Sliding constant-velocity universal joint

Abstract

In a double offset constant-velocity universal joint 1 having a normal angle of 5° or less, a grease with a coefficient of friction greater than 0.08 is used as a grease to be sealed inside an outer joint member 2. This arrangement allows for a reduction of the torque loss ratio in the operating angle range near the normal angle that is frequently used.

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 constant velocity universal joints, double offset 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 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

[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] However, even if the problem of idling vibration does not occur when applied to EVs or HEVs, as described above, the double offset constant velocity universal joint has sliding contact between its parts, and therefore has a larger torque loss rate (i.e., lower torque transmission efficiency) than a tripod constant velocity universal joint, in which the parts mainly contact by rolling. Therefore, if the torque loss rate of the double offset constant velocity universal joint can be reduced, that is, if the torque transmission efficiency can be improved, it may be possible to suitably use it in EVs and HEVs.

[0009] SUMMARY OF THE INVENTION It is therefore an object of the present invention to reduce the torque loss rate of a double offset type constant velocity universal joint.

[0010] The inventors investigated the relationship between the torque loss rate of a double offset constant velocity universal joint and the friction coefficient of the grease sealed inside. Specifically, two types of grease, A and B, with different friction coefficients were prepared. The components contained in grease A and B are shown in Table 1 below.

[0011]

[0012] The friction coefficients of Grease A and Grease B were measured using the SRV test standardized in ASTM D5706 and D5707. The test equipment used was an Optimol SRV5 vibration friction and wear tester. The test conditions were a ball (diameter 3 / 8 inch, material SUSJ2, hardness HRC 62) and a flat plate (material SCr450, hardness HRC 59, surface roughness Ra 0.7, polished finish) as test specimens. These were slid together under a pressure of 2 GPa, an amplitude of 3 mm, 25 Hz, and room temperature (25°C), and the friction coefficient between the two test specimens was measured. As shown in Figure 4, the friction coefficient of Grease A was 0.08 or higher (approximately 0.09) 5 minutes after the start of the test, while that of Grease B was less than 0.08 (approximately 0.06). Hereinafter, grease A having a relatively high coefficient of friction will be referred to as "high μ grease A," and grease B having a relatively low coefficient of friction will be referred to as "low μ grease B."

[0013] Torque loss rates were measured for double-offset constant velocity universal joints filled with high-μ grease A and double-offset constant velocity universal joints filled with low-μ grease B at various operating angles. As shown in Figure 5, torque loss rates were lower when low-μ grease B was used than when high-μ grease A was used at operating angles of 6° or greater. On the other hand, torque loss rates were lower when high-μ grease A was used than when low-μ grease B was used at operating angles of 5° or less. Because sliding contact occurs between internal components in double-offset constant velocity universal joints, as mentioned above, it is generally thought that a grease with a lower friction coefficient would result in lower torque loss rates. However, the results in Figure 5 reveal that, at low operating angles of 5° or less, torque loss rates are lower when a grease with a higher friction coefficient is used.

[0014] To explore the reasons for the above results, the inventors conducted a simulation analysis to determine how the contact load between the inner circumferential surface of the outer joint member and the outer circumferential surface of the cage changes with the working angle when the friction coefficients between the internal parts of a double-offset constant velocity universal joint are different. As a result, as shown in Figure 6, it was revealed that when the friction coefficient between the internal parts is high (see solid line), i.e., when a grease with a high friction coefficient is used, the working angle at which contact load begins to occur between the outer joint member and the cage shifts to a higher angle (right side in the figure) compared to when the friction coefficient between the internal parts is low (see dotted line), i.e., when a grease with a low friction coefficient is used. Thus, it is expected that the use of a grease with a high friction coefficient expands the working angle range where the contact load between the internal parts is small toward the higher angle side, which is one of the reasons for the reduction in torque loss rate at working angles of 5° or less.

[0015] Based on the above findings, 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 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; a cage having an outer peripheral spherical portion in sliding contact with the cylindrical inner peripheral surface of the outer joint member and an inner peripheral spherical portion in sliding contact with the spherical outer peripheral surface of the inner joint member, and holding the plurality of balls; and grease sealed inside the outer joint member, wherein the center of curvature of the outer peripheral spherical portion of the cage and the center of curvature of the inner peripheral spherical portion are offset by an equal distance on opposite axial sides from the joint center, wherein the common angle is 5° or less, and the friction coefficient of the grease is greater than 0.08.

[0016] As described above, in double-offset constant velocity universal joints with a common angle of 5° or less, the torque loss rate can be reduced in the operating angle range near the common angle, which is frequently used, by using a grease with a relatively high friction coefficient (greater than 0.08). The common angle of a constant velocity universal joint refers to the operating angle that occurs in each constant velocity universal joint of the drive shaft when the steering is in a straight-ahead position in a vehicle with one occupant on a level, flat road. The friction coefficient of the grease was measured using an SRV test, and the specific test conditions were the same as those for the friction coefficients of greases A and B described above.

[0017] In the above-described sliding type constant velocity universal joint, at least one of the track grooves of the outer joint member, the cylindrical inner peripheral surface, and the track grooves of the inner joint member can be a forged surface.

[0018] The number of balls in the sliding type constant velocity universal joint may be, for example, eight.

[0019] The sliding type constant velocity universal joint can be suitably mounted on 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] As described above, according to the present invention, the torque loss rate of a double offset constant velocity universal joint can be reduced.

[0021] FIG. 3 is a plan view of an electric vehicle (EV). FIG. 4 is an axial cross-sectional view of a sliding type constant velocity universal joint (double offset constant velocity universal joint) according to one embodiment of the present invention. FIG. 5 is an axial cross-sectional view of the sliding type constant velocity universal joint of FIG. 2. FIG. 6 is a graph showing measurement results of the friction coefficient of grease. FIG. 7 is a graph showing changes in torque loss rate when the operating angle of a double offset constant velocity universal joint is changed. FIG. 8 is a graph showing analysis results of contact load between an outer joint member and a cage when greases with different friction coefficients are used.

[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] 1 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.

[0024] Each drive shaft 64 has 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.

[0025] 2 and 3 , the 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. 2 ), 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. 1 ) 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. 2 is referred to as the "axial direction," the bottom side of the outer joint member 2 (right side in FIG. 2 ) 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. 2 ) is referred to as the "joint opening side."

[0026] Axial-direction 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 (see FIG. 3 ). Axial-direction 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 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.

[0027] The cage 5 is substantially cylindrical, with pockets 12 formed at equal intervals in the circumferential direction. Each pocket 12 holds one ball 4. The outer peripheral surface of the cage 5 is formed with an outer peripheral spherical portion 13 that slides against the cylindrical inner peripheral surface 6 of the outer joint member 2, and tapered surfaces 14 provided on both axial sides of the outer peripheral spherical portion 13 (see FIG. 2 ). The inner peripheral surface of the cage 5 is formed with an inner peripheral spherical portion 15 that slides against the spherical outer peripheral surface 8 of the inner joint member 3. Note that while the present embodiment shows a case where the number of track grooves 7, 9, balls 4, and pockets 12 is eight, the present invention is not limited to this, and the number of these may be, for example, six.

[0028] At least one of the track grooves 7 of the outer joint member 2, the cylindrical inner peripheral surface 6 of the outer joint member 2, and the track grooves 9 of the inner joint member 3 is a forged surface. In this embodiment, the track grooves 7 of the outer joint member 2, the cylindrical inner peripheral surface 6, and the track grooves 9 of the inner joint member 3 are all forged surfaces. The spherical outer peripheral surface 8 of the inner joint member 3, the outer peripheral spherical portion 13 and inner peripheral spherical portion 15 of the cage 5, and the inner peripheral surface of the pocket 12 are, for example, ground surfaces.

[0029] The center of curvature O1 of the outer spherical portion 13 of the cage 5 and the center of curvature O2 of the inner spherical portion 15 of the cage 5 (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 balls 4 and the axes of both joint members 2, 3), i.e., from the axial center of the pocket 12 (see FIG. 2 ). In the illustrated example, the center of curvature O1 of the outer spherical portion 13 of the cage 5 is offset toward the rear of the joint from the joint center O, and the center of curvature O2 of the inner spherical portion 15 of the cage 5 is offset toward the opening of the joint from the joint center O. Conversely, the center of curvature O1 of the outer spherical portion 13 of the cage 5 may be located toward the opening of the joint, and the center of curvature O2 of the inner spherical portion 15 of the cage 5 may be located toward the rear of the joint.

[0030] 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 in the cage 5 roll on the track grooves 7 of the outer joint member 2, allowing the internal part 10 to move axially relative to the outer joint member 2.

[0031] When installed in a vehicle, the operating angle of the sliding type constant velocity universal joint 1 is set to 5° or less. That is, the sliding type constant velocity universal joint 1 is mainly used with an operating angle of 5° or less.

[0032] A boot (not shown) is fitted between the intermediate shaft 66 inserted into the axial hole 11 of the inner joint member 3 and the opening of the outer joint member 2. Grease is sealed in the internal space of the outer joint member 2, which is sealed by the boot. In this embodiment, grease with a friction coefficient of more than 0.08 is used. The friction coefficient of the grease is set to, for example, 0.15 or less. The friction coefficient of the grease is measured by an SRV test, and is the friction coefficient 5 minutes after the start of the test. The detailed test conditions are the same as those for the test conditions for the friction coefficients of greases A and B described above.

[0033] In this way, by using a grease with a friction coefficient greater than 0.08, it is possible to suppress the torque loss rate when rotating with an operating angle of 5° or less, for example, the torque loss rate at an operating angle of 5° can be suppressed to 0.2% or less, preferably 0.1% or less. This suppresses the torque loss rate when the sliding type constant velocity universal joint 1 is set at an operating angle close to the common angle which is frequently used, thereby improving torque transmission efficiency. The torque loss rate was measured when a torque of 200 N·m was input to the outer joint member 2 of the sliding type constant velocity universal joint 1 and the torque loss rate was measured for 700 min. －１ The torque output to the inner joint member 3 when the inner joint member 3 is rotated at 100 rpm is measured, and the value is calculated as 100·(input torque−output torque) / (input torque).

[0034] 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.

[0035] 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 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 may be finished by cutting or rolling after heat treatment (quenching).

[0036] In the above-described sliding type constant velocity universal joint 1, the axial width of the pocket 12 of the cage 5 may be made larger than the diameter of the ball 4, thereby providing an axial gap (pocket 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 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).

[0037] Furthermore, the inner peripheral surface of the cage 5 and the spherical outer peripheral surface 8 of the inner joint member 3 may be made movable in the axial direction relative to each other. The amount of this relative axial movement (axial gap) can be set, for example, within a range of 0.6 to 1.5 mm. Alternatively, the amount of relative axial movement between the cage 5 and the inner joint member 3 may be made substantially zero. For example, the centers of curvature of the inner spherical surface portion 15 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 inner spherical surface portion 15 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 therebetween is substantially zero.

[0038] When the operating angle of the sliding type constant velocity universal joint 1 is greater than 5°, it is preferable to use grease with a small coefficient of friction, for example, grease with a coefficient of friction of 0.08 or less.

[0039] Furthermore, the double offset sliding constant velocity universal joint of the present invention is not limited to electric vehicles (EVs) that use only an electric motor as a drive source, but can also be applied to hybrid vehicles (HEVs) that use both an electric motor and an engine as a drive source, and to the power transmission systems of vehicles that use only an engine as a drive source.

[0040] REFERENCE SIGNS LIST 1 Sliding type constant velocity universal joint (double offset constant velocity universal joint) 2 Outer joint member 3 Inner joint member 4 Ball 5 Cage 7, 9 Track groove 10 Internal part O Joint center

Claims

1. A sliding-type constant velocity universal joint comprising: an outer joint member having a plurality of linear track grooves formed on its cylindrical inner peripheral surface; an inner joint member having a plurality of linear track grooves formed on its spherical outer peripheral surface; a plurality of balls arranged between the track grooves of said outer joint member and the track grooves of said inner joint member; a cage having an outer spherical surface portion in sliding contact with the cylindrical inner peripheral surface of said outer joint member and an inner spherical surface portion in sliding contact with the spherical outer peripheral surface of said inner joint member, and holding said plurality of balls; and grease sealed inside said outer joint member, wherein the center of curvature of the outer spherical surface portion of said cage and the center of curvature of the inner spherical surface portion are offset equidistantly in the axial direction on opposite sides of the joint center, wherein the common angle is 5° or less, and the friction coefficient of the grease is greater than 0.

08.

2. A sliding type constant velocity universal joint according to claim 1, wherein at least one of the track grooves of the outer joint member, the cylindrical inner peripheral surface, and the track grooves of the inner joint member is a forged surface.

3. A sliding type constant velocity universal joint according to claim 1, wherein the number of said balls is eight.

4. The sliding type constant velocity universal joint according to claim 1, which is mounted on a vehicle powered by an electric motor.

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