Gear housing and electric assist device

Abstract

To provide a gear housing that facilitates weight reduction while securing a necessary strength, and a power-assisting device provided with such a gear housing.SOLUTION: A wheel housing portion 25 of a gear housing 16 includes a wheel cylindrical portion 28, and a wheel bottom portion 29 in a circular wheel shape. As viewed from the one side of the wheel housing portion 25, when a virtual straight line L1 which has an origin that is the center of the wheel housing portion 25 and which is orthogonal to the center axis of a worm housing portion 26, and a reference virtual straight line Lb which passes through the center of the wheel housing portion 25, and which has an open angle relative to the first virtual straight line L1 being equal to or smaller than 60 degrees including 0 degree are taken into consideration, the thickness of at least a part of the wheel bottom portion 29 decreases from the near side of the worm housing portion 26 toward the distal side thereof with respect to the direction of the reference virtual straight line Lb.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an electric assist device and a gear housing that constitutes the electric assist device. [Background technology]

[0002] In the field of steering devices, electric power steering devices equipped with an electric assist device that applies an assist drive force to a steering force transmission path have become widespread in order to reduce the force required to rotate a steering wheel.

[0003] 20 to 22 show the structure described in International Publication No. 2019 / 087945 (Patent Document 1) as an example of a conventional structure of an electric power steering device.

[0004] The electric power steering device 100 includes an electric assist device 101 .

[0005] The electric assist device 101 includes an output shaft 105 connected to the front end of a steering shaft 102 via a torsion bar 104, and applies an assist driving force to the output shaft 105.

[0006] The electric assist device 101 further includes a gear housing 106 , a worm reducer 107 , an electric motor 108 , and a rear housing 109 .

[0007] The gear housing 106 is attached to the front end of the steering column 103 via a rear housing 109. The gear housing 106 integrally includes a wheel accommodating portion 110, a worm accommodating portion 111, and a hinge portion 112.

[0008] The wheel accommodating section 110 is a section that accommodates the worm wheel 113 of the worm reducer 107 inside. The wheel accommodating section 110 has a cylindrical wheel tubular section 114 that is arranged around the worm wheel 113, a circular wheel bottom section 115 that extends radially inward from the front end of the wheel tubular section 114, and a substantially cylindrical wheel fitting tubular section 116 that is connected to the radially inner end of the wheel bottom section 115.

[0009] The worm accommodating portion 111 is a portion that accommodates the worm 117 of the worm reducer 107 inside. The worm accommodating portion 111 is configured in a cylindrical shape with a bottom, an open end on one axial side (upper side in FIG. 21) and a closed end on the other axial side (lower side in FIG. 21). The worm accommodating portion 111 is connected to a circumferential portion of the radially outer end of the wheel accommodating portion 110. The central axis of the worm accommodating portion 111 is disposed in a twisted position with respect to the central axis of the wheel accommodating portion 110. The internal space of the worm accommodating portion 111 is in communication with the internal space of the wheel accommodating portion 110.

[0010] The hinge portion 112 is connected to a portion of the wheel accommodating portion 110 that is circumferentially separated from the worm accommodating portion 111 and protrudes forward from that portion. The hinge portion 112 has an insertion hole 119 through which a tilt shaft 120 attached to the vehicle body and extending in the left-right direction is inserted. The electric assist device 101, steering column 103, and steering shaft 102 are supported relative to the vehicle body so as to be able to pivot about the tilt shaft 120. This pivoting movement makes it possible to adjust the vertical position of the steering wheel. In the illustrated example, the gear housing 106 further includes reinforcing ribs 121a and 121b that span between the wheel accommodating portion 110 and the worm accommodating portion 111.

[0011] The worm 117 has worm teeth 122 on the outer peripheral surface of an axially intermediate portion thereof. The worm 117 is rotatably supported inside the worm accommodating portion 111 by ball bearings 123a and 123b.

[0012] The worm wheel 113 has wheel teeth 124 on its outer circumferential surface that mesh with the worm teeth 122. The worm wheel 113 is fitted and fixed to the outside of the axially intermediate portion of the output shaft 105. The output shaft 105 is rotatably supported by ball bearings 125a, 125b inside the gear housing 106 and rear housing 109.

[0013] The electric motor 108 is disposed in the axial direction of the worm housing portion 111. side The motor output shaft 118 of the electric motor 108 is connected to the base end of the worm 117 so as to be able to transmit torque. The electric motor 108 applies an assist driving force to the output shaft 105 via the worm reducer 107. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] International Publication No. 2019 / 087945 Summary of the Invention [Problem to be solved by the invention]

[0015] In the conventional electric assist device 101, when an assist driving force is generated, meshing reaction forces are applied to the worm 117 and the worm wheel 113 from the meshing portion between the worm teeth 122 and the wheel teeth 124. Furthermore, these meshing reaction forces are transmitted from the worm 117 to the worm housing portion 111 via ball bearings 123a and 123b, and from the worm wheel 113 to the wheel housing portion 110 via ball bearing 125a. As a result, forces acting in directions that move the worm housing portion 111 and the wheel housing portion 110 away from each other and forces acting in directions that twist the worm housing portion 111 and the wheel housing portion 110 relative to each other are applied to the gear housing 106.

[0016] Because such complex forces are applied to the gear housing 106, the wheel accommodating portion 110 requires higher strength (rigidity) particularly in areas close to the worm accommodating portion 111 compared to other areas. In other words, the wheel accommodating portion 110 requires different strengths at different locations. However, in the conventional structure, the thickness of the wheel bottom portion 115 (excluding the reinforcing ribs 121a and 121b) that constitutes the wheel accommodating portion 110 is uniform overall, so there is room for improvement in terms of reducing the weight of the gear housing 106.

[0017] An object of the present invention is to provide a gear housing that can be easily made lighter while ensuring the necessary strength, and an electric assist device that includes the gear housing. [Means for solving the problem]

[0018] A gear housing according to one aspect of the present invention includes a wheel receiving portion that receives a worm wheel therein, and a worm receiving portion that receives a worm therein.

[0019] The wheel accommodating portion has a wheel cylindrical portion arranged around the worm wheel, and a circular wheel bottom portion extending radially inward from one axial end of the wheel cylindrical portion.

[0020] The worm accommodating section is configured in a cylindrical shape with an open end on one axial side where the electric motor is fixed, and its central axis is positioned at a twisted position relative to the central axis of the wheel accommodating section, and is connected to a circumferential portion of the radially outer end of the wheel accommodating section.

[0021] When viewed from one axial side of the wheel accommodating portion, a first imaginary line that starts at the center of the wheel accommodating portion and is perpendicular to the central axis of the worm accommodating portion and a reference imaginary line that passes through the center of the wheel accommodating portion and has an angle between the first imaginary line and the reference imaginary line that is 75 degrees or less (including 0 degrees) (within the range of 0 degrees to 75 degrees), the thickness of at least a portion of the wheel bottom portion decreases from the side closer to the worm accommodating portion to the side farther from the axial side of the reference imaginary line. The angle between the first imaginary line and the reference imaginary line is preferably 60 degrees or less, more preferably 30 degrees or less, and even more preferably 15 degrees or less.

[0022] The gear housing according to one aspect of the present invention includes a hinge portion having an insertion hole for inserting a tilt shaft, the hinge portion being connected to a circumferential portion of the wheel accommodating portion and protruding from the circumferential portion toward the axial front side of the wheel accommodating portion.

[0023] In one aspect of the gear housing of the present invention, when viewed from one axial side of the wheel accommodating portion, considering a second imaginary line that has its origin at the center of the wheel accommodating portion and is perpendicular to the central axis of the insertion hole of the hinge portion, the opening angle between the first imaginary line and the second imaginary line is 135° or less, including 0° (within the range of 0° to 135°). The opening angle between the first imaginary line and the second imaginary line is preferably 90° or less, more preferably 60° or less, and even more preferably 30° or less.

[0024] In a gear housing according to one embodiment of the present invention, the reference imaginary line passes through a circumferential range sandwiched between the first imaginary line and the second imaginary line (including on the first imaginary line and / or on the second imaginary line).

[0025] In one embodiment of the gear housing of the present invention, the side surface on one axial side or the other axial side of at least a portion of the bottom portion for the wheel is configured as a flat surface or a curved surface that is inclined in a direction such that the thickness of at least a portion decreases as it moves from the side closer to the worm accommodating portion to the side farther from it in the direction of the reference virtual straight line.

[0026] In one embodiment of the gear housing of the present invention, at least a portion of a first connection portion, which is a connection portion of the worm accommodating portion to the wheel accommodating portion, and at least a portion of a second connection portion, which is a connection portion of the hinge portion to the wheel accommodating portion, are arranged at the same position relative to each other in the circumferential direction of the wheel accommodating portion, and when viewed from one axial side of the wheel accommodating portion, the reference imaginary line passes through both the first connection portion and the second connection portion.

[0027] In one embodiment of the gear housing of the present invention, the thickness of the wheel tubular portion at the end farthest from the worm accommodating portion in the direction of the reference virtual straight line decreases from one axial side to the other axial side.

[0028] An electric assist device of one embodiment of the present invention comprises a worm wheel having wheel teeth on its outer peripheral surface, a worm having worm teeth on its outer peripheral surface that mesh with the wheel teeth, a gear housing that houses the worm wheel and the worm inside, and an electric motor supported by the gear housing and that drives the worm to rotate, wherein the gear housing is constituted by a gear housing of one embodiment of the present invention. [Effects of the Invention]

[0029] According to one aspect of the present invention, it is possible to provide a gear housing that is easy to reduce in weight while ensuring the necessary strength, and an electric assist device that includes the gear housing. [Brief explanation of the drawings]

[0030] [Figure 1]FIG. 1 is a partial cutaway side view of an electric power steering device according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view of part A in FIG. [Figure 3] FIG. 3 is a cross-sectional view taken along the line BB in FIG. [Figure 4] FIG. 4 is a view of the gear housing of the first example as seen from the front side in the axial direction. [Figure 5] FIG. 5 is a cross-sectional view taken along CC line in FIG. 4, with some parts omitted. [Figure 6] FIG. 6 is a cross-sectional view taken along line DD of FIG. 4, with some parts omitted. [Figure 7] FIG. 7 is a schematic diagram showing the wheel bottom portion and the wheel fitting cylindrical portion extracted from FIG. [Figure 8] FIG. 8 is a schematic diagram showing the lower end portion of FIG. [Figure 9] FIG. 9 is a schematic diagram for explaining a mold for molding the portion shown in FIG. [Figure 10] FIG. 10 is a diagram corresponding to FIG. 8 and related to a second embodiment of the present invention. [Figure 11] FIG. 11 is a diagram corresponding to FIG. 9 and related to the second example. [Figure 12] FIG. 12 is a diagram corresponding to FIG. 8 and related to a third embodiment of the present invention. [Figure 13] FIG. 13 is a diagram corresponding to FIG. 8 and related to a fourth embodiment of the present invention. [Figure 14] FIG. 14 is a diagram corresponding to FIG. 8 and related to a fifth embodiment of the present invention. [Figure 15] FIG. 15 is a diagram corresponding to FIG. 8 and related to a sixth embodiment of the present invention. [Figure 16] FIG. 16 is a diagram corresponding to FIG. 4, showing a seventh embodiment of the present invention. [Figure 17] FIG. 17 is a cross-sectional view taken along the line E-E in FIG. [Figure 18] FIG. 18 is a diagram corresponding to FIG. 2 and shows an eighth example of an embodiment of the present invention. [Figure 19] FIG. 19 is a cross-sectional view taken along the line FF of FIG. 18, with some parts omitted. [Figure 20] FIG. 20 is a cross-sectional view showing an example of a conventional structure of an electric power steering device. [Figure 21] FIG. 21 is a cross-sectional view taken along line GG in FIG. [Figure 22] FIG. 22 is a perspective view of a gear housing constituting an example of a conventional structure. DETAILED DESCRIPTION OF THE INVENTION

[0031] [Example 1] A first embodiment of the present invention will be described with reference to FIGS. 1 to 9. FIG.

[0032] As shown in FIG. 1, the electric power steering device 1 of this example includes a steering wheel 2, a steering shaft 3, a steering column 4, a pair of universal joints 5a, 5b, an intermediate shaft 6, a steering gear unit 7, and an electric assist device 8.

[0033] In the following description of the electric power steering device 1, the front-rear direction means the front-rear direction of the vehicle, the left-right direction means the width direction of the vehicle, and the up-down direction means the up-down direction of the vehicle.

[0034] The steering wheel 2 is fixedly supported at the rear end of a steering shaft 3. The steering shaft 3 is rotatably supported inside a steering column 4, which is supported on the vehicle body. The front end of the steering shaft 3 is connected to a pinion shaft 9 of a steering gear unit 7 via a rear universal joint 5a, an intermediate shaft 6, and a front universal joint 5b. Therefore, when the driver turns the steering wheel 2, the rotation of the steering wheel 2 is transmitted to the pinion shaft 9 via the steering shaft 3, the pair of universal joints 5a and 5b, and the intermediate shaft 6. The rotation of the pinion shaft 9 is converted into linear motion of a rack shaft (not shown) of the steering gear unit 7 that meshes with the pinion shaft 9. As a result, a pair of tie rods 10 are pushed and pulled, and a steering angle corresponding to the amount of rotation of the steering wheel 2 is applied to the left and right steered wheels.

[0035] The electric power steering device 1 of this example is equipped with a tilt mechanism for adjusting the vertical position of the steering wheel 2 in accordance with the driver's physique and driving posture. To this end, an electric assist device 8 fixed to the front end of a steering column 4 is supported so as to be able to pivot about a tilt axis 11 in the left-right direction relative to the vehicle body. The vertical position of the steering wheel 2 is adjusted by pivoting the steering shaft 3, steering column 4, and electric assist device 8 up and down about the tilt axis 11.

[0036] 2 and 3, the electric assist device 8 includes a worm wheel 12 having wheel teeth 13 on its outer circumferential surface, a worm 14 having worm teeth 15 on its outer circumferential surface that mesh with the wheel teeth 13, a gear housing 16 that houses the worm wheel 12 and the worm 14 inside, and an electric motor 17 that is supported by the gear housing 16 and rotates the worm 14. The worm wheel 12 and the worm 14 are combined with each other to form a worm reducer.

[0037] 2, the electric assist device 8 of this example further includes a sensor housing 18 fixed to the front end of the steering column 4. In this example, the gear housing 16 located on the front side and the sensor housing 18 located on the rear side are combined together to form a housing 19.

[0038] The electric assist device 8 of this example further includes a torsion bar 20 and an output shaft 21. In this example, the front end of the steering shaft 3 is inserted inside the housing 19 and is connected to the output shaft 21 via the torsion bar 20. The front end of the output shaft 21 protrudes forward from the inside of the housing 19 and is connected to the intermediate shaft 6 via a rear universal joint 5a (see FIG. 1). The output shaft 21 is rotatably supported by ball bearings 22 and 23 at two points spaced apart in the axial direction relative to the housing 19.

[0039] The electrically power-assisted device 8 of this example further includes a torque sensor 24 housed inside the sensor housing 18 and arranged around the output shaft 21. The torque sensor 24 detects the direction and magnitude of the torque applied to the steering shaft 3 from the steering wheel 2. The electric motor 17 applies an assist driving force to the output shaft 21 via the worm wheel 12 by rotationally driving the worm 14 based on the detection signal from the torque sensor 24, a vehicle speed signal output from a vehicle speed sensor incorporated in the transmission, and the like. As a result, the force required by the driver to rotate the steering wheel 2 is reduced.

[0040] The gear housing 16, the worm 14, the worm wheel 12, and the electric motor 17 will be further described below.

[0041] In this example, the gear housing 16 includes a wheel receiving portion 25, a worm receiving portion 26, and a hinge portion 27, as shown in FIGS. 2 to 6. In this example, the gear housing 16 is integrally formed from a light alloy such as an aluminum alloy, or a thermoplastic resin. In other words, in the following description, the connection between the various parts that make up the gear housing 16 in this example means an integral connection. The gear housing 16 can be produced, for example, by die-casting a light alloy such as an aluminum alloy, or by injection molding a thermoplastic resin.

[0042] The wheel accommodating portion 25 is a portion that accommodates the worm wheel 12 therein. In this example, the axial front side (left side in FIGS. 2 and 5) of the wheel accommodating portion 25 corresponds to one axial side, and the axial rear side (right side in FIGS. 2 and 5) corresponds to the other axial side. In this example, the wheel accommodating portion 25 has a cylindrical wheel tubular portion 28 that is arranged around the worm wheel 12, a circular wheel bottom portion 29 that extends radially inward from the axial front end of the wheel tubular portion 28, and a substantially cylindrical wheel fitting tubular portion 30 that is connected to the radially inner end of the wheel bottom portion 29. In this example, the radially inner end of the wheel bottom portion 29 is connected to the axially middle portion of the wheel fitting tubular portion 30. In this example, when the gear housing 16 and the sensor housing 18 are combined to form the housing 19, the front end of the sensor housing 18 is fitted into the rear end of the wheel tubular portion 28, and the front end face of the sensor housing 18 abuts against the rear end face of the wheel tubular portion 28.

[0043] The worm accommodating portion 26 is a portion that accommodates the worm 14 inside. In this example, the worm accommodating portion 26 is configured as a cylindrical portion with a bottom, an open end on one axial side (the left side in FIGS. 3 and 4 ) and a closed end on the other axial side (the right side in FIGS. 3 and 4 ). However, when implementing the present invention, the worm accommodating portion may be configured as a cylindrical portion with open ends on one axial side and the other axial side, and the opening on the other axial side may be closed with a lid. The worm accommodating portion 26 is connected to a circumferential portion of the radially outer end of the wheel accommodating portion 25. In this example, the worm accommodating portion 26 is connected to the upper end of the wheel accommodating portion 25. The central axis of the worm accommodating portion 26 is disposed at a skewed position with respect to the central axis of the wheel accommodating portion 25. In this example, the central axis of the worm accommodating portion 26 is disposed in the left-right direction. The internal space of the worm accommodating portion 26 is in communication with the internal space of the wheel accommodating portion 25.

[0044] In this example, the worm accommodating portion 26 has a motor mounting flange 31 extending radially outward at one axial end. The motor mounting flange 31 is the portion where the electric motor 17 is mounted, and functions to increase the rigidity of the portion of the gear housing 16 surrounding the electric motor 17.

[0045] The hinge portion 27 has an insertion hole 32 for inserting the tilt shaft 11 (see FIG. 1). The hinge portion 27 is connected to a circumferential portion of the wheel accommodating portion 25, and protrudes from the circumferential portion toward the axial front side of the wheel accommodating portion 25.

[0046] When carrying out the present invention, if the gear housing 16 is provided with the hinge portion 27 as in this example, it is preferable to adopt a configuration in which the thickness of the wheel bottom portion 29 of the wheel accommodating portion 25 is increased at the portion close to the worm accommodating portion 26 and the hinge portion 27 and decreased at the portion far from the worm accommodating portion 26 and the hinge portion 27, from the viewpoint of ensuring the necessary strength and reducing weight. Furthermore, when adopting such a configuration, it is preferable that the worm accommodating portion 26 and the hinge portion 27 are not positioned far apart from each other in the circumferential direction of the wheel accommodating portion 25. Specifically, as shown in Fig. 4, when viewed from the axial front side of the wheel accommodating portion 25, a first imaginary line L1 is defined, starting from the center O of the wheel accommodating portion 25 and perpendicular to the central axis of the worm accommodating portion 26, and a second imaginary line L2 is defined, starting from the center O of the wheel accommodating portion 25 and perpendicular to the central axis of the insertion hole 32 of the hinge portion 27. The angle between the first imaginary line L1 and the second imaginary line L2 is preferably 135° or less, including 0° (within the range of 0° to 135°). More specifically, the angle is preferably 90° or less, more preferably 60° or less, and even more preferably 30° or less. In this example, the angle is 0°, and the first imaginary line L1 and the second imaginary line L2 overlap each other. Specifically, in this example, the hinge portion 27 is connected to the upper end of the wheel bottom portion 29 and the front side of the axially intermediate portion of the worm accommodating portion 26, and protrudes from these portions toward the axially front side of the wheel accommodating portion 25. In this example, the central axis of the insertion hole 32 of the hinge portion 27 is disposed in the left-right direction.

[0047] 4, when viewed from the axial front side of the wheel accommodating portion 25, a reference imaginary line Lb is considered which passes through the center O of the wheel accommodating portion 25 and has an opening angle with the first imaginary line L1 which is 75 degrees or less (within the range of 0 degrees to 75 degrees) including 0 degrees, the thickness of at least a portion of the wheel bottom portion 29 decreases from the side closer to the worm accommodating portion 26 to the side farther from the worm accommodating portion 26 in the direction of the reference imaginary line Lb. This point will be described in detail below.

[0048] When carrying out the present invention, the opening angle between the first imaginary line L1 and the reference imaginary line Lb is preferably 60° or less, more preferably 30° or less, and even more preferably 15° or less. In this example, the opening angle is 0°. That is, the reference imaginary line Lb overlaps the first imaginary line L1 and the second imaginary line L2 and extends in the vertical direction. In this example, as shown in FIG. 4 , a portion of the first connection portion J1 (= the portion within the range of opening angle α1) which is the connection portion of the worm accommodating portion 26 to the wheel accommodating portion 25, and the entire second connection portion J2 (= the portion within the range of opening angle α2) which is the connection portion of the hinge portion 27 to the wheel accommodating portion 25, are arranged at the same position relative to each other in the circumferential direction of the wheel accommodating portion 25. In this example, the reference imaginary line Lb passes through both the first connection portion J1 and the second connection portion J2.

[0049] 4, 5, and 7, i.e., in the direction of the reference imaginary line Lb, the overall thickness of the wheel bottom portion 29 decreases from the side closer to the worm accommodating portion 26 to the side farther from it. In other words, in this example, the overall thickness of the wheel bottom portion 29 decreases in the direction away from the central axis of the worm accommodating portion 26, and in further other words, the overall thickness of the wheel bottom portion 29 decreases from the top to the bottom. For this reason, in this example, as exaggeratedly shown in FIG. 7, the rear side surface 38 of the wheel bottom portion 29 is formed by a plane perpendicular to the central axis of the wheel accommodating portion 25 (= the central axis of the wheel bottom portion 29), and the front side surface 39 of the wheel bottom portion 29 is formed by a plane inclined in the direction in which the thickness of the wheel bottom portion 29 decreases in the direction indicated by arrow S1, i.e., toward the rear.

[0050] In this example, the thickness of the wheel bottom portion 29 does not change in a direction (left and right direction in FIGS. 4 and 6) perpendicular to both the central axis of the wheel accommodating portion 25 and the reference imaginary line Lb.

[0051] When implementing the structure of this example, the difference between the thickness t1 of the upper end portion, which is the thickest part of the wheel bottom portion 29, and the thickness t2 of the lower end portion, which is the thinnest part of the wheel bottom portion 29, can be set as desired. However, thickness t2 is preferably 90% or less of thickness t1, and more preferably 80% or less of thickness t1. However, when implementing the structure of this example, from the perspective of ensuring the strength of the gear housing 16, it is preferable to ensure that the thickness of the thinnest part of the gear housing 16 is 0.8 mm or more, and more preferably 1 mm or more.

[0052] In this example, the thickness of the wheel tubular portion 28 does not change in the axial direction in the upper half of the wheel tubular portion 28, i.e., in the half of the wheel tubular portion 28 that is above the central axis of the wheel accommodating portion 25 (= the central axis of the wheel tubular portion 28). However, when implementing the present invention, the thickness of the wheel tubular portion 28 in the upper half of the wheel tubular portion 28 can also be made smaller or larger from the rear side to the front side.

[0053] In this example, in the lower half of the wheel tubular portion 28, including the lower end of the wheel tubular portion 28, i.e., in the half of the wheel tubular portion 28 below the central axis of the wheel accommodating portion 25, the wall thickness of the wheel tubular portion 28 decreases from the front to the rear. For this reason, in this example, in the lower half of the wheel tubular portion 28, the inner circumferential surface 40 of the wheel tubular portion 28 is configured as a cylindrical surface whose diameter does not change in the axial direction, and the outer circumferential surface 41 of the wheel tubular portion 28 is configured as a tapered cylindrical surface whose diameter decreases from the front to the rear. When implementing the present invention, the portion of the wheel tubular portion where the wall thickness decreases from the front to the rear may be only the lower end of the wheel tubular portion. Here, the lower end of the wheel tubular portion may be, for example, a portion of the wheel tubular portion located below a position whose height from the lower edge of the wheel tubular portion is approximately 10% to 30% of the outer diameter of the wheel tubular portion.

[0054] When implementing the structure of this example, as shown in FIG. 8, in the lower half of the wheel tubular portion 28, the difference between the thickness t3 of the front end portion, which is the thickest part of the wheel tubular portion 28, and the thickness t4 of the rear end portion, which is the thinnest part of the wheel tubular portion 28, can be set arbitrarily. However, it is preferable that the thickness t4 be 80% or less of the thickness t3. As mentioned above, when the thickness t4 corresponds to the thinnest part of the gear housing 16, it is preferable that the thickness t4 be 0.8 mm or more, and accordingly, the thicknesses t2 and t3 are appropriately defined. Furthermore, when implementing the present invention, the thickness of the wheel tubular portion 28 in the lower half of the wheel tubular portion 28 can be constant in the axial direction or can increase from the front side to the rear side. In this example, the thickness t3 is set smaller than the thickness t2 (t3 <t2)。

[0055] In implementing the present invention, the gear housing may have protruding portions such as ribs or bosses protruding from the axial side surface of the wheel base portion, or protruding portions such as ribs or flanges protruding from the circumferential surface of the wheel tubular portion. However, these protruding portions are separate from the wheel base portion and the wheel tubular portion. In other words, the height dimensions of these protruding portions are independent of the wall thicknesses of the wheel base portion and the wheel tubular portion. In addition, in this example, a cylindrical wheel fitting tubular portion 30 is connected to the radially inner end of the wheel base portion 29, but the wheel fitting tubular portion 30 is also separate from the wheel base portion 29. In other words, the axial dimension of the wheel fitting tubular portion 30 is independent of the wall thickness of the wheel base portion.

[0056] In this example, as shown in Fig. 3, the worm 14 has worm teeth 15 that mesh with the wheel teeth 13 on the outer peripheral surface of an axially intermediate portion, and has a female spline 33 on the inner peripheral surface of one axial end portion. The worm 14 is rotatably supported inside the worm housing portion 26 by ball bearings 34, 35.

[0057] 2 and 3, the worm wheel 12 has wheel teeth 13 on its outer circumferential surface and is rotatably supported inside the wheel accommodating portion 25. For this reason, in this example, the worm wheel 12 is fitted and fixed to the outside of a portion of the output shaft 21 that is located between the two ball bearings 22, 23. The ball bearing 22, which is a wheel bearing, is fitted inside the wheel fitting cylindrical portion 30 of the wheel accommodating portion 25 and is fitted outside of a portion of the output shaft 21 that is adjacent to the front side of the worm wheel 12. The ball bearing 23 is fitted inside the front end of the sensor housing 18 and is fitted outside of a portion of the output shaft 21 that is adjacent to the rear side of the worm wheel 12.

[0058] In this example, the electric motor 17 is fixed to the motor mounting flange 31 of the worm housing 26, as shown in Fig. 3. In this example, the electric motor 17 has a male spline 37 on the outer peripheral surface of the tip of the motor output shaft 36. The motor output shaft 36 and the worm 14 are connected so as to be able to transmit torque by engaging the male spline 37 with the female spline 33 of the worm 14. When carrying out the present invention, the method of connecting the motor output shaft and the worm so as to be able to transmit torque is not particularly limited. For example, the motor output shaft and the worm can also be connected so as to be able to transmit torque via a coupling that can absorb (tolerate) misalignment of their respective axes.

[0059] In this example, when the gear housing 16 is produced by die-cast molding or injection molding, the gate of the mold, which serves as the supply port for the molten material, is arranged at one or more locations at the upper end of the worm housing molding space for molding the worm housing portion 26 (the portion corresponding to portion X1 in FIG. 4) in the cavity for molding the gear housing 16. In other words, the gate is arranged so that the worm housing portion molding space is on the upstream side. This allows the material that passes through the worm housing portion molding space to flow into the hinge portion molding space for molding the hinge portion 27 in the cavity and into the wheel housing portion molding space for molding the wheel housing portion 25 in the cavity.

[0060] In this example, when the gear housing 16 is produced by die-cast molding or injection molding, the molds used to form the lower half of the wheel accommodating section 25 are a fixed mold A1 that molds the front side surface 39 of the wheel bottom portion 29, a movable mold B1 that molds the rear side surface 38 of the wheel bottom portion 29 and the inner peripheral surface 40 of the wheel tubular portion 28, and a movable mold B2 that molds the outer peripheral surface 41 of the wheel tubular portion 28, as shown in Figure 9. The movable molds B1 and B2 can move toward and away from the fixed mold A1 in the axial direction (left and right direction in Figure 9). In this example, when the gear housing 16 is removed from the cavity after the material poured into the cavity has solidified, the slope of the outer peripheral surface 41 of the wheel tubular portion 28 is used as a draft angle for the movable mold B2.

[0061] In the electric assist device 8 of this example, when an assist driving force is generated, meshing reaction forces are applied to the worm 14 and the worm wheel 12 from the meshing portions between the worm teeth 15 and the wheel teeth 13. These meshing reaction forces are then transmitted from the worm 14 to the worm housing 26 via the ball bearings 34 and 35, and from the worm wheel 12 to the wheel housing 25 via the ball bearing 22. As a result, as shown in FIG. 4 , a force F1 that moves the worm housing 26 and the wheel housing 25 away from each other and a force F2 that twists the worm and wheel housing 25 relative to each other are applied to the gear housing 16. The twisting force F2 is generated because at least the worm teeth 15 of the worm teeth 15 and the wheel teeth 13 have a lead angle. The direction of the twisting force F2 reverses depending on the rotational direction of the worm 14.

[0062] Because the forces F1 and F2 are applied to the gear housing 16 in such a complex manner, the wheel accommodating portion 25 constituting the gear housing 16 is required to have higher strength (rigidity) than other portions, particularly in areas close to the worm accommodating portion 26 and in areas close to the hinge portion 27 that supports the torsional force F2. Therefore, in this example, it is desirable to increase the thickness of the upper half of the wheel accommodating portion 25 to ensure strength (rigidity) against the forces F1 and F2. On the other hand, it is desirable to decrease the thickness of the lower half of the wheel accommodating portion 25 from the perspective of weight reduction, as it is not significantly affected by the forces F1 and F2.

[0063] In this regard, in this example, as shown in FIG. 4, when viewed from the axial front side of the wheel accommodating portion 25, the opening angle between the first imaginary line L1 and the center line O of the wheel accommodating portion 25 is not 0°. 75 When the reference imaginary line Lb is considered to be equal to or less than 1°, the overall thickness of the wheel receiving bottom portion 29 decreases from the side closer to the worm accommodating portion 26 to the side farther from the worm accommodating portion 26 in the direction of the reference imaginary line Lb. More specifically, in this example, the opening angle is 0°, and the reference imaginary line Lb passes through both the first connection portion J1 and the second connection portion J2 and extends in the vertical direction. As a result, the overall thickness of the wheel receiving bottom portion 29 decreases from the top to the bottom. That is, the thickness of the upper half of the wheel accommodating portion 25 is greater, and the thickness of the lower half of the wheel accommodating portion 25 is smaller. This makes it easy to ensure the strength of the upper half of the wheel accommodating portion 25 while reducing the weight of the gear housing 16. In other words, in this example, it is easy to reduce the weight of the gear housing 16 while ensuring the necessary strength.

[0064] In this example, the thickness of the wheel tubular portion 28 in the lower half thereof decreases from the front to the rear, which also contributes to reducing the weight of the gear housing 16.

[0065] Furthermore, the gear housing 16 constituting the electric assist device 8 of this embodiment makes it easy to ensure formability during manufacturing.

[0066] That is, when a product is manufactured by die-casting or injection molding, if the flow area of ​​the material in the cavity decreases toward the downstream side, the flow rate of the material gradually increases, improving the moldability of the product. In this regard, in this example, the overall thickness of the wheel bottom portion 29 constituting the gear housing 16 decreases in the direction indicated by the arrow S1 in FIGS. 4, 5, and 7, i.e., from the side closer to the worm accommodating portion 26 toward the side farther from the worm accommodating portion 26 in the direction of the reference imaginary line Lb. Therefore, when the gear housing 16 is manufactured by die-casting or injection molding, the material flowing from the worm accommodating portion molding space to the wheel accommodating portion molding space can be efficiently flowed in the wheel accommodating portion molding space from the side closer to the worm accommodating portion molding space toward the side farther from the worm accommodating portion molding space in the direction of the reference imaginary line Lb. This makes it easier to ensure moldability during the manufacturing of the gear housing 16.

[0067] In particular, in this example, the front side surface 39 of the wheel bottom portion 29 is configured as a flat surface that slopes rearward in the direction indicated by arrow S1, i.e., a tapered surface. Therefore, compared to when the front side surface of the wheel bottom portion is configured as a stepped surface, the material that flows from the worm accommodating portion molding space to the wheel accommodating portion molding space can flow more smoothly along the front side surface 39 of the wheel bottom portion 29. Therefore, this also makes it easier to ensure moldability during manufacturing of the gear housing 16.

[0068] In this example, in the lower half of the wheel tubular portion 28 that constitutes the gear housing 16, the wall thickness of the wheel tubular portion 28 decreases from the front to the rear. Therefore, when the gear housing 16 is manufactured by die casting or injection molding, the material that flows from the space for molding the wheel bottom portion 29 to the space for molding the wheel tubular portion 28 in the space for molding the lower half of the wheel tubular portion 28 can flow efficiently from the front to the rear. This also makes it easy to ensure moldability during manufacturing of the gear housing 16.

[0069] [Example 2] A second embodiment of the present invention will be described with reference to FIGS.

[0070] 10, in the case of this example, in the lower half of the wheel tubular portion 28a of the wheel accommodating portion 25a that constitutes the gear housing 16a, the wall thickness of the wheel tubular portion 28a decreases from the front to the rear. For this reason, in this example, in the lower half of the wheel tubular portion 28a, the outer circumferential surface 41a of the wheel tubular portion 28a is configured as a cylindrical surface whose diameter does not change in the axial direction, and the inner circumferential surface 40a of the wheel tubular portion 28a is configured as a tapered cylindrical surface whose diameter increases from the front to the rear.

[0071] In this example, when the gear housing 16a is produced by die-casting or injection molding, the molds used to form the lower half of the wheel accommodating section 25a are a fixed mold A2 that forms the front side surface 39 of the wheel bottom portion 29 and the outer peripheral surface 41a of the wheel tubular portion 28a, and a movable mold B3 that forms the rear side surface 38 of the wheel bottom portion 29 and the inner peripheral surface 40a of the wheel tubular portion 28a, as shown in FIG. 11. The movable mold B3 can move toward and away from the fixed mold A2 in the axial direction (left and right in FIG. 11). In this example, after the material poured into the cavity has solidified, the inclination of the inner peripheral surface 40a of the wheel tubular portion 28a is used as a draft angle for the movable mold B3 when removing the gear housing 16a from the cavity. The other configurations, functions, and effects are the same as those of the first example.

[0072] [Example 3] A third embodiment of the present invention will be described with reference to FIG.

[0073] In this example, the overall thickness of the wheel bottom portion 29a of the wheel accommodating portion 25b constituting the gear housing 16b also decreases from top to bottom. For this reason, in this example, as shown in Fig. 12, the front side surface 39a of the wheel bottom portion 29a is configured as a plane perpendicular to the central axis of the wheel accommodating portion 25a, and the rear side surface 38a of the wheel bottom portion 29a is configured as a plane that slopes forward from top to bottom. The other configurations and effects are the same as those of the first example.

[0074] [Example 4] A fourth embodiment of the present invention will be described with reference to FIG.

[0075] In this example, the wheel accommodating portion 25c of the gear housing 16c has a wheel cylindrical portion 28a having the same structure as in Example 2, and a wheel bottom portion 29a having the same structure as in Example 3. The other configurations and effects are the same as in Example 2.

[0076] [Example 5] A fifth embodiment of the present invention will be described with reference to FIG.

[0077] In this example, the front side surface 39b of the wheel bottom portion 29b of the wheel accommodating portion 25d that constitutes the gear housing 16d is configured as a concave curved surface that slopes from top to bottom toward the rear. The other configurations and effects are the same as those of the second example.

[0078] [Example 6] A sixth embodiment of the present invention will be described with reference to FIG.

[0079] In this example, the front side surface 39c of the wheel bottom portion 29c of the wheel accommodating portion 25e that constitutes the gear housing 16e has an inclined surface portion 42 near the lower end that slopes rearward from top to bottom. The portion of the front side surface 39c above the inclined surface portion 42 and the portion below the inclined surface portion 42 are each formed by a plane perpendicular to the central axis of the wheel accommodating portion 25e. The other configurations and effects are the same as those of the second example.

[0080] When implementing the present invention, if an inclined surface portion is provided only on a portion of the side of the wheel bottom portion, as in the sixth example, the position and range of the inclined surface portion relative to the direction of the reference virtual straight line can be set arbitrarily.

[0081] [Example 7] A seventh embodiment of the present invention will be described with reference to FIGS.

[0082] In this example, as shown in Figure 16, when viewed from the axial front side of the wheel accommodating section 25f of the gear housing 16f, when considering a first imaginary line L1 that starts at the center O of the wheel accommodating section 25f and is perpendicular to the central axis of the worm accommodating section 26, and a second imaginary line L2 that starts at the center O of the wheel accommodating section 25f and is perpendicular to the central axis of the insertion hole 32 of the hinge portion 27, the opening angle θ between the first imaginary line L1 and the second imaginary line L2 is approximately 120 degrees.

[0083] As a result, in this example, the first connection portion J1, which is the connection portion of the worm accommodating portion 26 to the wheel accommodating portion 25f of the gear housing 16f, and the second connection portion J2, which is the connection portion of the hinge portion 27 to the wheel accommodating portion 25f, are arranged at different positions in the circumferential direction of the wheel accommodating portion 25f. Specifically, when viewed from the axial front side of the wheel accommodating portion 25f, the hinge portion 27 is arranged in the region a1, which includes one end of the worm accommodating portion 26 in the axial direction, of regions a1 and a2 on both sides of a third imaginary line L3 that passes through the center O of the wheel accommodating portion 25f and overlaps with the first imaginary line L1.

[0084] In this example, when viewed from the axial front side of the wheel accommodating section 25f, the reference imaginary line Lb passes through the circumferential center of the circumferential range (= range of opening angle θ) sandwiched between the first imaginary line L1 and the second imaginary line L2. That is, in this example, the opening angle between the first imaginary line L1 and the reference imaginary line Lb is approximately θ / 2 = 60°. When implementing the present invention, the reference imaginary line may also be a line inclined within a range of approximately ±15° with respect to the reference imaginary line Lb of this example.

[0085] 16 and 17, that is, from the side closer to the worm accommodating portion 26 to the side farther from the worm accommodating portion 26 in the direction of the reference imaginary line Lb. Also in this example, the thickness of the wheel tubular portion 28b in the half closer to the worm accommodating portion 26 in the direction of the reference imaginary line Lb does not change in the axial direction. In contrast, the thickness of the wheel tubular portion 28b in the half farther from the worm accommodating portion 26 in the direction of the reference imaginary line Lb decreases from the front side to the rear side.

[0086] In this example, when the gear housing 16f is manufactured by die-cast molding or injection molding, the gate of the mold, which serves as the supply port for the molten material, is arranged at one or more locations in the cavity for molding the gear housing 16f, in the space portion (corresponding to portion X2 in FIG. 16) that molds the worm accommodating portion 26 and one end portion of the hinge portion 27, in the axial direction of the worm accommodating portion 26. The other configurations and effects are the same as those of the first example.

[0087] [Example 8] An eighth embodiment of the present invention will be described with reference to FIGS.

[0088] In the housing 19a of the electric assist device 8a of this example, a gear housing 16g is integrally connected to the front end of a substantially cylindrical sensor housing 18a that is fixed to the front end of the steering column 4. That is, in this example, the sensor housing 18a and the gear housing 16g are made as a single component from a light alloy such as an aluminum alloy or a thermoplastic resin.

[0089] In this example, the gear housing 16g includes a wheel accommodating portion 25 and a worm accommodating portion 26, but does not include a hinge portion. The wheel accommodating portion 25 and the worm accommodating portion 26 have the same configuration as in the first example, but are oriented in the front-to-rear direction opposite to that of the first example. Therefore, in this example, with respect to the wheel accommodating portion 25, the axial rear side (right side in FIG. 18) corresponds to one axial side, and the axial front side (left side in FIG. 18) corresponds to the other axial side. The front end of the sensor housing 18a is connected to the rear end of the wheel fitting cylindrical portion 30 that constitutes the wheel accommodating portion 25.

[0090] The housing 19a in this example is formed by combining the sensor housing 18a and gear housing 16g, which are configured as a single component, with a cover 43. The cover 43 includes a main body 44 configured generally in a circular ring shape, and a hinge portion 27 that protrudes axially forward from the upper end of the main body 44. With the housing 19a configured, the rear end of the main body 44 is fitted into the front end of the wheel cylindrical portion 28.

[0091] In this example, the front ball bearing 22 is fitted inside the main body 44 of the cover 43, and the rear ball bearing 23 is fitted inside the wheel fitting cylindrical portion 30 of the gear housing 16g. The other configurations and effects are the same as those of the first example.

[0092] The present invention can be implemented by appropriately combining the structures of the above-described embodiments within the scope of not causing any contradiction. [Explanation of symbols]

[0093] 1 Electric power steering device 2 steering wheels 3 Steering shaft 4. Steering column 5a, 5b universal joint 6 Intermediate shaft 7 Steering gear unit 8, 8a Electric assist device 9 Pinion shaft 10 tie rod 11 Tilt axis 12 worm wheel 13 Wheel Teeth 14 Warm 15 worm teeth 16, 16a, 16b, 16c, 16d, 16e, 16f, 16g Gear housing 17 Electric motor 18, 18a Sensor housing 19, 19a Housing 20 Torsion bar 21 Output shaft 22 Ball bearing 23 Ball bearings 24 Torque sensor 25, 25a, 25b, 25c, 25d, 25e, 25f Wheel housing 26 Worm housing 27 Hinge part 28, 28a, 28b Wheel tube 29, 29a, 29b, 29c, 29d Wheel Bottoms 30 Wheel fitting tube 31 Motor mounting flange 32 Insertion hole 33 female spline 34 Ball bearing 35 Ball bearing 36 Motor output shaft 37 Male Spline 38, 38a Posterior side 39, 39a, 39b, 39c front side 40, 40a Inner surface 41, 41a Outer surface 42 Slope section 43 Lid 44 Main body 100 Electric power steering device 101 Electric assist device 102 steering shaft 103 Steering column 104 Torsion bar 105 output shaft 106 Gear housing 107 Worm reducer 108 Electric Motor 109 Rear housing 110 Wheel storage section 111 Worm housing 112 Hinge part 113 Worm Wheel 114 Wheel tube 115 Wheel base 116 Wheel fitting cylinder 117 Worm 118 Motor output shaft 119 Insertion hole 120 tilt axis 121a, 121b Reinforcing rib 122 worm teeth 123a, 123b ball bearings 124 wheel teeth 125a, 125b ball bearings

Claims

1. The gear is provided with a wheel accommodating portion that accommodates a worm wheel therein and a worm accommodating portion that accommodates a worm therein, the wheel accommodating portion has a wheel cylindrical portion disposed around the worm wheel, and a circular wheel bottom portion extending radially inward from one axial end of the wheel cylindrical portion, the worm accommodating portion is configured in a cylindrical shape with an open end on one axial side to which the electric motor is fixed, and its central axis is disposed at a position twisted with respect to the central axis of the wheel accommodating portion, and is connected to a circumferential portion of a radially outer end of the wheel accommodating portion, When viewed from one axial side of the wheel accommodating portion, a first imaginary line that starts at the center of the wheel accommodating portion and is perpendicular to the central axis of the worm accommodating portion and a reference imaginary line that passes through the center of the wheel accommodating portion and has an opening angle with the first imaginary line that is 75° or less and includes 0° are considered, the thickness of at least a portion of the wheel bottom portion decreases from the side closer to the worm accommodating portion to the side farther from the worm accommodating portion in the direction of the reference imaginary line, a hinge portion having an insertion hole for inserting a tilt shaft; the hinge portion is connected to a circumferential portion of the wheel accommodating portion and protrudes from the circumferential portion toward one axial side of the wheel accommodating portion, When a second imaginary line is considered, the second imaginary line has a starting point at the center of the wheel accommodating section and is perpendicular to the central axis of the insertion hole of the hinge section as viewed from one axial side of the wheel accommodating section, the opening angle between the first imaginary line and the second imaginary line is 135° or less and includes 0°, the reference virtual line passes through a circumferential range sandwiched between the first virtual line and the second virtual line; Gear housing.

2. The gear is provided with a wheel accommodating portion that accommodates a worm wheel therein and a worm accommodating portion that accommodates a worm therein, the wheel accommodating portion has a wheel cylindrical portion disposed around the worm wheel, and a circular wheel bottom portion extending radially inward from one axial end of the wheel cylindrical portion, the worm accommodating portion is configured in a cylindrical shape with an open end on one axial side to which the electric motor is fixed, and its central axis is disposed at a position twisted with respect to the central axis of the wheel accommodating portion, and is connected to a circumferential portion of a radially outer end of the wheel accommodating portion, When viewed from one axial side of the wheel accommodating portion, a first imaginary line that starts at the center of the wheel accommodating portion and is perpendicular to the central axis of the worm accommodating portion and a reference imaginary line that passes through the center of the wheel accommodating portion and has an opening angle with the first imaginary line that is 75° or less and includes 0° are considered, the overall thickness of the wheel bottom portion decreases from the side closer to the worm accommodating portion to the side farther from it in the direction of the reference imaginary line, The entire side surface of the wheel bottom portion on one axial side or the other axial side is configured by a flat surface or a curved surface that is inclined in a direction in which the thickness of the wheel bottom portion becomes smaller as it moves from the side closer to the worm accommodating portion to the side farther from the worm accommodating portion with respect to the direction of the reference virtual straight line. Gear housing.

3. a hinge portion having an insertion hole for inserting a tilt shaft; the hinge portion is connected to a circumferential portion of the wheel accommodating portion and protrudes from the circumferential portion toward one axial side of the wheel accommodating portion; The gear housing according to claim 2 .

4. at least a portion of a first connection portion, which is a connection portion of the worm accommodating portion with respect to the wheel accommodating portion, and at least a portion of a second connection portion, which is a connection portion of the hinge portion with respect to the wheel accommodating portion, are disposed at the same position relative to each other in the circumferential direction of the wheel accommodating portion, When viewed from one axial side of the wheel accommodating portion, the reference imaginary line passes through both the first connection portion and the second connection portion. The gear housing according to claim 3 .

5. The gear is provided with a wheel accommodating portion that accommodates a worm wheel therein and a worm accommodating portion that accommodates a worm therein, the wheel accommodating portion has a wheel cylindrical portion disposed around the worm wheel, and a circular wheel bottom portion extending radially inward from one axial end of the wheel cylindrical portion, the worm accommodating portion is configured in a cylindrical shape with an open end on one axial side to which the electric motor is fixed, and its central axis is disposed at a position twisted with respect to the central axis of the wheel accommodating portion, and is connected to a circumferential portion of a radially outer end of the wheel accommodating portion, When viewed from one axial side of the wheel accommodating portion, a first imaginary line that starts at the center of the wheel accommodating portion and is perpendicular to the central axis of the worm accommodating portion and a reference imaginary line that passes through the center of the wheel accommodating portion and has an opening angle with the first imaginary line that is 75° or less and includes 0° are considered, the thickness of at least a portion of the wheel bottom portion decreases from the side closer to the worm accommodating portion to the side farther from the worm accommodating portion in the direction of the reference imaginary line, a hinge portion having an insertion hole for inserting a tilt shaft; the hinge portion is connected to a circumferential portion of the wheel accommodating portion and protrudes from the circumferential portion toward one axial side of the wheel accommodating portion, at least a portion of a first connection portion, which is a connection portion of the worm accommodating portion with respect to the wheel accommodating portion, and at least a portion of a second connection portion, which is a connection portion of the hinge portion with respect to the wheel accommodating portion, are disposed at the same position relative to each other in the circumferential direction of the wheel accommodating portion, When viewed from one axial side of the wheel accommodating portion, the reference imaginary line passes through both the first connection portion and the second connection portion. Gear housing.

6. 6. The gear housing according to claim 3, wherein, when viewed from one axial side of the wheel accommodating portion, a second imaginary line is considered which has a starting point at the center of the wheel accommodating portion and is perpendicular to the central axis of the insertion hole of the hinge portion, an opening angle between the first imaginary line and the second imaginary line is 135° or less and includes 0°.

7. The gear housing according to claim 6 , wherein the reference imaginary line passes through a circumferential range sandwiched between the first imaginary line and the second imaginary line.

8. A gear housing as described in any one of claims 1 and 5, and claims 6 and 7 which directly or indirectly cite claim 5, wherein a side surface on one axial side or the other axial side of at least a portion of the wheel bottom portion is configured as a flat surface or a curved surface inclined in a direction such that the thickness of at least a portion decreases from the side closer to the worm accommodating portion to the side farther from the worm accommodating portion in the direction of the reference imaginary straight line.

9. A gear housing according to any one of claims 1 to 8, wherein the thickness of the wheel tubular portion at the end farther from the worm accommodating portion in the direction of the reference imaginary straight line decreases from one axial side to the other axial side.

10. a worm wheel having wheel teeth on its outer circumferential surface; a worm having worm teeth on its outer circumferential surface that mesh with the wheel teeth; a gear housing that houses the worm wheel and the worm therein; an electric motor supported by the gear housing and configured to rotate the worm, The gear housing is configured by the gear housing according to any one of claims 1 to 9. Electric assist device.

Images