WO2025205018A1 - Outer joint member for constant velocity universal joint - Google Patents

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

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

Constant velocity universal joints, which make up the power transmission systems of automobiles and various industrial machines, connect two shafts, one driving and one driven, 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, which allow only angular displacement, and sliding-type constant velocity universal joints, which allow both angular and axial displacement. For example, in the 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).

Regarding either the sliding or fixed type, constant velocity universal joints primarily comprise an outer joint member having a cup portion with track grooves formed on its inner circumferential surface that engage with the torque transmission element, and a shaft portion extending axially from the bottom of the cup portion. This outer joint member is often made by integrally forming the cup portion and shaft portion from a solid rod-shaped material through cold plastic processing such as forging or ironing, or machining such as cutting or grinding.

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 (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 made long-stem, this long stem is rotationally supported by a rolling bearing. The length of the long stem varies depending on the vehicle model, but is generally around 300 to 400 mm. Because the shaft portion of this outer joint member is long, it is difficult to precisely mold the cup portion and shaft portion into a single piece. For this reason, some outer joint members are constructed of two members: a cup member that forms the cup portion and a shaft member that forms the shaft portion, and the two members are joined by friction welding. Examples of such joint members joined by friction welding include a solid-stem type described in Patent Document 1 and a hollow-stem type described in Patent Document 2.

Japanese Patent Application Laid-Open No. 61-132284 Japanese Patent Application Laid-Open No. 2006-64060

When the shank of the long stem is joined to the cup portion by friction welding, the joint is subjected to thermal effects from 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 in the joint, such as the shank core, may become martensite. In this case, an imbalance in the degree of hardening occurs within the joint, making the quality and strength of the outer joint component unstable after friction welding. Furthermore, when the outer joint component is heat treated after friction welding, cracks (quench cracks) may occur in areas that have already been hardened by friction welding.

The present invention aims to stabilize the quality and strength of outer joint members in which the shaft portion and cup portion are joined by friction welding.

In order to achieve the above-mentioned object, the present invention provides an outer joint member for 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, the cup portion being cylindrical with one end open and having a cylindrical portion, a bottom, and a solid short shaft portion protruding from the bottom, the solid portion being formed at one end of the shaft portion, and the short shaft portion of the cup portion and the solid portion of the shaft being joined by friction welding, characterized in that the core hardness of the joint between the short shaft portion of the cup portion and the solid portion of the shaft is an average of Hv 350 or less and a maximum of Hv 390 or less.

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 variation in the degree of hardening in the joint, and stabilize the quality and strength of the outer joint member. It also makes it less likely for cracks to occur in the joint during heat treatment after friction welding.

When the carbon content of the shaft core at the joint between the short shaft portion of the cup portion and the solid portion of the shaft portion is C and the carbon content of the 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.

This suppresses central segregation at the joint, avoiding localized hardening during friction welding, reducing the difference in hardening between the axial core and peripheral areas, and stabilizing quality and strength. It also prevents cracks from occurring during heat treatment after friction welding.

The core hardness can be measured within a circular area of the joint with a diameter of 10 mm centered on the axis.

The joint between the short shaft portion of the cup portion and the solid portion of the shaft portion can be provided on the bearing mounting surface.

According to the present invention, it is possible to stabilize the quality and strength of outer joint members in which the shaft portion and cup portion are joined by friction welding.

1 is a cross-sectional view showing the overall structure of a drive shaft 1. FIG. FIG. FIG. 3 is a cross-sectional view showing the cup member and the shaft member before friction welding. FIG. 4 is a cross-sectional view showing the cup portion and the stem portion after friction welding. FIG. FIG. FIG. 4 is a front view showing a cut surface when the outer joint member is cut at a joint portion. FIG. 10 is a diagram showing the measurement results of hardness at the joint in this embodiment and a comparative example. FIG. 10 is a diagram showing test results evaluating whether or not cracks occur during heat treatment when the C/C0 value of the material is changed.

The following describes an embodiment of the present invention with reference to Figures 1 to 9.

Figure 1 shows 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 primarily composed of a sliding type constant velocity universal joint 10 located on the differential gear side (right side in the figure; hereinafter also referred to as the inboard side), a fixed type constant velocity universal joint 20 located on the drive wheel side (left side in the figure; hereinafter also referred to as the outboard side), and an intermediate shaft 2 that connects both constant velocity universal joints 10, 20 so that torque can be transmitted.

The sliding-type constant velocity universal joint 10 shown in Figure 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 within the inner periphery of the cup portion 12 of the outer joint member 11, and rollers 19 serving 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, spaced equally apart in the circumferential direction. The inner joint member 16 of a tripod-type constant velocity universal joint is also called a tripod member.

Three axially extending track grooves are formed on the inner peripheral surface of the outer joint member 11 at equal intervals in the circumferential direction. Rollers 19 fitted to the outside of each trunnion 18 engage with each track groove in the circumferential direction, thereby transmitting torque between the outer joint member 11 and the inner joint member 16. In addition to tripod-type constant velocity universal joints, other sliding-type constant velocity universal joints, such as double-offset-type constant velocity universal joints, can also be used as the sliding-type constant velocity universal joint 10.

The inner ring of the support bearing 6 is fixed to the outer peripheral surface of the shaft portion 13, and the 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 relative to the vehicle body by the support bearing 6. The provision of the support bearing 6 minimizes wobble of the outer joint member 11 during operation, etc. The outer joint member 11 is provided with a cylindrical bearing mounting surface 30 (see Figure 2) for fixing the inner ring of the support bearing 6.

The fixed type constant velocity universal joint 20 shown in Figure 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 within 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 type constant velocity universal joint 20.

The intermediate shaft 2 has splines (including serrations; the same applies below) 3, 3 for torque transmission on the outer diameter of both ends. 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. 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. While a solid type is shown as the intermediate shaft 2, a hollow type can also be used.

Grease is sealed inside both constant velocity universal joints 10, 20 as a lubricant. To prevent 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.

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 Figure 2, the outer joint member 11 comprises a cylindrical cup portion 12 with one open end and track grooves formed on the inner surface at three equal circumferential positions along the circumferential direction, along which rollers 19 (see Figure 1) roll; and a shaft portion 13 extending in the axial direction and having a spline Sp as a torque transmission connecting portion 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 with a partially hollow shaft portion 13 like this, a solid portion 32 is formed at the end of the shaft portion 13 on the cup portion 12 side.

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 indicated by dashed line A in Figure 2. The joint between the cup portion 12 and the shaft portion 13 exists at the 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.

Figure 3 shows the state before the cup portion 12 and shaft portion 13 of the outer joint member 11 are joined. The outer joint member 11 is made of 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. The end face 34 of the short shaft portion 31 of the cup member 12' and the end face 35 of the solid portion 32 of the shaft member 13' are both formed as flat surfaces extending radially without any irregularities.

Cup member 12' and shaft member 13' are formed from 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 as it does not provide the required strength and durability. Furthermore, a carbon content of more than 0.66 mass% is undesirable as it reduces forgeability and machinability and significantly increases hardness during air quenching after joining. By making the carbon content of cup member 12' and shaft member 13' different, it is possible to improve productivity during friction welding. For example, cup member 12' can be formed from S53C carbon steel for mechanical structures specified in JIS G4051, and shaft member 13' can be formed from S45C.

The cup member 12' shown in Figure 3 is manufactured through a forging process and a machining process. Forging involves heating a billet obtained by cutting a steel bar, then placing the billet in a forging die and applying pressure to the billet inside 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 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 boot mounting grooves, retaining ring grooves, etc. are formed by machining such as turning, to obtain the cup member 12' shown in Figure 3.

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 the approximate 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. The outer peripheral surface of the billet is also 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 the specified spline lower diameter. The spline Sp is then formed at the end of the shaft member 13' by rolling (see Figure 2).

The cup member 12' and shaft member 13' manufactured through the above steps 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.

Friction welding is carried out 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, softening the butted portion with the frictional heat generated. 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 pressure, and the two are integrated together as shown in Figure 4.

After joining the cup member 12' and the shaft member 13', the bearing mounting surface 30 formed by 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 other machining processes, and burrs and the like resulting from friction welding are removed, thereby obtaining the outer joint member 11 shown in Figure 2. The outer joint member 11 obtained in this manner is then subjected to a predetermined heat treatment. The heat treatment is performed, for example, by induction hardening the surface of the track grooves on the inner circumference of the cup portion 12 and the outer peripheral surface of the shaft portion 13 (including the splines Sp).

In the outer joint member 11 described above, at the joint between the short shaft portion 31 of the cup member 12' and the solid portion 32 of the shaft member 13', the rapid heating and cooling that accompanies friction welding may cause the steel structure to transform into martensite. The core, where heat from the surroundings is concentrated, is particularly prone to high temperatures, making it prone to transforming into martensite. When the core of the joint transforms into martensite in this way, the degree of hardening at the joint varies greatly, resulting in greater variation 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.

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. In order to keep the upper limit of the core hardness within the above range, it is possible to adopt methods such as reducing the pressing force of the two members 12', 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 variation in the degree of hardening at the joint, and stabilize the quality and strength of the outer joint member. It also makes the joint less susceptible to cracking during heat treatment after friction welding.

Friction welding can be performed using methods such as the brake method and the flywheel method, and the brake method can be used, for example. With 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 in Appendix JA.

The "core" in core hardness refers to the inner region of a circle of diameter Wr centered on the axial core of the outer joint member 11, as shown in Figure 4. The diameter Wr of the circle can be set to 10 mm, for example. The core hardness at the joint is also specified, and the joint referred to here refers to a region of 6 mm axial width Wa centered on 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 hardness measured at multiple locations within the circular region of diameter Wr on the cut surface.

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 area will have a high carbon content, which may result in the core portion of the shaft becoming locally hardened during friction welding. When the core portion of the shaft becomes locally hardened in this way, the difference in hardening degree compared to the surrounding area becomes large, resulting in a decrease in quality and strength. Furthermore, cracks are more likely to occur at the joint during heat treatment after friction welding.

To solve the above problems, it is desirable to minimize the occurrence of center segregation when manufacturing the steel bar (raw material) before forging. Therefore, in this embodiment, as shown in Figure 6, the steel bar used to form the cup member 12' and shaft member 13' is a material with a C/C0 ratio of 1.07 or less, where C is the carbon content at the shaft center of the material 40 and C0 is the carbon content at a distance of 1/4 of the material diameter d from the shaft center. A material with reduced center segregation like this can be obtained by, for example, using one or both of the following methods: feeding molten steel into a mold while stirring it during continuous casting in the steelmaking stage, or subjecting the material to diffusion annealing after continuous casting.

Note that, as shown in Figure 6, when a steel material with a round cross section is used as the raw material 40, "raw material diameter d" refers to the diameter of the raw material. As shown in Figure 7, a steel material with a square cross section can also be used as the raw material 40, in which case "raw material diameter d" refers to the diameter of the circle circumscribing the square steel material.

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 Figure 7, if 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 1/4 of the joint diameter D from the shaft core is C0, if C/C0 is 1.07 or less, it is 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 prevents localized hardening during friction welding, reduces the difference in hardness between the shaft core and peripheral portions, and stabilizes quality and strength. It also prevents cracks from occurring during heat treatment after friction welding.

Note that Figure 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 radially within the axial width Wa of the joint. 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 in Figure 7 both cut surfaces are represented by a common outline. 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 Figure 7.

Figure 8 shows the hardness measurement results at the joint for this embodiment (indicated by ○) and the comparative example (indicated by ◆). The hardness was measured on a cut surface created when the outer joint member 11 was cut radially at the joint. As is clear from Figure 8, in this embodiment, the average core hardness (within a 5 mm range from the center) is Hv 350 or less and the maximum value is Hv 390 or less, while in the comparative example, the average core hardness exceeds Hv 350 and the maximum value exceeds Hv 390. Cracks occurred during heat treatment in the comparative example, but no such cracks occurred in this embodiment. It can therefore be seen that cracks can be avoided by setting the average core hardness to Hv 350 or less and the maximum value to Hv 390 or less.

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 of the cup member 12' and 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 if this value is greater than this, and no cracks occurring if this value is less than this. Therefore, it can be seen that cracks can be avoided by forming the cup member 12' and shaft member 13' from a material 40 with a C/C0 of 1.07 or less.

In the embodiment described above, a fixed constant velocity universal joint with eight balls was used as an example, but this is not limited to this and ball numbers greater than eight can also be used as appropriate.

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, as well as all modifications within the scope of the claims.

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 surface 35 End surface

Claims (4)

a cup portion having a track groove formed on an inner periphery thereof with which a torque transmission element is engaged, and a shaft portion joined to the cup portion,
the cup portion is a cylindrical portion having an open end and a bottom, and includes a cylindrical portion, a bottom, and a solid short shaft portion protruding from the bottom, and a solid portion is formed at one end of the shaft portion;
In an outer joint member of a constant velocity universal joint in which a short shaft portion of the cup portion and a solid portion of the shaft portion are joined by friction welding,
1. An outer joint member of a constant velocity universal joint, 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 Hv 350 or less on average and Hv 390 or less on maximum. The outer joint member of a constant velocity universal joint according to 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 joint diameter is C0, C/C0 is 1.07 or less. The outer joint member of a constant velocity universal joint according to claim 1, wherein the core hardness is measured within a circular area of the joint having a diameter of 10 mm and centered on the axis. The outer joint member of a constant velocity universal joint according to claim 1, wherein the joint between the short shaft portion of the cup portion and the solid portion of the shaft portion is provided on a bearing mounting surface.