Plunging type constant velocity universal joint

Abstract

A film (manganese phosphate film 20) that reduces surface roughness to less than that of a base material surface is formed on at least one surface among a cylindrical inner peripheral surface 6 of an outside joint member 2 of a double offset type constant velocity universal joint 1, a surface of a track groove 7 of the outside joint member 2, a spherical outer peripheral surface 8 of an inside joint member 3, a surface of a track groove 9 of the inside joint member 3, a spherical surface section 13 of an outer peripheral surface of a retainer 5, and a spherical surface section 15 of an inner peripheral surface of the retainer 5.

Description

Sliding constant velocity universal joint

[0001] The present invention relates to a sliding type constant velocity universal joint.

[0002] Constant velocity universal joints used in automobile drive shafts and propeller shafts can be broadly divided into sliding types that allow both angular displacement and relative axial movement between two shafts, and fixed types that allow angular displacement between two shafts but do not allow relative axial movement between the two shafts.

[0003] Known sliding type constant velocity universal joints include double offset constant velocity universal joints (DOJs) that use balls as rolling elements for transmitting rotational torque, and tripod constant velocity universal joints (TJs) that use rollers as rolling elements. For example, Patent Document 1 listed below discloses a double offset constant velocity universal joint that is lightweight and compact by increasing the number of balls from six to eight. Patent Document 2 listed below also discloses a double offset constant velocity universal joint that has a maximum operating angle of 30° or more and is even lighter and more compact.

[0004] Compared to tripod-type constant velocity universal joints, double offset-type constant velocity universal joints have the advantages of less circumferential backlash, better responsiveness, and lower manufacturing costs, but they have the disadvantage of greater sliding resistance and greater susceptibility to vehicle vibrations, particularly engine vibrations during idling. Therefore, various countermeasures for idling vibrations have been studied for double offset-type constant velocity universal joints. For example, Patent Document 3 listed below discloses a technology for absorbing idling vibrations by providing a gap between the outer peripheral surface of the inner ring and the inner peripheral surface of the cage.

[0005] Japanese Patent Laid-Open No. 10-73129 Japanese Patent Laid-Open No. 2007-85488 Japanese Patent Laid-Open No. 2013-231518 Japanese Patent Laid-Open No. 2011-106534

[0006] However, as vehicle ride comfort continues to improve, the NVH (Noise, Vibration, Harshness) characteristics required of constant velocity universal joints are becoming more stringent. Because double offset constant velocity universal joints inevitably experience sliding contact between their components due to their structure, even if measures such as those shown in Patent Document 3 are implemented, it is difficult to reduce the sliding resistance to the same level as tripod-type sliding constant velocity universal joints, in which the components primarily contact each other by rolling. This sliding resistance problem, particularly the problem of idling vibration, has become a bottleneck, and in recent years the application range of double offset constant velocity universal joints has been largely limited to locations that are less susceptible to engine vibration (e.g., rear drive shafts).

[0007] In recent years, the electrification of vehicles has progressed, with an increase in electric vehicles (hereinafter referred to as EVs) that run solely on the power of electric motors and hybrid vehicles (hereinafter referred to as HEVs) that run on the power of both electric motors and engines. Because electric motors have superior responsiveness compared to engines, constant velocity universal joints that transmit the power of electric motors are also required to have low circumferential backlash and excellent responsiveness. Furthermore, since EVs do not generate engine vibrations and HEVs essentially stop their engines while the vehicle is stopped (idling), these vehicles do not experience the problem of idling vibration, which is the main factor limiting the application of double offset constant velocity universal joints. From these perspectives, the adoption of double offset sliding constant velocity universal joints, which have low circumferential backlash and excellent responsiveness, is being considered for use in EVs and HEVs as sliding constant velocity universal joints for power transmission systems such as drive shafts and propeller shafts.

[0008] On the other hand, in a sliding-type constant velocity universal joint, an axial load (induced thrust) is generated due to frictional forces between internal parts when rotating through a working angle. This induced thrust resonates with the vehicle body, which can cause various vibration problems such as swaying and muffled noise when starting. Such problems are particularly noticeable in EVs and HEVs, which are known for their quietness. Therefore, if the induced thrust can be reduced, double-offset constant velocity universal joints may be suitable for use in EVs and HEVs.

[0009] Therefore, an object of the present invention is to reduce the induced thrust of a double offset type constant velocity universal joint so that it can be suitably used in the power transmission system of a vehicle (EV or HEV) that uses an electric motor as a drive source.

[0010] [First Invention] As described above, induced thrust is an axial load generated due to frictional forces between internal components of a sliding-type constant velocity universal joint. Specifically, as shown in FIG. 23 , the induced thrust is the result of the axial resistance Qx resulting from the frictional forces at the contact points between the track grooves 101 a of the outer joint member 101 and the balls 102, and the axial resistance Hx resulting from the frictional forces at the contact points between the outer spherical surface 103 a of the cage 103 and the cylindrical inner peripheral surface 101 b of the outer joint member 101. FIG. 24 shows the results of mechanical analysis of the induced thrust Fx (= Qx + Hx) in a double offset constant velocity universal joint having eight balls. As shown in the figure, it was previously believed that a double offset constant velocity universal joint would mainly generate induced thrust of order components equal in number to the number of balls (eight in the figure).

[0011] Meanwhile, Figure 25 shows the results of measuring the actual induced thrust of an eight-ball double-offset constant velocity universal joint. In this test, assuming a vehicle starting, a torque of 900 Nm was input to the outer joint member of the constant velocity universal joint at a rotation speed of 150 rpm, while the working angle was changed from 0° to 12° at a rate of 10.7° / min. The axial load (induced thrust) applied to the outer joint member was measured. The X axis of Figure 25 represents the frequency of vibration generated in the constant velocity universal joint, the Y axis represents the working angle of the constant velocity universal joint, and the Z axis represents the magnitude of the axial load (induced thrust) applied to the outer joint member. The peak appearing at 2.5 Hz in the figure is the first-order component of the induced thrust (a vibration component that occurs once per rotation of the constant velocity universal joint), the peak appearing at 5 Hz is the second-order component, the peak appearing at 7.5 Hz is the third-order component, and so on, and the peak appearing at 20 Hz is the eighth-order component. From the actual measurement results shown in the figure, it can be confirmed that the induced thrust not only has an eighth-order component, but also many other order components, with the first-order component appearing particularly prominently.

[0012] As described above, the causes of the generation of induced thrusts of various orders are thought to be as follows: As described above, the induced thrust Fx is the resultant force of the axial resistance Qx between the balls and the outer joint member and the axial resistance Hx between the cage and the outer joint member (see FIG. 23 ). The axial resistances Qx and Hx are each the resultant force of the loads generated in each phase. Note that "each phase" refers to the phase of each ball and each track groove in contact with it (first to eighth phases in the case of an eight-ball double offset constant velocity universal joint) at the contact portion between the track grooves of the outer joint member and the balls, and refers to the phase of each spherical portion between the track grooves of the outer joint member and each column portion between the pockets of the cage in contact with them (first to eighth phases in the case of an eight-ball double offset constant velocity universal joint).

[0013] 26 and 27 show the results of a mechanical analysis of the axial loads applied to the track grooves of each phase of the outer joint member and their resultant force (Qx) in an eight-ball double-offset constant velocity universal joint. FIG. 26 shows the axial components of the track loads of each phase and their resultant force in an ideal state where all components have uniform dimensions and uniform surface properties in all phases. In this case, the axial loads (see dotted line) in the tracks of all phases are uniform, and the resultant force has a regular shape (see solid line) with a strong presence of a specific order component (eighth order component in the illustrated example). However, in reality, the contact state of the internal parts is non-uniform due to variations in the dimensions of each part and variations in the surface properties of the contact areas. Therefore, as shown in FIG. 27, variations occur in the axial loads generated in the tracks of each phase (see dotted line), and the resultant force has an irregular shape (see solid line). As a result, the induced thrust (Fx = Qx + Hx) generates various order components, including the first order component (see Figure 25). When induced thrust of various order components is generated in this way, the number of resonance points with the vehicle body increases, increasing the possibility of causing noise and vibration problems.

[0014] From the above viewpoints, the inventors have considered that it may be possible to suppress each order component of induced thrust by equalizing the axial load generated at the contact portion of each phase of a double offset constant velocity universal joint. Specifically, since it is well known that the roughness of the contact surface affects the frictional force when the contact portion between parts is boundary lubricated, they considered that suppressing the variation in surface roughness between phases would be effective in equalizing the axial load (i.e., frictional force) at the contact portion of each phase.

[0015] Therefore, a first invention provides a sliding type constant velocity universal joint comprising an outer joint member having a plurality of linear track grooves formed on a cylindrical inner peripheral surface thereof, an inner joint member having a plurality of linear track grooves formed on a spherical outer peripheral surface thereof, a plurality of balls arranged between the track grooves of the outer joint member and the track grooves of the inner joint member, an outer peripheral surface formed with spherical portions that come into sliding contact with the cylindrical inner peripheral surface of the outer joint member, an inner peripheral surface formed with spherical portions that come into sliding contact with the spherical outer peripheral surface of the inner joint member, and a cage having a plurality of pockets for holding the plurality of balls, wherein a coating that makes the surface roughness smaller than that of a surface of a base material is formed on at least one of the cylindrical inner peripheral surface of the outer joint member, the surfaces of the track grooves of the outer joint member, the spherical outer peripheral surface of the inner joint member, the surfaces of the track grooves of the inner joint member, the spherical portions of the outer peripheral surface of the cage, and the spherical portion of the inner peripheral surface of the cage.

[0016] In this way, by forming a coating (e.g., manganese phosphate coating) on ​​the contact surfaces between the components that reduces the surface roughness compared to the surface of the base material, it is possible to reduce the variation in surface quality between the phases. This makes the axial load generated at the contact points of each phase uniform, thereby reducing each order component of the induced thrust.

[0017] The roughness parameter Rsk of the surface on which the coating is formed is preferably a negative value, and the roughness parameter Rp of the surface on which the coating is formed is preferably 2 or less.

[0018] For example, the track groove surfaces and cylindrical inner peripheral surfaces of outer joint members and the track groove surfaces of inner joint members are often formed by forging from the viewpoint of manufacturing costs. Such forged surfaces have high surface roughness, which tends to result in large variations in surface properties for each phase. Therefore, when the surface of a base material is a forged surface, it is particularly preferable to form the above-described coating to suppress variations in roughness.

[0019] The sliding type constant velocity universal joint can be suitably incorporated into the power transmission system of a vehicle using an electric motor as a drive source. Specifically, a wheel drive device can be obtained that includes an electric motor, wheels, and a power transmission system that transmits the driving force of the electric motor to the wheels via the sliding type constant velocity universal joint.

[0020] [Second Invention] The components (outer joint member, inner joint member, and cage) that make up a double offset constant velocity universal joint move relative to one another during rotation. Therefore, when the constant velocity universal joint rotates, the sliding portions of the components generate axial frictional force (induced thrust), which can cause vibration problems. Through extensive research, the inventors have discovered that one of the causes of the vibration problem described above is the roughness of the sliding surfaces, and that appropriate control of this roughness can solve the vibration problem. Specifically, the surfaces of the components are formed by forging, rolling, or machining such as cutting, grinding, and polishing, and the peaks of the peaks are crushed or removed while leaving the bottoms of the valleys of the surface roughness curve. This reduces the axial frictional force at the sliding portions between the components, thereby reducing the induced thrust of the double offset constant velocity universal joint.

[0021] That is, a second invention is a sliding type constant velocity universal joint comprising: an outer joint member having a plurality of linear track grooves formed on a cylindrical inner peripheral surface thereof; an inner joint member having a plurality of linear track grooves formed on a spherical outer peripheral surface thereof; a plurality of balls arranged between the track grooves of the outer joint member and the track grooves of the inner joint member; and a cage having spherical surface portions formed on its outer peripheral surface which come into sliding contact with the cylindrical inner peripheral surface of the outer joint member and on its inner peripheral surface which come into sliding contact with the spherical outer peripheral surface of the inner joint member, and having pockets for holding the plurality of balls, wherein at least one surface of the cylindrical inner peripheral surface of the outer joint member, the surfaces of the track grooves of the outer joint member, the spherical outer peripheral surface of the inner joint member, the surfaces of the track grooves of the inner joint member, the spherical surface portions of the outer peripheral surface of the cage, and the spherical surface portions of the inner peripheral surface of the cage is formed by forging, rolling, or machining, The induced thrust is reduced by crushing or removing the peaks while leaving the bottoms of the valleys of the roughness curve of at least one of the surfaces, thereby making Rsk of this surface a negative value (or making Rp 2 or less).

[0022] In the above-described sliding-type constant velocity universal joint, the bottom of a valley of a roughness curve of at least one of the cylindrical inner peripheral surface of the outer joint member, the surface of a track groove of the outer joint member, the spherical outer peripheral surface of the inner joint member, the surface of a track groove of the inner joint member, the spherical portion of the outer peripheral surface of the cage, and the spherical portion of the inner peripheral surface of the cage is a forged surface, a rolled surface, or a machined surface, and Rsk of this surface is a negative value (or Rp is 2 or less).

[0023] The forged surface has numerous minute peaks and valleys that are connected around each peak all the way around. The machined surface has numerous streak-like peaks and valleys that extend in the same direction. In the present invention, the peaks of the peaks are crushed or removed while leaving the bottoms of the valleys, which are the forged or machined surface, to make Rsk a negative value (or make Rp 2 or less).

[0024] Incidentally, a technology is known in which a large number of minute dents are actively formed on the surfaces of track grooves, etc. of an outer joint member by barrel processing (see, for example, the above-mentioned Patent Document 4). In this case, the bottoms of the valleys (minute dents) formed on the surfaces of the track grooves, etc. become surfaces formed by the media used in the barrel processing. In contrast, the present invention crushes or removes only the tops of the peaks, leaving the bottoms of the valleys on the surfaces of the track grooves, etc. as the forged, rolled, or machined surface, and is completely different from the above-mentioned known technology in which valleys (minute dents) are actively formed.

[0025] When the cylindrical inner peripheral surface of the outer joint member, the surfaces of the track grooves of the outer joint member, and the surfaces of the track grooves of the inner joint member are formed by forging, the bottoms of the valleys of these surfaces become forged surfaces. The Ra of these surfaces is preferably 1.5 or less. The Rz of these surfaces is preferably 10 or less.

[0026] The present invention can be suitably applied to a sliding type constant velocity universal joint mounted on an electric vehicle.

[0027] As described above, according to the present invention, it is possible to reduce the induced thrust of a double offset sliding constant velocity universal joint which has little backlash and excellent responsiveness, and therefore this can be suitably used in the power transmission systems of EVs and HEVs.

[0028] FIG. 1 is an axial sectional view of a double offset sliding type constant velocity universal joint according to one embodiment of the present invention. FIG. 2 is an axial sectional view of the sliding type constant velocity universal joint of FIG. 1. FIG. 3 is an axial sectional view of the sliding type constant velocity universal joint of FIG. 1. FIG. 4 is an axial sectional view of the sliding type constant velocity universal joint of FIG. 1. FIG. 5 is an axial sectional view of the sliding type constant velocity universal joint of FIG. 1. FIG. 6 is an axial sectional view of the sliding type constant velocity universal joint of FIG. 1. FIG. 7 is an axial sectional view of the sliding type constant velocity universal joint of FIG. 1. FIG. 8 is an axial sectional view of the sliding type constant velocity universal joint of FIG. 1. FIG. 9 is an axial sectional view of the sliding type constant velocity universal joint of FIG. 1. FIG. 10 is an axial sectional view of the sliding type constant velocity universal joint of FIG. 1. 1 shows a roughness curve (left), a load curve (center), and an amplitude distribution curve (right) of the surface of another track groove of an outer joint member after manganese phosphate treatment has been applied.

[0034] FIG. 1 shows a graph showing measurement results of induced thrust of a double offset constant velocity universal joint in which an outer joint member has been subjected to manganese phosphate treatment.

[0035] FIG. 1 shows a roughness curve (left), a load curve (center), and an amplitude distribution curve (right) of the surface of one track groove of an inner joint member before manganese phosphate treatment has been applied.

[0036] FIG. 1 shows a roughness curve (left), a load curve (center), and an amplitude distribution curve (right) of the surface of another track groove of an inner joint member before manganese phosphate treatment has been applied.

[0037] FIG. 1 shows a roughness curve (left), a load curve (center), and an amplitude distribution curve (right) of the surface of one track groove of an inner joint member after manganese phosphate treatment has been applied.

[0038] FIG. 1 shows a roughness curve (left), a load curve (center), and an amplitude distribution curve (right) of the surface of another track groove of an inner joint member after manganese phosphate treatment has been applied.

[0039] 10 is a graph showing the measurement results of induced thrust of a double offset constant velocity universal joint in which an inner joint member is subjected to manganese phosphate treatment. It shows a roughness curve (left), a load curve (center), and an amplitude distribution curve (right) of the outer peripheral surface of one bar portion of the cage before being subjected to manganese phosphate treatment. It shows a roughness curve (left), a load curve (center), and an amplitude distribution curve (right) of the outer peripheral surface of another bar portion of the cage before being subjected to manganese phosphate treatment.1 shows a roughness curve (left), a load curve (center), and an amplitude distribution curve (right) of the outer peripheral surface of one bar portion of the cage after manganese phosphate treatment. 2 shows a roughness curve (left), a load curve (center), and an amplitude distribution curve (right) of the outer peripheral surface of another bar portion of the cage after manganese phosphate treatment. 3 shows a graph showing measurement results of induced thrust in a double offset constant velocity universal joint whose cage has been subjected to manganese phosphate treatment. 4 shows a cross-sectional view showing a portion where an axial load is applied when a double offset constant velocity universal joint rotates at an operating angle. 5 shows analysis results of induced thrust in a double offset constant velocity universal joint. 6 shows actual measurement results of induced thrust in a double offset constant velocity universal joint. 7 shows analysis results of axial load applied to track grooves of an outer joint member (when the axial load applied to the track grooves of each phase is uniform). 8 shows analysis results of axial load applied to track grooves of an outer joint member (when the axial load applied to the track grooves of each phase is non-uniform). 1 is a roughness curve of the surface of the track groove of an outer joint member before surface modification treatment is applied. FIG. 2 is a load curve of the surface of the track groove of an outer joint member before surface modification treatment is applied. FIG. 3 is an amplitude distribution curve of the surface of the track groove of an outer joint member before surface modification treatment is applied. FIG. 4 is a roughness curve of the surface of the track groove of an outer joint member that has been surface modified. FIG. 5 is a load curve of the surface of the track groove of an outer joint member that has been surface modified. FIG. 6 is an amplitude distribution curve of the surface of the track groove of an outer joint member that has been surface modified. FIG. 7 is a plan view of a forged surface that has been surface modified. FIG. 8 is a graph showing measurement results of induced thrust of a double offset constant velocity universal joint. FIG. 9 is a roughness curve of a cylindrical inner peripheral surface of an outer joint member that has been surface modified. 1 is a load curve of a cylindrical inner peripheral surface of an outer joint member that has been subjected to a surface modification treatment; 2 is an amplitude distribution curve of a cylindrical inner peripheral surface of an outer joint member that has been subjected to a surface modification treatment; 3 is a roughness curve of a spherical outer peripheral surface of an inner joint member that has been subjected to a surface modification treatment; 4 is a load curve of a spherical outer peripheral surface of an inner joint member that has been subjected to a surface modification treatment; and 5 is an amplitude distribution curve of a spherical outer peripheral surface of an inner joint member that has been subjected to a surface modification treatment.3 is a roughness curve of the spherical surface portion of the outer peripheral surface of a cage that has been subjected to surface modification treatment. 4 is a load curve of the spherical surface portion of the outer peripheral surface of a cage that has been subjected to surface modification treatment. 5 is an amplitude distribution curve of the spherical surface portion of the outer peripheral surface of a cage that has been subjected to surface modification treatment. 6 is a roughness curve of the spherical surface portion of the inner peripheral surface of a cage that has been subjected to surface modification treatment. 7 is a load curve of the spherical surface portion of the inner peripheral surface of a cage that has been subjected to surface modification treatment. 8 is an amplitude distribution curve of the spherical surface portion of the inner peripheral surface of a cage that has been subjected to surface modification treatment.

[0029] [Embodiment of First Invention] An embodiment of a sliding type constant velocity universal joint according to the first invention will be described in detail below with reference to Figs.

[0030] 7 shows an electric vehicle (EV) that runs solely on the power of an electric motor. This electric vehicle has a front wheel drive unit 61 that drives the front wheels 51 and a rear wheel drive unit 62 that drives the rear wheels 52. Each wheel drive unit 61, 62 has a drive unit 63 including an electric motor and a drive shaft 64 as a power transmission system that transmits the driving force of the electric motor to the front wheels 51 or rear wheels 52. In the illustrated example, the drive unit 63 of the front wheel drive unit 61 is connected to the left and right front wheels 51 via left and right drive shafts 64, and the drive unit 63 of the rear wheel drive unit 62 is connected to the left and right rear wheels 52 via left and right drive shafts 64.

[0031] Each drive shaft 64 has, for example, a sliding type constant velocity universal joint 1 provided on the inboard side (drive unit 63 side), a fixed type constant velocity universal joint 65 provided on the outboard side (wheels 51, 52 side), and an intermediate shaft 66 connecting both constant velocity universal joints 1, 65. A double offset constant velocity universal joint according to one embodiment of the present invention is applied as this sliding type constant velocity universal joint 1. The configuration of this sliding type constant velocity universal joint 1 will be described in detail below.

[0032] 1 and 2 , a sliding type constant velocity universal joint 1 of this embodiment includes a cup-shaped outer joint member 2 having one open axial end (the left end in FIG. 1 ), an inner joint member 3 disposed on the inner periphery of the outer joint member 2, a plurality of balls 4, and a cage 5 for holding the plurality of balls 4. An internal part 10 consisting of the inner joint member 3, the balls 4, and the cage 5 is housed on the inner periphery of the outer joint member 2 so as to be axially displaceable. An end of an intermediate shaft 66 (see FIG. 7 ) is coupled by spline fitting to an axial hole 11 of the inner joint member 3. In the following description, when the operating angle is 0° as shown in FIG. 1 , the axial direction of the outer joint member 2 and the inner joint member 3 is referred to as the "axial direction," the bottom side of the outer joint member 2 (right side in FIG. 1 ) in the axial direction is referred to as the "joint deep side," and the opening side of the outer joint member 2 (left side in FIG. 1 ) is referred to as the "joint opening side."

[0033] Axial-direction extending linear track grooves 7 are formed at equal intervals in a plurality of positions in the circumferential direction on the cylindrical inner peripheral surface 6 of the outer joint member 2. Axial-direction extending linear track grooves 9 are formed at equal intervals in a plurality of positions in the circumferential direction on the spherical outer peripheral surface 8 of the inner joint member 3. One ball 4 is disposed between each of the track grooves 7 of the outer joint member 2 and the track groove 9 of the inner joint member 3, which are opposed in the radial direction, to transmit rotational torque between the two joint members 2, 3.

[0034] The cage 5 is provided with a plurality of pockets 12, and each pocket 12 holds one ball 4. The outer peripheral surface of the cage 5 is formed with a spherical portion 13 that slides against the cylindrical inner peripheral surface 6 of the outer joint member 2, and tapered surfaces 14 that are provided on both axial sides of the spherical portion 13. The inner peripheral surface of the cage 5 is formed with a spherical portion 15 that slides against the spherical outer peripheral surface 8 of the inner joint member 3. In this embodiment, as shown in Fig. 2, the number of track grooves 7, 9, balls 4, and pockets 12 is eight, but the number of these may be six, for example.

[0035] The center of curvature O1 of the spherical portion 13 on the outer peripheral surface of the cage 5 and the center of curvature O2 of the spherical portion 15 on the inner peripheral surface (i.e., the center of curvature of the spherical outer peripheral surface 8 of the inner joint member 3) are offset by an equal distance F on opposite axial sides from the joint center O (the intersection of a plane passing through the centers of all the balls 4 and the axes of both joint members 2, 3) (see FIG. 1 ). In the illustrated example, the center of curvature O1 of the spherical portion 13 on the outer peripheral surface of the cage 5 is offset toward the back of the joint from the joint center O, and the center of curvature O2 of the spherical portion 15 on the inner peripheral surface of the cage 5 is offset toward the joint opening from the joint center O. As a result, when an operating angle is applied between the outer joint member 2 and the inner joint member 3, the balls 4 held in the pockets 12 of the cage 5 are always maintained within the plane bisecting the operating angle regardless of the operating angle, thereby ensuring uniform velocity between the outer joint member 2 and the inner joint member 3. Furthermore, the balls 4 held by the cage 5 roll on the track grooves 7 of the outer joint member 2, thereby allowing the internal part 10 to move axially relative to the outer joint member 2. Conversely to the above, the center of curvature O1 of the spherical portion 13 on the outer peripheral surface of the cage 5 may be located on the joint opening side, and the center of curvature O2 of the spherical portion 15 on the inner peripheral surface of the cage 5 may be located on the joint inner side.

[0036] The outer joint member 2 is manufactured through a forging process, a turning process, a rolling process, a heat treatment process, and a grinding process. In the forging process, an original outer joint member is formed by die-molding. In the turning process, the outer circumferential surface of the original outer joint member is turned. In the rolling process, male splines are formed on the shaft portion of the original outer joint member. In the heat treatment process, the original outer joint member is subjected to heat treatment (e.g., induction hardening and tempering). In the grinding process, the outer circumferential surface of the original outer joint member is ground. When manufactured according to these procedures, the cylindrical inner circumferential surface 6 and the track grooves 7 of the outer joint member 2 become forged surfaces.

[0037] The inner joint member 3 is manufactured through a forging process, a turning process, a broaching process, a heat treatment process, and a grinding process. In the forging process, a substantially cylindrical original form of the inner joint member is molded. In the turning process, the inner and outer circumferential surfaces of the original form of the inner joint member are turned. In the broaching process, a female spline is formed on the inner periphery of the original form of the inner joint member. In the heat treatment process, the original form of the inner joint member is heat treated (for example, carburized, quenched, and tempered). In the grinding process, the spherical outer circumferential surface of the original form of the inner joint member (regions excluding the track grooves) is ground. When manufactured through these procedures, the spherical outer circumferential surface 8 of the inner joint member 3 becomes a ground surface, the track grooves 9 become a forged surface, and the inner circumferential surface (female spline) becomes a machined surface.

[0038] The cage 5 is manufactured through a forging process, a turning process, a pocket punching process, a heat treatment process, and a grinding process. In the forging process, a substantially cylindrical cage original form is formed by die-molding. In the turning process, the inner and outer peripheral surfaces of the cage original form are turned. In the pocket punching process, the cage original form is punched in the radial direction to form the pockets 12. In the heat treatment process, the cage original form is subjected to heat treatment (e.g., carburizing, quenching, and tempering). In the grinding process, the spherical portion 13 on the outer peripheral surface, the spherical portion 15 on the inner peripheral surface, and the inner surfaces of the pockets 12 of the cage original form are ground. When manufactured using this procedure, the spherical portion 13 on the outer peripheral surface, the spherical portion 15 on the inner peripheral surface, and the inner surfaces of the pockets 12 of the cage 5 become ground surfaces, and the other areas become pocket punched surfaces or turned surfaces.

[0039] A coating that reduces the surface roughness compared to the surface of the base material (steel material) is formed on at least one of the cylindrical inner peripheral surface 6 of the outer joint member 2, the surfaces of the track grooves 7 of the outer joint member 2, the spherical outer peripheral surface 8 of the inner joint member 3, the surfaces of the track grooves 9 of the inner joint member 3, the spherical portion 13 of the outer peripheral surface of the cage 5, and the spherical portion 15 of the inner peripheral surface of the cage 5, which are produced by the above-described procedure. As such a coating treatment, a phosphate treatment such as manganese phosphate treatment can be performed. In this embodiment, manganese phosphate treatment is performed. Manganese phosphate treatment is a chemical conversion treatment that uses a treatment solution mainly composed of phosphate ions and manganese ions to form a manganese phosphate-based crystalline coating on the surface of a steel product. It is also called a "Parker treatment" or "Leubrite treatment." Manganese phosphate treatment is usually performed by immersing the components in the treatment solution. As a result, a manganese phosphate coating is formed on the entire surface of at least one of the outer joint member 2, the inner joint member 3, and the cage 5. Any of the outer joint member 2, the inner joint member 3, and the cage 5 may be subjected to the manganese phosphate treatment. However, whereas the inner joint member 3 and the cage 5 can be coated by immersing them in a treatment solution in large quantities at once, the outer joint member 2 needs to be coated only in the required area to avoid a situation in which the treatment solution comes into contact with the shaft portion that engages with the differential gear, etc., causing dimensional changes. Therefore, since the outer joint member 2 cannot be coated in large quantities at once, which requires time and cost, it is desirable to perform the manganese phosphate treatment on one or both of the inner joint member 3 and the cage 5, but not on the outer joint member 2.

[0040] Since manganese phosphate treatment is a chemical conversion treatment, a corrosion reaction occurs in the steel substrate in the initial stage of the treatment. Specifically, the anodic reaction (dissolution reaction of the material (Fe)) shown below is triggered by the cathodic reaction (2) shown below, which subsequently increases the pH at the material interface, and the film formation reaction (3) shown below proceeds. (1) Fe → Fe 2+ +2e (2) 2H + +2e→H2 (3)5Mn 2+ +10H2PO4 - →Mn5H2(PO4)4・4H2O+6H3PO4

[0041] This series of reactions begins with the dissolution of the raw material Fe in (1) above, but on a surface with roughness as shown in Figure 3, this reaction begins in the parts of the material with a microscopically large surface area (i.e., large protrusions). The raw material Fe that reacted in (1) above becomes ions and is released into the liquid, so the large protrusions on the surface roughness of the material inevitably dissolve and disappear (see Figure 4). Therefore, by performing manganese phosphate treatment, the rough surface disappears from the protrusions, and as a result, the uneven surface roughness is equalized.

[0042] Furthermore, the manganese phosphate coating easily smooths any remaining roughness variations when high surface pressure is applied, further reducing the variation in roughness between phases. That is, the manganese phosphate coating formed by manganese phosphate treatment exhibits a Mohs hardness of 5 and does not exhibit a "layered structure" or "cleavage," so it does not function as a solid lubricant. Furthermore, the manganese phosphate coating 20 has a crystalline structure and is a coating with microscopic irregularities (see FIG. 5). The protrusions of this manganese phosphate coating 20 easily crumble when high surface pressure is applied, thereby smoothing the surface of the component (see FIG. 6). In other words, as the manganese phosphate coating 20 is fractured or worn away by sliding against the mating material, the surface of the component enters a conformal state, with some of the manganese phosphate coating remaining and the surface of the steel material exposed.

[0043] The thickness of the manganese phosphate coating is not particularly limited, but is preferably 0.5 to 15 μm. If the thickness of the manganese phosphate coating exceeds 15 μm, the internal clearance required for a constant velocity universal joint will deviate, and the functionality of a constant velocity universal joint, including operability, may not be achieved. Furthermore, if the thickness of the manganese phosphate coating is less than 0.5 μm, the effects of uniforming the surface roughness and improving conformability may not be fully achieved.

[0044] The roughness (Ra) of the surface on which the manganese phosphate coating is formed is smaller than the roughness (Ra) of the surface of the base material covered with the coating. Specifically, the surface on which the manganese phosphate coating is formed satisfies at least one of the following: Rsk is a negative value, and Rp is 2 or less. In addition to the above conditions, it is desirable to satisfy at least one of Ra being 1.5 or less, preferably 0.6 or less, and Rz being 10 or less, preferably 6 or less. The surface on which the manganese phosphate coating of this embodiment is formed satisfies all of the above conditions. It is not necessary to set lower limits for the above surface roughness parameters, but, for example, the lower limit for Rsk is −3, the lower limit for Rp is 0.5, the lower limit for Ra is 0.2, and the lower limit for Rz is 1.0.

[0045] Note that Rsk, Rp, Ra, and Rz are the skewness, maximum peak height, arithmetic mean height, and maximum height of the roughness curve over the reference length as specified in JIS B 0601-2013, respectively. Rsk is positive when the amplitude distribution curve has many peaks relative to the mean line of the roughness curve, and negative when it has many valleys. Measurement of these parameters is performed at the contact point between each part, over a range of 4 mm in total, consisting of five sections of 0.8 mm reference length along the axial direction.

[0046] When the sliding-type constant velocity universal joint 1 has an operating angle, a rotational driving force from the electric motor is input to the outer joint member 2, and torque is transmitted to the inner joint member 3 via the cage 5 and the balls 4. At this time, axial sliding resistance occurs at the contact portions between the components, causing an axial load (induced thrust) to be generated in the outer joint member 2. However, due to variations in the dimensions and surface properties of the components, the axial load is not uniform in each phase and varies. In this embodiment, as described above, by forming a manganese phosphate coating on at least one of the outer joint member 2, the inner joint member 3, and the cage 5, the surface roughness of the component can be reduced and the variation in surface properties among the phases can be suppressed. This uniformizes the axial load generated at the contact portions of the phases, thereby reducing each order component of the induced thrust.

[0047] The present invention is not limited to the above-described embodiment. Other embodiments of the present invention will be described below, but redundant description of the same points as those in the above-described embodiment will be omitted.

[0048] In the above-described sliding type constant velocity universal joint 1, the axial dimension of the pocket 12 of the cage 5 may be made larger than the diameter of the ball 4, thereby providing an axial gap between the wall surface of the pocket 12 and the ball 4. This axial gap can be set, for example, within the range of 0.001 to 0.05 mm. Alternatively, the axial dimension of the pocket 12 of the cage 5 may be made smaller than the diameter of the ball 4, thereby bringing the wall surface of the pocket 12 into contact with the ball 4 via a negative gap (interference).

[0049] Furthermore, an axial gap may be provided between the spherical portion 15 on the inner peripheral surface of the cage 5 and the spherical outer peripheral surface 8 of the inner joint member 3. In this case, the cage 5 and the inner joint member 3 are able to move axially relative to each other, and the maximum amount of relative axial movement between them at this time is the axial gap. This axial gap can be set, for example, within a range of 0.6 to 1.5 mm. Alternatively, the axial gap between the cage 5 and the inner joint member 3 may be set to substantially zero. For example, the centers of curvature of the spherical portion 15 on the inner peripheral surface of the cage 5 and the spherical outer peripheral surface 8 of the inner joint member 3 may be aligned, and their radii of curvature may be made approximately the same. In this case, a slight radial gap is formed between the spherical portion 15 on the inner peripheral surface of the cage 5 and the spherical outer peripheral surface 8 of the inner joint member 3 to allow relative movement therebetween, but the amount of relative axial movement between them is substantially zero.

[0050] Although the track grooves 7 and the cylindrical inner peripheral surface 6 of the outer joint member 2 and the track grooves 9 of the inner joint member 3 are generally forged to reduce manufacturing costs, they may be finished by cutting or grinding after heat treatment (quenching). Furthermore, the outer peripheral surface and the inner peripheral surface of the cage 5 are generally ground to reduce manufacturing costs, but they may be finished by cutting or rolling after heat treatment (quenching).

[0051] Furthermore, the double offset sliding type constant velocity universal joint of the present invention is not limited to electric vehicles (EVs) that run solely on the driving force of an electric motor, but can also be applied to hybrid vehicles (HEVs) that run on the power of an electric motor and an engine, and to the power transmission systems of vehicles that run solely on the power of the engine.

[0052] Only the inner surface of the outer joint member 2 of the above-mentioned constant velocity universal joint 1 was subjected to manganese phosphate treatment, and the surface roughness and induced thrust were measured. Figures 8 and 9 show the roughness curves, load curves, and amplitude distribution curves of the surfaces of two of the eight track grooves 7 formed in the outer joint member 2 before the manganese phosphate treatment. The surfaces of these track grooves 7 are surfaces formed by forging. Figures 10 and 11 show the roughness curves, load curves, and amplitude distribution curves of the surfaces of the two track grooves 7 of the outer joint member 2 after the manganese phosphate treatment. As can be seen from these figures, the surface roughness of the track grooves 7 after the manganese phosphate treatment is reduced, and in particular, the roughness convexities are reduced (Rsk is a negative value).

[0053] Figure 12 shows the measurement results of induced thrust. The measurement conditions were the same as those of the test in Figure 25. Specifically, the axial load (induced thrust) applied to the outer joint member was measured when the working angle was changed from 0 deg to 12 deg at a rate of 10.7 deg / min while a torque of 900 Nm was input at a rotation speed of 150 rpm. As can be seen from this figure, it can be confirmed that Example 1, in which the outer joint member 2 was subjected to manganese phosphate treatment, had a reduced induced thrust, particularly at a high working angle, compared to the comparative example, in which the outer joint member 2 was not subjected to manganese phosphate treatment.

[0054] Only the surface of the inner joint member 3 of the above-described constant velocity universal joint 1 was subjected to manganese phosphate treatment, and the surface roughness and induced thrust were measured. Figures 13 and 14 show the roughness curves, load curves, and amplitude distribution curves of the surfaces of two of the eight track grooves 9 formed in the inner joint member 3 before the manganese phosphate treatment. The surfaces of these track grooves 9 are surfaces formed by forging. Figures 15 and 16 show the roughness curves, load curves, and amplitude distribution curves of the surfaces of the above-described two track grooves 9 of the inner joint member 3 after the manganese phosphate treatment. As can be seen from these figures, the surface roughness of the track grooves 9 after the manganese phosphate treatment is reduced, and in particular, the roughness convex portions are smaller (Rsk is a negative value).

[0055] Figure 17 shows the measurement results of the induced thrust. The measurement conditions were the same as those of the test in Figure 25. As can be seen from this figure, it can be confirmed that Example 2, in which the inner joint member 3 was subjected to manganese phosphate treatment, had a reduced induced thrust, particularly at a high working angle, compared to the Comparative Example, in which the inner joint member 3 was not subjected to manganese phosphate treatment.

[0056] Only the surface of the cage 5 of the above-mentioned constant velocity universal joint 1 was subjected to manganese phosphate treatment, and the surface roughness and induced thrust were measured. Figures 18 and 19 show the surface roughness curves, load curves, and amplitude distribution curves of two locations (two bar portions) with different phases on the outer peripheral surface of the cage 5 before the manganese phosphate treatment. The outer peripheral surface of the cage 5 is a surface that has been finished by polishing. Figures 20 and 21 show the surface roughness curves, load curves, and amplitude distribution curves of the above-mentioned two locations on the surface of the cage 5 after the manganese phosphate treatment. As can be seen from these figures, the surface roughness of the cage 5 after the manganese phosphate treatment is reduced, and in particular, the roughness peaks are smaller.

[0057] Figure 22 shows the measurement results of induced thrust. The measurement conditions were the same as those of the test in Figure 25. As can be seen from this figure, it can be confirmed that Example 3, in which the cage 5 was subjected to manganese phosphate treatment, had reduced induced thrust, particularly at high operating angles, compared to the comparative example, in which the cage 5 was not subjected to manganese phosphate treatment.

[0058] [Embodiment of the Second Invention] An embodiment of a sliding type constant velocity universal joint according to the second invention will be described in detail below with reference to Figs.

[0059] 1 and 2, the overall configuration of a double offset constant velocity universal joint 1 (hereinafter simply referred to as "constant velocity universal joint 1"), which is a type of sliding type constant velocity universal joint, is shown. This constant velocity universal joint 1 is mainly mounted on automobiles, and specifically, is incorporated into a power transmission shaft (drive shaft or propeller shaft) that transmits power to wheels. In particular, the constant velocity universal joint 1 of this embodiment is manufactured as a power transmission shaft suitable for electric automobiles.

[0060] The constant velocity universal joint 1 of this embodiment includes a cup-shaped outer joint member 2 having one open axial end, an inner joint member 3 disposed on the inner periphery of the outer joint member 2, a plurality of balls 4, and a cage 5 for holding the plurality of balls 4. An internal part 10 consisting of the inner joint member 3, the balls 4, and the cage 5 is housed on the inner periphery of the outer joint member 2 so as to be axially displaceable. An end of a shaft (not shown) is coupled by spline fitting to an axial hole 11 of the inner joint member 3. In the following description, the axial direction of the outer joint member 2 and the inner joint member 3 in the state of an operating angle of 0° shown in Fig. 1 will be referred to as the "axial direction," the bottom side of the outer joint member 2 (right side in Fig. 1) in the axial direction will be referred to as the "joint deep side," and the opening side of the outer joint member 2 (left side in Fig. 1) will be referred to as the "joint opening side."

[0061] Axial-direction linear track grooves 7 are formed at a plurality of positions in the circumferential direction at equal intervals on the cylindrical inner peripheral surface 6 of the outer joint member 2. Axial-direction linear track grooves 9 are formed at a plurality of positions in the circumferential direction at equal intervals on the spherical outer peripheral surface 8 of the inner joint member 3. One ball 4 is disposed between each of the track grooves 7 of the outer joint member 2 and the track groove 9 of the inner joint member 3, which are opposed in the radial direction, to transmit rotational torque between the two joint members 2, 3. In the illustrated example, eight track grooves 7, 9 are formed, and eight balls 4 are provided. The numbers of track grooves 7, 9 and balls 4 are not limited to those described above and may be, for example, five, six, or seven.

[0062] The cage 5 is provided with a plurality of pockets 12, and each pocket 12 holds one ball 4. The outer peripheral surface of the cage 5 is formed with a spherical portion 13 that slides against the cylindrical inner peripheral surface 6 of the outer joint member 2, and tapered surfaces 14 that are provided on both axial sides of the spherical portion 13. The inner peripheral surface of the cage 5 is formed with a spherical portion 15 that slides against the spherical outer peripheral surface 8 of the inner joint member 3.

[0063] The center of curvature O1 of the spherical portion 13 on the outer peripheral surface of the cage 5 and the center of curvature O2 of the spherical portion 15 on the inner peripheral surface (i.e., the center of curvature of the spherical outer peripheral surface 8 of the inner joint member 3) are offset by an equal distance F on opposite axial sides with respect to the joint center O (the intersection of a plane passing through the centers of all of the balls 4 and the axis of the joint). In the illustrated example, the center of curvature O1 of the spherical portion 13 on the outer peripheral surface of the cage 5 is offset toward the back of the joint with respect to the joint center O, and the center of curvature O2 of the spherical portion 15 on the inner peripheral surface of the cage 5 is offset toward the joint opening with respect to the joint center O. As a result, when an operating angle is applied between the outer joint member 2 and the inner joint member 3, the balls 4 held in the pockets 12 of the cage 5 are always maintained within the plane bisecting the operating angle regardless of the operating angle, thereby ensuring uniform velocity between the outer joint member 2 and the inner joint member 3. Furthermore, the balls 4 held by the cage 5 roll on the track grooves 7 of the outer joint member 2, thereby allowing the internal part 10 to slide axially relative to the outer joint member 2. Conversely to the above, the center of curvature O1 of the spherical portion 13 on the outer peripheral surface of the cage 5 may be located on the joint opening side, and the center of curvature O2 of the spherical portion 15 on the inner peripheral surface of the cage 5 may be located on the joint inner side.

[0064] Here, a method for manufacturing each of the components (the outer joint member 2, the inner joint member 3, and the cage 5) of the constant velocity universal joint 1 will be described.

[0065] The outer joint member 2 is manufactured through a forging process, a turning process, a rolling process, a heat treatment process, and a grinding process. In the forging process, an original outer joint member is formed by die-molding. In the turning process, the outer circumferential surface of the original outer joint member is turned. In the rolling process, male splines are formed on the shaft portion of the original outer joint member. In the heat treatment process, the original outer joint member is subjected to heat treatment (e.g., induction hardening and tempering). In the grinding process, the outer circumferential surface of the original outer joint member is ground. As a result of manufacturing using these procedures, the cylindrical inner circumferential surface 6 and the track grooves 7 of the outer joint member 2 become forged surfaces.

[0066] The inner joint member 3 is manufactured through a forging process, a turning process, a broaching process, a heat treatment process, and a grinding process. In the forging process, a substantially cylindrical original form of the inner joint member is molded. In the turning process, the inner and outer circumferential surfaces of the original form of the inner joint member are turned. In the broaching process, a female spline is formed on the inner periphery of the original form of the inner joint member. In the heat treatment process, the original form of the inner joint member is heat treated (for example, carburized, quenched, and tempered). In the grinding process, the spherical outer circumferential surface of the original form of the inner joint member (regions excluding the track grooves) is ground. As a result of manufacturing through these procedures, the spherical outer circumferential surface 8 of the inner joint member 3 becomes a ground surface, the track grooves 9 become a forged surface, and the inner circumferential surface (female spline) becomes a machined surface.

[0067] The cage 5 is manufactured through a forging process, a turning process, a pocket punching process, a heat treatment process, and a grinding process. In the forging process, a substantially cylindrical cage original form is formed by die-molding. In the turning process, the inner and outer peripheral surfaces of the cage original form are turned. In the pocket punching process, the cage original form is punched in the radial direction to form pockets 12. In the heat treatment process, the cage original form is subjected to heat treatment (e.g., carburizing, quenching, and tempering). In the grinding process, the spherical portion 13 on the outer peripheral surface, the spherical portion 15 on the inner peripheral surface, and the inner surfaces of the pockets 12 of the cage original form are ground. As a result of manufacturing using these procedures, the spherical portion 13 on the outer peripheral surface, the spherical portion 15 on the inner peripheral surface, and the inner surfaces of the pockets 12 of the cage 5 become ground surfaces, and the other areas become pocket punched surfaces or turned surfaces.

[0068] FIG. 28 shows a roughness curve of the surface of the track groove 7 of the outer joint member 2, which is a forged surface. FIG. 29 shows a load curve (BAC curve) of the same surface. FIG. 30 shows an amplitude distribution curve (ADC curve) of the same surface. The roughness curves are measured along the axial direction. Since the surface of the track groove 9 of the inner joint member 3 is a forged surface, it has generally similar properties to the surface of the track groove 7 of the outer joint member 2, which is also a forged surface, as shown in FIGS. 28 to 30 . As shown in FIGS. 28 to 30 , the surfaces of the track grooves 7 and 9 have slightly more peaks (portions above the mean line P of the roughness curve (see FIG. 28 )) than valleys (portions below the mean line P of the roughness curve (see FIG. 28 )). Specifically, Rsk of the surfaces of the track grooves 7 and 9 is a positive value, 0.56 in the roughness curve of FIG. 28 . Rp of the surfaces of the track grooves 7 and 9 is greater than 2, 4.77 in the roughness curve of FIG. 28 . The Ra of the surfaces of the track grooves 7 and 9 is greater than 0.6, which is 0.90 in the roughness curve of Fig. 28. The Rz of the surfaces of the track grooves 7 and 9 is greater than 6, which is 8.8 in the roughness curve of Fig. 28.

[0069] Note that Rsk, Rp, Ra, and Rz are the skewness, maximum peak height, arithmetic mean height, and maximum height of the roughness curve over the reference length as specified in JIS B 0601-2013, respectively. Rsk is positive when the amplitude distribution curve has many peaks relative to the mean line of the roughness curve, and negative when it has many valleys. Measurement of these parameters is performed at the contact point between each part, over a range of 4 mm in total, consisting of five sections of 0.8 mm reference length along the axial direction.

[0070] In this embodiment, the surfaces of the track grooves 7 of the outer joint member 2 and the surfaces of the track grooves 9 of the inner joint member 3, which are forged surfaces, are subjected to a surface modification treatment in which the crests are crushed or removed while the bottoms of the valleys remain. Examples of the surface modification treatment include mechanical processing such as honing and polishing. Since the surfaces of the track grooves 7 of the outer joint member 2 and the surfaces of the track grooves 9 of the inner joint member 3 in this embodiment are forged surfaces, the bottoms of the valleys on the surfaces after the surface modification treatment remain as forged surfaces, and the bottoms of all the valleys that appear on the roughness curve of the reference length are forged surfaces. On the other hand, because the crests of the peaks are crushed or removed by the surface modification treatment, the surfaces of the track grooves 7, 9 after the surface modification treatment have fewer peaks and more valleys.

[0071] Fig. 31 shows a surface roughness curve of the track grooves 7 of the outer joint member 2 after surface modification treatment, Fig. 32 shows a load curve (BAC curve) of the same surface, and Fig. 33 shows an amplitude distribution curve (ADC curve) of the same surface. Fig. 34 shows a surface roughness curve of the track grooves 9 of the inner joint member 3 after surface modification treatment, Fig. 35 shows a load curve (BAC curve) of the same surface, and Fig. 36 shows an amplitude distribution curve (ADC curve) of the same surface.

[0072] The surface modification process reduces the surface roughness parameters of the track grooves 7 and 9, such as Rsk, Rp, Ra, and Rz, by crushing or removing the peaks. Specifically, the Rsk of the track grooves 7 and 9 after the surface modification process is a negative value, −0.94 in the roughness curve of FIG. 31 and −1.65 in the roughness curve of FIG. 34. The lower limit of Rsk of the track grooves 7 and 9 is, for example, −3. The Rp of the track grooves 7 and 9 after the surface modification process is 2 or less, 0.90 in the roughness curve of FIG. 31 and 1.01 in the roughness curve of FIG. 34. The lower limit of Rp of the track grooves 7 and 9 after the surface modification process is 0.5, for example. The Ra of the track grooves 7 and 9 after the surface modification process is 1.5 or less, preferably 0.6 or less, 0.42 in the roughness curve of FIG. 31 and 0.45 in the roughness curve of FIG. 34. The lower limit of Ra of the surface is, for example, 0.2. The Rz of the surface of the track grooves 7, 9 after the surface modification process is 10 or less, preferably 6 or less, and is 3.0 in the roughness curve of Fig. 31 and 4.14 in the roughness curve of Fig. 34. The lower limit of Rz of the surface is, for example, 1.0.

[0073] The upper diagram of Fig. 37 shows the roughness curve of the surface of the track grooves 7, 9 after the surface modification treatment, and the lower diagram is a plan view of the region corresponding to this roughness curve. On the surface of the track grooves 7, 9 after the surface modification treatment, the peripheries of many minute peaks 21 (portions above the average line P of the roughness curve in the upper diagram), shown by hatching in the lower diagram of Fig. 37, are surrounded by valleys 22 (portions below the average line P of the roughness curve). In other words, while the many peaks 21 are formed independently, the valleys 22 are connected around the entire periphery of each peak 21.

[0074] By applying the above-described surface modification treatment to the surfaces of the track grooves 7 and 9 that slide against the balls 4, the frictional force caused by contact with the balls 4 is reduced, thereby making it possible to suppress induced thrust.

[0075] The surface of the ball 4 is ultra-finished, resulting in extremely low surface roughness (for example, Ra of approximately 0.01 to 0.1). Even if the above-described surface modification treatment were applied to such a surface, the surface characteristics would hardly change, and the effect of reducing induced thrust would not be achieved. Therefore, in this embodiment, the above-described surface modification treatment is not applied to the surface of the ball 4.

[0076] 38 shows the measurement results of induced thrust when the track grooves 7, 9 were subjected to surface modification treatment (Example) and when they were not (Comparative Example). As a result, no significant difference was observed when the working angle was small (3° or less), but when the working angle was large (especially 6° or more), the induced thrust of the Example with surface modification treatment was clearly smaller than the induced thrust of the Comparative Example without surface modification treatment, and was well below the vehicle vibration NG level. These results confirmed that the surface modification treatment can reduce induced thrust.

[0077] The present invention is not limited to the above-described embodiment. Other embodiments of the present invention will be described below, but redundant description of the same points as those in the above-described embodiment will be omitted.

[0078] The portions of the constant velocity universal joint 1 to which the surface modification treatment is applied are not limited to those described above, and the surface modification treatment may be applied to other sliding portions. For example, the surface modification treatment may be applied to at least one of the cylindrical inner peripheral surface 6 of the outer joint member 2, the spherical outer peripheral surface 8 of the inner joint member 3, and the spherical portion 13 on the outer peripheral surface and the spherical portion 15 on the inner peripheral surface of the cage 5. Since the cylindrical inner peripheral surface 6 of the outer joint member 2 is generally a forged surface, when the surface modification treatment is applied to this surface, the bottoms of the valleys are left as the forged surface, while the tops of the crests are crushed or removed, resulting in a surface having properties similar to those of the surfaces of the track grooves 7, 9 in the above embodiment.

[0079] Specifically, Fig. 39 shows the surface roughness curve of the cylindrical inner peripheral surface 6 of the outer joint member 2 after surface modification treatment, Fig. 40 shows the load curve (BAC curve) of the same surface, and Fig. 41 shows the amplitude distribution curve (ADC curve) of the same surface. The ranges of Rsk, Rp, Ra, and Rz of this surface are the same as the ranges of each parameter of the surface of the track grooves 7 and 9 after surface modification treatment in the above embodiment. In the roughness curve of Fig. 39, Rsk is -0.84, Rp is 1.50, Ra is 0.66, and Rz is 4.49.

[0080] On the other hand, the spherical outer peripheral surface 8 of the inner joint member 3, the spherical portion 13 on the outer peripheral surface of the cage 5, and the spherical portion 15 on the inner peripheral surface are generally ground surfaces, and a large number of streak-like peaks and valleys extending in the same direction are formed on these surfaces. By subjecting these ground surfaces to a surface modification treatment, the peaks of the peaks are crushed or removed while the bottoms of the valleys remain as the ground surface. In this embodiment, the bottoms of all the valleys that appear on the roughness curve of the reference length are the ground surface. The ground surface after the surface modification treatment satisfies at least one of the conditions that Rsk is a negative value and Rp is 2 or less.

[0081] Specifically, Fig. 42 shows a roughness curve of the spherical outer peripheral surface 8 of the inner joint member 3 after surface modification treatment, Fig. 43 shows a load curve (BAC curve) of the same surface, and Fig. 44 shows an amplitude distribution curve (ADC curve) of the same surface. Fig. 45 shows a roughness curve of the spherical portion 13 of the outer peripheral surface of the cage 5 after surface modification treatment, Fig. 46 shows a load curve (BAC curve) of the same surface, and Fig. 47 shows an amplitude distribution curve (ADC curve) of the same surface. Fig. 48 shows a roughness curve of the spherical portion 15 of the inner peripheral surface of the cage 5 after surface modification treatment, Fig. 49 shows a load curve (BAC curve) of the same surface, and Fig. 50 shows an amplitude distribution curve (ADC curve) of the same surface. The ranges of Rsk, Rp, Ra, and Rz of these surfaces are the same as the ranges of each parameter of the surfaces of the track grooves 7 and 9 after surface modification treatment in the above embodiment. In the roughness curve of Fig. 42, Rsk is -0.70, Rp is 1.24, Ra is 0.56, and Rz is 3.72. In the roughness curve of Fig. 45, Rsk is -1.19, Rp is 0.88, Ra is 0.45, and Rz is 3.15. In the roughness curve of Fig. 48, Rsk is -0.26, Rp is 1.30, Ra is 0.51, and Rz is 3.34.

[0082] The processing method of the sliding portion of the constant velocity universal joint 1 is not limited to the above. For example, the track grooves 7 and the cylindrical inner peripheral surface 6 of the outer joint member 2 and the track grooves 9 of the inner joint member 3 are generally forged to reduce manufacturing costs, but they may also be finished by cutting or grinding after heat treatment (quenching). The outer and inner peripheral surfaces of the cage 5 are generally ground to reduce manufacturing costs, but they may also be finished by cutting or rolling after heat treatment (quenching). Furthermore, the sliding portion of the constant velocity universal joint 1 (excluding the surface of the ball) may also be finished by polishing. The above-described machined surfaces, such as the cut, ground, and polished surfaces, and rolled surfaces may be subjected to the above-described surface modification treatment to satisfy at least one of the conditions that Rsk is a negative value and Rp is 2 or less. In this case, the bottoms of all valleys appearing in the roughness curve of the reference length can be machined or rolled surfaces.

[0083] Furthermore, at least one of the conditions that Rsk is a negative value and Rp is 2 or less may be satisfied without applying the above-mentioned surface modification treatment to any of the forged surfaces, rolled surfaces, or machined surfaces provided on the constant velocity universal joint 1.

[0084] The constant velocity universal joint according to the present invention is not limited to use in power transmission shafts for automobiles, but may also be used in power transmission shafts for industrial machinery, etc.

[0085] DESCRIPTION OF SYMBOLS 1 Sliding type constant velocity universal joint (double offset type constant velocity universal joint) 2 Outer joint member 3 Inner joint member 4 Ball 5 Cage 6 Cylindrical inner peripheral surface 7 Track groove 8 Spherical outer peripheral surface 9 Track groove 10 Internal part 12 Pocket 13 Spherical portion 14 Tapered surface 15 Spherical portion 20 Manganese phosphate coating 51 Front wheel 52 Rear wheel 61, 62 Wheel drive device 63 Drive unit 64 Drive shaft (power transmission system) 65 Fixed type constant velocity universal joint 66 Intermediate shaft O Joint center O1 Center of curvature of spherical portion of outer peripheral surface of cage O2 Center of curvature of spherical portion of inner peripheral surface of cage Fx Induced thrust Qx Axial resistance between outer joint member and ball Hx Axial resistance between outer joint member and cage

Claims

1. A sliding type constant velocity universal joint comprising an outer joint member having a plurality of straight track grooves formed on a cylindrical inner peripheral surface thereof, an inner joint member having a plurality of straight track grooves formed on a spherical outer peripheral surface thereof, a plurality of balls arranged between the track grooves of the outer joint member and the track grooves of the inner joint member, an outer peripheral surface formed with a spherical portion which comes into sliding contact with the cylindrical inner peripheral surface of the outer joint member, an inner peripheral surface formed with a spherical portion which comes into sliding contact with the spherical outer peripheral surface of the inner joint member, and a cage having a plurality of pockets for holding the plurality of balls, wherein a coating is formed on at least one of the cylindrical inner peripheral surface of the outer joint member, the surface of the track grooves of the outer joint member, the spherical outer peripheral surface of the inner joint member, the surface of the track grooves of the inner joint member, the spherical portion of the outer peripheral surface of the cage, and the spherical portion of the inner peripheral surface of the cage, the sliding type constant velocity universal joint.

2. A sliding type constant velocity universal joint according to claim 1, wherein the coating is a manganese phosphate coating.

3. A sliding type constant velocity universal joint according to claim 1, wherein the roughness parameter Rsk of the surface on which the coating is formed is a negative value.

4. A sliding type constant velocity universal joint according to claim 1, wherein the roughness parameter Rp of the surface on which the coating is formed is 2 or less.

5. A sliding type constant velocity universal joint according to claim 1, wherein the surface of the base material on which the coating is formed is a forged surface.

6. The sliding type constant velocity universal joint according to claim 1, which is provided in a power transmission system of a vehicle using an electric motor as a drive source.

7. A wheel drive device comprising the electric motor, a wheel, and the power transmission system that transmits the driving force of the electric motor to the wheel via the sliding type constant velocity universal joint according to claim 1.

8. A sliding type constant velocity universal joint comprising an outer joint member having a plurality of linear track grooves formed on a cylindrical inner peripheral surface thereof, an inner joint member having a plurality of linear track grooves formed on a spherical outer peripheral surface thereof, a plurality of balls arranged between the track grooves of the outer joint member and the track grooves of the inner joint member, an outer peripheral surface formed with a spherical portion which comes into sliding contact with the cylindrical inner peripheral surface of the outer joint member, an inner peripheral surface formed with a spherical portion which comes into sliding contact with the spherical outer peripheral surface of the inner joint member, and a cage having a plurality of pockets for holding the plurality of balls, wherein the bottom of a valley of a roughness curve on at least one of the cylindrical inner peripheral surface of the outer joint member, the surface of the track grooves of the outer joint member, the spherical outer peripheral surface of the inner joint member, the surface of the track grooves of the inner joint member, the spherical portion of the outer peripheral surface of the cage, and the spherical portion of the inner peripheral surface of the cage is any one of a forged surface, a rolled surface, and a machined surface; A sliding type constant velocity universal joint, wherein Rsk of the at least one surface is a negative value.

9. A sliding type constant velocity universal joint comprising an outer joint member having a plurality of linear track grooves formed on a cylindrical inner peripheral surface thereof, an inner joint member having a plurality of linear track grooves formed on a spherical outer peripheral surface thereof, a plurality of balls arranged between the track grooves of the outer joint member and the track grooves of the inner joint member, an outer peripheral surface formed with a spherical portion which comes into sliding contact with the cylindrical inner peripheral surface of the outer joint member, an inner peripheral surface formed with a spherical portion which comes into sliding contact with the spherical outer peripheral surface of the inner joint member, and a cage having a plurality of pockets for holding the plurality of balls, wherein the bottom of a valley of a roughness curve on at least one of the cylindrical inner peripheral surface of the outer joint member, the surface of the track grooves of the outer joint member, the spherical outer peripheral surface of the inner joint member, the surface of the track grooves of the inner joint member, the spherical portion of the outer peripheral surface of the cage, and the spherical portion of the inner peripheral surface of the cage is any one of a forged surface, a rolled surface, and a machined surface; The sliding type constant velocity universal joint has Rp of 2 or less on said at least one surface.

10. A sliding type constant velocity universal joint comprising an outer joint member having a plurality of linear track grooves formed on a cylindrical inner peripheral surface thereof, an inner joint member having a plurality of linear track grooves formed on a spherical outer peripheral surface thereof, a plurality of balls arranged between the track grooves of the outer joint member and the track grooves of the inner joint member, an outer peripheral surface formed with a spherical portion which comes into sliding contact with the cylindrical inner peripheral surface of the outer joint member, an inner peripheral surface formed with a spherical portion which comes into sliding contact with the spherical outer peripheral surface of the inner joint member, and a cage having a plurality of pockets for holding the plurality of balls, wherein at least one of the cylindrical inner peripheral surface of the outer joint member, the surface of the track grooves of the outer joint member, the spherical outer peripheral surface of the inner joint member, the surface of the track grooves of the inner joint member, the spherical portion of the outer peripheral surface of the cage, and the spherical portion of the inner peripheral surface of the cage has a large number of minute ridges and valleys provided so as to be connected around each ridge all around, A sliding type constant velocity universal joint, wherein Rsk of the at least one surface is a negative value.

11. A sliding type constant velocity universal joint comprising an outer joint member having a plurality of linear track grooves formed on a cylindrical inner peripheral surface thereof, an inner joint member having a plurality of linear track grooves formed on a spherical outer peripheral surface thereof, a plurality of balls arranged between the track grooves of the outer joint member and the track grooves of the inner joint member, an outer peripheral surface formed with a spherical portion which comes into sliding contact with the cylindrical inner peripheral surface of the outer joint member, an inner peripheral surface formed with a spherical portion which comes into sliding contact with the spherical outer peripheral surface of the inner joint member, and a cage having a plurality of pockets for holding the plurality of balls, wherein at least one of the cylindrical inner peripheral surface of the outer joint member, the surface of the track grooves of the outer joint member, the spherical outer peripheral surface of the inner joint member, the surface of the track grooves of the inner joint member, the spherical portion of the outer peripheral surface of the cage, and the spherical portion of the inner peripheral surface of the cage has a large number of minute ridges and valleys provided so as to be connected around each ridge all around, The sliding type constant velocity universal joint has Rp of 2 or less on said at least one surface.

12. A sliding type constant velocity universal joint comprising an outer joint member having a plurality of straight track grooves formed on a cylindrical inner peripheral surface thereof, an inner joint member having a plurality of straight track grooves formed on a spherical outer peripheral surface thereof, a plurality of balls arranged between the track grooves of said outer joint member and the track grooves of said inner joint member, an outer peripheral surface formed with a spherical portion which comes into sliding contact with the cylindrical inner peripheral surface of said outer joint member, an inner peripheral surface formed with a spherical portion which comes into sliding contact with the spherical outer peripheral surface of said inner joint member, and a cage having a plurality of pockets for holding said plurality of balls, wherein at least one of the cylindrical inner peripheral surface of said outer joint member, the surface of the track grooves of said outer joint member, the spherical outer peripheral surface of said inner joint member, the surface of the track grooves of said inner joint member, the spherical portion of the outer peripheral surface of the cage, and the spherical portion of the inner peripheral surface of the cage has a plurality of striated peaks and valleys extending in the same direction, and Rsk of said at least one surface is a negative value.

13. A sliding type constant velocity universal joint comprising an outer joint member having a plurality of straight track grooves formed on its cylindrical inner peripheral surface, an inner joint member having a plurality of straight track grooves formed on its spherical outer peripheral surface, a plurality of balls arranged between the track grooves of the outer joint member and the track grooves of the inner joint member, an outer peripheral surface formed with a spherical portion which comes into sliding contact with the cylindrical inner peripheral surface of the outer joint member, an inner peripheral surface formed with a spherical portion which comes into sliding contact with the spherical outer peripheral surface of the inner joint member, and a cage having a plurality of pockets for holding the plurality of balls, wherein at least one of the cylindrical inner peripheral surface of the outer joint member, the surface of the track grooves of the outer joint member, the spherical outer peripheral surface of the inner joint member, the surface of the track grooves of the inner joint member, the spherical portion of the outer peripheral surface of the cage, and the spherical portion of the inner peripheral surface of the cage has a plurality of striated peaks and valleys extending in the same direction, and Rp of at least one of the surfaces is 2 or less.

14. The sliding type constant velocity universal joint according to any one of claims 8 to 11, wherein the at least one surface is any one of a cylindrical inner circumferential surface of the outer joint member, a surface of a track groove of the outer joint member, and a surface of a track groove of the inner joint member, and the bottom of the valley portion is a forged surface.

15. A sliding type constant velocity universal joint according to claim 14, wherein the Ra of said at least one surface is 1.5 or less.

16. A sliding type constant velocity universal joint according to claim 14, wherein Rz of said at least one surface is 10 or less.

17. The sliding type constant velocity universal joint according to any one of claims 8 to 16, which is mounted on an electric vehicle.

Images