JP2008196592A - Power transmission shaft - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance the fatigue strength of a material itself which forms a power transmission shaft besides enhancing the fatigue strength of a male spline portion by easing stress concentration of both tensile stress and shear stress in the male spline of a power transmission shaft. <P>SOLUTION: A male spline portion Sm is formed in an outer periphery of a power transmission shaft. An enlarged diameter portion 21b gradually enlarging in the outer diameter toward the opposite shaft end side is arranged in a portion on the opposite shaft end side in a valley portion 21 of the male spline portion Sm. A sectionally arc-shaped rounded portion 21b1 is provided on both circumferential sides of the enlarged diameter portion 21b, and the curvature radius of the rounded portion 21b1 is gradually increased toward the opposite shaft end side. A material of the power transmission shaft is steel formed of predetermined components, wherein an average grain size of retained austenite in a surface layer of a hardened layer after induction hardening/annealing is 10 μm or smaller. <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, with 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 further weight reduction and strength improvement. In this type of power transmission shaft, a male spline is formed on the outer periphery, and a female spline is formed on the inner peripheral surface of the inner joint member of the constant velocity universal joint. By fitting the male spline portion on the outer periphery of the power transmission shaft and the female spline portion on the inner peripheral surface of the inner joint member, the power transmission shaft and the inner joint member are coupled so as to be able to transmit torque.

  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 end portion of the valley portion 100 on the opposite axis end side (left side in the drawing) connected to an outer peripheral surface (smooth portion) 101 via a diameter-expanded portion 102 whose outer diameter is gradually increased. It is a top view which shows a male spline part of a round-up type. 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.

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.

Even if the fatigue strength of the male spline part is increased, if the fatigue strength of the material itself forming the power transmission shaft is low, fatigue fracture such as cracks may occur in other parts. For example, as shown in FIG. 1, when the constant velocity universal joints J ₁ and J ₂ are coupled to both ends of the power transmission shaft 2, the outer joint member when the joint of the power transmission shaft 2 takes a large operating angle. In some cases, the minimum diameter portions 2a and 2b are formed in the portions that interfere with the open end portions of the four and fourteen portions, thereby increasing the operating angle of the joint. At this time, fatigue breakage such as cracks may occur in the minimum diameter portions 2a and 2b of the power transmission shaft 2 having relatively low strength.

  Therefore, in 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 relaxed to improve the fatigue strength of the male spline portion, and the fatigue of the material itself forming the power transmission shaft is increased. 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. The fatigue curve decreased with the decrease of the stress amplitude until the number of loadings until the break exceeded 10 ^5, and a clear fatigue limit phenomenon was observed in which the fracture did not occur when the stress amplitude was 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 ασ obtained in the rotating bending fatigue test and the fatigue limit strength. As shown in the figure, the fatigue strength improved as ασ decreased. However, as shown by the broken line in the figure, the slope of the fatigue curve was large when ασ ≦ 2.7, and the fatigue strength when ασ was decreased was large. It has been found that the improvement appears more pronounced.

  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

FIG. 13 shows the relationship between the stress concentration coefficient ατ obtained in the above-mentioned swing-torsion fatigue test and the fatigue strength at 10 ⁵ times. As shown in the figure, the fatigue strength improved as ατ decreased, but as shown by the broken line in the figure, the fatigue curve gradient was large when ατ ≦ 2.1, and the fatigue strength when ατ was decreased It has been found that the improvement appears more pronounced.

From the above, the fatigue strength is improved by stress concentration relaxation regardless of whether the crack initiation is governed by tensile stress or shear stress, and ασ ≦ 2.7 particularly for tensile stress, and shearing It has been found that the stress concentration relaxation effect is further enhanced when ατ ≦ 2.1 against stress. Accordingly, in the diameter-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 of axial shear stress τθ _zmax is 2.1 times or less of the reference stress τ ₀ (τθ _zmax ≦ 2.1τ ₀ ). Here, the reference stress τ ₀ is the torque T, the diameter d _{o of the} bottom of the valley of the male spline shown in FIG. 6, and the inner diameter d _{i of the} male spline (when the male spline is hollow. 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. As a result, it was found that σ _1max ≦ 2.7τ _o and τθ _zmax ≦ 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 in which the shape of the enlarged diameter portion was 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.

Next, a shaft-shaped test piece shown in FIG. 28 (b) is manufactured using steel A (steel for the present invention) and steel B (steel for conventional products) having the components shown in FIG. 28 (a). A double torsional fatigue test was performed using test pieces A-1, A-2, A-3, and B-1 obtained by performing different treatments (see 28 (c)). The both ends of the test piece have male spline portions corresponding to the invented spline specifications shown in FIG. In the middle part in the axial direction of the test piece, there is a small diameter part with a diameter D _o = 19 mm and a smooth outer peripheral surface. The test pieces A-1 to A-3 made of steel A were quenched by induction heating at a relatively low temperature of about 900 ° C. in order to make the prior austenite grain size fine. On the other hand, the test piece B-1 made of B steel was quenched by induction heating at a conventional condition, that is, at a relatively high temperature of about 1000 ° C. Conditions other than the high-frequency heating temperature were adjusted so that the hardened layer ratio was about 0.6 for both the A steel product and the B steel product. Regarding the subsequent tempering, the steel A product has three levels of 170 ° C. × 1 hour (A-1), 150 ° C. × 1 hour (A-2), and no tempering (A-3), and the steel B product has 170 levels. One level (B-1) of ° C. × 1 hour was set. This double torsional fatigue test was conducted at four levels in the range of 850 to 1300 Nm, and the obtained T / N diagrams are shown in FIGS. 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.

  From the test results shown in FIGS. 29A to 29C, among A-1 to A-3 using A steel, A-1 and A-2 having a low tempering temperature and a correspondingly high surface layer hardness are all A high intensity exceeding the target value in the load range was shown (see FIGS. 29A and 29B). In addition, A-3, which has a high tempering temperature and a correspondingly low surface layer hardness, shows a result on average approximately along the target value (see FIG. 29C). On the other hand, from the test results shown in FIG. 29 (d), B-1 shows fatigue strength superior to that of the conventional product (solid line) because the diameter-expanded portion of the male spline portion is the shape of the present invention. It became clear that the target value (dotted line) was not reached. From the above, when steel A having the components defined in the present invention is used, a target value for improving fatigue strength can be achieved. In addition, from the result of A-3, in order to demonstrate the intensity | strength more than a target value in a full load area, the surface layer hardness of HV690 or more is required in consideration of the variation in surface layer hardness shown in FIG.28 (c). I can say that.

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

  (I) In the power transmission shaft having a male spline portion provided on the outer periphery and having a diameter-expanded portion whose outer diameter is gradually increased on one axial end side of the valley portion of the male spline portion, Round portions are provided on both sides in the circumferential direction of the enlarged diameter portion, and the radius of curvature of the round portion is gradually increased toward one end side in the axial direction.

(II) When the torque T is applied, the first principal stress acting on the diameter-expanded portion of the male spline portion and the maximum value of the shear stress in the axial direction are σ _1max and τθ _zmax , respectively. When the reference stress τ ₀ given by the equation (1) is set to the diameter d _{o of} the trough portion and the inner diameter d _i of the male spline portion, the following equations (2) and (3) are satisfied simultaneously.

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

σ _1max ≦ 2.7τ _o … 2)

τθ _zmax ≦ 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 °.

  Further, from the above test results, the power transmission shaft of the present invention has C: 0.4 mass% or more and 0.5 mass% or less, Si: 0.35 mass% or more, 0.8 mass% or less, Mn: 0.5 mass%. Or more, 0.8 mass% or less, Al: 0.005 mass% or more, 0.05 mass% or less, Ti: 0.005 mass% or more and 0.05 mass% or less, Mo: 0.3 mass% or more, 0.5 mass% or less, B : 0.0005 mass% or more, 0.005 mass% or less, Cu: 0.05 mass% or more, 0.5 mass% or less, S: 0.005 mass% or more, 0.025 mass% or less, P: 0.02 mass% or less, Cr : 0.2% by mass or less, the balance being made of steel consisting of Fe and inevitable impurities, induction hardening and quenching of this steel It is preferable prior austenite average particle size in the cured layer surface after it is 10μm or less.

  From the above test results, it is preferable that the steel base material structure forming the power transmission shaft has a bainite structure of 50% or more in terms of the structure fraction. Furthermore, the hardened layer ratio, which is the ratio of the hardened layer depth after induction hardening and tempering of this steel to the axial radius, is preferably 0.55 or more.

  The reason for limiting the components of the steel forming the power transmission shaft is as follows.

(1) C: 0.4 mass% or more, 0.5 mass% or less C is an element having the greatest influence on hardenability, and increases the hardness and depth of the hardened layer after induction hardening and tempering. Effectively contributes to strength improvement. However, if the content is less than 0.4 mass%, the cured layer ratio must be considerably increased in order to ensure the required strength, in which case the occurrence of burning cracks becomes significant, and bainite Tissues are also difficult to generate. On the other hand, when the content exceeds 0.5 mass%, the grain boundary strength is lowered, and the machinability, cold forgeability, and fire cracking resistance are also lowered. From the above, the C content is desirably 0.4 mass% or more and 0.5 mass% or less, preferably 0.42 mass% or more and 0.46 mass% or less.

(2) Si: 0.35 mass% or more and 0.8 mass% or less Si is an element useful for generating a bainite structure. Moreover, it has the effect | action which refines | miniaturizes the particle size of a hardening hardening layer. Furthermore, it is an element that improves the temper softening resistance, and increases the hardness of the hardened layer after induction hardening. Furthermore, the carbide | carbonized_material production | generation is suppressed and the fall of the grain boundary strength by the carbide | carbonized_material precipitation to a grain boundary is suppressed. By these actions, the strength and the resistance to fire cracking are improved. If the Si content is less than 0.35 mass%, the bainite structure fraction is lowered, the prior austenite grain size of the hardened layer surface layer cannot be made 10 μm or less, and the hardened layer hardness is lowered. Strength decreases. On the other hand, when the Si content exceeds 0.8 mass%, the material hardness increases due to the solid solution hardening of ferrite, and the machinability, cold forgeability, and fire cracking resistance decrease. From the above, the Si content is desirably in the range of 0.35 mass% to 0.8 mass%, preferably in the range of 0.4 mass% to 0.8 mass%.

(3) Mn: 0.5 mass% or more and 0.8 mass% or less Mn is an element that improves hardenability and is indispensable for ensuring the depth of the hardened layer after induction hardening. When the content of Mn is less than 0.5 mass%, the effect of addition is small. When the content of Mn exceeds 0.8 mass%, the material hardness increases, and the rolling property and machinability deteriorate, and the resistance to fire cracking. Also decreases. From the above, the Mn content is desirably in the range of 0.5 mass% to 0.8 mass%, preferably in the range of 0.5 mass% to 0.7 mass%.

(4) Al: 0.005 mass% or more and 0.05 mass% or less Al is an element effective for deoxidation. In addition, there is an effect of suppressing austenite grain growth during induction hardening. However, when the Al content is less than 0.005 mass%, the effect is small. On the other hand, even if the Al content exceeds 0.05 mass%, the effect is saturated, and there is a disadvantage that the component cost increases. From the above, the content of Al is desirably in the range of 0.005 mass% to 0.05 mass%, preferably in the range of 0.02 mass% to 0.04 mass%.

(5) Ti: 0.005 mass% or more and 0.05 mass% or less Ti is combined with N mixed as an inevitable impurity, thereby preventing B from becoming BN and improving the hardenability of B. be able to. In order to obtain this effect, it is necessary to contain at least 0.005 mass%. However, if it contains more than 0.05 mass%, a large amount of TiN is formed. As a result, this becomes a starting point of fatigue fracture and causes a decrease in strength. From the above, the Ti content is desirably in the range of 0.005 mass% to 0.05 mass%, preferably in the range of 0.015 mass% to 0.03 mass%. Furthermore, it is preferable to satisfy Ti (mass%) / N (mass%) ≧ 3.42 from the viewpoint of securely fixing N and sufficiently exhibiting the effect of improving hardenability by B. .

(6) Mo: 0.3 mass% or more and 0.5 mass% or less Mo has an action of promoting the formation of a bainite structure. Moreover, there exists an effect | action which refines | miniaturizes the prior austenite grain size of a hardening hardening layer by suppressing the austenite grain growth at the time of induction hardening heating. Furthermore, since it is an element useful for improving hardenability, it is used for adjusting hardenability. If the Mo content is less than 0.3 mass%, the prior austenite grain size of the hardened layer cannot be reduced to 10 μm or less, regardless of how the production conditions and quenching conditions are adjusted. On the other hand, when Mo is contained exceeding 0.5 mass%, the hardness of the material is remarkably increased, resulting in a decrease in workability and a decrease in fire cracking resistance. From the above, the Mo content is desirably in the range of 0.30 mass% to 0.5 mass%, preferably in the range of 0.35 mass% to 0.45 mass%.

(7) B: 0.0005 mass% or more and 0.005 mass% or less B has an effect of promoting the formation of a bainite structure or a martensite structure. Further, B has the effect of improving the hardenability by adding a small amount and increasing the quenching depth to improve the strength. Further, B has an action of preferentially segregating at the grain boundary and reducing the concentration of P segregating at the grain boundary and improving the grain boundary strength. Also, the crack resistance is improved by strengthening the grain boundaries. If the content of B is less than 0.0005 mass%, the effect of addition is small. On the other hand, if the content of B exceeds 0.005 mass%, the effect is saturated and the cost of components is increased. From the above, the B content is desirably 0.0005 mass% or more and 0.005 mass% or less, and preferably 0.001 mass% or more and 0.003 mass% or less.

(8) Cu: 0.05 mass% or more and 0.5 mass% or less Cu is effective in improving hardenability, and has the effect of solid-dissolving in ferrite and improving the strength by solid solution strengthening. Moreover, by suppressing the production | generation of a carbide | carbonized_material, the fall of the grain boundary intensity | strength by a carbide | carbonized_material is suppressed, and a strength improvement is carried out. For that purpose, it is necessary to add 0.05 mass% or more. However, if the Cu content exceeds 0.5 mass%, cracks occur during hot working, and the fire cracking resistance decreases. As described above, the Cu content is desirably in the range of 0.05 mass% to 0.5 mass%, preferably in the range of 0.05 mass% to 0.3 mass%.

(9) S: 0.005 mass% or more, 0.025 mass% or less S is a useful element that forms MnS in steel and improves the machinability, and is contained by 0.005 mass% or more, but 0.025 mass%. If the content exceeds V, the amount of MnS increases and the strength decreases. Therefore, the content of S is desirably in the range of 0.005 mass% to 0.025 mass%.

(10) P: 0.02 mass% or less P segregates at the prior austenite grain boundaries and lowers the grain boundary strength. In addition, there is a harmful effect that promotes burning cracks. Therefore, it is desirable to reduce the P content as much as possible, but 0.02 mass% is allowed.

(11) Cr: 0.2 mass% or less Cr stabilizes carbides, promotes the formation of residual carbides, and lowers the grain boundary strength. Furthermore, Cr promotes fire cracking. Therefore, the Cr content is desirably reduced as much as possible, but is allowed up to 0.2 mass%, preferably 0.05 mass% or less.

  Further, the steel base material structure forming the power transmission shaft, that is, the structure before quenching has a bainite structure, and the structure fraction is 50% or more for the following reasons. The bainite structure is a structure in which carbides are finely dispersed as compared with the ferrite-pearlite structure, and the total interfacial area between ferrite and carbides that become nucleation sites of austenite during quenching heating increases. Therefore, since austenite becomes fine, the prior austenite grain size of the hardened layer after induction hardening can be made fine. The grain boundary strength is increased by the refinement of the prior austenite grain size, and the strength and resistance to fire cracking can be improved. Such an effect can be obtained when the bainite structure has a structure fraction of 50% or more, but preferably the bainite structure has a structure fraction of 60% or more.

  Further, if the prior austenite grain size of the hardened layer surface layer after induction hardening of the steel forming the power transmission shaft exceeds 10 μm, sufficient grain boundary strength cannot be obtained, so this prior austenite grain size is 10 μm or less, preferably It is desirable that the thickness is 8 μm or less. In addition, a hardened layer surface layer shall point to the part to the depth of 500 micrometers from the surface.

  The reason why the hardened layer ratio, which is the ratio of the hardened layer depth to the shaft radius, is 0.55 or more in the hardened layer after induction hardening and tempering of the minimum smooth portion of the steel forming the power transmission shaft, This is to keep the static torsional strength and torsional fatigue strength stable and high. When the hardened layer ratio is less than 0.55, the inside undergoes plastic deformation when a torsional load is applied, resulting in an increase in stress on the surface and a decrease in strength. In addition, the hardened layer depth here shall mean the depth which has the hardness of HV450 or more (JIS conformity).

  Moreover, from the above test results, in order to achieve the target value for improving the fatigue strength of the steel forming the power transmission shaft, that is, to improve the strength by 15% compared to the conventional product, the hardened layer after induction hardening and tempering of the steel. The hardness of the surface layer is desirably HV690 or more.

  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 relaxed to increase the fatigue strength of the male spline portion, and the power transmission shaft is formed. The fatigue strength of the material itself can be increased.

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

FIG. 1 shows a drive shaft 1 of a vehicle incorporating a power transmission shaft 2 according to the present invention. The drive shaft 1 in the illustrated example includes a power transmission shaft 2 and a fixed type constant velocity universal joint J ₁ that is attached to the end of the power transmission shaft 2 on the outboard side (the outside in the vehicle width direction when mounted on the vehicle). And a sliding-type constant velocity universal joint (tripod type constant-velocity universal joint) J ₂ mounted on the end portion on the inboard side of the power transmission shaft 2.

The fixed type constant velocity universal joint J 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 the inner joint member 3 and the outer joint member. A ball 5 that transmits torque to and from 4 is a main component. The balls 5 are arranged on a ball track formed by the track grooves 3a formed on the outer periphery of the inner joint member 3 and the track grooves 4a formed on the inner periphery of the outer joint member 4, and are arranged at equidistant positions in the circumferential direction. The plurality of balls 5 are held by the cage 6.

The tripod type constant velocity universal joint J ₂ includes an inner joint member 13 coupled to the power transmission shaft 2, an outer joint member 14 disposed on the outer diameter side of the inner joint member 13, and the inner joint member 13 and the outer joint member. A roller 15 as a torque transmission member that transmits torque to and from 14 is a main component. Leg shafts 13 a are projected from three locations in the circumferential direction of the inner joint member 13. A track groove 14a extending in the axial direction is formed at a position of the inner periphery of the outer joint member 14 in the circumferential direction, and the roller 15 rolls along the track groove 14a.

  The power transmission shaft 2 is formed of steel having the following components. That is, C: 0.4 mass% or more, 0.5 mass% or less, Si: 0.35 mass% or more, 0.8 mass% or less, Mn: 0.5 mass% or more, 0.8 mass% or less, Al: 0.005 mass% Or more, 0.05 mass% or less, Ti: 0.005 mass% or more and 0.05 mass% or less, Mo: 0.3 mass% or more, 0.5 mass% or less, B: 0.0005 mass% or more, 0.005 mass% or less, Cu : 0.05 mass% or more, 0.5 mass% or less, S: 0.005 mass% or more, 0.025 mass% or less, P: 0.02 mass% or less, Cr: formed with steel containing 0.2 mass% or less, For example, it is made of steel having the components of steel A shown in FIG. The balance of the above components of this steel consists of Fe and inevitable impurities. This steel has an old austenite average grain size of 10 μm or less in the surface layer of the hardened layer after induction hardening and tempering, and the base metal structure has a bainite structure of 50% or more in terms of the structure fraction. -Hardened layer ratio which is ratio with respect to axial radius of hardened layer depth after tempering is 0.55 or more. Moreover, the hardness of the hardened layer surface layer after induction hardening and tempering of this steel is HV690 or more.

  The power transmission shaft 2 is formed solid. Note that the power transmission shaft 2 may be formed hollow, and in this case, the weight can be reduced.

  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 members 3 and 13 as shown in FIG. 3, the power transmission shaft 2 and the inner joint members 3 and 13 are connected. It is connected so that torque can be transmitted. The inner joint members 3 and 13 have the inner diameter end on the opposite shaft end side (left side in FIG. 3) abutted against the shoulder 24 on the outer periphery of the power transmission shaft 2 and the inner diameter on the shaft end side (right side in FIG. 3). For example, the end portion is locked with a retaining ring (not shown) to be positioned and fixed in the axial direction with respect to the power transmission shaft 2.

Minimum diameter portions 2a and 2b are formed in a region on the opposite shaft end side from the male spline portion Sm formed on the outer periphery of both shaft ends of the power transmission shaft 2, that is, a region between the male spline portions Sm to which a torque load is applied. (See FIG. 1). The minimum diameter portions 2a and 2b are formed in the shape of a cylindrical surface having a smooth outer peripheral surface and the smallest diameter in the region between the male spline portions Sm at both shaft ends. The minimum diameter portions 2a and 2b are provided at portions that interfere with the opening end portions of the outer joint members 4 and 14 when the constant velocity universal joints J ₁ and J ₂ provided at the shaft ends are bent at the maximum. Thereby, interference with the power transmission shaft 2 and the outer joint members 4 and 14 can be delayed, and the operating angle of the joint can be increased.

  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.

  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 dotted 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 curvature radius R _A of the rounded portion connecting the straight portion 21a of the valley portion 21 and the tooth surface 23 is constant until reaching the boundary portion with the enlarged diameter portion 21b. As shown in FIGS. 5b to 5d, in the enlarged diameter portion 21b, the radius of curvature of the rounded portion 21b1 is larger than the radius of curvature R _A of the boundary portion and gradually increases toward the opposite shaft end side (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 part Sm (product of the present invention) shown in FIG. 2 and the male spline part 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. The maximum value τθ _zmax was obtained, and a value obtained by dividing these 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 and 21 ′ of the male spline portions Sm and 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 at the main portion P (including the male spline portions Sm and Sm ′) (minimum element length was 0.05 mm), and 0.5 mm except at 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 direction axis passing through the center of the valley portion 21 is an axis of symmetry, and all the nodes on both side surfaces W in the circumferential direction are periodically 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 τθ _z 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, in the conventional product, σ _1max / τ ₀ = 3.03, whereas in the product of the present invention, σ _1max / τ ₀ = 2.48, which is higher than the conventional product in _terms of stress concentration against tensile stress. 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 ασ with respect to the tensile stress is 2.7 or less, the stress concentration mitigating effect becomes significant. Therefore, if the product of the present invention _satisfies σ _1max / τ ₀ ≦ 2.7, Compared to conventional products, the fatigue strength against tensile stress can be greatly increased.

Further, in the conventional product, τθ _zmax / τ ₀ = 2.28, whereas in the product of the present invention, τθ _zmax / τ ₀ = 1.74, which is a stress relaxation effect on the axial shear stress compared to the conventional product. It was also found to increase. As described above, if the stress concentration coefficient ατ 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 τθ _zmax / τ ₀ ≦ 2.1 is compared with the conventional product. In comparison, 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 round 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 τθ _zmax / τ ₀ ≦ 2.1 can be satisfied.

As shown in FIG. 14, in the conventional product, the maximum shear stress τθ _zmax 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 τθ _zmax is generated at 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.

  With the above measures, the male spline portion Sm can obtain high fatigue strength against both tensile stress and shear stress, and can increase the fatigue strength of the material itself forming the power transmission shaft 2.

  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.

It is a partial step view of the power transmission shaft according to the present invention. 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. It is a figure which shows the measurement result of the torsional fatigue strength in 10 < ⁵ > time calculated | required by the 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 component table | surface of A steel and B steel. It is a side view which shows a test piece. It is a table | surface which shows the specification and material of a test piece. It is a T / N diagram obtained by the double torsional fatigue test.

Explanation of symbols

1 drive shaft 2 power transmission shaft 2a, 2b minimum diameter portion J _1, J ₂ constant velocity universal joint 3, 13 inner joint member 4,14 outer joint member 21 troughs 21a straight portion 21b enlarged diameter portion 21b1 rounded portion 21b2 flat portion 22 Mountain part 23 Tooth surface 24 Shoulder part 25 Smooth part Sm Male spline part Sf Female spline part

Claims (6)

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.
Providing rounded portions on both sides in the circumferential direction of the enlarged portion of the male spline portion, gradually increasing the radius of curvature of the rounded portion toward one end in the axial direction; and
C: 0.4 mass% or more, 0.5 mass% or less,
Si: 0.35 mass% or more, 0.8 mass% or less,
Mn: 0.5 mass% or more, 0.8 mass% or less,
Al: 0.005 mass% or more, 0.05 mass% or less,
Ti: 0.005 mass% or more and 0.05 mass% or less,
Mo: 0.3 mass% or more, 0.5 mass% or less,
B: 0.0005 mass% or more, 0.005 mass% or less,
Cu: 0.05 mass% or more, 0.5 mass% or less,
S: 0.005 mass% or more, 0.025 mass% or less,
P: 0.02 mass% or less,
Cr: 0.2 mass% or less is contained, and the balance is formed of steel composed of Fe and inevitable impurities, and the prior austenite average grain size in the hardened layer surface layer after induction hardening and tempering of this steel is 10 μm or less. A power transmission shaft characterized by that. When the torque T is applied, the first principal stress acting on the enlarged diameter portion of the male spline portion and the maximum value of the shear stress in the axial direction are σ _1max and τθ _zmax , respectively. 2. The power transmission shaft according to claim 1, wherein when the reference stress τ ₀ given by the equation (1) is set to the diameter d _{o of} the portion and the inner diameter d _i of the male spline portion, the following equations (2) and (3) are satisfied simultaneously.
τ ₀ = 16 Td _o / [π (d _o ⁴ −d _i ⁴ )]... 1)
σ _1max ≦ 2.7τ _o … 2)
τθ _zmax ≦ 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 any one of claims 1 to 3, wherein the base material structure of the steel has a bainite structure having a structure fraction of 50% or more.   The power transmission shaft according to any one of claims 1 to 4, wherein a hardened layer ratio, which is a ratio of a hardened layer depth of the steel after induction hardening and tempering, to an axial radius is 0.55 or more.   The power transmission shaft according to any one of claims 1 to 5, wherein the hardness of the surface layer of the hardened layer after induction hardening and tempering of the steel is HV690 or more.

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