Electric power steering device and rack shaft for electric power steering device

Abstract

To facilitate ensuring a contact area between a tooth flank of a rack tooth and a tooth flank of a pinion tooth in each on a steering side and an assist side in spite of a manufacturing error of a rack shaft.SOLUTION: Of a tooth flank of a tooth of a steering-side rack 28, a tooth flank of a tooth in a range in a tooth width direction range overlapping with the steering-side rack 28 of a steering-side pinion 25, a tooth flank of a tooth of an assist-side rack 29, and a tooth flank of a tooth in a tooth width direction range overlapping with the assist-side rack 29 of an assist-side pinion 41, in at least a part in a tooth muscle direction of the tooth flank of the tooth of the steering-side rack 28 and / or a part of the tooth flank of a tooth in a tooth width direction range overlapping with the steering-side rack 28 of the steering-side pinion 25, crowning processing is executed.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a dual pinion type electric power steering apparatus and a rack shaft constituting the apparatus.

Background Art

[0002] An automobile steering apparatus is configured to apply a steering angle to left and right steering wheels based on transmitting the rotation of a steering wheel rotated by a driver through a steering shaft, an intermediate shaft, etc. to a steering side pinion shaft of a steering gear unit, and further converting the rotational motion of the steering side pinion shaft into a linear motion of a rack shaft of the steering gear unit that meshes with the steering side pinion shaft.

[0003] In the field of steering apparatuses, electric power steering apparatuses configured to apply an assist driving force to a steering force transmission path have become widespread in order to reduce the force required for rotating the steering wheel. There are various types of electric power steering apparatuses in which the method of applying the assist driving force to the steering force transmission path is different.

[0004] The dual pinion type electric power steering apparatus targeted by the present invention includes, in addition to a steering side pinion shaft, an assist side pinion shaft as a pinion shaft that meshes with a rack shaft, as described in, for example, Japanese Patent Application Laid-Open No. 2002-154442 (Patent Document 1). That is, the rack shaft has a steering side rack on a part of the circumferential direction of the outer peripheral surface of one end portion in the axial direction and an assist side rack on a part of the circumferential direction of the outer peripheral surface of the other end portion in the axial direction, and both end portions in the axial direction are connected to the steering wheels. The steering side pinion shaft has a steering side pinion that meshes with the steering side rack on the outer peripheral surface and is rotationally driven by a rotational operation of the steering wheel. The assist side pinion shaft has an assist side pinion that meshes with the assist side rack on the outer peripheral surface and is rotationally driven by an electric motor. During driving of an automobile, the force required for rotating the steering wheel is reduced by applying an assist driving force to the rack shaft from the electric motor via the assist side pinion shaft.

[0005] Furthermore, a prior art document related to the present invention is Japanese Patent Application Publication No. 2005-53327 (Patent Document 2). This publication describes an electric power steering device of a type (hereinafter referred to as a ball screw type) that applies assist driving force from an electric motor to a rack shaft via a ball screw mechanism. In addition, this publication describes a technique for applying crowning to the tooth surfaces of the teeth of the steering-side pinion in a ball screw type electric power steering device. In the structure described in this publication, by employing such a technique, the amount of inclination between the central axes of the male screw portion and the nut of the ball screw mechanism is suppressed, based on allowing the contact position between the tooth surfaces of the teeth of the steering-side pinion and the tooth surfaces of the teeth of the steering-side rack to move in the direction of the tooth trace, thereby making the operation of the ball screw mechanism smoother. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2002-154442 [Patent Document 2] Japanese Patent Publication No. 2005-53327 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] In a conventional dual-pinion electric power steering system, if the positional relationship between the steering-side rack and the steering-side pinion in the circumferential direction of the rack shaft, and the positional relationship between the assist-side rack and the assist-side pinion in the circumferential direction of the rack shaft, are in a predetermined positional relationship as defined in the design, then on both the steering-side and assist-side, as shown in Figure 18(a), the tooth tip surface S of the rack 101, which is provided on a part of the circumferential outer surface of the rack shaft 100, and the central axis C of the pinion 103, which is provided on the outer surface of the pinion shaft 102, mesh with each other so that they are parallel to each other. In this case, as shown in Figure 19(a), the tooth surface 105 of the teeth 104 of the rack 101 makes surface contact with the tooth surface 107 of the teeth 106 of the pinion 103 with a large contact area.

[0008] However, in reality, for example, due to manufacturing tolerances of the rack shaft, the relative positional relationship between the steering rack and the assist rack in the circumferential direction may deviate from the predetermined positional relationship. As a result, at least one of the relative positional relationships between the steering rack and the steering pinion in the circumferential direction of the rack shaft, and the relative positional relationship between the assist rack and the assist pinion in the circumferential direction of the rack shaft, may deviate from the predetermined positional relationship defined in the design. In this case, on at least one of the steering side and the assist side, the rack 101 and the pinion 103 mesh such that the tooth tip surface S of the rack 101 and the central axis C of the pinion 103 are inclined relative to each other, as shown in Figure 18(b). In this case, as shown in Figure 19(b), the tooth surface 105 of the tooth 104 of the rack 101 tends to make contact with the tooth surface 107 of the tooth 106 of the pinion 103 at the edge portion (P portion) in the tooth trace direction, that is, contact with a narrow contact area. As a result, the stress acting on each tooth surface 105 and 107 increases.

[0009] However, even in such cases, increasing the overall tooth thickness of each tooth 104 and 106 will ensure the durability of each tooth 104 and 106. However, increasing the overall tooth thickness of each tooth 104 and 106 tends to lead to an increase in the weight of the rack shaft 100 and the pinion shaft 102.

[0010] Furthermore, the technique for crowning the tooth surfaces of the teeth of the steering pinion, as described in Japanese Patent Publication No. 2005-53327, is a technique intended for ball screw type electric power steering devices.

[0011] The present invention aims to provide an electric power steering device and a rack shaft that constitutes the device, which facilitates securing the contact area between the tooth surfaces of the rack teeth and the tooth surfaces of the pinion teeth on both the steering and assist sides, regardless of manufacturing tolerances of the rack shaft. [Means for solving the problem]

[0012] An electric power steering device according to one aspect of the present invention comprises a rack shaft, a steering-side pinion shaft, and an assist-side pinion shaft.

[0013] The rack shaft has a steering rack on a portion of the outer surface of one axial side and an assist rack on a portion of the outer surface of the other axial side, and both ends on the axial side are connected to the steering wheels.

[0014] The steering-side pinion shaft has a steering-side pinion on its outer circumference that meshes with the steering-side rack, and is rotationally driven by the rotational operation of the steering wheel.

[0015] The assist-side pinion shaft has an assist-side pinion on its outer circumferential surface that meshes with the assist-side rack, and is rotationally driven by an electric motor.

[0016] Crowning is applied to at least a portion of the tooth trace direction of the tooth surface of the teeth of the steering rack, and / or a portion of the tooth trace direction of the tooth surface of the teeth of the steering pinion in the tooth width direction range that overlaps with the steering rack, the tooth surface of the teeth of the assist rack, and the tooth surface of the teeth of the assist pinion in the tooth width direction range that overlaps with the assist rack. The crowned portion has a crowning shape that is inclined in the direction that the tooth thickness decreases towards the end with respect to the tooth trace direction.

[0017] In one embodiment of the present invention, the tooth surfaces of the teeth of the steering rack are crowned over the entire range in the tooth trace direction.

[0018] In an electric power steering device according to one aspect of the present invention, the rack shaft comprises a steering-side shaft portion having the steering-side rack on a portion of its outer peripheral surface in the circumferential direction, an assist-side shaft portion having the assist-side rack on a portion of its outer peripheral surface in the circumferential direction, and a connecting portion connecting the axial end of the steering-side shaft portion and the axial end of the assist-side shaft portion.

[0019] A rack shaft for an electric power steering device according to one aspect of the present invention has a steering-side rack on a portion of the outer circumferential surface of one axial side portion and an assist-side rack on a portion of the outer circumferential surface of the other axial side portion, and of the tooth surfaces of the teeth of the steering-side rack and the tooth surfaces of the teeth of the assist-side rack, only the tooth surfaces of the teeth of the steering-side rack are crowned in at least a portion of the tooth trace direction.

[0020] A rack shaft for an electric power steering device according to one aspect of the present invention comprises a steering-side shaft portion having the steering-side rack on a portion of its outer peripheral surface in the circumferential direction, an assist-side shaft portion having the assist-side rack on a portion of its outer peripheral surface in the circumferential direction, and a connecting portion that connects the axial end of the steering-side shaft portion to the axial end of the assist-side shaft portion. [Effects of the Invention]

[0021] According to one aspect of the present invention, even when the circumferential positional relationship between the steering side rack and the assist side rack deviates from the specified positional relationship, on each side, an electric power steering device that can easily secure the contact area between the tooth surface of the rack teeth and the tooth surface of the pinion teeth, and a rack shaft constituting the device can be provided.

Brief Description of the Drawings

[0022] [Figure 1] FIG. 1 is a schematic diagram showing an electric power steering device according to a first example of an embodiment of the present invention. [Figure 2] FIG. 2 is an exploded perspective view showing some members taken out from the electric power steering device of the first example. [Figure 3] FIG. 3 is a cross-sectional view showing one axial side portion of the rack shaft of the first example and members arranged around it. [Figure 4] FIG. 4 is a cross-sectional view showing the other axial side portion of the rack shaft of the first example and members arranged around it. [Figure 5] FIG. 5 is a cross-sectional view taken along the line A-A of FIG. 3. [Figure 6] FIG. 6(a) is a view of the teeth constituting the steering side pinion of the first example as seen from the radially outer side, and FIG. 6(b) is a cross-sectional view taken along the line B-B of FIG. 6(a). [Figure 7] FIG. 7 is a perspective view of the rack shaft of the first example. [Figure 8] FIG. 8 is a plan view of the rack shaft of the first example. [Figure 9] FIG. 9 is a perspective view of the teeth constituting the steering side rack of the first example. [Figure 10] FIG. 10 is a perspective view of the teeth constituting the assist side rack of the first example. [Figure 11] FIG. 11 is a cross-sectional view taken along the line C-C of FIG. 4. [Figure 12] FIG. 12(a) is a view of the teeth constituting the assist side pinion of the first example as seen from the radially outer side, and FIG. 12(b) is a cross-sectional view taken along the line D-D of FIG. 12(a). [Figure 13] Figure 13(a) shows the state in which the assist rack and assist pinion mesh together in the first example, such that the tooth tip surface S2 of the assist rack and the central axis C2 of the assist pinion are parallel to each other. Figure 13(b) shows the state in which the steering rack and steering pinion mesh together in the first example, such that the tooth tip surface S1 of the steering rack and the central axis C1 of the steering pinion are inclined relative to each other. [Figure 14] Figure 14(a) is a schematic diagram showing the contact state between the tooth surfaces of the assist-side rack teeth and the tooth surfaces of the assist-side pinion teeth in the state shown in Figure 13(a), and Figure 14(b) is a schematic diagram showing the contact state between the tooth surfaces of the steering-side rack teeth and the tooth surfaces of the steering-side pinion teeth in the state shown in Figure 13(b). [Figure 15] Figure 15 is a diagram corresponding to Figure 6(a), showing a second example of an embodiment of the present invention. [Figure 16] Figure 16 is a diagram corresponding to Figure 9, showing a third embodiment of the present invention. [Figure 17] Figure 17 is a diagram corresponding to Figure 9, showing a fourth example of an embodiment of the present invention. [Figure 18] Figure 18(a) shows a conventional electric power steering system in which the rack and pinion mesh together such that the tooth surface S of the rack and the central axis C of the pinion are parallel to each other, and Figure 18(b) shows a state in which the rack and pinion mesh together such that the tooth surface S of the rack and the central axis C of the pinion are inclined relative to each other. [Figure 19] Figure 19(a) is a schematic diagram showing the contact state between the tooth surfaces of the rack teeth and the tooth surfaces of the pinion teeth in the state shown in Figure 18(a), and Figure 19(b) is a schematic diagram showing the contact state between the tooth surfaces of the rack teeth and the tooth surfaces of the pinion teeth in the state shown in Figure 18(b). [Modes for carrying out the invention]

[0023] [Example 1] A first example of an embodiment of the present invention will be described with reference to Figures 1 to 14.

[0024] In the following description, the front-rear direction refers to the front-rear direction of the vehicle, the up-down direction refers to the up-down direction of the vehicle, and the left-right direction refers to the width direction of the vehicle. The left-right direction coincides with the axial direction of the rack axis 11 and rack housing 14, which will be described later. With respect to the axial direction of the rack axis 11 and rack housing 14, one side is the left side in Figures 1, 3, 4, and 8, and the other side with respect to the axial direction of the rack axis 11 and rack housing 14 is the right side in Figures 1, 3, 4, and 8.

[0025] The electric power steering system 1 in this example is a dual-pinion type. That is, the electric power steering system 1 in this example comprises one rack shaft 11 and two pinion shafts, a steering-side pinion shaft 10 and an assist-side pinion shaft 13. The steering force is input from the steering-side pinion shaft 10 to the rack shaft 11, and the assist-side pinion shaft 13 is input to the rack shaft 11.

[0026] The electric power steering system 1 in this example comprises a steering wheel 2, a steering shaft 3, a steering column 4, a pair of universal joints 5a and 5b, an intermediate shaft 6, a housing 7 (not shown in Figure 1, see Figure 2), a steering mechanism 8, and an assist mechanism 9, as shown in Figure 1.

[0027] The steering shaft 3 is rotatably supported inside the steering column 4, which is supported by the vehicle body. The steering wheel 2, which is rotated by the driver, is attached to the rear end of the steering shaft 3. The front end of the steering shaft 3 is connected to the steering-side pinion shaft 10, which constitutes the steering mechanism 8, via a universal joint 5a, an intermediate shaft 6, and another universal joint 5b. As a result, the rotational motion of the steering wheel 2 is transmitted to the steering-side pinion shaft 10. The rotational motion of the steering-side pinion shaft 10 is converted into axial linear motion of the rack shaft 11, which constitutes the steering mechanism 8. This imparts a steering angle to the left and right steering wheels 12 according to the amount of rotation of the steering wheel 2. The electric power steering device 1 reduces the steering force required for the driver to rotate the steering wheel 2 by applying assist driving force to the rack shaft 11 via the assist-side pinion shaft 13, which constitutes the assist mechanism 9.

[0028] In this example, the housing 7 is a casting integrally made by die-casting a light alloy such as an aluminum alloy, and is fixed to the vehicle body. When implementing the present invention, the housing can also be constructed by joining and fixing multiple parts together. In this example, as shown in Figure 2, the housing 7 includes a rack housing section 14, a steering-side pinion housing section 15, an assist-side pinion housing section 16, a steering-side guide housing section 17, an assist-side guide housing section 18, a gear housing section 19, and a plurality of mounting sections 20a, 20b.

[0029] The rack housing section 14 is a part that houses the axial intermediate portion of the rack shaft 11 that constitutes the steering mechanism section 8, and has a cylindrical shape and extends in the left-right direction.

[0030] The steering-side pinion housing 15 is the portion that houses the leading half of the steering-side pinion shaft 10, and is located on a circumferential portion of the axial side of the rack housing 14. More specifically, the steering-side pinion housing 15 is located on the front side of the axial side of the rack housing 14 and has a bottomed cylindrical shape with only the upper end open. The steering-side pinion housing 15 is positioned in a torsional positional relationship with the rack housing 14. In other words, the central axis of the steering-side pinion housing 15 and the central axis of the rack housing 14 are in a torsional positional relationship. When viewed from the front-rear direction, which is perpendicular to both the central axis of the rack housing 14 and the central axis of the steering-side pinion housing 15, the central axis of the steering-side pinion housing 15 is not positioned perpendicular to the central axis of the rack housing 14, but is inclined with respect to that perpendicular direction. The internal space of the steering-side pinion housing 15 is in communication with the internal space of the rack housing 14.

[0031] The assist-side pinion housing 16 is the portion that houses the assist-side pinion shaft 13 and is located on a circumferential portion of the other axial portion of the rack housing 14. More specifically, in this example, the assist-side pinion housing 16 is located on the front side of the other axial portion of the rack housing 14 and has a cylindrical shape with open ends on both the upper and lower sides. The assist-side pinion housing 16 is positioned in a torsion relationship with respect to the rack housing 14. That is, the central axis of the assist-side pinion housing 16 and the central axis of the rack housing 14 are in a torsion relationship. When viewed from the front-to-back direction, which is perpendicular to both the central axis of the rack housing 14 and the central axis of the assist-side pinion housing 16, the central axis of the assist-side pinion housing 16 is not positioned perpendicular to the central axis of the rack housing 14, but is inclined with respect to that perpendicular direction. The internal space of the assist-side pinion housing 16 is in communication with the internal space of the rack housing 14. When implementing the present invention, the central axis of the assist-side pinion housing can also be positioned perpendicular to the central axis of the rack housing.

[0032] The steering-side guide housing 17 is the portion that houses the steering-side rack guide 24, which will be described later, and is located in the part of the rack housing 14 that is diametrically opposite to the steering-side pinion housing 15. In other words, in this example, the steering-side guide housing 17 is located in the rear part of the rack housing 14 at the same axial position as the steering-side pinion housing 15. The steering-side guide housing 17 has a cylindrical shape and extends radially around the central axis of the rack housing 14. In other words, in this example, the steering-side guide housing 17 extends in the front-rear direction. The internal space of the steering-side guide housing 17 is also in communication with the internal space of the rack housing 14.

[0033] The assist-side guide housing 18 is the portion that houses the assist-side rack guide 37, which will be described later, and is located in the part of the rack housing 14 that is diametrically opposite to the assist-side pinion housing 16. In other words, in this example, the assist-side guide housing 18 is located in the rear part of the rack housing 14 at the same axial position as the assist-side pinion housing 16. The assist-side guide housing 18 has a cylindrical shape and extends radially around the central axis of the rack housing 14. In other words, in this example, the assist-side guide housing 18 extends in the front-rear direction. The internal space of the assist-side guide housing 18 is also in communication with the internal space of the rack housing 14.

[0034] The gear housing section 19 is the part that houses the worm reducer 38, which will be described later and constitute the assist mechanism section 9, and includes a worm housing section 21 and a wheel housing section 22.

[0035] The wheel housing section 22 is the part that houses the worm wheel 50 that constitutes the worm reducer 38, and is arranged adjacent to the assist-side pinion housing section 16 in the axial direction. Specifically, in this example, the wheel housing section 22 is located above the assist-side pinion housing section 16. The wheel housing section 22 has a substantially cylindrical shape and is arranged coaxially with the assist-side pinion housing section 16.

[0036] The worm housing 21 is the part that houses the worm 49 that constitutes the worm reducer 38, and is located in a circumferential part of the wheel housing 22. Specifically, in this example, the worm housing 21 is located in the front part of the wheel housing 22. The worm housing 21 has a bottomed cylindrical shape, and in this example, it has an opening at one end with respect to the axial direction of the rack housing 14. The worm housing 21 has a mounting flange 23 that protrudes radially outward at the opening end, that is, at one end with respect to the axial direction of the rack housing 14. The internal space of the worm housing 21 and the internal space of the wheel housing 22 are in communication with each other.

[0037] Multiple mounting sections 20a and 20b are used to fix the housing 7 to the vehicle body. In this example, the multiple mounting sections 20a and 20b consist of two mounting sections 20a located on the front side of the axial ends of the rack housing section 14, and two mounting sections 20b located on the rear side of the axial middle section of the rack housing section 14. The housing 7 is fixed to the vehicle body using fastening members such as bolts or studs inserted through each of the mounting sections 20a and 20b.

[0038] The steering mechanism 8 includes a steering-side pinion shaft 10, a rack shaft 11, and a steering-side rack guide 24, and converts the rotational motion of the steering wheel 2 into linear motion in the axial direction of the rack shaft 11.

[0039] The steering-side pinion shaft 10 is a metal shaft member such as carbon steel. As shown in Figures 1 and 5, the steering-side pinion shaft 10 has a steering-side pinion 25 on the outer circumferential surface of its front half that meshes with the steering-side rack 28 of the rack shaft 11.

[0040] In this example, the steering pinion 25 has crowning applied to at least one half of the tooth surface in the tooth trace direction (the tip side of the steering pinion shaft 10, the lower side in Figure 5) in the tooth width direction range α (see Figure 5) that overlaps with the steering rack 28. This point will be explained using Figures 6(a) and 6(b).

[0041] Figure 6(a) is a view of the teeth 55 constituting the steering pinion 25 from the radially outer direction in the tooth width range α, and Figure 6(b) is a cross-sectional view of BB in Figure 6(a). Note that in Figure 6(a), the teeth 55, which actually extend in the helical direction, are conveniently depicted as extending in the linear direction. The steering pinion 25 has a predetermined twist angle. That is, the tooth trace direction of the teeth 55 is inclined with respect to the axial direction of the steering pinion shaft 10 by the amount of this twist angle. In this example, the tooth surfaces 56, which are the sides of the teeth 55 in the tooth thickness direction, are curved in a direction in which the tooth thickness of the teeth 55 decreases from the tooth root side to the tooth tip side. In particular, in this example, the tooth surface 56 is crowned on one half of the tooth trace direction (the lower side in Figure 6(a)) in the tooth width range α. In other words, one half of the tooth surface 56 in the tooth trace direction has a crowning shape that slopes in a direction in which the tooth thickness decreases curvilinearly or linearly (curvilinearly in the illustrated example) toward one side in the tooth trace direction. Therefore, in this example, the tooth thickness of tooth 55 is constant in the other half (upper side in Figure 6(a)) in the tooth trace direction within the tooth width direction range α, and decreases toward one side in the tooth trace direction. The tooth surface 56 is smoothly continuous along the entire length in the tooth trace direction within the tooth width direction range α, and does not have any sharp corners in the middle of the tooth trace direction, including the boundary between one half and the other half. Regarding the tooth surface of the steering pinion teeth, crowning can only be applied to one end (the tip side of the steering pinion shaft) of the ends on both sides in the tooth trace direction, and not to the other end (the base end side of the steering pinion shaft). Therefore, in this example, within the tooth width range α, crowning is applied only to one half of the tooth surface 56 of the teeth 55 of the steering pinion 25 in the tooth trace direction (the side toward the tip of the steering pinion shaft).

[0042] The steering pinion shaft 10 is rotatably supported inside the steering pinion housing 15 using bearings 26a and 26b. The central axis of the steering pinion shaft 10 is coaxial with the central axis of the steering pinion housing 15. The steering pinion shaft 10 is connected to the steering wheel 2 via universal joints 5a and 5b and an intermediate shaft 6, and is rotationally driven by the rotational operation of the steering wheel 2. The rotational motion of the steering pinion shaft 10 is converted into linear motion of the rack shaft 11, pushing and pulling the tie rods 27 connected to the axial ends of the rack shaft 11. This imparts a steering angle to the left and right steering wheels 12.

[0043] The rack shaft 11 is a rod-shaped member made of metal such as carbon steel. The rack shaft 11 is positioned with its axial direction oriented left to right. As shown in Figures 7 and 8, the rack shaft 11 has a steering rack 28 on a portion of the outer circumferential surface of one axial side portion that meshes with the steering pinion 25 of the steering pinion shaft 10, and an assist rack 29 on a portion of the outer circumferential surface of the other axial side portion that meshes with the assist pinion 41 of the assist pinion shaft 13. In this example, the phases of the circumferential arrangement of the steering rack 28 and the assist rack 29 of the rack shaft 11 are substantially equal, except for phase shifts due to manufacturing errors. Specifically, in this example, the steering rack 28 and the assist rack 29 are positioned on the front surface of the rack shaft 11. However, when implementing the present invention, the phases of the circumferential arrangement of the steering rack 28 and the assist rack 29 of the rack shaft 11 can be made to differ from each other by a magnitude exceeding the phase shift due to manufacturing errors.

[0044] In this example, the steering rack 28 does not have crowning applied to the tooth surfaces in the tooth width direction range α (see Figure 5) that overlaps with the steering pinion 25, that is, in the entire tooth width direction range. This point will be explained using Figure 9.

[0045] Figure 9 is a perspective view of the teeth 57 that make up the steering rack 28. The steering rack 28 has a predetermined twist angle. That is, the tooth trace direction of the teeth 57 is not perpendicular to the axial direction of the rack axis 11, but is inclined by the amount of the twist angle with respect to the direction perpendicular to the axial direction of the rack axis 11 (the vertical direction in Figure 8). In this example, the tooth surfaces 58, which are the sides of the tooth 57 in the tooth thickness direction, are inclined linearly in a direction in which the tooth thickness of the tooth 57 decreases from the tooth root side to the tooth tip side. In this example, the tooth surfaces 58 are not crowned overall. That is, the tooth surfaces 58 are not inclined with respect to the tooth trace direction. Therefore, the tooth thickness of the tooth 57 is constant over the entire length in the tooth trace direction.

[0046] In this example, the assist rack 29 does not have crowning applied to the tooth surfaces in the tooth width direction range β (see Figure 11) that overlaps with the assist pinion 41, that is, in the entire tooth width direction range. This point will be explained using Figure 10.

[0047] Figure 10 is a perspective view of the teeth 59 that make up the assist rack 29. In this example, the assist rack 29 also has a predetermined twist angle, and the tooth surface 60 of the teeth 59 is linearly inclined in a direction in which the tooth thickness of the teeth 59 decreases from the tooth root side to the tooth tip side. Furthermore, the tooth surface 60 is not crowned overall and is not inclined with respect to the tooth trace direction.

[0048] The rack shaft 11 has screw holes 30 that open to the axial end faces at each of its axial ends.

[0049] In this example, a structure is adopted that allows the rack shaft 11 to be manufactured in two separate shaft sections, taking into consideration the ease of manufacturing the rack shaft 11. Specifically, in this example, the rack shaft 11 comprises a steering-side shaft section 61 having a steering-side rack 28 on a part of its outer circumferential surface, an assist-side shaft section 62 having an assist-side rack 29 on a part of its outer circumferential surface, and a connecting section 63 that connects the axial end of the steering-side shaft section 61 and the axial end of the assist-side shaft section 62. That is, in this example, the rack shaft 11 is manufactured by separately manufacturing the steering-side shaft section 61 having the steering-side rack 28 and the assist-side shaft section 62 having the assist-side shaft section 62, and then connecting the axial end of the steering-side shaft section 61 and the axial end of the assist-side shaft section 62 with the connecting section 63. In this example, the connecting section 63 is made of a friction-pressure-welded section. However, when implementing the present invention, any connection method can be used for the connection portion 63, such as a welded joint, a friction stir joint, or a crimped joint, as long as the necessary strength for the connection portion 63 can be secured. The screw holes 30 provided at both ends of the rack shaft 11 in the axial direction can be formed before the connection portion 63 is constructed, or they can be formed after the connection portion 63 is constructed. When implementing the present invention, the rack shaft can also be made from a single rod-shaped material.

[0050] The rack shaft 11 is supported inside the rack housing 14, allowing for reciprocating movement in the axial direction, with its axial ends protruding from openings on both the left and right sides of the rack housing 14. The axial ends of the rack shaft 11 are connected to tie rods 27 via spherical joints 31. Specifically, a male threaded portion 32 provided at the base of the spherical joint 31 is screwed into each of the threaded holes 30 provided at the axial ends of the rack shaft 11, and the base end of the tie rod 27 is pivotably supported at the tip of the spherical joint 31. The rack shaft 11 is connected to the left and right steering wheels 12 via a link mechanism including the spherical joints 31 and tie rods 27.

[0051] The steering rack guide 24 is a guide that presses the rack shaft 11 toward the steering pinion shaft 10, and is located inside the steering guide housing 17. That is, the steering rack guide 24 is positioned to sandwich the rack shaft 11 between itself and the steering pinion shaft 10. As shown in Figures 3 and 5, the steering rack guide 24 in this example is a sliding rack guide and comprises a pad 33 and an elastic member 34.

[0052] The pad 33 has a substantially cylindrical shape and is positioned inside the steering-side guide housing 17, allowing for movement relative to the rack shaft 11. The pad 33 has a concave cylindrical pressing surface 35 on the surface facing the convex cylindrical rear surface of the rack shaft 11, with a shape that matches the rear surface of the rack shaft 11. The pressing surface 35 is made of a synthetic resin or the like with excellent lubricity. The elastic member 34, in the illustrated example, is a torsion coil spring and is elastically compressed and sandwiched between the pad 33 and the steering-side cap 36 that closes the opening of the steering-side guide housing 17. As a result, the elastic member 34 presses the pad 33 toward the rack shaft 11.

[0053] The steering rack guide 24 reduces backlash at the meshing portion between the steering pinion 25 and the steering rack 28 by pressing the rack shaft 11 toward the steering pinion shaft 10. This suppresses the generation of abnormal noise at the meshing portion between the steering pinion 25 and the steering rack 28.

[0054] The assist mechanism 9 reduces the steering force required for the driver to rotate the steering wheel 2 by applying an assist driving force to the rack shaft 11. The assist mechanism 9 comprises an assist-side pinion shaft 13, an assist-side rack guide 37, a worm gear reducer 38, an electric motor 39, and a torque sensor 40.

[0055] The assist-side pinion shaft 13 is a metal shaft member such as carbon steel. As shown in Figures 1 and 11, the assist-side pinion shaft 13 has an assist-side pinion 41 on the outer circumferential surface of its front half that meshes with the assist-side rack 29 of the rack shaft 11.

[0056] In this example, the assist pinion 41 does not have crowning applied to the tooth surface in at least the tooth width direction range β (see Figure 11) that overlaps with the assist rack 29. This point will be explained using Figures 12(a) and 12(b).

[0057] Figure 12(a) is a view of the teeth 64 constituting the assist pinion 41 from the radially outer direction in the tooth width direction range β, and Figure 12(b) is a cross-sectional view DD of Figure 12(a). Note that in Figure 12(a), the teeth 64, which actually extend in the helical direction, are conveniently depicted as extending in the linear direction. The assist pinion 41 has a predetermined twist angle. That is, the tooth trace direction of the teeth 64 is inclined with respect to the axial direction of the assist pinion shaft 13 by the amount of this twist angle. In this example, the tooth surfaces 65, which are the sides of the tooth 64 in the tooth thickness direction, are curved in a direction in which the tooth thickness of the tooth 64 decreases from the tooth root side to the tooth tip side. In this example, the tooth surfaces 65 are not crowned over the entire range in the tooth trace direction in the tooth width direction range β. That is, the tooth surfaces 65 are not inclined with respect to the tooth trace direction. Therefore, the tooth thickness of tooth 64 is constant along its entire length in the direction of the tooth trace.

[0058] The assist-side pinion shaft 13 is rotatably supported inside the assist-side pinion housing 16 using bearings 42a and 42b. The central axis of the assist-side pinion shaft 13 is coaxial with the central axis of the assist-side pinion housing 16. The assist-side pinion shaft 13 is rotationally driven by an electric motor 39 via a worm gear reducer 38. In this example, the opening of the assist-side pinion housing 16 opposite the wheel housing 22 in the axial direction is closed by a cap 51.

[0059] The assist-side rack guide 37 is a guide that presses the rack shaft 11 toward the assist-side pinion shaft 13, and is located inside the assist-side guide housing 18. That is, the assist-side rack guide 37 is positioned to sandwich the rack shaft 11 between itself and the assist-side pinion shaft 13. As shown in Figures 4 and 11, the assist-side rack guide 37 is a rolling-type rack guide and includes a roller 43, a holder 44, a pin 45, a rolling bearing 46, and an elastic member 47.

[0060] The roller 43 has a substantially annular shape and is rotatably supported on the holder 44 via a pin 45 and a rolling bearing 46 arranged axially in the vertical direction. As a result, the outer circumferential surface of the roller 43 is in rolling contact with the middle portion in the width direction of the rear side surface of the rack shaft 11. The outer circumferential surface of the roller 43 has a concave arc generatrix shape that substantially matches the contour shape of the rear side surface of the rack shaft 11. The holder 44 is positioned inside the assist-side guide housing 18, allowing for movement toward and from the rack shaft 11. The elastic member 47, in the illustrated example, is a disc spring and is positioned between the holder 44 and the assist-side cap 48 that closes the opening of the assist-side guide housing 18. The elastic member 47 presses the holder 44 toward the rack shaft 11.

[0061] The assist-side rack guide 37 reduces backlash at the meshing portion between the assist-side pinion 41 and the assist-side rack 29 by pressing the rack shaft 11 toward the assist-side pinion shaft 13. This suppresses the generation of abnormal noise at the meshing portion between the assist-side pinion 41 and the assist-side rack 29.

[0062] In the dual-pinion type electric power steering system 1, the assist driving force acting on the meshing portion between the assist-side rack 29 and the assist-side pinion 41 is significantly larger than the steering force acting on the meshing portion between the steering-side rack 28 and the steering-side pinion 25. As a result, the assist-side rack guide 37 receives a much larger load from the rack shaft 11 than the steering-side rack guide 24. Therefore, if the pressing portions of the steering-side rack guide 24 and the assist-side rack guide 37 are configured with sliding surfaces that slide against the outer circumferential surface of the rack shaft 11, the frictional force acting between the outer circumferential surface of the rack shaft 11 and the pressing portion of the assist-side rack guide 37 tends to be large. Consequently, the resistance to linear movement of the rack shaft 11 in the axial direction tends to be large, and wear is likely to occur on the sliding surfaces of the rack shaft 11 and the assist-side rack guide 37. Therefore, in order to resolve these problems, the structure of this example is configured such that the pressing portion of the steering-side rack guide 24 is made of a sliding surface that slides against the outer circumferential surface of the rack shaft 11, while the pressing portion of the assist-side rack guide 37 is made of a rolling surface that makes rolling contact with the outer circumferential surface of the rack shaft 11. By adopting this configuration, the frictional force acting between the outer circumferential surface of the rack shaft 11 and the pressing portion of the assist-side rack guide 37 can be kept to a minimum.

[0063] As shown in Figure 11, the worm gear reducer 38 comprises a worm 49 and a worm wheel 50, and reduces the rotation of the electric motor 39, that is, increases the driving torque of the electric motor 39, and transmits it to the assist side pinion shaft 13.

[0064] The worm 49 has worm teeth on its outer circumference and is rotatably supported inside the worm housing 21. The base end of the worm 49 is connected to the motor output shaft of the electric motor 39 via a coupling (not shown) or the like, enabling torque transmission.

[0065] The worm wheel 50 has wheel teeth on its outer circumference that mesh with the worm teeth and is located inside the wheel housing 22. The worm wheel 50 is externally fitted and fixed to the base end of the assist-side pinion shaft 13 so as not to rotate relative to it. In this example, the opening of the wheel housing 22 opposite to the assist-side pinion housing 16 in the axial direction is closed by a cap 52.

[0066] The electric motor 39 is fixed to the mounting flange 23 of the worm housing 21.

[0067] The torque sensor 40 is positioned around the steering-side pinion shaft 10 and detects the magnitude and direction of the torque input to the steering-side pinion shaft 10. The torque sensor 40 then outputs a signal corresponding to the torque input to the steering-side pinion shaft 10 to the electronic control unit of the electric motor 39. Various types of torque sensors can be used as the torque sensor 40, such as a non-contact torque sensor utilizing the magnetostrictive effect.

[0068] The assist mechanism 9 controls the drive of the electric motor 39 based on the output signal of the torque sensor 40. This transmits the drive torque generated by the electric motor 39 to the rack shaft 11 as an assist drive force via the worm gear reducer 38 and the assist-side pinion shaft 13. As a result, the steering force required for the driver to rotate the steering wheel 2 is reduced.

[0069] The structure in this example includes a pair of rack bushings 53 and 54 that support the rack shaft 11 so that it can be displaced axially without rattling relative to the rack housing 14. These rack bushings 53 and 54 are made of synthetic resin such as polyacetal resin or polyamide resin and have a substantially cylindrical shape.

[0070] The rack bush 53, located on one axial side near the steering-side pinion shaft 10, is fitted inside near the opening on one axial side of the rack housing 14, as shown in Figure 3. The rack bush 54, located on the other axial side near the assist-side pinion shaft 13, is fitted inside near the opening on the other axial side of the rack housing 14, as shown in Figure 4. The rack bushes 53 and 54 support the outer circumferential surface of the rack shaft 11 so that it can slide in the axial direction. The steering-side rack guide 24 that presses against the rack shaft 11 is a sliding rack guide, and sufficient holding force in the width direction (vertical direction in Figure 5) of the rack shaft 11 can be secured, so the rack bush 53 located on one axial side can be omitted.

[0071] As described above, in the electric power steering device 1 of this example, in the tooth width range α (see Figure 5) where the steering rack 28 and the steering pinion 25 overlap each other, the tooth surface of at least one of the teeth of the steering rack 28 and the steering pinion 25 has a crowning shape. Specifically, in this example, of the steering rack 28 and the steering pinion 25, only the tooth surface 56 of the tooth 55 of the steering pinion 25 (see Figures 6(a) and 6(b)) has a crowning shape on one half of the tooth trace direction, while the tooth surface 58 of the tooth 57 of the steering rack 28 (see Figure 9) does not have a crowning shape. On the other hand, in the tooth width direction range β (see Figure 11) where the assist rack 29 and the assist pinion 41 overlap each other, the tooth surfaces 60 of the teeth 59 of the assist rack 29 (see Figure 10) and the tooth surfaces 65 of the teeth 64 of the assist pinion 41 (see Figures 12(a) and 12(b)) do not have a crowning shape.

[0072] Therefore, for example, even if the relative positions of the steering rack 28 and the assist rack 29 in the circumferential direction of the rack shaft 11 deviate from the predetermined relative positions defined in the design due to a manufacturing error of the rack shaft 11, or if the relative positions of the steering pinion 25 and the assist pinion 41 in the circumferential direction of the rack shaft 11 deviate from the predetermined relative positions defined in the design due to a manufacturing error of the housing 7, the assist rack 29 and the assist pinion 41 can be meshed together such that the tooth tip surface S2 of the assist rack 29 and the central axis C2 of the assist pinion 41 are parallel to each other, as shown in Figure 13(a). In this case, as shown in Figure 14(a), the tooth surface 60 of the teeth 59 of the assist rack 29 can be made to make surface contact with the tooth surface 65 of the teeth 64 of the assist pinion 41 with a wide contact area. Therefore, the stress acting on these teeth 59 and 64 can be kept small. Therefore, the durability of teeth 59 and 64 can be ensured without excessively increasing the tooth thickness of these teeth 59 and 64.

[0073] On the other hand, if the aforementioned positional misalignment occurs, the steering rack 28 and the steering pinion 25 mesh so that, for example, as shown in Figure 13(b), the tooth tip surface S1 of the steering rack 28 and the central axis C1 of the steering pinion 25 are inclined relative to each other. However, in this example, even in such a case, since one half of the tooth surface 56 of the tooth 55 of the steering pinion 25 has a crowning shape in the tooth trace direction, as shown in Figure 14(b), the tooth surface 58 of the tooth 57 of the steering rack 28 contacts the tooth surface 56 of the tooth 55 of the steering pinion 25 with a relatively wide contact area in a portion offset to one side from the center in the tooth trace direction. Therefore, the stress acting on these teeth 55 and 57 can be kept small. As a result, the durability of these teeth 55 and 57 can be ensured without excessively increasing the tooth thickness of these teeth 55 and 57.

[0074] In this example, the rack shaft 11 is constructed by separately manufacturing a steering-side shaft portion 61 having a steering-side rack 28 and an assist-side shaft portion 62 having an assist-side shaft portion 62, and then connecting the axial end of the steering-side shaft portion 61 and the axial end of the assist-side shaft portion 62 with a connecting portion 63. In such a rack shaft 11, the relative positional relationship between the steering-side rack 28 and the assist-side rack 29 in the circumferential direction of the rack shaft 11 is prone to deviation from the predetermined positional relationship defined in the design, particularly due to manufacturing errors when constructing the connecting portion 63. Therefore, the technique of applying crowning to the tooth surface of at least one of the teeth of the steering-side rack 28 and the steering-side pinion 25, thereby ensuring a wide contact area between the tooth surfaces of the rack and pinion on both the steering side and the assist side, is particularly effective.

[0075] Incidentally, one possible method to ensure sufficient contact area between the tooth surfaces of the teeth on the steering rack and the teeth on the steering pinion, and also between the tooth surfaces of the teeth on the assist rack and the teeth on the assist pinion, is to perform crowning on the tooth surfaces of at least one of the teeth on the assist rack and the assist pinion.

[0076] However, the assist driving force acting on the meshing portion between the assist-side rack and the assist-side pinion is significantly larger than the steering force acting on the meshing portion between the steering-side rack and the steering-side pinion. Specifically, it is about 10 to 20 times larger than the steering force. For this reason, the durability of the teeth of the assist-side rack 29 and the assist-side pinion 41 is considered more important than that of the steering-side rack 28 and the steering-side pinion 25.

[0077] On the other hand, applying crowning to the tooth surface reduces the tooth thickness at the end of the tooth in the direction of the tooth trace. Therefore, applying crowning to the tooth surface reduces the durability of the tooth compared to a tooth that has not been crowned. Consequently, from the perspective of prioritizing the durability of the teeth of the assist-side rack and assist-side pinion, that is, from the perspective of making it easier to ensure the durability of the teeth, applying crowning to the tooth surfaces of the teeth of the assist-side rack and assist-side pinion is not appropriate.

[0078] In this regard, in this example, crowning is applied only to the tooth surface of at least one of the teeth of the steering rack 28 and the steering pinion 25, while crowning is not applied to the tooth surface 60 of the tooth 59 of the assist rack 29 and the tooth surface 65 of the tooth 64 of the assist pinion 41. Therefore, it is easier to ensure the durability of the tooth 59 of the assist rack 29 and the tooth 64 of the assist pinion 41.

[0079] Furthermore, in this example, under conditions where the assist driving force acting on the meshing portion between the assist-side rack 29 and the assist-side pinion 41 is significantly larger than the steering force acting on the meshing portion between the steering-side rack 28 and the steering-side pinion 25, as shown in Figure 14(a), the tooth surfaces 65 of the teeth 64 of the assist-side pinion 41, which have not undergone crowning, make surface contact with the tooth surfaces 60 of the teeth 59 of the assist-side rack 29 with a wide contact area, without tilting or making uneven contact. Therefore, this also provides an advantage in ensuring the durability of the teeth 59 of the assist-side rack 29 and the teeth 64 of the assist-side pinion 41.

[0080] In this example, crowning is applied only to the tooth surfaces 56 of the teeth 55 of the steering side pinion 25, out of the steering side rack 28 and steering side pinion 25. This reduces the processing costs of the steering side rack 28 and steering side pinion 25. Specifically, the crowning of the tooth surfaces 56 of the teeth 55 of the steering side pinion 25 can be done using the low-cost hobbing method, but the crowning of the tooth surfaces 58 of the teeth 57 of the steering side rack 28 cannot be done using hobbing and requires the high-cost forging process. In this example, since crowning is not applied to the tooth surfaces 58 of the teeth 57 of the steering side rack 28, the steering side rack 28 can be formed using hobbing. This reduces the processing costs of the steering side pinion 25 and steering side rack 28.

[0081] The reason why crowning the tooth surfaces 58 of the teeth 57 of the steering rack 28 cannot be done by hobbing is that the teeth 57 of the steering rack 28 have a helix angle, and the tooth surfaces of these teeth 57 are arranged on the same virtual plane. In other words, if one attempts to impart a crowning shape to the tooth surfaces 58 of such teeth 57 of the steering rack 28 by hobbing, the positions in the tooth width direction where the tooth thickness is maximum will differ between adjacent teeth 57, making it impossible to impart the same crowning shape to the tooth surfaces 58 of each tooth 57.

[0082] [Example 2] A second example of an embodiment of the present invention will be described with reference to Figure 15.

[0083] Figure 15 is a diagram corresponding to Figure 6(a) in this example. In this example, crowning is applied only to the end 67a on one side (the lower side in Figure 15) of the tooth surface 56a of the tooth 55a constituting the steering pinion 25a. That is, in this example, crowning is not applied to the middle portion 66 and the other end 67b (the upper side in Figure 15) of the tooth surface 56a of the tooth 55a constituting the steering pinion 25a. In other words, in this example, the middle portion 66 and the other end 67b are not inclined with respect to the tooth surface direction, and only the one end 67a has a crowning shape that is inclined in a direction in which the tooth thickness of the tooth 55a decreases curvilinearly or linearly (curvilinearly in the illustrated example) toward one side in the tooth surface direction. Therefore, in this example, the tooth thickness of the tooth 55a decreases toward one side in the tooth surface direction, within the same range as the one end 67a toward the tooth surface direction. The tooth surface 56a is smoothly continuous along its entire length in the tooth trace direction, and does not have any sharp corners along the tooth trace direction, including the boundary between the intermediate portion 66 and one end portion 67a.

[0084] In the structure of this example, if the relative positions of the steering rack 28 and the assist rack 29 (see Figure 1) in the circumferential direction of the rack shaft 11 are in a predetermined positional relationship as defined in the design, and the relative positions of the steering pinion 25 and the assist pinion 41 (see Figure 1) in the circumferential direction of the rack shaft 11 are also in a predetermined positional relationship as defined in the design, then the tooth surface 56a of the teeth 55 of the steering pinion 25a can be made to contact the tooth surface 58 of the teeth 57 of the steering rack 28 (see Figure 9) with a wider contact area at the intermediate portion 66. Other configurations and effects are the same as in the first example.

[0085] [Example 3] A third embodiment of the present invention will be described with reference to Figure 16.

[0086] In this example, the steering-side pinion (not shown) that meshes with the steering-side rack 28a, like the assist-side pinion 41 shown in Figures 12(a) and 12(b), does not have crowning applied to the entire tooth surface. That is, the tooth surface is not inclined with respect to the tooth trace direction. Therefore, the tooth thickness is constant along the entire length in the tooth trace direction.

[0087] Instead, in this example, as shown in Figure 16, the steering rack 28a has crowning applied to the entire tooth surface 58a of the tooth 57a. That is, the tooth surface 58a has a crowning shape that slopes in a direction in which the tooth thickness of the tooth 57a decreases curvilinearly or linearly (curvilinearly in the illustrated example) from the center of the tooth trace direction toward both sides. Therefore, in this example, the tooth thickness of the tooth 57a is maximum in the center of the tooth trace direction and decreases toward both sides toward both sides toward the tooth trace direction. The tooth surface 58a is smoothly continuous along its entire length toward the tooth trace direction and does not have any sharp corners in the middle of the tooth trace direction.

[0088] In the structure of this example, since the tooth surfaces 58a of the teeth 57a of the steering rack 28a are crowned, even if the relative positions of the steering rack 28a and the assist rack 29 (see Figure 1) in the circumferential direction of the rack shaft 11 deviate from the predetermined relative positions defined in the design due to manufacturing tolerances of the rack shaft 11, or if the relative positions of the steering pinion and the assist pinion 41 (see Figure 1) in the circumferential direction of the rack shaft 11 deviate from the predetermined relative positions defined in the design due to manufacturing tolerances of the housing 7 (see Figure 2), the tooth surfaces 58a of the teeth 57a of the steering rack 28a can be made to contact the tooth surfaces of the teeth of the steering pinion with a reasonably wide contact area in a portion offset from the center in the tooth trace direction. The other configurations and effects are the same as in the first example.

[0089] [Example 4] A fourth embodiment of the present invention will be described with reference to Figure 17.

[0090] In this example, of the tooth surfaces 58b of the teeth 57b constituting the steering rack 28b, the intermediate portion 68 in the tooth trace direction is not crowned, and crowning is applied only to the ends 69 on both sides of the intermediate portion 68 in the tooth trace direction. That is, in this example, the intermediate portion 68 is not inclined with respect to the tooth trace direction, and only the ends 69 on both sides have a crowned shape that is inclined in a direction in which the tooth thickness of the tooth 57b decreases curvilinearly or linearly (curvilinearly in the illustrated example) from the center to both sides in the tooth trace direction. Therefore, in this example, the tooth thickness of the tooth 57b is maximum in the same range as the intermediate portion 68 in the tooth trace direction, and decreases from the center to both sides in the same range as the ends 69 in the tooth trace direction. The tooth surface 58b is smoothly continuous along its entire length in the tooth trace direction and does not have any sharp corners in the middle of the tooth trace direction, including the boundary between the intermediate portion 68 and the ends 69.

[0091] In the structure of this example, if the relative positions of the steering rack 28b and the assist rack 29 (see Figure 1) in the circumferential direction of the rack shaft 11 are in a predetermined positional relationship as defined in the design, and the relative positions of the steering pinion and the assist pinion 41 (see Figure 1) in the circumferential direction of the rack shaft 11 are also in a predetermined positional relationship as defined in the design, then the tooth surface 58b of the teeth 57b of the steering rack 28b can be brought into contact with the tooth surface of the teeth of the steering pinion at the intermediate portion 68 with a wider contact area. Other configurations and effects are the same as in the third example.

[0092] [Example 5] A fifth embodiment of the present invention will now be described. In this example, crowning is applied to both the tooth surface 56 of the teeth 55 of the steering-side pinion 25 (see Figure 6) and the tooth surface 58a of the teeth 57a of the steering-side rack 28a (see Figure 16). The other configurations and effects are the same as in the first example.

[0093] The present invention can be implemented by appropriately combining the structures of each embodiment described above, to the extent that it does not create inconsistencies. [Explanation of symbols]

[0094] 1. Electric power steering system 2 Steering Wheel 3. Steering shaft 4. Steering column 5a, 5b Universal joint 6 Intermediate shaft 7 Housing 8. Steering mechanism 9. Assist mechanism 10 Steering side pinion shaft 11 Rack axis 12 Steering wheels 13. Assist-side pinion shaft 14 Rack storage section 15 Steering side pinion housing 16. Assist side pinion housing 17 Steering side guide housing 18 Assist side guide housing 19 Gear housing section 20a, 20b Mounting parts 21 Worm containment section 22 Wheel housing 23 Mounting flange 24 Steering side rack guide 25, 25a Steering pinion 26a, 26b bearings 27 Tie Rod 28, 28a, 28b Steering rack 29 Assist side rack 30 screw holes 31 Spherical joint 32 Male threaded section 33 pads 34 Elastic members 35 Pressing surface 36 Steering side cap 37 Assist-side rack guide 38 Worm gear reducer 39 Electric motor 40 Torque Sensor 41 Assist side pinion 42a, 42b bearings 43 Laura 44 holder 45 pins 46 Rolling bearings 47 Elastic members 48 Assist side cap 49 Warm 50 Worm Wheel 51 Cap 52 caps 53 Rack bush 54 Rack bush 55, 55a teeth 56, 56a Tooth surface Teeth 57, 57a, 57b 58, 58a, 58b tooth surface 59 teeth 60 Tooth surface 61 Steering shaft 62 Assist-side shaft 63 Connection part 64 teeth 65 Tooth surface 66 Middle section 67a, 67b end 68 Middle section 69 End

Claims

1. a rack shaft having a steering-side rack on a circumferential portion of an outer peripheral surface of one axial side portion and an assist-side rack on a circumferential portion of an outer peripheral surface of the other axial side portion, the rack shaft having both axial ends connected to steered wheels; a steering-side pinion shaft having a steering-side pinion on its outer circumferential surface that meshes with the steering-side rack, the steering-side pinion shaft being rotationally driven by the rotation of a steering wheel; an assist-side pinion shaft having an assist-side pinion on its outer circumferential surface that meshes with the assist-side rack and that is rotationally driven by an electric motor; Among the tooth flanks of the teeth of the steering side rack, the tooth flanks of the teeth of the steering side pinion in a tooth width direction range overlapping with the steering side rack, the tooth flanks of the teeth of the assist side rack, and the tooth flanks of the teeth of the assist side pinion in a tooth width direction range overlapping with the assist side rack, crowning is applied only to at least a part in the tooth trace direction of the tooth flanks of the teeth of the steering side rack and / or only to a part in the tooth trace direction of the tooth flanks of the teeth of the steering side pinion in the tooth width direction range overlapping with the steering side rack. Electric power steering device.

2. 2. The electric power steering device according to claim 1, wherein crowning is applied to the tooth flanks of the teeth of the steering-side rack over the entire range in the tooth trace direction.

3. 3. The electric power steering device according to claim 1, wherein the rack shaft comprises: a steering-side shaft portion having the steering-side rack on a circumferential portion of an outer circumferential surface thereof; an assist-side shaft portion having the assist-side rack on a circumferential portion of an outer circumferential surface thereof; and a connecting portion connecting an axial end portion of the steering-side shaft portion and an axial end portion of the assist-side shaft portion.

4. a steering-side rack on a circumferential portion of an outer peripheral surface of the one axial side portion, and an assist-side rack on a circumferential portion of an outer peripheral surface of the other axial side portion, Among the tooth surfaces of the teeth of the steering side rack and the tooth surfaces of the teeth of the assist side rack, only the tooth surfaces of the teeth of the steering side rack are subjected to crowning processing in at least a part in the tooth trace direction. Rack shaft for electric power steering device.

5. 5. The rack shaft for an electric power steering device according to claim 4, comprising: a steering-side shaft portion having the steering-side rack on a circumferential portion of an outer circumferential surface thereof; an assist-side shaft portion having the assist-side rack on a circumferential portion of an outer circumferential surface thereof; and a connecting portion connecting an axial end portion of the steering-side shaft portion and an axial end portion of the assist-side shaft portion.

Images