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 device and a rack shaft that constitutes the device. [Background technology]

[0002] An automobile steering device is configured to transmit the rotation of the steering wheel, which is rotated by the driver, to the steering-side pinion shaft of the steering gear unit via a steering shaft, an intermediate shaft, etc., and to apply a steering angle to the left and right steered wheels by converting the rotational movement of the steering-side pinion shaft into linear movement of the rack shaft of the steering gear unit, which meshes with the steering-side pinion shaft.

[0003] In the field of steering devices, electric power steering devices that are configured to apply an assist drive force to a steering force transmission path in order to reduce the force required to turn a steering wheel are widely used. There are various types of electric power steering devices that differ in the way in which the assist drive force is applied to the steering force transmission path.

[0004] A dual-pinion electric power steering device, which is the subject of the present invention, includes a steering-side pinion shaft and an assist-side pinion shaft as pinion shafts meshing 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 circumferential portion of the outer peripheral surface of one axial side portion and an assist-side rack on a circumferential portion of the outer peripheral surface of the other axial side portion, and both axial ends are connected to steered wheels. The steering-side pinion shaft has a steering-side pinion on its outer peripheral surface that meshes with the steering-side rack and is rotationally driven by rotation of the steering wheel. The assist-side pinion shaft has an assist-side pinion on its outer peripheral surface that meshes with the assist-side rack and is rotationally driven by an electric motor. When driving a vehicle, the electric motor applies an assist driving force to the rack shaft via the assist-side pinion shaft, thereby reducing the force required to rotate the steering wheel.

[0005] Incidentally, Japanese Patent Laid-Open Publication No. 2005-53327 (Patent Document 2) is a prior art document related to the present invention. This publication describes an electric power steering device of a type in which an assist driving force is applied from an electric motor to a rack shaft via a ball screw mechanism (hereinafter referred to as a ball screw type in this specification). Furthermore, this publication also describes a technique for applying crowning to the tooth flanks of the teeth of the steering-side pinion in a ball screw type electric power steering device. By employing this technique, the structure described in this publication allows the contact position between the tooth flanks of the teeth of the steering-side pinion and the tooth flanks of the teeth of the steering-side rack to move in the tooth trace direction, thereby reducing the amount of inclination between the central axes of the male thread portion of the ball screw mechanism and the nut, thereby smoothing the operation of the ball screw mechanism. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-154442 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-53327 Summary of the Invention [Problem to be solved by the invention]

[0007] In a conventional dual-pinion electric power steering device, when 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 predetermined positional relationships determined by design, the rack 101 and the pinion 103 mesh together on both the steering and assist sides so that the tooth tip surface S of the rack 101 provided on a part of the circumferential surface of the rack shaft 100 and the central axis C of the pinion 103 provided on the outer circumferential surface of the pinion shaft 102 are parallel to each other, as shown in Fig. 18(a). In this case, as shown in Fig. 19(a), tooth flanks 105 of teeth 104 of the rack 101 come into surface contact with tooth flanks 107 of teeth 106 of the pinion 103 over a wide contact area.

[0008] However, in reality, for example, due to manufacturing errors in the rack shaft, the relative positional relationship between the steering-side rack and the assist-side rack in the circumferential direction may deviate from the predetermined positional relationship. As a result, at least one of 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 may deviate from the predetermined positional relationship determined 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 so 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 FIG. 18(b). In this case, as shown in FIG. 19(b), the tooth flank 105 of the tooth 104 of the rack 101 tends to make one-sided contact with the tooth flank 107 of the tooth 106 of the pinion 103 at an end edge portion (portion P) in the tooth trace direction, i.e., the contact area tends to be small. As a result, the stress acting on each of the tooth surfaces 105 and 107 increases.

[0009] However, even in such a case, if the tooth thickness of each of the teeth 104, 106 is increased overall, it is possible to ensure the durability of each of the teeth 104, 106. However, if the tooth thickness of each of the teeth 104, 106 is increased overall, this tends to increase the weight of the rack shaft 100 and the pinion shaft 102.

[0010] The technique of crowning the tooth surfaces of the steering-side pinion teeth, described in JP-A-2005-53327, is a technique targeted at ball screw type electric power steering devices.

[0011] The present invention aims to provide an electric power steering device that can easily ensure a sufficient contact area between the tooth surfaces of the rack teeth and the tooth surfaces of the pinion teeth on both the steering side and the assist side, regardless of manufacturing errors of the rack shaft, and a rack shaft that constitutes the device. [Means for solving the problem]

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

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

[0014] The steering-side pinion shaft has a steering-side pinion on its outer circumferential surface that meshes with the steering-side rack, and is rotationally driven by the rotation 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] Of the tooth flanks of the teeth of the steering side rack, the tooth flanks of the teeth of the steering side pinion in a range in the tooth width direction that overlaps 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 range in the tooth width direction that overlaps with the assist side rack, crowning is applied to at least a portion in the tooth trace direction of the tooth flanks of the teeth of the steering side rack and / or only a portion in the tooth trace direction of the tooth flanks of the teeth of the steering side pinion in the range in the tooth width direction that overlaps with the steering side rack. The crowned portion has a crowning shape that is inclined in the direction in which the tooth thickness decreases toward the end in the tooth trace direction.

[0017] In the electric power steering device according to one aspect of the present invention, crowning is applied to the tooth flanks of the teeth of the steering-side rack over the entire range in the tooth trace direction.

[0018] In one embodiment of the electric power steering device of the present invention, the rack shaft includes a steering-side shaft portion having the steering-side rack on a circumferential portion of its outer peripheral surface, an assist-side shaft portion having the assist-side rack on a circumferential portion of its outer peripheral surface, 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 embodiment of the present invention has a steering-side rack on a circumferential portion of the outer peripheral surface of one axial side portion, and an assist-side rack on a circumferential portion of the outer peripheral surface of the other axial side portion, and of the tooth surfaces of the teeth of the steering-side rack and 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 portion in the tooth trace direction.

[0020] A rack shaft for an electric power steering device according to one embodiment of the present invention comprises a steering-side shaft portion having the steering-side rack on a circumferential portion of its outer peripheral surface, an assist-side shaft portion having the assist-side rack on a circumferential portion of its outer peripheral surface, 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. [Effects of the Invention]

[0021] According to one aspect of the present invention, it is possible to provide an electric power steering device and a rack shaft that constitutes the device, which make it easy to ensure a contact area between the tooth surfaces of the rack teeth and the tooth surfaces of the pinion teeth on each side, even if the relative positional relationship between the steering side rack and the assist side rack in the circumferential direction deviates from the specified positional relationship. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a schematic diagram showing an electric power steering device according to a first embodiment of the present invention. [Figure 2] FIG. 2 is an exploded perspective view showing some members extracted 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 the portion. [Figure 4] FIG. 4 is a cross-sectional view showing the other axial side portion of the rack shaft of the first example and the members arranged around it. [Figure 5] FIG. 5 is a cross-sectional view taken along line AA in FIG. [Figure 6] FIG. 6(a) is a diagram showing the teeth that make up the steering-side pinion of the first example, as viewed from the radially outside, and FIG. 6(b) is a cross-sectional view taken along line BB 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 that constitute the steering-side rack of the first example. [Figure 10] FIG. 10 is a perspective view of teeth that constitute the assist-side rack of the first example. [Figure 11] FIG. 11 is a cross-sectional view taken along CC in FIG. [Figure 12] FIG. 12(a) is a diagram showing the teeth that make up the assist-side pinion of the first example as viewed from the outside in the radial direction, and FIG. 12(b) is a DD cross-sectional view of FIG. 12(a). [Figure 13] FIG. 13(a) is a diagram showing, for the first example, a state in which the assist side rack and the assist side pinion mesh together so that the tooth tip surface S2 of the assist side rack and the central axis C2 of the assist side pinion are parallel to each other, and FIG. 13(b) is a diagram showing, for the first example, a state in which the steering side rack and the steering side pinion mesh together so that the tooth tip surface S1 of the steering side rack and the central axis C1 of the steering side pinion are inclined to each other. [Figure 14] FIG. 14(a) is a schematic diagram showing the contact state between the tooth flanks of the teeth of the assist side rack and the tooth flanks of the teeth of the assist side pinion in the state shown in FIG. 13(a), and FIG. 14(b) is a schematic diagram showing the contact state between the tooth flanks of the teeth of the steering side rack and the tooth flanks of the teeth of the steering side pinion in the state shown in FIG. 13(b). [Figure 15] FIG. 15 is a diagram corresponding to FIG. 6(a) and shows a second embodiment of the present invention. [Figure 16] FIG. 16 is a diagram corresponding to FIG. 9 and shows a third embodiment of the present invention. [Figure 17] FIG. 17 is a diagram corresponding to FIG. 9 and shows a fourth embodiment of the present invention. [Figure 18] FIG. 18(a) is a diagram showing a state in which the rack and pinion mesh with each other so that the rack tooth tip surface S and the pinion central axis C are parallel to each other in a conventional electric power steering device, and FIG. 18(b) is a diagram showing a state in which the rack and pinion mesh with each other so that the rack tooth tip surface S and the pinion central axis C are inclined to each other. [Figure 19] FIG. 19(a) is a schematic diagram showing the state of contact between the tooth flanks of the rack teeth and the tooth flanks of the pinion teeth in the state shown in FIG. 18(a), and FIG. 19(b) is a schematic diagram showing the state of contact between the tooth flanks of the rack teeth and the tooth flanks of the pinion teeth in the state shown in FIG. 18(b). DETAILED DESCRIPTION OF THE INVENTION

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

[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 corresponds to the axial direction of the rack shaft 11 and the rack housing unit 14, which will be described later. One side in the axial direction of the rack shaft 11 and the rack housing unit 14 is the left side in Figures 1, 3, 4, and 8, and the other side in the axial direction of the rack shaft 11 and the rack housing unit 14 is the right side in Figures 1, 3, 4, and 8.

[0025] The electric power steering device 1 of this example is a dual pinion type electric power steering device that includes one rack shaft 11 and two pinion shafts, a steering-side pinion shaft 10 and an assist-side pinion shaft 13, and inputs a steering force from the steering-side pinion shaft 10 to the rack shaft 11, and inputs an assist driving force from the assist-side pinion shaft 13 to the rack shaft 11.

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

[0027] The steering shaft 3 is rotatably supported inside a steering column 4 supported on the vehicle body. A steering wheel 2 that 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 a steering-side pinion shaft 10 that constitutes a 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 linear axial motion of a rack shaft 11 that constitutes the steering mechanism 8. As a result, a steering angle corresponding to the amount of rotation of the steering wheel 2 is applied to the left and right steered wheels 12. The electric power steering device 1 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 via an assist-side pinion shaft 13 that constitutes an assist mechanism 9.

[0028] In this example, the housing 7 is a casting made as a single unit 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 configured by joining and fixing a plurality of parts to each other. In this example, as shown in FIG. 2, the housing 7 includes a rack accommodating portion 14, a steering-side pinion accommodating portion 15, an assist-side pinion accommodating portion 16, a steering-side guide accommodating portion 17, an assist-side guide accommodating portion 18, a gear housing portion 19, and a plurality of mounting portions 20a, 20b.

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

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

[0031] The assist-side pinion accommodating portion 16 accommodates the assist-side pinion shaft 13 and is disposed at a circumferential portion of the other axial side portion of the rack accommodating portion 14. More specifically, in this example, the assist-side pinion accommodating portion 16 is disposed at the front side of the other axial side portion of the rack accommodating portion 14 and has a cylindrical shape with both ends in the vertical direction open. The assist-side pinion accommodating portion 16 is disposed in a twisted position relative to the rack accommodating portion 14. That is, the central axis of the assist-side pinion accommodating portion 16 and the central axis of the rack accommodating portion 14 are in a twisted positional relationship. When viewed from the front-rear direction, which is a direction perpendicular to both the central axis of the rack accommodating portion 14 and the central axis of the assist-side pinion accommodating portion 16, the central axis of the assist-side pinion accommodating portion 16 is not disposed in a direction perpendicular to the central axis of the rack accommodating portion 14 but is inclined relative to the perpendicular direction. The internal space of the assist-side pinion accommodating portion 16 is in communication with the internal space of the rack accommodating portion 14. When carrying out the present invention, the central axis of the assist-side pinion accommodating portion can also be disposed in a direction perpendicular to the central axis of the rack accommodating portion.

[0032] The steering side guide accommodating portion 17 is a portion that accommodates a steering side rack guide 24, which will be described later, and is arranged in a portion of the rack accommodating portion 14 that is diametrically opposite to the steering side pinion accommodating portion 15. That is, in this example, the steering side guide accommodating portion 17 is arranged in the rear portion of the rack accommodating portion 14 at the same axial position as the steering side pinion accommodating portion 15. The steering side guide accommodating portion 17 has a cylindrical shape and extends in radial directions about the central axis of the rack accommodating portion 14. That is, in this example, the steering side guide accommodating portion 17 extends in the front-rear direction. The internal space of the steering side guide accommodating portion 17 also communicates with the internal space of the rack accommodating portion 14.

[0033] The assist-side guide accommodating portion 18 is a portion that accommodates an assist-side rack guide 37, which will be described later, and is disposed in a portion of the rack accommodating portion 14 that is diametrically opposite the assist-side pinion accommodating portion 16. That is, in this example, the assist-side guide accommodating portion 18 is disposed in the rear portion of the rack accommodating portion 14 at the same axial position as the assist-side pinion accommodating portion 16. The assist-side guide accommodating portion 18 has a cylindrical shape and extends in radial directions about the central axis of the rack accommodating portion 14. That is, in this example, the assist-side guide accommodating portion 18 extends in the front-rear direction. The internal space of the assist-side guide accommodating portion 18 also communicates with the internal space of the rack accommodating portion 14.

[0034] The gear housing portion 19 is a portion that accommodates a worm reducer 38 (to be described later) that constitutes the assist mechanism portion 9, and includes a worm accommodating portion 21 and a wheel accommodating portion 22.

[0035] The wheel accommodating portion 22 is a portion that accommodates the worm wheel 50 that constitutes the worm reducer 38, and is disposed adjacent to the assist side pinion accommodating portion 16 in the axial direction. Specifically, in this example, the wheel accommodating portion 22 is disposed above the assist side pinion accommodating portion 16. The wheel accommodating portion 22 has a substantially cylindrical shape, and is disposed coaxially with the assist side pinion accommodating portion 16.

[0036] The worm accommodating section 21 is a section that accommodates the worm 49 that constitutes the worm reducer 38, and is disposed at a portion of the wheel accommodating section 22 in the circumferential direction. Specifically, in this example, the worm accommodating section 21 is disposed at the front portion of the wheel accommodating section 22. The worm accommodating section 21 has a cylindrical shape with a bottom, and in this example, has an opening at one end in the axial direction of the rack accommodating section 14. The worm accommodating section 21 has a mounting flange 23 that protrudes radially outward at the end on the opening side, i.e., at one end in the axial direction of the rack accommodating section 14. The internal space of the worm accommodating section 21 and the internal space of the wheel accommodating section 22 are in communication with each other.

[0037] The multiple mounting portions 20a, 20b are used to secure the housing 7 to the vehicle body. In this example, the multiple mounting portions 20a, 20b consist of two mounting portions 20a arranged on the front side of both axial ends of the rack housing portion 14, and two mounting portions 20b arranged on the rear side of the axial middle portion of the rack housing portion 14. The housing 7 is secured to the vehicle body using fastening members such as bolts or studs inserted through the mounting portions 20a, 20b, respectively.

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

[0039] The steering-side pinion shaft 10 is a shaft member made of a metal 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 peripheral surface of its tip half that meshes with the steering-side rack 28 of the rack shaft 11.

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

[0041] FIG. 6(a) is a view of the teeth 55 constituting the steering-side pinion 25 in the tooth width direction range α as seen from the radial outside, and FIG. 6(b) is a cross-sectional view taken along the line B-B of FIG. 6(a). Note that in FIG. 6(a), the teeth 55, which actually extend in a spiral direction, are depicted as extending in a linear direction for convenience. The steering-side pinion 25 has a predetermined helix angle. That is, the tooth trace direction of the teeth 55 is inclined by the helix angle with respect to the axial direction of the steering-side pinion shaft 10. In this example, tooth flanks 56, which are the side surfaces on both sides of the tooth 55 in the tooth thickness direction, are curvedly inclined in a direction in which the tooth thickness of the tooth 55 decreases from the root side toward the tip side. In particular, in this example, the tooth flanks 56 are crowned on one half of the tooth trace direction (the lower side in FIG. 6(a)) within the tooth width direction range α. That is, one half of the tooth flank 56 in the tooth trace direction has a crowning shape inclined in a direction in which the tooth thickness decreases curvedly or linearly (curvedly in the illustrated example) toward one side in the tooth trace direction. Therefore, in this example, the tooth thickness of the tooth 55 is constant in the other half of the tooth trace direction (the upper side in FIG. 6( a)) within the tooth width direction range α, and decreases toward one side in the tooth trace direction. Within the tooth width direction range α, the tooth flank 56 is smoothly continuous over the entire length in the tooth trace direction, and does not have any sharp corners along the tooth trace direction, including the boundary between the one half and the other half. Regarding the tooth flank of the steering-side pinion, only the end on one side (the tip end side of the steering-side pinion shaft) of both ends in the tooth trace direction can be crowned, and the end on the other side (the base end side of the steering-side pinion shaft) cannot be crowned. For this reason, in this example, in the tooth width direction range α, crowning is applied to only half of the tooth surface 56 of the tooth 55 of the steering side pinion 25 on one side in the tooth trace direction (the tip side of the steering side pinion shaft).

[0042] The steering side pinion shaft 10 is rotatably supported inside the steering side pinion accommodating portion 15 using bearings 26a and 26b. The central axis of the steering side pinion shaft 10 is arranged coaxially with the central axis of the steering side pinion accommodating portion 15. The steering side 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 rotation of the steering wheel 2. The rotational movement of the steering side pinion shaft 10 is converted into linear movement of the rack shaft 11, which pushes and pulls tie rods 27 connected to both axial ends of the rack shaft 11. This imparts a steering angle to the left and right steered wheels 12.

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

[0044] In this example, the steering-side rack 28 is not subjected to crowning on the tooth surfaces of the teeth in the range α in the tooth width direction (see FIG. 5) that overlaps with the steering-side pinion 25, i.e., over the entire range in the tooth width direction. This point will be explained using FIG. 9.

[0045] FIG. 9 is a perspective view of a tooth 57 constituting the steering-side rack 28. The steering-side rack 28 has a predetermined helix angle. That is, the tooth trace direction of the tooth 57 is not perpendicular to the axial direction of the rack shaft 11, but is inclined by the helix angle with respect to the direction perpendicular to the axial direction of the rack shaft 11 (the up-and-down direction in FIG. 8). In this example, tooth flanks 58, which are the side surfaces on both sides of the tooth 57 in the tooth thickness direction, are linearly inclined in a direction in which the tooth thickness of the tooth 57 decreases from the root side toward the tip side. In this example, the tooth flanks 58 are not entirely crowned. That is, the tooth flanks 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 tooth surfaces of the assist-side rack 29 are not crowned in the tooth width direction range β (see FIG. 11) that overlaps with the assist-side pinion 41, i.e., over the entire range in the tooth width direction. This point will be explained using FIG. 10.

[0047] 10 is a perspective view of a tooth 59 that constitutes the assist-side rack 29. In this example, the assist-side rack 29 also has a predetermined helix angle, and the tooth flank 60 of the tooth 59 is linearly inclined in a direction in which the tooth thickness of the tooth 59 decreases from the tooth base side toward the tooth tip side. Moreover, the tooth flank 60 is not entirely crowned, and is not inclined relative to the tooth trace direction.

[0048] The rack shaft 11 has a threaded hole 30 at each of both axial ends thereof, the threaded hole 30 opening to the axial end face.

[0049] In this example, in consideration of ease of manufacturing the rack shaft 11, a structure is adopted in which the rack shaft 11 can be manufactured separately into two shaft portions. Specifically, in this example, the rack shaft 11 includes a steering-side shaft portion 61 having a steering-side rack 28 on a circumferential portion of its outer circumferential surface, an assist-side shaft portion 62 having an assist-side rack 29 on a circumferential portion of its outer circumferential surface, and a connecting portion 63 connecting the axial end of the steering-side shaft portion 61 to the axial end of the assist-side shaft portion 62. That is, in the rack shaft 11 of this example, the steering-side shaft portion 61 having the steering-side rack 28 and the assist-side shaft portion 62 having the assist-side shaft portion 62 are manufactured separately, and then the axial end of the steering-side shaft portion 61 and the axial end of the assist-side shaft portion 62 are connected by the connecting portion 63. In this example, the connecting portion 63 is configured as a friction-welded portion. However, when implementing the present invention, any connection part such as a welded joint, a friction stir welded joint, or a caulked joint can be used as the connection part 63 as long as the strength required for the connection part 63 can be ensured. The threaded holes 30 provided at both axial ends of the rack shaft 11 can be formed before or after the connection part 63 is formed. 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 so as to be capable of reciprocating axially, with both axial ends protruding from both left and right openings of the rack housing 14. Both axial ends of the rack shaft 11 are connected to tie rods 27 via spherical joints 31. That is, male threads 32 provided at the base of the spherical joint 31 are threadedly engaged with threaded holes 30 provided at both axial ends of the rack shaft 11, and the base end of the tie rod 27 is supported to be swingable at the tip of the spherical joint 31. The rack shaft 11 is connected to the left and right steered wheels 12 via a link mechanism including the spherical joint 31 and the tie rod 27.

[0051] The steering side rack guide 24 is a guide that presses the rack shaft 11 toward the steering side pinion shaft 10, and is disposed inside the steering side guide accommodating portion 17. In other words, the steering side rack guide 24 is disposed so as to sandwich the rack shaft 11 between itself and the steering side pinion shaft 10. As shown in FIGS. 3 and 5, the steering side rack guide 24 of this example is a sliding type rack guide, and includes a pad 33 and an elastic member 34.

[0052] The pad 33 has a generally cylindrical shape and is disposed inside the steering-side guide accommodating portion 17 so as to be movable toward and away from the rack shaft 11. The pad 33 has a pressing surface 35 in the shape of a concave cylindrical surface that matches the shape of the rear side surface of the rack shaft 11 on the surface facing the convex cylindrical rear side surface of the rack shaft 11. The pressing surface 35 is made of a synthetic resin or the like that has excellent sliding properties. In the illustrated example, the elastic member 34 is a torsion coil spring and is sandwiched in an elastically compressed state between the pad 33 and a steering-side cap 36 that closes the opening of the steering-side guide accommodating portion 17. In this way, the elastic member 34 presses the pad 33 toward the rack shaft 11.

[0053] The steering side rack guide 24 presses the rack shaft 11 toward the steering side pinion shaft 10, thereby reducing backlash at the meshing portion between the steering side pinion 25 and the steering side rack 28. This suppresses the generation of abnormal noise at the meshing portion between the steering side pinion 25 and the steering side rack 28.

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

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

[0056] In this example, the tooth surfaces of the assist-side pinion 41 are not crowned at least in the tooth width direction range β (see FIG. 11) that overlaps with the assist-side rack 29. This point will be explained using FIGS. 12(a) and 12(b).

[0057] FIG. 12(a) is a view of the teeth 64 constituting the assist-side pinion 41 in the tooth width direction range β as viewed from the radial outside, and FIG. 12(b) is a DD cross-sectional view of FIG. 12(a). Note that in FIG. 12(a), the teeth 64, which actually extend in a spiral direction, are depicted as extending in a linear direction for convenience. The assist-side pinion 41 has a predetermined helix angle. That is, the tooth trace direction of the teeth 64 is inclined by the helix angle with respect to the axial direction of the assist-side pinion shaft 13. In this example, tooth flanks 65, which are the side surfaces on both sides of the tooth 64 in the tooth thickness direction, are inclined in a curved manner such that the tooth thickness of the tooth 64 decreases from the root side to the tip side. In this example, the tooth flanks 65 are not crowned over the entire tooth trace direction within the tooth width direction range β. That is, the tooth flanks 65 are not inclined with respect to the tooth trace direction. Therefore, the tooth thickness of the tooth 64 is constant over the entire length in the tooth trace direction.

[0058] The assist side pinion shaft 13 is rotatably supported inside the assist side pinion accommodating portion 16 using bearings 42a and 42b. The central axis of the assist side pinion shaft 13 is arranged coaxially with the central axis of the assist side pinion accommodating portion 16. The assist side pinion shaft 13 is rotationally driven by an electric motor 39 via a worm reducer 38. In this example, an opening of the assist side pinion accommodating portion 16 on the opposite side of the wheel accommodating portion 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 disposed inside the assist-side guide accommodating portion 18. In other words, the assist-side rack guide 37 is disposed so as to sandwich the rack shaft 11 between itself and the assist-side pinion shaft 13. As shown in FIGS. 4 and 11 , the assist-side rack guide 37 is a rolling-type rack guide, and has a roller 43, a holder 44, a pin 45, a rolling bearing 46, and an elastic member 47.

[0060] The roller 43 has a generally annular shape and is rotatably supported relative to the holder 44 via a pin 45 and a rolling bearing 46, whose axial direction is oriented in the vertical direction. As a result, the outer peripheral surface of the roller 43 is in rolling contact with the middle portion of the rear side of the rack shaft 11 in the width direction. The outer peripheral surface of the roller 43 has a generatrix-like concave arc shape that generally matches the contour shape of the rear side of the rack shaft 11. The holder 44 is disposed inside the assist-side guide accommodating portion 18 so as to be able to move toward and away from the rack shaft 11. In the illustrated example, the elastic member 47 is a disc spring and is disposed between the holder 44 and an assist-side cap 48 that closes the opening of the assist-side guide accommodating portion 18. The elastic member 47 presses the holder 44 toward the rack shaft 11.

[0061] The assist side rack guide 37 presses the rack shaft 11 toward the assist side pinion shaft 13, thereby reducing backlash at the meshing portion between the assist side pinion 41 and the assist side rack 29. This prevents abnormal noise from being generated at the meshing portion between the assist side pinion 41 and the assist side rack 29.

[0062] In the dual-pinion electric power steering device 1, the assist drive force acting on the meshing portion between the assist-side rack 29 and the assist-side pinion 41 is much 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. For this reason, 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 is likely to be large. As a result, the resistance to linear movement of the rack shaft 11 in the axial direction is likely 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. To solve this problem, in the structure of this example, the pressing portion of the steering-side rack guide 24 is configured with a sliding surface that slides against the outer peripheral surface of the rack shaft 11, while the pressing portion of the assist-side rack guide 37 is configured with a rolling surface that comes into rolling contact with the outer peripheral surface of the rack shaft 11. By adopting this configuration, the frictional force acting between the outer peripheral surface of the rack shaft 11 and the pressing portion of the assist-side rack guide 37 can be kept small.

[0063] As shown in FIG. 11, the worm reducer 38 includes a worm 49 and a worm wheel 50, and reduces the rotation speed of the electric motor 39, i.e., increases the drive 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 circumferential surface and is rotatably supported inside the worm accommodating portion 21. The base end of the worm 49 is connected to the motor output shaft of the electric motor 39 via a coupling or the like (not shown) so as to transmit torque.

[0065] The worm wheel 50 has wheel teeth on its outer circumferential surface that mesh with the worm teeth, and is disposed inside the wheel accommodating portion 22. The worm wheel 50 is fitted and fixed to the base end of the assist-side pinion shaft 13 so as not to rotate relative to the assist-side pinion accommodating portion 16. In this example, an opening of the wheel accommodating portion 22 on the opposite side of the assist-side pinion accommodating portion 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 portion 21 .

[0067] The torque sensor 40 is disposed around the steering side pinion shaft 10 and detects the magnitude and direction of the torque input to the steering side pinion shaft 10. As a result, the torque sensor 40 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. As the torque sensor 40, various types of torque sensors can be used, such as a non-contact torque sensor that utilizes the magnetostrictive effect.

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

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

[0070] The rack bush 53, which is located on one axial side closer to the steering-side pinion shaft 10, is fitted internally near the opening on one axial side of the rack housing portion 14, as shown in Fig. 3. The rack bush 54, which is located on the other axial side closer to the assist-side pinion shaft 13, is fitted internally near the opening on the other axial side of the rack housing portion 14, as shown in Fig. 4. The rack bushes 53, 54 support the outer peripheral surface of the rack shaft 11 so that it can slide in the axial direction. Note that the steering-side rack guide 24 that presses against the rack shaft 11 is a sliding rack guide and can ensure sufficient holding force in the width direction of the rack shaft 11 (the up-and-down direction in Fig. 5), 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 range α in the tooth width direction (see FIG. 5) where the steering side rack 28 and the steering side pinion 25 overlap each other, the tooth flanks of the teeth of at least one of the steering side rack 28 and the steering side pinion 25 have a crowning shape. Specifically, in this example, of the steering side rack 28 and the steering side pinion 25, only the tooth flank 56 of the tooth 55 of the steering side pinion 25 (see FIGS. 6(a) and 6(b)) has a crowning shape in one half portion in the tooth trace direction, and the tooth flank 58 of the tooth 57 of the steering side rack 28 (see FIG. 9) does not have a crowning shape. On the other hand, in the tooth width direction range β (see Figure 11) where the assist side rack 29 and the assist side pinion 41 overlap each other, the tooth surface 60 of the tooth 59 of the assist side rack 29 (see Figure 10) and the tooth surface 65 of the tooth 64 of the assist side pinion 41 (see Figures 12(a) and 12(b)) do not have a crowning shape.

[0072] Therefore, for example, even if a manufacturing error in the rack shaft 11 causes 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 to deviate from a predetermined positional relationship defined by design, or even if a manufacturing error in the housing 7 causes the relative positional relationship between the steering-side pinion 25 and the assist-side pinion 41 in the circumferential direction of the rack shaft 11 to deviate from a predetermined positional relationship defined by design, the assist-side rack 29 can be meshed with the assist-side pinion 41 so that the tooth tip surface S2 of the assist-side rack 29 and the central axis C2 of the assist-side pinion 41 are parallel to each other, as shown in FIG. 13(a). In this case, as shown in FIG. 14(a), the tooth flanks 60 of the teeth 59 of the assist-side rack 29 can be brought into surface contact with the tooth flanks 65 of the teeth 64 of the assist-side pinion 41 over a wide contact area. Therefore, the stress acting on these teeth 59, 64 can be kept small. Therefore, the durability of the teeth 59, 64 can be ensured without making the tooth thickness of the teeth 59, 64 excessively large.

[0073] On the other hand, if a deviation in the positional relationship occurs, the steering side rack 28 and the steering side pinion 25 mesh with each other so that the tooth tip surface S1 of the steering side rack 28 and the central axis C1 of the steering side pinion 25 are inclined relative to each other, as shown in FIG. 13(b), for example. However, in this example, even in such a case, since one half of the tooth flank 56 of the tooth 55 of the steering side pinion 25 in the tooth trace direction has a crowning shape, as shown in FIG. 14(b), the tooth flank 58 of the tooth 57 of the steering side rack 28 contacts the tooth flank 56 of the tooth 55 of the steering side pinion 25 over 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, 57 can be kept small. As a result, the durability of these teeth 55, 57 can be ensured without excessively increasing the tooth thickness of these teeth 55, 57.

[0074] In this example, the rack shaft 11 is formed 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 by 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 likely to deviate from the predetermined positional relationship determined by the design, particularly due to manufacturing errors that may occur when the connecting portion 63 is formed. Therefore, the technique of applying crowning to the tooth surfaces of the teeth of at least one of the steering-side rack 28 and the steering-side pinion 25 as described above, thereby ensuring a wide contact area between the tooth surfaces of the rack and the pinion on both the steering side and the assist side, is particularly effective.

[0075] Incidentally, one possible method for ensuring a sufficient contact area between the tooth surfaces of the steering side rack and the steering side pinion, and for ensuring a sufficient contact area between the tooth surfaces of the teeth of the assist side rack and the assist side pinion, is to apply crowning processing to the tooth surfaces of at least one of the teeth of the assist side rack and the assist side pinion.

[0076] However, the assist drive force acting on the meshing portion between the assist side rack and the assist side pinion is much larger than the steering force acting on the meshing portion between the steering side rack and the steering side pinion, and specifically, for example, is about 10 to 20 times the magnitude of the steering force. For this reason, the durability of the teeth of the assist side rack 29 and the assist side pinion 41 is emphasized more than that of the steering side rack 28 and the steering side pinion 25.

[0077] On the other hand, when the tooth flanks are crowned, the tooth thickness of the teeth is reduced at the ends of the teeth in the tooth trace direction. Therefore, when the tooth flanks are crowned, the durability of the teeth is reduced compared to when the tooth flanks are not crowned. Therefore, from the perspective of prioritizing the durability of the teeth of the assist-side rack and the assist-side pinion, that is, from the perspective of making it easier to ensure the durability of the teeth, it is not appropriate to crown the tooth flanks of the teeth of the assist-side rack and the assist-side pinion.

[0078] In this regard, in this example, crowning is performed only on the tooth surfaces of the teeth of at least one of the steering side rack 28 and the steering side pinion 25, and crowning is not performed on either the tooth surfaces 60 of the teeth 59 of the assist side rack 29 or the tooth surfaces 65 of the teeth 64 of the assist side pinion 41. This makes it easier to ensure the durability of the teeth 59 of the assist side rack 29 and the teeth 64 of the assist side pinion 41.

[0079] Furthermore, in this example, under conditions in which the assist driving force acting on the meshing portion between the assist-side rack 29 and the assist-side pinion 41 is much greater 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 flanks 65 of the teeth 64 of the assist-side pinion 41, which have not been subjected to crowning, come into surface contact over a wide contact area with the tooth flanks 60 of the teeth 59 of the assist-side rack 29, without coming into tilted, one-sided contact. Therefore, this is also advantageous 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, of the steering side rack 28 and the steering side pinion 25, only the tooth flanks 56 of the teeth 55 of the steering side pinion 25 are crowned. This reduces the machining costs of the steering side rack 28 and the steering side pinion 25. That is, the crowning of the tooth flanks 56 of the teeth 55 of the steering side pinion 25 can be performed by hobbing, which is a low-cost machining method, but the crowning of the tooth flanks 58 of the teeth 57 of the steering side rack 28 cannot be performed by hobbing and must be performed by a costly forging process. In this regard, in this example, the crowning is not performed on the tooth flanks 58 of the teeth 57 of the steering side rack 28, so the forming of the steering side rack 28 can be performed by hobbing. This reduces the machining costs of the steering side pinion 25 and the steering side rack 28.

[0081] The reason why the crowning process cannot be performed on the tooth flanks 58 of the teeth 57 of the steering-side rack 28 by hobbing is that the teeth 57 of the steering-side rack 28 have a twist angle and the tooth flanks of these teeth 57 are arranged on the same imaginary plane. In other words, if a crowning shape is to be imparted to the tooth flanks 58 of the teeth 57 of such a steering-side rack 28 by hobbing, the positions in the tooth width direction at which the tooth thickness is maximum will differ between adjacent teeth 57, making it impossible to impart the same crowning shape to the tooth flanks 58 of each tooth 57.

[0082] [Example 2] A second embodiment of the present invention will be described with reference to FIG.

[0083] FIG. 15 is a diagram of this example, corresponding to FIG. 6(a). In this example, of the tooth flanks 56a of the teeth 55a constituting the steering-side pinion 25a, only the end 67a on one side (lower side in FIG. 15) in the tooth trace direction is crowned. That is, in this example, of the tooth flanks 56a of the teeth 55a constituting the steering-side pinion 25a, the middle portion 66 in the tooth trace direction and the end 67b on the other side (upper side in FIG. 15) are not crowned. That is, in this example, the middle portion 66 and the end 67b on the other side are not inclined with respect to the tooth trace direction, and only the end 67a on one side has a crowning shape inclined in a direction in which the tooth thickness of the tooth 55a decreases curvedly or linearly (curved in the illustrated example) toward one side in the tooth trace direction. Therefore, in this example, the tooth thickness of the tooth 55a decreases toward one side in the tooth trace direction within the same range as the end 67a on one side in the tooth trace direction. The tooth surface 56a is smooth and continuous over the 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, when the steering-side rack 28 and the assist-side rack 29 (see FIG. 1) are manufactured so that the relative positional relationship between them in the circumferential direction of the rack shaft 11 is a predetermined positional relationship determined by design, and when the steering-side pinion 25 and the assist-side pinion 41 (see FIG. 1) are manufactured so that the relative positional relationship between them in the circumferential direction of the rack shaft 11 is a predetermined positional relationship determined by design, the teeth of the steering-side pinion 25a 55a This allows the tooth surface 56a of the tooth 57 to come into contact with the tooth surface 58 (see FIG. 9) of the steering-side rack 28 over a wider contact area in the intermediate portion 66. The other configurations and effects are the same as those of the first example.

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

[0086] In this example, the steering-side pinion (not shown) that meshes with the steering-side rack 28a is not crowned over the entire tooth surface, similar to the assist-side pinion 41 shown in Figures 12(a) and 12(b). In other words, the tooth surface is not inclined relative to the tooth trace direction. Therefore, the tooth thickness of the tooth is constant over the entire length in the tooth trace direction.

[0087] Instead, in this example, as shown in Fig. 16, the steering-side rack 28a has a crowning process performed on the entire tooth flank 58a of each tooth 57a. That is, the tooth flank 58a has a crowning shape that is inclined in a direction in which the tooth thickness of the tooth 57a decreases in a curved or linear manner (curved in the illustrated example) from the center of the tooth 57a in the tooth trace direction toward both sides. Therefore, in this example, the tooth thickness of the tooth 57a is greatest at the center of the tooth trace direction and decreases toward both sides in the tooth trace direction. The tooth flank 58a is smoothly continuous over the entire length in the tooth trace direction and does not have any sharp corners along the tooth trace direction.

[0088] In the structure of this example, because the tooth flanks 58a of the teeth 57a of the steering-side rack 28a are crowned, even if a manufacturing error in the rack shaft 11 causes the relative positional relationship between the steering-side rack 28a and the assist-side rack 29 (see FIG. 1) in the circumferential direction of the rack shaft 11 to deviate from the predetermined positional relationship determined by design, or even if a manufacturing error in the housing 7 (see FIG. 2) causes the relative positional relationship between the steering-side pinion and the assist-side pinion 41 (see FIG. 1) in the circumferential direction of the rack shaft 11 to deviate from the predetermined positional relationship determined by design, the tooth flanks 58a of the teeth 57a of the steering-side rack 28a can be brought into contact with the tooth flanks of the teeth of the steering-side pinion over a relatively wide contact area in a portion away from the center in the tooth trace direction. The other configurations, functions, and effects are the same as those of the first example.

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

[0090] In this example, the tooth flanks 58b of the teeth 57b constituting the steering-side rack 28b are not crowned in the intermediate portion 68 in the tooth trace direction, but are crowned only in the end portions 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 relative to the tooth trace direction, and only the end portions 69 on both sides have a crowning shape inclined in a direction in which the tooth thickness of the tooth 57b decreases curvedly or linearly (curved in the illustrated example) from the center toward 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 in the same range as the end portions 69 in the tooth trace direction from the center toward both sides in the tooth trace direction. The tooth flanks 58b are smoothly continuous over the entire length in the tooth trace direction, and do not have any sharp corners along the tooth trace direction, including the boundary between the intermediate portion 68 and the end portions 69.

[0091] In the structure of this example, when the positional relationship between the steering-side rack 28b and the assist-side rack 29 (see FIG. 1) in the circumferential direction of the rack shaft 11 is a predetermined positional relationship determined by design, and when the positional relationship between the steering-side pinion and the assist-side pinion 41 (see FIG. 1) in the circumferential direction of the rack shaft 11 is manufactured to be a predetermined positional relationship determined by design, the tooth surfaces 58b of the teeth 57b of the steering-side rack 28b can be brought into contact with the tooth surfaces of the teeth of the steering-side pinion over a wider contact area at the intermediate portion 68. The other configurations and effects are the same as those of the third example.

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

[0093] The present invention can be implemented by appropriately combining the structures of the above-described embodiments within the scope of not causing any contradiction. [Explanation of symbols]

[0094] 1 Electric power steering device 2 steering wheels 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 wheel 13 Assist side pinion shaft 14 Rack storage area 15 Steering side pinion housing 16 Assist side pinion housing 17 Steering side guide housing 18 Assist side guide housing 19 Gear housing 20a, 20b Mounting parts 21 Worm housing 22 Wheel housing 23 Mounting flange 24 Steering side rack guide 25, 25a Steering side pinion 26a, 26b bearings 27 tie rod 28, 28a, 28b Steering rack 29 Assist side rack 30 screw holes 31 Spherical joint 32 Male thread 33 Pad 34 Elastic member 35 Pressing surface 36 Steering side cap 37 Assist side rack guide 38 Worm reducer 39 Electric Motor 40 Torque sensor 41 Assist side pinion 42a, 42b bearings 43 Laura 44 Holder 45-pin 46 Rolling bearings 47 Elastic member 48 Assist side cap 49 Warm 50 worm wheel 51 Cap 52 Cap 53 Luck Bush 54 Luck Bush 55, 55a teeth 56, 56a tooth surface 57, 57a, 57b teeth 58, 58a, 58b tooth surface 59 teeth 60 Tooth surface 61 Steering side shaft 62 Assist side shaft 63 Connection 64 teeth 65 Tooth surface 66 Middle section 67a, 67b end 68 Middle 69 End 100 rack axis 101 Luck 102 Pinion shaft 103 Pinion 104 teeth 105 Tooth surface 106 teeth 107 Tooth surface

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