Steering apparatus

Abstract

A steering apparatus (20) comprises a motor (22B), a power transmission device (22C), and a steering gearbox (22A). The power transmission device includes only a first transmission device (61), a second transmission device (62), and a transmission device housing (60). The first transmission device has a first input element (61A) and a first output element (61B) connected to each other so as to be capable of transmitting power. The second transmission device has a second input element (62A) and a second output element (62B) connected to each other so as to be capable of transmitting power. The first input element is coaxially connected to a motor shaft (22B1). The first output element is coaxially connected to the second input element. The second output element is coaxially connected to an input shaft (42) of the steering gearbox. The motor, the steering gearbox, and the first transmission device are disposed on the same side with respect to the second transmission device.

Description

Steering apparatus

[0001] The present disclosure relates to a steering apparatus.

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

[0003] The speed reducer is a two-stage gear speed reducer. The speed reducer includes a first gear set and a second gear set. The steering gear box, the motor, and the second gear set are arranged on the same side with respect to the first gear set. The same side refers to the side closer to the steering wheel with respect to the first gear set. The first gear set is a combination of a pinion provided on a motor shaft and an intermediate gear. The second gear set is a combination of a pinion coaxially connected to the intermediate gear and a helical gear connected to the ball screw shaft.

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

[0005] The electric power steering apparatus of Patent Document 1 has the following concerns. That is, depending on the specifications of the electric power steering apparatus or constraints on the mounting layout, it is required to secure a longer inter-shaft distance between the motor and the ball screw shaft. In this case, as the inter-shaft distance increases, for example, it is necessary to increase the size of the intermediate gear. However, when the size of the intermediate gear is increased, the size of the housing that accommodates the first gear set and the second gear set also increases. Therefore, there is a risk that a part of the housing interferes with vehicle components.

[0006] A steering system according to one aspect of the present disclosure comprises a motor having a motor shaft, a power transmission device configured to transmit the rotation of the motor, an input shaft to which rotation is supplied from the power transmission device, and an output shaft configured to be linked to the steering wheels of a vehicle, wherein the steering gearbox is configured to convert the rotation of the input shaft to the rotation of the output shaft. The power transmission device includes only a first transmission device, a second transmission device, and a transmission device housing that houses the first transmission device and the second transmission device. The first transmission device has a first input element and a first output element connected to each other in a power-transmitting manner. The second transmission device has a second input element and a second output element connected to each other in a power-transmitting manner. The first input element is connected coaxially to the motor shaft. The first output element is connected coaxially to the second input element. The second output element is connected coaxially to the input shaft. The motor, the steering gearbox, and the first transmission device are located on the same side as the second transmission device.

[0007] Figure 1 is a schematic diagram of an electric power steering device according to the first embodiment. Figure 2 is a cross-sectional view of the steering actuator in Figure 1, cut in the axial direction. Figure 3 is a front view showing the main part of the power transmission device in Figure 2. Figure 4 is a front view showing the main part of the power transmission device according to the second embodiment. Figure 4 is a front view showing the main part of the power transmission device according to the third embodiment. Figure 4 is a front view showing the main part of the power transmission device according to the fourth embodiment.

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

[0009] The steering system 20 includes a steering shaft 21 and a steering actuator 22 provided on the steering shaft 21. The steering actuator 22 includes a steering gearbox 22A, a motor 22B, a power transmission device 22C, and a torque sensor 22D. The power transmission device 22C is located at the bottom of the steering gearbox 22A. The torque sensor 22D is located at the top of the steering gearbox 22A.

[0010] The first end of the steering shaft 21 is connected to the steering wheel 23. The second end of the steering shaft 21 is connected to the steering gearbox 22A via a torque sensor 22D. The motor 22B is connected to the steering gearbox 22A via a power transmission device 22C. The steering gearbox 22A is of the RBS type (Recirculating Ball Screw type).

[0011] The steering gearbox 22A is connected to the steering wheels 13 via a linkage mechanism 30. The linkage mechanism 30 includes a pitman arm 31, a drag link 32, and a tie rod 33. The base end of the pitman arm 31 is connected to the side of the steering gearbox 22A. The pitman arm 31 is pivotable in the longitudinal direction of the vehicle about its base end. The first end of the drag link 32 is rotatably connected to the tip of the pitman arm 31. The second end of the drag link 32 is rotatably connected to the knuckle arm 34 of the right steering wheel 13. Both ends of the tie rod 33 are connected to the left and right steering wheels 13 via tie rod arms 35.

[0012] The rotation of the steering wheel 23 is transmitted to the steering gearbox 22A via the steering shaft 21 and torque sensor 22C. The steering gearbox 22A converts the rotational motion of the steering shaft 21 into the oscillating motion of the pitman arm 31. The oscillating motion of the pitman arm 31 drives the drag link 32 in the longitudinal direction of the vehicle. The knuckle arm 34 oscillates in conjunction with the drag link 32, causing the steering wheels 13 to turn. In other words, the steering gearbox 22A converts the rotation of the steering wheel 23 into the steering motion of the steering wheels 13.

[0013] Motor 22B generates torque in response to the steering torque detected by torque sensor 22D. The torque from motor 22B is transmitted to the steering shaft 21 via power transmission device 22C and steering gearbox 22A. Power transmission device 22C converts the torque from motor 22B into assist torque. The assist torque is torque in the same direction as the steering direction of the steering wheel 23. By applying assist torque to the steering shaft 21, the steering of the steering wheel 23 is assisted.

[0014] <Configuration of Steering Gearbox 22A> Next, the configuration of the steering gearbox 22A will be described in detail. As shown in Figure 2, the steering gearbox 22A has a housing 41, a ball screw shaft 42, a ball screw nut 43, a plurality of balls 44, a sector shaft 45, and a sector gear 46. The housing 41 houses the ball screw shaft 42, the ball screw nut 43, the plurality of balls 44, the sector shaft 45, and the sector gear 46 inside.

[0015] The ball screw shaft 42 is maintained in an orientation that extends vertically within the vehicle 10 when the steering gearbox 22A is mounted to the vehicle 10. The ball screw shaft 42 has a first thread groove 42A, which is a helical groove provided on the outer circumferential surface of the ball screw shaft 42. The ball screw shaft 42 is rotatably supported relative to the housing 41 via a first bearing 47 and a second bearing 48.

[0016] The first bearing 47 rotatably supports the first end of the ball screw shaft 42 relative to the housing 41. The first end is the end of the ball screw shaft 42 that faces upward towards the vehicle 10 when the steering gearbox 22 is mounted on the vehicle 10. The second bearing 48 rotatably supports the second end of the ball screw shaft 42 relative to the housing 41. The second end is the end of the ball screw shaft 42 that faces downward towards the vehicle 10 when the steering gearbox 22 is mounted on the vehicle 10.

[0017] The first bearing 47 is, for example, a radial ball bearing, which is a type of rolling bearing. Examples of radial ball bearings include a single-row deep groove ball bearing and a single-row four-point contact ball bearing. The first bearing 47 has an inner ring 47A, an outer ring 47B, a plurality of balls 47C, and a cage (not shown). The inner ring 47A is provided on the outer circumferential surface of the ball screw shaft 42. The inner ring 47A may be formed integrally with the ball screw shaft 42, for example. The outer ring 47B faces the inner ring 47A in the radial direction. The plurality of balls 47C are interposed between the inner ring 47A and the outer ring 47B.

[0018] The second bearing 48 is, for example, a roller bearing, which is a type of rolling bearing. Examples of roller bearings include needle bearings. However, the second bearing 48 is not limited to a roller bearing and may be any type of rolling bearing.

[0019] The ball screw nut 43 is a cylindrical body having a circular cross-sectional shape and has a second screw groove 43A. The second screw groove 43A is a helical groove provided on the inner circumferential surface of the ball screw nut 43. The second screw groove 43A is opposite the first screw groove 42A in the radial direction of the ball screw nut 43. The ball screw nut 43 is screwed onto the ball screw shaft 42 via a plurality of balls 44. The helical space surrounded by the first screw groove 42A and the second screw groove 43A functions as a rolling path for the balls 44 to roll. The ball screw nut 43 has a plurality of rack teeth 43B. The rack teeth 43B are provided on the outer circumferential surface of the ball screw nut 43. The plurality of rack teeth 43B are arranged in the axial direction of the ball screw nut 43.

[0020] The ball screw shaft 42, the ball screw nut 43, and the multiple balls 44 constitute a ball screw mechanism. The sector shaft 45 extends in a direction perpendicular to the axial direction of the ball screw nut 43 (perpendicular to the plane of the paper in Figure 2). The sector shaft 45 is rotatably supported by the housing 41 via a bearing (not shown). The sector shaft 45 has an outer end that penetrates the housing 41 and is exposed to the outside. The base end of the pitman arm 31 is fixed to the outer end of the sector shaft 45. The sector shaft 45 is the output shaft of the steering gearbox 22 and is linked to the steering wheel 13.

[0021] The sector gear 46 is rotatably mounted integrally with the sector shaft 45. The sector gear 46 is a fan-shaped gear having multiple teeth 46A. The teeth 46A of the sector gear 46 mesh with the rack teeth 43B of the ball screw nut 43.

[0022] The rotation of the steering wheel 23 is transmitted to the ball screw shaft 42 via the steering shaft 21 and torque sensor 22D. As the ball screw shaft 42 rotates, the ball screw nut 43 moves axially relative to the ball screw shaft 42. As a result, the sector gear 46 oscillates around the sector shaft 45 in response to the axial movement of the ball screw nut 43. When the sector gear 46 oscillates, the sector shaft 45 rotates, causing the pitman arm 31 to oscillate around the sector shaft 45.

[0023] The torque sensor 22D is provided at the first end of the housing 41. The first end is the end of the housing 41 that faces upward when the steering gearbox 22A is mounted on the vehicle 10. The first end of the ball screw shaft 42 is rotatably connected to the steering shaft 21 via the torque sensor 22D. The first end of the ball screw shaft 42 is the end of the ball screw shaft 42 that faces upward when the steering gearbox 22 is mounted on the vehicle 10.

[0024] The torque sensor 22D includes a sensor housing 51, an input shaft 52, a torsion bar 53, and a detector 54. The sensor housing 51 is a cylindrical body with a circular cross-section and is attached to the first end of the housing 41. The sensor housing 51 has an insertion portion 51A. The insertion portion 51A is a cylindrical body with a circular cross-section and is inserted into the first end of the housing 41. An oil seal 55 is interposed between the inner circumferential surface of the insertion portion 51A and the outer circumferential surface of the first end of the ball screw shaft 42. The oil seal 55 is a cylindrical body with a circular cross-section and seals the space between the outer circumferential surface of the first end of the ball screw shaft 42 and the inner circumferential surface of the insertion portion 51A. The inner circumferential surface of the oil seal 55 has a lip portion. The lip portion is in slidable contact with the outer circumferential surface of the first end of the ball screw shaft 42.

[0025] The input shaft 52 is a hollow cylindrical body with a circular cross-sectional shape. The input shaft 52 is rotatably supported in the housing 41 via a third bearing 56. The first end of the input shaft 52 protrudes outward through the end wall of the housing 41 in the axial direction. The first end of the input shaft 52 is connected to the steering wheel 23 via the steering shaft 21. The second end of the input shaft 52 is inserted into the first end of the ball screw shaft 42. A radial gap is formed between the outer circumferential surface of the second end of the input shaft 52 and the inner circumferential surface of the first end of the ball screw shaft 42. The second end of the input shaft 52 and the first end of the ball screw shaft 42 are rotatable relative to each other.

[0026] The torsion bar 53 is inserted into the input shaft 52. The first end of the torsion bar 53 is fixed in a press-fitted or inserted state on the input shaft 52. The second end of the torsion bar 53 protrudes non-contact from the second end of the input shaft 52. The second end of the torsion bar 53 is fixed in a press-fitted or inserted state on the first end of the ball screw shaft 42. In other words, the input shaft 52 is connected to the ball screw shaft 42 via the torsion bar 53.

[0027] The steering torque applied to the steering wheel 23 is transmitted to the input shaft 52 via the steering shaft 21. This causes the input shaft 52 to rotate. The first end of the torsion bar 53 is rotatable integrally with the input shaft 51. The second end of the torsion bar 53 is rotatable integrally with the ball screw shaft 42. Therefore, the rotation of the input shaft 52 is transmitted to the ball screw shaft 42 via the torsion bar 53. This causes the ball screw shaft 42 to rotate in the same direction as the input shaft 51. Also, when the input shaft 52 rotates, the torsion bar 53 twists in accordance with the steering torque. The detector 54 detects the steering torque based on the amount of twist of the torsion bar 53. The detector 54 is provided, for example, so as to surround the outer circumferential surface of the input shaft 52.

[0028] The power transmission device 22C is provided at the second end of the housing 41. The second end of the housing 41 is the end of the housing 41 that faces downwards from the vehicle 10 when the steering gearbox 22A is mounted on the vehicle 10. The second end of the ball screw shaft 42 is connected to the power transmission device 22C. The second end of the ball screw shaft 42 is the end of the ball screw shaft 42 that faces downwards from the vehicle 10 when the steering gearbox 22 is mounted on the vehicle 10.

[0029] The torque of the motor 22B is transmitted to the ball screw shaft 42 via the power transmission device 22C. The power transmission device 22C converts the torque of the motor 22B into assist torque. The assist torque is torque in the same direction as the steering direction of the steering wheel 23. By applying assist torque to the ball screw shaft 42, the steering of the steering wheel 23 is assisted. The ball screw shaft 42 corresponds to the input shaft to which rotation is supplied from the power transmission device 22C.

[0030] The power transmission device 22C is a combination of multiple transmission devices that transmit rotational motion. A transmission device is, for example, a gear transmission device having multiple gears that mesh with each other. The power transmission device 22C has a transmission device housing 60 that houses the multiple transmission devices. The transmission device housing 60 is attached to the second end of the housing 41.

[0031] <Configuration of Power Transmission System 22C> Next, the configuration of the power transmission system 22C will be described in detail. As shown in Figure 3, the power transmission system 22C has a first transmission system 61 and a second transmission system 62. The steering gearbox 22A, the motor 22B, and the first transmission system 61 are located on the same side as the second transmission system 62. The same side is the side of the second transmission system 62 that is closer to the steering wheel 23.

[0032] The first transmission device 61 has a first gear 61A and a second gear 61B that mesh with each other. The first gear 61A corresponds to the first input element, and the second gear 61B corresponds to the first output element. The first gear 61A and the second gear 61B are, for example, helical gears.

[0033] The second transmission device 62 has a third gear 62A, a fourth gear 62B, and a fifth gear 62C that mesh with each other. The third gear 62A corresponds to a second input element, the fourth gear 62B corresponds to a second output element, and the fifth gear 62C corresponds to a transmission element. The third gear 62A, the fourth gear 62B, and the fifth gear 62C are, for example, helical gears.

[0034] The first gear 61A is mounted on the motor shaft 22B1. The motor shaft 22b1 is the output shaft mounted on the motor 22B. The motor shaft 22B1 and the first gear 61A are arranged coaxially. The motor shaft 22B1 and the first gear 61A rotate together as a unit around the first axis O1 in conjunction with the drive of the motor 22B.

[0035] The second gear 61B is provided on the first shaft 63. The first shaft 63 is positioned between the motor shaft 22B1 of the motor 22B and the ball screw shaft 42 in the direction in which the steering gearbox 22A and the motor 22B are aligned (the vertical direction in Figure 3). The first shaft 63 is rotatably supported by the transmission housing 60 via bearings. The second gear 61B is positioned, for example, at the first end of the first shaft 63. The first end is the end of the first shaft 63 that is closer to the motor 22B in the axial direction. The second gear 61B meshes with the first gear 61A.

[0036] The third gear 62A is mounted on the first shaft 63. The third gear 62A is positioned closer to the second end of the first shaft 63 than the second gear 61B. The third gear 62A is coaxial with the second gear 61B. The second gear 61B and the third gear 62A rotate together as a single unit about the second axis O2.

[0037] The fourth gear 62B is located at the second end of the ball screw shaft 42. The second end is the end of the ball screw shaft 42 that faces downwards when the steering gearbox 22 is mounted on the vehicle 10. The fourth gear 62B is arranged coaxially with the ball screw shaft 42. The ball screw shaft 42 and the fourth gear 62B rotate integrally around the third axis O3.

[0038] The fifth gear 62C is mounted on the second shaft 64. The second shaft 64 is positioned between the first shaft 63 and the ball screw shaft 42 in the direction in which the steering gearbox 22A and the motor 22B are aligned (the vertical direction in Figure 3). The second shaft 64 is rotatably supported by the transmission housing 60 via bearings. The fifth gear 62C meshes with the third gear 62A and the fourth gear 62B, which are spaced apart from each other. In other words, the fifth gear 62C is an idler gear that fills the gap between the third gear 62A and the fourth gear 62B. An idler gear is also called an intermediate gear. The fifth gear 62C rotates about the fourth axis O4.

[0039] The first axis O1, the second axis O2, the third axis O3, and the fourth axis O4 are parallel to each other. Also, the outer diameter of the second gear 61B is larger than the outer diameter of the first gear 61A. The outer diameters of the third to fifth gears 62A, 62B, and 62C increase in the order of the third gear 62A, the fifth gear 62C, and the fourth gear 62B. Both the first transmission device 61 and the second transmission device 62 function as speed reducers.

[0040] The torque of the motor 22B is transmitted to the ball screw shaft 42 via the first gear 61A, the second gear 61B, the third gear 62A, the fifth gear 62C, and the fourth gear 62B. By applying torque to the ball screw shaft 42 in the same direction as the steering direction of the steering wheel 23, the steering of the steering wheel 23 is assisted.

[0041] <Effects of the First Embodiment> The first embodiment provides the following effects. (1-1) Depending on the specifications of the steering device 20 or constraints on the mounting layout, it may be necessary to secure a longer axial distance between the motor 22B and the ball screw shaft 42. In this regard, according to the first embodiment, the steering gearbox 22A, the motor 22B, and the first transmission device 61 are arranged on the same side as the second transmission device 62. The same side is the side closer to the steering wheel 23 relative to the second transmission device 62. Therefore, it is possible to secure a longer axial distance between the motor 22 and the ball screw shaft 42 while suppressing the enlargement of the first gear 61A and the second gear 61B, as well as the enlargement of the third gear 62A, the fourth gear 62B, and the fifth gear 62C. Since the size of the transmission housing 60, which houses the first transmission device 61 and the second transmission device 62, is also suppressed, interference between a part of the transmission housing 60 and parts of the vehicle is reduced.

[0042] For example, consider the case where the steering gearbox 22A, motor 22B, and second transmission device 62 are located on the same side as the first transmission device 61, as a comparative example. If, due to the specifications of the steering device 20 or constraints on the mounting layout, it is necessary to ensure a longer distance between the motor 22B and the ball screw shaft 42, then, for example, the size of the second gear 61B of the first transmission device 61 needs to be increased in proportion to the increase in distance between the axes. This may increase the size of the transmission device housing 60. For example, as shown by the dashed line in Figure 2, in order to accommodate the larger second gear 61B, it becomes necessary to form a protrusion 60A that projects radially outward in the portion of the transmission device housing 60 corresponding to the second gear 61B. This protrusion 60A may interfere with vehicle components.

[0043] (1-2) The first transmission device 61 includes a first gear 61A and a second gear 61B that mesh with each other. The first gear 61A is coupled to a motor shaft 22B1 of the motor 22B. The second transmission device 62 includes a third gear 62A, a fourth gear 62B, and a fifth gear 62C that mesh with each other. The third gear 62A is coaxially coupled to the second gear 61B via a first shaft 63. The fourth gear 62B is coupled to an end portion of a ball screw shaft 42. The fifth gear 62C meshes with the third gear 62A and the fourth gear 62B. That is, the gap between the third gear 62A and the fourth gear 62B is filled by the fifth gear 62C. Accordingly, the torque of the motor 22B can be appropriately transmitted to the ball screw shaft 42 via the first transmission device 61 and the second transmission device 62.

[0044] (1-3) Since both the first transmission device 61 and the second transmission device 62 are configured only by gears, it is easy to ensure strength. <Second Embodiment> Next, a steering device according to a second embodiment will be described. This embodiment basically has the same configuration as the first embodiment shown in FIGS. 1 to 3. Therefore, the same members and configurations as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted. In this embodiment, the configuration of the second transmission device 62 is different from that of the first embodiment.

[0045] As shown in FIG. 4, the second transmission device 62 is a belt transmission device including a first pulley 71, a second pulley 72, and a belt 73. The first pulley 71 corresponds to a second input element, the second pulley 72 corresponds to a second output element, and the belt 73 corresponds to a transmission element.

[0046] The first pulley 71 is provided at a second end portion of the first shaft 63. The second end portion is an end portion of the first shaft 63 opposite to a first end portion where the second gear 61B is provided. The first pulley 71 and the second gear 61B rotate integrally around a second axis O2. The first pulley 71 may be a toothed pulley having teeth on an outer circumferential surface thereof.

[0047] The second pulley 72 is provided at the second end of the ball screw shaft 42. The second end is an end of the ball screw shaft 42 that faces downward of the vehicle 10 when the steering gear box 22 is mounted on the vehicle 10. The ball screw shaft 42 and the second pulley 72 rotate integrally about the third axis O3. The outer diameter of the second pulley 72 is larger than the outer diameter of the first pulley 71. The second pulley 72 may be a toothed pulley having teeth on its outer peripheral surface.

[0048] The belt 73 is made of rubber, for example. The belt 73 is endless and is wound between the first pulley 71 and the second pulley 72. That is, the belt 73 connects the first pulley 71 and the second pulley 72, which are arranged spaced apart from each other, such that power can be transmitted therebetween. The belt 73 may be a toothed belt having teeth on an inner peripheral surface thereof. The teeth of the belt 73 mesh with the teeth of the first pulley 71 and the second pulley 72.

[0049] Torque of the motor 22B is transmitted to the ball screw shaft 42 via the first gear 61A, the second gear 61B, the first pulley 71, the belt 73, and the second pulley 72. By applying torque in the same direction as the steering direction of the steering wheel 23 to the ball screw shaft 42, steering of the steering wheel 23 is assisted. Both the first transmission device 61 and the second transmission device 62 function as reduction gears.

[0050] <Effects of Second Embodiment> According to the second embodiment, the following effects are exhibited. (2-1) The steering gear box 22A, the motor 22B, and the first transmission device 61 are arranged on the same side with respect to the second transmission device 62. The same side refers to the side closer to the steering wheel 23 with respect to the second transmission device 62. Therefore, the same effects as those described in the (1-1) section of the first embodiment can be obtained.

[0051] (2-2) The first transmission device 61 has a first gear 61A and a second gear 61B that mesh with each other. The first gear 61A is connected to the motor shaft 22B1 of the motor 22B. The second transmission device 62 has a first pulley 71, a second pulley 72, and a belt 73. The first pulley 71 is connected coaxially to the second gear 61B via the first shaft 63. The second pulley 72 is connected to the end of the ball screw shaft 42. The belt 73 is wrapped between the first pulley 71 and the second pulley 72. That is, the belt 73 connects the first pulley 71 and the second pulley 72, which are spaced apart from each other, in a way that enables power transmission. As a result, the torque of the motor 22B can be properly transmitted to the ball screw shaft 42 via the first transmission device 61 and the second transmission device 62.

[0052] (2-3) Since the second transmission device 62 is a belt transmission device, it is less expensive than when both the first transmission device 61 and the second transmission device 62 are composed only of gears. <Third Embodiment> Next, a steering device according to the third embodiment will be described. This embodiment basically has the same configuration as the first embodiment shown in Figures 1 to 3. Therefore, the same reference numerals are used for the same components and components as in the first embodiment, and detailed explanations are omitted. In this embodiment, the configuration of the second transmission device 62 differs from that of the first embodiment.

[0053] As shown in Figure 5, the second transmission device 62 is a chain transmission device and includes a first sprocket 81, a second sprocket 82, and a chain 83. The first sprocket 81 corresponds to the second input element, the second sprocket 82 corresponds to the second output element, and the chain 83 corresponds to the transmission element.

[0054] The first sprocket 81 is provided at the second end of the first shaft 63. The second end is the end of the first shaft 63 opposite to the first end on which the second gear 61B is provided. The first sprocket 81 and the second gear 61B rotate integrally around the second axis O2. The first sprocket 81 is a type of gear and has teeth on its outer surface.

[0055] The second sprocket 82 is provided at the second end of the ball screw shaft 42. The second end is the end of the ball screw shaft 42 that faces downwards when the steering gearbox 22 is mounted on the vehicle 10. The ball screw shaft 42 and the second sprocket 82 rotate integrally around the third axis O3. The second sprocket 82 is a type of gear and has teeth on its outer surface. The outer diameter of the second sprocket 82 is larger than the outer diameter of the first sprocket 81.

[0056] The chain 83 is, for example, an endless roller chain, wrapped between the first sprocket 81 and the second sprocket 82. The chain 83 meshes with the teeth of the first sprocket 81 and the second sprocket 82. In other words, the chain 83 connects the first sprocket 81 and the second sprocket 82, which are spaced apart from each other, in a way that enables power transmission.

[0057] The torque of the motor 22B is transmitted to the ball screw shaft 42 via the first gear 61A, the second gear 61B, the first sprocket 81, the chain 83, and the second sprocket 82. By applying torque to the ball screw shaft 42 in the same direction as the steering direction of the steering wheel 23, the steering of the steering wheel 23 is assisted. Both the first transmission device 61 and the second transmission device 62 function as reduction gears.

[0058] <Effects of the Third Embodiment> The third embodiment provides the following effects: (3-1) The steering gearbox 22A, motor 22B, and first transmission device 61 are arranged on the same side as the second transmission device 62. The same side is the side closer to the steering wheel 23 relative to the second transmission device 62. Therefore, the same effects as those described in section (1-1) of the first embodiment can be obtained.

[0059] (3-2) The first transmission device 61 has a first gear 61A and a second gear 61B that mesh with each other. The first gear 61A is connected to the motor shaft 22B1 of the motor 22B. The second transmission device 62 has a first sprocket 81, a second sprocket 82, and a chain 83. The first sprocket 81 is connected coaxially to the second gear 61B via the first shaft 63. The second sprocket 82 is connected to the end of the ball screw shaft 42. The chain 83 is wrapped between the first sprocket 81 and the second sprocket 82. That is, the chain 83 connects the first sprocket 81 and the second sprocket 82, which are spaced apart from each other, in a way that enables power transmission. As a result, the torque of the motor 22B can be properly transmitted to the ball screw shaft 42 via the first transmission device 61 and the second transmission device 62. The strength of chain 83 is greater than the strength of belt 73.

[0060] (3-1) Since the second transmission device 62 is a chain transmission device, it is less expensive than when both the first transmission device 61 and the second transmission device 62 are composed only of gears. <Fourth Embodiment> Next, a steering device according to the fourth embodiment will be described. This embodiment basically has the same configuration as the first embodiment shown in Figures 1 to 3. Therefore, the same reference numerals are used for the same components and components as in the first embodiment, and detailed explanations are omitted. In this embodiment, the configuration of the second transmission device 62 differs from that of the first embodiment.

[0061] As shown in Figure 6, the second transmission device 62 is a gear transmission device and includes a third gear 62A and a fourth gear 62B. However, in the second transmission device 62, the fifth gear 62C in the first embodiment is omitted. Because the fifth gear 62C is omitted, the size of the third gear 62A is increased. For example, the outer diameter of the third gear 62A is larger than the outer diameter of the fourth gear 62B. As a result, the third gear 62A meshes with the fourth gear 62B.

[0062] The torque of the motor 22B is transmitted to the ball screw shaft 42 via the first gear 61A, the second gear 61B, the third gear 62A, and the fourth gear 62B. By applying torque to the ball screw shaft 42 in the same direction as the steering direction of the steering wheel 23, the steering of the steering wheel 23 is assisted. The first transmission 61 functions as a reduction gear, and the second transmission 62 functions as a speed increaser. However, by making the outer diameter of the fourth gear 62B larger than the outer diameter of the third gear 62A, it is also possible to make the second transmission 62 function as a reduction gear.

[0063] <Effects of the Fourth Embodiment> The fourth embodiment provides the following effects: (4-1) The steering gearbox 22A, motor 22B, and first transmission device 61 are arranged on the same side as the second transmission device 62. The same side is the side closer to the steering wheel 23 relative to the second transmission device 62. Therefore, the same effects as those described in section (1-1) of the first embodiment can be obtained.

[0064] (4-2) The first transmission device 61 has a first gear 61A and a second gear 61B that mesh with each other. The first gear 61A is connected to the motor shaft 22B1 of the motor 22B. The second transmission device 62 has a third gear 62A and a fourth gear 62B that mesh with each other. The third gear 62A is connected coaxially to the second gear 61B via the first shaft 63. The fourth gear 62B is connected to the end of the ball screw shaft 42. As a result, the torque of the motor 22B can be properly transmitted to the ball screw shaft 42 via the first transmission device 61 and the second transmission device 62.

[0065] (4-3) Since both the first transmission device 61 and the second transmission device 62 are composed solely of gears, it is easy to ensure strength. <Other Embodiments> This embodiment may be implemented with the following modifications.

[0066] In the first to fourth embodiments, the first transmission device 61 may be a belt transmission device. In this case, the first transmission device 61 has a third pulley that replaces the first gear 61A, a fourth pulley that replaces the second gear 61B, and a second belt that is wrapped between the third pulley and the fourth pulley. The outer diameter of the fourth pulley is larger than the outer diameter of the third pulley.

[0067] In the first to fourth embodiments, the first transmission device 61 may be a chain transmission device. In this case, the first transmission device 61 has a third sprocket replacing the first gear 61A, a fourth sprocket replacing the second gear 61B, and a second chain wrapped between the third sprocket and the fourth sprocket. The outer diameter of the fourth sprocket is larger than the outer diameter of the third sprocket.

Claims

1. A steering gearbox comprising: a motor having a motor shaft; a power transmission device configured to transmit the rotation of the motor; an input shaft to which rotation is supplied from the power transmission device; and an output shaft configured to be linked to the steering wheels of a vehicle, wherein the steering gearbox is configured to convert the rotation of the input shaft into the rotation of the output shaft, the power transmission device includes a first transmission device, a second transmission device, and a transmission device housing housing the first transmission device and the second transmission device, the first transmission device having a first input element and a first output element connected in a power-transmitting manner, the second transmission device having a second input element and a second output element connected in a power-transmitting manner, the first input element being connected coaxially to the motor shaft, the first output element being connected coaxially to the second input element, and the second output element being connected coaxially to the input shaft. A steering system comprising the motor, the steering gearbox, and the first transmission device, arranged on the same side as the second transmission device.

2. The steering device according to claim 1, wherein the second transmission device further comprises a transmission element in addition to the second input element and the second output element, the transmission element being configured to transmit the motion of the second input element to the second output element.

3. The steering device according to claim 2, wherein the second input element and the second output element are gears arranged spaced apart from each other, and the transmission element is an intermediate gear that meshes with the two gears.

4. The steering device according to claim 2, wherein the second input element and the second output element are pulleys spaced apart from each other, and the transmission element is a belt wrapped between the two pulleys.

5. The steering device according to claim 2, wherein the second input element and the second output element are sprockets spaced apart from each other, and the transmission element is a chain wrapped between the two sprockets.

6. The steering device according to claim 1, wherein the first input element and the first output element are gears that mesh with each other, and the second input element and the second output element are gears that mesh with each other.

Images