Steering gear

Abstract

A steering device (20) comprises: an actuator including a motor (23) configured to generate torque corresponding to steering of a steering wheel (26) of a vehicle (10), and a speed reducer (25) configured to decrease the speed of rotation of the motor (23); and a ball screw mechanism (48) configured to convert rotation imparted via the speed reducer (25) into rotation of an output shaft (44) linked to a steered wheel (13). The speed reducer (25) and the ball screw mechanism (48) are configured such that load torque increase / decrease characteristics with respect to a steering direction that arise when the steering wheel (26) is steered are mutually opposite in the speed reducer (25) and the ball screw mechanism (48).

Description

[Technical Field]

[0001] The present disclosure relates to a steering device. [Background technology]

[0002] Conventionally, so-called cab-over vehicles, such as trucks, in which the driver's seat is located forward of the front axle, often use an axle-type suspension in which the left and right wheels are connected by an axle. Vehicles with an axle-type suspension often use a ball screw type steering gearbox. The steering gearbox is mounted on, for example, the vehicle frame.

[0003] The electric power steering device in Patent Document 1 has a motor and a ball screw 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 worm reducer. This assists the steering of the steering wheel. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2021 / 0261188 Summary of the Invention [Problem to be solved by the invention]

[0005] Conventional electric power steering devices with a ball screw type steering gearbox, including the one disclosed in Patent Document 1, have the following concerns: When the steering gearbox is mounted on a vehicle, the ball screw shaft is maintained in a position extending in the vertical direction of the vehicle. The ball screw nut moves along the ball screw shaft extending in the vertical direction of the vehicle. At this time, gravity acts on the ball screw nut according to its mass. Therefore, when the ball screw mechanism is viewed alone, the load torque of the ball screw mechanism differs depending on the steering direction of the steering wheel, i.e., the direction of movement of the ball screw nut. The load torque is a torque generated according to the resistance generated via the balls at the meshing portion between the ball screw nut and the ball screw shaft. [Means for solving the problem]

[0006] A steering device according to one aspect of the present disclosure includes an actuator having a motor configured to generate torque in response to steering of a steering wheel of a vehicle, a reducer configured to decelerate rotation of the motor, and a ball screw mechanism configured to convert the rotation imparted through the reducer into rotation of an output shaft that is linked to steered wheels. The reducer and the ball screw mechanism are configured so that the increase and decrease characteristics of the load torque with respect to the steering direction generated when the steering wheel is steered are opposite to each other. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a schematic diagram of a steering device according to an embodiment; [Figure 2] FIG. 2 is a cross-sectional view of the steering gear box of FIG. 1. [Figure 3A] 3 is a cross-sectional view of the reducer of FIG. 2 cut in a direction perpendicular to the axis of the worm wheel. [Figure 3B] FIG. 3B is an enlarged cross-sectional view of the portion indicated by the reference numeral 3B in FIG. 3A. [Figure 4] 3B is a schematic diagram of the reducer of FIG. 3A, the reducer having a worm disposed in a first orientation. FIG. [Figure 5] 3B is a schematic diagram of the reducer of FIG. 3A, with the worm oriented in a second direction. FIG. [Figure 6] FIG. 3 is a cross-sectional view of the steering gear box of FIG. 2, which has a right-hand screw type ball screw shaft. [Figure 7] FIG. 3 is a cross-sectional view of the steering gear box of FIG. 2, which has a left-handed ball screw shaft. DETAILED DESCRIPTION OF THE INVENTION

[0008] A steering device according to one embodiment will be described. The steering device is an electric power steering device. <Overall structure> As shown in FIG. 1, the steering device 20 is mounted on, for example, a cab-over vehicle 10. The vehicle 10 is equipped with an axle-mounted 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 longitudinal direction of the vehicle. Both ends of the leaf spring 14 are attached to a body frame 16 via support members 15, such as shackles.

[0009] The steering device 20 has a steering shaft 21, a steering gearbox 22, a motor 23, a torque sensor 24, and a reducer 25. The steering shaft 21 and the steering gearbox 22 constitute a steering mechanism of the vehicle 10. A first end of the steering shaft 21 is connected to a steering wheel 26. A second end of the steering shaft 21 is connected to the steering gearbox 22 via the torque sensor 24 and the reducer 25. The motor 23 is connected to the steering gearbox 22 via the reducer 25. The steering gearbox 22 is, for example, an RBS type (recirculating ball screw type) steering gearbox.

[0010] The steering gear box 22 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 22. The pitman arm 31 is swingable 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 26 is transmitted to the steering gear box 22 via the steering shaft 21. The steering gear box 22 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 23 generates torque in the same direction as the steering direction of the steering wheel 26, according to the steering torque detected by the torque sensor 24. The torque of the motor 23 is transmitted to the steering gear box 22 via the reducer 25. That is, the reducer 25 is configured to impart torque to the steering gear box 22. The application of torque in the same direction as the steering direction to the steering gear box 22 assists the steering of the steering wheel 26. The motor 23 and the reducer 25 constitute an actuator.

[0013] <Configuration of steering gear box 22> 2, the steering gear box 22 has a first housing 40. Inside the first housing 40, a ball screw shaft 41, a ball screw nut 42, a plurality of balls 43, a sector shaft 44, and a sector gear 45 are provided.

[0014] When the steering gearbox 22 is mounted on the vehicle 10, the ball screw shaft 41 is maintained in a position extending in the vertical direction relative to the vehicle 10. The ball screw shaft 41 is rotatably supported by the first housing 40 via a first bearing 46 and a second bearing 47. A first end of the ball screw shaft 41 passes through the first housing 40 and is connected to the steering shaft 21 via the reducer 25 and the torque sensor 24. The first end is an end of the ball screw shaft 41 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 41, opposite the first end, is an end of the ball screw shaft 41 that faces downward of the vehicle 10 when the steering gearbox 22 is mounted on the vehicle 10. The second end faces an end wall of the first housing 40.

[0015] The ball screw shaft 41 has a first screw groove 41A. The first screw groove 41A is a spiral groove provided on the outer peripheral surface of the ball screw shaft 41. A screw portion is formed by providing the first screw groove 41A. The ball screw shaft 41 is a right-hand screw type or a left-hand screw type. The screw portion of a right-hand screw type ball screw shaft 41 is a right-hand screw, and the winding direction of the first screw groove 41A is clockwise when viewed from the first end side. The screw portion of a left-hand screw type ball screw shaft 41 is a left-hand screw, and the winding direction of the first screw groove 41A is counterclockwise when viewed from the first end side.

[0016] The ball screw nut 42 is cylindrical and has a second screw groove 42A. The second screw groove 42A is a spiral groove provided on the inner peripheral surface of the ball screw nut 42. The second screw groove 42A faces the first screw groove 41A in the radial direction. The ball screw nut 42 is screwed onto the ball screw shaft 41 via a plurality of balls 43. A spiral space surrounded by the first screw groove 41A and the second screw groove 42A functions as a rolling path along which the balls 43 roll. The ball screw shaft 41, the ball screw nut 42, and the balls 43 constitute a ball screw mechanism 48.

[0017] The ball screw nut 42 has a plurality of rack teeth 42B. The rack teeth 42B are provided on the outer peripheral surface of the ball screw nut 42. The rack teeth 42B are aligned in the axial direction of the ball screw nut 42.

[0018] The sector shaft 44 extends in a direction perpendicular to the axis of the ball screw nut 42 (a direction perpendicular to the plane of the paper in FIG. 2 ). The sector shaft 44 is rotatably supported in the first housing 40 via a bearing (not shown). The sector shaft 44 has an outer end that penetrates the first housing 40 and is exposed to the outside. The base end of the pitman arm 31 is fixed to the outer end of the sector shaft 44. The sector shaft 44 is the output shaft of the steering gear box 22 and is connected to the steered wheels 13.

[0019] The sector gear 45 is provided so as to be rotatable integrally with the sector shaft 44. The sector gear 45 is a sector-shaped gear and has a plurality of teeth 45A. The teeth 45A of the sector gear 45 mesh with the rack teeth 42B of the ball screw nut 42.

[0020] The rotation of the steering wheel 26 is transmitted to the ball screw shaft 41 via the steering shaft 21 and the reducer 25. As the ball screw shaft 41 rotates, the ball screw nut 42 moves axially relative to the ball screw shaft 41. This causes the sector gear 45 to swing around the sector shaft 44. As the sector gear 45 swings, the sector shaft 44 rotates, causing the pitman arm 31 to swing around the sector shaft 44.

[0021] If the ball screw shaft 41 is a right-hand thread type, when the ball screw shaft 41 rotates clockwise as viewed from the first end side, the ball screw nut 42 moves upward. Also, if the ball screw shaft 41 is a right-hand thread type, when the ball screw shaft 41 rotates counterclockwise as viewed from the first end side, the ball screw nut 42 moves downward.

[0022] If the ball screw shaft 41 is a left-handed type, when the ball screw shaft 41 rotates counterclockwise as viewed from the first end side, the ball screw nut 42 moves upward. Also, if the ball screw shaft 41 is a left-handed type, when the ball screw shaft 41 rotates clockwise as viewed from the first end side, the ball screw nut 42 moves downward.

[0023] When the steering wheel 26 is steered to the right, the ball screw shaft 41 rotates clockwise as seen from the first end, relative to the neutral steering position. When the steering wheel 26 is steered to the left, the ball screw shaft 41 rotates counterclockwise as seen from the first end, relative to the neutral steering position. The neutral steering position is the position of the steering wheel 26 that corresponds to the vehicle traveling straight ahead.

[0024] <Configuration of the reducer 25> As shown in FIG. 2 , the reducer 25 is connected to a first end of the ball screw shaft 41. The reducer 25 is configured to apply torque to the ball screw mechanism 48. The reducer 25 has a second housing 50. The second housing 50 is connected to the first housing 40 of the steering gearbox 22. The interior of the first housing 40 and the interior of the second housing 50 are in communication with each other. The motor 23 is attached to the outside of the second housing 50. The output shaft 23A of the motor 23 extends, for example, in a direction perpendicular to the axis of the ball screw shaft 41 and parallel to the axis of the sector shaft 44. The output shaft 23A penetrates the second housing 50 and is inserted into the second housing 50.

[0025] A shaft 51, a worm wheel 52, and a worm 53 are provided inside the second housing 50. The worm wheel 52 is integrally rotatably connected to a first end of the ball screw shaft 41. Specifically, the shaft 51 is rotatably supported relative to the second housing 50 via a third bearing 54 and a fourth bearing 55. The first end of the shaft 51 passes through the second housing 50 and is connected to the steering shaft 21 via the torque sensor 24. The second end of the shaft 51 is integrally rotatably connected to the first end of the ball screw shaft 41. The worm wheel 52 is attached to the outer peripheral surface of the shaft 51. The worm wheel 52 is rotatable integrally with the shaft 51. The worm 53 is provided integrally rotatably with the output shaft 23A of the motor 23. The axis of the output shaft 23A and the axis of the worm 53 coincide with each other. The worm 53 is in mesh with the worm wheel 52. The axial angle between the worm 53 and the worm wheel 52 is, for example, 90°.

[0026] The torque of the motor 23 is transmitted to the ball screw shaft 41 via the reducer 25. A torque is applied to the ball screw shaft 41 in the same direction as the steering direction of the steering wheel 26, thereby assisting the steering of the steering wheel 26.

[0027] 3A, the second housing 50 has a worm accommodating portion 61 and a worm wheel accommodating portion 62. The worm accommodating portion 61 is a cylindrical space that accommodates the worm 53 and extends in the axial direction of the worm 53. The worm wheel accommodating portion 62 is a cylindrical space that accommodates the worm wheel 52 and extends in a direction perpendicular to the axial direction of the worm 53. The worm wheel accommodating portion 62 is continuous with the worm accommodating portion 61.

[0028] A first end of the worm accommodating portion 61 communicates with a recessed motor mounting portion 63 provided on the outer surface of the second housing 50. The motor 23 is fixed to the motor mounting portion 63. The output shaft 23A of the motor 23 is inserted into the first end of the worm accommodating portion 61. A second end of the worm accommodating portion 61 opens to the outside of the second housing 50. The opening is closed by an end cover 64.

[0029] The worm 53 is rotatably supported on the inner circumferential surface of the worm accommodating portion 61 via a fifth bearing 65 and a sixth bearing 66. A first end of the worm 53 is supported by the fifth bearing 65. A second end of the worm 53 is supported by the sixth bearing 66. A cylindrical fixing plug 54A is attached to the first end of the worm accommodating portion 61. An end wall of the fixing plug 54A abuts against the outer ring of the fifth bearing 65 in the axial direction. This restricts movement of the fifth bearing 65, and therefore the worm 53, in a direction approaching the motor 23.

[0030] <Shaft Coupling 70> A first end of the worm 53 is connected to the output shaft 23A of the motor 23 via a shaft coupling 70. The shaft coupling 70 has a first joint 71, a second joint 72, and an elastic body 73. The elastic body 73 is interposed between the first joint 71 and the second joint 72.

[0031] The first joint 71 is connected to a first end of the worm 53 so as to be rotatable together with the worm. The first joint 71 has a plurality of first engaging claws 71A. The first engaging claws 71A are provided on the side surface of the first joint 71 that comes into contact with the elastic body 73. The plurality of first engaging claws 71A are provided at equal intervals in the circumferential direction.

[0032] The second joint 72 is connected to the output shaft 23A of the motor 23 so as to be rotatable together with the second joint 72. The second joint 72 has a plurality of second engagement claws 72A. The second engagement claws 72A are provided on the side surface of the second joint 72 that comes into contact with the elastic body 73. The plurality of second engagement claws 72A are provided at equal intervals in the circumferential direction.

[0033] The elastic body 73 has a plurality of first engagement recesses 73A and a plurality of second engagement recesses 73B. The first engagement recesses 73A are provided on the side surface of the elastic body 73 that comes into contact with the first joint 71. The second engagement recesses 73B are provided on the side surface of the elastic body 73 that comes into contact with the second joint 72. The first engagement recesses 73A and the second engagement recesses 73B are provided alternately in the circumferential direction of the elastic body 73.

[0034] The first engagement claw 71A meshes with the first engagement recess 73A. The second engagement claw 72A meshes with the second engagement recess 73B. The first engagement claw 71A circumferentially engages with the first engagement recess 73A, and the second engagement claw 72A circumferentially engages with the second engagement recess 73B. This enables torque transmission between the worm 53 and the output shaft 23A.

[0035] The elastic body 73 is maintained in a slightly compressed state by the first joint 71 and the second joint 72. When an external force is applied to the worm 53 in the radial direction, the elastic body 73 elastically deforms, allowing the worm 53 to tilt relative to the output shaft 23A. The fifth bearing 65 has an internal gap. The internal gap is the amount of play between the outer ring, inner ring, and balls of the fifth bearing 65. The worm 53 can move tilting relative to the axis of the output shaft 23A, centered on the part of the worm 53 supported by the fifth bearing 65. The oscillation center O of the worm 53 coincides with the center of the fifth bearing 65 when viewed from the axial direction of the worm wheel 52.

[0036] <Biasing mechanism 80> 3A, the reducer 25 has a biasing mechanism 80. The biasing mechanism 80 is configured to adjust the backlash of the reducer 25 by elastically biasing the worm 53 toward the worm wheel 52.

[0037] As shown in FIG. 3B , the biasing mechanism 80 includes a bolt 81, a compression coil spring 82, and a pressing member 83. The bolt 81 includes a head 81A and a shaft 81B. The shaft 81B is fastened to a threaded hole 50A in the second housing 50. The threaded hole 50A is a cylindrical hole that penetrates the peripheral wall of the second housing 50 and opens to the inner peripheral surface of the worm accommodating portion 61 corresponding to the sixth bearing 66. The compression coil spring 82 is interposed between the shaft 81B and the pressing member 83 in an axially compressed state. The pressing member 83 is fixed to the end of the compression coil spring 82 opposite the shaft 81B.

[0038] The compression coil spring 82 constantly biases the sixth bearing 66, and therefore the second end of the worm 53, toward the worm wheel 52 via the pressing member 83. As a result, a moment acts on the second end of the worm 53 in the counterclockwise direction in FIG. 3A around the oscillation center O of the worm 53. The counterclockwise tilting of the worm 53 is restricted by the worm 53 abutting against the worm wheel 52.

[0039] The worm 53 is pressed against the worm wheel 52, thereby making it possible to reduce the amount of backlash at the meshing portion between the worm 53 and the worm wheel 52. It is also expected that the teeth of the worm 53 and the teeth of the worm wheel 52 will wear over time. However, even in this case, the worm 53 tilts by the amount of wear of the teeth, thereby suppressing an increase in backlash at the meshing portion between the worm 53 and the worm wheel 52.

[0040] <Load torque of ball screw mechanism 48> Next, a supplementary explanation will be given regarding the load torque of the ball screw mechanism 48. When the steering gearbox 22 is mounted on the vehicle 10, the ball screw shaft 41 is maintained in a position extending in the vertical direction of the vehicle 10. The ball screw nut 42 moves along the ball screw shaft 41 extending in the vertical direction of the vehicle 10. At this time, gravity acts on the ball screw nut 42 in accordance with its mass. For this reason, when the ball screw mechanism 48 is viewed alone, the load torque of the ball screw mechanism 48 differs depending on the steering direction of the steering wheel 26, i.e., the movement direction of the ball screw nut 42. The load torque is a torque generated in accordance with the resistance generated via the balls 43 at the meshing portion between the ball screw nut 42 and the ball screw shaft 41.

[0041] <Load torque for right-hand thread type> As shown in FIG. 6, the load torque of the ball screw mechanism 48 when using a right-hand screw type ball screw shaft 41 will be considered.

[0042] In this case, when the steering wheel 26 is steered to the right, the ball screw shaft 41 rotates in the clockwise direction DR2 as viewed from the first end side. As a result, the ball screw nut 42 rises along the ball screw shaft 41. In other words, the ball screw nut 42 moves in the direction opposite to the direction in which gravity acts. As a result, the load torque of the ball screw mechanism 48 increases.

[0043] When the steering wheel 26 is steered to the left, the ball screw shaft 41 rotates in the counterclockwise direction DL2 as viewed from the first end side. As a result, the ball screw nut 42 descends along the ball screw shaft 41. That is, the ball screw nut 42 moves in the same direction as the direction in which gravity acts. As a result, the load torque of the ball screw mechanism 48 decreases.

[0044] Therefore, as shown in the following equation (3), the load torque T generated when the steering wheel 26 is steered to the right is br is the load torque T generated when the steering wheel 26 is steered to the left. bl becomes larger than the absolute value of

[0045] │T bl │<│T br │ …(3) <Load torque for left-hand thread type> Next, as shown in FIG. 7, the load torque of the ball screw mechanism 48 when a left-handed ball screw shaft 41 is used will be considered.

[0046] In this case, when the steering wheel 26 is steered to the left, the ball screw shaft 41 rotates in the counterclockwise direction DR2 as viewed from the first end side. As a result, the ball screw nut 42 rises along the ball screw shaft 41. In other words, the ball screw nut 42 moves in the direction opposite to the direction in which gravity acts. As a result, the load torque of the ball screw mechanism 48 increases.

[0047] When the steering wheel 26 is steered to the right, the ball screw shaft 41 rotates in the clockwise direction DR2 as viewed from the first end side. As a result, the ball screw nut 42 descends along the ball screw shaft 41. That is, the ball screw nut 42 moves in the same direction as the direction in which gravity acts. As a result, the load torque of the ball screw mechanism 48 decreases.

[0048] Therefore, as shown in the following equation (4), the load torque T generated when the steering wheel 26 is steered to the left is blThe absolute value of the load torque T br becomes larger than the absolute value of

[0049] │T bl │>│T br │ …(4) <Reaction force acting on worm 53> Next, the reaction force acting on the worm 53 will be described.

[0050] The tooth flanks of the worm 53 are twisted relative to the axis of the worm 53. The tooth flanks of the worm wheel 52 are twisted relative to the axis of the worm wheel 52. A pressure angle exists between the tooth flanks of the worm 53 and the worm wheel 52. The pressure angles of the two tooth flanks of the worm 53 that are located opposite each other in the axial direction of the worm 53 are the same. The pressure angles of the two tooth flanks of the worm wheel 52 that are located opposite each other in the rotational direction of the worm wheel 52 are the same. Therefore, as the worm 53 rotates, a reaction force is applied from the worm wheel 52 to the worm 53. The position where the reaction force is applied is the meshing portion between the worm 53 and the worm wheel 52.

[0051] 4, the reaction force acting from the worm wheel 52 to the worm 53 includes an axial component force F1, which is a force applied in the axial direction of the worm 53, and a radial component force F2, which is a force applied in the radial direction of the worm 53. When the steering wheel 26 is steered, the reaction force including the axial component force F1 and the radial component force F2 acts as resistance.

[0052] The direction of the radial component force F2 is the same when the steering wheel 26 is steered to the right and when it is steered to the left. If the magnitude of the torque applied to the steering wheel 26 is the same, the magnitude of the radial component force F2 is the same when the steering wheel 26 is steered to the right and when it is steered to the left.

[0053] The direction of the axial component force F1 is opposite when the steering wheel 26 is steered to the right and when it is steered to the left. If the magnitude of the torque applied to the steering wheel 26 is the same, the magnitude of the axial component force F1 is the same when the steering wheel 26 is steered to the right and when it is steered to the left.

[0054] When the steering wheel 26 is steered to the right, the worm wheel 52 rotates in a clockwise direction DR1 as viewed from the axial direction. At this time, as shown by the solid line in Fig. 4, the direction of the axial component force F1 is from the meshing portion between the worm 53 and the worm wheel 52 toward the first end of the worm 53 (to the right in Fig. 4). The axial component force F1 generated when the steering wheel 26 is steered to the right becomes, for example, a positive value.

[0055] When the steering wheel 26 is steered to the left, the worm wheel 52 rotates in a counterclockwise direction DL1 as viewed from the axial direction. At this time, as shown by the dashed line in Fig. 4, the direction of the axial component force F1 is from the meshing portion between the worm 53 and the worm wheel 52 toward the second end of the worm 53 (to the left in Fig. 4). When the axial component force F1 generated when the steering wheel 26 is steered to the right is a positive value, the axial component force F1 generated when the steering wheel 26 is steered to the left is a negative value.

[0056] The position where the reaction force from the worm wheel 52 is applied to the worm 53 is the meshing portion between the worm 53 and the worm wheel 52. The meshing portion is a position that is shifted from the axis of the worm 53 toward the worm wheel 52. Therefore, when the oscillation center O of the worm 53 is located on the axis of the worm 53, a moment about the oscillation center O acts on the worm 53 due to the axial component force F1. The direction of the moment about the oscillation center O is reversed depending on the rotation direction of the worm wheel 52.

[0057] For this reason, the load torque of the reducer 25 may differ depending on the steering direction of the steering wheel 26, i.e., the rotation direction of the worm wheel 52. The load torque is generated in response to the resistance generated in the meshing portion between the worm wheel 52 and the worm 53.

[0058] <Load Torque of the Reducer 25 When the Orientation of the Worm 53 is in the First Orientation> First, consider the load torque of the reducer 25 generated when the worm 53 is disposed in the first orientation shown in FIG. 4. The first orientation is the orientation of the worm 53 in which the second end of the worm 53 is located to the left of the center line L1 of the worm wheel 52 when viewed from the first end of the shaft 51 to which the steering shaft 21 is connected. The center line L1 is a straight line that passes through the center of the worm wheel 52 and is perpendicular to the center line L2 of the worm 53 when viewed from the axial direction of the worm wheel 52. In other words, the second end of the worm 53 is located to the left of the center of the worm wheel 52 in the axial direction of the worm 53. The biasing mechanism 80 applies a preload F0 to the second end of the worm 53. The preload F0 is an elastic force generated by the compression coil spring 82 of the biasing mechanism 80 in a direction toward the worm wheel 52.

[0059] When the worm wheel 52 rotates in the clockwise direction DR1, a moment M1 acts on the worm 53 in the counterclockwise direction around the oscillation center O due to the axial component force F1. The moment M1 acts to tilt the worm 53 around the oscillation center O in a direction approaching the worm wheel 52. Therefore, the force pressing the tooth surfaces of the worm 53 and the worm wheel 52 against each other increases, and the load torque of the reducer 25 increases.

[0060] When the worm wheel 52 rotates in the counterclockwise direction DL1, a clockwise moment M2 about the oscillation center O acts on the worm 53 due to the axial component force F1. The moment M2 acts to tilt the worm 53 in a direction away from the worm wheel 52 about the oscillation center O. Therefore, the force pressing the tooth surfaces of the worm 53 and the worm wheel 52 against each other becomes smaller, and the load torque of the reducer 25 decreases.

[0061] In other words, the reducer 25 is configured so that the preload F0 applied by the biasing mechanism 80 to the worm 53 when the steering direction is such that the ball screw nut 42 descends is greater than the preload F0 applied by the biasing mechanism 80 to the worm 53 when the steering direction is such that the ball screw nut 42 ascends.

[0062] Therefore, as shown in the following equation (1), the load torque T of the reducer 25 generated when the steering wheel 26 is steered to the right is dr The absolute value of the load torque T dl The load torque is also a loss torque that occurs when the steering wheel 26 is steered.

[0063] │T dr │>│T dl │ …(1) In this way, if the load torque of the reducer 25 varies depending on the steering direction of the steering wheel 26, the force required for the driver to operate the steering wheel 26 may vary depending on the steering direction. That is, there is a concern that the difference between the force required to steer the steering wheel 26 to the right and the force required to steer the steering wheel 26 to the left may become large.

[0064] <Load Torque of the Reducer 25 When the Orientation of the Worm 53 is in the Second Orientation> Next, the load torque of the reducer 25 when the worm 53 is provided in a second orientation opposite to the first orientation shown in FIG. 4 will be considered.

[0065] 5, the second orientation is the orientation of the worm 53 in which the second end of the worm 53 is located to the right of the center line L1 of the worm wheel 52 when viewed from the first end of the shaft 51 to which the steering shaft 21 is connected. In other words, the second end of the worm 53 is located to the right of the center of the worm wheel 52 in the axial direction of the worm 53.

[0066] When the worm wheel 52 rotates in the clockwise direction DR1, a moment M1 acts on the worm 53 in the counterclockwise direction around the oscillation center O due to the axial component force F1. The moment M1 acts to tilt the worm 53 around the oscillation center O in a direction away from the worm wheel 52. Therefore, the force pressing the tooth surfaces of the worm 53 and the worm wheel 52 against each other becomes smaller, and the load torque of the reducer 25 decreases.

[0067] When the worm wheel 52 rotates in the counterclockwise direction DL1, a clockwise moment M2 about the oscillation center O acts on the worm 53 due to the axial component force F1. The moment M2 acts to tilt the worm 53 in a direction approaching the worm wheel 52 about the oscillation center O. Therefore, the force pressing the tooth surfaces of the worm 53 and the worm wheel 52 against each other increases, and the load torque of the reducer 25 increases.

[0068] In other words, the reducer 25 is configured so that the preload F0 applied by the biasing mechanism 80 to the worm 53 when the steering direction is such that the ball screw nut 42 descends is greater than the preload F0 applied by the biasing mechanism 80 to the worm 53 when the steering direction is such that the ball screw nut 42 ascends.

[0069] Therefore, as shown in the following equation (2), the load torque T generated when the steering wheel 26 is steered to the left is dl The absolute value of the load torque T dr becomes larger than the absolute value of

[0070] │T dr │<│T dl │ …(2) When the worm 53 is provided in the second direction, the load torque T generated when the steering wheel 26 is steered to the right is dr and the absolute value of the load torque T generated when the steering wheel 26 is steered to the left. dl The magnitude relationship between the absolute value of the worm 53 and the absolute value of the worm 53 is opposite to that when the worm 53 is provided in the first orientation.

[0071] Even in this case, the load torque of the reducer 25 may differ depending on the steering direction of the steering wheel 26, which may cause the force required for the driver to operate the steering wheel 26 to differ depending on the steering direction. That is, there is a concern that the difference between the force required to steer the steering wheel 26 to the right and the force required to steer the steering wheel 26 to the left may become large.

[0072] Therefore, in this embodiment, by focusing on the fact that the load torque of the ball screw mechanism 48 varies depending on the steering direction of the steering wheel 26, the combination of the orientation of the worm 53 and the type of the ball screw shaft 41 is determined. There are two patterns of combinations.

[0073] <First combination> As shown in Table 1, when the orientation of the worm 53 is set to the first direction shown in FIG. 4, the left-hand thread type ball screw shaft 41 shown in FIG. 7 is used.

[0074] [Table 1]

[0075] When the first combination shown in Table 1 is used, when the steering wheel 26 is steered to the right, the load torque of the reducer 25 increases while the load torque of the ball screw shaft 41 decreases. Furthermore, when the steering wheel 26 is steered to the left, the load torque of the reducer 25 decreases while the load torque of the ball screw shaft 41 increases. In other words, the increase / decrease characteristics of the load torque generated when the steering wheel 26 is steered are opposite for the reducer 25 and the ball screw mechanism 48. Therefore, the difference in the total load torque generated by the reducer 25 and the ball screw mechanism 48 becomes smaller when the steering wheel 26 is steered to the right and when the steering wheel 26 is steered to the left.

[0076] <Second combination> As shown in Table 2, when the orientation of the worm 53 is set to the second direction shown in FIG. 5, the right-hand thread type ball screw shaft 41 shown in FIG. 6 is used.

[0077] [Table 2]

[0078] When the second combination shown in Table 2 is adopted, when the steering wheel 26 is steered to the right, the load torque of the reducer 25 decreases while the load torque of the ball screw shaft 41 increases. Furthermore, when the steering wheel 26 is steered to the left, the load torque of the reducer 25 increases while the load torque of the ball screw shaft 41 decreases. In other words, the increase / decrease characteristics of the load torque generated when the steering wheel 26 is steered are opposite for the reducer 25 and the ball screw mechanism 48. Therefore, the difference in the total load torque generated by the reducer 25 and the ball screw mechanism 48 becomes smaller when the steering wheel 26 is steered to the right and when the steering wheel 26 is steered to the left.

[0079] <Effects of the embodiment> This embodiment has the following advantages. (1) The reducer 25 and the ball screw mechanism 48 are configured so that the increase and decrease characteristics of the load torque generated when the steering wheel 26 is steered are opposite to each other in the reducer 25 and the ball screw mechanism 48. Therefore, the difference in the total load torque generated by the reducer 25 and the ball screw mechanism 48 between when the steering wheel 26 is steered to the right and when the steering wheel 26 is steered to the left is reduced. Therefore, it is possible to reduce the difference between the force required to steer the steering wheel 26 to the right and the force required to steer the steering wheel 26 to the left.

[0080] (2) When the orientation of the worm 53 is set to the first direction shown in Fig. 4, a left-hand thread type ball screw shaft 41 shown in Fig. 7 is used. In this way, the load torque increase / decrease characteristics of the speed reducer 25 and the ball screw mechanism 48 can be reversed when the steering wheel 26 is steered to the right and when the steering wheel 26 is steered to the left.

[0081] (3) When the orientation of the worm 53 is set to the second direction shown in Fig. 5, a right-hand thread type ball screw shaft 41 shown in Fig. 6 is used. Even in this way, the load torque increase / decrease characteristics of the speed reducer 25 and the ball screw mechanism 48 can be reversed when the steering wheel 26 is steered to the right and when the steering wheel 26 is steered to the left.

[0082] (4) Depending on the specifications of the steering device 20, either the first combination shown in Table 1 or the second combination shown in Table 2 can be selected as the combination of the orientation of the worm 53 and the type of the ball screw shaft 41. This increases the product variation of the steering device 20.

[0083] <Other embodiments> This embodiment may be modified as follows. The posture of the steering gear box 22 when mounted on the vehicle 10 may be changed as appropriate as long as gravity acts in the direction of movement of the ball screw nut 42.

[0084] In order to further reduce the difference between the left and right load torques in the steering direction that occurs when the steering wheel 26 is steered, ideally to zero, the motor 23 may be controlled as follows. That is, the motor 23 is controlled so that the torque applied to the ball screw mechanism 48 when the steering direction is such that the ball screw nut 42 descends is smaller than the torque applied to the ball screw mechanism 48 when the steering direction is such that the ball screw nut 42 ascends. The motor 23 is controlled, for example, by a control device of the steering device 20.

[0085] This reduces the difference between the left and right load torques generated in the ball screw mechanism 48 when the steering wheel 26 is steered. The difference between the left and right load torques is the difference between the load torque when the steering wheel 26 is steered to the right and the load torque when the steering wheel 26 is steered to the left. Therefore, compared to a case where the speed reducer 25 and the ball screw mechanism 48 are simply configured so that the increase and decrease characteristics of the load torque generated when the steering wheel 26 is steered are opposite to each other, the difference between the left and right load torques generated in the total load torques generated in the speed reducer 25 and the ball screw mechanism 48 can be further reduced.

[0086] The reducer 25 may have a configuration in which the biasing mechanism 80 is omitted. Also, the worm 53 does not have to be tiltable relative to the output shaft 23A of the motor 23. In this case, the difference between the left and right load torques generated in the reducer 25 when the steering wheel 26 is steered can basically be ignored. Therefore, it is necessary to focus only on the difference between the left and right load torques generated in the ball screw mechanism 48 when the steering wheel 26 is steered.

[0087] Therefore, in order to reduce the difference between the left and right load torques generated in the ball screw mechanism 48 in the steering direction when the steering wheel 26 is steered, ideally to zero, the motor 23 may be controlled as follows. That is, the motor 23 is controlled so that the torque applied to the ball screw mechanism 48 when the steering direction is such that the ball screw nut 42 descends is smaller than the torque applied to the ball screw mechanism 48 when the steering direction is such that the ball screw nut 42 ascends. In this way, it is possible to reduce the difference between the left and right load torques generated in the ball screw mechanism 48 when the steering wheel 26 is steered, ideally to zero. The load torque increase / decrease characteristic of the ball screw mechanism 48 is generated by controlling the motor 23.

[0088] In this case, the combination of the orientation of the worm 53 and the type of ball screw shaft 41 (left-hand thread type and right-hand thread type) can be different from the combinations shown in Tables 1 and 2. For example, when the orientation of the worm 53 is in the first direction, a right-hand thread type ball screw shaft 41 may be used. Also, when the orientation of the worm 53 is in the second direction, a left-hand thread type ball screw shaft 41 may be used.

[0089] The combinations of the orientation of the worm 53 and the type (left-hand thread type and right-hand thread type) of the ball screw shaft 41 do not necessarily have to be those shown in Tables 1 and 2. In this case, the motor 23 may be controlled as follows to reduce the difference between the left and right load torques relative to the steering direction generated when the steering wheel 26 is steered, ideally to zero. That is, the motor 23 is controlled so that the torque applied to the ball screw mechanism 48 when the steering direction is such that the ball screw nut 42 descends is smaller than the torque applied to the ball screw mechanism 48 when the steering direction is such that the ball screw nut 42 ascends. In this way, the difference between the left and right load torques generated in the ball screw mechanism 48 when the steering wheel 26 is steered can be reduced, ideally to zero. The load torque increase / decrease characteristics of the ball screw mechanism 48 are generated by controlling the motor 23. Therefore, the difference between the left and right load torques generated in the ball screw mechanism 48 can be reduced by the amount that the difference between the left and right load torques generated in the speed reducer 25 and the ball screw mechanism 48 is reduced.

[0090] The steering device 20 may be a steer-by-wire type steering device. The steering device 20 may have a so-called linkless structure in which the power transmission between the steering wheel 26 and the steered wheels 13 is separated. The steering device 20 may also have a structure in which the power transmission between the steering wheel 26 and the steered wheels 13 can be separated by a clutch. When the clutch is disengaged, the power transmission between the steering wheel 26 and the steered wheels 13 is disconnected. When the clutch is engaged, the power transmission between the steering wheel 16 and the steered wheels 13 is connected.

Claims

1. an actuator having a motor configured to generate torque in response to steering of a steering wheel of a vehicle and a reducer configured to decelerate rotation of the motor; a ball screw mechanism configured to convert the rotation imparted through the reducer into rotation of an output shaft interlocked with the steered wheels, the speed reducer and the ball screw mechanism are configured such that the load torque increase / decrease characteristics with respect to the steering direction generated when the steering wheel is steered are opposite to each other in the speed reducer and the ball screw mechanism, the ball screw mechanism includes a ball screw shaft to which rotation is imparted through the reducer, the ball screw shaft extending in a vertical direction relative to the vehicle, a plurality of balls, and a ball screw nut screwed onto the ball screw shaft via the balls, The reducer is a worm having a first end tiltably coupled to the motor and a second end opposite the first end; a worm wheel that is rotatable integrally with the ball screw shaft and configured to mesh with the worm; a biasing mechanism that applies a preload to the second end of the worm in a direction toward the worm wheel, The reduction gear is configured so that the preload applied to the worm by the biasing mechanism when the steering direction is such that the ball screw nut descends is greater than the preload applied to the worm by the biasing mechanism when the steering direction is such that the ball screw nut ascends.

2. the load torque increase / decrease characteristic of the ball screw mechanism is generated by controlling the motor; 2. The steering device according to claim 1, wherein the motor is controlled so that the torque applied to the ball screw mechanism when the steering direction is such that the ball screw nut descends is smaller than the torque applied to the ball screw mechanism when the steering direction is such that the ball screw nut ascends, in order to eliminate a difference in load torque depending on the steering direction when the steering wheel is steered.

3. The second end of the worm is located to the left of the center of the worm wheel in the axial direction of the worm when viewed from the end of the ball screw shaft to which the worm wheel is connected, and the ball screw shaft is a left-handed screw type and has a spiral screw groove that winds in a counterclockwise direction when viewed from the end of the ball screw shaft to which the worm wheel is connected, or 3. The steering device according to claim 1 or 2, wherein the second end of the worm is located to the right of the center of the worm wheel in the axial direction of the worm when viewed from the end side of the ball screw shaft to which the worm wheel is connected, and the ball screw shaft is a right-hand thread type and has a spiral screw groove that winds in a clockwise direction when viewed from the end side of the ball screw shaft to which the worm wheel is connected.

Images