Slidable constant velocity universal joint

Abstract

To reduce an induction thrust of a double offset type slidable constant velocity universal joint, to make it suitable for use in a power transmission system of a vehicle (EV or HEV) with an electric motor as a drive source.SOLUTION: A coating (manganese phosphate coating 20) that makes the surface roughness smaller than a surface of a base material, is formed on at least one surface of 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 part 13 of an outer peripheral surface of a holder 5, and a spherical part 15 of an inner peripheral surface of the holder 5.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to a slide type constant velocity universal joint. [Background technology]

[0002] Constant velocity universal joints applied to automobile drive shafts and propeller shafts can be broadly divided into sliding types, which allow both angular displacement and relative axial movement between two shafts, and fixed types, which 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 (DOJ) that use balls as rolling elements for transmitting rotational torque, and tripod constant velocity universal joints (TJ) that use rollers as rolling elements. For example, the following Patent Document 1 shows a double offset constant velocity universal joint that is lightweight and compact by increasing the number of balls from six to eight. Also, the following Patent Document 2 shows a double offset constant velocity universal joint that has a maximum operating angle of 30° or more and is further lightweight and compact.

[0004] A double offset constant velocity universal joint has the advantages of less circumferential backlash, better response, and lower manufacturing costs than a tripod constant velocity universal joint, but has the disadvantage of greater sliding resistance and greater transmission of vehicle vibrations, particularly engine vibrations during idling. Therefore, various measures against idling vibrations have been considered for double offset constant velocity universal joints. For example, Patent Document 3 below shows 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. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 10-73129 [Patent Document 2] JP 2007-85488 A [Patent Document 3] JP 2013-231518 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, as improvements in vehicle ride comfort progress, the NVH (Noise, Vibration, Harshness) characteristics required of constant velocity universal joints are becoming more stringent. Because double offset constant velocity universal joints are structurally unable to avoid the occurrence of sliding contact between parts, even if measures such as those shown in Patent Document 3 are taken, it is difficult to reduce the sliding resistance to the same level as tripod-type sliding constant velocity universal joints in which parts mainly come into contact with each other by rolling. Such problems of sliding resistance, particularly idling vibration, have become a bottleneck, and in recent years, the application range of double offset constant velocity universal joints has been limited to areas that are less susceptible to engine vibration (for example, rear drive shafts).

[0007] Meanwhile, in recent years, the electrification of vehicles has progressed, and electric vehicles (hereinafter referred to as EVs) that run only on the power of electric motors and hybrid vehicles (hereinafter referred to as HEVs) that run on the power of electric motors and engines are on the rise. Electric motors have better response than engines, so constant velocity joints that transmit the power of electric motors are also required to have less circumferential backlash and better response. In addition, since there is no engine vibration in EVs and the engine is basically stopped in HEVs while the vehicle is not moving (during idling), these vehicles do not have the problem of idling vibration, which is the main factor limiting the application of double offset type constant velocity joints. From the above perspective, the adoption of double offset types that have less circumferential backlash and have better response is being considered as sliding type constant velocity joints to be provided in the power transmission systems of drive shafts, propeller shafts, etc. in EVs and HEVs.

[0008] On the other hand, in a sliding type constant velocity universal joint, an axial load (induced thrust) is generated due to friction between internal parts when rotating through an operating angle. This induced thrust resonates with the vehicle body and can cause various vibration problems such as swaying and muffled noise when starting off. Such problems are particularly noticeable in EVs and HEVs, which are quiet. Therefore, if the induced thrust can be reduced, there is a possibility that double offset type constant velocity universal joints can be used suitably 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. [Means for solving the problem]

[0010] As described above, the induced thrust is an axial load generated due to the frictional force between the internal parts of the sliding type constant velocity universal joint. Specifically, as shown in FIG. 23, the resultant force of the axial resistance Qx caused by the frictional force of the contact portion between the track groove 101a of the outer joint member 101 and the ball 102, and the axial resistance Hx caused by the frictional force of the contact portion between the outer spherical surface 103a of the cage 103 and the cylindrical inner peripheral surface 101b of the outer joint member 101 becomes the induced thrust. FIG. 24 shows the result of the induced thrust Fx (=Qx+Hx) of a double offset type constant velocity universal joint having eight balls, which was obtained by mechanical analysis. As shown in the figure, it was thought that in the double offset type constant velocity universal joint, the induced thrust of the order component of the same number as the number of balls (8 in the figure) is mainly generated.

[0011] Meanwhile, Fig. 25 shows the results of measuring the actual induced thrust of an eight-ball double offset constant velocity universal joint. In this test, assuming the start of a vehicle, 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 deg to 12 deg at a rate of 10.7 deg / min, and the axial load (induced thrust) applied to the outer joint member was measured. The X-axis in Fig. 25 shows the frequency of vibration generated in the constant velocity universal joint, the Y-axis shows the working angle of the constant velocity universal joint, and the Z-axis shows the magnitude of the axial load (induced thrust) applied to the outer joint member. The peak that appears at 2.5 Hz in the figure is the first-order component of the induced thrust (the vibration component that occurs once per rotation of the constant velocity universal joint), the peak that appears at 5 Hz is the second-order component, the peak that appears at 7.5 Hz is the third-order component, and the peak that appears at 20 Hz is the eighth-order component. From the actual measurement results shown in the figure, it can be seen that the induced thrust not only has an 8th-order component, but also many other order components, with the 1st-order component being particularly prominent.

[0012] As described above, the causes of the occurrence of induced thrusts of various orders are considered 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 the resultant forces of the loads occurring in each phase. Note that "each phase" refers to the phases of each ball and each track groove in contact with it (first to eighth phases in the case of an eight-ball double offset type constant velocity universal joint) at the contact portion between the track grooves of the outer joint member and the balls, and refers to the phases 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 it (first to eighth phases in the case of an eight-ball double offset type constant velocity universal joint).

[0013] Here, in an eight-ball double offset type constant velocity universal joint, the mechanical analysis results of the axial load applied to the track grooves of each phase of the outer joint member and its resultant force (Qx) are shown in Figs. 26 and 27. Fig. 26 shows the axial components of the track loads of each phase and their resultant force in an ideal state in which all parts have uniform dimensions and the surface properties of all phases are uniform. In this case, the axial loads (see dotted line) in the tracks of all phases become uniform, and these resultant forces have a regular shape in which a specific order component (the 8th order component in the illustrated example) is strongly expressed (see solid line). However, in reality, the contact state of the internal parts becomes non-uniform due to the variation in the dimensions of each part and the variation in the surface properties of the contact parts. Therefore, as shown in Fig. 27, the axial loads generated in the tracks of each phase vary (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 occurs in this way, the number of resonance points with the car body increases, and the possibility of causing noise and vibration problems increases.

[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 type 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 for each phase would be effective in equalizing the axial load (i.e., the frictional force) at the contact portion of each phase.

[0015] Therefore, the present 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 circumferential surface thereof, an inner joint member having a plurality of linear track grooves formed on a spherical outer circumferential 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 circumferential surface formed with a spherical portion which comes into sliding contact with the cylindrical inner circumferential surface of the outer joint member, an inner circumferential surface formed with a spherical portion which comes into sliding contact with the spherical outer circumferential surface of the inner joint member, and a cage having a plurality of pockets which hold the plurality of balls, The present invention provides a sliding type constant velocity universal joint, in which a coating is formed on at least one of the cylindrical inner joint member, the surface of a track groove of the outer joint member, the spherical outer 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, thereby making the surface roughness smaller than that of the surface of a base material.

[0016] In this way, by forming a coating (e.g., manganese phosphate coating) on ​​the contact surfaces between parts that has a smaller surface roughness than the surface of the base material, it is possible to suppress the variation in surface properties for each phase. 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 surface of the track groove and the cylindrical inner peripheral surface of the outer joint member and the surface of the track groove of the inner joint member are often formed by forging from the viewpoint of manufacturing costs. Such forged surfaces have a high surface roughness, and therefore tend to have a large variation in surface properties for each phase. Therefore, when the surface of the base material is a forged surface, it is particularly preferable to form the above-mentioned coating to suppress the variation in roughness.

[0019] The above-mentioned sliding type constant velocity universal joint can be suitably incorporated into a 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 above-mentioned sliding type constant velocity universal joint. Effect of the Invention

[0020] As described above, according to the present invention, it is possible to reduce the induced thrust of a double offset sliding type 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. [Brief description of the drawings]

[0021] [Figure 1] 1 is an axial cross-sectional view of a double offset type sliding type constant velocity universal joint according to one embodiment of the present invention. FIG. [Diagram 2] 2 is a cross-sectional view of the sliding type constant velocity universal joint of FIG. 1 taken in a direction perpendicular to the axis. [Diagram 3] FIG. 2 is an enlarged cross-sectional view of the surface before manganese phosphate treatment. [Figure 4] FIG. 2 is an enlarged cross-sectional view of a surface undergoing manganese phosphate treatment. [Diagram 5] FIG. 2 is an enlarged cross-sectional view of the surface after manganese phosphate treatment. [Figure 6] FIG. 2 is an enlarged cross-sectional view of a surface from which a portion of the manganese phosphate coating has been removed due to sliding against a counter material. [Figure 7] FIG. 2 is a plan view of an electric vehicle (EV) on which the constant velocity universal joint of FIG. 1 is mounted. [Figure 8] 4 shows a roughness curve (left), a load curve (center), and an amplitude distribution curve (right) of the surface of one track groove of an outer joint member before being subjected to manganese phosphate treatment. [Figure 9] 11A and 11B show a roughness curve (left), a load curve (center), and an amplitude distribution curve (right) of the surface of a track groove of another outer joint member before being subjected to manganese phosphate treatment. [Figure 10]4A shows a roughness curve (left), a load curve (center), and an amplitude distribution curve (right) of the surface of one track groove of an outer joint member after being subjected to a manganese phosphate treatment. [Figure 11] 13A and 13B show 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 being subjected to manganese phosphate treatment. [Figure 12] 13 is a graph showing measurement results of induced thrust of a double offset type constant velocity universal joint in which an outer joint member is subjected to manganese phosphate treatment. [Figure 13] 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 being subjected to manganese phosphate treatment. [Figure 14] 13A and 13B show 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 being subjected to manganese phosphate treatment. [Figure 15] 11 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 being subjected to manganese phosphate treatment. [Figure 16] 13A and 13B show 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 being subjected to manganese phosphate treatment. [Figure 17] 13 is a graph showing measurement results of induced thrust of a double offset type constant velocity universal joint in which an inner joint member is subjected to manganese phosphate treatment. [Figure 18] The roughness curve (left), load curve (center), and amplitude distribution curve (right) of the outer peripheral surface of one bar portion of the cage before manganese phosphate treatment. [Figure 19] The roughness curve (left), load curve (center), and amplitude distribution curve (right) of the outer peripheral surface of another bar portion of the cage before manganese phosphate treatment are shown. [Figure 20] The roughness curve (left), load curve (center), and amplitude distribution curve (right) of the outer peripheral surface of one bar portion of the cage after manganese phosphate treatment. [Figure 21]The roughness curve (left), load curve (center), and amplitude distribution curve (right) of the outer peripheral surface of the other bar portion of the cage after manganese phosphate treatment. [Figure 22] 13 is a graph showing the measurement results of induced thrust of a double offset type constant velocity universal joint in which a cage is subjected to manganese phosphate treatment. [Figure 23] 4 is a cross-sectional view showing a portion to which an axial load is applied when a double offset type constant velocity universal joint rotates at an operating angle. FIG. [Figure 24] FIG. 11 is a diagram showing the analysis results of induced thrust in a double offset type constant velocity universal joint. [Diagram 25] FIG. 13 is a diagram showing the results of measuring induced thrust in a double offset type constant velocity universal joint. [Figure 26] FIG. 11 is a diagram showing an analysis result of an axial load applied to a track groove of an outer joint member (when the axial load applied to the track groove of each phase is uniform). [Figure 27] FIG. 11 is a diagram showing an analysis result of an axial load applied to a track groove of an outer joint member (when the axial load applied to the track groove of each phase is non-uniform). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a sliding type constant velocity universal joint according to the present invention will be described in detail below with reference to the drawings.

[0023] 7 shows an electric vehicle (EV) that runs only on the power of an electric motor. This electric vehicle has a front wheel drive device 61 that drives the front wheels 51, and a rear wheel drive device 62 that drives the rear wheels 52. Each wheel drive device 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 the rear wheels 52. In the illustrated example, the drive unit 63 of the front wheel drive device 61 is connected to the left and right front wheels 51 via the left and right drive shafts 64, and the drive unit 63 of the rear wheel drive device 62 is connected to the left and right rear wheels 52 via the left and right drive shafts 64.

[0024] 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 type 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.

[0025] As shown in Figs. 1 and 2, the sliding type constant velocity universal joint 1 of this embodiment includes a cup-shaped outer joint member 2 with one axial end (left end in Fig. 1) open, an inner joint member 3 arranged on the inner circumference of the outer joint member 2, a plurality of balls 4, and a retainer 5 for holding the plurality of balls 4. An internal part 10 consisting of the inner joint member 3, the balls 4, and the retainer 5 is accommodated on the inner circumference of the outer joint member 2 so as to be displaceable in the axial direction. An end of an intermediate shaft 66 (see Fig. 7) is coupled to an axial hole 11 of the inner joint member 3 by spline fitting. In the following description, the axial direction of the outer joint member 2 and the inner joint member 3 in a state of an operating angle of 0° as shown in Fig. 1 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 inner side", and the opening side of the outer joint member 2 (left side in Fig. 1) is referred to as the "joint opening side".

[0026] Axial-extending linear track grooves 7 are formed at multiple locations in the circumferential direction at equal intervals on the cylindrical inner peripheral surface 6 of the outer joint member 2. Axial-extending linear track grooves 9 are formed at multiple locations 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 face each other in the radial direction, to transmit rotational torque between the two joint members 2, 3.

[0027] The cage 5 is provided with a plurality of pockets 12, and each pocket 12 holds one ball 4. The cage 5 has an outer circumferential surface formed with a spherical portion 13 that slides against the cylindrical inner circumferential surface 6 of the outer joint member 2, and tapered surfaces 14 provided on both axial sides of the spherical portion 13. The cage 5 has an inner circumferential surface formed with a spherical portion 15 that slides against the spherical outer circumferential 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 present invention is not limited to this, and the number of these may be six, for example.

[0028] 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 the opposite axial side 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 and 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 inner side 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 side 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 bisecting plane of the operating angle at any 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. Note that, 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 disposed 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 disposed on the joint inner side.

[0029] 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, the original outer joint member is molded. In the turning process, the outer circumferential surface of the original outer joint member is turned. In the rolling process, a male spline is 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 through these procedures, the cylindrical inner circumferential surface 6 and the track grooves 7 of the outer joint member 2 become forged surfaces.

[0030] 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 circumference of the original form of the inner joint member. In the heat treatment process, the original form of the inner joint member is subjected to heat treatment (e.g., carburizing, quenching, and tempering). In the grinding process, the spherical outer circumferential surface (area excluding the track grooves) of the original form of the inner joint member 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.

[0031] 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 roughly cylindrical cage original is formed by die forming. In the turning process, the inner and outer peripheral surfaces of the cage original are turned. In the pocket punching process, the cage original is punched in the radial direction to form pockets 12. In the heat treatment process, the cage original is heat treated (for example, carburized, quenched, and tempered). 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 are ground. When manufactured according to 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.

[0032] A coating is formed on at least one of the cylindrical inner peripheral surface 6 of the outer joint member 2, the surface of the track groove 7 of the outer joint member 2, the spherical outer peripheral surface 8 of the inner joint member 3, the surface of the track groove 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 prepared by the above procedure, to reduce the surface roughness compared to the surface of the base material (steel material). As such a coating treatment, a phosphate treatment such as manganese phosphate treatment can be performed, and 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 generate a manganese phosphate-based crystalline coating on the surface of a steel product, and is also called "Parker treatment" or "Lubrite treatment". Manganese phosphate treatment is usually performed by immersing the member in the treatment solution. Therefore, 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 retainer 5 may be subjected to the manganese phosphate treatment. However, while the inner joint member 3 and the retainer 5 can be coated by immersing them in a treatment liquid in large quantities at once, the outer joint member 2 needs to be coated only in a necessary area to avoid a situation in which the treatment liquid is applied to the shaft portion that fits with the differential gear and causes dimensional changes. Therefore, since the outer joint member 2 cannot be coated in large quantities at once and it requires time and cost, it is preferable to apply the manganese phosphate treatment to one or both of the inner joint member 3 and the retainer 5 without applying the manganese phosphate treatment to the outer joint member 2.

[0033] 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 (1) is triggered, followed by the cathodic reaction (2) below, which increases the pH at the material interface and advances the film formation reaction (3) below. (1)Fe →Fe 2+ +2e (2) 2H + +2e→H 2 (3) 5Mn 2+ +10H2 PO 4 - →Mn 5 H 2 (PO 4 ) 4 4H 2 O+6H 3 PO 4

[0034] This series of reactions begins with the dissolution of the Fe material in (1) above, but in the case of a surface with roughness as shown in Figure 3, this reaction begins in the parts of the material that have a large microscopic surface area (i.e., large protrusions). The Fe material that reacted in (1) above becomes ions and is released into the liquid, so the large protrusions in 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.

[0035] Furthermore, the slight remaining variation in roughness is easily smoothed by the presence of the manganese phosphate coating when a high surface pressure is applied, so the variation in roughness between the phases is further reduced. That is, the manganese phosphate coating formed by the manganese phosphate treatment has a Mohs hardness of 5 and does not exhibit a "layered structure" or "cleavage", so it does not function as a solid lubricant. Also, the manganese phosphate coating 20 has a crystalline structure and is a coating having microscopic irregularities (see FIG. 5). The convex parts of this manganese phosphate coating 20 are easily broken when a high surface pressure is applied, so that the surface of the member is smoothed (see FIG. 6). That is, the manganese phosphate coating 20 is broken or worn down by sliding with the mating material, so that the surface of the member is in a conformal state in which the manganese phosphate coating remains to a certain extent and the surface of the steel material is exposed.

[0036] 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 of the constant velocity universal joint will deviate from the proper range, and the constant velocity universal joint's functions, including operability, may not be obtained. If the thickness of the manganese phosphate coating is less than 0.5 μm, the effects of uniforming the surface roughness and improving the conformability may not be obtained sufficiently.

[0037] 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 Rsk being a negative value and Rp being 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 value of Rsk is -3, the lower limit value of Rp is 0.5, the lower limit value of Ra is 0.2, and the lower limit value of Rz is 1.0.

[0038] Rsk, Rp, Ra, and Rz are respectively the skewness, maximum peak height, arithmetic mean height, and maximum height of the roughness curve of the reference length specified in JIS B 0601-2013. Rsk is a positive value when there are many peaks in the amplitude distribution curve relative to the average line of the roughness curve, and a negative value when there are many valleys. Measurement of these parameters is performed at the contact position between each part, over a range of 4 mm in total, consisting of 5 sections of 0.8 mm × 0.8 mm reference length along the axial direction.

[0039] When the above-mentioned sliding type constant velocity universal joint 1 has an operating angle, a torque is inputted to the outer joint member 2 by a rotational driving force from an electric motor, and the torque is transmitted to the inner joint member 3 via the cage 5 and the balls 4. At this time, an axial load (induced thrust) is generated in the outer joint member 2 due to an axial sliding resistance at the contact portion between each part. However, the axial load in each phase is not uniform and varies due to variations in the dimensions and surface properties of each part. 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 member can be reduced and the variation in the surface properties for each phase can be suppressed. As a result, the axial load generated at the contact portion of each phase is uniform, and therefore each order component of the induced thrust can be reduced.

[0040] The present invention is not limited to the above embodiment. Other embodiments of the present invention will be described below, but redundant description of the same points as the above embodiment will be omitted.

[0041] In the above-mentioned 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 to provide an axial gap between the wall surface of the pocket 12 and the ball 4. This axial gap can be set within the range of 0.001 to 0.05 mm, for example. Alternatively, the axial dimension of the pocket 12 of the cage 5 may be made smaller than the diameter of the ball 4 to bring the wall surface of the pocket 12 into contact with the ball 4 via a negative gap (interference).

[0042] Also, 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 value of the relative axial movement between them at this time is the above-mentioned axial gap. This axial gap can be set within a range of, for example, 0.6 to 1.5 mm. In addition, the axial gap between the cage 5 and the inner joint member 3 may be 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 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 the relative movement between them, but the relative axial movement between them is approximately zero.

[0043] In addition, 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 finished by forging as described above in order to reduce manufacturing costs, but may be finished by cutting or grinding after heat treatment (hardening). In addition, the outer peripheral surface and the inner peripheral surface of the cage 5 are generally finished by grinding as described above, but may be finished by cutting or rolling after heat treatment (hardening).

[0044] Furthermore, the double offset type sliding constant velocity universal joint of the present invention can be applied not only to electric vehicles (EVs) that run only on the driving force of an electric motor, but also 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 only on the power of the engine. EXAMPLES

[0045] Only the inner surface of the outer joint member 2 of the 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 surface roughness curves, load curves, and amplitude distribution curves 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 surface roughness curves, load curves, and amplitude distribution curves 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 projections are reduced (Rsk is a negative value).

[0046] Figure 12 shows the measurement results of the 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 10.7 deg / min while inputting a torque of 900 Nm 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. EXAMPLES

[0047] Only the surface of the inner joint member 3 of the 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 track grooves 9 out 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 two track grooves 9 of the inner joint member 3 after the manganese phosphate treatment. As can be seen from these figures, the surface of the track groove 9 after the manganese phosphate treatment has a smaller roughness, and in particular, the convex parts of the roughness are smaller (Rsk has a negative value).

[0048] Figure 17 shows the measurement results of the induced thrust. The measurement conditions are the same as those of the test in Figure 25. As can be seen from this figure, it can be confirmed that in Example 2 in which the inner joint member 3 was subjected to the manganese phosphate treatment, the induced thrust, particularly the induced thrust at a high working angle, was reduced compared to the comparative example in which the manganese phosphate treatment was not performed. EXAMPLES

[0049] Only the surface of the retainer 5 of the 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 pillars) with different phases on the outer peripheral surface of the retainer 5 before the manganese phosphate treatment was applied. The outer peripheral surface of the retainer 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 retainer 5 after the manganese phosphate treatment was applied. As can be seen from these figures, the surface roughness of the retainer 5 after the manganese phosphate treatment is reduced, and in particular, the convex parts of the roughness are reduced.

[0050] The measurement results of the induced thrust are shown in Figure 22. 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 in Example 3 in which the cage 5 was subjected to manganese phosphate treatment, the induced thrust, especially the induced thrust at a high operating angle, was reduced compared to the comparative example in which the cage 5 was not subjected to manganese phosphate treatment. [Explanation of symbols]

[0051] 1. Sliding 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 surface 7 Track groove 8 Spherical outer surface 9 Track groove 10 Internal parts 12 Pockets 13 Spherical part 14 Tapered surface 15 Spherical part 20 Manganese phosphate coating 51 Front wheel 52 Rear wheel 61, 62 Wheel drive unit 63 Drive unit 64 Drive shaft (power transmission system) 65 Fixed constant velocity joint 66 Intermediate shaft O Joint center O1 Center of curvature of the spherical part of the outer periphery of the cage O2 Center of curvature of the spherical part of the inner circumference of the 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 linear track grooves formed on a cylindrical inner peripheral surface; an inner joint member having a plurality of linear track grooves formed on a 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 which hold the plurality of balls, a coating having a smaller surface roughness than that of a surface of a base material is formed on at least one of a cylindrical inner peripheral surface of the outer joint member, a surface of a track groove of the outer joint member, a spherical outer peripheral surface of the inner joint member, a surface of a track groove of the inner joint member, a spherical portion of an outer peripheral surface of the cage, and a spherical portion of an inner peripheral surface of the cage.

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

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

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

5. 2. 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. 2. 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; wheels; and the power transmission system that transmits the driving force of the electric motor to the wheels via the sliding type constant velocity universal joint according to claim 1.

Images