Electric power steering device, pinion shaft, and method for manufacturing pinion shaft

Abstract

This pinion shaft comprises: a pinion part having an outer periphery on which pinion teeth are provided; a bearing holding part which has an outer peripheral surface of which the distance from a center axis is smaller than the distance between the teeth bottom of the pinion teeth and the center axis, and holds a bearing on the outer peripheral surface; and a male screw part having a male screw. The distance between the ridge section of the thread of the male screw and the center axis is smaller than the distance between the outer peripheral surface of the bearing holding part and the center axis. The hardness of a pinion part-side first region on the outer peripheral surface of the bearing holding part is greater than the hardness of a male-screw-side second region on the outer peripheral surface of the bearing holding part.

Description

[Technical Field]

[0001] The present disclosure relates to an electric power steering device, a pinion shaft, and a method for manufacturing a pinion shaft. [Background technology]

[0002] The electric power steering device of Patent Document 1 includes a pinion shaft, a worm wheel, a worm shaft, a rack shaft, and a housing. One axial portion of the central axis of the pinion shaft is rotatably supported relative to the housing via a first bearing, and the other axial portion is rotatably supported relative to the housing via a second bearing. Pinion teeth are provided on the pinion shaft between the portion where the first bearing is attached and the portion where the second bearing is attached, and the pinion teeth mesh with rack teeth on the rack shaft. The rack shaft is pressed against the pinion teeth.

[0003] A worm wheel is attached to the pinion shaft on the other axial side of the portion where the second bearing is attached, and the worm wheel meshes with the worm shaft. The worm shaft is rotatably attached to the output shaft of the motor. In addition, to reduce the weight of the pinion shaft, the outer diameter of the portion where the first bearing is attached is smaller than the outer diameter of the portion where the second bearing is attached. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-023796 Summary of the Invention [Problem to be solved by the invention]

[0005] As described above, a pushing force from the rack shaft is applied to the axial center of the pinion shaft. Support forces that resist the force from the rack shaft are applied to one and the other axial sides of the pinion shaft via the first bearing and the second bearing, causing a bending moment to act on both sides of the pinion shaft. Here, the outer diameter of the pinion shaft's portion where the first bearing is attached is smaller than the outer diameter of the portion where the second bearing is attached. Therefore, the rigidity of the portion where the first bearing is attached is lower than the rigidity of the portion where the second bearing is attached. Therefore, the bearing attachment portion where the first bearing is attached may be damaged over long-term use, and so it is necessary to increase the rigidity of this bearing attachment portion.

[0006] The present disclosure has been made in consideration of the above-mentioned problems, and has an object to provide an electric power steering device, a pinion shaft, and a method for manufacturing a pinion shaft in which the rigidity of a bearing mounting portion of the pinion shaft is higher. [Means for solving the problem]

[0007] In order to achieve the above object, a pinion shaft according to one embodiment is a pinion shaft extending in the axial direction of a central axis, and comprising: a pinion portion having pinion teeth on its outer periphery; a bearing holder portion located on one side of the pinion portion in the axial direction and having an outer periphery surface that is closer to the central axis than the distance between the bottoms of the pinion teeth and the central axis, and a bearing is held on the outer periphery surface; and a male thread portion located on one side of the bearing holder portion in the axial direction and having a male thread, wherein the distance between the crest of the thread of the male thread and the central axis is smaller than the distance between the outer periphery of the bearing holder portion and the central axis, and the hardness of a first region of the outer periphery surface of the bearing holder portion that is closer to the pinion portion is greater than the hardness of a second region of the outer periphery surface of the bearing holder portion that is closer to the male thread portion.

[0008] As mentioned above, in the pinion shaft of Patent Document 1, the bearing mounting portion to which the first bearing is attached may be damaged by long-term use, so it is necessary to increase the rigidity of the bearing mounting portion.

[0009] In the pinion shaft of the present disclosure, the distance between the outer peripheral surface of the bearing holder and the central axis is smaller than the distance between the bottom of the pinion teeth and the central axis. Furthermore, the outer diameter of the male thread is smaller than the outer diameter of the bearing holder. Therefore, if the surfaces of the pinion portion, bearing holder, and male thread are all the same in hardness, the rigidity decreases in the order of the pinion portion, the bearing holder, and the male thread. However, by subjecting the bearing holder to, for example, carburizing and quenching, the hardness of the first region of the bearing holder is greater than the hardness of the second region. Therefore, the surface layer of the bearing holder is harder than if the first region and the second region had the same hardness, and the rigidity of the bearing holder is also greater.

[0010] In a preferred aspect, the bearing holder further comprises: a groove portion located between the pinion portion and the bearing holder and having a bottom surface located more inward than the outer peripheral surface of the bearing holder; and a tapered portion located at one end of the second region of the bearing holder in the axial direction and having a diameter that decreases toward the one side in the axial direction, wherein the hardness of the bottom surface of the groove portion is greater than the hardness of the surface of the tapered portion.

[0011] According to this, because the pinion shaft has a groove, bending moments acting on one axial side of the pinion shaft tend to concentrate on the groove, making the groove more susceptible to damage. However, because the hardness of the bottom surface of the groove is greater than the hardness of the surface of the tapered portion, the groove has high rigidity and is less susceptible to damage even when bending moments are concentrated.

[0012] Furthermore, the fact that the bottom surface of the groove is harder than the surface of the tapered portion means that the surface of the tapered portion is softer than the bottom surface of the groove. Therefore, when the tapered portion is formed by cutting, for example, the workability of the cutting process is improved.

[0013] In a preferred embodiment, the Vickers hardness of the first region of the bearing holder is 450 or more, and the Vickers hardness of the second region is 400 or less, so that the hardness of the surface layer of the bearing holder is greater than when the first region and the second region have the same hardness, and the rigidity of the bearing holder is also increased. Furthermore, when the second region is subjected to cutting, for example, the workability of the cutting is improved.

[0014] In a preferred embodiment, the hardness of the first region of the bearing holder is greater than the hardness of the surface of the thread of the male thread portion.

[0015] In other words, the hardness of the surface of the thread in the male thread portion is lower than the hardness of the first region, which improves the workability of the cutting process when forming the thread in the male thread portion.

[0016] An electric power steering device according to one aspect includes: a pinion shaft extending in the axial direction of a central axis and provided with a pinion portion having pinion teeth; a first bearing attached to a first bearing holder provided on one side of the pinion shaft in the axial direction relative to the pinion portion and rotatably supporting the pinion shaft in a housing; a worm wheel attached to the other side of the pinion shaft in the axial direction relative to the pinion portion and provided with a wheel tooth portion on an outer periphery; and a second bearing holder provided between the pinion portion on the pinion shaft and the worm wheel. a second bearing attached to a first bearing holder and rotatably supporting the pinion shaft on the housing, and a rack guide having rack teeth that mesh with the pinion teeth and that is pressed toward the pinion teeth, wherein the outer diameter of the first bearing holder is smaller than the outer diameter of the second bearing holder, the hardness of a first region on the other side in the axial direction of the outer peripheral surface of the first bearing holder is greater than the hardness of a second region on one side in the axial direction of the outer peripheral surface of the first bearing holder, and a tapered portion having a diameter that decreases toward the one side in the axial direction is provided at an end of the second region.

[0017] As described above, a pushing force from the rack shaft is applied to the axial center of the pinion shaft. Because a support force that resists the force from the rack shaft is applied to both axial sides of the pinion shaft, a bending moment acts on both axial sides of the pinion shaft. Here, the outer diameter of the first bearing holder on the pinion shaft is smaller than the outer diameter of the second bearing holder. However, the first bearing holder is, for example, carburized and quenched, so that the hardness of the first region of the first bearing holder is greater than the hardness of the second region. Therefore, the hardness of the surface layer of the first bearing holder is greater than when the first region and the second region have the same hardness, and the rigidity of the first bearing holder is also increased.

[0018] A method of manufacturing a pinion shaft according to one aspect is a method of manufacturing a pinion shaft extending in the axial direction of a central axis and having a male thread portion and a bearing retaining portion at an axial end portion, the method including: a blank material preparation step of preparing a columnar blank material having at least pinion teeth; an anti-carburization agent application step of applying an anti-carburization agent to the axial end portion of the blank material after the blank material preparation step; a carburization and quenching step of carburizing and quenching the blank material after the carburization and quenching step; a cutting step of cutting the axial end portion of the blank material to which the anti-carburization agent has been applied, to form the male thread portion and the bearing retaining portion; and a polishing step of polishing the bearing retaining portion after the cutting step. Another aspect of the method for manufacturing a pinion shaft is a method for manufacturing a pinion shaft that extends in the axial direction of a central axis and has a male thread portion and a bearing retaining portion at an axial end thereof, the method including: a blank material preparation step of preparing a columnar blank material having at least pinion teeth; an anti-carburization agent application step of applying an anti-carburization agent to the axial end of the blank material after the blank material preparation step; a carburization and quenching step of carburizing and quenching the blank material after the anti-carburization agent application step; a polishing step of grinding the bearing retaining portion after the carburizing and quenching step; and a cutting step of cutting the axial end of the blank material to which the anti-carburization agent has been applied, to form the male thread portion and the bearing retaining portion after the polishing step.

[0019] In this way, the areas of the blank where the male thread and bearing retaining portion will be formed are coated with a carburization inhibitor in advance, and then carburized and quenched. As a result, the surface layer of the areas coated with the carburization inhibitor is softer than that of areas not coated with the carburization inhibitor, making it easier to form the male thread and bearing retaining portion by cutting, for example, and improving the workability of the cutting process. In addition, the surface layer of the areas coated with the carburization inhibitor is harder, improving rigidity. [Effects of the Invention]

[0020] According to the present disclosure, it is possible to provide an electric power steering device, a pinion shaft, and a method for manufacturing a pinion shaft in which the bearing mounting portion of the pinion shaft has higher rigidity. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a schematic cross-sectional view of an electric power steering device according to an embodiment. [Figure 2A] FIG. 2A is a side view of a pinion shaft according to an embodiment. [Figure 2B] FIG. 2B is a side view of a portion of a pinion shaft according to a comparative example. [Figure 3] FIG. 3 is a schematic diagram showing an enlarged portion of FIG. 2A. [Figure 4] FIG. 4 is a schematic diagram of carburizing and quenching the surface of a metal, where (a) shows the surface of the metal that has been subjected to anti-carburizing treatment, (b) shows the surface of the metal that has been subjected to anti-carburizing treatment and then carburizing, (c) shows the state in which carbon has penetrated into the surface of the metal due to the carburizing treatment, and (d) shows the state in which the metal has been quenched. [Figure 5] FIG. 5 is a schematic diagram showing a step of preparing a blank in the manufacturing process of a pinion shaft. [Figure 6] FIG. 6 is a schematic diagram showing a state in which an anti-carburization agent is applied to the tip end of a blank material in the manufacturing process of a pinion shaft. [Figure 7] FIG. 7 is a schematic diagram showing a state in which the tip end of a blank has been machined in the manufacturing process of a pinion shaft. [Figure 8] FIG. 8 is a schematic diagram showing a state in which a part of the cutting-processed portion has been polished in the manufacturing process of the pinion shaft. [Figure 9] FIG. 9 is a schematic diagram showing forces and moments acting on the pinion shaft. [Figure 10] FIG. 10 is a schematic diagram showing a steering device equipped with an electric power steering device according to this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0022] Modes (embodiments) for carrying out the present invention will be described in detail with reference to the drawings. The present invention is not limited to the contents described in the following embodiments. Furthermore, the components described below include those that can be easily imagined by a person skilled in the art and those that are substantially identical. Furthermore, the components described below can be combined as appropriate. Furthermore, parts with the same structure are given the same reference numerals and their description will be omitted. Note that in this embodiment, the Z direction is the axial direction of the central axis AX1, the Z2 side is one side of the axial direction, and the Z1 side is the other side of the axial direction. The Y direction is perpendicular to (intersects with) the Z direction.

[0023] [Embodiment] An electric power steering device according to an embodiment will be described below. Fig. 1 is a schematic cross-sectional view of an electric power steering device according to an embodiment.

[0024] As shown in Fig. 1, the electric power steering device 100 includes a housing 200, a pinion shaft 3, a worm wheel 23, a worm shaft 27, and a rack shaft 15. The electric power steering device 100 according to this embodiment is, for example, a dual-pinion type steering device, and although Fig. 1 shows the assist side of the steering side and the assist side, it may also be applied to the steering side. The electric power steering device according to the present invention is not limited to the dual-pinion type, and may also be applied to a single-pinion type.

[0025] The housing 200 accommodates the pinion shaft 3, the worm wheel 23, the worm shaft 27, and the rack shaft 15. The housing 200 includes a first housing 4 and a second housing 5. The first housing 4 is adjacent to the second housing 5 on the Z2 side. The first housing 4 is fixed to the second housing 5 via bolts.

[0026] The first housing 4 has a main body 41, a flange 42, a tip end 43, and a rack guide accommodating portion 412. The main body 41 and the tip end 43 are cylindrical members extending in the Z direction. The flange 42 is provided at the end of the main body 41 on the Z1 side. An annular protrusion 401 is provided on the radially inner side of the flange 42. The annular protrusion 401 protrudes toward the Z1 side. The second housing 5 is spigot-joined to the annular protrusion 401. Specifically, the side surface 51 of the second housing 5 is fitted into the outer peripheral surface 401a of the annular protrusion 401, and the outer peripheral surface 401a and the side surface 51 abut against each other. An inner peripheral surface 401b and a bottom surface 401c are provided on the inner peripheral side of the annular protrusion 401. The inner peripheral surface 401b is a cylindrical surface extending circumferentially around the central axis AX1. Bottom surface 401c is a plane that is perpendicular to (intersects with) central axis AX1. Second bearing 242 is provided on the inner peripheral side of annular convex portion 401. Specifically, an outer ring of second bearing 242 abuts against inner peripheral surface 401b and bottom surface 401c, and an inner ring abuts against the outer peripheral surface of pinion shaft 3. As a result, the Z1 side of pinion shaft 3 is rotatably supported by flange 42 via second bearing 242.

[0027] Furthermore, a first bearing 241 is provided on an inner peripheral surface 43a of the tip portion 43. As a result, the Z2 side of the pinion shaft 3 is rotatably supported on the tip portion 43 via the first bearing 241. A cap 26 is provided on the Z2 side of the first bearing 241. Specifically, a male thread is formed on the outer periphery of the cap 26, and a female thread is formed on the inner periphery of the tip portion 43, and the male thread of the cap 26 meshes with the female thread of the tip portion 43.

[0028] The rack guide accommodating portion 412 protrudes from the side surface of the main body portion 41 toward the Y2 side. A rack guide 153 and a spring 154 are accommodated inside the rack guide accommodating portion 412. A sealing member 155 is fitted into the opening on the Y2 side of the rack guide accommodating portion 412. Because the rack guide 153 and the spring 154 are accommodated in a compressed state, when the spring 154 presses the rack guide 153 toward the Y1 side, the rack guide 153 presses the rack shaft 15 toward the Y1 side, and the rack shaft 15 is pressed against the pinion shaft 3. This maintains the meshing between the rack teeth 152 of the rack shaft 15 and the pinion teeth 321 of the pinion shaft 3.

[0029] The second housing 5 includes a side surface portion 51, a top surface portion 52, and a flange 53. The second housing 5 accommodates the worm wheel 23 and the worm shaft 27. The side surface portion 51 extends from the outer peripheral end of the top surface portion 52 toward the Z2 side. The flange 53 is fixed to the flange 42 via bolts.

[0030] The worm wheel 23 includes a core metal portion 231 and wheel teeth 232. The wheel teeth 232 mesh with shaft teeth 271 of the worm shaft 27. The worm shaft 27 has a central axis AX2. The worm shaft 27 is rotatably attached to the output shaft of a motor (not shown). The worm shaft 27 rotates around the central axis AX2.

[0031] Next, the pinion shaft will be described in detail. Fig. 2A is a side view of the pinion shaft according to the embodiment. Fig. 3 is a schematic view showing an enlarged portion of Fig. 2A.

[0032] 2A, pinion shaft 3 has an end portion 31, a pinion portion 32, a first groove portion (groove portion) 33, a first bearing holder portion (bearing holder portion) 34, a second groove portion 35, and a male thread portion 36. Pinion shaft 3 has a center axis AX1.

[0033] The end 31 is located at the end on the Z1 side of the pinion shaft 3. The end 31 has a flange 311 and a worm wheel holding portion 312. As described with reference to FIG. 1, the worm wheel 23 is attached to the outer circumferential side of the worm wheel holding portion 312. The flange 311 protrudes outward beyond the outer circumferential surface of the worm wheel holding portion 312. As shown in FIG. 1, the core metal portion 231 of the worm wheel 23 abuts against the flange 311.

[0034] The pinion portion 32 is adjacent to the end portion 31 on the Z2 side. A second bearing holder 323 is provided at the end portion on the Z1 side of the pinion portion 32. As described with reference to FIG. 1, the second bearing 242 is attached to the outer periphery of the second bearing holder 323. The pinion portion 32 is provided with pinion teeth 321. As described above, the pinion teeth 321 mesh with the rack teeth 152 of the rack shaft 15 shown in FIG. 1. A protrusion 322 is provided at the end portion on the Z2 side of the pinion portion 32. The outer circumferential surface of the protrusion 322 is located outer circumferentially of the cutting edge of the pinion teeth 321. The protrusion 322 extends annularly in the circumferential direction around the central axis AX1.

[0035] The first groove portion 33 is adjacent to the pinion portion 32 on the Z2 side. A bottom surface 331 of the first groove portion 33 extends annularly in the circumferential direction around the central axis AX1. As shown in FIG. 3, the bottom surface 331 of the first groove portion 33 has a diameter smaller than that of the outer peripheral surface of the protruding portion 322 and also smaller than that of the outer peripheral surface 344 of the first bearing holder 34. The cross-sectional shape of the bottom surface 331 is a concave shape that is recessed toward the inner periphery. Specifically, as shown in FIG. 3, the cross-sectional shape of the bottom surface 331 has an R-shaped portion 331a, a straight portion 331b, and a tapered portion 331c.

[0036] As shown in FIG. 2A, the first bearing holder 34 has an outer peripheral surface 344 and a tapered portion 341. The first bearing 241 shown in FIG. 1 is attached to the first bearing holder 34. The outer peripheral surface 344 extends annularly in the circumferential direction around the central axis AX1. The distance between the outer peripheral surface 344 of the first bearing holder 34 and the central axis AX1 is greater than the distance between the tooth bottoms of the pinion teeth 321 and the central axis AX1. Furthermore, the distance between the outer peripheral surface of the second bearing holder 323 and the central axis AX1 is greater than the distance between the outer peripheral surface 344 of the first bearing holder 34 and the central axis AX1. As shown in FIG. 3, the outer peripheral surface 344 is divided into two portions across a boundary line 345 that passes through the axial center of the central axis AX1 and is perpendicular to the central axis AX1. Specifically, the outer peripheral surface 344 has a first region 342 located on the Z1 side of the boundary line 345, and a second region 343 located on the Z2 side of the boundary line 345. In other words, the first region 342 is a portion of the outer peripheral surface 344 closer to the pinion portion 32, and the second region 343 is a portion of the outer peripheral surface 344 closer to the male thread portion 36. The first region 342 is harder than the second region 343.

[0037] The tapered portion 341 is also referred to as a guide portion. As shown in Fig. 3, the diameter of the tapered portion 341 decreases toward the Z2 side. The tapered portion 341 is provided to facilitate smooth insertion from the tapered portion (guide portion) 341 when inserting the first bearing 241 shown in Fig. 1 into the first bearing holder 34 from the Z2 side toward the Z1 side.

[0038] The second groove portion 35 is adjacent to the first bearing holder 34 on the Z2 side. A bottom surface 351 of the second groove portion 35 extends annularly in the circumferential direction around the central axis AX1. As shown in FIG. 3, the bottom surface 351 of the second groove portion 35 has a smaller diameter than the Z2-side end of the tapered portion 341, and is located more inward than the tip of the male thread 361. The cross-sectional shape of the bottom surface 351 is concave.

[0039] The male thread portion 36 is adjacent to the second groove portion 35 on the Z2 side. A male thread 361 is formed on the outer periphery of the male thread portion 36. The tip of the male thread 361 is located outer periphery of the bottom surface 351 and inner periphery of the end of the tapered portion 341 on the Z2 side. The distance between the crest of the thread of the male thread 361 and the central axis AX1 is smaller than the distance between the outer periphery 344 of the first bearing holder 34 and the central axis AX1. A nut 25 shown in FIG. 1 is fastened to the male thread portion 36. Specifically, a female thread is provided on the inner periphery of the nut 25, and the female thread engages with the male thread 361. The nut 25 presses the inner ring of the first bearing 241 toward the Z1 side. As a result, the inner ring of the first bearing 241 is sandwiched between the protrusion 322 (see FIG. 2A) and the nut 25 and held in the Z direction.

[0040] Next, a pinion shaft 3A according to a comparative example will be described. Fig. 2B is a side view of a portion of the pinion shaft according to the comparative example.

[0041] As described above, the pinion shaft 3 according to the embodiment includes the first groove portion 33 and the second groove portion 35, but the pinion shaft 3A according to the comparative example does not include the first groove portion 33 and the second groove portion 35. That is, as shown in FIG. 2B , in the pinion shaft 3A according to the comparative example, a first connecting portion 33A is provided between the protruding portion 322 of the pinion portion 32 and the outer peripheral surface 344 of the first bearing holder 34. Furthermore, a second connecting portion 35A is provided between the outer peripheral surface 344 of the first bearing holder 34 and the male thread portion 36. The first connecting portion 33A extends annularly in the circumferential direction around the center axis AX1. The outer peripheral surface 33Aa of the first connecting portion 33A connects the outer peripheral surface 344 of the first bearing holder 34 and the outer peripheral surface of the protruding portion 322. In a cross section including the central axis AX1, the outer peripheral surface 33Aa has a curved shape that is concave toward the central axis AX1. The second connecting portion 35A extends annularly in the circumferential direction about the central axis AX1. The outer peripheral surface 35Aa of the second connecting portion 35A connects the outer peripheral surface 344 of the first bearing holder 34 and the tip of the male thread 361 of the male thread portion 36. In a cross section including the central axis AX1, the outer peripheral surface 35Aa has a curved shape that is concave toward the central axis AX1.

[0042] The effects of the first groove portion 33 and the second groove portion 35 will be briefly described below. The first bearing 241 shown in FIG. 1 is attached to the first bearing holder 34. Specifically, the inner ring of the first bearing 241 is inserted and fitted onto the outer peripheral surface 344 of the first bearing holder 34. If the first connecting portion 33A were provided as in the comparative example, the corners of the inner ring of the first bearing 241 on the Z1 side would abut against the outer peripheral surface 33Aa of the first connecting portion 33A, which could cause the axial position of the inner ring of the first bearing 241 to shift from its proper position. However, by providing the first groove portion 33 as in this embodiment, the corners of the inner ring of the first bearing 241 do not abut, and the axial position of the inner ring of the first bearing 241 is positioned properly.

[0043] 1 is fastened to the male thread portion 36. Specifically, a female thread is provided on the inner periphery of the nut 25, and the female thread meshes with the male thread 361. The nut 25 presses the inner ring of the first bearing 241 toward the Z1 side, thereby holding the inner ring of the first bearing 241 in the Z direction. Here, if the second connecting portion 35A is provided as in the comparative example, the Z1-side corner of the nut 25 may abut against the outer peripheral surface 35Aa of the second connecting portion 35A, and the nut 25 may not abut against the inner ring of the first bearing 241. However, by providing the second groove portion 35 as in the present embodiment, the nut 25 can press the inner ring of the first bearing 241 toward the Z1 side.

[0044] Next, we will briefly explain carburizing and quenching. Figure 4 is a schematic diagram of carburizing and quenching the surface of a metal, where (a) shows the metal surface after anti-carburizing treatment, (b) shows the carburizing treatment on the anti-carburizing metal surface, (c) shows the state in which carbon has penetrated the metal surface due to carburizing treatment, and (d) shows the state in which the metal has been quenched. Carburizing is a process in which low-carbon steel with a low carbon content is heated to a high temperature (for example, about 900°C) to diffuse and penetrate carbon (C) from the surface. Carburizing can be done by gas carburizing, but other methods such as solid carburizing can also be used.

[0045] As shown in Fig. 4(a), first, a steel material 600 is prepared, with a carburization inhibitor 630 applied to a surface 610. The steel material 600 is, for example, low-carbon steel 621. That is, before heat treatment, the interior 620 of the steel material is low-carbon steel 621. Then, the carburization inhibitor 630 is applied to the surface 610 of the portion 640 to be carburized. In this state, the entire steel material 600 is heated to, for example, about 900°C.

[0046] 4(b) and (c), carbon 631 is diffused and penetrated from the surface 610 of the steel material 600. The carbon 631 does not penetrate into the area where the carburization inhibitor 630 is applied, but the carbon 631 penetrates and diffuses into the area where the carburization inhibitor 630 is not applied.

[0047] Then, as shown in Fig. 4(d), the steel material 600 is quenched. Specifically, the steel material 600 is cooled. The surface layer portion where the carburization inhibitor 630 is applied remains low-carbon steel 621, and the surface layer portion where the carburization inhibitor 630 is not applied becomes high-carbon steel 622. Note that the portion inside the surface layer portion is low-carbon steel 621.

[0048] Next, a method for manufacturing a pinion shaft will be described with reference to FIGS. 5 to 8. FIG. 5 is a schematic diagram showing the step of preparing a blank in the process of manufacturing a pinion shaft. FIG. 6 is a schematic diagram showing the state in which an anti-carburization agent has been applied to the tip end of the blank in the process of manufacturing a pinion shaft. FIG. 7 is a schematic diagram showing the state in which a cutting process has been applied to the tip end of the blank in the process of manufacturing a pinion shaft. FIG. 8 is a schematic diagram showing the state in which a portion of the cutting process has been polished in the process of manufacturing a pinion shaft. The method for manufacturing a pinion shaft according to this embodiment includes a blank preparation step, a carburization inhibitor application step, a carburizing and quenching step, a cutting step, and a polishing step. These steps will be described in detail below.

[0049] (1) Blank material preparation step In the blank preparation step, a blank 700 shown in Fig. 5 is prepared. The blank 700 differs from the pinion shaft 3 shown in Fig. 2A in that the first bearing retaining portion 34, the second groove portion 35, and the male thread portion 36 are not formed in the blank 700. In other words, the end portion 31, the pinion portion 32, and the first groove portion 33 of the blank 700 are formed in advance, and the portions corresponding to the first bearing retaining portion 34, the second groove portion 35, and the male thread portion 36 remain as a cylindrical portion 710. In other words, the blank preparation step is a step of preparing a columnar blank 700 having at least pinion teeth.

[0050] (2) Carburization prevention agent application step After the blank preparation step, a carburization inhibitor application step is performed. In the carburization inhibitor application step, as shown in Fig. 6, a carburization inhibitor 630 (shown by hatching) is applied to the surface of the cylindrical portion 710 on the Z2 side of the boundary line 345. In other words, the carburization inhibitor 630 is applied to the surface of the cylindrical portion 710 on the parts corresponding to the second region 343, the tapered portion 341, the second groove portion 35, and the male thread portion 36 shown in Fig. 3. The carburization inhibitor 630 is applied, for example, manually by an operator.

[0051] (3) Carburizing and quenching step After the carburization inhibitor application step, a carburization and quenching step is performed. In the carburization and quenching, as described with reference to Fig. 4, the entire blank 700 is heated to, for example, about 900°C, and carbon 631 is diffused and penetrated from the surface of the blank 700. Carbon 631 does not penetrate into the areas where the carburization inhibitor 630 is applied, but carbon 631 penetrates and diffuses into areas where the carburization inhibitor 630 is not applied. Then, the blank 700 is cooled and quenched.

[0052] (4) Cutting process step After the carburizing and quenching step, a second cutting step is performed in which the tapered portion 341, the second groove portion 35, and the male thread portion 36 shown in Figs. 3 and 7 are formed by cutting.

[0053] (5) Polishing step After the second cutting step, a polishing step is performed. In the polishing step, the entire surface of the first bearing holder 34 is polished, as shown in Fig. 8. Specifically, the portions (areas) to be polished are the portions indicated by hatching in Fig. 8. The order of the cutting step and the polishing step described above may be reversed. In other words, the cutting step may be performed after the polishing step.

[0054] Next, the hardness and metal structure of each portion of the pinion shaft 3 will be described. The portion to which the carburization inhibitor 630 is applied, i.e., the portion on the Z2 side of the boundary line 345 shown in FIG. 7, has a Vickers hardness HV of approximately 300 and a Rockwell HRC of approximately 30, for example. The metal structure of the surface is, for example, a mixed structure of ferrite and pearlite. Note that pearlite is a layered structure formed by a eutectoid reaction in which thin plate-like ferrite and cementite are precipitated alternately.

[0055] The hardness of the portion where the carburization inhibitor 630 is not applied, i.e., the portion on the Z1 side of the boundary line 345 shown in Fig. 7, is, for example, a Vickers hardness HV of about 720 and a Rockwell hardness HRC of about 61. The metal structure of the surface is, for example, martensite.

[0056] 3 is harder than the second region 343, tapered portion 341, second groove portion 35, and male thread portion 36, which are located on the Z2 side of boundary line 345. The hardness of the portion near boundary line 345 is softer than the Z1 side end of first region 342 and harder than the Z2 side end of second region 343. That is, the hardness decreases from the Z1 side to the Z2 side, from the Z1 side end of first region 342 through the portion near boundary line 345 to the Z2 side end of second region 343.

[0057] Next, we will explain the forces and moments applied to the pinion shaft 3. Figure 9 is a schematic diagram showing the forces and moments applied to the pinion shaft.

[0058] 9, at the center side of the pinion shaft 3 in the Z direction (axial direction), the rack teeth 152 of the rack shaft 15 mesh with the pinion teeth 321 of the pinion shaft 3. The rack shaft 15 is pressed toward the pinion shaft 3 by the rack guide 153. Therefore, a force F1 is applied to the center side of the pinion shaft 3 in the Z direction from the rack shaft 15, pressing it toward the Y1 side.

[0059] 2A , a first bearing holder 34 and a second bearing holder 323 are provided on the Z2 side and the Z1 side of the pinion shaft 3, respectively. A first bearing 241 is attached to the first bearing holder 34, and a second bearing 242 is attached to the second bearing holder 323. Therefore, forces F2 and F3 act on the Z2 side and the Z1 side of the pinion shaft 3 as reaction forces to the force F1. Specifically, force F2 is a force applied to the Z2 side of the pinion shaft 3 via first bearing 241, and force F3 is a force applied to the Z1 side of the pinion shaft 3 via second bearing 242. Therefore, a moment M2 acts on the Z2 side of the pinion shaft 3, and a moment M1 acts on the Z1 side of the pinion shaft 3.

[0060] Next, the position of the electric power steering device 100 in the steering device 80 will be described. FIG. 10 is a schematic diagram showing a steering device equipped with the electric power steering device according to this embodiment. As shown in FIG. 10, the steering device 80 includes, in the order in which a force applied by an operator is transmitted, a steering wheel 81, a steering shaft 82, a universal joint 84, an intermediate shaft 85, a universal joint 86, a shaft 87, a steering gear 88, and a tie rod 89. The steering device 80 also includes a control device (hereinafter referred to as an ECU (Electronic Control Unit)) 100A, a torque sensor 10, and an electric motor 102. A vehicle speed sensor 101 is provided in the vehicle and outputs a vehicle speed signal V to the ECU 100A via CAN (Controller Area Network) communication. The steering gear 88 includes a pinion shaft 3, a rack shaft 15, and another pinion shaft 88c, all of which are included in the electric power steering device 100. The pinion shaft 3 and the other pinion shaft 88c are meshed with the rack shaft 15, respectively.

[0061] As described above, in this embodiment, the pinion shaft 3 includes the pinion portion 32 having pinion teeth 321 provided on its outer periphery, the first bearing holder (bearing holder) 34 having an outer circumferential surface 344 whose distance from the central axis AX1 is shorter than the distance between the bottom of the pinion teeth 321 and the central axis AX1, and which holds the first bearing (bearing) 241 on the outer circumferential surface 344, and the male thread portion 36 having the male thread 361. The distance between the crest of the thread of the male thread 361 and the central axis AX1 is shorter than the distance between the outer circumferential surface 344 of the first bearing holder 34 and the central axis AX1. The hardness of a first region 342 on the outer circumferential surface 344 of the first bearing holder 34 that is closer to the pinion portion 32 is greater than the hardness of a second region 343 on the outer circumferential surface 344 of the first bearing holder 34 that is closer to the male thread portion 36.

[0062] As mentioned above, in the pinion shaft of Patent Document 1, the bearing mounting portion to which the first bearing is attached may be damaged by long-term use, so it is necessary to increase the rigidity of the bearing mounting portion.

[0063] In this embodiment, the distance between the outer peripheral surface 344 of the first bearing holder 34 and the central axis AX1 is smaller than the distance between the bottom of the pinion tooth 321 and the central axis AX1. Furthermore, the outer diameter of the male thread 361 is smaller than the outer diameter of the first bearing holder 34. Therefore, if the hardness of the surface layers of the pinion portion 32, the first bearing holder 34, and the male thread portion 36 are all the same, the rigidity decreases in the order of the pinion portion 32, the first bearing holder 34, and the male thread portion 36.

[0064] However, the first bearing holder 34 is subjected to, for example, carburizing and quenching, so that the hardness of the first region 342 of the first bearing holder 34 is greater than the hardness of the second region 343. Therefore, the hardness of the surface layer of the first bearing holder 34 is greater than when the first region 342 and the second region 343 have the same hardness, and the rigidity of the first bearing holder 34 is also greater.

[0065] The first bearing holder 34 further includes a first groove 33 having a bottom surface 331 located on the inner circumferential side of the outer circumferential surface 344 of the first bearing holder 34, and a tapered portion 341 located at the end of the second region 343 on the Z2 side and having a diameter that decreases toward the Z2 side. The bottom surface 331 of the first groove 33 is harder than the surface of the tapered portion 341.

[0066] Because the first groove portion 33 is provided, the bending moment acting on the Z2 side portion of the pinion shaft 3 tends to concentrate on the first groove portion 33, making the first groove portion 33 more susceptible to damage. However, the hardness of the bottom surface 331 of the first groove portion 33 is greater than the hardness of the surface of the tapered portion 341, so the rigidity of the first groove portion 33 is increased, making it less susceptible to bending moment concentration and damage.

[0067] Furthermore, the fact that the hardness of the bottom surface 331 of the first groove portion 33 is harder than the hardness of the surface of the tapered portion 341 means that the hardness of the surface of the tapered portion 341 is softer than the hardness of the bottom surface 331 of the first groove portion 33. Therefore, when the tapered portion 341 is formed by, for example, cutting, the workability of the cutting is improved.

[0068] Since the Vickers hardness of the first region 342 is, for example, 450 or more and the Vickers hardness of the second region 343 is, for example, 400 or less, the hardness of the surface layer of the first bearing holder 34 is greater than when the first region 342 and the second region 343 have the same hardness, and the rigidity of the first bearing holder 34 is also increased. Furthermore, when the portion of the second region 343 is subjected to cutting, for example, the workability of the cutting is improved.

[0069] The hardness of the first region 342 is greater than the hardness of the surface of the threads of the male thread portion 36. In other words, the hardness of the surface of the threads of the male thread portion 36 is less than the hardness of the first region 342. Therefore, the workability of the cutting process when forming the threads on the male thread portion 36 is improved.

[0070] The electric power steering device 100 includes a pinion shaft 3 provided with a pinion portion 32 having pinion teeth 321, a first bearing 241 attached to a first bearing holder 34 and rotatably supporting the pinion shaft 3 in a housing 200, a worm wheel 23 provided with a wheel tooth portion 232 on its outer periphery, a second bearing 242 attached to a second bearing holder 323 and rotatably supporting the pinion shaft 3 in the housing 200, and a rack shaft 15 having rack teeth 152 meshing with the pinion teeth 321 and pressed toward the pinion teeth 321. The outer diameter of the first bearing holder 34 is smaller than the outer diameter of the second bearing holder 323. The hardness of a first region 342 of the first bearing holder 34 is greater than the hardness of a second region 343 of the first bearing holder 34.

[0071] As described above, a pushing force from rack shaft 15 is applied to the center side of pinion shaft 3 in the Z direction (axial direction). A supporting force that resists the force from rack shaft 15 is applied to the Z2 side and Z1 side of pinion shaft 3, so a bending moment acts on the Z2 side and Z1 side of pinion shaft 3. Here, in pinion shaft 3, the outer diameter of first bearing holder 34 is smaller than the outer diameter of second bearing holder 323.

[0072] However, the first bearing holder 34 is subjected to, for example, carburizing and quenching, so that the hardness of the first region 342 of the first bearing holder 34 is greater than the hardness of the second region 343. Therefore, the hardness of the surface layer of the first bearing holder 34 is greater than when the first region 342 and the second region 343 have the same hardness, and the rigidity of the first bearing holder 34 is also greater.

[0073] The method for manufacturing a pinion shaft includes a blank preparation step of preparing a columnar blank 700 having at least pinion teeth 321, an anti-carburization agent application step of applying an anti-carburization agent 630 to an axial end of the blank 700 after the blank preparation step, a carburization and quenching step of carburizing and quenching the blank 700 after the anti-carburization agent application step, a cutting step of cutting the axial end of the blank 700 to which the anti-carburization agent 630 has been applied to form the male thread portion 36 and the first bearing holder 34 after the carburization and quenching step, and a polishing step of polishing the first bearing holder 34 after the cutting step. Note that, as described above, the cutting step may be performed after the polishing step.

[0074] In this way, the carburization inhibitor 630 is applied in advance to the portions of the blank 700 where the male thread portion 36 and the first bearing holder 34 will be formed, and then the blank 700 is carburized and quenched. Here, as described above, the area to which the carburization inhibitor 630 is applied is the region on the Z2 side of the boundary line 345 shown in FIGS. 3 and 7 , and the axial position of the boundary line 345 is the middle portion of the first bearing holder 34 in the axial direction. This makes the surface layer of the portion to which the carburization inhibitor 630 is applied softer than the portion to which the carburization inhibitor 630 is not applied. This makes it easier to form the male thread portion 36 and the first bearing holder 34, for example, by cutting, improving the workability of the cutting process. Furthermore, the surface layer of the portion to which the carburization inhibitor 630 is applied is harder, improving rigidity.

[0075] 2A, the distance between the outer circumferential surface of the second bearing holder 323 and the central axis AX1 is greater than the distance between the outer circumferential surface 344 of the first bearing holder 34 and the central axis AX1. That is, since the second bearing holder 323 has a larger diameter than the first bearing holder 34, the strength of the second bearing holder 323 is greater than the strength of the first bearing holder 34. The pinion teeth 321 are disposed between the first bearing holder 34 and the second bearing holder 323.

[0076] As shown in FIG. 9 , a force F1 is applied to the pinion teeth 321 from the rack shaft 15 toward the pinion teeth 321. Forces F2 and F3 act on the Z2 and Z1 sides of the pinion shaft 3 as reaction forces to the force F1. Specifically, force F2 is applied to the Z2 side of the pinion shaft 3 via the first bearing 241, and force F3 is applied to the Z1 side of the pinion shaft 3 via the second bearing 242. Here, because the strength of the second bearing holder 323 is greater than the strength of the first bearing holder 34, force F3 from the second bearing holder 323, acting as a reaction force against the rack shaft 15, is greater than force F2 from the first bearing holder 34. Therefore, the load burden on the first bearing holder 34 is reduced, and the durability of the first bearing holder 34 is improved.

[0077] Furthermore, dual-pinion electric power steering devices generally have a larger motor output than single-pinion electric power steering devices. Therefore, the pinion shaft 3 according to this embodiment, which has improved durability of the first bearing holder 34, has improved durability against forces input from the road surface while the vehicle is traveling, and is more suitable for dual-pinion electric power steering devices with large motor output. [Explanation of symbols]

[0078] 3, 3A pinion shaft 4. First Housing 5 Second Housing 15 Rack axis 23 Worm Wheel 25 Nut 26 Cap 27 Worm shaft 31 End 32 Pinion section 33 First groove 33A 1st connection part 33Aa Outer surface 34 First bearing retainer 35 Second groove 35A 2nd connection part 35Aa outer surface 36 Male thread 41 Main body 42 flange 43 Tip 43a Inner surface 51 Side part 52 Top section 53 flange 100 Electric power steering device 152 rack teeth 153 Rack guide 154 Spring 155 Sealing member 200 Housing 231 Core metal part 232 Wheel tooth 241 First bearing 242 Second bearing 271 Shaft teeth 311 flange 312 Worm wheel holder 321 Pinion teeth 322 Protrusion 323 Second bearing retainer 331 bottom 341 Tapered section (guide section) 342 First area 343 Second area 344 Outer surface 345 Borderline 351 bottom 361 Male thread 401 Annular convex part 401a Outer surface 401b Inner surface 401c bottom 412 Rack guide housing 600 Steel 610 Surface 620 Steel interior 621 low carbon steel 622 high carbon steel 630 Carburization inhibitor 631 Carbon 640 Carbon-proofing target area 700 blank material 710 Cylinder AX1, AX2 center axis

Claims

1. A pinion shaft extending in an axial direction of a central axis, a pinion portion having pinion teeth on an outer periphery thereof; a bearing holding portion that is located on one side of the pinion portion in the axial direction, has an outer circumferential surface that is closer to the central axis than a distance between the bottom of the pinion teeth and the central axis, and that holds a bearing on the outer circumferential surface; a male threaded portion that is located on one side of the bearing holder in the axial direction and has a male thread, a distance between a crest of the male screw and the central axis is smaller than a distance between the outer circumferential surface of the bearing holder and the central axis; The hardness of a first region of the outer peripheral surface of the bearing holder that is closer to the pinion portion is the hardness is harder than the hardness of a second region of the outer circumferential surface of the bearing holder that is closer to the male thread portion; Pinion shaft.

2. a groove portion located between the pinion portion and the bearing holder portion and having a bottom surface located on the inner peripheral side of the outer peripheral surface of the bearing holder portion; a tapered portion located at one end of the second region of the bearing holder in the axial direction, the tapered portion having a diameter that decreases toward the one end in the axial direction; Further provided with The hardness of the bottom surface of the groove portion is The hardness is harder than the surface of the tapered portion. The pinion shaft according to claim 1 .

3. the first region of the bearing holder has a Vickers hardness of 450 or more, and the second region has a Vickers hardness of 400 or less; The pinion shaft according to claim 1 or 2.

4. The hardness of the first region of the bearing holder is greater than the hardness of the surface of the thread of the male thread portion. The pinion shaft according to claim 1 or 2.

5. a pinion shaft extending in the axial direction of a central axis and provided with a pinion portion having pinion teeth; a first bearing attached to a first bearing holder provided on the pinion shaft on one side of the pinion portion in the axial direction, and which rotatably supports the pinion shaft in a housing; a worm wheel attached to the pinion shaft on the other side of the pinion portion in the axial direction and having a wheel tooth portion on an outer periphery thereof; a second bearing attached to a second bearing holder provided on the pinion shaft between the pinion portion and the worm wheel, and which rotatably supports the pinion shaft in the housing; a rack shaft having rack teeth that mesh with the pinion teeth and that is pressed toward the pinion teeth; Equipped with an outer diameter of the first bearing retaining portion is smaller than an outer diameter of the second bearing retaining portion; The hardness of a first region on the other side in the axial direction of the outer peripheral surface of the first bearing retaining portion is the hardness is greater than the hardness of a second region on one side in the axial direction of the outer peripheral surface of the first bearing retaining portion, a tapered portion having a diameter that decreases toward one end of the second region in the axial direction; Electric power steering device.

6. A method for manufacturing a pinion shaft that extends in an axial direction of a central axis and has a male thread portion and a bearing retaining portion at an end portion in the axial direction, comprising: a blank preparation step of preparing a columnar blank having at least pinion teeth; a carburization inhibitor application step of applying a carburization inhibitor to an axial end of the blank material after the blank material preparation step; a carburizing and quenching step of carburizing and quenching the blank material after the carburizing inhibitor application step; a cutting step of, after the carburizing and quenching step, performing cutting on the axial end of the blank material to which the carburization inhibitor has been applied, to form the male thread portion and the bearing holder; a polishing step of polishing the bearing holder after the cutting step; Including, A method for manufacturing a pinion shaft.

7. A method for manufacturing a pinion shaft that extends in an axial direction of a central axis and has a male thread portion and a bearing retaining portion at an end portion in the axial direction, comprising: a blank preparation step of preparing a columnar blank having at least pinion teeth; a carburization inhibitor application step of applying a carburization inhibitor to an axial end of the blank material after the blank material preparation step; a carburizing and quenching step of carburizing and quenching the blank material after the carburizing inhibitor application step; a polishing step of polishing the bearing holder after the carburizing and quenching step; a cutting step of, after the polishing step, performing a cutting process on the axial end of the blank material to which the carburization inhibitor has been applied, to form the male thread portion and the bearing holder; Including, A method for manufacturing a pinion shaft.

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