Steering device

Abstract

This steering device comprises: a steering shaft (1); an electric motor (4) that, out of both end parts (1a, 1b) of the steering shaft (1), is disposed on the end part (1a) side on which a steering wheel is provided; a speed reducer (5) that reduces the rotational force of the electric motor (4); a ball screw mechanism (2) provided on the steering shaft (1); a sector gear (3) having a tooth part (3a) that meshes with rack teeth (20c) provided on the outer peripheral surface of a nut (20); and a four-point contact ball bearing (21) that, out of both ends parts (1a, 1b) of the steering shaft (1), is disposed on a fifth shaft part (12e) on the end part (1b) side on the opposite side from the side on which the electric motor (4) is provided. The four-point contact ball bearing (21) receives thrust force and radial force acting on the steering shaft (1).

Description

Steering device

[0001] The present invention relates to a steering device.

[0002] As a steering device, for example, the steering device described in Patent Document 1 below is known.

[0003] In the steering device of Patent Document 1, it is configured as an integral type steering device used for large vehicles and the like. In this steering device, an electric motor that applies a steering assist force to the steering shaft and a speed reducer that decelerates the rotational force of the electric motor are arranged on the ground side.

[0004] When an electric motor or a speed reducer is arranged on the ground side as in the steering device of Patent Document 1, there is a risk that the electric motor and the speed reducer interfere with vehicle peripheral devices, and there is a problem of poor mountability.

[0005] Therefore, in order to solve this problem, it is conceivable to arrange the electric motor and the speed reducer on the driver's seat side. However, in this case, the axial dimension on the driver's seat side becomes long, and for example, there is a risk that a part of the housing interferes with the floor panel.

[0006] The present invention has been devised in view of the above technical problems, and one of its purposes is to provide a steering device capable of shortening the dimensions of the steering device along the axial direction of the steering shaft while mounting an electric motor and a speed reducer on the driver's seat side.

[0007] Japanese Patent Application Laid-Open No. 2019-156082

[0008] The steering device of the present invention comprises a steering shaft to which rotation from a steering wheel is input; an electric motor positioned at the end of the steering shaft on which the steering wheel is provided and which applies a steering assist force to the steering shaft; a reduction gear for reducing the rotational force of the electric motor; a ball screw mechanism provided on the steering shaft, having a first ball screw groove which is a helical groove formed on the outer circumferential surface of the steering shaft; a second ball screw groove which is a helical groove formed on the inner circumferential surface of a nut provided around the steering shaft; and a plurality of balls positioned between the first ball screw groove and the second ball screw groove; a sector gear having a second tooth portion that meshes with a first tooth portion provided on the outer circumferential surface of the nut; and a four-point contact bearing positioned at the end of the steering shaft opposite to the side on which the electric motor is provided and which rotatably supports the steering shaft.

[0009] According to the present invention, it is possible to mount an electric motor and a reduction gear on the driver's side while shortening the dimensions of the steering device along the axial direction of the steering shaft.

[0010] This is a longitudinal cross-sectional view of the steering device of the first embodiment. This is an explanatory diagram showing various steps in the assembly method of the steering device of the first embodiment, where (a) shows the press-fitting step of the worm wheel, (b) shows the arrangement step of the connecting member and the arrangement step of the second housing, and (c) shows the press-fitting step of the detection part of the torque sensor and the crimping step of the sensor magnet part. This is an explanatory diagram showing various steps in the assembly method of the steering device of the first embodiment, where (a) shows the assembly step of the first housing and the assembly step of the nut, (b) shows the assembly step of the assembly including the steering shaft, and (c) shows the assembly step of the sector gear, fixing member, closing member and electric motor. This is a longitudinal cross-sectional view of the steering device of the second embodiment.

[0011] The embodiments of the steering device of the present invention will be described below with reference to the drawings. In the embodiments described below, the steering device is shown as an integral type steering device used in large vehicles and the like.

[0012] [First Embodiment] Figure 1 is a longitudinal cross-sectional view of the steering device according to the first embodiment. In Figure 1, for the sake of explanation, the side of the steering shaft 1 that is linked to the steering wheel (upper side in the figure) in the direction of the rotation axis Z is referred to as the "first end 1a," and the side to which the four-point contact ball bearing 21 is attached (lower side in the figure) is referred to as the "second end 1b." Note that in Figure 1, for the sake of explanation, the internal structure of the detection unit 16 and the signal processing unit 23 of the torque sensor 14 is omitted from the illustration. Furthermore, the shape of the housing that houses the detection unit 16 and the signal processing unit 23 will be mainly described.

[0013] This steering device mainly consists of a steering shaft 1 linked to a steering wheel (not shown), a sector gear 3 connected to the steering shaft 1 via a ball screw mechanism 2 and used for steering the steering wheels, an electric motor 4 that provides steering assist force to the steering shaft 1, and a reduction gear 5 that reduces the rotation of the electric motor 4.

[0014] The steering shaft 1 extends into a cylindrical third housing (nut-side housing) 9 provided on the second end 1b side of the connecting member 8, passing through a cylindrical first housing 6, a cylindrical second housing (reducer-side housing) 7 to which the first housing 6 is attached, and a cylindrical connecting member 8 provided on the end face 7a on the second end 1b side of the second housing 7. The steering shaft 1 includes an input shaft 10 to which rotational force from a steering wheel (not shown) is transmitted, and an output shaft 12 connected to the input shaft 10 via a torsion bar 11.

[0015] The input shaft 10 has a one-end shaft portion 10a located on the first end 1a side, an intermediate shaft portion 10b formed integrally with the one-end shaft portion 10a and having a larger diameter than the one-end shaft portion 10a, and a other-end shaft portion 10c formed integrally with the intermediate shaft portion 10b and having a smaller diameter than the intermediate shaft portion 10b. An annular dust seal 13 is provided on the outer circumferential surface of the intermediate shaft portion 10b at a position closer to the one-end shaft portion 10a to prevent dust and dirt from entering the inside of the first housing 6 or the like from the outside. Furthermore, a cylindrical magnet portion 15, whose magnetic change is detected by a torque sensor 14 described later, is crimped and fixed to the outer circumferential surface of the intermediate shaft portion 10b at a position adjacent to the other-end shaft portion 10c in the axial direction.

[0016] The output shaft 12 has a cylindrical first shaft portion 12a located on the first end portion 1a side, a cylindrical second shaft portion 12b formed integrally with the first shaft portion 12a and having a larger diameter than the first shaft portion 12a, a cylindrical third shaft portion 12c formed integrally with the second shaft portion 12b and having a larger diameter than the second shaft portion 12b, a cylindrical fourth shaft portion 12d formed integrally with the third shaft portion 12c and having a smaller diameter than the third shaft portion 12c, a fifth shaft portion 12e formed integrally with the fourth shaft portion 12d and having a smaller diameter than the fourth shaft portion 12d, and a sixth shaft portion 12f formed integrally with the fifth shaft portion 12e and having a smaller diameter than the fifth shaft portion 12e.

[0017] The reduced-diameter portion 16b of the detection unit 16 of the torque sensor 14 (described later) is press-fitted and fixed to the outer circumferential surface of the first shaft portion 12a. The core metal portion 18 of the worm wheel 17 of the reduction gear 5 (described later) is press-fitted and fixed to the outer circumferential surface of the second shaft portion 12b. A deep groove ball bearing 19 is provided on the outer circumferential surface of the third shaft portion 12c to rotatably support the third shaft portion 12c. A nut 20, which constitutes part of the ball screw mechanism 2, is provided around the fourth shaft portion 12d. A four-point contact ball bearing 21 is provided on the outer circumferential surface of the fifth shaft portion 12e to rotatably support the fifth shaft portion 12e. A fixing member 22 is provided around the sixth shaft portion 12f to fix the four-point contact ball bearing 21 to the fifth shaft portion 12e.

[0018] The first housing 6 is formed in a generally cylindrical shape from a metal material and mainly houses the torque sensor 14 which is provided around the input shaft 10.

[0019] The second housing 7 is formed in a generally cylindrical shape from a metal material and mainly houses the reduction gear 5, which is arranged around the second shaft portion 12b of the output shaft 12.

[0020] The connecting member 8 is formed in a generally cylindrical shape from a metal material. The connecting member 8 connects the end face 7a on the second end 1b side of the second housing 7 and the end face 9a on the first end 1a side of the third housing 9, and also holds a shielded or rubber-sealed deep groove ball bearing 19 between itself and the outer circumferential surface of the third shaft portion 12c of the output shaft 12.

[0021] The third housing 9 is formed in a generally cylindrical shape from a metal material. The third housing 9 has a nut housing portion 9b that mainly houses the nut 20 provided around the output shaft 12, and a sector gear housing portion 9c that communicates with the nut housing portion 9b and houses the sector gear 3.

[0022] The torque sensor 14 is configured as a well-known magnetic torque sensor using a magnet portion 15 that is crimped and fixed to the outer circumferential surface of the input shaft 10. The torque sensor 14 has a detection unit 16 that detects a signal corresponding to the change in magnetism of the magnet portion 15 that rotates together with the input shaft 10, and a signal processing unit 23 that is arranged around the detection unit 16 and processes the signal detected by the detection unit 16 to calculate the steering torque. The signal processing unit 23 may also be configured to calculate the steering angle instead of the steering torque, or in addition to the steering torque.

[0023] The detection unit 16 (the housing of the detection unit) is formed in an annular shape from a synthetic resin material and is arranged around the magnet unit 15. On the axial end face of the first end 1a of the detection unit 16, adjacent to the inner circumferential surface of the detection unit 16, an annular projection 16a is formed that protrudes toward the first end 1a in the direction of the rotation axis Z of the steering shaft 1. Also, on the axial end face of the second end 1b of the detection unit 16, adjacent to the inner circumferential surface of the detection unit 16, an annular reduced-diameter portion 16b is formed that protrudes toward the second end 1b in a stepped-reduced-diameter shape. This reduced-diameter portion 16b is press-fitted and fixed to the outer circumferential surface of the first shaft portion 12a of the output shaft 12 and rotates together with the output shaft 12.

[0024] The signal processing unit 23 (the housing of the signal processing unit) is formed in an annular shape from a synthetic resin material and is arranged around the detection unit 16. Two cylindrical rotation restricting portions 23a are formed on the axial end face of the signal processing unit 23 on the second end 1b side, adjacent to the outer circumferential surface of the signal processing unit 23, to restrict the rotation of the signal detection unit 16 from being dragged along by the rotation of the detection unit 16. As shown in Figure 1, the two rotation restricting portions 23a are arranged radially symmetrically with respect to the output shaft 12. Each rotation restricting portion 23a has a columnar expansion portion 23b that protrudes with a constant diameter towards the second end 1b side along the direction of the rotation axis Z of the steering shaft 1 and then tapers into a cone shape, and a cylindrical rotation restricting cylindrical portion 23c that protrudes from the tip of the expansion portion 23b towards the second end 1b side along the direction of the rotation axis Z of the steering shaft 1. The rotation-restricting cylindrical portion 23c is formed to be elastically deformable and has an engaging portion (not shown) on its tip side (located on the far side in Figure 1). This engaging portion elastically engages with a cylindrical portion 7d which is integrally formed with a protruding portion 7c that extends radially inward from the inner circumference on the first end 1a side of the peripheral wall portion 7b of the second housing 7. More specifically, it elastically engages with the edge portion 7g on the second end 1b side of the circular hole portion 7f of the cylindrical portion 7d. The signal detection unit 16 is held in the second housing 7 by this elastic engagement.

[0025] The reduction gear 5 comprises a worm shaft 24 and a worm wheel 17 that meshes with the worm shaft 24. The worm shaft 24 is connected to an output shaft (not shown) of the electric motor 4. The worm wheel 17 is formed by insert molding a cylindrical metal core portion 18 into a gear forming portion 25 made of synthetic resin. This core portion 18 is press-fitted onto the outer circumferential surface of the second shaft portion 12b of the output shaft 12, so that the core portion 18 abuts against the axial end face on the first end 1a side of the third shaft portion 12c. An annular recess 26 is provided between the gear forming portion 25 and the core portion 18, which is continuous in the circumferential direction of the steering shaft 1. As shown in Figure 1, the axial end face 25a on the first end 1a side of the gear forming portion 25 is located closer to the first end 1a than the axial end face 18a on the first end 1a side of the core portion 18.

[0026] The axial end of the cylindrical portion 7d of the second housing 7 on the second end 2a side extends into the annular recess 26 along the direction of the rotation axis Z of the steering shaft 1. Therefore, the axial end face 7h on the second end 1b side of the cylindrical portion 7d is located on the second end 1b side than the axial end face 18a on the first end 1a side of the core metal portion 18. As described above, since the engaging portion of the rotation-restricting cylindrical portion 23c elastically engages with the edge 7g of the circular hole portion 7f of the cylindrical portion 7d, the portion of the rotation-restricting cylindrical portion 23c on the second end 1b side also extends into the annular recess 26. In other words, the portion of the rotation-restricting cylindrical portion 23c on the second end 1b side is inserted into the annular recess 26 to about half the depth of the annular recess 26 with respect to the axial end face 18a of the core metal portion 18. In other words, the portion of the rotation-restricting cylindrical part 23c on the second end 1b side overlaps with the internal space of the annular recess 26 in the radial direction of the steering shaft 1 by an axial length of about half the depth of the annular recess 26.

[0027] Furthermore, an annular support portion 7i is formed on the protruding portion 7c of the second housing 7 at a position radially outward from the cylindrical portion 7d, projecting toward the first end portion 1a. As shown in Figure 1, when the first housing 6 is assembled to the second housing 7, the tip of the annular support portion 7i abuts against the annular portion 6a that protrudes radially outward from the outer circumferential surface of the first housing 6.

[0028] Furthermore, a flange portion 7j is formed to protrude radially outward from the axial end of the peripheral wall portion 7b of the second housing 7, on the side of the second end portion 1b. A first hole portion 7k is formed through this flange portion 7j, along the direction of the rotation axis Z of the steering shaft 1, into which a fixing member, such as a bolt 27, is inserted.

[0029] The electric motor 4 is driven and controlled based on the steering torque detected by the torque sensor 14. The electric motor 4 is located on the first end 1a side of the steering shaft 1, where the steering wheel is provided. More specifically, the electric motor 4 is located on the opposite side of the steering shaft 1 from the sector gear 3, and is adjacent to the first housing 6, second housing 7, and connecting member 8, which are relatively close to the steering wheel provided in the driver's seat (not shown).

[0030] The deep groove ball bearing 19 is a bearing positioned between the third shaft portion 12c of the output shaft 12 and the connecting member 8, and receives radial forces, which are radial forces acting on the output shaft 12. The deep groove ball bearing 19 is positioned on the opposite side of the four-point contact ball bearing 21, with the ball screw mechanism 2 in between, in the axial direction of the steering shaft 1.

[0031] Furthermore, an annular groove is formed at the axial center of the outer circumferential surface of the third shaft portion 12c, into which an O-ring 28, an annular sealing member made of, for example, rubber, is fitted. The O-ring 28 provides a liquid-tight seal between the outer circumferential surface of the third shaft portion 12c and the inner circumferential surface of the deep groove ball bearing 19, thereby accumulating lubricant near the reducer 5 inside the third housing 9 and improving the lubrication of the meshing portion between the worm shaft 24 and the worm wheel 17.

[0032] The connecting member 8 has a cylindrical body portion 8a, an annular projection 8b projecting radially outward from the axial center position of the outer circumferential surface of the cylindrical body portion 8a, an annular projection 8c projecting radially inward from the position on the inner circumferential surface of the cylindrical body portion 8a that is closest to the first end 1a, a cylindrical extension portion 8d extending from the axial end of the annular projection portion 8c on the first end 1a side toward the first end 1a side along the direction of the rotation axis Z of the steering shaft 1 toward the first end 1a side, and an annular ceiling wall portion 8e extending radially inward from the position on the inner circumferential surface of the extension portion 8d that is closest to the first end 1a side.

[0033] Of the annular projection 8b, the portion 8f facing the third housing 9 in the direction of the rotation axis Z of the sector gear 3 and the steering shaft 1 is radially expanded outward compared to the remaining portion. A second hole 8g into which a bolt 27 is inserted is formed through this portion 8f along the direction of the rotation axis Z of the steering shaft 1. The second housing 7 and the connecting member 8 are fastened together to the third housing 9 by screwing the bolt 27 into the second hole 8g of portion 8f, the first hole 7k of the flange portion 7j of the second housing 7, and the screw hole 9d provided in portion 8f and the sector gear housing portion 9c of the third housing 9.

[0034] The inner circumferential surfaces of the annular projection 8c and the expanded portion 8d hold the deep groove ball bearing 19 between themselves and the outer circumferential surface of the third shaft portion 12c.

[0035] The ceiling wall portion 8e has a radial length such that it covers the deep groove ball bearing 19 from the first end portion 1a side.

[0036] The ball screw mechanism 2 is composed of a steering shaft side ball screw groove 12g, which is a helical groove provided on the outer circumferential surface of the fourth shaft portion 12d of the output shaft 12; a nut side ball screw groove 20a, which is a helical groove provided on the inner circumferential surface of the nut 20; and a plurality of balls 29 arranged between the ball screw grooves 12g and 20a. The balls 29 support the nut 20 so that it can rotate relative to the fourth shaft portion 12d. As shown in Figure 1, the back surface 20b of the nut 20, which is on the side opposite to the sector gear 3, is spaced apart from the inner circumferential surface of the nut housing portion 9b of the third housing 9 that houses the nut 20.

[0037] Furthermore, multiple rack teeth 20c are formed on the outer circumference of the nut 20 on the sector gear housing portion 9c side. These rack teeth (tooth portions) 20c mesh with the teeth 3a of the sector gear 3. As shown in Figure 1, the surface 20e of the outer circumference of the nut 20 that is on the second end portion 1b side of the rack teeth 20c is spaced apart from the inner surface of the third housing 9.

[0038] The four-point contact ball bearing 21 is located at the second end 1b of the steering shaft 1, on the side opposite to the side where the electric motor 4 is installed (the ground side). More specifically, the four-point contact ball bearing 21 is located around the fifth shaft portion 12e of the output shaft 12. The four-point contact ball bearing 21 has larger axial and radial dimensions than the deep groove ball bearing 19. The four-point contact ball bearing 21 receives both thrust force, which acts in the axial direction of the output shaft 12, i.e., in the direction of the rotation axis Z, and radial force, which acts in the radial direction of the output shaft 12. More specifically, let's consider the force acting on one of the four contact points P of the four-point contact ball bearing 21, for example, the force F acting on the second end 1b side of the outer race 30 of the four-point contact ball bearing 21 as shown in Figure 1. This force F can be decomposed into a thrust force F1, which is a force along the direction of the rotation axis Z of the steering shaft 1, and a radial force F2, which is a force directed radially outward from the steering shaft 1. Therefore, the four-point contact ball bearing 21 receives a thrust force F1 acting through the contact point P, and a radial force F2, which also acts through the contact point P and is smaller than the thrust force F1. In one example, the thrust force F1 is about 1.5 to 2 times larger than the radial force F2. Therefore, the four-point contact ball bearing 21 is configured such that its contribution to receiving the thrust force F1 is much larger than its contribution to receiving the radial force F2. As in this embodiment, in a large vehicle having a sector gear 3, the thrust force acting on the steering shaft 1 is very large compared to a vehicle of a typical size. Therefore, it is advantageous to use a four-point contact ball bearing 21 to simultaneously withstand such a large thrust force and radial force.

[0039] Furthermore, the four-point contact ball bearing 21 includes an inner race 31 positioned on the outer circumference of the fifth shaft portion 12e, an outer race 30 positioned on the outer circumference of the inner race 31, and a plurality of balls 32 positioned between the inner race 31 and the outer race 30. The four-point contact ball bearing 21 is press-fitted into the inner circumferential surface of the third housing 9. Of the inner circumferential corners 31a and 31b of the inner race 31, the corner 31a on the first end 1a side abuts against a stepped portion 12h formed between the fourth shaft portion 12d and the fifth shaft portion 12e of the output shaft 12. On the other hand, of the outer circumferential corners 30a and 30b of the outer race 30, the corner 30a on the first end 1a side abuts against a stepped portion 9e provided on the inner circumferential surface of the third housing 9.

[0040] The fixing member 22 is configured as a nut member and has a fixing member side female thread portion 22a formed on its inner circumferential surface. This fixing member side female thread portion 22a screws into the steering shaft side male thread portion 12i formed on the outer circumferential surface of the sixth shaft portion 12f of the output shaft 12. This screwing presses the corner portion 31b of the inner race 31, and by pressing the corner portion 31a of the inner race 31 against the stepped portion 12h provided on the output shaft 12, the inner race 31 is fixed to the output shaft 12.

[0041] A disc-shaped closing member 33 is provided at a position on the second end 1b side of the four-point contact ball bearing 21 in the direction of the rotation axis Z of the steering shaft 1. This closing member 33 closes the opening of the third housing 9 from the four-point contact ball bearing 21 side and fixes the outer race 30 of the four-point contact ball bearing 21 to the third housing 9. The closing member 33 is made of metal or synthetic resin. The closing member 33 has a circular plate-shaped bottom wall portion 33a, a cylindrical inner peripheral wall portion 33b rising from the outer edge of the bottom wall portion 33a toward the first end 1a side, an annular projection portion 33c extending radially outward from the end of the inner peripheral wall portion 33b toward the first end 1a side, and a cylindrical outer peripheral wall portion 33d extending from the outer edge of the annular projection portion 33c toward the second end 1b side.

[0042] The space surrounded by the bottom wall portion 33a and the inner peripheral wall portion 33b serves as a fixed member accommodating portion 34 for accommodating the fixed member 22. Also, the length of the outer peripheral wall portion 33d along the direction of the rotation axis Z of the steering shaft 1 is greater than the length of the inner peripheral wall portion 33b along the direction of the rotation axis Z of the steering shaft 1. The axial end face 33e on the first end portion 1a side of the outer peripheral wall portion 33d is located closer to the first end portion 1a side than the axial end face 33f on the first end portion 1a side of the annular projecting portion 33c. For this reason, the axial end face 33e is in contact with the axial end face 30c on the second end portion 1b side of the outer race 30 of the four-point contact ball bearing 21, while the axial end face 33f is separated from the axial end face 31c on the second end portion 1b side of the inner race 31 of the four-point contact ball bearing 21.

[0043] Also, an annular seal groove into which an O-ring 35, which is a seal member, is fitted is formed at a position on the outer peripheral surface of the outer peripheral wall portion 33d closer to the axial end face 33e. The O-ring 35 seals the space between the inner peripheral surface of the third housing 9 and the outer peripheral surface of the outer peripheral wall portion 33d in an airtight manner.

[0044] Also, a male screw portion on the closure member side 33g is formed at a position on the outer peripheral surface of the outer peripheral wall portion 33d on the second end portion 1b side of the O-ring 35, and this male screw portion on the closure member side 33g is screwed into a female screw portion 9f formed on the inner peripheral surface of the end portion on the second end portion 1b side of the third housing 9. By this screwing, the axial end face 33e of the outer race 30 is pressed, and by pressing the corner portion 30a of the outer race 30 against the stepped portion 9e of the third housing 9, the outer race 30 is fixed to the third housing 9.

[0045] The sector gear 3 is provided in a sector gear accommodating portion 9c provided in the third housing 9 so as to be swingable. One axial end side of the sector gear 3 is connected to the nut 20 via the tooth portion 3a, and the other end side is linked to a steering wheel (not shown) via a pitman arm (not shown).

[0046] In such a steering apparatus, when a driver rotates the steering wheel, the input shaft 10 rotates and the torsion bar 11 is twisted. Due to the elastic force of the torsion bar 11 generated thereby, the output shaft 12 rotates. As the output shaft 12 rotates, the nut 20 moves in the direction of the rotation axis Z of the steering shaft 1, causing the sector gear 3 to rotate. As a result, the pitman arm is pulled in the vehicle body width direction, changing the direction of the steered wheels.

[0047] FIG. 2 is an explanatory diagram showing various steps of the assembly method of the steering apparatus according to the first embodiment. FIG. 2(a) shows the press-fitting step of the worm wheel 17, FIG. 2(b) shows the arrangement step of the connecting member 8 and the arrangement step of the second housing 7, and FIG. 2(c) shows the press-fitting step of the detection part 16 of the torque sensor 14 and the caulking step of the magnet part 15. FIG. 3 is an explanatory diagram showing various steps of the assembly method of the steering apparatus according to the first embodiment. FIG. 3(a) shows the assembly step of the first housing 6 and the assembly step of the nut 20, which are subsequent steps to FIG. 2(c), FIG. 3(b) shows the assembly step of the assembly including the steering shaft 1, and FIG. 3(c) shows the assembly step of the sector gear 3, the fixing member 22, the closing member 33, and the electric motor 4.

[0048] First, in the press-fitting step of the worm wheel 17 in FIG. 2(a), the core metal part 18 provided with the gear forming part 25 is press-fitted onto the outer peripheral surface of the second shaft part 12b of the output shaft 12.

[0049] Next, in the arrangement step of the connecting member 8 in which the outer race of the deep groove ball bearing 19 shown in FIG. 2(b) is press-fitted and fixed, the connecting member 8 is arranged from the second end part 1b side of the steering shaft 1 so as to be adjacent to the worm wheel 17. Then, in the arrangement step of the second housing 7, the second housing 7 is arranged on the annular protrusion part 8b of the connecting member 8 from the first end part 1a side of the steering shaft 1.

[0050] After the placement process of the second housing 7, in the assembly process of the torque sensor 14 shown in Figure 2(c), the reduced diameter portion 16b of the detection unit 16 is press-fitted onto the outer circumferential surface of the first shaft portion 12a of the output shaft 12. After this press-fitting, the signal processing unit 23 is attached to the cylindrical portion 7d of the second housing 7 by engaging the engaging portion of the rotation-restricting cylindrical portion 23c of the signal processing unit 23 with the edge portion 7g of the circular hole portion 7f of the cylindrical portion 7d.

[0051] Then, in the crimping process of the magnet portion 15 shown in Figure 2(c), the magnet portion 15 is crimped to the outer circumferential surface of the intermediate shaft portion 10b of the input shaft 10.

[0052] Next, in the assembly process of the first housing 6 shown in Figure 3(a), the annular portion 6a of the first housing 6 is assembled onto the annular support portion 7i of the second housing 7.

[0053] Furthermore, in the assembly process of the nut 20 shown in Figure 3(a), the nut 20 is assembled around the fourth shaft portion 12d of the output shaft 12 of the steering shaft 1.

[0054] After the assembly process of the nut 20, the four-point contact ball bearing 21 is press-fitted into the inner circumferential surface of the third housing 9.

[0055] Next, in the assembly process of the steering shaft 1 shown in Figure 3(b), the assembly assembled as shown in Figure 3(a) is inserted into the third housing 9 equipped with a four-point contact ball bearing 21. With the assembly inserted, the fifth shaft portion 12e of the output shaft 12 is inserted into the inner race 31 of the four-point contact ball bearing 21. Furthermore, in this state, the end face 9a on the first end 1a side of the third housing 9 is in contact with the annular projection 8b of the connecting member 8.

[0056] Then, in the assembly process of the sector gear 3 shown in Figure 3(c), the sector gear 3 is assembled so that the teeth 3a of the sector gear 3 mesh with the rack teeth 20c of the nut 20.

[0057] Furthermore, in the assembly process of the fixing member 22 shown in Figure 3(c), the fixing member 22 is assembled to the sixth shaft portion 12f by screwing the fixing member side female thread portion 22a of the fixing member 22 to the steering shaft side male thread portion 12i of the sixth shaft portion 12f of the output shaft 12. As a result, the inner race 31 of the four-point contact ball bearing 21 is fixed to the stepped portion 12h provided on the output shaft 12.

[0058] Furthermore, after the assembly process of the fixing member 22, the closing member 33 is assembled to the third housing 9 by screwing the male threaded portion 33g of the closing member side of the outer peripheral wall portion 33d of the closing member 33 to the female threaded portion 9f of the housing side of the third housing 9. As a result, the outer race 30 of the four-point contact ball bearing 21 is fixed to the stepped portion 9e of the third housing 9.

[0059] Furthermore, in the assembly process of the electric motor 4 shown in Figure 3(c), the electric motor 4 is assembled into the second housing 7 such that the worm shaft 24 meshes with the gear forming portion 25 of the worm wheel 17.

[0060] [Effects of the First Embodiment] As described above, in the first embodiment, the electric motor 4 is located on the first end 1a side of the steering shaft 1 where the steering wheel is provided, that is, on the first end 1a side on the driver's side. The four-point contact ball bearing 21 is located around the fifth shaft portion 12e on the second end 1b side of the steering shaft 1, which is opposite to the side where the electric motor 4 is provided. If the electric motor 4 and the reduction gear 5 attached to the electric motor 4 are located on the driver's side, the structure of the steering device on the driver's side becomes complex. If further components are added to the driver's side, the axial dimension of the steering device on the driver's side increases, which may cause interference between, for example, the first housing 6 and a floor panel (not shown). Therefore, in this embodiment, the four-point contact ball bearing 21 is located on the second end 1b side of the steering shaft 1, where there is relatively more space, so that it receives most of the force applied to the steering shaft 1 of a steering device used in a large vehicle. This makes it possible to shorten the axial dimension of the steering device. In particular, the axial dimension of the steering device on the driver's side can be shortened, and therefore interference between the first housing 6 and a floor panel (not shown) can be suppressed.

[0061] Furthermore, in this embodiment, the deep groove ball bearing 19 is positioned on the opposite side of the four-point contact ball bearing 21 from the ball screw mechanism 2 in the axial direction of the steering shaft 1. In other words, the deep groove ball bearing 19 is positioned on the driver's side, adjacent to the electric motor 4 and the reduction gear 5. The four-point contact ball bearing 21, located on the second end 1b side of the steering shaft 1, receives both thrust force and radial force, and receives thrust force, which is particularly large in large vehicles. Therefore, a smaller deep groove ball bearing 19 is provided on the driver's side to receive relatively small radial force. Accordingly, compared to the case where four-point contact ball bearings 21 are provided on both the driver's side and the ground side, the axial space on the driver's side of the steering device can be efficiently secured.

[0062] Furthermore, the four-point contact ball bearing 21 is larger than ball bearings or deep groove ball bearings 19, as it is subjected to both thrust and radial forces. Therefore, the four-point contact ball bearing 21 needs to be firmly fixed, requiring a relatively large fixing mechanism. Consequently, placing the four-point contact ball bearing 21 on the driver's side would reduce the axial space available for the steering system on the driver's side.

[0063] Furthermore, in this embodiment, the connecting member 8 connects the end face 7a on the second end 1b side of the peripheral wall portion 7b of the second housing 7 to the end face 9a on the first end 1a side of the third housing 9, and also holds the deep groove ball bearing 19 between itself and the outer circumferential surface of the third shaft portion 12c of the output shaft 12. For this reason, the steering device can be assembled relatively easily by consolidating the first housing 6, second housing 7, torque sensor 14, worm wheel 17, etc. above the connecting member 8, forming an assembly by attaching a nut 20 to the steering shaft 1, and then simply inserting this assembly into the third housing 9. In addition, since the connecting member 8 has the function of consolidating the first housing 6, etc., and connecting the second housing 7 and the third housing 9, as well as the function of holding the deep groove ball bearing 19, there is no need to provide a separate component for holding the deep groove ball bearing 19.

[0064] Furthermore, if there is no separate connecting member 8, the second housing 7, the connecting member 8, and the third housing 9 are molded together as one elongated housing, making it impossible to press-fit the core metal portion 18 of the reduction gear 5 into the steering shaft 1 inserted into such a housing.

[0065] Furthermore, in this embodiment, the steering device further includes a disc-shaped closing member 33 that fixes the outer race 30 of the four-point contact ball bearing 21 to the third housing 9. The closing member 33 fixes the outer race 30 of the four-point contact ball bearing 21 to the stepped portion 9e provided on the third housing 9 by screwing the housing-side female thread portion 9f of the third housing 9 to the closing member-side male thread portion 33g of the closing member 33. As a result, the four-point contact ball bearing 21, which is larger than the deep groove ball bearing 19 and is subjected to large forces, can be firmly fixed to the third housing 9.

[0066] Furthermore, in this embodiment, the steering device further includes an annular fixing member 22 for fixing the inner race 31 of the four-point contact ball bearing 21 to the steering shaft 1. The fixing member 22 fixes the inner race 31 of the four-point contact ball bearing 21 to the stepped portion 12h provided on the steering shaft 1 by screwing together the steering shaft side male thread portion 12i of the output shaft 12 of the steering shaft 1 and the fixing member side female thread portion 22a. As a result, the four-point contact ball bearing 21, which is subjected to large forces, can be firmly fixed to the steering shaft 1.

[0067] Furthermore, in this embodiment, the torque sensor 14 includes a detection unit 16 that detects changes in the magnetism of a magnet unit 15 fixed to the steering shaft 1, and a signal processing unit 23 that processes signals corresponding to the changes in the magnetism of the magnet unit 15. The signal processing unit 23 has a rotation restricting unit 23a that extends in the axial direction of the steering shaft 1, and the rotation restricting unit 23a restricts the rotation of the signal processing unit 23 in conjunction with the rotation of the detection unit 16. The tip of the rotation restricting unit 23a is inserted into an annular recess 26 provided between the core metal unit 18 and the gear forming unit 25 of the reduction gear 5. In other words, in the radial direction of the steering shaft 1, the tip side of the rotation restricting unit 23a overlaps with the internal space of the annular recess 26. For this reason, the axial dimension of the steering device can be shortened compared to the case where the tip side of the rotation restricting unit 23a does not overlap with the internal space of the annular recess 26 in the radial direction of the steering shaft 1.

[0068] [Second Embodiment] Figure 4 is a longitudinal cross-sectional view of the steering device according to the second embodiment. Note that, for the sake of explanation, the internal structure of the detection unit 16 and the signal processing unit 23 of the torque sensor 14 is omitted from Figure 4.

[0069] The steering device of the second embodiment eliminates the deep groove ball bearing 19 of the first embodiment, and is further configured such that the back surface 20b of the nut 20 slidably contacts the inner circumferential surface of the third housing 9. In addition, in the second embodiment, due to the elimination of the deep groove ball bearing 19, the dimension of the third shaft portion 12c along the direction of the rotation axis Z of the steering shaft 1 is shorter compared to the first embodiment. Consequently, the dimension of the connecting member 8 along the direction of the rotation axis Z of the steering shaft 1 is also shorter.

[0070] The connecting member 8 has a cylindrical body portion 8a, an annular projection 8b projecting radially outward from the axial center of the outer circumferential surface of the cylindrical body portion 8a, and an annular projection wall portion 8h projecting radially inward from the position on the inner circumferential surface of the cylindrical body portion 8a closest to the first end 1a, and extending to the vicinity of the outer circumferential surface of the third shaft portion 12c of the output shaft 12. The inner circumferential surface of the annular projection wall portion 8h has a gap between it and the outer circumferential surface of the third shaft portion 12c that is sufficient to maintain airtightness between the second housing 7 and the third housing 9, that is, to seal the second housing 7 and the third housing 9, but not to support the third shaft portion 12c.

[0071] The back surface 20b of the nut 20, which is on the side opposite to the sector gear 3, has a recess 20d formed at the axial center. In this embodiment, the length of the recess 20d along the direction of the rotation axis Z of the steering shaft 1 is set to be less than one-third of the length of the nut 20 along the direction of the rotation axis Z of the steering shaft 1. As shown in Figure 4, the portion of the back surface 20b excluding the recess 20d is in slidable contact with the inner surface of the third housing 9. This contact transmits the radial force acting on the output shaft 12 to the third housing 9 via the nut 20. Also, as shown in Figure 4, the surface 20e of the nut 20 that is on the second end 1b side of the rack teeth 20c is in slidable contact with the inner surface of the third housing 9.

[0072] [Effects of the Second Embodiment] In the second embodiment, the steering device does not have a deep groove ball bearing 19, and the back surface 20b of the nut 20 slides against the inner circumferential surface of the third housing 9. In this embodiment, the radial force that was received by the deep groove ball bearing 19 in the first embodiment is received by the third housing 9 via the back surface 20b of the nut 20. Therefore, by eliminating the deep groove ball bearing 19 and shortening the axial dimension of the connecting member 8, the axial dimension of the steering device can be shortened.

Claims

1. A steering device comprising: a steering shaft to which rotation from a steering wheel is input; an electric motor positioned at the end of the steering shaft on which the steering wheel is provided and which provides steering assist force to the steering shaft; a reduction gear for reducing the rotational force of the electric motor; a ball screw mechanism provided on the steering shaft, the ball screw mechanism having a first ball screw groove which is a helical groove formed on the outer circumferential surface of the steering shaft; a second ball screw groove which is a helical groove formed on the inner circumferential surface of a nut provided around the steering shaft; and a plurality of balls positioned between the first ball screw groove and the second ball screw groove; a sector gear having a second tooth portion that meshes with a first tooth portion provided on the outer circumferential surface of the nut; and a four-point contact ball bearing positioned at the end of the steering shaft opposite to the side on which the electric motor is provided and which rotatably supports the steering shaft.

2. A steering device according to claim 1, further comprising a deep groove ball bearing that rotatably supports the steering shaft, wherein the deep groove ball bearing is positioned in the axial direction of the steering shaft on the opposite side of the ball screw mechanism from the four-point contact ball bearing.

3. A steering device according to claim 2, further comprising a cylindrical reduction gear side housing for housing the reduction gear, a cylindrical nut side housing for housing the nut, and a cylindrical connecting member connecting the reduction gear side housing and the nut side housing, wherein the connecting member has an annular projection that protrudes radially inward from the inner circumferential surface of the connecting member and holds the deep groove ball bearing.

4. A steering device according to claim 1, further comprising a cylindrical nut-side housing for housing the nut, wherein the nut has a back surface on its outer circumferential surface that is on the opposite side of the sector gear with respect to the steering shaft, and the back surface of the nut slides against the inner circumferential surface of the nut-side housing.

5. A steering device according to claim 4, further comprising a cylindrical reduction gear side housing for housing the reduction gear, and a cylindrical connecting member for connecting the reduction gear side housing and the nut side housing, wherein the connecting member seals the reduction gear side housing and the nut side housing.

6. A steering device according to claim 1, further comprising: a cylindrical nut-side housing for housing the nut; and a disc-shaped closing member that closes the nut-side housing from the four-point contact ball bearing side and fixes the outer race of the four-point contact ball bearing to the nut-side housing, wherein the outer circumferential surface of the closing member has a closing member-side male thread that screws into a housing-side female thread formed on the inner circumferential surface of the nut-side housing; and the closing member fixes the outer race of the four-point contact ball bearing to a stepped portion provided on the nut-side housing by screwing the housing-side female thread and the closing member-side male thread.

7. A steering device according to claim 6, further comprising an annular fixing member for fixing the inner race of the four-point contact ball bearing to the steering shaft, wherein the inner circumferential surface of the fixing member has a fixing member side female thread that screws into a steering shaft side male thread formed on the outer circumferential surface of the steering shaft, and the fixing member fixes the inner race of the four-point contact ball bearing to a stepped portion provided on the steering shaft by screwing the steering shaft side male thread and the fixing member side female thread.

8. A steering device according to claim 1, wherein the reduction gear has a worm wheel, the worm wheel has a core metal portion fixed to the steering shaft and a gear forming portion fixed to the core metal portion, and further comprises a torque sensor adjacent to the reduction gear in the axial direction of the steering shaft and detecting the steering torque of the steering shaft, the torque sensor having a detection unit fixed to the steering shaft and detecting a change in the magnetism of a magnet fixed to the steering shaft, and a signal processing unit arranged around the detection unit and processing a signal corresponding to the change in magnetism, the signal processing unit extending in the axial direction of the steering shaft and having a rotation restricting unit that restricts the rotation of the signal processing unit in conjunction with the rotation of the detection unit, and the tip of the rotation restricting unit is inserted into an annular recess provided between the core metal portion and the gear forming portion of the reduction gear.

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