake method, flywheel method, etc. For example, the brake method can be used. In the brake method, it is necessary to adjust the friction pressure, peripheral speed, friction margin, upset pressure, upset margin, etc., and these conditions can be determined based on JIS Z3607:2016 "Friction Welding of Metallic Materials," specifically, Table JA.1 of Annex JA.

[0036] As shown in FIG. 4, the "core" in the core hardness refers to the inner region of a circle having a diameter Wr and centered on the axial core of the outer joint member 11. The diameter Wr of the circle can be set to, for example, 10 mm. The core hardness at the joint is specified, and the joint here refers to a region having an axial width Wa of 6 mm and centered on the interface A. The outer joint member 11 is cut radially within this axial width Wa, and the hardness measured at the core (within the circular region of diameter Wr) on the cut surface is the core hardness. The average core hardness refers to the average of hardness values ​​measured at multiple locations within the circular region of diameter Wr on the cut surface.

[0037] Furthermore, in the outer joint member 11 described above, if a large amount of center segregation occurs during steelmaking of the raw material, the segregated portion will have a large amount of carbon, and the axial core portion may be locally hardened during friction welding. If the axial core portion is locally hardened in this manner, the difference in hardness between the axial core portion and the peripheral portion will increase, resulting in a decrease in quality and strength. Furthermore, cracks are more likely to occur at the joint during heat treatment after friction welding.

[0038] To solve the above problems, it is desirable to minimize the occurrence of centerline segregation during the manufacturing stage of the steel bar (raw material) before forging. Therefore, in this embodiment, as the steel bar used for the cup member 12′ and the shaft member 13′, a material is used in which, as shown in FIG. 6 , the carbon content at the shaft core of the raw material 40 is C and the carbon content at a distance of ¼ of the raw material diameter d from the shaft core is C, such that C / C0 is 1.07 or less. A raw material with reduced centerline segregation can be obtained, for example, by supplying molten steel to a mold while stirring it during continuous casting in the steelmaking stage, or by subjecting the raw material to diffusion annealing after continuous casting.

[0039] As shown in Fig. 6, when a steel material with a round cross section is used as the raw material 40, the "raw material diameter d" refers to the diameter of the raw material. As shown in Fig. 7, a steel material with a square cross section can also be used as the raw material 40, and in this case, the "raw material diameter d" refers to the diameter of a circle circumscribing the square steel material.

[0040] By using a steel material with minimal centerline segregation as the raw material 40, centerline segregation at the joint can be suppressed in the outer joint member after friction welding. For example, in the completed outer joint member 11, as shown in FIG. 7 , when the carbon content at the shaft core at the joint between the short shaft portion 31 of the cup member 12′ and the solid portion 32 of the shaft member 13′ is C and the carbon content at a distance of ¼ of the joint diameter D from the shaft core is C0, if the C / C0 ratio is 1.07 or less, it becomes clear that a steel material with minimal centerline segregation was used as the raw material 40. Therefore, if the C / C0 value of the outer joint member 11 is 1.07 or less, centerline segregation at the joint is suppressed, which results in avoiding a localized hardening state during friction welding, reducing the difference in hardness between the shaft core and peripheral portions, and stabilizing quality and strength. Furthermore, cracking during heat treatment after friction welding can also be avoided.

[0041] 7 shows the shapes of the cut surfaces on the cup portion 12 side and the shaft portion 13 side when the outer joint member 11 is cut in the radial direction within the range of the axial width Wa of the joint portion. In this embodiment, the shapes of the cut surfaces on the cup portion 12 side and the shaft portion 13 side are the same, and therefore both cut surfaces are represented by a common contour line in FIG. 7. When measuring the carbon contents of the outer joint member 11, the carbon contents C and C0 can be measured on the cut surfaces shown in FIG. 7.

[0042] FIG. 8 shows the hardness measurement results at the joint of the present embodiment (indicated by ◯) and the comparative example (indicated by ♦). The hardness was measured on a cross section obtained by cutting the outer joint member 11 in the radial direction at the joint. As is clear from FIG. 8 , in the present embodiment, the average core hardness (within a 5 mm range from the center) is Hv 350 or less and the maximum core hardness is Hv 390 or less. However, in the comparative example, the average core hardness exceeds Hv 350 and the maximum core hardness exceeds Hv 390. While cracks occurred during heat treatment in the comparative example, no such cracks occurred in the present embodiment. Therefore, it can be seen that the occurrence of cracks can be avoided by setting the average core hardness to Hv 350 or less and the maximum core hardness to Hv 390 or less.

[0043] Figure 9 shows the results of a test evaluating whether or not cracks occur during heat treatment when the C / C0 value of the material 40 for the cup member 12' and the shaft member 13' is changed. From Figure 9, it can be seen that the C / C0 value of the material 40 is set at 1.07, with cracks occurring when the value is greater than this, and no cracks occurring when the value is less than this. Therefore, it can be seen that cracks can be avoided by forming the cup member 12' and the shaft member 13' from a material 40 with a C / C0 value of 1.07 or less.

[0044] In the embodiment described above, the fixed type constant velocity universal joint has been exemplified as having eight balls, but this is not limitative and the number of balls may be more than eight as appropriate.

[0045] The present invention is not limited to the above-described embodiments, and can of course be embodied in various other forms without departing from the spirit of the present invention. The scope of the present invention is indicated by the claims, and further includes the equivalent meanings set forth in the claims, and all modifications within the scope of the claims.

[0046] REFERENCE SIGNS LIST 10 Sliding type constant velocity universal joint 11 Outer joint member 12 Cup portion 12' Cup member 13 Shaft portion 13' Shaft member 16 Inner joint member 19 Roller (torque transmission element) 20 Fixed type constant velocity universal joint 30 Bearing mounting surface 31 Short shaft portion 32 Solid portion 34 End face 35 End face

Claims

1. An outer joint member of a constant velocity universal joint comprising: a cup portion having track grooves formed on its inner circumference with which a torque transmission element engages; and a shaft portion joined to the cup portion, wherein the cup portion is cylindrical with one end open and has a cylindrical portion, a bottom, and a solid short shaft portion protruding from the bottom, and wherein a solid portion is formed at one end of the shaft portion, and the short shaft portion of the cup portion and the solid portion of the shaft portion are joined by friction welding, wherein the core hardness of the joint between the short shaft portion of the cup portion and the solid portion of the shaft portion is an average of Hv 350 or less and a maximum of Hv 390 or less.

2. An outer joint member of a constant velocity universal joint as set forth in claim 1, wherein, at the joint between the short shaft portion of the cup portion and the solid portion of the shaft portion, when the carbon content of the shaft core is C and the carbon content of a portion away from the shaft core by a distance of 1 / 4 of the diameter of the joint is C0, C / C0 is 1.07 or less.

3. The outer joint member of a constant velocity universal joint according to claim 1, wherein the core hardness is measured within a circular region of the joint having a diameter of 10 mm and centered on the axis.

4. An outer joint member for a constant velocity universal joint according to claim 1, wherein the joint between the short shaft portion of said cup portion and the solid portion of said shaft portion is provided on a bearing mounting surface.

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