JP7756810B2 - Electric power steering device, pinion shaft, and method for manufacturing pinion shaft - Google Patents

Description

The present disclosure relates to an electric power steering device, a pinion shaft, and a method for manufacturing a pinion shaft.

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 pinion shaft's central axis 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 these pinion teeth mesh with rack teeth on the rack shaft. The rack shaft is pressed against the pinion teeth.

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. Furthermore, to reduce the weight of the pinion shaft, the outer diameter of the portion of the pinion shaft where the first bearing is attached is smaller than the outer diameter of the portion where the second bearing is attached.

JP 2010-023796 A

As mentioned 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 and second bearings, 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, so it is necessary to increase the rigidity of this bearing attachment portion.

This disclosure has been made in consideration of the above-mentioned problems, and aims to provide an electric power steering device, a pinion shaft, and a method for manufacturing a pinion shaft that have higher rigidity in the bearing mounting portion of the pinion shaft.

In order to achieve the above-mentioned objective, one embodiment of a pinion shaft 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 whose distance from the central axis is shorter than the distance between the roots of the pinion teeth and the central axis, and on which a bearing is held; 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 male thread and the central axis is shorter 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 closer to the pinion portion is greater than the hardness of a second region of the outer periphery surface of the bearing holder portion closer to the male thread portion.

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 with long-term use, so it is necessary to increase the rigidity of the bearing mounting portion.

In the pinion shaft of the present disclosure, the distance between the outer peripheral surface of the bearing holder and the center axis is smaller than the distance between the bottom of the pinion teeth and the center 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 portion, and male thread portion all have the same hardness, the rigidity decreases in the order of the pinion portion, bearing holder portion, and male thread portion. 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.

In a preferred embodiment, the bearing holder further comprises a groove portion located between the pinion portion and the bearing holder portion and having a bottom surface located more inward than the outer peripheral surface of the bearing holder portion, and a tapered portion located at one end of the second region of the bearing holder portion in the axial direction and having a diameter that decreases toward the one end 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.

As a result, because a groove is provided in the pinion shaft, bending moments acting on one axial side of the pinion shaft tend to concentrate in the groove, making the groove more susceptible to damage. However, because the bottom surface of the groove is harder than the surface of the tapered portion, the groove is more rigid, making it less susceptible to damage even when bending moments are concentrated.

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.

In a preferred embodiment, the Vickers hardness of the first region of the bearing holder is 450 or higher, and the Vickers hardness of the second region is 400 or lower. This results in a harder surface layer of the bearing holder and higher rigidity than if the first region and the second region had the same hardness. Furthermore, when machining the second region, for example, the workability of the machining is improved.

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 in the male thread portion.

In other words, the hardness of the surface of the thread in the male thread portion is softer than the hardness in the first region. This improves the workability of the cutting process when forming the thread in the male thread portion.

An electric power steering device according to one embodiment 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 portion in the axial direction on the pinion shaft and rotatably supporting the pinion shaft in a housing; a worm wheel attached to the other side of the pinion portion in the axial direction on the pinion shaft and provided with wheel teeth on its outer periphery; and a second bearing holder provided between the pinion portion and the worm wheel on the pinion shaft. a second bearing attached to a first portion and rotatably supporting the pinion shaft on the housing, and a rack guide having rack teeth that mesh with the pinion teeth and 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 harder 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.

As mentioned 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 one and the other axial sides of the pinion shaft, a bending moment acts on each of the two axial sides of the pinion shaft. Here, the outer diameter of the first bearing retaining portion of the pinion shaft is smaller than the outer diameter of the second bearing retaining portion. However, the first bearing retaining portion is, for example, carburized and quenched, so that the hardness of the first region of the first bearing retaining portion is greater than the hardness of the second region. Therefore, the hardness of the surface layer of the first bearing retaining portion is greater than if the first region and the second region had the same hardness, and the rigidity of the first bearing retaining portion is also higher.

One embodiment of a pinion shaft manufacturing method 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 its axial end, and includes the following steps: a blank material preparation step of preparing a cylindrical 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 carburization and quenching step; 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 cutting step; 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.

In this way, carburization inhibitor is applied in advance to the areas of the blank material where the male thread and bearing retainer will be formed, and then carburized and quenched. This makes the surface layer of the areas coated with carburization inhibitor softer than areas not coated with carburization inhibitor, making it easier to form the male thread and bearing retainer by cutting, for example, and improving the workability of the cutting process. Furthermore, the surface layer of the areas coated with carburization inhibitor is harder, improving rigidity.

This disclosure makes it possible to provide an electric power steering device, pinion shaft, and method for manufacturing a pinion shaft that have higher rigidity in the bearing mounting portion of the pinion shaft.

FIG. 1 is a schematic cross-sectional view of an electric power steering device according to an embodiment. FIG. 2A is a side view of a pinion shaft according to an embodiment. FIG. 2B is a side view of a portion of a pinion shaft according to a comparative example. FIG. 3 is a schematic diagram showing an enlarged portion of FIG. 2A. FIG. 4 is a schematic diagram showing the process of carburizing and quenching a metal surface, where (a) shows the metal surface after anti-carburization treatment, (b) shows the carburizing treatment performed on the anti-carburized metal surface, (c) shows the state in which carbon has penetrated into the metal surface due to the carburization treatment, and (d) shows the state in which the metal has been quenched. FIG. 5 is a schematic diagram showing a step of preparing a blank in the manufacturing process of a pinion shaft. 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. FIG. 7 is a schematic diagram showing a state in which the tip end of the blank has been machined in the manufacturing process of the pinion shaft. 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. FIG. 9 is a schematic diagram showing forces and moments acting on the pinion shaft. FIG. 10 is a schematic diagram showing a steering device including an electric power steering device according to this embodiment.

Forms (embodiments) for implementing 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 a person skilled in the art would easily imagine 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 explanations 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.

[Embodiment]
1 is a schematic cross-sectional view of an electric power steering device according to an embodiment of the present invention.

As shown in FIG. 1, the electric power steering device 100 comprises 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 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, but can also be applied to a single-pinion type.

The housing 200 accommodates the pinion shaft 3, worm wheel 23, worm shaft 27, and rack shaft 15. The housing 200 comprises 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.

The first housing 4 has a main body portion 41, a flange 42, a tip portion 43, and a rack guide accommodating portion 412. The main body portion 41 and the tip portion 43 are cylindrical members extending in the Z direction. The flange 42 is provided at the Z1-side end of the main body portion 41. 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 outer peripheral surface 401a of the annular protrusion 401 is fitted with the side surface portion 51 of the second housing 5, and the outer peripheral surface 401a and the side surface portion 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. The bottom surface 401c is a plane perpendicular to (intersecting with) the central axis AX1. The second bearing 242 is provided on the inner peripheral side of the annular convex portion 401. Specifically, the outer ring of the second bearing 242 abuts against the inner peripheral surface 401b and the bottom surface 401c, and the inner ring abuts against the outer peripheral surface of the pinion shaft 3. As a result, the Z1 side of the pinion shaft 3 is rotatably supported by the flange 42 via the second bearing 242.

A first bearing 241 is provided on the 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, with the male thread of the cap 26 meshing with the female thread of the tip portion 43.

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 spring 154 are accommodated when the spring 154 is compressed, 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.

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

The worm wheel 23 has a core metal portion 231 and a wheel tooth portion 232. The wheel tooth portion 232 meshes with the shaft tooth portion 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 in a direction around the central axis AX2.

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

As shown in FIG. 2A, the pinion shaft 3 has an end portion 31, a pinion portion 32, a first groove portion (groove portion) 33, a first bearing retaining portion (bearing retaining portion) 34, a second groove portion 35, and a male thread portion 36. The pinion shaft 3 has a center axis AX1.

The end 31 is located at the Z1 side end 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 periphery of the worm wheel holding portion 312. The flange 311 protrudes outward beyond the outer periphery 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.

The pinion portion 32 is adjacent to the end portion 31 on the Z2 side. A second bearing holder 323 is provided at the Z1 side end 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 Z2 side end of the pinion portion 32. The outer peripheral surface of the protrusion 322 is located outer than the cutting edge of the pinion teeth 321. The protrusion 322 extends annularly in the circumferential direction around the central axis AX1.

The first groove portion 33 is adjacent to the pinion portion 32 on the Z2 side. The bottom surface 331 of the first groove portion 33 extends annularly in the circumferential direction around the center axis AX1. As shown in FIG. 3, the bottom surface 331 of the first groove portion 33 has a smaller diameter than the outer peripheral surface of the protrusion 322 and also a smaller diameter than the outer peripheral surface 344 of the first bearing holder 34. The cross-sectional shape of the bottom surface 331 is a concave surface 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.

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 center axis AX1. The distance between the outer peripheral surface 344 of the first bearing holder 34 and the center axis AX1 is greater than the distance between the tooth bottom of the pinion tooth 321 and the center axis AX1. Furthermore, the distance between the outer peripheral surface of the second bearing holder 323 and the center axis AX1 is greater than the distance between the outer peripheral surface 344 of the first bearing holder 34 and the center axis AX1. As shown in FIG. 3, the outer peripheral surface 344 is divided into two portions by a boundary line 345 that passes through the axial center of the center axis AX1 and is perpendicular to the center 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.

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

The second groove portion 35 is adjacent to the first bearing holder portion 34 on the Z2 side. The bottom surface 351 of the second groove portion 35 extends annularly in the circumferential direction around the central axis AX1. As shown in Figure 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.

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 Z2-side end of the tapered portion 341. The distance between the crest of the male thread 361 and the center axis AX1 is smaller than the distance between the outer periphery 344 of the first bearing holder 34 and the center axis AX1. The nut 25 shown in FIG. 1 is fastened to the male thread portion 36. Specifically, a female thread is formed on the inner periphery of the nut 25, and this 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.

Next, we will explain the pinion shaft 3A related to the comparative example. Figure 2B is a side view of a portion of the pinion shaft related to the comparative example.

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.

The effects of the first groove portion 33 and the second groove portion 35 are 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, potentially causing 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 properly positioned.

The 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 this female thread engages 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 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 this embodiment, the nut 25 can press the inner ring of the first bearing 241 toward the Z1 side.

Next, we will briefly explain carburizing and quenching. Figure 4 is a schematic diagram of carburizing and quenching on a metal surface, 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 (e.g., approximately 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.

As shown in Figure 4(a), first, a steel material 600 is prepared with a carburization inhibitor 630 applied to its 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, for example, to approximately 900°C.

Next, as shown in Figures 4(b) and (c), carbon 631 is diffused and penetrated from the surface 610 of the steel material 600. Carbon 631 does not penetrate into areas where carburization inhibitor 630 has been applied, but carbon 631 penetrates and diffuses into areas where carburization inhibitor 630 has not been applied.

Then, as shown in Figure 4(d), the steel material 600 is hardened. Specifically, the steel material 600 is cooled. The surface layer where the carburization inhibitor 630 is applied remains low-carbon steel 621, while the surface layer where the carburization inhibitor 630 is not applied becomes high-carbon steel 622. Note that the area inside the surface layer is low-carbon steel 621.

Next, a method for manufacturing a pinion shaft will be described with reference to Figures 5 to 8. Figure 5 is a schematic diagram showing the step of preparing a blank in the pinion shaft manufacturing process. Figure 6 is a schematic diagram showing the state in which an anti-carburization agent has been applied to the tip of the blank in the pinion shaft manufacturing process. Figure 7 is a schematic diagram showing the state in which the tip of the blank has been machined in the pinion shaft manufacturing process. Figure 8 is a schematic diagram showing the state in which a portion of the machined section has been polished in the pinion shaft manufacturing process. The pinion shaft manufacturing method 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 are described in detail below.

(1) Blank 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 holder 34, the second groove 35, and the male thread 36 are not formed in the blank 700. In other words, the end 31, the pinion portion 32, and the first groove 33 of the blank 700 are formed in advance, and the portions corresponding to the first bearing holder 34, the second groove 35, and the male thread 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.

(2) Carburization Inhibitor Application Step After the blank preparation step, the 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 in the areas corresponding to the second region 343, tapered portion 341, second groove portion 35, and male thread portion 36 shown in FIG. 3 . The carburization inhibitor 630 is applied, for example, manually by an operator.

(3) Carburization and Quenching Step After the carburization inhibitor application step, the carburization and quenching step is performed. As described with reference to Fig. 4, the carburization and quenching involves heating the entire blank 700 to, for example, approximately 900°C, to diffuse and penetrate carbon 631 from the surface of the blank 700. Carbon 631 does not penetrate into the areas where the carburization inhibitor 630 is applied, but penetrates and diffuses into areas where the carburization inhibitor 630 is not applied. The blank 700 is then cooled and quenched.

(4) Cutting Step After the carburizing and quenching step, a second cutting step is performed. In the second cutting step, 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.

(5) Polishing Step After the second cutting step, the 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 hatched portions 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.

Next, the hardness and metal structure of each portion of the pinion shaft 3 will be explained. The hardness of the portion where the carburization inhibitor 630 is applied, i.e., the portion on the Z2 side of the boundary line 345 shown in Figure 7, is, for example, approximately 300 Vickers hardness HV and approximately 30 Rockwell HRC. 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 precipitate in an alternating state.

In addition, the hardness of the area where the carburization inhibitor 630 is not applied, i.e., the area on the Z1 side of the boundary line 345 shown in Figure 7, is, for example, a Vickers hardness HV of approximately 720 and a Rockwell HRC of approximately 61. The metal structure of the surface is, for example, martensite.

As such, the hardness of the first region 342 shown in Figure 3 is greater than the hardness of the second region 343, tapered portion 341, second groove portion 35, and male thread portion 36, which are located on the Z2 side of the boundary line 345. Furthermore, the hardness of the portion near the boundary line 345 is softer than the Z1 side end of the first region 342 and harder than the Z2 side end of the second region 343. In other words, the hardness decreases from the Z1 side to the Z2 side, from the Z1 side end of the first region 342 through the portion near the boundary line 345 to the Z2 side end of the second region 343.

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.

As shown in Figure 9, at the center 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 also pressed toward the pinion shaft 3 by the rack guide 153. Therefore, a force F1 is applied to the center of the pinion shaft 3 in the Z direction, pushing it toward the Y1 side from the rack shaft 15.

As shown in FIG. 2A , the Z2 side and Z1 side of the pinion shaft 3 are provided with a first bearing holder 34 and a second bearing holder 323, 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 Z1 side of the pinion shaft 3 as reaction forces to 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. Therefore, moment M2 acts on the Z2 side of the pinion shaft 3, and moment M1 acts on the Z1 side of the pinion shaft 3.

Next, the position of the electric power steering device 100 in the steering device 80 will be explained. Figure 10 is a schematic diagram showing a steering device equipped with the electric power steering device according to this embodiment. As shown in Figure 10, the steering device 80 includes, in order of transmission of force applied by the operator, 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 the pinion shaft 3, the rack shaft 15, and another pinion shaft 88c, which are included in the electric power steering device 100. The pinion shaft 3 and the other pinion shaft 88c each mesh with the rack shaft 15.

As described above, in this embodiment, the pinion shaft 3 includes a pinion portion 32 having pinion teeth 321 provided on its outer periphery, a first bearing retaining portion (bearing retaining portion) 34 having an outer peripheral 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 retains a first bearing (bearing) 241 on the outer peripheral surface 344, and a male threaded portion 36 having a 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 peripheral surface 344 of the first bearing retaining portion 34 and the central axis AX1. The hardness of a first region 342 on the outer peripheral surface 344 of the first bearing retaining portion 34 that is closer to the pinion portion 32 is greater than the hardness of a second region 343 on the outer peripheral surface 344 of the first bearing retaining portion 34 that is closer to the male threaded portion 36.

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 with long-term use, so it is necessary to increase the rigidity of the bearing mounting portion.

In this embodiment, the distance between the outer peripheral surface 344 of the first bearing holder 34 and the center axis AX1 is smaller than the distance between the bottom of the pinion teeth 321 and the center 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, first bearing holder 34, and male thread portion 36 are all the same, the rigidity decreases in the order of the pinion portion 32, first bearing holder 34, and male thread portion 36.

However, if the first bearing holder 34 is subjected to, for example, carburizing and quenching, 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 if the first region 342 and the second region 343 had the same hardness, and the rigidity of the first bearing holder 34 is also greater.

The first bearing holder 34 further includes a first groove 33 having a bottom surface 331 located radially inward relative to the outer peripheral surface 344 of the first bearing holder 34, and a tapered portion 341 located at the Z2-side end of the second region 343 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.

Because the first groove portion 33 is provided, bending moments acting on the Z2 side portion of the pinion shaft 3 tend to concentrate on the first groove portion 33, making the first groove portion 33 more susceptible to damage. However, because 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, the rigidity of the first groove portion 33 is increased, making it less susceptible to bending moments and less susceptible to damage.

Furthermore, the fact that the bottom surface 331 of the first groove portion 33 is harder than the surface hardness of the tapered portion 341 means, in other words, that the surface hardness of the tapered portion 341 is softer than 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 process is improved.

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 surface layer of the first bearing holder 34 is harder and the rigidity of the first bearing holder 34 is higher than when the first region 342 and the second region 343 have the same hardness. Furthermore, when cutting is performed on the portion of the second region 343, the workability of the cutting process is improved.

The hardness of the first region 342 is greater than the hardness of the surface of the threads in the male thread portion 36. In other words, the hardness of the surface of the threads in the male thread portion 36 is less than the hardness of the first region 342. This improves the workability of the cutting process when forming the threads in the male thread portion 36.

The electric power steering device 100 includes a pinion shaft 3 having a pinion portion 32 with pinion teeth 321, a first bearing 241 attached to a first bearing holder 34 and rotatably supporting the pinion shaft 3 in the housing 200, a worm wheel 23 having 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 that mesh with the pinion teeth 321 and that is 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 the first region 342 of the first bearing holder 34 is greater than the hardness of the second region 343 of the first bearing holder 34.

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

However, if the first bearing holder 34 is subjected to, for example, carburizing and quenching, 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 if the first region 342 and the second region 343 had the same hardness, and the rigidity of the first bearing holder 34 is also greater.

The pinion shaft manufacturing method includes a blank preparation step of preparing a cylindrical blank 700 having at least pinion teeth 321; a carburization inhibitor application step of applying an anti-carburization agent 630 to the axial end of the blank 700 after the blank preparation step; a carburization quenching step of carburizing and quenching the blank 700 after the carburization inhibitor 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 quenching step; and a polishing step of polishing the first bearing holder 34 after the cutting step. As mentioned above, the cutting step may be performed after the polishing step.

In this way, the carburization inhibitor 630 is applied 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 is carburized and quenched. As mentioned 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 Figures 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 results in the surface layer of the portion to which the carburization inhibitor 630 is applied being 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.

Furthermore, as described above with reference to Figure 2A, 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. In other words, because 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. Furthermore, the pinion teeth 321 are arranged between the first bearing holder 34 and the second bearing holder 323.

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

In addition, dual-pinion electric power steering devices generally have a higher motor output than single-pinion electric power steering devices. Therefore, the pinion shaft 3 according to this embodiment, which has improved durability in the first bearing retaining portion 34, has improved durability against forces input from the road surface when the vehicle is traveling, and is more suitable for dual-pinion electric power steering devices with high motor output.

3, 3A Pinion shaft 4 First housing 5 Second housing 15 Rack shaft 23 Worm wheel 25 Nut 26 Cap 27 Worm shaft 31 End 32 Pinion portion 33 First groove portion 33A First connecting portion 33Aa Outer peripheral surface 34 First bearing holding portion 35 Second groove portion 35A Second connecting portion 35Aa Outer peripheral surface 36 Male thread portion 41 Main body portion 42 Flange 43 Tip portion 43a Inner peripheral surface 51 Side portion 52 Top surface portion 53 Flange 100 Electric power steering device 152 Rack teeth 153 Rack guide 154 Spring 155 Sealing member 200 Housing 231 Core metal portion 232 Wheel tooth portion 241 First bearing 242 Second bearing 271 Shaft tooth portion 311 Flange 312 Worm wheel holding portion 321 Pinion teeth 322 Protrusion 323 Second bearing holder 331 Bottom surface 341 Tapered portion (guide portion)
342 First region 343 Second region 344 Outer peripheral surface 345 Boundary line 351 Bottom surface 361 Male thread 401 Annular convex portion 401a Outer peripheral surface 401b Inner peripheral surface 401c Bottom surface 412 Rack guide accommodating portion 600 Steel material 610 Surface 620 Steel material interior 621 Low carbon steel 622 High carbon steel 630 Carburization inhibitor 631 Carbon 640 Carburization prevention target portion 700 Blank material 710 Cylindrical portions AX1, AX2 Central axis

Claims (7)

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 whose distance from the central axis is smaller than the 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. 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 . 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. 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. 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. 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. 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.