Gear assembly with rotating shaft and method of assembling the same

Abstract

The present application realizes a structure in which the coaxiality of a gear and a rotating shaft can be easily ensured. A pinion shaft (8) has a serration shaft portion (25) having an outer serration portion (28) on an outer peripheral surface and being press-fitted into a central hole (30) of a worm wheel (29), and a guide shaft portion (26) provided adjacent to an axial side of the serration shaft portion (25) and having an outer diameter capable of being fitted into the central hole (30) in a clearance fit.

Description

Technical Field

[0001] The present invention relates to an assembly of a gear and a rotating shaft, such as an assembly of a worm gear and a pinion shaft, and an assembling method thereof. Background Art

[0002] As a device for reducing the force required to operate the steering wheel when applying a steering angle to the steering wheel of a car, an electric power steering device that uses an electric motor as an auxiliary power source is widely used. Depending on the installation position of the electric motor, the electric power steering device is roughly divided into a column-assisted type that applies auxiliary power to a steering shaft that is rotatably supported on the inside of the steering column, a pinion-assisted type that applies auxiliary power to a pinion shaft that serves as the input shaft of the steering gear unit, and a double-pinion type that provides a pinion shaft different from the pinion shaft that serves as the input shaft in the steering gear unit and applies auxiliary power to the pinion shaft. In any of the structures, the auxiliary power of the electric motor is applied to a shaft component that rotates or moves linearly with the operation of the steering wheel via a speed reducer. As such a speed reducer, a worm speed reducer is widely used. The worm speed reducer that constitutes the electric power steering device includes a worm that is driven to rotate by the electric motor and a worm wheel that meshes with the worm.

[0003] Figure 15 and Figure 16 Japanese Patent Application Laid-Open No. 2013-119890 discloses a pinion-assisted electric power steering device 100. In the electric power steering device 100, the rotation of a steering wheel 101 is transmitted to a pinion shaft 106 of a steering gear unit 105 via a steering shaft 102, an intermediate shaft 103, and a pair of universal joints 104a and 104b. As the rack shaft 107 of the steering gear unit 105 rotates and displaces axially, a pair of tie rods 108 connected to the axial ends of the rack shaft 107 are pushed and pulled, imparting a steering angle to a pair of steering wheels 109.

[0004] In the pinion-assisted electric power steering device 100 , the torque of the electric motor 110 is applied to the pinion shaft 106 via the worm reducer 111 to reduce the force required for the driver to operate the steering wheel 101 . The worm reducer 111 includes a worm 112 and a worm wheel 113 .

[0005] The worm 112 has thread-shaped worm teeth 114 on its outer peripheral surface and is driven to rotate by the electric motor 110 .

[0006] The worm wheel 113 has a plurality of teeth 115 on its outer circumference that mesh with the worm teeth 114. It also has a center hole 116 extending axially through the center of the worm wheel 113 and having a constant inner diameter. The worm wheel 113 is secured to the pinion shaft 106 by press-fitting the axially intermediate portion of the pinion shaft 106 into the center hole 116.

[0007] The pinion shaft 106 is pressed into the axial middle portion of the center hole 116 of the worm wheel 113 at one axial side portion ( Figure 16 The upper part) has an outer peripheral surface of a cylindrical surface centered on the central axis of the pinion shaft 106, that is, a cylindrical shaft portion 117, and the other axial side portion ( Figure 16 The lower portion of the pinion shaft 106 is provided with a serrated shaft portion 118 having external serrations on its outer circumferential surface. Before the axially intermediate portion of the pinion shaft 106 is press-fitted into the center hole 116 of the worm wheel 113, the outer diameter of the cylindrical shaft portion 117 is larger than the inner diameter of the center hole 116, and the outer diameter of the serrated shaft portion 118, that is, the diameter of the tooth tip circle of the serrations forming the external serrations provided on the outer circumferential surface of the serrated shaft portion 118, is larger than the outer diameter of the cylindrical shaft portion 117.

[0008] When fastening the pinion shaft 106 to the worm wheel 113, the pinion shaft 106, serving as the rotating shaft, is inserted into the center hole 116 of the worm wheel 113, serving as the gear, with the axial end thereof at the leading end. Furthermore, after the cylindrical shaft portion 117 is press-fitted into the center hole 116, the pinion shaft 106 is further displaced axially to one side relative to the worm wheel 113, and the serrated shaft portion 118 is press-fitted into the center hole 116, thereby ensuring the fastening strength between the pinion shaft 106 and the worm wheel 113.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent Application Laid-Open No. 2013-119890 Summary of the Invention

[0012] Technical problem that the invention aims to solve

[0013] The electric power steering device 100 described in Japanese Patent Application Laid-Open No. 2013-119890 has room for improvement in terms of reliably maintaining an appropriate meshing state between the worm teeth 114 of the worm 112 and the gear teeth 115 of the worm wheel 113 .

[0014] In the electric power steering device 100 described in Japanese Patent Application Laid-Open No. 2013-119890, when the pinion shaft 106 as a rotating shaft and the worm wheel 113 as a gear are connected and fixed, the pinion shaft 106 is first aligned with the worm wheel 113 by press-fitting the cylindrical shaft portion 117 into the center hole 116 .

[0015] Therefore, if the central axis of the pinion shaft 106 is tilted or offset relative to the central axis of the worm wheel 113 during the initial stages of press-fitting the cylindrical shaft portion 117 into the center hole 116 to axially align the pinion shaft 106 with the worm wheel 113, there is a risk that the tilt and / or offset of the central axis of the pinion shaft 106 relative to the central axis of the worm wheel 113 cannot be corrected thereafter. In other words, the pinion shaft 106 and the worm wheel 113 may be coupled and fixed while the central axis of the pinion shaft 106 is tilted or offset relative to the central axis of the worm wheel 113.

[0016] If the center axis of the pinion shaft 106 is tilted or offset relative to the center axis of the worm wheel 113, there is a risk of increased meshing resistance between the worm 114 and the gear teeth 115, or abnormal noise may be generated at the meshing portion between the worm 114 and the gear teeth 115. Therefore, in the electric power steering device 100 described in Japanese Patent Application Laid-Open No. 2013-119890, it is necessary to precisely align the pinion shaft 106 and the worm wheel 113 before starting the press-fitting operation of the cylindrical shaft portion 117 into the center hole 116, which is quite laborious.

[0017] Furthermore, in the electric power steering device 100 described in Japanese Patent Application Laid-Open No. 2013-119890, the dimensional tolerances of the center hole 116 of the worm wheel 113 and the dimensional tolerances of the cylindrical shaft portion 117 and the serrated shaft portion 118 of the pinion shaft 106 must be restricted with high precision. The reason for this is as follows.

[0018] In the electric power steering device 100 described in Japanese Patent Application Laid-Open No. 2013-119890, both the cylindrical shaft portion 117 for axial alignment with the worm wheel 113 and the serrated shaft portion 118 for ensuring the connection strength with the worm wheel 113 are press-fitted into the center hole 116. Therefore, before the cylindrical shaft portion 117 is press-fitted into the center hole 116, the outer diameter D of the cylindrical shaft portion 117 is 117 Than the inner diameter d of the center hole 116 116 Slightly larger (D 117 >d 116 ). In addition, if the height of the serrations constituting the outer serrations provided on the outer peripheral surface of the serration shaft 118, i.e., the tooth height, is t, then the outer diameter of the serration shaft 118, i.e., the tooth tip diameter D of the serrations is 118 Approximately equal to the outer diameter D of the cylindrical shaft portion 117 117 and twice the height t of the sawtooth (D 118 ≒D 117 +2t). Therefore, if the inner diameter d of the center hole 116 is 116 If it is too small, it will be difficult to press the serrated shaft portion 118 into place.

[0019] Therefore, in order to press both the cylindrical shaft portion 117 and the serrated shaft portion 118 of the pinion shaft 106 into the center hole 116 , the dimensional tolerances of the center hole 116 and the dimensional tolerances of the cylindrical shaft portion 117 and the serrated shaft portion 118 need to be restricted with high precision, which increases costs.

[0020] Furthermore, if the height t of the serrations is reduced, it is not necessary to precisely limit the dimensional tolerances of the center hole 116 , the cylindrical shaft portion 117 , and the serrated shaft portion 118 . However, this may not fully ensure the effect of preventing the worm wheel 113 from creeping relative to the pinion shaft 106 .

[0021] The present invention has been made to solve the above-mentioned problems and aims to ensure good coaxiality between a gear and a rotating shaft in an assembly of a worm wheel and a pinion shaft, etc. However, the purpose of the present invention is not limited thereto.

[0022] Technical means to solve the problem

[0023] A gear and rotating shaft assembly according to one embodiment of the present invention includes a gear and a rotating shaft.

[0024] The gear has a plurality of teeth on an outer peripheral surface and a center hole penetrating in an axial direction at a center portion.

[0025] The rotating shaft includes: a serrated shaft portion having an outer serration portion on an outer peripheral surface and being pressed into the center hole; and a guide shaft portion, the guide shaft portion being arranged adjacent to one axial side of the serrated shaft portion and having an outer diameter that can be embedded in the center hole in a gap-fitting manner.

[0026] In the gear and rotating shaft assembly according to one embodiment of the invention, only the serrated shaft portion of the rotating shaft can be fitted into the center hole of the gear. That is, the guide shaft portion of the rotating shaft can be positioned axially to one side of the center hole.

[0027] Alternatively, at least one axial side portion of the serrated shaft portion and the other axial side portion of the guide shaft portion may be fitted into the center hole of the gear.

[0028] In the gear and rotary shaft assembly according to one embodiment of the invention, the outer diameter of the guide shaft portion can be set smaller than the inner diameter of the center hole by 0.01% to 0.5%, preferably by 0.01% to 0.2%.

[0029] In the gear and rotating shaft assembly according to one embodiment of the invention, the rotating shaft includes an auxiliary guide shaft portion at a portion adjacent to one axial side of the guide shaft portion. The auxiliary guide shaft portion has an outer diameter smaller than that of the guide shaft portion.

[0030] In this case, the outer diameter of the auxiliary guide shaft portion can be made smaller toward one side in the axial direction.

[0031] In the assembly of the gear and the rotating shaft involved in one embodiment of the invention, the center hole has a first chamfered portion at the end on the other axial side of the inner circumferential surface, and a second chamfered portion is provided at a portion of the inner circumferential surface adjacent to the axial side of the first chamfered portion, and the chamfer angle of the second chamfered portion is smaller than the chamfer angle of the first chamfered portion.

[0032] In the gear and rotary shaft assembly according to one embodiment of the invention, the center hole has a small-diameter portion on one axial side and a large-diameter portion on the other axial side having an inner diameter larger than that of the small-diameter portion. In this case, the serrated shaft portion is press-fitted into the large-diameter portion, and the guide shaft portion is loosely fitted into the small-diameter portion.

[0033] In the gear and rotating shaft assembly according to one aspect of the invention, the guide shaft portion has an external serration portion on an outer peripheral surface.

[0034] In the assembly of a gear and a rotating shaft involved in one embodiment of the invention, the gear includes: a hub having the center hole in the center portion; and a synthetic resin gear portion having the multiple teeth on the outer peripheral surface, which is combined and fixed to the hub in a manner covering the radially outer end portion of the hub.

[0035] In the gear and rotating shaft assembly according to one embodiment of the present invention, the gear may be formed of a worm wheel. In other words, the plurality of teeth may each be formed of a gear tooth that meshes with threaded worm teeth of a worm.

[0036] In the assembly method of the gear and rotating shaft assembly involved in one embodiment of the present invention, in order to assemble the gear and rotating shaft assembly involved in one embodiment of the present invention, the assembly method has the following steps: inserting the rotating shaft from the other axial side of the center hole with the end on one axial side as the front end into the center hole of the gear, embedding the guide shaft portion in the center hole in a clearance fit manner, thereby aligning the rotating shaft and the gear shaft, and then pressing the serrated shaft portion into the center hole, thereby combining the gear and the rotating shaft.

[0037] Effects of the Invention

[0038] According to the assembly of the gear and the rotating shaft of one embodiment of the present invention, it is possible to realize a structure that can easily and satisfactorily ensure the coaxiality between the gear and the rotating shaft. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1This is a schematic diagram showing an electric power steering device including an assembly of a worm wheel (gear) and a pinion shaft (rotating shaft) according to a first example of an embodiment of the present invention.

[0040] Figure 2 1 is a cross-sectional view showing a main part of an electric power steering device according to a first example.

[0041] Figure 3 This is a side view showing an assembly of a worm wheel and a pinion shaft according to a first example.

[0042] Figure 4 yes Figure 3 AA cross-sectional view.

[0043] Figure 5 This is an enlarged cross-sectional view of the main parts of the first example, showing a state before the worm wheel and the pinion shaft are coupled together.

[0044] Figure 6 yes Figure 5 Enlarged view of part B.

[0045] Figure 7 This is a side view showing an assembly of a worm wheel and a pinion shaft according to a second example of an embodiment of the present invention.

[0046] Figure 8 yes Figure 7 CC cross-sectional view.

[0047] Figure 9 This is a side view showing an assembly of a worm wheel and a pinion shaft according to a third example of an embodiment of the present invention.

[0048] Figure 10 yes Figure 9 DD cross-sectional view.

[0049] Figure 11 This is the equivalent of the worm gear and pinion shaft assembly of the third example. Figure 5 Picture.

[0050] Figure 12 This is a side view showing an assembly of a worm wheel and a pinion shaft according to a fourth example of the embodiment of the present invention.

[0051] Figure 13 yes Figure 12 EE cross-sectional view.

[0052] Figure 14 The worm gear and pinion shaft assembly of the fourth example is equivalent to Figure 5 Picture.

[0053] Figure 15This is a schematic diagram showing an example of a conventional structure of a pinion-assist electric power steering device.

[0054] Figure 16 yes Figure 15 A cross-sectional view of the main parts of an example of a conventional structure is shown.

[0055] Explanation of symbols

[0056] 1 Electric power steering

[0057] 2 Steering wheel

[0058] 3 Steering axles

[0059] 4a, 4b universal joints

[0060] 5 intermediate shaft

[0061] 6 Steering gear unit

[0062] 7 Electric assist device

[0063] 8, 8a, 8b, 8c pinion shaft

[0064] 9 Housing

[0065] 10 Rack shaft

[0066] 11 Rack guide

[0067] 12 tie rod

[0068] 13 Steering Wheel

[0069] 14 Rack accommodating portion

[0070] 15 Pinion housing

[0071] 16 Guide accommodating portion

[0072] 17 Gear housing

[0073] 18 Installation

[0074] 19 Worm reducer

[0075] 20 wheel storage unit

[0076] 21 Worm housing

[0077] 22 pinion teeth

[0078] 23a, 23b cylindrical surface

[0079] 24a, 24b bearings

[0080] 25 serrated shaft

[0081] 26, 26a, 26b, 26c guide shaft

[0082] 27 Auxiliary guide shaft

[0083] 28 External serrations

[0084] 29, 29a worm gear

[0085] 30, 30a center hole

[0086] 31 External serrations

[0087] 32 Chamfer

[0088] 33 Hollow Shaft

[0089] 34 Torsion bar

[0090] 35 rack teeth

[0091] 36 pads

[0092] 36a Pressing surface

[0093] 37 Elastic components

[0094] 38 Cover

[0095] 39 Electric Motor

[0096] 40 Torque sensor

[0097] 41 Worm

[0098] 42 teeth

[0099] 43 First chamfer

[0100] 44 Second chamfer

[0101] 45 wheel hub

[0102] 46 Gear Department

[0103] 47 Inner diameter side cylinder

[0104] 48 Outer diameter side cylinder

[0105] 49 connection

[0106] 50 assembly

[0107] 51 small trail section

[0108] 52 Large diameter part

[0109] 53 Slanted Face

[0110] 100 Electric power steering

[0111] 101 Steering Wheel

[0112] 102 steering shaft

[0113] 103 intermediate shaft

[0114] 104a, 104b universal joints

[0115] 105 Steering gear unit

[0116] 106 pinion shaft

[0117] 107 rack shaft

[0118] 108 tie rod

[0119] 109 Steering Wheel

[0120] 110 Electric Motor

[0121] 111 Worm reducer

[0122] 112 worm

[0123] 113 Worm gear

[0124] 114 worm teeth

[0125] 115 gear teeth

[0126] 116 center hole

[0127] 117 Cylindrical shaft

[0128] 118 Serrated shaft DETAILED DESCRIPTION

[0129] [First example]

[0130] Figures 1 to 6 The first embodiment of the present invention is shown. This example is an example in which the gear and rotating shaft assembly and the assembly method thereof according to one embodiment of the present invention are applied to an assembly 50 of the pinion shaft 8 constituting the steering gear unit 6 and the worm wheel 29 constituting the worm reducer 19 of a pinion-assisted electric power steering device 1 and the assembly method thereof.

[0131] In the following description, the front-to-back direction refers to the front-to-back direction of the vehicle, the up-down direction refers to the up-down direction of the vehicle, and the left-to-right direction refers to the width direction of the vehicle. The left-to-right direction coincides with the axial direction of the rack shaft 10 and the axial direction of the rack housing 14 described later. With respect to the axial direction of the rack shaft 10 and the rack housing 14, one axial side refers to the Figure 1 On the left side, for the axial direction of the rack shaft 10 and the rack accommodating portion 14, the other axial side refers to Figure 1 In this example, the up-down direction is consistent with the width direction of the rack shaft 10.

[0132] The electric power steering system 1 of this example includes a steering wheel 2 , a steering shaft 3 , two universal joints 4 a and 4 b , an intermediate shaft 5 , a steering gear unit 6 , and an electric assist device 7 .

[0133] The steering shaft 3 is rotatably supported inside a steering column (not shown) supported by the vehicle body. A steering wheel 2, which the driver uses to steer the vehicle, is attached to the rear end of the steering shaft 3. The front end of the steering shaft 3 is connected to a pinion shaft 8 constituting a steering gear unit 6 via a universal joint 4a, an intermediate shaft 5, another universal joint 4b, a hollow shaft 33 (described later), and a torsion bar 34. Thus, the rotational motion of the steering wheel 2 is transmitted to the pinion shaft 8.

[0134] The steering gear unit 6 includes a housing 9, a pinion shaft 8, a rack shaft 10, and a rack guide 11. The steering gear unit 6 converts the rotational motion of the pinion shaft 8 into axial linear motion of the rack shaft 10. This pushes and pulls a pair of tie rods 12 connected to the axial ends of the rack shaft 10, imparting a steering angle to a pair of left and right steering wheels 13.

[0135] The housing 9 includes a rack housing portion 14 , a pinion housing portion 15 , a guide housing portion 16 , and a gear housing portion 17 .

[0136] The rack housing portion 14 has a cylindrical shape with both ends opened in the axial direction, and has a Figure 1 Mounting portions 18 are provided at two axially separated locations on the near front side of the vehicle body. The rack housing 14 is secured to the vehicle body with its axial direction oriented left and right and approximately parallel, preferably parallel, to the road surface using fixing members such as bolts and studs inserted through the mounting portions 18.

[0137] The pinion accommodating portion 15 has a cylindrical shape and is arranged in a manner extending in the vertical direction on the front side of the rack accommodating portion 14 and on one axial side of the rack accommodating portion 14. The central axis of the pinion accommodating portion 15 and the central axis of the rack accommodating portion 14 are in a twisted positional relationship. In addition, in this example, when viewed from the front-to-back direction, the central axis of the pinion accommodating portion 15 and the central axis of the rack accommodating portion 14 are obliquely intersected. In other words, the central axis of the pinion accommodating portion 15 and the central axis of the rack accommodating portion 14 form an acute angle. The internal space of the pinion accommodating portion 15 is connected to the internal space of the rack accommodating portion 14.

[0138] Guide housing 16 is cylindrical and is located behind rack housing 14 at the same position as pinion housing 15 in the axial direction of rack housing 14, i.e., on one side of the axial direction, with the axial direction facing the front-rear direction. The interior of guide housing 16 communicates with the interior of rack housing 14.

[0139] The gear housing portion 17 is a portion that houses a worm speed reducer 19 of the electric assist device 7 described later, and includes a wheel housing portion 20 and a worm housing portion 21 .

[0140] The wheel accommodating portion 20 has a cylindrical shape and is provided on the lower side of the pinion accommodating portion 15 coaxially with the pinion accommodating portion 15 .

[0141] The worm housing 21 is cylindrical and extends horizontally behind the wheel housing 20. The central axis of the worm housing 21 is in a twisted relationship with the central axis of the wheel housing 20. The interior of the worm housing 21 communicates with the interior of the wheel housing 20.

[0142] The pinion shaft 8 has a plurality of pinion teeth 22 on its outer circumference in the axially intermediate portion, and cylindrical surface portions 23a and 23b on its outer circumference on either side of the portion having the pinion teeth 22. The pinion shaft 8 is coaxially and rotatably supported within the pinion accommodating portion 15 via two bearings 24a and 24b. Specifically, the inner rings of the two bearings 24a and 24b are externally fitted and fixed to the cylindrical surface portions 23a and 23b of the pinion shaft 8, while their outer rings are internally fitted and fixed to the inner circumference of the pinion accommodating portion 15, thereby rotatably supporting the pinion shaft 8 within the pinion accommodating portion 15.

[0143] The axial direction of the pinion shaft 8 is Figures 2 to 6 In this example, with respect to the pinion shaft 8 and the worm wheel 29 fixedly coupled to the pinion shaft 8, the axial side refers to the end side of the pinion shaft 8, i.e. Figure 2-Figure 6 The lower side of the axial direction refers to the base end side of the pinion shaft 8. Figure 2-Figure 6 upper side.

[0144] The pinion shaft 8 includes a serrated shaft portion 25, a guide shaft portion 26, and an auxiliary guide shaft portion 27 in the axially opposite portion of the cylindrical surface portion 23a, 23b. In other words, the pinion shaft 8 includes the serrated shaft portion 25 and the guide shaft portion 26 disposed adjacent to one axial side of the serrated shaft portion 25. Furthermore, in this example, the pinion shaft 8 includes the auxiliary guide shaft portion 27 in the axially adjacent portion of the guide shaft portion 26.

[0145] The serrated shaft portion 25 has an outer serrated portion 28 on its outer peripheral surface. As will be described later, the serrated shaft portion 25 is fixed to the center hole 30 of the worm wheel 29 constituting the worm reducer 19 by interference fit. That is, the serrated shaft portion 25 is pressed into the center hole 30. Therefore, in the case of Figure 5 Before the serrated shaft portion 25 is pressed into the center hole 30 of the worm wheel 29 as shown, the serrated shaft portion 25 has an inner diameter d that is larger than the inner diameter d of the center hole 30.30 The slightly larger outer diameter, that is, the tip diameter D of the serrations constituting the outer serration portion 28 25 Specifically, the outer diameter D of the serrated shaft portion 25 is 25 than the inner diameter d of the center hole 30 of the worm wheel 29 30 The content is preferably 0.5% to 1.5%, and preferably 0.7% to 1.0%.

[0146] The axial length L of the serrated shaft portion 25 25 There is no particular limitation as long as the connection strength of the pinion shaft 8 to the worm wheel 29 can be ensured. However, the axial length L of the serrated shaft portion 25 is 25 It is preferable that the axial length of the serrated shaft portion 25 is equal to or greater than the axial length of the axial middle portion of the center hole 30 of the worm wheel 29, excluding the chamfered portions provided at the axial ends on both sides, specifically, the first chamfered portion 43 and the second chamfered portion 44 provided at the end on the other axial side, and the chamfered portion provided at the end on one axial side. However, as long as the connection strength of the pinion shaft 8 to the worm wheel 29 can be ensured, the axial length L of the serrated shaft portion 25 may be greater than or equal to the axial length L of the serrated shaft portion 25. 25 It may also be shorter than the axial length of the axial middle portion of the center hole 30. In the example shown in the figure, the axial length L of the serrated shaft portion 25 is 25 The axial length L30 of the serrated shaft portion 25 is about 0.8 times the total axial length L30 of the center hole 30, including the chamfered portions provided at both ends in the axial direction. 25 The upper limit of the length L of the serrated shaft portion 25 is limited. 25 The total axial length L of the center hole 30 is preferably 30 1.2 times or less, more preferably 1 time or less.

[0147] As will be described later, the guide shaft portion 26 functions as a guide for axially aligning the pinion shaft 8 with respect to the worm wheel 29 when the pinion shaft 8 is inserted into the center hole 30 of the worm wheel 29 with one axial end thereof as the leading end.

[0148] The guide shaft 26 has an outer serration 31 on its outer circumference and has an outer diameter capable of being fitted into the center hole 30 of the worm wheel 29 with clearance, that is, a tooth tip diameter D of the serrations constituting the outer serration 31. 26 In other words, the guide shaft portion 26 has an outer diameter D that allows for relative axial displacement in the center hole 30 of the worm wheel 29 and is inserted into the center hole 30 of the worm wheel 29 by inserting and fitting to minimize radial play. 26 Specifically, the outer diameter D of the guide shaft portion 26 is 26 than the inner diameter d of the center hole 30 of the worm wheel 29 30The content is 0.01% to 0.5%, preferably 0.01% to 0.2%.

[0149] The axial length L of the guide shaft portion 26 26 There are no particular restrictions as long as the pinion shaft 8 can be aligned with the worm wheel 29 when the pinion shaft 8 is inserted into the center hole 30 of the worm wheel 29. However, the axial length L of the center hole 30 of the worm wheel 29 is preferably set to 30 In the example shown in the figure, the axial length L of the guide shaft portion 26 is 0.3 times or more, and more preferably 0.5 times or more. 26 is the axial length L of the center hole 30 of the worm wheel 29 30 In addition, in order to prevent the axial length of the pinion shaft 8 from being unnecessarily lengthened, the axial length L of the guide shaft portion 26 is 26 Specifically, for example, the axial length L of the guide shaft portion 26 is limited. 26 The axial length L of the central hole 30 is preferably 30 1.0 times or less, more preferably 0.8 times or less.

[0150] As will be described later, the auxiliary guide shaft portion 27 functions as an auxiliary guide for substantially aligning the pinion shaft 8 with respect to the worm wheel 29 when the pinion shaft 8 is inserted into the center hole 30 of the worm wheel 29 with one axial end as the leading end. The outer peripheral surface of the auxiliary guide shaft portion 27 is formed by excluding the chamfered portion provided at the end portion on one axial side and having an axial outer diameter D of 1. 27 Unchanging cylindrical surface composition.

[0151] The auxiliary guide shaft portion 27 has a cylindrical outer peripheral surface centered on the central axis of the pinion shaft 8 and has an outer diameter D 1 1 1 larger than that of the guide shaft portion 26 . 26 Small outer diameter D 27 Specifically, the outer diameter D of the auxiliary guide shaft 27 is 27 than the outer diameter D of the guide shaft portion 26 26 The auxiliary guide shaft portion 27 has a chamfered portion 32 on the outer peripheral surface of the end portion on one axial side. In this example, the chamfered portion 32 is formed of a C-chamfer having a linear generatrix shape.

[0152] The axial length L of the auxiliary guide shaft portion 27 27 There are no particular restrictions as long as the pinion shaft 8 can be substantially aligned with the worm wheel 29 when the pinion shaft 8 is inserted into the center hole 30 of the worm wheel 29. However, the axial length L of the center hole 30 of the worm wheel 29 is preferably set to 30 In the example shown in the figure, the axial length L of the auxiliary guide shaft portion 27 is27 is the axial length L of the center hole 30 of the worm wheel 29 30 In addition, in order to prevent the axial length of the pinion shaft 8 from being unnecessarily lengthened, the axial length L of the auxiliary guide shaft portion 27 is about 0.6 times. 27 Specifically, for example, the axial length L of the auxiliary guide shaft portion 27 is limited. 27 The axial length L of the central hole 30 is preferably 30 2.0 times or less, more preferably 1.5 times or less.

[0153] Furthermore, the outer circumferential surface of the serrated shaft portion 25, i.e., the tooth top surfaces of the serrations forming the external serrated portion 28, and the outer circumferential surface of the guide shaft portion 26, i.e., the tooth top surfaces of the serrations forming the external serrated portion 31, are connected via an inclined surface whose outer diameters gradually decrease toward one side in the axial direction. Furthermore, the outer circumferential surface of the guide shaft portion 26 and the outer circumferential surface of the auxiliary guide shaft portion 27 are connected via an inclined surface whose outer diameters gradually decrease toward one side in the axial direction.

[0154] When manufacturing the pinion shaft 8 , the serrated shaft portion 25 including the external serration portion 28 and the guide shaft portion 26 including the external serration portion 31 can be formed, for example, as follows.

[0155] First, the outer peripheral surface of the portion of the pinion shaft 8 where the serrated shaft portion 25 and the guide shaft portion 26 are to be formed is formed by rolling, broaching, etc. into a raw material external serration whose outer diameter, i.e., the tooth tip diameter, does not change in the axial direction.

[0156] Next, the tooth tops of the raw material external serrations are ground. At this time, the amount of grinding of the tooth tops on one axial side of the raw material external serrations is greater than the amount of grinding of the tooth tops on the other axial side, thereby processing the raw material external serrations into the external serrations 28 of the serration shaft 25 and the external serrations 31 of the guide shaft 26. For example, the raw material external serrations are ground using a forming grinding wheel having a stepped cylindrical outer peripheral surface in which the outer diameter of the axial side portion is larger than the outer diameter of the axial side portion. When the external serrations 28 and the external serrations 31 are machined in this way, the external serrations 28 and the external serrations 31 differ only in the tooth top circle diameter, while the tooth bottom circle diameter, tooth width, and phase are the same as each other.

[0157] The other axial end of the pinion shaft 8 is connected to one axial end of the hollow shaft 33 via a torsion bar 34. The other axial end of the hollow shaft 33 is connected to the front end of the steering shaft 3 via a universal joint 4a, an intermediate shaft 5, and another universal joint 4b. Therefore, the pinion shaft 8 rotates in response to the operation of the steering wheel 2.

[0158] The rack shaft 10 is cylindrical and has a plurality of rack teeth 35 on a portion of its outer circumference that mesh with the pinion teeth 22 of the pinion shaft 8. In this example, the rack shaft 10 has the plurality of rack teeth 35 on the front side of its axially intermediate portion.

[0159] The rack shaft 10 is supported coaxially with the rack housing 14 by rack bushings (not shown), such as sliding bearings, for axial reciprocation, with its axial direction oriented horizontally and its axial ends protruding from the left and right openings of the rack housing 14. The rack shaft 10 is connected to the tie rods 12 via spherical joints at both axial ends.

[0160] The rack guide 11 has a function of pressing the rack shaft 10 toward the pinion shaft 8. The rack guide 11 includes a packing 36 and an elastic member 37.

[0161] The liner 36 has a generally cylindrical shape and is positioned inside the guide housing 16 so as to be movable relative to the rack shaft 10. The liner 36 has a concave cylindrical pressing surface 36a on the surface facing the rear side of the rack shaft 10. This pressing surface 36a is shaped to guide the rear side of the rack shaft 10 in the axial direction of the rack shaft 10. The pressing surface 36a is made of a synthetic resin with excellent sliding properties. The elastic member 37 is sandwiched in an elastically compressed state between the liner 36 and a cover 38 that covers the opening of the guide housing 16. Thus, the elastic member 37 presses the liner 36 toward the rack shaft 10. In the illustrated example, the elastic member 37 is formed of a torsion coil spring, but may alternatively be formed of a disc spring or the like.

[0162] The rack guide 11 presses the rack shaft 10 toward the pinion shaft 8 to reduce the backlash at the meshing portion between the pinion teeth 22 and the rack teeth 35 , thereby preventing abnormal sound from being generated at the meshing portion between the pinion teeth 22 and the rack teeth 35 .

[0163] The electric assist device 7 applies auxiliary power to the pinion shaft 8, reducing the steering effort required by the driver to operate the steering wheel 2. The electric assist device 7 includes a worm reducer 19, an electric motor 39, and a torque sensor 40. Specifically, the electric assist device 7 amplifies the torque of the output shaft of the electric motor 39. In other words, it decelerates the rotation of the output shaft and transmits it to the pinion shaft 8.

[0164] The worm speed reducer 19 includes a worm wheel 29 and a worm 41 .

[0165] The worm wheel 29, which functions as a gear, has a plurality of teeth 42 on its outer circumference, corresponding to a plurality of teeth, and a center hole 30 extending axially through its center. The center hole 30 has a first chamfered portion 43 on the inner circumferential surface of the end portion on the other axial side, and a second chamfered portion 44 on the inner circumferential portion adjacent to one axial side of the first chamfered portion 43. Both the first chamfered portion 43 and the second chamfered portion 44 are C-chamfered with a linear generatrix shape, with the chamfer angle θ2 of the second chamfered portion 44 being smaller than the chamfer angle θ1 of the first chamfered portion 43. The chamfer angle θ1 of the first chamfered portion 43 refers to the inclination angle of the generatrix of the first chamfered portion 43 relative to the central axis of the worm wheel 29, while the chamfer angle θ2 of the second chamfered portion 44 refers to the inclination angle of the generatrix of the second chamfered portion 44 relative to the central axis of the worm wheel 29.

[0166] The chamfer angle θ1 of the first chamfered portion 43 is preferably 10.0 degrees to 20.0 degrees, more preferably 14.0 degrees to 16.0 degrees. The chamfer angle θ2 of the second chamfered portion 44 is smaller than the chamfer angle θ1 of the first chamfered portion 43 and is preferably 2.5 degrees to 7.5 degrees, more preferably 4.0 degrees to 6.0 degrees.

[0167] In this example, the inner peripheral surface of the center hole 30 is composed of a cylindrical surface whose inner diameter does not change in the axial direction except for the first chamfered portion 43 and the second chamfered portion 44 provided at the other axial end and the chamfered portion provided at the one axial end.

[0168] The worm wheel 29 is secured to the pinion shaft 8 by press-fitting the serrated shaft portion 25 of the pinion shaft 8 into the center hole 30. It is then positioned inside the wheel housing 20 of the gear housing 17. In this embodiment, when the pinion shaft 8 and worm wheel 29 are combined to form the assembly 50, the portion of the pinion shaft 8 from the axial middle of the guide shaft portion 26 to the axially opposite end of the serrated shaft portion 25 engages with the center hole 30 of the worm wheel 29. Specifically, the axially opposite portion of the guide shaft portion 26 engages with the axially opposite end of the center hole 30 with minimal play, while the serrated shaft portion 25 is press-fitted into the axially opposite portion of the center hole 30. Furthermore, the axially opposite portion of the guide shaft portion 26 is located axially further to the side than the axially opposite opening of the center hole 30. The serrated shaft portion 25 bites into the inner circumferential surface of the center hole 30.

[0169] In this example, the worm wheel 29 includes a metal hub 45 and a gear portion 46 made of synthetic resin.

[0170] The hub 45 includes an inner diameter side cylinder portion 47 having a center hole 30 at its center, an outer diameter side cylinder portion 48 arranged around the inner diameter side cylinder portion 47 and coaxially with the inner diameter side cylinder portion 47 , and a hollow circular plate-shaped connecting portion 49 connecting the inner diameter side cylinder portion 47 and the outer diameter side cylinder portion 48 .

[0171] The gear portion 46 has a plurality of gear teeth 42 at its radially outer end and is fixedly coupled to the hub 45 so as to cover the radially outer end of the hub 45. Specifically, the gear portion 46 is formed by injection molding, specifically insert molding, and embeds the radially outer ends of the outer diameter side cylindrical portion 48 and the connecting portion 49 of the hub 45.

[0172] The worm 41 has threaded worm teeth on its outer peripheral surface that mesh with the gear teeth 42 of the worm wheel 29 , and is supported inside the worm accommodating portion 21 so as to be slightly oscillatory about its base end.

[0173] The electric motor 39 has an output shaft connected to a base end portion of the worm 41 in a torque-transmittable manner, and is supported and fixed to the worm accommodating portion 21 .

[0174] The torque sensor 40 is disposed around the hollow shaft 33 and the pinion shaft 8 to detect the rotational direction of the hollow shaft 33 and the pinion shaft 8, as well as the torque transmitted between the hollow shaft 33 and the pinion shaft 8. The torque sensor 40 outputs a signal corresponding to the rotational direction of the hollow shaft 33 and the pinion shaft 8, as well as the torque transmitted between the hollow shaft 33 and the pinion shaft 8, to the electronic control unit of the electric motor 39. In this example, a sensor that detects torque by detecting a phase difference using an encoder is used as the torque sensor 40. However, a non-contact torque sensor utilizing the magnetostrictive effect, for example, can also be used as the torque sensor 40. In this case, the hollow shaft 33 and the torsion bar 34 can be omitted, and the base end of the pinion shaft 8 can be directly connected to the universal joint 4b.

[0175] The electric assist device 7 controls the driving of the electric motor 39 based on the output signal of the torque sensor 40. As a result, the driving torque generated by the electric motor 39 is transmitted to the pinion shaft 8 as a steering assist force via the worm reducer 19. As a result, the steering effort required by the driver to operate the steering wheel 2 is reduced.

[0176] According to this example, it is possible to ensure good coaxiality between the pinion shaft 8 and the worm wheel 29. The reason for this will be described below.

[0177] When the pinion shaft 8 is combined with the worm gear 29, as shown in FIG. Figure 5As shown by the middle arrow, the pinion shaft 8 is inserted into the center hole 30 of the worm wheel 29 with one axial end as the leading end. In the initial stages of inserting the pinion shaft 8 into the center hole 30, the auxiliary guide shaft portion 27 of the pinion shaft 8 is fitted into the other axial end of the center hole 30 with minimal play. This allows the center axis of the pinion shaft 8 to be roughly aligned with the center axis of the worm wheel 29, achieving rough axial alignment between the pinion shaft 8 and the worm wheel 29.

[0178] After the auxiliary guide shaft portion 27 is engaged with the center hole 30, the pinion shaft 8 is further displaced axially toward one side relative to the worm wheel 29, increasing the amount of insertion of the pinion shaft 8 into the center hole 30. This allows the guide shaft portion 26 of the pinion shaft 8 to be fitted into the axially opposite portion of the center hole 30 with a clearance fit that eliminates play, i.e., with less play than the play between the auxiliary guide shaft portion 27 and the center hole 30. This allows the center axes of the pinion shaft 8 and the worm wheel 29 to be aligned with high precision, achieving high-precision axial alignment between the pinion shaft 8 and the worm wheel 29.

[0179] After the pinion shaft 8 is aligned with the worm wheel 29 by fitting the guide shaft portion 26 into the center hole 30, the pinion shaft 8 is further displaced axially to one side relative to the worm wheel 29 to increase the amount of insertion of the pinion shaft 8 into the center hole 30. As a result, the serrated shaft portion 25 of the pinion shaft 8 is pressed into the center hole 30, thereby combining the pinion shaft 8 with the worm wheel 29 to obtain the following: Figure 3 The assembly 50 is shown.

[0180] As described above, by fitting the guide shaft portion 26 of the pinion shaft 8 into the center hole 30 of the worm wheel 29 with no play, the axial alignment of the worm wheel 29 and the pinion shaft 8 can be achieved with high precision. This allows for highly precise alignment of the worm wheel 29 and the pinion shaft 8 before the serrated shaft portion 25 can be press-fitted into the center hole 30. Furthermore, with the guide shaft portion 26 fitted into the center hole 30, even if the center axis of the pinion shaft 8 is slightly tilted or offset relative to the center axis of the worm wheel 29, the slight clearance between the inner circumferential surface of the center hole 30 and the outer circumferential surface of the guide shaft portion 26 (the tooth top surfaces of the external serrations 31) can correct this tilt and / or offset. This prevents the pinion shaft 8 and worm wheel 29 from being coupled and fixed while the center axis of the pinion shaft 8 is tilted or offset relative to the center axis of the worm wheel 29. Consequently, maintaining coaxiality between the pinion shaft 8 and the worm wheel 29 is facilitated.

[0181] In this example, the guide shaft portion 26 of the pinion shaft 8 for axial alignment with the worm wheel 29 is fitted into the center hole 30 of the worm wheel 29 with clearance. Therefore, there is no need to strictly limit the dimensional tolerance of the center hole 30, the dimensional tolerance of the serrated shaft portion 25, and the dimensional tolerance of the guide shaft portion 26 as in the conventional structure. Figure 16 As shown, the cylindrical shaft portion 117 for axial alignment with the worm wheel 113 and the serrated shaft portion 118 for ensuring coupling strength with the worm wheel 113 are both press-fitted into the center hole 116 of the worm wheel 113. This can suppress an increase in the manufacturing cost of the assembly 50 of the pinion shaft 8 and the worm wheel 29.

[0182] In this example, the outer circumference of the portion where the serrated shaft portion 25 and the guide shaft portion 26 are to be formed is formed with a raw material outer serration whose outer diameter does not change in the axial direction. Then, a grinding wheel having a stepped cylindrical outer circumference is used for grinding, thereby simultaneously finishing the outer circumference of the serrated shaft portion 25 and the outer circumference of the guide shaft portion 26. Therefore, the outer diameter D of the serrated shaft portion 25 can be controlled with high precision. 25 The outer diameter D of the guide shaft 26 26 The difference between the guide shaft portion 26 and the center hole 30 can achieve both high-precision axis alignment by fitting the guide shaft portion 26 into the center hole 30 with clearance and ensuring coupling strength by press-fitting the serrated shaft portion 25 into the center hole 30 at a high level.

[0183] However, it is also possible to form raw external serrations with a constant outer diameter in the axial direction on the outer circumferential surface of the portion forming the serrated shaft portion 25 and the guide shaft portion 26, and then grind the tooth tips of the raw external serrations on one axial side and the tooth tips on the other axial side separately or at different times using different grinding wheels. In this case, the feed rate of the grinding wheel grinding the tooth tips on the one axial side portion where the guide shaft portion 26 is formed is set to be greater than the feed rate of the grinding wheel grinding the tooth tips on the other axial side portion where the serrated shaft portion 25 is formed. Alternatively, the grinding process may not be performed on the other axial side portion where the serrated shaft portion 25 is formed.

[0184] In this example, a first chamfered portion 43 is provided on the inner circumferential surface of the end portion on the other axial side of the center hole 30, and a second chamfered portion 44 is provided on the inner circumferential surface adjacent to one axial side of the first chamfered portion 43. The chamfer angle θ2 of the second chamfered portion 44 is smaller than the chamfer angle θ1 of the first chamfered portion 43. Therefore, with the guide shaft portion 26 loosely fitted into the center hole 30, even if the center axis of the pinion shaft 8 is slightly tilted or offset relative to the center axis of the worm wheel 29, the tilt and / or offset of the center axis of the pinion shaft 8 relative to the center axis of the worm wheel 29 can be corrected when the serrated shaft portion 25 is press-fitted into the center hole 30. That is, as the pinion shaft 8 is further displaced axially toward the side relative to the worm wheel 29 from the state in which the guide shaft portion 26 is fitted into the center hole 30 with clearance, the outer peripheral surface of the axial end portion of the serrated shaft portion 25 is guided by the first chamfered portion 43 and the second chamfered portion 44, thereby correcting the inclination and / or offset of the center axis of the pinion shaft 8 relative to the center axis of the worm wheel 29. This also facilitates maintaining the coaxiality of the pinion shaft 8 and the worm wheel 29.

[0185] In this example, an auxiliary guide shaft portion 27 is provided at a portion adjacent to one axial side of the guide shaft portion 26. The auxiliary guide shaft portion 27 has an outer diameter D that is larger than that of the guide shaft portion 26. 26 Small outer diameter D 27 In the initial stages of the coupling operation between the pinion shaft 8 and the worm wheel 29, that is, before the guide shaft portion 26 and the center hole 30 are fitted with a clearance fit to achieve high-precision alignment of the worm wheel 29 and the pinion shaft 8, the auxiliary guide shaft portion 27 is fitted with the center hole 30 with a clearance fit with minimal play, thereby enabling rough alignment of the worm wheel 29 and the pinion shaft 8. This facilitates the coupling operation between the pinion shaft 8 and the worm wheel 29.

[0186] That is, the radial clearance between the outer circumference of the guide shaft portion 26 and the inner circumference of the center hole 30 is extremely small. Therefore, in a structure without an auxiliary guide shaft portion, the guide shaft portion cannot be inserted into the center hole unless the center axis of the pinion shaft and the worm wheel are precisely aligned before the pinion shaft and worm wheel are joined. In contrast, in this example, by fitting the auxiliary guide shaft portion 27 into the center hole 30 with a small clearance, the guide shaft portion 26 can be inserted into the center hole 30 after the worm wheel 29 and pinion shaft 8 are roughly aligned. In short, there is no need to precisely align the center axis of the pinion shaft 8 with the center axis of the worm wheel 29 before the pinion shaft 8 and worm wheel 29 are joined. This facilitates the joining of the pinion shaft 8 and worm wheel 29.

[0187] In this example, the outer peripheral surface of the auxiliary guide shaft portion 27 is formed by the outer diameter D 27 The outer circumference of the auxiliary guide shaft portion is formed of a cylindrical surface that does not change in the axial direction. However, the outer circumference of the auxiliary guide shaft portion may also be formed of a frustum of a cone whose outer diameter increases toward the other axial side. In this case, the outer diameter of the end portion of the auxiliary guide shaft portion on the other axial side is smaller than the outer diameter of the guide shaft portion.

[0188] Alternatively, the auxiliary guide shaft portion may be omitted when the center axis of the pinion shaft and the center axis of the worm wheel can be aligned with each other with high accuracy before the operation of coupling the pinion shaft and the worm wheel begins.

[0189] In this example, the axially oriented portion of the guide shaft portion 26 of the pinion shaft 8, which is used for axial alignment with the worm wheel 29, is positioned axially toward the center hole 30. In other words, the axially oriented portion of the guide shaft portion 26 protrudes axially toward the center hole 30 from the axially oriented opening of the center hole 30. This facilitates ensuring the required engagement length between the serrated shaft portion 25 and the center hole 30, which is required to ensure sufficient bonding strength with the worm wheel 29. Consequently, sufficient bonding strength between the pinion shaft 8 and the worm wheel 29 can be maintained.

[0190] In contrast, in Figure 15 as well as Figure 16 In the conventional structure shown, not only the serrated shaft portion 118 of the pinion shaft 106, which ensures the bonding strength with the worm wheel 113, but also the entire axial length of the cylindrical shaft portion 117, which is used for axial alignment with the worm wheel 113, is engaged with the center hole 116 of the worm wheel 113. Therefore, it is difficult to ensure the engagement length of the serrated shaft portion 118 with the center hole 116, and there is a possibility that the bonding strength between the pinion shaft 106 and the worm wheel 113 cannot be sufficiently ensured.

[0191] On the other hand, in the existing structure, in order to ensure the coupling strength between the pinion shaft 106 and the worm wheel 113, the axial length of the cylindrical shaft portion 117 is shortened while the axial length of the serrated shaft portion 118 is fully ensured. During the coupling operation between the pinion shaft 106 and the worm wheel 113, it may be impossible to fully align the pinion shaft 106 with respect to the worm wheel 113.

[0192] [Second example]

[0193] Figure 7 and Figure 8A second example of an embodiment of the present invention is shown. In this example, the serrated shaft portion 25 of the pinion shaft 8a is engaged with the center hole 30 of the worm wheel 29 over its entire axial length. In other words, only the serrated shaft portion 25 of the pinion shaft 8a is press-fitted into the center hole 30 of the worm wheel 29, and the entire axial length of the guide shaft portion 26a is located axially to the side of the center hole 30 of the worm wheel 29. In other words, the entire axial length of the guide shaft portion 26a protrudes axially from the axial opening of the center hole 30.

[0194] In this example, the outer peripheral surface of the guide shaft portion 26a of the pinion shaft 8a is formed of a cylindrical surface whose outer diameter does not change in the axial direction, except for the connection portion with the serrated shaft portion 25 and the connection portion with the auxiliary guide shaft portion 27, that is, the end portions on both axial sides.

[0195] In this example, since the serrated shaft portion 25 of the pinion shaft 8a is fully axially engaged with the center hole 30 of the worm wheel 29, the connection strength between the pinion shaft 8a and the worm wheel 29 can be improved compared to the first example. The other structures and effects are the same as those of the first example.

[0196] [Third example]

[0197] Figures 9 to 11 The third embodiment of the present invention is shown. In this embodiment, the center hole 30a of the worm wheel 29a has a small diameter portion 51 on one axial side and a large diameter portion 52 on the other axial side. The large diameter portion 52 has an inner diameter d smaller than that of the small diameter portion 51. 51 Large inner diameter d 52 The small diameter portion 51 and the large diameter portion 52 are connected by a truncated cone-shaped inclined surface portion 53 that is inclined in a direction in which the inner diameter becomes smaller as it moves toward one axial side. That is, the center hole 30a is composed of a stepped hole. The inner diameter d of the small diameter portion 51 is 51 and the inner diameter d of the large diameter portion 52 52 There is no particular limitation, and each can be set to any size.

[0198] Axial length L of the small diameter portion 51 51 and the axial length L of the large diameter portion 52 52 There are no particular restrictions as long as the connection strength between the pinion shaft 8b and the worm wheel 29 can be ensured, but it is preferable to set the axial length L of the large diameter portion 52 to 52 The axial length L of the small diameter portion 51 is 51 The ratio of the total weight of the catalyst is 0.3 times or more and 1.0 times or less, more preferably 0.5 times or more and 0.8 times or less.

[0199] The pinion shaft 8b includes a serrated shaft portion 25, a guide shaft portion 26b, and an auxiliary guide shaft portion 27 in this order from the other axial side, on a portion of the pair of cylindrical surface portions 23a, 23b that is closer to the axial side than the cylindrical surface portion 23a on the axial side.

[0200] The serrated shaft portion 25 has an inner diameter d that is larger than the inner diameter d of the large diameter portion 52 in the center hole 30a of the worm wheel 29a. 52 Slightly larger outer diameter D 25 Specifically, before the serrated shaft portion 25 is pressed into the large diameter portion 52, the outer diameter D of the serrated shaft portion 25 is 25 than the inner diameter d of the large diameter portion 52 52 The content is preferably 0.5% to 1.5%, and preferably 0.7% to 1.0%.

[0201] In this example, the outer peripheral surface of the guide shaft portion 26b is formed of a cylindrical surface whose outer diameter does not change in the axial direction except for the connection portion with the serrated shaft portion 25 and the connection portion with the auxiliary guide shaft portion 27, that is, the end portions on both axial sides.

[0202] The guide shaft portion 26b has an outer diameter that can be fitted into the small diameter portion 51 in the center hole 30a of the worm wheel 29a with clearance. That is, the guide shaft portion 26b has an inner diameter d that is larger than the inner diameter d of the small diameter portion 51. 51 Slightly smaller outer diameter D 26 Specifically, the outer diameter D of the guide shaft portion 26b is 26 Than the inner diameter d of the small diameter portion 51 51 The content is 0.01% to 0.5%, preferably 0.01% to 0.2%.

[0203] In addition, the axial length L of the guide shaft portion 26b is 26 The axial length L of the large diameter portion 52 of the center hole 30a is greater than 52 Specifically, the axial length L of the guide shaft portion 26b is 26 is the axial length L of the large diameter portion 52 52 Thus, during the operation of coupling the pinion shaft 8b and the worm wheel 29a described later, the serrated shaft portion 25 is prevented from being pressed into the large diameter portion 52 before the guide shaft portion 26b and the small diameter portion 51 are fitted with clearance without play.

[0204] When the pinion shaft 8b is combined with the worm gear 29a, as shown in FIG. Figure 11 As indicated by the hollow arrow, the pinion shaft 8b is inserted into the center hole 30a of the worm wheel 29a, with one axial end facing the front end. Furthermore, the auxiliary guide shaft portion 27 and the small-diameter portion 51 are fitted together with minimal play. This allows the central axis of the pinion shaft 8b to be roughly aligned with the central axis of the worm wheel 29a, achieving rough alignment of the pinion shaft 8b and the worm wheel 29a.

[0205] From this position, the pinion shaft 8b is further displaced axially to one side relative to the worm wheel 29a, so that the guide shaft portion 26b is fitted with the small-diameter portion 51 with clearance and without play. This allows the central axis of the pinion shaft 8b and the central axis of the worm wheel 29b to be aligned with high precision, achieving high-precision axis alignment between the pinion shaft 8b and the worm wheel 29a.

[0206] With the guide shaft 26b fitted into the small diameter portion 51, the pinion shaft 8b is further displaced axially toward one side relative to the worm wheel 29a, and the serrated shaft 25 is pressed into the large diameter portion 52, thereby fastening the pinion shaft 8b to the worm wheel 29a.

[0207] According to this embodiment, the inner diameter of the center hole 30a of the worm wheel 29a can be made different at the small diameter portion 51 for axial alignment with the pinion shaft 8b and the large diameter portion 52 for ensuring the connection strength with respect to the pinion shaft 8b. 51 There is no need to consider the outer diameter D of the serrated shaft 25 25 The relationship between the outer diameter D of the guide shaft portion 26b can be 26 In addition, the inner diameter D of the large diameter portion 52 is 52 There is no need to consider the outer diameter D of the guide shaft portion 26b. 26 The relationship between the outer diameter D of the serrated shaft 25 can be 25 The structure and effects of other parts are the same as those of the first example of the embodiment.

[0208] [Fourth example]

[0209] Figures 12 to 14 A fourth example of an embodiment of the present invention is shown. In this example, the center hole 30a provided in the worm wheel 29a is formed as a stepped hole having a small diameter portion 51 on one axial side and a large diameter portion 52 on the other axial side.

[0210] Furthermore, the pinion shaft 8c includes a serrated shaft portion 25 and a guide shaft portion 26c, in order from the other axial side, on the portion of the pair of cylindrical surface portions 23a and 23b located axially closer to the cylindrical surface portion 23a on the axial side. In other words, the pinion shaft 8c of this example omits the auxiliary guide shaft portion 27 compared to the pinion shaft 8b of the third example. This minimizes the amount of protrusion of the pinion shaft 8c from the axial opening of the center hole 30a of the worm wheel 29a. The remaining structure and effects are identical to those of the first and third examples.

[0211] The embodiments of the present invention have been described above, but the present invention is not limited thereto and can be modified appropriately without departing from the technical concept of the present invention. In addition, the first to fourth examples can be combined appropriately without causing any contradiction.

[0212] In the first to fourth examples, an assembly 50 has been described in which the worm wheel 29 is supported and fixed to a portion of the pinion shaft 8, 8a, 8b, or 8c located closer to the distal end than the axially intermediate portion having the plurality of pinion teeth 22. However, the present invention is also applicable to an assembly in which the worm wheel is supported and fixed to a portion of the pinion shaft located closer to the proximal end than the axially intermediate portion having the plurality of pinion teeth.

[0213] In the first to fourth examples, the present invention is applied to an assembly 50 of the pinion shafts 8, 8a, 8b, and 8c constituting the steering gear unit 6 and the worm wheels 29 and 29a constituting the worm reducer 19 of a pinion-assisted electric power steering device 1, and a method for assembling the assembly. However, the present invention can also be applied to an assembly of a rotating shaft and a worm wheel in electric power steering devices other than pinion-assisted electric power steering devices. That is, in the case of a column-assisted electric power steering device, the present invention can be applied to an assembly of a steering shaft and a worm wheel supported and fixed to the steering shaft, and a method for assembling the assembly. In the case of a dual-pinion electric power steering device, the present invention can be applied to an assembly of a second pinion shaft that is different from the pinion shaft connected to the steering shaft in a torque-transmitting manner and that meshes with the rack shaft, and a worm wheel supported and fixed to the second pinion shaft, and a method for assembling the assembly.

[0214] Furthermore, the present invention is not limited to electric power steering devices but can also be applied to assemblies of rotating shafts and worm wheels in worm reducers of various machines, and to methods for assembling the same. Furthermore, the present invention is not limited to assemblies of rotating shafts and worm wheels, and to methods for assembling the same, but can also be applied to assemblies of rotating shafts and gears other than worm gears, such as spur gears, and to methods for assembling the same.

Claims

1. An assembly of a gear and a rotating shaft, characterized in that: have: a gear having a plurality of teeth on an outer peripheral surface and a central hole extending therethrough in an axial direction at a central portion; and A rotating shaft comprising: a serrated shaft portion having an outer serration on an outer peripheral surface thereof and press-fitted into the center hole; and a guide shaft portion disposed adjacent to one axial side of the serrated shaft portion and having an outer diameter capable of being fitted into the center hole with clearance. The guide shaft portion has an outer diameter that allows for relative displacement in the axial direction in the center hole and can be fitted into the center hole by inserting and fitting with radial play suppressed as much as possible. The outer diameter of the guide shaft portion is smaller than the inner diameter of the center hole by 0.01% or more and 0.5% or less. The axial length of the guide shaft portion is not less than 0.3 times and not more than 1.0 times the axial length of the center hole.

2. The gear and rotating shaft assembly according to claim 1, wherein: The rotating shaft has an auxiliary guide shaft portion at a portion adjacent to an axial side of the guide shaft portion, and the auxiliary guide shaft portion has an outer diameter smaller than the outer diameter of the guide shaft portion. The outer diameter of the auxiliary guide shaft portion can be embedded in the center hole in a gap-fitting manner with little shaking, and the outer diameter of the auxiliary guide shaft portion is smaller than the outer diameter of the guide shaft portion by more than 2.0% and less than 10.0%.

3. The gear and rotating shaft assembly according to claim 2, wherein: The outer diameter of the auxiliary guide shaft portion decreases toward one side in the axial direction.

4. The gear and rotating shaft assembly according to claim 1, wherein: The center hole has a first chamfered portion at the end portion on the other axial side of the inner circumferential surface, and has a second chamfered portion at a portion of the inner circumferential surface adjacent to one axial side of the first chamfered portion, wherein the chamfer angle of the second chamfered portion is smaller than the chamfer angle of the first chamfered portion.

5. The assembly of a gear and a rotating shaft according to any one of claims 1 to 4, characterized in that: The center hole has a small diameter portion on one axial side and a large diameter portion on the other axial side with an inner diameter larger than that of the small diameter portion. The serrated shaft portion is pressed into the large diameter portion. The guide shaft portion is embedded in the small diameter portion in a gap-fitting manner. The outer diameter of the guide shaft portion is smaller than the inner diameter of the small diameter portion by 0.01% or more and 0.5% or less. The guide shaft portion has an axial length longer than that of the large diameter portion.

6. The gear and rotating shaft assembly according to any one of claims 1 to 4, characterized in that: The guide shaft portion has an external serration portion on an outer peripheral surface.

7. A method for assembling a gear and a rotating shaft assembly, characterized in that: A method for assembling a gear and a rotating shaft assembly according to any one of claims 1 to 6, comprising the following steps: The rotating shaft is inserted into the center hole of the gear from the other axial side of the center hole with the end on one axial side as the front end, and the guide shaft portion is embedded in the center hole in a clearance fit manner consisting of a plug-in fit that can perform axial relative displacement in the center hole and can suppress radial shaking as much as possible, thereby aligning the rotating shaft and the gear shaft, and then the serrated shaft portion is pressed into the center hole to combine the gear and the rotating shaft.

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