JP2008128408A - Power transmission shaft - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent a crack in a small diametrical portion of a power transmission shaft as well as increasing fatigue strength of a male spline portion by alleviating stress concentration of both tensile stress and shear stress in a male spline portion of the power transmission shaft. <P>SOLUTION: The male spline portion Sm is formed in an outer circumference of the power transmission shaft. A diameter enlarging portion 21b whose outer diametrical size is gradually enlarged toward a portion opposite to a shaft end is provided in the portion opposite to a shaft end among cores 21 of the male spline portion Sm. An r portion 21b1 whose cross section is circular-arc-shaped is provided on either side of a circumferential direction of the diameter enlarging portion 21b and then a curvature radius of the r portion 21b1 is gradually enlarged toward the portion opposite to the shaft end. Further, a hardened layer by shot peening process or the like is provided in a small diametrical portion of the power transmission shaft. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a power transmission shaft coupled to a female member through a spline (including serrations, the same applies hereinafter).

  In recent years, due to increasing interest in environmental issues, for example, automobiles are strongly required to tighten exhaust gas regulations and improve fuel efficiency. As part of these measures, power transmission shafts used for drive shafts, propeller shafts, etc. There is a strong demand for weight reduction and strength improvement. Many of these power transmission shafts have spline portions on the outer peripheral surface. By fitting the spline part (male spline part) of the power transmission shaft and the spline part (female spline part) of the female side member fitted on the power transmission shaft, the power transmission shaft and the female side member are coupled, Rotational power is transmitted.

  Since a power transmission shaft with a male spline part requires strength, usually steel is used as the material, and after forming the male spline part by rolling or pressing, at least the male spline part is hardened and hardened. Used. As a method of quenching and hardening after molding, induction hardening is often used, but there are also cases of submerged hardening and carburizing and hardening.

  FIG. 8 shows a so-called round-up male spline in which the end on the opposite axis end side (left side in the drawing) of the valley portion 100 is connected to the outer peripheral surface (smooth portion) 101 of the male side member via the enlarged diameter portion 102. It is a top view of a part. In the enlarged diameter portion 102, the outer diameter dimension is gradually increased in the range of the region C. The fatigue failure of the male spline portion of this form usually occurs near the joint between the valley portion 100 and the enlarged diameter portion 102 or at the enlarged diameter portion 102. There are two crack generation modes at that time, one is due to the tensile stress concentrated in the A portion, and the other is due to the shear stress concentrated in the B portion. In the case of steel, cracks are mainly governed by shear stress below Vickers hardness 700 as a guide, and if it is more than that, and if it is swung torsional fatigue, it is governed by tensile stress. The shear stress is substantially parallel to the axial direction, and tends to concentrate and act near the starting point of the enlarged diameter portion (B portion).

So far, several methods have been proposed as means for improving the fatigue strength of the male spline part. For example, Patent Literature 1 discloses a technique for reducing stress concentration by blunting the boundary between the enlarged diameter portion and the tooth surface. Further, Patent Document 2 discloses a high strength technology in which two or more diameter-expanded portions are usually arranged side by side in the axial direction.
JP 2005-147367 A Japanese National Patent Publication No. 11-514079

  However, in the technique described in Patent Document 1, an effect is recognized in reducing the tensile stress concentration, but the effect of reducing the shear stress concentration is insufficient. Further, in the technique of Patent Document 2, the shear stress concentration can be reduced, but the effect of reducing the tensile stress concentration is insufficient. As described above, there is a technology that can relieve one of the two stresses that govern crack initiation, but there is no technology that relieves both simultaneously, and there is room for improvement in order to achieve further improvement in fatigue strength. was there.

  Further, even if the fatigue strength of the male spline part is increased, there is a possibility that fatigue fracture such as a crack may occur in other parts of the power transmission shaft. For example, as shown in FIG. 1, when the constant velocity universal joints 1, 1 ′ are coupled to both ends of the power transmission shaft 2, the outer joint member when the joint takes a large operating angle in the power transmission shaft 2. Small diameter portions 2a and 2a 'may be formed at portions that interfere with the open end portions of 4, 4' to increase the operating angle of the joint. At this time, there is a possibility that fatigue fracture such as a crack may occur in the small diameter portions 2a and 2a 'having relatively low strength in the power transmission shaft 2.

  Therefore, in the present invention, the stress concentration of both the tensile stress and the shear stress at the male spline portion of the power transmission shaft is alleviated to improve the fatigue strength of the male spline portion, and the fatigue of the small diameter portion of the power transmission shaft is reduced. The purpose is to increase the strength.

  The inventors of the present invention manufactured a test piece having a notch in a parallel portion, and used it for a rotational bending fatigue test and a torsional fatigue test, respectively, to determine the relationship between the stress concentration factor and the fatigue strength.

  As a test piece, a medium carbon steel having the same chemical composition shown in FIG. 9 was used, and a test piece having the shape and dimensions (unit: mm) shown in FIGS. 10a and 11a was produced. FIG. 10 a is a test piece for a rotating bending fatigue test, and FIG. 11 a is a test piece for a torsional fatigue test. In the specimen of the rotating bending fatigue test, the radius of curvature of the notch tip is set to five levels of 0.10, 0.15, 0.25, 0.50, and 1.40, and the stress concentration coefficient α is 3.5. , 3.0, 2.5, 2.0, 1.5 (see FIG. 10c). In the torsional fatigue test specimen, the radius of curvature of the notch tip is set to four levels of 0.15, 0.25, 0.50, 1.40, and the stress concentration coefficient α is 3.0, 2.5, 2.0 and 1.5 were set (see FIG. 11c). All of these test pieces were subjected to induction quenching in parallel portions including the notches and then subjected to low temperature tempering. All the test pieces had a surface hardness of about HV650 after the heat treatment.

  First, the rotating bending fatigue test was performed with an Ono type rotating bending fatigue tester in a room temperature atmosphere at a load frequency of 50 Hz.

As a result of the rotating bending fatigue test, cracks occurred along the bottom of the notch regardless of the level of the notch, leading to fracture. The crack initiation mode in this case is governed by tensile stress. Until around exceeding the load count is 10 ⁵ up to the break, fatigue curve drops with decreasing stress amplitude, stress amplitude showed clear fatigue limit phenomena no longer fracture and below a certain value. The stress amplitude here is a nominal stress amplitude that does not depend on the level of the notch, and a smooth round bar having a notch bottom diameter (φ6.5 mm) was given a bending moment of the same size as the fatigue test. It means the maximum tensile stress amplitude that sometimes acts on the surface.

FIG. 12 shows the relationship between the stress concentration factor α _σ obtained in the rotating bending fatigue test and the fatigue limit strength. As shown in the figure, the fatigue strength improved as α _σ decreased. However, as shown by the broken line in the figure, when α _σ ≦ 2.7, the fatigue curve has a large gradient, and when α _σ is decreased. It has been found that the improvement in fatigue strength appears more prominently.

  Next, the torsional fatigue test was carried out under the conditions of a load frequency of 2 Hz and a full swing (stress ratio R = -1) in a normal temperature atmosphere by torque control using an electric hydraulic servo fatigue tester.

As a result of the torsional fatigue test, cracks occurred along the bottom of the notch regardless of the level of the notch, leading to fracture. The crack initiation mode in this case is governed by shear stress. Both reversed torsional fatigue test is load count went until near 10 ⁶ times at most in the range with decreasing stress amplitude fatigue curve drops. The stress amplitude here is a nominal stress amplitude that does not depend on the level of the notch, and is applied to a smooth round bar having a notch bottom diameter (φ17 mm) when a torsion torque of the same magnitude as that in the fatigue test is applied. Means the maximum shear stress amplitude acting on

13 shows a relationship between fatigue strength of the both reversed torsional fatigue test with the resulting stress concentration factor alpha _tau and 10 ⁵ times. As shown in the figure, the fatigue strength improved as α _τ decreased. However, as shown by the broken line in the figure, when α _τ ≦ 2.1, the fatigue curve gradient was large, and when α _τ was decreased, It has been found that the improvement in fatigue strength appears more prominently.

From the above, whether the crack initiation is governed by either tensile stress or shear stress, the fatigue strength is improved by stress concentration relaxation, particularly α _σ ≦ 2.7 for tensile stress, It has been found that the stress concentration relaxation effect is further enhanced when α _τ ≦ 2.1 against shear stress. Accordingly, in the expanded portion of the male spline portion that undergoes fatigue failure in both failure modes, the maximum value σ _1max of the first principal stress concentrated there is not more than 2.7 times the reference stress τ ₀ (σ _1max ≦ 2. 7τ _o ), and it is desirable to tune the shape so that the maximum value τ _θzmax of the shear stress in the axial direction is 2.1 times or less (τ _θzmax ≦ 2.1τ ₀ ) of the reference stress τ ₀ . The reference stress tau ₀ is the torque T, the diameter d _o of the valley bottom of the male spline section shown in FIG. 6, when the inner diameter d _{i (male} spline portion of the male spline portion of the hollow case. In real Is given by d _i = 0).

τ ₀ = 16 Td _o / [π (d _o ⁴ −d _i ⁴ )]

As a result of various tunings of the shape of the enlarged diameter portion by the present inventors, rounded portions are provided on both sides in the circumferential direction of the enlarged diameter portion of the male spline portion, and the curvature radius of the rounded portion is gradually increased toward the opposite shaft end side. Then, it was found that σ _1max ≦ 2.7τ _o and τ _θzmax ≦ 2.1τ ₀ can be satisfied.

  Next, a shaft-shaped test piece having male spline portions at both shaft ends is manufactured using a material having the same component (see FIG. 9) as the notched fatigue test piece of FIGS. 10 (a) and 11 (a). (See FIG. 17a) Using this test piece, a double torsional fatigue test and a single torsional fatigue test were performed. According to the involute spline specifications shown in FIG. 17b, two types of test pieces were produced, equivalent to the product of the present invention and equivalent to the conventional product. These test pieces are subjected to induction hardening and tempering under the same conditions in the atmosphere as a whole. The double torsional fatigue test is conducted at four levels in the range of 850 to 1300 Nm, and the single torsional fatigue test gives a maximum torsional torque of four levels in the range of 1250 to 2000 Nm. FIG. 18 shows a T / N diagram obtained in the double swing torsional fatigue test, and FIG. 19 shows a T / N diagram obtained in the single swing fatigue test. As is clear from both figures, the product of the present invention can achieve a significant improvement in fatigue strength in both the double torsional fatigue and the single swing torsional fatigue compared to the conventional product.

  Furthermore, using the material of the same component (see FIG. 9) as the notched fatigue test piece of FIG. 10 (a) and FIG. 11 (a), it has a male spline part at both shaft ends, and at its axial middle part. A shaft-shaped test piece having a small-diameter portion with a smooth outer peripheral surface was manufactured (see FIG. 28a), and a double-twisted torsional fatigue test was performed using this test piece. The test piece is subjected to induction hardening and tempering under the same conditions in the atmosphere according to the involute spline specifications shown in FIG. 17b. In this test, four types of test pieces having different shapes and shapes of the enlarged diameter portion of the male spline portion, presence / absence of shot peening to the small diameter portion, and strength were manufactured (see FIG. 28b). This double torsional fatigue test was conducted at four levels in the range of 850 to 1300 Nm, and the obtained T / N diagram is shown in FIG. The solid line in FIG. 29 is a reference regression line indicating the fatigue strength level of the conventional product. The target is to improve the strength by 15% from the conventional product indicated by the solid line, and the target value is indicated by a dotted line in FIG.

  As a result of the test, in the case where the comparative product 3 was used, all the samples were broken from the spline part, and the result almost along the standard regression line of the conventional product was shown. On the other hand, the implementation product, comparative product 1 and comparative product 2 (those having a diameter-enlarged portion corresponding to the product of the present invention in the male spline portion) were broken from the small-diameter portion. Moreover, the implementation product and comparative product 1 which performed shot peening showed higher strength than the comparative product 2 which did not perform shot peening. Further, the product with a relatively long shot peening injection time (40 s) shows higher strength than the comparative product 1 with a relatively short injection time (20 s), and only this product has a target value indicated by a dotted line in FIG. The result that almost cleared was shown.

  FIG. 30 shows the residual stress distribution of these test pieces. From this figure, it can be seen that the comparative products 2 and 3 not subjected to shot peening do not generate a portion having a high residual stress, and that the actual product generates a deeper residual stress than the comparative product 1. From the difference in the depth of the residual stress, it is considered that the comparative product 1 did not reach the target value indicated by the dotted line in FIG. As a guideline, the generation depth of the residual stress of 1000 MPa reaches a depth of about 0.05 mm in the implementation product, but reaches only about half or less of the implementation product in the comparison product 1. Therefore, it can be said that the work hardened layer (residual stress of 1000 MPa or more) is preferably generated to a depth of 0.05 mm or more.

  As described above, the present invention is characterized by the following matters.

  (I) In a power transmission shaft having a male spline portion on the outer periphery and having a diameter-expanded portion whose outer diameter is gradually expanded on one end side in the axial direction of the valley portion of the male spline portion, Provide round portions on both sides in the circumferential direction of the diameter portion, gradually increase the radius of curvature of the round portion toward one end in the axial direction, and on the surface of the smallest diameter portion on the one end side in the axial direction from the male spline portion A work hardened layer is provided.

(II) When the torque T is loaded, the first principal stress acting on the diameter-expanded portion of the male spline portion and the maximum values of the shear stress in the axial direction are σ _1max and τ _θzmax , respectively. valley diameter d _o of the _parts, with respect to the inner diameter d _i of the male spline portion, 1) when the reference stress tau ₀ given by equation to satisfy the following 2) and 3) at the same time.

τ ₀ = 16 Td _o / [π (d _o ⁴ −d _i ⁴ )]... 1)

σ _1max ≦ 2.7τ _o ... 2)

τ _θzmax ≦ 2.1τ ₀ ... 3)

  As a result of verification by the inventor, in the above configuration, the rate of increase in the radius of curvature of the rounded portion is dR / dL, and the angle of the straight line connecting the inner diameter end and the outer diameter end of the axial section of the enlarged diameter portion is θ. It was found that it is desirable to set the respective values in the ranges of 0.05 ≦ dR / dL ≦ 0.60 and 5 ° ≦ θ ≦ 20 °.

  Moreover, in this invention, by providing a work hardening layer on the surface of the small diameter part of a power transmission shaft, the fatigue strength of a shaft can be improved and generation | occurrence | production of the crack in a small diameter part can be suppressed. When the generation depth of the work hardened layer is 0.05 mm or more, a sufficient strength improvement effect can be obtained. Moreover, this work hardening layer can be formed by shot peening, for example.

  As described above, according to the present invention, the stress concentration of both the tensile stress and the shear stress in the male spline portion of the power transmission shaft is alleviated to improve the fatigue strength of the male spline portion, and the power transmission shaft. It is possible to increase the fatigue strength of the small diameter portion.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

  FIG. 1 shows a partial cross-sectional view of a drive shaft using a power transmission shaft 2. The drive shaft of the illustrated example has a fixed type constant velocity universal joint 1 mounted on an end portion of the power transmission shaft 2 on the outboard side (the side that is outside in the vehicle width direction when mounted on the vehicle) and the inboard side (mounted on the vehicle). A sliding type constant velocity universal joint 1 ′ such as a tripod type is attached to the end of the vehicle width direction center side).

  The fixed type constant velocity universal joint 1 includes an inner joint member 3 fixed to the power transmission shaft 2, an outer joint member 4 disposed on the outer diameter side of the inner joint member 3, and the inner joint member 3 and the outer joint member 4. A ball 5 serving as a torque transmission member that transmits torque between them is a main component. The constant velocity universal joint of the illustrated example is called a zepper type, and is formed of a track groove 3 a formed on the outer periphery of the inner joint member 3 and a track groove 4 a formed on the inner periphery of the outer joint member 4. The balls 5 are arranged on a ball track, and a plurality of balls 5 arranged at equal circumferential positions are held by a cage 7.

  The tripod type constant velocity universal joint 1 ′ includes an inner joint member 3 ′ coupled to the power transmission shaft 2, an outer joint member 4 ′ disposed on the outer diameter side of the inner joint member 3 ′, and an inner joint member 3 ′. And a roller 5 ′ as a torque transmitting member for transmitting torque between the outer joint member 4 ′ and the outer joint member 4 ′. Leg shafts 3a 'are projected from three locations in the circumferential direction of the inner joint member 3'. A track groove 4a 'extending in the axial direction is formed at a position of the inner circumference of the outer joint member 4' in the circumferential direction, and a roller 5 'rolls along the track groove 4a'.

  The power transmission shaft 2 is formed hollow with medium carbon steel (for example, carbon steel S35C for machine structure defined in JIS G4051) having a carbon content of about 0.30 to 0.60 mass%, for example. If the C content is less than 0.30 mass%, stable high hardness cannot be obtained even by induction hardening, and if it exceeds 0.60 mass%, the material hardness increases and the workability such as rolling is significantly reduced. To do. Thus, by forming the power transmission shaft 2 hollow, it is possible to reduce the weight and the cost. The power transmission shaft 2 may be formed solid, and in this case, an effect of improving strength can be obtained.

  Male spline portions Sm are formed on the outer circumferences of both shaft ends of the power transmission shaft 2. By fitting this male spline portion Sm with a female spline portion Sf formed on the inner periphery of the inner joint member 3, 3 'as a male side member as shown in FIG. 3, the power transmission shaft 2 and the inner joint The members 3 and 3 'are coupled so as to be able to transmit torque. The inner joint members 3, 3 ′ have the inner diameter end on the opposite shaft end side (left side in FIG. 3) abutted with the shoulder 24 on the outer periphery of the power transmission shaft 2, and on the shaft end side (right side in FIG. 3). The inner diameter end portion is locked with a retaining ring (not shown), for example, so as to be positioned and fixed with respect to the shaft 2 in the axial direction.

  On the opposite shaft end side of the male spline portions Sm, Sm provided at both shaft ends of the power transmission shaft 2, cylindrical small diameter portions 2a, 2a 'having a smaller diameter than other portions and a smooth outer peripheral surface are provided. Formed (see FIG. 1). This delays the interference between the open ends of the outer joint members 4 and 4 ′ of the constant velocity universal joints 1 and 1 ′ and the power transmission shaft 2, thereby increasing the maximum operating angle of the joint. A surface hardened layer by shot peening is formed on the small diameter portions 2a and 2a '.

  As shown in FIGS. 2, 3, and 6, the male spline portion Sm of the power transmission shaft 2 has trough portions 21 and crest portions 22 extending in the axial direction alternately in the circumferential direction. The male spline portion Sm of this embodiment is a so-called up-round type formed by rolling, and each valley portion 21 is formed on the straight portion 21a having the same diameter in the axial direction and on the opposite end side. It is comprised with the enlarged diameter part 21b. Similarly, each peak portion 22 includes a straight portion 22a having the same diameter in the axial direction and a reduced diameter portion 22b formed on the opposite end side. As shown in FIG. 4, the starting ends of the enlarged diameter portion 21 b and the reduced diameter portion 22 b are at the same position in the axial direction, and the terminal ends are also at the same position in the axial direction. This male spline part Sm can also be formed by cold forging. In this case, normally, the reduced diameter part 22b of the peak part 22 is not formed, and the entire opposite end side of the peak part 22 has the same outer diameter. It becomes. The male spline part Sm after molding is subjected to heat treatment by induction hardening or the like, and further shot peened as necessary.

  As shown in FIG. 3, the valley 31 of the female spline portion Sf has the same diameter and is formed to the end on the opposite shaft end side. On the other hand, the peak portion 32 has a small diameter portion 32a, a large diameter portion 32b, and a rising portion 32c between the small diameter portion 32a and the large diameter portion 32b. The inner diameter dimension of the large diameter part 32b is smaller than the maximum outer diameter dimension (outer diameter dimension of the straight part 22a) of the peak part 22 of the male spline part Sm, and the power transmission formed on the opposite end side of the male spline part Sm. It is larger than the outer diameter of the smooth portion 25 of the shaft 2.

  When the male spline portion Sm and the female spline portion Sf are fitted to each other, the tooth surface 23 of the male spline portion Sm and the tooth surface (not shown) of the female spline portion Sf are in strong pressure contact. At this time, the fitting portions (represented by a dot pattern) of both tooth surfaces extend to the outer diameter side region of the enlarged diameter portion 21b as shown in FIG.

  In addition, although FIG. 3 illustrates the case where both the axial sections of the enlarged diameter portion 21b and the reduced diameter portion 22b are formed in a linear taper shape, both axial sections are formed in a curved shape. You can also. Moreover, it can also be set as the composite shape of a linear form and a curvilinear form.

  As shown in FIG. 2, in the present invention, the enlarged diameter portion 21b of the male spline portion Sm is formed between the rounded portion 21b1 (shown by a dotted pattern) formed on both sides in the circumferential direction and the rounded portion 21b1. And a planar flat portion 21b2. The radius portion 21b1 has a circular cross section in the radial direction, and both circumferential sides thereof are smoothly connected to the tooth surface 23 and the flat portion 21b2.

4 is a plan view showing the vicinity of the enlarged diameter portion 21b in the male spline portion Sm, and FIGS. 5a to 5d are the AA, BB, CC, and DD lines in FIG. FIG. As shown in FIG. 5a, the radius of curvature R _A of the round portion connecting the straight portion 21a and the tooth surface 23 of the trough portion 21 is constant up to the boundary between the enlarged diameter portion 21b. As shown in FIG 5b~ Figure 5d, the enlarged diameter portion 21b, the curvature radius of the rounded portion 21b1 is larger than the radius of curvature R _A of the boundary portion, and the more Hanjiku end gradually increases (R _A <R _B <R _C <R _D ). Further, as shown in FIG. 4, the width of the round portion 21b1 in the circumferential direction is on the side opposite the axis (upward in the drawing) until the position where the boundary line of the round portion 21b1 intersects the ridge line of the mountain portion and the tooth surface 23 disappears. ) Gradually expands toward (), and beyond this, the width dimension gradually decreases. The width dimension in the circumferential direction of the flat portion 21b2 is also gradually increased toward the opposite shaft end side.

  L in FIG. 4 indicates coordinates taken in the direction of a line passing through the center of the radius of curvature in the rounded portion 21b1 of the enlarged diameter portion 21b. The increasing rate of the radius of curvature of the rounded portion 21b1 is represented by dR / dL, and is set to dR / dL = 0.18 in this embodiment. Further, θ in FIG. 4 represents the inclination angle of a straight line connecting the inner diameter end and the outer diameter end of the axial section of the enlarged diameter portion 21b, and is set to θ = 8.3 ° in the present embodiment.

  14 to 16, the male spline portion Sm ′ described in Patent Document 1 (Japanese Patent Laid-Open No. 2005-147367), that is, the rounded portion 21b1 ′ is formed at the boundary between the enlarged diameter portion 21b ′ and the tooth surface 23 ′. A male spline portion Sm ′ formed and having a radius of curvature of the rounded portion 21b1 ′ constant over the entire length in the axial direction is shown (in FIGS. 14 to 16, a portion corresponding to the portion shown in FIGS. 2 to 4) (The same sign with (') added to it).

FEM analysis is performed for each of the male spline portion Sm (product of the present invention) shown in FIG. 2 and the male spline portion Sm ′ (conventional product) shown in FIG. 14, and the maximum value σ _1max of the first principal stress and the shear stress of each are analyzed. A maximum value τ _θzmax was obtained, and a value obtained by dividing the maximum value τ _θzmax by the reference stress τ ₀ was calculated.

  This FEM analysis is a three-dimensional linear elastic analysis, and “I-deas Ver. 10” was used as analysis software. As shown in FIG. 20, the analysis model is a linear elastic body including one valley portion 21, 21 ′ of the male spline portions Sm, Sm ′, and the model length is 100 mm. FIG. 21 shows a mesh attached to this analysis model. Each element is a tetrahedral secondary element, the total number of elements is about 200,000, and the total number of contacts is about 300,000. The element length was 0.2 mm or less (the minimum element length was 0.05 mm) at the main portion P (including the male spline portions Sm and Sm ′), and 0.5 mm at the portions other than the main portion P. FIG. 22 is an enlarged view showing the mesh of the main part P. FIG. 22A shows the product of the present invention corresponding to FIG. 2, and FIG. 22B shows the conventional product corresponding to FIG. . As shown in FIG. 23, a Rigid element was created on the end face M on the opposite end side of the analysis model, and a torque T was loaded on the central axis O of the end face M. However, since a 1 / tooth number model is used as a model, the load torque is 1 / tooth number of actual torque. As shown in FIG. 24, the analysis model has a shape in which the radial axis passing through the center of the valley portion 21 is an axis of symmetry, and all the contacts on both side surfaces W in the circumferential direction are cyclically symmetric. In addition, as shown in FIG. 25, the displacement of the normal direction is restrained in the contact surface (it shows with a dotted pattern) with the other party member of an analysis model.

The analysis result of the first principal stress σ ₁ is shown in FIG. 26, and the analysis result of the axial shear stress τ _θz is shown in FIG. 26A and 27B, FIG. 26A shows the product model of the present invention, and FIG. 26B shows the conventional product model. The reference stress τ ₀ in both figures is τ ₀ = 16 Td _o / [π (d _o ⁴ − −) with respect to the torque T, the diameter d _o of the valley of the male spline part Sm, and the inner diameter d _i of the male spline part. d _i ⁴ )].

From the above analysis results, σ _1max / τ ₀ = 3.03 in the conventional product, whereas σ _1max / τ ₀ = 2.48 in the product of the present invention. It has been found that the relaxation effect is enhanced. This is presumably because the radius of curvature of the rounded portion 21b1 in the vicinity of the end of the tooth surface 23 is larger than the radius of curvature at the corresponding portion of the conventional product in the product of the present invention. As described above, if the stress concentration coefficient α _σ with respect to the tensile stress is 2.7 or less, the stress concentration mitigating effect becomes significant, so that the present invention product of σ _1max / τ ₀ ≦ 2.7 can be used. For example, it is possible to greatly increase the fatigue strength against tensile stress compared to conventional products.

Further, in the conventional product, τ _θzmax / τ ₀ = 2.28, whereas in the product of the present invention, τ _θzmax / τ ₀ = 1.74, which is a stress relaxation effect on the shear stress in the axial direction as compared with the conventional product. It was also found to increase. As described above, if the stress concentration coefficient α _τ with respect to the shear stress is 2.1 or less, the stress concentration relaxation effect becomes significant. Therefore, the product of the present invention in which τ _θzmax / τ ₀ ≦ 2.1 is a conventional product. Compared to the above, the fatigue strength against shear stress can be greatly improved. Thus, according to the present invention, it is possible to obtain a high stress concentration relaxation effect with respect to both tensile stress and shear stress in the male spline portion Sm. Therefore, the fatigue strength of the power transmission shaft 2 can be increased.

As a result of further analysis by the present inventor, the rate of increase dR / dL of the radius of curvature of the rounded portion 21b1 shown in FIG. 4 is 0.05 ≦ dR / dL ≦ 0.60, and the inclination angle θ of the enlarged diameter portion 21b is In the range of 5 ° ≦ θ ≦ 20 °, it was found that σ _1max / τ ₀ ≦ 2.7 and τ _θzmax / τ ₀ ≦ 2.1 can be satisfied.

As shown in FIG. 14, in the conventional product, the maximum shear stress τ _θzmax occurs on the center line of the starting point of the enlarged diameter portion 21b ′. In this way, when the maximum shear stress is generated on the center line, when the power transmission shaft 2 transmits torque in both forward and reverse directions, the maximum shear stress is generated in the same part during both forward and reverse rotations, so that fatigue failure is caused accordingly. Easy to progress. On the other hand, in the product of the present invention, as shown in FIG. 2, the maximum shear stress τ _θzmax is generated in both rounded portions 21b1 on the side opposite to the axial end from the starting point of the enlarged diameter portion 21b. For this reason, the generation site of the maximum shear stress differs between the forward rotation and the reverse rotation, and therefore the progress rate of fatigue fracture can be suppressed. From the above, the product of the present invention is particularly suitable for an application in which the torque transmission direction is frequently switched, for example, an application in which the torque transmission direction is reversed in accordance with forward / backward movement of the vehicle.

  The enlarged diameter portion 21b having the rounded portion 21b1 described above is formed by forming a molded portion having a shape corresponding to the enlarged diameter portion 21b on a rolling rack used during rolling, thereby forming the teeth of the male spline portion Sm. It can be formed at the same time. Similarly, when the male spline part is cold forged by press working, the round part is formed simultaneously with the teeth of the male spline part Sm by previously forming a molding part corresponding to the shape of the enlarged diameter part 21b on the die for press working. 21b1 can be molded.

  By the above measures, high fatigue strength can be obtained for both tensile stress and shear stress in the male spline portion Sm. As described above, when the strength of the male spline portion Sm, which has been most likely to cause fatigue fracture, is increased in the shaft 2, cracks or the like are caused in the small-diameter portions 2a and 2a ′ of the shaft 2 that have been reduced in diameter to increase the operating angle of the joint. There is a risk of fatigue fracture. Therefore, in the present invention, shot peening is performed on the small diameter portions 2a and 2a 'to form a hardened surface layer, thereby increasing the fatigue strength of these portions and suppressing the occurrence of cracks. Moreover, as shown above, it became clear by verification by the present inventors that sufficient strength can be obtained by setting the generation depth of the surface hardened layer to 0.05 mm or more.

  FIG. 7 shows another embodiment of the present invention. This embodiment is an embodiment in which either one of the male spline part Sm or the female spline part Sf (male spline part Sm in the drawing) has a twist angle β with respect to the axial direction. This is an effective method for loosening between the spline portions Sm and Sf after the combination. When the torsion angle β is provided, the contact pressure between the tooth surfaces on the torque transmission side increases, and as a result, the tensile stress and the shear stress concentrated on the enlarged diameter portion also increase, resulting in a decrease in fatigue strength. From this point of view, the conventional product has been limited to a torsion angle β of substantially 15 °. On the other hand, in the present invention product, the fatigue strength of the power transmission spline can be greatly increased as described above, so that a twist angle β of 15 ° or more can be obtained, and a high backlash effect can be obtained. is there.

  In the above-described embodiment, as the male spline portion Sm, a so-called “saddle type” in which the circumferential width of the enlarged diameter portion 21b is gradually enlarged on the opposite shaft end side is illustrated, but not limited thereto. The present invention can also be applied to a so-called “boat type” male spline portion Sm in which the circumferential width of the enlarged diameter portion 21b is constant. Also in this case, the same effects as those of the present invention can be obtained by providing rounded portions on both sides in the circumferential direction of the enlarged diameter portion 21b and gradually increasing the radius of curvature of the rounded portion toward the opposite end side.

  In the drive shaft described above, the male spline portion Sm is provided integrally with or separately from the outer joint members 4 and 4 ′ as shown in the drawing in addition to the joint portion of the shaft 2 with the inner joint members 3 and 3 ′. It can also be formed on the outer periphery of the stem portion.

It is sectional drawing of a drive shaft. It is a perspective view which shows a non-shaft end side part (FIG. 1 code | symbol X part) among the male spline parts formed in the power transmission shaft. It is sectional drawing which expands and shows the code | symbol X part of FIG. (A) A figure is a top view which shows the opposite-axis end side part of a male spline part, (b) A figure is the YY sectional view taken on the line in (a) figure. 4A is a cross-sectional view taken along line AA in FIG. 4A, FIG. 4B is a cross-sectional view taken along line BB, FIG. 4C is a cross-sectional view taken along line CC, and FIG. Is a sectional view taken along the line DD. It is a circumferential direction sectional view of a male spline part. It is a top view which shows schematic structure of the male spline part which has a twist angle. It is a top view of a male spline part. It is a table | surface which shows the chemical composition of the test piece used by a fatigue test. It is a side view which shows the test piece of a rotation bending fatigue test. It is the side view to which the notch part A of the said test piece was expanded. It is a table | surface which shows the relationship between the dimension of a notch part, and a stress concentration factor. It is a side view which shows the test piece of a torsional fatigue test. It is the side view to which the notch part A of the said test piece was expanded. It is a table | surface which shows the relationship between the dimension of a notch part, and a stress concentration factor. It is a figure which shows the measurement result of the fatigue limit strength calculated | required by the rotation bending fatigue test. Is a graph showing measurement results of the torsional fatigue strength at 10 ⁵ times determined in torsional fatigue test. It is a perspective view which shows the anti-shaft end side part of the conventional male spline part. It is sectional drawing which shows the anti-shaft end side part of the conventional male spline part. It is a top view which shows the non-axis end side part of the conventional male spline part. It is a side view which shows a test piece. It is a table | surface which shows the involute spline specification of a test piece. It is a T / N diagram obtained by the double torsional fatigue test. FIG. 3 is a T / N diagram obtained in a single swing torsional fatigue test. It is a perspective view which shows a FEM analysis model. It is a perspective view which shows the analysis model which attached | subjected the mesh. (A) The figure is a perspective view of the principal part P of this invention goods which attached | subjected the mesh, The figure (b) is a perspective view of the principal part P of a conventional product similarly. It is a perspective view of the edge part by the side of the non-axis end of an analysis model. It is a front view of the analysis model seen from the arrow direction of FIG. It is a perspective view of an analysis model. It is a figure which shows the analysis result of a 1st principal stress. It is a figure which shows the analysis result of an axial direction shear stress. It is a side view which shows a test piece. It is a table | surface which shows an implementation product and a comparison product. It is a T / N diagram obtained by the double torsional fatigue test. It is a graph which shows the depth direction distribution of the residual stress of a test piece.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Constant velocity universal joint 2 Power transmission shaft 2a Small diameter part 3 Inner joint member 4 Outer joint member 5 Ball 7 Cage 21 Valley part 21a Straight part 21b Wide diameter part 21b1 Earl part 21b2 Flat part 22 Mountain part 23 Tooth surface 24 Shoulder part 25 Smooth part Sm Male spline part Sf Female spline part

Claims (5)

In the power transmission shaft provided with a male spline part on the outer periphery, and having a diameter-expanded part gradually expanding the outer diameter dimension on one end side in the axial direction of the valley part of the male spline part.
R-parts are provided on both sides in the circumferential direction of the enlarged diameter part of the male spline part, the radius of curvature of the rounded part is gradually increased toward one end in the axial direction, and the smallest diameter on one end side in the axial direction from the male spline part A power transmission shaft, characterized in that a work hardened layer is provided on the surface of each part. When the torque T is applied, the first principal stress acting on the diameter-expanded portion of the male spline portion and the maximum values of the shear stress in the axial direction are σ _1max and τ _θzmax , respectively. 2. The power transmission shaft according to claim 1, wherein the following 2) and 3) are satisfied at the same time when the reference stress τ ₀ given by the equation (1) is used for the diameter d _{o of} the portion and the inner diameter d _i of the male spline portion.
τ ₀ = 16 Td _o / [π (d _o ⁴ −d _i ⁴ )]... 1)
σ _1max ≦ 2.7τ _o ... 2)
τ _θzmax ≦ 2.1τ ₀ ... 3) When the rate of increase in the radius of curvature of the radius portion is dR / dL and the angle of the straight line connecting the inner diameter end and the outer diameter end of the axial section of the enlarged diameter portion is θ, each value is 0.05 ≦ dR / dL ≦ 0.60,
5 ° ≦ θ ≦ 20 °
The power transmission shaft according to claim 2, which is in the range of.   The power transmission shaft according to claim 1, wherein a generation depth of the work hardened layer is 0.05 mm or more.   The power transmission shaft according to claim 1, wherein the work hardened layer is formed by shot peening.

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