Steering device

Abstract

A steering device (20) includes a steering gear box (22A) having a ball screw shaft (62), a first power assist unit (22B) connected to a first end of the ball screw shaft, and a second power assist unit (22C) connected to a second end of the ball screw shaft. The first power assist unit includes a first motor (41) having a first motor shaft (41A), and a first speed reducer (42) for reducing rotational speed of the first motor. The second power assist unit includes a second motor (51) having a second motor shaft (51A), and a second speed reducer (52) for reducing rotational speed of the second motor. The first speed reducer has a worm wheel (42A) and a worm (42B). The second speed reducer includes a first parallel-axis cylindrical gear pair (71) having a first gear (71A) and a second gear (71B), and a second parallel-axis cylindrical gear pair (72) having a third gear (72A) and a fourth gear (72B).

Description

Steering gear

[0001] The present disclosure relates to a 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 as an assist force 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 connected coaxially and rotatably supported inside the same housing.

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

[0005] Depending on the size of the vehicle in which the steering device is installed, the assist force may be insufficient. In such cases, it is necessary to ensure the assist force while preventing the steering device from becoming too large.

[0006] A steering device according to one aspect of the present disclosure includes a steering gearbox having a ball screw shaft configured to convert rotation of the ball screw shaft into steering motion of steered wheels of a vehicle, a first power assist unit coupled to a first end of the ball screw shaft, and a second power assist unit coupled to a second end of the ball screw shaft. The first end is an end of the ball screw shaft that faces upward of the vehicle when the steering gearbox is mounted on the vehicle. The second end is an end of the ball screw shaft that faces downward of the vehicle when the steering gearbox is mounted on the vehicle. The first power assist unit includes a first motor having a first motor shaft and a first reducer configured to decelerate rotation of the first motor. The second power assist unit includes a second motor having a second motor shaft and a second reducer configured to decelerate rotation of the second motor. The first reducer includes a worm wheel coupled to the first end of the ball screw shaft and a worm coupled to the first motor shaft and meshing with the worm wheel. The second reducer includes a first pair of parallel-axis cylindrical gears having a first gear and a second gear meshing with each other and a second pair of parallel-axis cylindrical gears having a third gear and a fourth gear meshing with each other. The first gear is coupled to the second 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. The fourth gear is coupled to the second end of the ball screw shaft and configured to rotate about a third axis.

[0007] Fig. 1 is a schematic diagram of a steering device according to one embodiment, Fig. 2 is a cross-sectional view of the steering actuator of Fig. 1 cut in the axial direction, and Fig. 3 is a schematic diagram of the steering actuator of Fig. 2.

[0008] A steering device according to one embodiment will be described. <Overall Configuration> As shown in Fig. 1, the steering device 20 is an electric power steering device and 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. The leaf spring 14 is located, for example, 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 steering device 20 includes a steering actuator 22. The steering actuator 22 is provided on a steering shaft 21. The steering actuator 22 has a steering gearbox 22A, a first power assist unit 22B, and a second power assist unit 22C. The first power assist unit 22B is provided on an upper part of the steering gearbox 22A. The upper part is a portion of the steering gearbox 22A that is close to the steering shaft 21. The second power assist unit 22C is provided on a lower part of the steering gearbox 22A. 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 via the first power assist unit 22B.

[0010] The steering gearbox 22A is of the RBS (recirculating ball screw) type. The steering gearbox 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 gearbox 22A. The pitman arm 31 is capable of swinging in the fore-and-aft 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, for example, 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 first power assist unit 22B has a first motor 41, a first reducer 42, and a torque sensor 43. The first motor 41 is connected to an upper part of the steering gear box 22A via the first reducer 42. The first motor 41 operates to apply torque to the steering shaft 21 in the same direction as the steering direction of the steering wheel 23, in accordance with the steering torque detected via the torque sensor 43. The torque of the first motor 41 is transmitted to the steering shaft 21 via the first reducer 42 as a first assist force. This assists the steering of the steering wheel 23.

[0013] The second power assist unit 22C has a second motor 51 and a second reducer 52. The second motor 51 is connected to a lower part of the steering gearbox 22A via the second reducer 52. The second motor 51 operates to apply torque to the steering shaft 21 in the same direction as the steering direction of the steering wheel 23, in accordance with the steering torque detected by the torque sensor 43. The torque of the second motor 51 is transmitted to the steering shaft 21 via the second reducer 52 and the steering gearbox 22A as a second assist force. This assists the steering of the steering wheel 23.

[0014] <Configuration of Steering Gear Box 22A> Next, the configuration of the steering gear box 22A will be described in detail. As shown in Fig. 2, the steering gear box 22A has a first housing 61, a ball screw shaft 62, a ball screw nut 63, a plurality of balls 64, a sector shaft 65, and a sector gear 66. The first housing 61 accommodates the ball screw shaft 62, the ball screw nut 63, the plurality of balls 64, the sector shaft 65, and the sector gear 66.

[0015] When the steering gearbox 22A is mounted on the vehicle 10, the ball screw shaft 62 is maintained in a position extending in the vertical direction relative to the vehicle 10. The ball screw shaft 62 is rotatably supported in the first housing 61 via a first bearing 67 and a second bearing 68. The first bearing 67 is, for example, a ball bearing, which is a type of rolling bearing, and has an inner ring, an outer ring, and a plurality of balls interposed between the inner ring and the outer ring. The inner ring may be formed integrally with the ball screw shaft 62. The second bearing 68 is, for example, a needle bearing, which is a type of rolling bearing. However, the second bearing 68 may also be a ball bearing or a sliding bearing.

[0016] A first end of the ball screw shaft 62 is connected to the steering shaft 21 via the first power assist unit 22B. The first end is the end of the ball screw shaft 62 that faces upward of the vehicle 10 when the steering gearbox 22 is mounted on the vehicle 10. A second end of the ball screw shaft 62 opposite the first end is the end of the ball screw shaft 62 that faces downward of the vehicle 10 when the steering gearbox 22 is mounted on the vehicle 10. The ball screw shaft 62 has a first screw groove 62A. The first screw groove 62A is a spiral groove provided on the outer circumferential surface of the ball screw shaft 62.

[0017] The ball screw nut 63 is a cylindrical body and has a second screw groove 63A. The second screw groove 63A is a spiral groove provided on the inner peripheral surface of the ball screw nut 63. The second screw groove 63A faces the first screw groove 62A in the radial direction of the ball screw nut 63. The ball screw nut 63 is threadedly engaged with the ball screw shaft 62 via a plurality of balls 64. The spiral space surrounded by the first screw groove 62A and the second screw groove 63A functions as a rolling path along which the balls 64 roll. The ball screw nut 63 has a plurality of rack teeth 63B. The rack teeth 63B are provided on the outer peripheral surface of the ball screw nut 63. The rack teeth 63B are aligned in the axial direction of the ball screw nut 63.

[0018] The ball screw shaft 62, the ball screw nut 63, and the plurality of balls 64 constitute a ball screw mechanism. The sector shaft 65 extends in a direction perpendicular to the axis of the ball screw nut 63 (a direction perpendicular to the plane of the paper in FIG. 2 ). The sector shaft 65 is rotatably supported by the first housing 61 via a bearing (not shown). The sector shaft 65 has an outer end that penetrates the first housing 61 and is exposed to the outside. The base end of the pitman arm 31 is fixed to the outer end of the sector shaft 65. The sector shaft 65 is an output shaft of the steering gear box 22 and is connected to the steered wheels 13.

[0019] The sector gear 66 is provided so as to be rotatable integrally with the sector shaft 65. The sector gear 66 is a sector-shaped gear and has a plurality of teeth 66A. The teeth 66A of the sector gear 66 mesh with the rack teeth 63B of the ball screw nut 63.

[0020] The rotation of the steering wheel 23 is transmitted to the ball screw shaft 62 via the steering shaft 21 and the first power assist unit 22B. As the ball screw shaft 62 rotates, the ball screw nut 63 moves in the axial direction relative to the ball screw shaft 62. This causes the sector gear 66 to swing around the sector shaft 65. As the sector gear 66 swings, the sector shaft 65 rotates, causing the pitman arm 31 to swing around the sector shaft 65.

[0021] <First Power Assist Unit 22B> Next, the configuration of the first power assist unit 22B will be described in detail. As shown in FIG. 2 , the first power assist unit 22B has a second housing 44. The second housing 44 is connected to a first end of the first housing 61. The first end is the end of the first housing 61 that faces upward of the vehicle 10 when the steering gearbox 22 is mounted on the vehicle 10. The second housing 44 houses the first reducer 42. The second housing 44 also supports the first motor 41 and the torque sensor 43.

[0022] The first motor 41 is attached to the outside of the second housing 44. The first motor 41 has a first motor shaft 41A. The axis of the first motor shaft 41A is, for example, perpendicular to the axis of the ball screw shaft 41 and parallel to the axis of the sector shaft 44. The first motor shaft 41A penetrates the peripheral wall of the second housing 44 and is inserted into the second housing 44.

[0023] The torque sensor 43 has the following configuration: The torque sensor 43 has an input shaft 43A, an output shaft 43B, a torsion bar 43C, and a detector 43D. The input shaft 43A and the output shaft 43B are connected to each other via the torsion bar 43C. The input shaft 43A and the output shaft 43B are hollow cylindrical bodies with circular cross sections.

[0024] A first end of the input shaft 43A is connected to the steering wheel 23 via the steering shaft 21. A second end of the input shaft 43A is inserted into a first end of the output shaft 43B. The outer circumferential surface of the input shaft 43A is kept out of contact with the inner circumferential surface of the output shaft 43B. A plain bearing 43E is interposed between the outer circumferential surface of the input shaft 43A and the inner circumferential surface of the output shaft 43B. The input shaft 43A and the output shaft 43B are capable of relative rotation via the plain bearing 43E.

[0025] The second end of the output shaft 43B is coupled to the first end of the ball screw shaft 62 so as to be rotatable together. The first end of the ball screw shaft 62 has a coupling hole 52B. The second end of the output shaft 43B is axially fitted into the coupling hole 52B. The serration coupling between the output shaft 43B and the ball screw shaft 62 restricts relative rotation between the output shaft 43B and the ball screw shaft 62. The serration coupling is an example of a meshing coupling between an inner member having external teeth and an outer member having internal teeth. The output shaft 43B is rotatably supported relative to the second housing 44 via a third bearing 43F.

[0026] Steering torque applied to the steering wheel 23 is transmitted to the input shaft 43A via the steering shaft 21, causing the input shaft 43A to rotate. The rotation of the input shaft 43A is transmitted to the output shaft 43B via the torsion bar 43C, causing the output shaft 43B to rotate. The torsion bar 43C twists in response to the steering torque. The detector 43D is provided, for example, to surround the outer circumferential surface of the input shaft 43A. The detector 43D detects the steering torque based on the amount of twist of the torsion bar 43C.

[0027] The first reducer 42 is configured as follows. The first reducer 42 has a worm wheel 42A and a worm 42B. The worm wheel 42A is a disk-shaped body with a circular cross section and is attached to the outer circumferential surface of the output shaft 43B. The worm wheel 42A can rotate integrally with the output shaft 43B. The worm 42B is connected to the first motor shaft 41A. The worm 42B can rotate integrally with the first motor shaft 41A. The axes of the first motor shaft 41A and the worm 42B are aligned. The worm 42B meshes with the worm wheel 42A. The axial angle between the worm 42B and the worm wheel 42A is, for example, 90°.

[0028] The torque of the first motor 41 is transmitted to the ball screw shaft 41 via the first reducer 42. A torque is applied to the ball screw shaft 41 in the same direction as the steering direction of the steering wheel 23. This assists the steering of the steering wheel 23.

[0029] <Second power assist unit 22C> Next, the configuration of the second power assist unit 22C will be described in detail. As shown in Fig. 2, the second power assist unit 22C has a third housing 53. The third housing 53 has a base portion 53A and a fitting portion 53B.

[0030] The base 53A is a plate-shaped portion of the third housing 53 that extends in a direction perpendicular to the axial direction of the ball screw shaft 62. The base 53A has a first end face and a second end face. The first end face is an end face that is closer to the first housing 61 in the axial direction of the ball screw shaft 62. The second end face is an end face opposite to the first end face and is an end face that is farther from the first housing 61 in the axial direction of the ball screw shaft 62.

[0031] The fitting portion 53B is provided on a first end surface of the base portion 53A. The fitting portion 53B is a cylindrical body having a circular cross section. The outer peripheral surface of the second end portion of the first housing 61 is fitted into the inner peripheral surface of the fitting portion 53B. The first housing 61 is fixed to the third housing 53 with bolts (not shown) via mounting portions (not shown) provided on the outer peripheral surface of the second end portion of the first housing 61.

[0032] A portion of the base 53A protrudes radially outward from the outer circumferential surface of the fitting portion 53B. The second motor 51 is attached to a portion of the base 53A that protrudes radially outward from the outer circumferential surface of the fitting portion 53B. The second motor 51 is attached to the base 53A from the first end face side of the base 53A. The second motor 51 has a second motor shaft 51A. The second motor shaft 51A passes through the base 53A in the axial direction.

[0033] The second power assist unit 22C has a fourth housing 54. The fourth housing 54 is attached to the base 53A. The fourth housing 54 is a box-shaped body that covers the second end surface of the base 53A. The contour of the fourth housing 54 corresponds to the contour of the base 53A. The tip of the second motor shaft 51A is located inside the fourth housing 54.

[0034] The second reducer 52 is configured as follows. As shown in FIG. 3 , the second reducer 52 has two parallel-axis cylindrical gear pairs. That is, the second reducer 52 has a first parallel-axis cylindrical gear pair 71 and a second parallel-axis cylindrical gear pair 72. The first parallel-axis cylindrical gear pair 71 has a first gear 71A and a second gear 71B that mesh with each other. The second parallel-axis cylindrical gear pair 72 has a third gear 72A and a fourth gear 72B that mesh with each other. The first gear 71A, the second gear 71B, the third gear 72A, and the fourth gear 72B are, for example, helical gears.

[0035] The first gear 71A is housed inside the fourth housing 54. The first gear 71A is coupled to the second motor shaft 51A. The second motor shaft 51A and the first gear 71A are arranged coaxially. The second motor shaft 51A and the first gear 71A rotate integrally about the first axis O1 as the second motor 51 is driven.

[0036] The second gear 71B is accommodated inside the fourth housing 54. The second gear 71B meshes with the first gear 71A. The third gear 72A is accommodated inside the third housing 53. The third gear 72A is connected to the second gear 71B via a shaft 73. The second gear 71B and the third gear 72A are arranged coaxially. The second gear 71B and the third gear 72A rotate integrally about the second axis O2.

[0037] The fourth gear 72B is accommodated inside the first housing 61. The fourth gear 72B is coupled to a second end of the ball screw shaft 62. The ball screw shaft 62 and the fourth gear 72B are arranged coaxially. The ball screw shaft 62 and the fourth gear 72B rotate integrally about the third axis O3.

[0038] The first axis O1, the second axis O2, and the third axis O3 are parallel to one another. The outer diameters of the first gear 71A, the second gear 71B, the third gear 72A, and the fourth gear 72B are largest in this order: first gear 71A, third gear 72A, fourth gear 72B, and second gear 71B. That is, the first gear 71A has the smallest outer diameter, and the second gear 72B has the largest outer diameter.

[0039] The torque of the second motor 52 is transmitted to the ball screw shaft 62 via the first gear 71A, the second gear 71B, the third gear 72A, and the fourth gear 72B. Steering of the steering wheel 23 is assisted by applying torque to the ball screw shaft 62 in the same direction as the steering direction of the steering wheel 23. The ball screw shaft 62 corresponds to an input shaft to which rotation is applied from the first reducer 42 and the second reducer 52. The sector shaft 65 corresponds to an output shaft or an interlocking member that interlocks with the steered wheels 13.

[0040] <Effects of this embodiment> This embodiment provides the following effects. (1) The steering device 20 has a steering gear box 22A, a first power assist unit 22B, and a second power assist unit 22C. The steering gear box 22A has a ball screw shaft 62 and converts the rotation of the ball screw shaft 62 into a steering operation of the steered wheels 13 of the vehicle. Converting the rotation of the ball screw shaft 62 into a steering operation of the steered wheels 13 of the vehicle means converting the rotation of the ball screw shaft 62 into the rotation of the sector shaft 65. The first power assist unit 22B applies a first assist force to the ball screw shaft 62. The second power assist unit 22C applies a second assist force to the ball screw shaft 62. Therefore, it is easier to ensure the assist force to be applied to the ball screw shaft 62 compared to when only the first assist force or the second assist force is applied to the ball screw shaft 62.

[0041] (2) The first power assist unit 22B is connected to a first end of the ball screw shaft 62. The first end is the end of the ball screw shaft 62 that faces upward when the steering gearbox 22A is mounted on the vehicle. The second power assist unit 22C is connected to a second end of the ball screw shaft 62. The second end is the end of the ball screw shaft 62 that faces downward when the steering gearbox 22A is mounted on the vehicle. In this way, the first assist force and the second assist force can be applied appropriately and in a balanced manner from both ends of the ball screw shaft 62.

[0042] (3) The first power assist unit 22B includes a first motor 41 and a first reducer 42. The first motor 41 has a first motor shaft 41A. The first reducer 42 reduces the rotation speed of the first motor 41. The second power assist unit 22C includes a second motor 51 and a second reducer 52. The second motor 51 has a second motor shaft 51A. The second reducer 52 reduces the rotation speed of the second motor 51. In this manner, the outputs of the first motor 41 and the second motor 51 and the reduction ratios of the first reducer 42 and the second reducer 52 can be individually set. This allows for a high degree of freedom in designing the output characteristics of the first power assist unit 22B and the second power assist unit 22C, and ultimately the specifications of the steering device 20.

[0043] (4) The first reducer 42 has a worm wheel 42A and a worm 42B. The worm wheel 42A is connected to a first end of the ball screw shaft 62. The worm 42B is connected to the first motor shaft 41A and meshes with the worm wheel 42A. The first reducer 42 is a so-called worm reducer.

[0044] The second reducer 52 has a first pair of parallel-axis cylindrical gears 71 and a second pair of parallel-axis cylindrical gears 72. The first pair of parallel-axis cylindrical gears 71 has a first gear 71A and a second gear 71B that mesh with each other. The second pair of parallel-axis cylindrical gears 72 has a third gear 72A and a fourth gear 72B that mesh with each other. The first gear 71A is connected to the second motor shaft 51A and rotates about a first axis O1. The second gear 71B and the third gear 72A are coaxially connected and rotate about a second axis O2. The fourth gear 72B is connected to the second end of the ball screw shaft 62 and rotates about a third axis O3. The second reducer 52 is a so-called parallel-axis reducer.

[0045] A worm reducer can achieve a higher reduction ratio. A parallel shaft reducer can set a wide range of reduction ratios by combining multiple gear stages. However, the second reducer 52, which is a parallel shaft reducer, tends to be larger than the first reducer 42, which is a worm reducer. Therefore, compared to when a parallel shaft reducer is used as the first reducer 42, the parallel shaft reducer can be placed in the lower part of the engine compartment, where there is relatively more space, which is advantageous in terms of ease of installation in a vehicle.

[0046] Other Embodiments This embodiment may be modified as follows: The torque sensor 43 may be provided separately from the first power assist unit 22B.

[0047] The steering device 20 can be applied to a steer-by-wire type steering device in which the steering wheel 23 and the steered wheels 13 are mechanically separated. In this case, for example, the ball screw shaft 60 and the steering wheel 23 are separated so that mechanical power transmission is not possible. Furthermore, the first motor 41 and the second motor 51 function as steering motors that generate a steering force. The steering force is a force for steering the steered wheels 13.

Claims

1. A steering gear box having a ball screw shaft, the steering gear box being configured to convert rotation of the ball screw shaft into steering motion of steered wheels of a vehicle; a first power assist unit connected to a first end of the ball screw shaft; and a second power assist unit connected to a second end of the ball screw shaft, the first end being an end of the ball screw shaft that faces upward of the vehicle when the steering gear box is mounted on the vehicle, and the second end being an end of the ball screw shaft that faces downward of the vehicle when the steering gear box is mounted on the vehicle, the first power assist unit comprising a first motor having a first motor shaft and a first reducer configured to decelerate rotation of the first motor, the second power assist unit comprising a second motor having a second motor shaft and a second reducer configured to decelerate rotation of the second motor, the first reducer being a worm wheel connected to the first end of the ball screw shaft; and a worm connected to the first motor shaft and meshing with the worm wheel; the second reducer includes: a first parallel-axis cylindrical gear pair having a first gear and a second gear meshing with each other; and a second parallel-axis cylindrical gear pair having a third gear and a fourth gear meshing with each other, wherein the first gear is connected to the second motor shaft and configured to rotate about a first axis line, the second gear and the third gear are coaxially connected to each other and configured to rotate about a second axis line, and the fourth gear is connected to the second end of the ball screw shaft and configured to rotate about a third axis line.

2. A steering device as claimed in claim 1, wherein the ball screw shaft and the steering wheel of the vehicle are connected to each other so as to enable mechanical power transmission.

3. A steering device according to claim 1, wherein the ball screw shaft and the steering wheel of the vehicle are separated so that mechanical power transmission is not possible.

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