Electric power steering device

Abstract

This electric power steering device (20) comprises a motor (22B), a speed reducer (22C), and a steering gear box (22A). The steering gear box has an input shaft (61) and a first housing (41). The speed reducer has first to fourth gears (51-54), a second housing (42), and a third housing (43). The first gear is coupled to a motor shaft (22B1) and rotates about a first axis (O1), the second gear and the third gear are coaxially coupled and rotate about a second axis (O2), and the fourth gear is coupled to an axial end of the input shaft and rotates about a third axis (O3). The relative positions of the first axis (O1) and the third axis (O3) are determined by attaching the motor (22B) and the first housing (41) to the second housing (42) from the same attachment direction in the axial direction of the input shaft.

Description

Electric power steering device

[0001] The present disclosure relates to an electric power steering device.

[0002] For example, the electric power steering device disclosed in Patent Document 1 includes a motor and a recirculating ball screw (RBS) type steering gearbox. The steering gearbox converts the rotational motion of the steering shaft into the swinging motion of a pitman arm. The direction of the steered wheels is changed in conjunction with the pitman arm. The torque of the motor is transmitted to the ball screw shaft of the steering gearbox via a reducer. This assists the steering of the steering wheel.

[0003] The reducer is a two-stage gear reducer. The reducer has a first gear set and a second gear set. The first gear set is a combination of a pinion provided on the output shaft of the motor and an intermediate gear. The second gear set is a combination of a pinion provided on the intermediate gear and a helical gear connected to the ball screw shaft. The intermediate gear and the pinion of the intermediate gear are rotatably supported inside the same housing via two bearings.

[0004] US Patent Application Publication No. 2022 / 0135118

[0005] An electric power steering device is required to operate smoothly.

[0006] An electric power steering device according to one aspect of the present disclosure includes: a motor having a motor shaft; a speed reducer configured to reduce rotation of the motor; and a steering gearbox having an input shaft to which rotation is applied from the speed reducer and an output shaft configured to move in conjunction with steered wheels of a vehicle, the steering gearbox configured to convert rotation of the input shaft into rotation of the output shaft. The steering gearbox has a first housing extending in the axial direction of the input shaft and accommodating at least a portion of the input shaft. The speed reducer includes: a first pair of parallel-axis cylindrical gears having a first gear and a second gear meshing with each other; a second pair of parallel-axis cylindrical gears having a third gear and a fourth gear meshing with each other; a second housing connected to an axial end of the first housing and accommodating at least the fourth gear of the second pair of parallel-axis cylindrical gears; and a third housing connected to the second housing on the opposite side of the input shaft from the first housing and configured to cover the first pair of parallel-axis cylindrical gears. The first gear is coupled to the motor shaft and configured to rotate about a first axis, the second gear and the third gear are coaxially coupled to each other and configured to rotate about a second axis, and the fourth gear is coupled to an axial end of the input shaft and configured to rotate about a third axis, and the relative positions of the first axis and the third axis are determined by attaching the motor and the first housing to the second housing from the same mounting direction in the axial direction of the input shaft.

[0007] FIG. 1 is a schematic diagram of an electric power steering device according to a first embodiment. FIG. 2 is a perspective view of a steering actuator in FIG. 1. FIG. 3 is a perspective view showing a main portion of the steering actuator in FIG. 2. FIG. 4 is a perspective view of the steering actuator in FIG. 2 as seen from the third housing side. FIG. 5 is a perspective view showing the internal structure of the steering actuator in FIG. 2. FIG. 6 is an exploded perspective view of the steering actuator in FIG. 2. FIG. 7 is a perspective view showing a main portion of a steering gearbox in FIG. 2. FIG. 8 is a schematic diagram of the steering actuator in FIG. 2. FIG. 9 is a cross-sectional view of the steering actuator in FIG. 2 cut in the axial direction. FIG. 10 is a cross-sectional view of a steering actuator according to a second embodiment cut in the axial direction. FIG. 11 is a cross-sectional view showing a main portion of the steering actuator in FIG. 10. FIG. 12 is a front view of the fourth gear in FIG. 10 as seen from the axial direction.

[0008] <First embodiment> An electric power steering device according to a first embodiment will be described. <Overall configuration> As shown in Fig. 1, an electric power steering device 20 is mounted on, for example, a cab-over vehicle 10. The vehicle 10 is equipped with an axle-suspended suspension 11. The suspension 11 supports a front axle 12. Steerable wheels 13, which are front wheels, are connected to both ends of the front axle 12. The suspension 11 has a leaf spring 14. As an example, the leaf spring 14 is located above the front axle 12. The leaf spring 14 extends in the fore-and-aft direction of the vehicle. Both ends of the leaf spring 14 are attached to a vehicle body frame 16 via support members 15, such as shackles.

[0009] The electric power steering device 20 includes a steering actuator 22 provided on a steering shaft 21. The steering actuator 22 has a steering gearbox 22A, a motor 22B, and a reducer 22C. A first end of the steering shaft 21 is connected to a steering wheel 23. A second end of the steering shaft 21 is connected to the steering gearbox 22A. The motor 22B is connected to the steering gearbox 22A via the reducer 22C. The steering gearbox 22A is of an RBS (recirculating ball screw) type.

[0010] The steering gear box 22A is connected to the steered wheels 13 via a link mechanism 30. The link mechanism 30 has a pitman arm 31, a drag link 32, and a tie rod 33. The base end of the pitman arm 31 is connected to a side portion of the steering gear box 22A. The pitman arm 31 is able to swing in the front-to-rear direction of the vehicle around its base end. A first end of the drag link 32 is rotatably connected to the tip of the pitman arm 31. A second end of the drag link 32 is rotatably connected to a knuckle arm 34 of the right steered wheel 13. Both ends of the tie rod 33 are connected to the left and right steered wheels 13 via tie rod arms 35.

[0011] The rotation of the steering wheel 23 is transmitted to the steering gear box 22A via the steering shaft 21. The steering gear box 22A converts the rotational motion of the steering shaft 21 into the swinging motion of a pitman arm 31. The swinging motion of the pitman arm 31 drives a drag link 32 in the front-to-rear direction of the vehicle. The knuckle arm 34 swings in conjunction with the drag link 32, causing the steered wheels 13 to turn.

[0012] The motor 22B operates to apply torque to the steering shaft 21 in the same direction as the steering direction of the steering wheel 23. The torque of the motor 22B is transmitted to the steering shaft 21 via the reducer 22C and the steering gear box 22A, thereby assisting the steering of the steering wheel 23.

[0013] <Configuration of Steering Actuator 22> Next, the configuration of the steering actuator 22 will be described in detail. As shown in FIG. 2, the steering gearbox 22A has a first housing 41. The first housing 41 has a cylindrical portion with a circular cross-sectional shape. A torque sensor 44 is provided at a first end of the first housing 41. The torque sensor 44 detects the torque applied to the steering shaft 21. The first end of the first housing 41 rotatably supports the steering shaft 21 via the torque sensor 44. A third housing 43 is attached to a second end of the first housing 41 via a second housing 42.

[0014] The reducer 22C has a second housing 42. The second housing 42 has a base 42A and a fitting portion 42B. The base 42A is a plate-shaped portion of the second housing 42 that extends in a direction perpendicular to the axial direction of the first housing 41. A portion of the base 42A protrudes radially outward from the first housing 41.

[0015] The base portion 42A has a first end face and a second end face. The first end face is an end face closer to the first housing 41 in the axial direction of the first housing 41. The second end face is an end face opposite to the first end face and is an end face farther from the first housing 41 in the axial direction of the first housing 41.

[0016] A first end of the motor 22B is attached to a portion of the base 42A that protrudes radially outward from the first housing 41. The first end is the end of the motor 22B from which a motor shaft (not shown), which is the output shaft of the motor 22B, protrudes. The motor 22B is attached from the first end face side of the base 42A. A control device 45 is attached to a second end of the motor 22B. The control device 45 controls the driving of the motor 22B.

[0017] The fitting portion 42B is provided on the first end surface of the base portion 42A. The fitting portion 42B is a cylindrical body with a circular cross-sectional shape. The outer peripheral surface of the second end portion of the first housing 41 is fitted into the inner peripheral surface of the fitting portion 42B. The first housing 41 is fixed to the second housing 42 with bolts (not shown) via mounting portions (not shown) provided on the outer peripheral surface of the second end portion. The axis of the first housing 41 is parallel to the axis of the motor 22B.

[0018] The reducer 22C has a third housing 43. The third housing 43 is attached to the base 42A. The third housing 43 is a box-shaped body that covers the second end surface of the base 42A. The contour of the third housing 43 corresponds to the contour of the base 42A.

[0019] 3, the motor shaft 22B1 passes through the base portion 42A without contacting the base portion 42A. A first gear 51 is integrally rotatably connected to the tip of the motor shaft 22B1. The first gear 51 is, for example, a helical gear.

[0020] 4, the first gear 51 is housed in a non-contact state inside the third housing 43. The first gear 51 rotates about a first axis O1 as the motor 22B is driven. The first axis O1 coincides with the axis of the motor shaft 22B1.

[0021] As shown in FIG. 5 , the reducer 22C includes a second gear 52, a third gear 53, a fourth gear 54, and a shaft 55 in addition to the second housing 42 and the third housing 43. The second gear 52, the third gear 53, and the fourth gear 54 are, for example, helical gears. The second gear 52 and the third gear 53 are integrally rotatably coupled to the shaft 55. The second gear 52 is disposed near a first end of the shaft 55. The third gear 53 is disposed near a second end of the shaft 55. A first bearing 56 is attached to the first end of the shaft 55. A second bearing 57 is attached to the second end of the shaft 55. The second gear 52 is located inward of the first bearing 56 in the axial direction of the shaft 55. The third gear 53 is located inward of the second bearing 57 in the axial direction of the shaft 55. The second gear 52, the shaft 55, and the third gear 53 are positioned coaxially. The second gear 52, the shaft 55, and the third gear 53 are rotatable about a second axis O2. The fourth gear 54 will be described later.

[0022] The outer diameters of the first to fourth gears 51 to 54 are set to increase in the order of the first gear 51, the third gear 53, the fourth gear 54, and the second gear 52. That is, the first gear 51 has the smallest outer diameter, and the second gear 52 has the largest outer diameter.

[0023] As shown in FIG. 3 , the second housing 42 has a shaft insertion hole 42C. The shaft insertion hole 42C is a concave portion of the second housing 42 having an arcuate surface extending in the axial direction of the fitting portion 42B. The interior of the shaft insertion hole 42C is in communication with the interior of the fitting portion 22B. The shaft insertion hole 42C passes through the second housing 43 in the axial direction of the fitting portion 42B. The shaft 55 is inserted into the shaft insertion hole 42C. The third gear 53 is disposed in a non-contact state inside the shaft insertion hole 42C. In other words, the third gear 53 is accommodated inside the second housing 42.

[0024] As shown in FIG. 4 , a first bearing support 43A is provided on the inner surface of the end wall of the third housing 43. The first bearing support 43A is a cylindrical body having a circular cross-sectional shape. The outer peripheral surface of a first bearing 56 is fitted into the inner peripheral surface of the first bearing support 43A. That is, the first end of the shaft 55 is rotatably supported relative to the third housing 43 via the first bearing 56. The second gear 52 is accommodated inside the third housing 43 in a non-contact state. The second gear 52 meshes with the first gear 51. The first gear 51 and the second gear 52 constitute a first parallel-axis cylindrical gear pair.

[0025] As shown in FIG. 6 , the second housing 42 has a second bearing support portion 42D. The second bearing support portion 42D is part of the shaft insertion hole 42C. The second bearing support portion 42D is coaxial with the shaft insertion hole 42C and is provided near the tip of the fitting portion 42B. The second bearing support portion 42D is a concave portion of the second housing 42 having an arcuate surface extending in the axial direction of the fitting portion 42B. The second bearing support member 42D has a smaller diameter than the shaft insertion hole 42C and is in communication with the shaft insertion hole 42C. The outer peripheral surface of the second bearing 57 is fitted into the inner peripheral surface of the second bearing support portion 42D. In other words, the second end of the shaft 55 is rotatably supported relative to the second housing 42 via the second bearing 57.

[0026] 5, the steering gear box 22A has, in addition to the first housing 41, a ball screw shaft 61, a ball screw nut 62, a sector shaft 63, and a sector gear 64. The ball screw shaft 61, the ball screw nut 62, the sector shaft 63, and the sector gear 64 are housed inside the first housing 41.

[0027] The ball screw shaft 61 is rotatably supported with respect to the first housing 41 via a third bearing 65 and a fourth bearing 66. The third bearing 65 is provided at a first end of the ball screw shaft 61. The fourth bearing 66 is provided at a second end of the ball screw shaft 61. The first end of the ball screw shaft 61 is connected to the steering shaft 21 via the torque sensor 44. The second end of the ball screw shaft 61 passes through the first housing 41 in the axial direction and is exposed to the outside (see FIG. 3 ).

[0028] The ball screw nut 62 is threadedly engaged with the ball screw shaft 61 via a plurality of circulable balls (not shown). The ball screw nut 62 has a plurality of rack teeth 62A. The rack teeth 62A are aligned in the axial direction of the ball screw nut 62 on the outer peripheral surface of the ball screw nut 62.

[0029] The sector shaft 63 extends in a direction perpendicular to the axis of the ball screw nut 62. The sector shaft 63 is rotatably supported by the first housing 41 via a bearing (not shown). The sector shaft 63 has an outer end. The outer end is the end of the sector shaft 63 that penetrates the first housing 41 and protrudes to the outside. The base end of the pitman arm 31 (see FIG. 1) is fixed to the outer end of the sector shaft 63.

[0030] The sector gear 64 is provided so as to be rotatable integrally with the sector shaft 63. The sector gear 64 is a sector-shaped gear and has a plurality of teeth 64A. The teeth 64A of the sector gear 64 mesh with the rack teeth 62A of the ball screw nut 62.

[0031] The rotation of the steering wheel 23 is transmitted to the ball screw shaft 61 via the steering shaft 21. As the ball screw shaft 61 rotates, the ball screw nut 62 moves axially relative to the ball screw shaft 61. This causes the sector gear 64 to swing around the sector shaft 63. As the sector gear 64 swings, the sector shaft 63 rotates, causing the pitman arm 31 to swing around the sector shaft 63.

[0032] A fourth gear 54 is integrally rotatably coupled to the ball screw shaft 61. The fourth gear 54 is located in a portion of the ball screw shaft 61 between the ball screw nut 62 and the fourth bearing 66, and is located close to the third bearing 65. The fourth gear 54 is located coaxially with the ball screw shaft 61. The fourth gear 54 and the ball screw shaft 61 are rotatable about a third axis O3.

[0033] As shown in Fig. 7, the first housing 41 has an opening 41A. The opening 41A is provided by cutting out a corner where an axial end wall and a peripheral wall intersect at the second end of the first housing 41. A portion of the fourth gear 54 is exposed to the outside of the first housing 41 through the opening 41A. The fourth gear 54 meshes with the third gear 53 through the opening 41A. The third gear 53 and the fourth gear 54 constitute a second pair of parallel-axis cylindrical gears.

[0034] The torque of the motor 22B is transmitted to the ball screw shaft 61 via the first gear 51, the second gear 52, the third gear 53, and the fourth gear 54. The torque applied to the ball screw shaft 61 in the same direction as the steering direction of the steering wheel 23 assists the steering of the steering wheel 23. The ball screw shaft 61 corresponds to the input shaft to which rotation is applied from the reducer 22C. The sector shaft 63 corresponds to the output shaft that moves in conjunction with the steered wheels 13.

[0035] <Method of Assembling the Steering Actuator 22> Next, a method of assembling the steering actuator 22 will be described. The steering gearbox 22A, the motor 22B, and the reducer 22C are subassemblies that are pre-assembled. A subassembly is an assembly of parts that make up a specific functional part so that they can be handled as a single unit. However, the reducer 22C as a subassembly does not include the fourth gear 54. The fourth gear 54 is housed inside the first housing 41.

[0036] 8, when assembling the steering actuator 22, first, the steering gearbox 22A is attached to the second housing 42 of the reducer 22C in a first attachment direction D1. The first attachment direction D1 is a direction from the second housing 42 toward the third housing 43 in the axial direction of the fitting portion 42B.

[0037] Next, the motor 22B is attached to the second housing 42 from the first attachment direction D1. That is, the steering gear box 22A and the motor 22B are attached to the second housing 42 from the same direction. The steering gear box 22A and the motor 22B are located on the opposite side of the second housing 42 from the third housing 43.

[0038] This completes the assembly of the steering actuator 22. After the assembly is complete, oil is sealed inside the first to third housings 41 to 43. The steering gearbox 22A and the motor 22B are fixed to the second housing 42, thereby defining the relative positions of the first axis O1 and the third axis O3. Once the relative positions of the first axis O1 and the third axis O3 are determined, the relative positions of the first axis O1 and the second axis O2, and the relative positions of the third axis O3 and the second axis O2 are also determined. This is because the position of the second axis O2 relative to the second housing 42 has already been determined during the assembly of the reducer 22C.

[0039] The third housing 43 is attached to the second housing 42, for example, in the following state. That is, a first end of a shaft 55 is rotatably supported in the third housing 43 via a first bearing 56. A second gear 52 is housed inside the third housing 43. A second bearing 57 is attached to the second end of the shaft 55. In this state, the third housing 43 is attached to the second housing 42 in a second attachment direction D2. The second attachment direction D2 is opposite to the first attachment direction D1.

[0040] By attaching the third housing 43 to the second housing 42, the second bearing 57 is fitted into the second bearing support portion 42D of the second housing 42. That is, the positions of both ends of the shaft 55 are determined by the first bearing 56 and the second bearing 57. Once the positions of both ends of the shaft 55 are determined, the position of the second axis O2 relative to the second housing 42 is determined.

[0041] Therefore, by fixing the third housing 43, the steering gearbox 22A, and the motor 22B to the second housing 42, all relative positional relationships between the first axis O1, the second axis O2, and the third axis O3 are determined. The first axis O1, the second axis O2, and the third axis O3 are parallel to one another. The second housing 42 is a member that serves as a reference when assembling the steering actuator 22.

[0042] Advantages of the First Embodiment The first embodiment provides the following advantages. (1-1) The center-to-center distance between the gears of the reduction gear 22C is important for the gears to mesh properly. The center-to-center distance includes the distance between the first axis O1 and the third axis O3. In this embodiment, the steering gearbox 22A and the motor 22B are fixed to the second housing 42 from the first mounting direction D1, thereby defining the relative positions of the first axis O1 and the third axis O3. Because the distance between the first axis O1 and the third axis O3 is maintained at an appropriate distance, the steering actuator 22 can be operated smoothly. Consequently, the electric power steering device 20 can be operated smoothly. Noise and vibration (NV) performance is also improved.

[0043] (1-2) When assembling the reducer 22C, the third housing 43 is fixed to the second housing 42 from the second mounting direction D2, thereby determining the position of the second axis O2 relative to the second housing 42. In this state, the steering gearbox 22A and the motor 22B are fixed to the second housing 42 from the first mounting direction D1, thereby determining all of the relative positional relationships among the first to third axes O1 to O3. That is, the distance between the first axis O1 and the second axis O2, the distance between the second axis O2 and the third axis O3, and the distance between the third axis O2 and the first axis O1 are maintained at appropriate distances. This allows the steering actuator 22 to operate smoothly.

[0044] (1-3) In the axial direction of the fitting portion 42B of the second housing 42, the outer peripheral surface of the second end of the first housing 41 is fitted to the inner peripheral surface of the fitting portion 42B. Therefore, the axial length of the steering actuator 22 can be shortened compared to, for example, a case in which the second end of the first housing 41 is butted against the axial tip of the fitting portion 42B when connecting. The axial length of the steering actuator 22 is shortened by the fitting depth of the second end of the first housing 41 relative to the fitting portion 42B. Furthermore, the axial length of the shaft 55 can be shortened by the fitting depth of the second end of the first housing 41 relative to the fitting portion 42B. This increases the rigidity of the shaft 55. Furthermore, the shaft 55 is less likely to tilt relative to the second axis O2. This improves the rigidity, strength, durability, and NV performance of the steering actuator 22.

[0045] (1-4) The second gear 52, the third gear 53, and the shaft 55 are rotatable integrally. The second gear 52, the third gear 53, and the shaft 55 are supported across the second housing 42 and the third housing 43 via the first bearing 56 and the second bearing 57. The first bearing 56 is fixed to the third housing 43, and the second bearing 57 is fixed to the second housing 42. Fixing the first bearing 56 and the second bearing 57 to different housings facilitates the centering of the first bearing 56 and the second bearing 57 when assembling the reducer 22C. The centering is the process of adjusting the positions of the first axis 56 and the second bearing 57 so that the axis of the first bearing 56 and the axis of the second bearing 57 are aligned on the same straight line. Performing the centering process can eliminate misalignment of the gear axes due to processing tolerances. It is also advantageous in terms of strength, durability, and NV performance.

[0046] (1-5) The first bearing 56 is disposed axially outward relative to the second gear 52. The axially outward position is on the opposite side of the second gear 52 from the third gear 53. Unlike a case in which the first bearing 56 is provided between the second gear 52 and the third gear 53 on the shaft 55, the axial length of the shaft 55 can be shortened, which is advantageous in terms of NV performance. Furthermore, the axial distance between the second gear 52 and the third gear 53 can be shortened. This allows the axial lengths of the second housing 42 and the third housing 43 to be shortened. This allows the steering actuator 22 to be made more compact. Furthermore, oil is sealed inside the first to third housings 41 to 43. Therefore, the amount of oil required for the steering actuator 22 is reduced as the steering actuator 22 is made more compact. Therefore, the weight of the steering actuator 22 can be reduced.

[0047] Second Embodiment Next, an electric power steering device according to a second embodiment will be described. This embodiment basically has the same configuration as the first embodiment shown in Figures 1 to 8. This embodiment differs from the first embodiment in part of the configuration of the steering actuator 22. Therefore, the same members and configurations as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted.

[0048] 9 , in the first embodiment, the fourth gear 54 is housed inside the first housing 42. The fourth gear 54 is located more inward in the axial direction of the ball screw shaft 61 than the fourth bearing 66. However, depending on the product specifications, the position of the fourth gear 54 may be changed as follows.

[0049] 10 , the fourth gear 54 is disposed outside the first housing 42. The fourth gear 54 is also disposed further outward in the axial direction of the ball screw shaft 61 than the fourth bearing 66. The axial length of the first housing 42 is shorter than that of the first embodiment by the axial length of the fourth gear 54. The fourth gear 54 is accommodated inside the second housing 42. A third gear 53 (not shown) that meshes with the fourth gear 54 is also accommodated inside the second housing 42.

[0050] The second housing 42 has a base 42A and a mating portion 42B, similar to the first embodiment. However, the shape of the base 42A differs from that of the first embodiment. That is, the base 42A is a box-shaped body having a predetermined contour and opens at a second end face of the base 42A. The second end face is the end face opposite the first end face and is the end face farther from the first housing 41 in the axial direction of the first housing 41. The interior of the base 42A is connected to the interior of the mating portion 42B. A second gear 52 is housed inside the base 42A. A first gear 51 (not shown) that meshes with the second gear 52 is also housed inside the base 42A.

[0051] The second end of the ball screw shaft 61 axially penetrates the first housing 41 and protrudes into the mating portion 42B. The second end is the end of the ball screw shaft 61 opposite the torque sensor 44. The fourth gear 54 is integrally rotatably coupled to the second end of the ball screw shaft 61 inside the mating portion 42B. That is, the fourth gear 54 is housed inside the mating portion 42B. A third gear 53 (not shown) that meshes with the fourth gear 54 is also housed inside the mating portion 42B. The axial length of the mating portion 42B is longer than the axial length of the mating portion 42B in the first embodiment.

[0052] The third housing 43 is a plate-shaped member and has an outline corresponding to the outline of the base 42 A. The third housing 43 covers the second end surface of the base 42 A. <Support Structure for Second Gear 52 and Third Gear 53> Next, the support structure for the second gear 52 and the third gear 53 will be described.

[0053] As shown in Fig. 11 , the second gear 52 and the third gear 53 are coupled to a shaft 55 so as to be rotatable integrally with the shaft 55. The second gear 52 is disposed near a first end of the shaft 55. The third gear 53 is disposed near a second end of the shaft 55. The third gear 53 may be a separate member from the shaft 55, or may be formed integrally with the shaft 55. A fifth bearing 58 is attached to the shaft 55 in a portion between the second gear 52 and the third gear 53. In this embodiment, the first bearing 56 and the second bearing 57 (see Fig. 5 ) of the first embodiment are omitted.

[0054] A third bearing support portion 42B1 is provided inside the fitting portion 42B. The third bearing support portion 42B1 is a cylindrical portion of the fitting portion 42B having a circular cross-sectional shape. The outer peripheral surface of the fifth bearing 58 is fitted into the inner peripheral surface of the third bearing support portion 42B1. The shaft 55 is rotatably supported by the fitting portion 42B and, by extension, the second housing 42 via the fifth bearing 58. The third gear 53 meshes with the fourth gear 54 inside the fitting portion 42B.

[0055] <Regarding Deflection of Ball Screw Shaft 61> However, the steering actuator 22 configured as above has the following concerns. That is, as shown in FIG. 10 , by disposing the fourth gear 54 outside the first housing 42, the ball screw shaft 61 may be more prone to deflection than in the first embodiment. The direction of deflection of the ball screw shaft 61 is a direction intersecting the axial direction of the ball screw shaft 61. When the ball screw shaft 61 deflects, the fourth gear 54 is displaced. Depending on the direction of displacement of the fourth gear 54, this may affect the meshing state between the fourth gear 54 and the third gear 53.

[0056] For example, it is conceivable that a first load F1 from the sector gear 64 is applied to the ball screw shaft 61 via the ball screw nut 62. The first load F1 is, for example, a force in a direction perpendicular to the axial direction of the ball screw shaft 61. When the ball screw shaft 61 receives the first load F1, it bends in the same direction as the direction of the first load F1, i.e., in the direction away from the sector gear 64.

[0057] When the ball screw shaft 61 bends, the axial center of the ball screw shaft 61 is slightly displaced in the same direction as the first load F1, with the third bearing 65 and the fourth bearing 66 as fulcrums. Furthermore, both ends of the ball screw shaft 61 are slightly displaced in the opposite direction to the direction of the first load F1, with the third bearing 65 and the fourth bearing 66 as fulcrums. A second load F2 is applied to the fourth gear 54 due to the slight displacement of both ends of the ball screw shaft 61. The second load F2 is a force in the opposite direction to the first load F1. Under the second load F2, the fourth gear 54 is displaced in the same direction as the second load F2.

[0058] Depending on the direction of the second load F2, for example, the fourth gear 54 may be displaced in a direction away from the third gear 53. In this case, the gap between the teeth of the fourth gear 54 and the teeth of the second gear 53 widens. This is one cause of the generation of abnormal noise. Therefore, in this embodiment, the arrangement of the third gear 53 is determined taking into consideration the bending direction of the ball screw shaft 61.

[0059] <Arrangement of Third Gear 53> Next, the arrangement of the third gear 53 will be described. As shown in Fig. 12, when viewed in the axial direction of the fourth gear 54, the third gear 53 is arranged in an area on the opposite side of the direction of the first load F1 with respect to a line L1 that passes through the center of the fourth gear 54 and is perpendicular to the direction of the first load F1. When viewed in the axial direction of the fourth gear 54, the center of the fourth gear 54 coincides with the third axis O3. The direction of the first load F1 is also the deflection direction of the ball screw shaft 61.

[0060] As described above, when the first load F1 is applied to the ball screw shaft 61 and the ball screw shaft 61 is bent, the fourth gear 54 receives the second load F2 and is displaced in the same direction as the second load F2. In contrast, the third gear 53 is disposed in an area opposite the direction of the first load F1 with respect to the line L1. In other words, the third gear 53 is located in an area on the same side of the line L1 as the direction of displacement of the fourth gear 54.

[0061] Therefore, when the fourth gear 54 is displaced in the same direction as the second load F2, the fourth gear 54 approaches the third gear 53, narrowing the gap between the teeth of the fourth gear 54 and the teeth of the second gear 53. This maintains an appropriate meshing state between the fourth gear 54 and the third gear 53. Furthermore, the generation of abnormal noise is also suppressed.

[0062] Incidentally, as viewed from the axial direction of the fourth gear 54, the third gear 53 may be provided in any position as long as the center of the third gear 53 is located in the region opposite the direction of the first load F1 with respect to the line L1 and within the region of "0°" to "180°" with respect to the line L1. As viewed from the axial direction of the fourth gear 54, the center of the third gear 53 coincides with the second axis O2.

[0063] For example, when viewed from the axial direction of the fourth gear 54, the center of the third gear 53 may be located on the line L1. In this case, a portion of the third gear 53 may extend beyond the line L1 into an area on the same side as the direction of the first load F1. However, even in this case, when the fourth gear 54 is displaced in the same direction as the second load F2, the gap between the teeth of the fourth gear 54 and the teeth of the second gear 53 narrows.

[0064] This embodiment may be applied to the first embodiment described above. In the first embodiment as well, the first load F1 from the sector gear 64 is applied to the ball screw shaft 61 via the ball screw nut 62, which may cause the ball screw shaft 61 to bend in the same direction as the first load F1.

[0065] Advantages of the Second Embodiment The second embodiment has the following advantages: (2-1) When viewed from the axial direction of the fourth gear 54, the third gear 53 is disposed in a region opposite the direction of the first load F1 with respect to the line L1, and the center of the third gear 53 is positioned within a region from 0° to 180° with respect to the line L1. This makes it possible to prevent deflection of the ball screw shaft 61 from affecting the meshing state between the teeth of the fourth gear 54 and the teeth of the third gear 53.

[0066] (2-2) The fourth gear 54 is provided outside the first housing 41. The fourth gear 54 meshes with the third gear 53 inside the second housing 42. Therefore, unlike the first embodiment, there is no need to provide an opening 41A (see FIG. 7) in the first housing 41 to expose the fourth gear 54. This allows for a simplified configuration of the first housing 41.

[0067] (2-3) The second housing 42 rotatably supports the second gear 52. With the second gear 532 supported by the second housing 42, the position of the second axis O2 relative to the second housing 42 is determined. With the steering gearbox 22A and the motor 22B fixed to the second housing 42 in this state, all of the relative positional relationships between the first to third axes O1 to O3 are determined. That is, the distance between the first axis O1 and the second axis O2, the distance between the second axis O2 and the third axis O3, and the distance between the third axis O2 and the first axis O1 are maintained at appropriate distances. This allows the steering actuator 22 to operate smoothly.

[0068] Other Embodiments The first and second embodiments may be modified as follows: The first to fourth gears 51 to 54 may each be a spur gear.

[0069] The second end of the steering gearbox 22A may be configured to fit onto the outer peripheral surface of the fitting portion 42B. It is sufficient that there is a fitting relationship between the second end of the steering gearbox 22A and the fitting portion 42B in the axial direction of the fitting portion 42B. However, the contour of the inner peripheral surface of the second end of the steering gearbox 22A is configured to match the contour of the outer peripheral surface of the fitting portion 42B. Even in this case, the same effect as in (3) above can be obtained.

[0070] The assembly method of the reducer 22C may be modified as appropriate. For example, the third housing 43 may be attached to the second housing 42 in a state where the shaft 55 is rotatably supported by the second housing 42 via the second bearing 57.

[0071] As used herein, the term "cylindrical" may refer to any structure having a peripheral wall. For example, but not limited to, the term "cylindrical" may refer to any structure having a cross-sectional shape that is circular, oval, and polygonal with sharp or rounded corners.

[0072] The expression "at least a part of A" used in this specification means "a part of A or all of A." Additionally, the expression "at least B of A and B" used in this specification means "only B, or both A and B."

[0073] The electric power steering device described in this specification can be applied to a steering device of a steer-by-wire steering system in which the steering device and the steering device are not mechanically connected. The steering device is a part of the steering system that includes the steering shaft 21. The steering device is a part of the steering system that includes the steering actuator 22. In this case, the motor 22B functions as a steering motor that generates a steering force. The steering force is a force for steering the steered wheels 13.

Claims

1. An electric power steering device comprising: a motor having a motor shaft; a speed reducer configured to reduce the speed of rotation of the motor; and a steering gearbox having an input shaft to which rotation is given from the speed reducer, and an output shaft configured to be interlocked with steered wheels of a vehicle, the steering gearbox configured to convert the rotation of the input shaft into the rotation of the output shaft, wherein the steering gearbox has a first housing extending in the axial direction of the input shaft and accommodating at least a portion of the input shaft, and the speed reducer has: a first parallel-axis cylindrical gear pair having a first gear and a second gear that mesh with each other; a second parallel-axis cylindrical gear pair having a third gear and a fourth gear that mesh with each other; a second housing connected to an axial end of the first housing and accommodating at least the fourth gear of the second parallel-axis cylindrical gear pair; and a third housing connected to the side of the second housing opposite to the first housing in the axial direction of the input shaft, configured to cover the first parallel-axis cylindrical gear pair, An electric power steering device, wherein the first gear is connected to the motor shaft and configured to rotate about a first axis, the second gear and the third gear are coaxially connected to each other and configured to rotate about a second axis, the fourth gear is connected to an axial end of the input shaft and configured to rotate about a third axis, and the relative positions of the first axis and the third axis are determined by attaching the motor and the first housing to the second housing from the same mounting direction in the axial direction of the input shaft.

2. An electric power steering device as described in claim 1, wherein the third housing is configured to rotatably support the second gear, and the relative positions of the first axis, the second axis, and the third axis are determined by attaching the third housing to the second housing from a direction opposite to the attachment direction of the motor and the first housing.

3. An electric power steering device as described in claim 1, wherein the second housing is configured to rotatably support the second gear, and the relative positions of the first axis, the second axis, and the third axis are determined by mounting the motor and the first housing to the second housing.

4. An electric power steering device as claimed in any one of claims 1 to 3, wherein the second housing has a cylindrical fitting portion, and the fitting portion and the axial end portion of the first housing are fitted together.

5. An electric power steering device according to claim 4, wherein the outer peripheral surface of the axial end of the first housing is fitted into the inner peripheral surface of the fitting portion.

6. An electric power steering device as described in claim 1 or claim 2, wherein the reducer has a shaft having a first end and a second end, the second gear is rotatably connected to a portion of the shaft near the first end, the third gear is rotatably connected to a portion of the shaft near the second end, the third housing has a first bearing configured to rotatably support the first end of the shaft, the second housing has a second bearing configured to rotatably support the second end of the shaft, and the second gear, the third gear and the shaft are supported between the second housing and the third housing via the first bearing and the second bearing.

7. The steering gearbox is a recirculating ball screw type steering gearbox having: a ball screw shaft rotatably supported inside the first housing and to which rotation is imparted from the reducer; a ball screw nut screwed onto the ball screw shaft and having a plurality of rack teeth arranged on its outer circumferential surface along the axial direction of the ball screw shaft; a sector shaft configured to interlock with the steered wheels; and a sector gear rotatably connected integrally to the sector shaft, meshing with the rack teeth and oscillating about the sector shaft as the ball screw nut moves in the axial direction, wherein the ball screw shaft corresponds to the input shaft and the sector shaft corresponds to the output shaft. An electric power steering device according to any one of claims 1 to 3.

8. An electric power steering device as described in claim 7, wherein the ball screw shaft is configured so that a load from the sector gear acts through the ball screw nut, and when viewed in the axial direction of the fourth gear, the third gear is disposed in a region on the opposite side of the load direction with respect to a line that passes through the center of the fourth gear and is perpendicular to the direction of the load, and the center of the third gear is located within a region from 0° to 180° with respect to the line.

Images