Steering gear

Abstract

This steering device (20) comprises: a shaft (24B2) which rotates in an interlocked manner with steering of a steering wheel (26); a ball screw mechanism (48) which converts rotation of the shaft into rotation of an output shaft (44); and a speed reducer (25) which applies a torque to the shaft. The ball screw mechanism has a first housing (40), a ball screw shaft (41) which is connected to the shaft in an intermeshed manner, and first and second bearings (46, 47) which support the ball screw shaft. The speed reducer has a second housing (50) and a third bearing (53) which supports the shaft. An outer diameter (φ1) of the shaft is set such that a third centerline (O3) passing through the cross-sectional center of the third bearing is positioned radially outside a first centerline (O1) passing through the cross-sectional center of the first bearing or a second centerline (O2) passing through the cross-sectional center of the second bearing.

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 rotation of an output shaft that rotates in conjunction with the steered wheels. The direction of the steered wheels is changed in conjunction with the rotation of the output shaft. 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.

[0004] The steering shaft has an input shaft, an output shaft, and a torsion bar. The input shaft and the output shaft are connected to each other via the torsion bar. Steering torque applied to the steering wheel is transmitted to the output shaft via the input shaft and the torsion bar. The torsion bar twists in response to the steering torque. The end of the output shaft opposite the input shaft is connected to the end of the ball screw shaft. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Chinese Patent Application Publication No. 111284556 Summary of the Invention [Problem to be solved by the invention]

[0006] The electric power steering device of Patent Document 1 has the following concerns. Specifically, the output shaft and the ball screw shaft are connected to each other by a serration connection. A serration connection is a method of connecting a rotating shaft to a part having a fitting portion into which the rotating shaft is fitted. The serrated teeth on the outer peripheral surface of the rotating shaft and the sawtooth grooves on the inner peripheral surface of the fitting portion fit together to restrict relative rotation between the rotating shaft and the part.

[0007] In the electric power steering device of Patent Document 1, sawtooth teeth extending in the axial direction are provided on the outer peripheral surface of the end of the output shaft coupled to the ball screw shaft. The end of the ball screw shaft to which the output shaft is coupled has a fitting portion. A sawtooth groove extending in the axial direction is provided on the inner peripheral surface of the fitting portion. The end of the output shaft is press-fitted into the fitting portion, so that the sawtooth teeth and the sawtooth groove fit together.

[0008] However, there is a slight amount of play between the end of the output shaft and the fitting portion of the ball screw shaft in the rotational direction or radial direction. With long-term use of the electric power steering device, there is a risk that the play between the end of the output shaft and the fitting portion of the ball screw shaft will gradually increase. In this case, there is a risk that the output shaft will tilt due to the play between the end of the output shaft and the fitting portion of the ball screw shaft. [Means for solving the problem]

[0009] A steering device according to one aspect of the present disclosure includes a shaft that rotates in conjunction with steering of a vehicle steering wheel, a ball screw mechanism configured to convert the rotation of the shaft into the rotation of an output shaft that rotates in conjunction with the steered wheels, and a reducer configured to apply torque to the shaft. The ball screw mechanism includes a first housing, a ball screw shaft housed inside the first housing, the ball screw shaft having a first end and a second end and integrally rotatably connected to the shaft by meshing connection between the first end and the shaft, a first bearing that rotatably supports the first end of the ball screw shaft relative to the first housing, and a second bearing that rotatably supports the second end of the ball screw shaft relative to the first housing. The reducer includes a second housing axially connected to the first housing and into which the shaft is axially inserted, and a third bearing that rotatably supports the shaft relative to the second housing. A line passing through the center of a cross section obtained by cutting the first bearing in the axial direction and parallel to the central axis of the shaft is a first center line. A line passing through the center of a cross section obtained by cutting the second bearing in the axial direction and parallel to the central axis of the shaft is a second center line. A line passing through the center of a cross section obtained by cutting the third bearing in the axial direction and parallel to the central axis of the shaft is a third center line. The outer diameter of the shaft is set so that the third center line is located outside the first center line or the second center line in the radial direction relative to the central axis of the shaft. [Brief explanation of the drawings]

[0010] [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] FIG. 3 is a cross-sectional view showing a first comparative example of the output shaft of FIG. 2. [Figure 3B]FIG. 3 is a cross-sectional view showing a second comparative example of the output shaft of FIG. 2. [Figure 4A] FIG. 3 is a cross-sectional view showing the inclination of the output shaft of FIG. 2 in a first comparative example. [Figure 4B] FIG. 3 is a cross-sectional view showing the inclination of a second comparative example of the output shaft of FIG. 2. DETAILED DESCRIPTION OF THE INVENTION

[0011] A steering device according to an embodiment will be described. The steering device is, for example, 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.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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 shaft 21 via the reducer 25. That is, the reducer 25 is configured to apply torque to the steering shaft 21. By applying torque in the same direction as the steering direction to the steering shaft 21, the steering of the steering wheel 26 is assisted.

[0016] <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.

[0017] 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. The first bearing 46 and the second bearing 47 are, for example, the same product.

[0018] The first bearing 46 supports a first end of the ball screw shaft 41. The first end is the 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. The first end of the ball screw shaft 41 is connected to the steering shaft 21 via the torque sensor 24.

[0019] The second bearing 47 supports a second end of the ball screw shaft 41. The second end is the end of the ball screw shaft 41 opposite the first end. In other words, the second end is the 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 the end wall of the first housing 40.

[0020] 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. The provision of the first screw groove 41A forms a threaded portion. The ball screw shaft 41 also has a connecting hole 41B at a first end. The connecting hole 41B extends in the axial direction. A plurality of serration grooves extending in the axial direction are provided on the inner peripheral surface of the connecting hole 41B. The serration grooves are arranged at equal intervals around the circumferential direction of the connecting hole 41B. When viewed in the axial direction, the serration grooves have, for example, a triangular shape.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] The rotation of the steering wheel 26 is transmitted to the ball screw shaft 41 via the steering shaft 21 and the torque sensor 24. 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.

[0026] <Configuration of torque sensor 24> 2, the torque sensor 24 has an input shaft 24A, an output shaft 24B, a torsion bar 24C, and a detector 24D. The input shaft 24A and the output shaft 24B are connected to each other via the torsion bar 24C. The input shaft 24A and the output shaft 24B are each a hollow cylinder.

[0027] A first end of the input shaft 24A is connected to the steering shaft 21. A second end of the input shaft 24A is inserted into a first end of the output shaft 24B. The input shaft 24A is held in a non-contact state with respect to the output shaft 24B. A plain bearing 24E is interposed between the outer peripheral surface of the input shaft 24A and the inner peripheral surface of the output shaft 24B. The input shaft 24A and the output shaft 24B can rotate relative to each other via the plain bearing 24E.

[0028] The second end of the output shaft 24B penetrates the end wall of the first housing 40 and fits into the connecting hole 41B. A plurality of serration teeth extending in the axial direction are provided on the outer peripheral surface of the second end of the output shaft 24B. The serration teeth are arranged at equal intervals in the circumferential direction. When viewed in the axial direction, the serration teeth have, for example, a triangular shape. The serration teeth of the output shaft 24B and the serration grooves of the connecting hole 41B fit together. This serration connection between the output shaft 24B and the ball screw shaft 41 restricts relative rotation between the output shaft 24B and the ball screw shaft 41. The output shaft 24B can rotate integrally with the ball screw shaft 41.

[0029] Although a connecting structure using so-called triangular tooth serrations has been exemplified, any structure may be used as long as it is an intermeshing connection between an inner member with external teeth and an outer member with internal teeth, and a square spline or an involute spline may also be used.

[0030] The second end of the output shaft 24B is an end of the output shaft 24B located on the opposite side of the bearing mounting portion 24B1 with respect to the worm wheel mounting portion 24B2. The second end of the output shaft 24B is axially fitted into the connecting hole 41B and is indirectly supported by the first bearing 46 via the peripheral wall of the connecting hole 41B.

[0031] The output shaft 24B has a bearing mounting portion 24B1 and a worm wheel mounting portion 24B2. The bearing mounting portion 24B1 and the worm wheel mounting portion 24B2 are located between the first end and the second end of the output shaft 24B. The outer diameter of the bearing mounting portion 24B1 is larger than the outer diameters of the other portions of the output shaft 24B except for the worm wheel mounting portion 24B2. The outer diameter of the worm wheel mounting portion 24B2 is slightly larger than the outer diameter of the bearing mounting portion 24B1.

[0032] The bearing mounting portion 24B1 and the worm wheel mounting portion 24B2 are adjacent to each other in the axial direction. The bearing mounting portion 24B1 is located on the opposite side of the worm wheel mounting portion 24B2 from the ball screw shaft 41. The bearing mounting portion 24B1 is a portion of the output shaft 24B that is located above the worm wheel mounting portion 24B2 when the steering gear box 22 is mounted on the vehicle 10. The worm wheel mounting portion 24B2 is a portion of the output shaft 24B that is located below the bearing mounting portion 24B1 when the steering gear box 22 is mounted on the vehicle 10.

[0033] The torsion bar 24C is inserted into the input shaft 24A and the output shaft 24B. A first end of the torsion bar 24C is fixed to the inner circumferential surface of the first end of the input shaft 24A. A second end of the torsion bar 24C is fixed to the inner circumferential surface of the second end of the output shaft 24B. A gap exists between the outer circumferential surface of the portion of the torsion bar 24C between the first end and the second end and the inner circumferential surface of the output shaft 24B.

[0034] Steering torque applied to the steering wheel 26 is transmitted to the input shaft 24A via the steering shaft 21, causing the input shaft 24A to rotate. The rotation of the input shaft 24A is transmitted to the output shaft 24B via the torsion bar 24C, causing the output shaft 24B to rotate. The output shaft 24B corresponds to a shaft that rotates in conjunction with the steering of the steering wheel 26. The torsion bar 24C twists in response to the steering torque.

[0035] The detector 24D is provided, for example, to surround the outer circumferential surface of the input shaft 24A. The detector 24D detects the steering torque based on the amount of twist of the torsion bar 24C. <Configuration of the reducer 25> 2, 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 second housing 50 is connected to a portion of the first housing 40 that is closer to the first bearing 46 than to the second bearing 47 in the axial direction.

[0036] The first housing 40 has a cylindrical connecting portion 40A that extends in the axial direction. The connecting portion 40A is provided at an end of the first housing 40 that faces upward of the vehicle 10 when the steering gearbox 22 is mounted on the vehicle 10. The connecting portion 40A is closer to the first bearing 46 than to the second bearing 47 in the axial direction.

[0037] The second housing 50 is cylindrical and has an end wall at a first end in the axial direction. The first end is the end of the second housing 50 that faces upward of the vehicle 10 when the steering gearbox 22 is mounted on the vehicle 10. The end wall has a first opening 50A. The input shaft 24A and the output shaft 24B pass through the first opening 50A in the axial direction without contacting each other. The second housing 50 has a second opening 50B at a second end opposite the first end. The second end is the end of the second housing 50 that faces downward of the vehicle 10 when the steering gearbox 22 is mounted on the vehicle 10.

[0038] The connecting portion 40A of the first housing 40 and the second opening 50B of the second housing 50 are fitted together in the axial direction. Specifically, the inner peripheral surface of the second opening 50B is fitted into the outer peripheral surface of the connecting portion 40A. There is no step between the outer peripheral surfaces of the first housing 40 and the second housing 50. The interior of the first housing 40 and the interior of the second housing 50 are in communication with each other.

[0039] 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 passes through the second housing 50 and is inserted into the interior of the second housing 50.

[0040] A worm wheel 51 and a worm 52 are housed inside the second housing 50. The output shaft 24B passes through the second housing 50 in the axial direction. The output shaft 24B is rotatably supported on the inner peripheral surface of the first opening 50A of the second housing 50 via a third bearing 53. The third bearing 53 is mounted on the outer peripheral surface of a bearing mounting portion 24B1 of the output shaft 24B. The third bearing 53 is, for example, a ball bearing. The inner ring of the third bearing 53 is fitted with the outer peripheral surface of the bearing mounting portion 24B1 by a clearance fit. That is, the inner diameter of the inner ring of the third bearing 53 is slightly larger than the outer diameter of the bearing mounting portion 24B1. The outer ring of the third bearing 53 is fitted with the inner peripheral surface of the first opening 50A by a clearance fit. That is, the outer diameter of the outer ring of the third bearing 53 is slightly smaller than the inner diameter of the first opening 50A.

[0041] The worm wheel 51 is cylindrical and is mounted on the outer circumferential surface of the worm wheel mounting portion 24B2 of the output shaft 24B. The worm wheel 51 can rotate integrally with the output shaft 24B. A first end face of the worm wheel 51 and the third bearing 53 are spaced apart from each other in the axial direction. A second end face of the worm wheel 51 and the connecting portion 40A of the first housing 40 are spaced apart from each other in the axial direction. The second end face is the end face of the worm wheel 51 opposite to the first end face. The second end face of the worm wheel 51 has a recess 51A. The recess 51A is, for example, frustum-shaped. The recess 51A is a region including the center of the second end face as viewed from the axial direction, and is provided in a region from the center of the second end face to near the tooth bottom. The recess 51A opens on the side of the worm wheel mounting portion 24B2 opposite the bearing mounting portion 24B1. A portion of the connecting portion 40A of the first housing 40 is inserted into the recess 51A.

[0042] The worm 52 is connected to the output shaft 23A of the motor 23 so as to be rotatable together with the output shaft 23A. The axis of the output shaft 23A and the axis of the worm 52 coincide with each other. The worm 52 meshes with the worm wheel 51. The axial angle between the worm 52 and the worm wheel 51 is, for example, 90°.

[0043] 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.

[0044] <Suppression of tilt of output shaft 24B> As described above, the output shaft 24B and the ball screw shaft 41 are axially coupled to each other via serration teeth and serration grooves. In this case, due to the structure, there is slight play between the output shaft 24B and the ball screw shaft 41 in the rotational direction or radial direction thereof. Furthermore, the fit between the outer ring of the third bearing 53 and the first opening 50A is a clearance fit. Therefore, there is a concern that the output shaft 24B may tilt around the coupling portion with the ball screw shaft 41 as a fulcrum depending on the gap between the outer peripheral surface of the outer ring of the third bearing 53 and the inner peripheral surface of the first opening 50A.

[0045] Therefore, in this embodiment, the following configuration is adopted to suppress the tilt of the output shaft 24B. 2, a line passing through the center of a cross section obtained by cutting the first bearing 46 in the axial direction and parallel to the central axis O of the output shaft 24B is a first center line O1. A line passing through the center of a cross section obtained by cutting the second bearing 47 in the axial direction and parallel to the central axis O of the output shaft 24B is a second center line O2. A line passing through the center of a cross section obtained by cutting the third bearing 53 in the axial direction and parallel to the central axis O of the output shaft 24B is a third center line O3. The outer diameter φ1 of the bearing mounting portion 24B1 of the output shaft 24B is set so that the third center line O3 is located outside the first center line O1 in the radial direction relative to the central axis O of the output shaft 24B. In other words, the outer diameter φ1 of the bearing mounting portion 24B1 of the output shaft 24B is set so that the radial distance from the central axis O of the output shaft 24B to the third center line O3 is greater than the radial distance from the central axis O of the output shaft 24B to the first center line O1. In the following description, the terms "radially outer" and "radially inner" mean the outer and inner sides, respectively, in the radial direction relative to the central axis O of the output shaft 24B.

[0046] This configuration can suppress tilt of the output shaft 24B more effectively than when, for example, the outer diameter φ1 of the bearing mounting portion 24B1 of the output shaft 24B is set so that the third center line O3 is positioned radially inward of the first center line O1. This is because the larger the outer diameter φ1 of the bearing mounting portion 24B1, the smaller the amount of tilt of the output shaft 24B.

[0047] Furthermore, the first bearing 46, the second bearing 47, and the third bearing 53 are all ball bearings, but the diameter of the balls in the third bearing 53 is smaller than those in the other bearings 46 and 47. This allows the inner diameter of the third bearing 53 to be larger, and the outer diameter φ1 of the bearing mounting portion 24B1 to be larger.

[0048] Incidentally, the third center line O3 is located radially outward from the second center line O2. The first bearing 46 and the second bearing 47 are identical and are located coaxially. Therefore, the second center line O2 coincides with the first center line O1. Therefore, if the third center line O3 is located radially outward from the first center line O1, then naturally the third center line O3 is located radially outward from the second center line O2.

[0049] Additionally, the outer diameter φ1 of the bearing mounting portion 24B1 is larger than the outer diameter φ2 of the ball screw shaft 41. The outer diameter φ2 is the outer diameter of the portion of the ball screw shaft 41 other than the portions supported by the first bearing 46 and the second bearing 47.

[0050] <Relationship between outer diameter and inclination of output shaft 24B> Consider a first comparative example of an output shaft 24B shown in FIG. 3A and a second comparative example of an output shaft 24B shown in FIG. 3B.

[0051] 3A, in the first comparative example, the outer diameter φ1 of the bearing mounting portion 24B1 is set to a first outer diameter φ11. It is also assumed that a virtual wall WL is present axially opposing a first end face of the third bearing 53. The first end face is the end face of the third bearing 53 opposite the worm wheel mounting portion 24B2, and faces upward of the vehicle 10 when the steering gearbox 22 is mounted on the vehicle 10.

[0052] The first end face of the third bearing 53 and the imaginary wall WL are separated axially by a distance Y1. The center axis O of the output shaft 24B and the outer peripheral surface of the outer ring of the third bearing 53 are separated radially by a distance X1. The output shaft 24B can tilt up to a position where a corner P1 of the outer ring of the third bearing 53 contacts the imaginary wall WL. The corner P1 is a corner formed by the first end face of the third bearing 53 and the outer peripheral surface of the outer ring of the third bearing 53. The amount of tilt of the output shaft 24B is equal to the amount of tilt of the first end face of the third bearing 53. Therefore, the amount of tilt θ1 of the output shaft 24B in the first comparative example is expressed by the following equation (1).

[0053] θ1=tan -1 (Y1 / X1)…(1) As shown in FIG. 3B, in the second comparative example, the outer diameter φ1 of the bearing mounting portion 24B1 is set to a second outer diameter φ12. The second outer diameter φ12 is larger than the first outer diameter φ11. Similar to the first comparative example, it is assumed that a virtual wall WL is present axially facing the first end face of the third bearing 53. Similar to the first comparative example, the first end face of the third bearing 53 and the virtual wall WL are separated axially by a distance Y1. However, the center axis O of the output shaft 24B and the outer peripheral surface of the outer ring of the third bearing 53 are separated radially by a distance X2. The distance X2 is longer than the distance X1 in the first comparative example. The tilt amount θ2 of the output shaft 24B in the second comparative example is expressed by the following equation (2).

[0054] θ2=tan -1 (Y1 / X2)…(2) As described above, the distance X2 of the second comparative example of the output shaft 24B is longer than the distance X1 of the first comparative example of the output shaft 24B. Therefore, as shown in Figures 4A and 4B, the amount of tilt θ2 of the second comparative example of the output shaft 24B is smaller than the amount of tilt θ1 of the first comparative example of the output shaft 24B.

[0055] Therefore, if the gap distance between the outer peripheral surface of the outer ring of the third bearing 53 and the inner peripheral surface of the first opening 50A is the same, the larger the outer diameter φ1 of the bearing mounting portion 24B1, the smaller the amount of tilt of the output shaft 24B.

[0056] <Effects of the embodiment> This embodiment has the following advantages. (1) The outer diameter of the output shaft 24B is set so that a third center line O3 passing through the center of a cross section obtained by axially cutting the third bearing 53 is located radially outward of a first center line O1 passing through the center of a cross section obtained by axially cutting the first bearing 46. The outer diameter is the outer diameter φ1 of the bearing mounting portion 24B1. In this way, tilt of the output shaft 24B can be suppressed more effectively than when, for example, the outer diameter φ1 of the bearing mounting portion 24B1 is set so that the third center line O3 is located radially inward of the first center line O1. This is because the larger the outer diameter φ1 of the bearing mounting portion 24B1, the smaller the amount of tilt of the output shaft 24B. The first center line O1 is the reference for setting the outer diameter φ1 of the bearing mounting portion 24B1.

[0057] (2) When the first center line O1 is located radially outward from the second center line O2, the outer diameter φ1 of the bearing mounting portion 24B1 is larger than the outer diameter φ1 of the bearing mounting portion 24B1 when the first center line O1 is located radially inward from the second center line O2. The larger the outer diameter φ1 of the bearing mounting portion 24B1, the larger the inner diameter of the bearing mounting portion 24B1 is, and the larger the number of balls in the third bearing 53 is. The larger the number of balls, the smaller the load on each ball. This ensures the long service life of the third bearing 53.

[0058] (3) The output shaft 24B is rotatably supported on the inner circumferential surface of the second housing 50 via a single third bearing 53. That is, the output shaft 24B only needs to be provided with a single bearing mounting portion 24B1. Therefore, the axial length of the output shaft 24B can be shortened compared to when the output shaft 24B is supported by two bearings spaced apart from each other in the axial direction. This makes it possible to shorten the axial length of the reducer 25, and therefore the total axial length of the reducer 25 and the steering gearbox 22.

[0059] Incidentally, when the output shaft 24B is supported by two bearings, it is necessary to provide two bearing mounting portions 24B1 that are spaced apart in the axial direction on the output shaft 24B. This increases the axial length of the output shaft 24B. This may increase the axial length of the reducer 25, and thus the total axial length of the reducer 25 and the steering gearbox 22.

[0060] (4) The bearing mounting portion 24B1 is located on the opposite side of the ball screw shaft 41 with respect to the worm wheel mounting portion 24B2. The ball screw shaft 41 has a connecting hole 41B extending in the axial direction at a first end. The end of the output shaft 24B located on the opposite side of the bearing mounting portion 24B1 with respect to the worm wheel mounting portion 24B2 is axially fitted into the connecting hole 41B and is indirectly supported by the first bearing 46 via the peripheral wall of the connecting hole 41B. Therefore, the output shaft 24B can be stably supported by the third bearing 53 and the first bearing 46.

[0061] (5) The first housing 40 and the second housing 50 are fitted together in the axial direction so as to have overlapping portions in a direction perpendicular to the axial direction. Therefore, the total axial length of the reducer 25 and the steering gearbox 22 can be shortened compared to when the first housing 40 and the second housing 50 are connected together, for example, by butting them together in the axial direction.

[0062] (6) The second end surface of the worm wheel 51 has a recess 51A. A portion of the first housing 40 is axially inserted into the recess 51A. The portion of the first housing 40 is part of the connecting portion 40A. Therefore, compared to when the second end surface of the worm wheel 51 is a flat surface without the recess 51A, for example, it is possible to shorten the total axial length of the reducer 25 and the steering gearbox 22.

[0063] (7) The configurations (5) and (6) above enable the axial distance between the first bearing 46 and the third bearing 53 to be further shortened. In addition, the first bearing 46 can be positioned corresponding to the axial position of the second end of the output shaft 24B.

[0064] (8) The total axial length of the reducer 25 and the steering gear box 22 is shorter, which makes it easier to mount the reducer 25 and the steering gear box 22 on a vehicle. <Other embodiments> This embodiment may be modified as follows.

[0065] The fitting relationship between the first housing 40 and the second housing 50 may be reversed. That is, the outer peripheral surface of the second housing 50 may be fitted axially into the inner peripheral surface of the first housing 40. Even in this case, the first housing 40 and the second housing 50 are fitted axially together so as to have overlapping portions in a direction perpendicular to the axis.

[0066] The first center line O1 does not have to be located radially outward from the third center line O3. If the first bearing 46 and the second bearing 47 are not identical, the third center line O3 and the second center line O2 may not coincide with each other. For example, the third center line O3 may be located radially outward or radially inward from the second center line O2. Even in this case, the outer diameter φ1 of the bearing mounting portion 24B1 may be set so that the first center line O1 is located radially outward from the second center line O2.

[0067] When the third center line O3 is located radially outward of the second center line O2, the outer diameter φ1 of the bearing mounting portion 24B1 may be set so that the first center line O1 is located radially outward of the third center line O3. By setting the outer diameter φ of the bearing mounting portion 24B1 to a larger value, tilt of the output shaft 24B can be further suppressed. The third center line O3 can be used as a reference when setting the outer diameter φ1 of the bearing mounting portion 24B1.

[0068] The first housing and the second housing do not have to be fitted together in the axial direction so as to have overlapping portions in a direction perpendicular to the axial direction. For example, the first housing 40 and the second housing 50 may be connected so as to butt against each other in the axial direction.

[0069] A portion of the first housing 40 does not have to be axially inserted into the recess 51A of the worm wheel 51. In this case, the worm wheel 51 may be configured without the recess 51A. The second end surface of the worm wheel 51 is provided, for example, as a flat surface.

[0070] The axial position of the first bearing 46 and the axial position of the second end of the output shaft 42B do not have to correspond to each other. In this case, the output shaft 24B may be supported by two bearings that are axially separated from each other. Also, the first bearing 46 may support the ball screw shaft 41 at a position axially displaced from the second end of the output shaft 42B.

[0071] The outer diameter of the bearing mounting portion 24B1 and the outer diameter of the worm wheel mounting portion 24B2 may be the same. The outer diameter of the output shaft 24B may also be the same over its entire length. Even in this case, the outer diameter of the output shaft 24B is set so that the first center line O1 is positioned radially outward of the second center line O2. This can prevent the output shaft 24B from tilting.

[0072] The position of the torque sensor 24 may be changed as appropriate. For example, the torque sensor 24 may be provided midway along the steering shaft 21. In this case, the reducer 25 may have a shaft similar to the output shaft 24B. The steering shaft 21 is connected to the ball screw shaft 41 via a shaft. The shaft rotates in conjunction with the steering of the steering wheel 26.

Claims

1. a shaft that rotates in conjunction with steering of a steering wheel of a vehicle; a ball screw mechanism configured to convert rotation of the shaft into rotation of an output shaft that is interlocked with the steered wheels; a reducer configured to impart a torque to the shaft; The ball screw mechanism a first housing; a ball screw shaft accommodated inside the first housing, the ball screw shaft having a first end and a second end, and coupled to the shaft so as to be rotatable integrally with the shaft by meshing connection between the first end and the shaft; a first bearing that rotatably supports the first end of the ball screw shaft with respect to the first housing; a second bearing that rotatably supports the second end of the ball screw shaft relative to the first housing, the reducer includes a second housing axially connected to the first housing and into which the shaft is inserted in the axial direction, and a third bearing that rotatably supports the shaft relative to the second housing, a first center line is a line that passes through the center of a cross section obtained by cutting the first bearing in the axial direction and is parallel to a central axis of the shaft; a second center line is a line that passes through the center of a cross section obtained by cutting the second bearing in the axial direction and is parallel to the central axis of the shaft; a third center line is a line that passes through the center of a cross section obtained by cutting the third bearing in the axial direction and is parallel to the central axis of the shaft; A steering device wherein an outer diameter of the shaft is set so that the third center line is located outside the first center line or the second center line in a radial direction relative to the central axis of the shaft.

2. the reducer includes, inside the second housing, a worm wheel attached to an outer circumferential surface of the shaft, and a worm meshing with the worm wheel; the shaft has a bearing mounting portion on which the third bearing is mounted, and a worm wheel mounting portion that is adjacent to the bearing mounting portion in the axial direction and on which the worm wheel is mounted, 2. The steering device according to claim 1, wherein an outer diameter of the bearing mounting portion is set so that the third center line is located outside the first center line or the second center line in a radial direction relative to the central axis of the shaft.

3. the bearing mounting portion is located on the opposite side of the worm wheel mounting portion from the ball screw shaft, the ball screw shaft has a connecting hole extending in the axial direction at the first end, 3. The steering device according to claim 2, wherein an end of the shaft located on the opposite side of the bearing mounting portion with respect to the worm wheel mounting portion is indirectly supported by the first bearing via a peripheral wall of the connecting hole while being axially fitted into the connecting hole.

4. the worm wheel has a recess that opens to the opposite side of the worm wheel mounting portion from the bearing mounting portion, 4. The steering apparatus according to claim 3, wherein a portion of the first housing is axially inserted into the recess.

5. A steering device according to any one of claims 1 to 4, wherein the first housing and the second housing are fitted together in the axial direction so as to have overlapping portions in a direction perpendicular to the axial direction.

6. a torque sensor configured to detect a steering torque applied to the steering wheel; the torque sensor includes an input shaft to which the steering torque is transmitted, an output shaft provided rotatable relative to the input shaft, and a torsion bar connecting the input shaft and the output shaft; The steering device according to any one of claims 1 to 4, wherein the shaft is the output shaft.

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