JP7762082B2 - Electric power steering device and rack shaft for electric power steering device - Google Patents

Description

The present invention relates to a dual-pinion electric power steering device and the rack shaft that constitutes the device.

An automobile steering system is configured to transmit the rotation of the steering wheel, which is turned by the driver, to the steering-side pinion shaft of the steering gear unit via the steering shaft, intermediate shaft, etc., and then 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.

In the field of steering devices, electric power steering devices are widely used, which are configured to apply an assist drive force to the steering force transmission path in order to reduce the force required to rotate the steering wheel. There are various types of electric power steering devices, each with a different way of applying an assist drive force to the steering force transmission path.

A dual-pinion electric power steering device, which is the subject of this invention, as described in, for example, Japanese Patent Application Laid-Open No. 2002-154442 (Patent Document 1), includes a steering-side pinion shaft and an assist-side pinion shaft as pinion shafts that mesh with the rack shaft. That is, the rack shaft has a steering-side rack on one axial side portion of its outer circumferential surface, and an assist-side rack on the other axial side portion of its outer circumferential surface, with both axial ends connected to steered wheels. 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 rotation of the steering wheel. 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. 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.

Prior art related to the present invention is JP 2005-53327 A (Patent Document 2). This publication describes an electric power steering device of the 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). Furthermore, this publication describes a technique for applying crowning to the tooth flanks of the steering-side pinion teeth 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 steering-side pinion teeth and the tooth flanks of the steering-side rack teeth 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, and smoothing the operation of the ball screw mechanism.

Japanese Patent Application Laid-Open No. 2002-154442 Japanese Patent Application Laid-Open No. 2005-53327

In a conventional dual-pinion electric power steering device, when the positional relationship between the steering-side rack and steering-side pinion in the circumferential direction of the rack shaft, and the positional relationship between the assist-side rack and assist-side pinion in the circumferential direction of the rack shaft, are each predetermined as determined by design, the rack 101 and pinion 103 mesh on both the steering and assist sides so that the tooth tip surface S of the rack 101, which is provided on a portion of the circumferential surface of the rack shaft 100, and the central axis C of the pinion 103, which is provided on the 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), the tooth flanks 105 of the teeth 104 of the rack 101 come into surface contact with the tooth flanks 107 of the teeth 106 of the pinion 103 over a wide contact area.

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 specified positional relationship. As a result, at least one of the relative positional relationship between the steering-side rack and the steering-side pinion in the circumferential direction of the rack shaft and the relative positional relationship between the assist-side rack and the assist-side pinion in the circumferential direction of the rack shaft may deviate from the specified 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 the end edge (portion P) in the tooth trace direction, i.e., the contact area tends to be small. As a result, the stress acting on each tooth surface 105, 107 increases.

However, even in such cases, the durability of each tooth 104, 106 can be ensured by increasing the overall tooth thickness of each tooth 104, 106. However, increasing the overall tooth thickness of each tooth 104, 106 can easily lead to an increase in the weight of the rack shaft 100 and pinion shaft 102.

The technology described in JP 2005-53327 A for applying crowning to the tooth surfaces of the steering-side pinion teeth is intended for ball-screw type electric power steering devices.

The present invention aims to provide an electric power steering device and a rack shaft that constitutes said device, which makes it easy to ensure a sufficient contact area between the tooth surfaces of the rack and the pinion on both the steering and assist sides, regardless of manufacturing errors in the rack shaft.

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.

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

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 rotating the steering wheel.

The assist side pinion shaft has an assist side pinion on its outer surface that meshes with the assist side rack, and is driven to rotate by an electric motor.

Of the tooth flanks of the teeth of the steering side rack, the tooth flanks of the teeth of the steering side pinion in the tooth width direction that overlap 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 the tooth width direction that overlap with the assist side rack, crowning is applied to at least a portion of the tooth flanks of the teeth of the steering side rack in the tooth trace direction and/or only a portion of the tooth flanks of the teeth of the steering side pinion in the tooth width direction that overlap with the steering side rack. The crowned portions have a crowning shape that is inclined in the direction in which the tooth thickness decreases toward the end in the tooth trace direction.

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

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 connection portion connecting the axial end of the steering-side shaft portion to the axial end of the assist-side shaft portion.

One embodiment of the rack shaft for an electric power steering device 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 flanks of the teeth of the steering-side rack and the assist-side rack, only the tooth flanks of the teeth of the steering-side rack are crowned in at least a portion of the tooth trace direction.

One embodiment of the rack shaft for an electric power steering device 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 the axial end of the steering-side shaft portion to the axial end of the assist-side shaft portion.

One aspect of the present invention provides an electric power steering device and a rack shaft that constitutes the device, which can easily ensure a sufficient contact area between the tooth surfaces of the rack and the pinion on each side, even when the circumferential positional relationship between the steering-side rack and the assist-side rack deviates from the specified positional relationship.

FIG. 1 is a schematic diagram showing an electric power steering device according to a first embodiment of the present invention. FIG. 2 is an exploded perspective view showing some components of the electric power steering device of the first example. FIG. 3 is a cross-sectional view showing one axial side portion of the rack shaft of the first example and the members arranged around it. 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. FIG. 5 is a cross-sectional view taken along line AA in FIG. 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). FIG. 7 is a perspective view of the rack shaft of the first example. FIG. 8 is a plan view of the rack shaft of the first example. FIG. 9 is a perspective view of teeth that constitute the steering-side rack of the first example. FIG. 10 is a perspective view of teeth that constitute the assist-side rack of the first example. FIG. 11 is a cross-sectional view taken along the line CC in FIG. FIG. 12(a) is a diagram showing the teeth that make up the assist-side pinion of the first example as viewed from the radially outer side, and FIG. 12(b) is a cross-sectional view taken along line DD of FIG. 12(a). 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 such 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 such 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. 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). FIG. 15 is a diagram corresponding to FIG. 6(a) and shows a second embodiment of the present invention. FIG. 16 is a diagram corresponding to FIG. 9 and shows a third embodiment of the present invention. FIG. 17 is a diagram corresponding to FIG. 9 and shows a fourth embodiment of the present invention. 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. 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).

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

In the following description, the front-to-rear direction refers to the front-to-rear direction of the vehicle, the up-down direction refers to the up-down direction of the vehicle, and the left-to-right direction refers to the width direction of the vehicle. The left-to-right direction corresponds to the axial direction of the rack shaft 11 and rack housing unit 14, which will be described later. One side in the axial direction of the rack shaft 11 and 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 rack housing unit 14 is the right side in Figures 1, 3, 4, and 8.

The electric power steering device 1 of this example is a dual pinion type. That is, the electric power steering device 1 of this example has one rack shaft 11 and two pinion shafts, a steering side pinion shaft 10 and an assist side pinion shaft 13, and is an electric power steering device in which steering force is input from the steering side pinion shaft 10 to the rack shaft 11, and assist driving force is input from the assist side pinion shaft 13 to the rack shaft 11.

The electric power steering device 1 of this example, as shown in the overall configuration in Figure 1, comprises 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 Figure 1, see Figure 2), a steering mechanism 8, and an assist mechanism 9.

The steering shaft 3 is rotatably supported inside a steering column 4 supported on 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 a steering-side pinion shaft 10, which constitutes a steering mechanism 8, via a universal joint 5a, an intermediate shaft 6, and another universal joint 5b. Therefore, 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 a rack shaft 11, which constitutes the steering mechanism 8. This imparts a steering angle to the left and right steered wheels 12 in accordance with 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 an assist driving force to the rack shaft 11 via an assist-side pinion shaft 13, which constitutes an assist mechanism 9.

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 constructed by joining and fixing multiple parts together. In this example, as shown in FIG. 2, the housing 7 includes a rack housing portion 14, a steering side pinion housing portion 15, an assist side pinion housing portion 16, a steering side guide housing portion 17, an assist side guide housing portion 18, a gear housing portion 19, and multiple mounting portions 20a, 20b.

The rack housing 14 houses the axially middle portion of the rack shaft 11 that constitutes the steering mechanism 8. It has a cylindrical shape and extends in the left-right direction.

The steering side pinion accommodating portion 15 accommodates the tip half of the steering side pinion shaft 10 and is located circumferentially on one axial side of the rack accommodating portion 14. More specifically, the steering side pinion accommodating portion 15 is located on the front side of one axial side 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 positioned in a twisted 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 relationship. When viewed from the fore-and-aft 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 positioned in a direction perpendicular to the central axis of the rack accommodating portion 14 but is tilted relative to the perpendicular direction. The internal space of the steering-side pinion housing 15 is connected to the internal space of the rack housing 14.

The assist-side pinion accommodating section 16 accommodates the assist-side pinion shaft 13 and is located at a circumferential portion of the other axial side of the rack accommodating section 14. More specifically, in this example, the assist-side pinion accommodating section 16 is located at the front of the other axial side of the rack accommodating section 14 and has a cylindrical shape with open ends on both the top and bottom sides. The assist-side pinion accommodating section 16 is positioned in a twisted relationship with respect to the rack accommodating section 14. In other words, the central axis of the assist-side pinion accommodating section 16 and the central axis of the rack accommodating section 14 are in a twisted relationship. When viewed from the front-to-rear direction, which is a direction perpendicular to both the central axis of the rack accommodating section 14 and the central axis of the assist-side pinion accommodating section 16, the central axis of the assist-side pinion accommodating section 16 is not positioned in a direction perpendicular to the central axis of the rack accommodating section 14 but is inclined relative to the perpendicular direction. The internal space of the assist-side pinion accommodating section 16 is connected to the internal space of the rack accommodating section 14. When implementing this invention, the central axis of the assist side pinion housing can also be positioned perpendicular to the central axis of the rack housing.

The steering side guide accommodating portion 17 accommodates the steering side rack guide 24 (described below), and is located in a portion of the rack accommodating portion 14 that is diametrically opposite the steering side pinion accommodating portion 15. That is, in this example, the steering side guide accommodating portion 17 is located 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 fore-and-aft direction. The internal space of the steering side guide accommodating portion 17 is also connected to the internal space of the rack accommodating portion 14.

The assist side guide accommodating portion 18 accommodates the assist side rack guide 37 (described below), and is located 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 located 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 centered on 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-to-rear direction. The internal space of the assist side guide accommodating portion 18 is also connected to the internal space of the rack accommodating portion 14.

The gear housing portion 19 houses the worm reducer 38 (described below) that constitutes the assist mechanism portion 9, and includes a worm housing portion 21 and a wheel housing portion 22.

The wheel accommodating portion 22 accommodates the worm wheel 50 that constitutes the worm reducer 38, and is disposed axially adjacent to the assist side pinion accommodating portion 16. 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 generally cylindrical shape and is disposed coaxially with the assist side pinion accommodating portion 16.

The worm accommodating section 21 accommodates the worm 49 that constitutes the worm reducer 38, and is located at a portion of the wheel accommodating section 22 in the circumferential direction. Specifically, in this example, the worm accommodating section 21 is located 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 open end, 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 connected to each other.

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 located on the front side of both axial ends of the rack housing portion 14, and two mounting portions 20b located 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 that pass through each of the mounting portions 20a, 20b.

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.

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.

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

Figure 6(a) is a view of the teeth 55 constituting the steering side pinion 25 in the tooth width direction range α, viewed from the radial outside, and Figure 6(b) is a cross-sectional view taken along the line B-B of Figure 6(a). Note that in Figure 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 relative to the axial direction of the steering side pinion shaft 10. In this example, the tooth flanks 56, which are the side surfaces on both sides of the tooth 55 in the tooth thickness direction, are curved and inclined in a direction in which the tooth thickness of the teeth 55 decreases from the tooth root side toward the tooth 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 Figure 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. 6A ) 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 one end (the tip end side of the steering-side pinion shaft) of both ends in the tooth trace direction can be crowned, and the other end (the base end side of the steering-side pinion shaft) cannot be crowned. For this reason, in this example, within the tooth width direction range α, crowning is applied to only half of the tooth flanks 56 of the teeth 55 of the steering side pinion 25 on one side in the tooth trace direction (the tip side of the steering side pinion shaft).

The steering side pinion shaft 10 is rotatably supported inside the steering side pinion housing 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 housing 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 rotating the steering wheel 2. The rotational motion of the steering side pinion shaft 10 is converted into linear motion 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.

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 phase deviations 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 phase of the steering side rack 28 and the assist side rack 29 in the circumferential direction of the rack shaft 11 can be made to differ from each other by an amount that exceeds the phase shift due to manufacturing errors.

In this example, the steering side rack 28 has no crowning applied to the tooth surfaces of the teeth in the tooth width direction range α (see Figure 5) that overlaps with the steering side pinion 25, i.e., the entire range in the tooth width direction. This point will be explained using Figure 9.

Figure 9 is a perspective view of a tooth 57 that constitutes 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 relative to the direction perpendicular to the axial direction of the rack shaft 11 (the up-and-down direction in Figure 8). In this example, the 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 tooth base side toward the tooth tip side. In this example, the tooth flanks 58 are not entirely crowned. That is, the tooth flanks 58 are not inclined relative to the tooth trace direction. Therefore, the tooth thickness of the tooth 57 is constant over the entire length in the tooth trace direction.

In this example, the tooth surfaces of the assist side rack 29 are not crowned over the tooth width direction range β (see Figure 11) that overlaps with the assist side pinion 41, i.e., over the entire tooth width direction range. This point will be explained using Figure 10.

Figure 10 is a perspective view of the teeth 59 that make up the assist-side rack 29. In this example, the assist-side rack 29 also has a predetermined helix angle, and the tooth flanks 60 of the teeth 59 are linearly inclined in a direction in which the tooth thickness of the teeth 59 decreases from the tooth base side toward the tooth tip side. Furthermore, the tooth flanks 60 are not crowned overall, and are not inclined relative to the tooth trace direction.

The rack shaft 11 has a threaded hole 30 that opens onto the axial end face at each of its two axial ends.

In this example, in consideration of ease of manufacturing the rack shaft 11, a structure is adopted that allows the rack shaft 11 to be manufactured in two separate 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. In other words, the rack shaft 11 in this example is formed by separately manufacturing 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, and then connecting the axial end of the steering-side shaft portion 61 to the axial end of the assist-side shaft portion 62 via 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 type of connection can be used as the connection 63, such as a welded joint, a friction stir weld, or a caulked joint, as long as the required strength can be ensured for the connection 63. The threaded holes 30 provided at both axial ends of the rack shaft 11 can be formed before or after the connection 63 is constructed. When implementing the present invention, the rack shaft can also be made from a single rod-shaped material.

The rack shaft 11 is supported inside the rack housing 14 so that both axial ends protrude from the left and right openings of the rack housing 14 and can move back and forth in the axial direction. Both axial ends of the rack shaft 11 are connected to tie rods 27 via spherical joints 31. Specifically, male threads 32 provided at the base of the spherical joints 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 so that it can swing 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 that includes the spherical joints 31 and the tie rods 27.

The steering side rack guide 24 is a guide that presses the rack shaft 11 toward the steering side pinion shaft 10 and is arranged inside the steering side guide accommodating portion 17. In other words, the steering side rack guide 24 is arranged so that the rack shaft 11 is sandwiched between it and the steering side pinion shaft 10. As shown in Figures 3 and 5, the steering side rack guide 24 in this example is a sliding rack guide and includes a pad 33 and an elastic member 34.

The pad 33 has a generally cylindrical shape and is positioned inside the steering side guide housing 17 so that it can move toward and away from the rack shaft 11. The pad 33 has a concave cylindrical pressing surface 35, whose shape matches 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 other material with excellent slip 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 housing 17. This allows the elastic member 34 to press the pad 33 toward the rack shaft 11.

The steering side rack guide 24 presses the rack shaft 11 toward the steering side pinion shaft 10, reducing backlash at the meshing portion between the steering side pinion 25 and the steering side rack 28. This prevents abnormal noise from being generated at the meshing portion between the steering side pinion 25 and the steering side rack 28.

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.

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 on the outer peripheral surface of the tip half that meshes with the assist side rack 29 of the rack shaft 11.

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 Figure 11) that overlaps with the assist side rack 29. This point will be explained using Figures 12(a) and 12(b).

Figure 12(a) is a view of the teeth 64 constituting the assist-side pinion 41 in the tooth width direction range β, viewed from the radial outside, and Figure 12(b) is a cross-sectional view taken along the line D-D of Figure 12(a). Note that in Figure 12(a), the teeth 64, which actually extend in a spiral direction, are depicted as extending linearly 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 relative to the axial direction of the assist-side pinion shaft 13. In this example, the tooth flanks 65, which are the side surfaces on both sides of the tooth 64 in the tooth thickness direction, are curvedly inclined in a direction in which the tooth thickness of the teeth 64 decreases from the tooth root side toward the tooth tip side. In this example, the tooth flanks 65 are not crowned across the entire tooth trace direction within the tooth width direction range β. That is, the tooth flanks 65 are not inclined relative to the tooth trace direction. As a result, the tooth thickness of the teeth 64 is constant over the entire length in the tooth trace direction.

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 arranged coaxially with the central axis of the assist side pinion housing 16. The assist side pinion shaft 13 is driven to rotate by an electric motor 39 via a worm reducer 38. In this example, the opening of the assist side pinion housing 16 on the axially opposite side from the wheel housing 22 is closed by a cap 51.

The assist side rack guide 37 is a guide that presses the rack shaft 11 toward the assist side pinion shaft 13, and is arranged inside the assist side guide accommodating portion 18. In other words, the assist side rack guide 37 is arranged so that the rack shaft 11 is sandwiched between it 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 has a roller 43, a holder 44, a pin 45, a rolling bearing 46, and an elastic member 47.

The roller 43 has a generally annular shape and is rotatably supported on the holder 44 via a pin 45 and a rolling bearing 46, whose axial direction is oriented vertically. This causes the outer peripheral surface of the roller 43 to be 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 that roughly matches the contour of the rear side of the rack shaft 11. The holder 44 is positioned inside the assist side guide accommodating section 18 so that it can move toward and away from the rack shaft 11. In the illustrated example, the elastic member 47 is a disc spring and is positioned between the holder 44 and an assist side cap 48 that closes the opening of the assist side guide accommodating section 18. The elastic member 47 presses the holder 44 toward the rack shaft 11.

The assist side rack guide 37 presses the rack shaft 11 toward the assist side pinion shaft 13, 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.

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 greater 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 greater 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 as sliding surfaces that slide against the outer peripheral surface of the rack shaft 11, 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 is likely to be large. As a result, the resistance to linear axial movement of the rack shaft 11 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 as 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 as 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.

As shown in Figure 11, the worm reducer 38 includes a worm 49 and a worm wheel 50, and reduces the rotation 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.

The worm 49 has worm teeth on its outer circumferential surface 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 to enable torque transmission.

The worm wheel 50 has wheel teeth on its outer circumferential surface that mesh with the worm teeth, and is positioned inside the wheel accommodating section 22. The worm wheel 50 is fitted onto the base end of the assist-side pinion shaft 13 so that it cannot rotate relative to the assist-side pinion accommodating section 16. In this example, the opening of the wheel accommodating section 22 on the axially opposite side from the assist-side pinion accommodating section 16 is closed by a cap 52.

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

The torque sensor 40 is arranged around the steering side pinion shaft 10 and detects the magnitude and direction of the torque input to the steering side pinion shaft 10. This causes the torque sensor 40 to output 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 that utilizes the magnetostrictive effect.

The assist mechanism 9 controls the drive of the electric motor 39 based on the output signal of the torque sensor 40. As a result, the drive torque generated by the electric motor 39 is transmitted as assist drive 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 rotate the steering wheel 2 is reduced.

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 roughly cylindrical shape.

The rack bushing 53, located on one axial side closer to the steering-side pinion shaft 10, is fitted within the rack housing 14 near the opening on one axial side, as shown in FIG. 3. The rack bushing 54, located on the other axial side closer to the assist-side pinion shaft 13, is fitted within the rack housing 14 near the opening on the other axial side, as shown in FIG. 4. The rack bushings 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, which can ensure sufficient holding force in the width direction of the rack shaft 11 (the up-and-down direction in FIG. 5). Therefore, the rack bushing 53 located on one axial side can be omitted.

As described above, in the electric power steering device 1 of this example, in the tooth width direction range α (see Figure 5) where the steering side rack 28 and the steering side pinion 25 overlap each other, the tooth flanks of at least one of the teeth 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 Figures 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 Figure 9) does not have a crowning shape. On the other hand, in the tooth width direction range β (see FIG. 11) where the assist side rack 29 and the assist side pinion 41 overlap, the tooth flank 60 of the tooth 59 of the assist side rack 29 (see FIG. 10) and the tooth flank 65 of the tooth 64 of the assist side pinion 41 (see FIGS. 12(a) and 12(b)) do not have a crowning shape.

Therefore, even if, for example, 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 the predetermined positional relationship defined by the design, or 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 the predetermined positional relationship defined by the design, the assist-side rack 29 and the assist-side pinion 41 can be meshed 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 flank 60 of the tooth 59 of the assist-side rack 29 can be in surface contact with the tooth flank 65 of the tooth 64 of the assist-side pinion 41 over a wide contact area. This allows the stress acting on these teeth 59, 64 to be kept low. This ensures the durability of these teeth 59, 64 without making the tooth thickness of these teeth 59, 64 excessively large.

On the other hand, if the above-mentioned positional relationship is misaligned, the steering side rack 28 and the steering side pinion 25 mesh together 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 Figure 13(b), for example. However, in this example, even in such a case, because 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 Figure 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 at a portion offset to one side from the center in the tooth trace direction. This allows the stress acting on these teeth 55, 57 to be kept low. As a result, the durability of these teeth 55, 57 can be ensured without excessively increasing the tooth thickness of these teeth 55, 57.

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 via a connecting portion 63. With this type of rack shaft 11, manufacturing errors, particularly when constructing the connecting portion 63, can easily cause 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 the predetermined positional relationship determined by the design. Therefore, the technique of crowning the tooth surfaces of at least one of the teeth of the steering-side rack 28 and the steering-side pinion 25, as described above, to ensure 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.

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 also between the tooth surfaces of the teeth of the assist side rack and the assist side pinion, is to apply crowning to the tooth surfaces of at least one of the teeth of the assist side rack and the assist side pinion.

However, the assist drive force acting on the meshing portion between the assist side rack and assist side pinion is much larger than the steering force acting on the meshing portion between the steering side rack and steering side pinion; specifically, it is, for example, approximately 10 to 20 times the magnitude of the steering force. For this reason, tooth durability is given more importance to the assist side rack 29 and assist side pinion 41 than to the steering side rack 28 and steering side pinion 25.

On the other hand, when crowning is applied to the tooth flanks, the tooth thickness of the teeth becomes smaller at the ends of the teeth in the tooth trace direction. Therefore, when crowning is applied to the tooth flanks, the durability of the teeth decreases 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, i.e., 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.

In this regard, in this example, crowning is applied only to the tooth surfaces of at least one of the teeth of the steering side rack 28 and the steering side pinion 25, and crowning is not applied to 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.

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 with the tooth flanks 60 of the teeth 59 of the assist side rack 29 over a wide contact area, without making an inclined, one-sided contact. Therefore, this also has 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.

In this example, of the steering side rack 28 and the steering side pinion 25, crowning is performed only on the tooth flanks 56 of the teeth 55 of the steering side pinion 25. This reduces the processing costs for the steering side rack 28 and the steering side pinion 25. That is, while crowning the tooth flanks 56 of the teeth 55 of the steering side pinion 25 can be performed by hobbing, a low-cost processing method, crowning the tooth flanks 58 of the teeth 57 of the steering side rack 28 cannot be performed by hobbing and requires a costly forging process. In this regard, in this example, crowning is not performed on the tooth flanks 58 of the teeth 57 of the steering side rack 28, so the steering side rack 28 can be formed by hobbing. This reduces the processing costs for the steering side pinion 25 and the steering side rack 28.

The reason why crowning the tooth flanks 58 of the teeth 57 of the steering-side rack 28 cannot be achieved by hobbing is because 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 were to be achieved by hobbing on the tooth flanks 58 of the teeth 57 of such a steering-side rack 28, the positions in the tooth width direction at which the tooth thickness is greatest would differ between adjacent teeth 57, making it impossible to achieve the same crowning shape on the tooth flanks 58 of each tooth 57.

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

Figure 15 is a diagram of this example, corresponding to Figure 6(a). In this example, crowning is applied to only the end 67a on one side (lower side in Figure 15) of the tooth flank 56a of the tooth 55a constituting the steering-side pinion 25a in the tooth trace direction. That is, in this example, crowning is not applied to the middle portion 66 in the tooth trace direction and the end 67b on the other side (upper side in Figure 15) of the tooth flank 56a of the tooth 55a constituting the steering-side pinion 25a. In other words, in this example, the middle portion 66 and the end 67b on the other side are not inclined relative 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 in a curved or linear manner (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 middle portion 66 and one end portion 67a.

In the structure of this example, if 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 if 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 tooth flanks 56a of the teeth 55a of the steering-side pinion 25a can be brought into contact with the tooth flanks 58 of the teeth 57 of the steering-side rack 28 (see FIG. 9) over a wider contact area at the intermediate portion 66. The other configurations, functions, and effects are the same as those of the first example.

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

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 in the tooth trace direction. As a result, the tooth thickness of the tooth is constant over the entire length in the tooth trace direction.

Instead, in this example, as shown in Figure 16, the steering-side rack 28a has a crowning process performed on the entire tooth flank 58a of the 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 flank 57a toward both sides in the tooth flank direction. Therefore, in this example, the tooth thickness of the tooth 57a is greatest in the center of the tooth flank direction and decreases toward both sides in the tooth flank direction. The tooth flank 58a is smoothly continuous over the entire length in the tooth flank direction and does not have any sharp corners along the tooth flank direction.

In the structure of this example, the tooth flanks 58a of the teeth 57a of the steering-side rack 28a are crowned. This allows the tooth flanks 58a of the teeth 57a of the steering-side rack 28a to come into contact with the tooth flanks of the steering-side pinion over a relatively wide contact area in a portion off-center in the tooth trace direction, 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 defined by the design, or 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 defined by the design. Other configurations, functions, and effects are the same as those of the first example.

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

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 either side 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 either side have a crowning shape inclined in a direction in which the tooth thickness of the tooth 57b decreases in a curved or linear manner (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 their 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.

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

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

The present invention can be implemented by appropriately combining the structures of the above-mentioned embodiments to the extent that no contradictions arise.

REFERENCE SIGNS LIST 1 electric power steering device 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 shaft 12 steering wheel 13 assist side pinion shaft 14 rack accommodating section 15 steering side pinion accommodating section 16 assist side pinion accommodating section 17 steering side guide accommodating section 18 assist side guide accommodating section 19 gear housing section 20a, 20b mounting section 21 worm accommodating section 22 wheel accommodating section 23 mounting flange 24 steering side rack guide 25, 25a steering side pinion 26a, 26b bearing 27 tie rod 28, 28a, 28b steering side rack 29 assist side rack 30 threaded hole 31 Spherical joint 32 Male thread portion 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 Bearing 43 Roller 44 Holder 45 Pin 46 Rolling bearing 47 Elastic member 48 Assist side cap 49 Worm 50 Worm wheel 51 Cap 52 Cap 53 Rack bush 54 Rack bush 55, 55a Teeth 56, 56a Tooth surface 57, 57a, 57b Teeth 58, 58a, 58b Tooth surface 59 Tooth 60 Tooth surface 61 Steering side shaft portion 62 Assist side shaft portion 63 Connection portion 64 Tooth 65 tooth surface 66 intermediate portion 67a, 67b end portions 68 intermediate portion 69 end portions
100 rack shaft
101 Lack
102 Pinion shaft
103 Pinion
104 teeth
105 Tooth surface
106 teeth
107 Tooth surface

Claims (5)

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. The electric power steering device of claim 1, wherein crowning is applied to the entire tooth surface of the steering-side rack in the tooth trace direction. The electric power steering device of claim 1 or 2, wherein the rack shaft 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 the axial end of the steering-side shaft portion to the axial end of the assist-side shaft portion. 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. 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 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 to the axial end of the assist-side shaft portion.