Worm wheel and manufacturing method thereof

Abstract

To provide a structure which can suppress the wear of worm wheel teeth by suppressing the contact face pressure of the worm wheel teeth with respect to worm wheel.SOLUTION: A metal-made hub 4 comprises a hub-side recess 9 having the same cross section shape over an entire periphery at an external peripheral face of an intermediate part of an outside-diameter side cylinder part 7 in an axial direction. A synthetic resin-made gear part 5 has a gear-side recess 15 which is more recessed than a portion adjoining both sides in the axial direction at the intermediate part in the axial direction including a portion located outside the hub-side recess 9 in a radial direction. The gear-side recess 15 is formed by the formation of a concave called as a sink mark at an external peripheral face of an intermediate part of the gear part 5 in the axial direction accompanied by molding contraction after the injection molding of the gear part 5.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to a worm wheel having a gear portion made of synthetic resin fixedly connected to the radially outer end of a hub. [Background technology]

[0002] Electric power steering devices using an electric motor as an auxiliary power source are widely used as devices for reducing the force required to operate the steering wheel when applying a steering angle to the steering wheels of an automobile. Electric power steering devices are broadly classified according to the mounting position of the electric motor. Specifically, various structures have been proposed, such as a column assist type that applies auxiliary power to a steering shaft rotatably supported inside a steering column, a pinion assist type that applies auxiliary power to a pinion shaft that is an input shaft of a steering gear unit, and a dual pinion type that provides auxiliary power to a pinion shaft that is separate from the pinion shaft that is an input shaft of the steering gear unit. In any of the structures, the auxiliary power of the electric motor is applied to a shaft member that rotates or moves linearly by the operation of the steering wheel via a reduction gear. As such a reduction gear, a worm reduction gear is widely used. The worm reduction gear constituting the electric power steering device includes a worm that is rotationally driven by an electric motor and a worm wheel that meshes with the worm.

[0003] In a worm reducer, in order to suppress the teeth rattle noise that occurs at the meshing point between the worm and the worm wheel when operating the steering wheel or driving on rough roads, and to reduce weight and / or costs, it has been proposed to make the radially inner part of the worm wheel that is fitted onto the rotating shaft from a metal material and the radially outer part that meshes with the worm from a synthetic resin material.

[0004] 20 shows a structure described in WO 2017 / 135140 (Patent Document 1) as an example of a worm wheel having a radially inner portion made of a metal material and a radially outer portion made of a synthetic resin material. The worm wheel 100 includes a metal hub 101 and a gear portion 102 made of a synthetic resin.

[0005] The hub 101 includes an inner diameter side cylinder portion 103, an outer diameter side cylinder portion 104 arranged around the inner diameter side cylinder portion 103 and coaxially with the inner diameter side cylinder portion 103, and a hollow circular plate-shaped connecting portion 105 that connects the inner diameter side cylinder portion 103 and the outer diameter side cylinder portion 104. Of these, the inner diameter side cylinder portion 103 is fitted and fixed to a rotating shaft such as a steering shaft or pinion shaft that rotates with the rotation of the steering wheel, or a pinion shaft that meshes with a rack shaft.

[0006] The gear portion 102 has a plurality of worm wheel teeth 106 on its radially outer end, and is joined and fixed to the hub 101 so as to cover the radially outer end of the hub 101. That is, the gear portion 102 embeds the outer diameter side cylinder portion 104 of the hub 101 and the radially outer end of the connection portion 105 during injection molding (by insert molding). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2017 / 135140 Brochure Summary of the Invention [Problem to be solved by the invention]

[0008] The worm wheel 100 described in WO 2017 / 135140 has room for improvement in terms of reducing wear on the worm wheel teeth 106.

[0009] In the worm wheel 100 described in WO 2017 / 135140, the tooth surface of each of the multiple worm wheel teeth 106 is configured as a flat surface or a convex curved surface that is smooth in the axial direction.

[0010] When the worm wheel teeth 106 of such a worm wheel 100 are meshed with threaded worm teeth provided on the worm, the tooth surfaces of the worm wheel teeth 106 and the worm teeth come into point contact, resulting in high contact pressure, making the worm wheel teeth 106 more susceptible to wear.

[0011] The present invention has been made to solve the above-mentioned problems, and has an object to provide a worm wheel that can reduce the contact pressure of the worm wheel teeth against the worm teeth, thereby suppressing wear of the worm wheel teeth. [Means for solving the problem]

[0012] A worm wheel according to one aspect of the present invention includes a hub and a gear portion made of synthetic resin. The hub comprises an inner diameter side cylindrical portion, an outer peripheral surface having a hub side recess having the same cross-sectional shape around the entire circumference, and an outer diameter side cylindrical portion arranged coaxially around the inner diameter side cylindrical portion, and a connecting portion connecting the inner diameter side cylindrical portion and the outer diameter side cylindrical portion. The gear portion has a plurality of worm wheel teeth arranged at equal intervals in the circumferential direction, and is fixedly coupled to the hub so as to cover the radially outer end of the hub. In particular, in one embodiment of the worm wheel of the present invention, the gear portion has a gear side recess that is recessed more than adjacent portions on both axial sides, in an axial middle portion including a portion of the outer peripheral surface of at least the tooth surfaces of each of the worm wheel teeth that is located radially outside the hub side recess.

[0013] In one aspect of the present invention, the gear-side recess can be formed over the entire outer circumferential surface of the gear portion, that is, over the entire tooth flanks and tooth tip surfaces of the worm wheel teeth and the tooth bottom surface between a pair of circumferentially adjacent worm wheel teeth, including the portions located radially outward of the hub-side recess in the axial direction.

[0014] In one aspect of the present invention, at least the radially outer portion of at least one of the side surfaces on both axial sides of the hub side recess can be configured by a truncated cone surface having a linear generatrix shape inclined with respect to the central axis of the hub.

[0015] In one aspect of the present invention, the bottom surface of the hub side recess can be configured as a cylindrical surface centered on the central axis of the hub.

[0016] In one aspect of the present invention, the hub-side recess can have an arc-shaped cross-sectional shape.

[0017] In one aspect of the present invention, the outer diameter of a portion of the outer peripheral surface of the outer diameter side cylindrical portion that is located on one axial side of the hub side recess can be made different from the outer diameter of a portion of the outer peripheral surface of the outer diameter side cylindrical portion that is located on the other axial side of the hub side recess. Alternatively, the outer diameter of a portion of the outer peripheral surface of the outer diameter side cylindrical portion that is located on one axial side of the hub side recess and the outer diameter of a portion of the outer peripheral surface of the outer diameter side cylindrical portion that is located on the other axial side of the hub side recess can be made the same.

[0018] In one aspect of the present invention, the gear portion can be made of a synthetic resin mainly composed of polyamide 66. In this case, it is preferable to mix reinforcing fibers such as glass fibers, polyethylene fibers, carbon fibers, and / or aramid fibers into the synthetic resin. Alternatively, the gear portion may be made of a synthetic resin containing, as a main component, polyamide 6, polyamide 46, polyamide 9T, polyphenylene sulfide, polyethylene terephthalate, polyacetal, phenol, etc. In this case, it is also preferable to mix reinforcing fibers into the synthetic resin.

[0019] A method for manufacturing a worm wheel according to one aspect of the present invention includes the steps of: a step of setting a radially outer end of the hub in a cavity of a mold device having a tooth forming concave and convex portion on an inner peripheral surface thereof, and filling the cavity with molten synthetic resin; a step of forming the gear-side recess by cooling the synthetic resin to cause the synthetic resin to shrink and solidify, thereby generating a sink mark on an outer peripheral surface of an axially intermediate portion of the gear portion; Equipped with. Effect of the Invention

[0020] According to the worm wheel of one aspect of the present invention, the contact pressure of the worm wheel teeth against the worm teeth can be kept low, thereby suppressing wear of the worm wheel teeth. [Brief description of the drawings]

[0021] [Figure 1] FIG. 1 is a perspective view showing a worm reducer including a worm wheel according to a first example of the embodiment. [Diagram 2] FIG. 2 is a view of a worm reducer including a worm wheel according to a first example of the embodiment, viewed from a direction perpendicular to the central axis of the worm and the central axis of the worm wheel. [Diagram 3] FIG. 3 is a cross-sectional view taken along line AA of FIG. [Figure 4] FIG. 4 is a perspective view showing a worm wheel according to a first example of the embodiment. [Diagram 5] FIG. 5(A) is a view of a worm wheel according to a first example of an embodiment as viewed from the axial direction, and FIG. 5(B) is a cross-sectional view taken along line BB of FIG. 5(A). [Figure 6]FIG. 6 is an enlarged view of the upper part of FIG. 5(B). [Figure 7] FIG. 7(A) is a diagram showing a worm wheel according to a first example of the embodiment as viewed from a radial direction, and FIG. 7(B) is a cross-sectional view taken along line CC of FIG. 7(A). [Figure 8] FIG. 8 is a perspective view showing the hub according to the first example of the embodiment. [Figure 9] FIG. 9 is a cross-sectional view showing a method for manufacturing the worm wheel according to the first example of the embodiment. [Figure 10] FIG. 10 is a diagram showing a second example of the embodiment, and corresponds to FIG. 5(B). [Figure 11] FIG. 11 is a diagram showing a second example of the embodiment, and corresponds to FIG. [Figure 12] FIG. 12 is a perspective view showing the hub according to the second example of the embodiment. [Figure 13] FIG. 13 is a cross-sectional view showing a method for manufacturing a worm wheel according to a second example of the embodiment. [Figure 14] FIG. 14 is a diagram showing a third example of the embodiment, and corresponds to FIG. 5(B). [Figure 15] FIG. 15 is a diagram showing a third example of the embodiment, and corresponds to FIG. [Figure 16] FIG. 16 is a diagram showing a fourth example of the embodiment, and corresponds to FIG. 5(B). [Figure 17] FIG. 17 is a diagram showing a fourth example of the embodiment, and corresponds to FIG. [Figure 18] FIG. 18 is a diagram showing a fifth example of the embodiment, and corresponds to FIG. 5(B). [Figure 19] FIG. 19 is a diagram showing a fifth example of the embodiment, and corresponds to FIG. [Figure 20] FIG. 20 is a cross-sectional view showing an example of a conventional worm wheel structure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] [First Example of Implementation] A first embodiment of the present invention will be described with reference to Figs. 1 to 9. A worm wheel 1 of this embodiment is combined with a worm 2 to form a worm reduction gear 3. The worm 2 has threaded worm teeth (not shown in Figs. 1 to 3) on the outer circumferential surface of the axially intermediate portion. When the worm reduction gear 3 is incorporated into an electric power steering device, an output shaft of an electric motor is connected to a base end (the end on the lower left side in Fig. 2) of the worm 2 by spline engagement or via a joint. In other words, the worm 2 is configured to be rotatable by an electric motor. In the worm reduction gear 3 of this embodiment, when viewed from a direction perpendicular to the central axis of the worm wheel 1 and the central axis of the worm 2, the central axis of the worm wheel 1 and the central axis of the worm 2 intersect obliquely (form an acute angle).

[0023] The worm wheel 1 includes a metal hub 4 and a gear portion 5 made of synthetic resin.

[0024] The hub 4 comprises an inner diameter side cylindrical portion 6, an outer diameter side cylindrical portion 7 arranged coaxially around the inner diameter side cylindrical portion 6, and a connecting portion 8 connecting the inner diameter side cylindrical portion 6 and the outer diameter side cylindrical portion 7.

[0025] The outer diameter side cylindrical portion 7 has a hub side recess 9 having the same cross-sectional shape over the entire circumference on the outer peripheral surface of the axially middle portion. That is, the hub side recess 9 has no irregularities in the circumferential direction on either the bottom surface 10 constituting the inner surface or the side surfaces 11 on both axial sides. In this example, the hub side recess 9 has a substantially U-shaped cross-sectional shape.

[0026] In this example, the bottom surface 10 of the hub side recess 9 is formed of a cylindrical surface centered on the central axis of the hub 4, except for both axial ends that are connected to the side surfaces 11 on both axial sides. Moreover, the side surfaces 11 on both axial sides of the hub side recess 9 are formed of flat surfaces perpendicular to the central axis of the hub 4, except for both radial ends.

[0027] In this example, the outer diameter of the portion of the outer peripheral surface of the outer diameter side tube portion 7 located on one axial side of the hub side recess 9 (left side in Figures 5(B) and 6) and the outer diameter of the portion of the outer peripheral surface of the outer diameter side tube portion 7 located on the other axial side of the hub side recess 9 (right side in Figures 5(B) and 6) are the same.

[0028] Furthermore, the outer diameter side cylindrical portion 7 has a groove 12 provided around the entire circumference on the inner surface of one axial side portion, and a gear-shaped hub side uneven portion 13 consisting of concave and convex portions arranged alternately around the entire circumference.

[0029] The recessed groove 12 has an approximately V-shaped cross-sectional shape, and is formed around the entire circumference of the inner surface of the outer diameter side tube portion 7, in a portion located on one axial side of the portion to which the radially outer end of the connection portion 8 is connected, so as to be recessed radially outward.

[0030] The hub side uneven portion 13 is provided on the inner circumferential surface of the outer diameter side cylindrical portion 7 in a portion adjacent to one side of the recessed groove 12 in the axial direction.

[0031] The connection part 8 connects the other axial side part of the inner diameter side cylinder part 6 and the other axial side part of the outer diameter side cylinder part 7. In this example, the radially outer end part of the connection part 8 is connected to a part closer to one axial side than the other axial end part of the outer diameter side cylinder part 7. That is, the outer diameter side cylinder part 7 has a step part 37 facing radially inward on the inner peripheral surface of the end part on the other axial side. Also, in this example, the connection part 8 is configured in the shape of a hollow circular plate. The connection part may be configured of a plurality of connection pieces arranged radially at intervals in the circumferential direction, like spokes.

[0032] The hub 4 is made of a metal material such as steel, such as carbon steel or alloy steel, or a light alloy, such as aluminum, magnesium, or an alloy thereof. The hub 4 is formed by subjecting the metal material to cutting and / or plastic processing. From the standpoint of forming the hub 4 at low cost with good material yield, it is preferable to form the hub 4 by subjecting the metal material to plastic processing, such as forging, pressing, or flow forming.

[0033] The gear portion 5 has a plurality of worm wheel teeth 14 arranged at equal intervals in the circumferential direction, and is joined and fixed to the hub 4 so as to cover the radially outer end of the hub 4. In other words, the gear portion 5 is joined and fixed to the hub 4 by embedding the outer diameter side cylindrical portion 7 of the hub 4 and the radially outer end of the connection portion 8 over the entire circumference during injection molding (by insert molding). Each of the worm wheel teeth 14 is composed of a straight tooth whose tooth trace is parallel to the central axis of the hub 4.

[0034] The gear portion 5 has a gear side recess 15 recessed more than adjacent portions on both axial sides over the entire circumference in an axial middle portion including a portion located radially outside the hub side recess 9 among the outer peripheral surface including the tooth tip surface and tooth surface of each worm wheel tooth 14. In other words, the gear portion 5 has a gear side recess 15 recessed more than adjacent portions on both axial sides over the entire circumference in an axial middle portion including a portion that coincides with the hub side recess 9 in the axial direction among the tooth tip surface (radially outer end surface) and tooth surface (circumferential side surface) of each worm wheel tooth 14 and the tooth bottom surface between a pair of circumferentially adjacent worm wheel teeth 14. That is, the gear side recess 15 is composed of a tooth surface recess 15a provided on each tooth surface of the worm wheel tooth 14, a tooth tip surface recess 15b provided on each tooth tip surface of the worm wheel tooth 14, and a tooth bottom surface recess 15c provided on a tooth bottom surface between a pair of circumferentially adjacent worm wheel teeth 14. The gear-side recess 15 has a portion in the axial middle where the depth is constant or nearly constant, and the depth becomes gradually shallower from that portion toward both axial sides.

[0035] As described later, the gear side recess 15 is formed by generating a depression called a sink mark on the outer peripheral surface of the axial middle part of the gear portion 5 due to molding shrinkage after injection molding of the gear portion 5. That is, in this example, the outer peripheral surface of the gear portion 5 including the gear side recess 15 is formed by an injection molding surface.

[0036] The gear portion 5 has a gear side protrusion 16 that engages with the hub side recess 9 over the entire circumference of an outer diameter side cover portion 5a that covers the outer peripheral surface of the outer diameter side cylinder portion 7 of the hub 4. The gear side protrusion 16 is formed by injecting part of the synthetic resin that constitutes the gear portion 5 into the hub side recess 9 and cooling and solidifying it when the gear portion 5 is manufactured by injection molding.

[0037] Furthermore, the gear portion 5 has a protrusion 17 that engages with the groove 12 and a gear side uneven portion 18 that engages with the hub side uneven portion 13, on a first inner diameter side cover portion 5b that covers the inner peripheral surface of one axial side portion of the outer diameter side cylinder portion 7 of the hub 4. The protrusion 17 is formed by feeding a part of the synthetic resin that constitutes the gear portion 5 into the groove 12 and cooling and solidifying it when the gear portion 5 is manufactured by injection molding. The gear side uneven portion 18 is formed by feeding a part of the synthetic resin that constitutes the gear portion 5 into the recess that constitutes the hub side uneven portion 13 and cooling and solidifying it when the gear portion 5 is manufactured by injection molding.

[0038] In this example, the second inner diameter side cover portion 5c covering the inner peripheral surface (step portion 37) at the other axial end of the outer diameter side cylinder portion 7 of the hub 4 is engaged in the radial direction with the step portion 37 of the outer diameter side cylinder portion 7. In short, the gear portion 5 is joined to the hub 4 in a manner that wraps around the radially outer end of the hub 4, thereby ensuring the joining strength of the gear portion 5 to the hub 4.

[0039] The gear portion 5 is made of a synthetic resin mainly composed of polyamide 66. The synthetic resin of the gear portion 5 is preferably mixed with reinforcing fibers such as glass fiber, polyethylene fiber, carbon fiber, and / or aramid fiber. Alternatively, the gear portion 5 can be made of a synthetic resin mainly composed of polyamide 6, polyamide 46, polyamide 9T, polyphenylene sulfide, polyethylene terephthalate, polyacetal, phenol, etc. In this case, it is also preferable to mix reinforcing fibers such as glass fiber, polyethylene fiber, carbon fiber, and / or aramid fiber into the synthetic resin of the gear portion 5.

[0040] The worm wheel 1 has an inner diameter side cylindrical portion 6 of the hub 4 fitted and fixed to the outside of a rotating shaft. Specifically, for example, when the worm reduction gear 3 is incorporated into a column-assist type electric power steering device, the worm wheel 1 is supported and fixed to the steering shaft by fitting and fixing the inner diameter side cylindrical portion 6 of the hub 4 to the outside of the steering shaft. In this way, the column-assist type electric power steering device is configured to be able to apply auxiliary power output from an electric motor to the steering shaft via the worm reduction gear.

[0041] When the worm reduction gear 3 is incorporated into a pinion-assist electric power steering device, the worm wheel 1 is supported and fixed to the pinion shaft by externally fitting and fixing the inner diameter side cylindrical portion 6 of the hub 4 to the pinion shaft which is connected to the steering shaft via a universal joint or an intermediate shaft so as to be capable of transmitting torque. In this way, the pinion-assist electric power steering device is configured so that auxiliary power output from the electric motor can be applied to the pinion shaft via the worm reduction gear.

[0042] When the worm reduction gear 3 is incorporated into a dual pinion type electric power steering device, the worm wheel 1 is supported and fixed to the second pinion shaft by externally fitting and fixing the inner diameter side cylindrical portion 6 of the hub 4 to the second pinion shaft that is separate from the pinion shaft connected to the steering shaft so as to be able to transmit torque and that meshes with the rack shaft. This allows the dual pinion type electric power steering device to apply auxiliary power output from the electric motor to the rack shaft via the worm reduction gear and the second pinion shaft.

[0043] In the process of manufacturing the worm wheel 1 by insert molding the gear portion 5 onto the radially outer end of the hub 4, a die apparatus 19 as shown in Fig. 9 is used. The die apparatus 19 includes a fixed die 20 and a movable die 21 that is arranged so as to be movable toward and away from the fixed die 20 in the axial direction.

[0044] The fixed mold 20 has a runner 23 in the center for feeding molten synthetic resin into a cavity 22 defined between the fixed mold 20 and the movable mold 21 and the hub 4. The runner 23 can be configured, for example, by a hot runner provided with a heater (not shown) around the periphery.

[0045] The movable mold 21 is composed of three mold parts 24a, 24b, and 24c.

[0046] The first mold part 24a has a stepped cylindrical shape. Specifically, the first mold part 24a has a large diameter portion 25 on one axial side and a small diameter portion 26 on the other axial side.

[0047] The second mold part 24b has a cylindrical shape and is fitted onto the large diameter part 25 of the first mold part 24a without any rattle. The second mold part 24b has a protrusion 27 at the radially inner part of the other axial end part, which protrudes further axially to the other axial side than the radially outer part.

[0048] The third mold part 24c has a cylindrical shape and has, on its inner peripheral surface, a tooth-forming uneven portion 28 for forming a plurality of worm wheel teeth 14 of the gear portion 5. The tooth-forming uneven portion 28 has a surface shape that roughly matches the outer peripheral surface shape of the gear portion 5 to be formed. However, the tooth-forming uneven portion 28 does not have a convex portion having a surface shape that matches the gear-side recess 15 in a portion where the gear-side recess 15 will be provided in the completed state. In other words, the surface of the tooth-forming uneven portion 28 does not have any unevenness in the axial direction. Specifically, of the tooth forming uneven portion 28, the portion which forms the tip surface of each of the worm wheel teeth 14 is composed of a flat surface or a partial cylindrical surface that has no unevenness in the axial direction, the portion which forms the tooth surface of each of the worm wheel teeth 14 is composed of a flat surface or a concave curved surface that has no unevenness in the axial direction, and the portion which forms the bottom surface between adjacent worm wheel teeth 14 in the circumferential direction is composed of a flat surface or a partial cylindrical surface that has no unevenness in the axial direction.

[0049] In this example, the movable mold 21 is composed of three mold parts 24a, 24b, and 24c, but the movable mold can be constructed as a whole integrally (as one part), or can be constructed of two or four or more mold parts.

[0050] When insert-molding the gear portion 5 on the radially outer end of the hub 4 using the mold device 19, the inner diameter side cylinder portion 6 of the hub 4 is fitted onto the other axial end of the small diameter portion 26 of the first mold part 24a without any rattle. In addition, the tip face (the other axial end face) of the protrusion 27 of the second mold part 24b is abutted against the radially middle part of the side face on one axial side of the connection part 8 of the hub 4. Furthermore, the third mold part 24c is disposed around the outer diameter side cylinder portion 7 of the hub 4 and is axially sandwiched between the fixed mold 20 and the second mold part 24b. That is, the side face on one axial side of the radially outer part of the fixed mold 20 is abutted against the end face on the other axial side of the third mold part 24c, and the side face on the other axial side of the radially outer part of the second mold part 24b is abutted against the end face on one axial side of the third mold part 24c.

[0051] With the hub 4 set inside the mold device 19, a cavity 22 for molding the gear portion 5 is defined by a side surface on one axial direction side of the radial intermediate portion of the fixed mold 20, a side surface on the other axial direction side of the radial intermediate portion of the second mold part 24b and the outer peripheral surface of the protrusion 27, an inner peripheral surface of the third mold part 24c, and a radial outer portion of the hub 4. Furthermore, a disk gate 29 is defined between a side surface on one axial direction side of the radial inner portion of the fixed mold 20 and a side surface on the other axial direction side of the connection portion 8 of the hub 4.

[0052] As described above, with the hub 4 set inside the mold device 19, molten synthetic resin (molten resin) is sent into the cavity 22 through the runner 23 and the disk gate 29. The synthetic resin sent into the cavity 22 flows from a portion located on the other axial side of the radial outer portion of the hub 4, via a portion located radially outward of the outer diameter side cylindrical portion 7, toward a portion located on one axial side of the radial outer portion of the hub 4, thereby filling the entire cavity 22. Immediately after the synthetic resin has been filled entirely inside the cavity 22, the outer peripheral surface of the gear portion 5 is not provided with the gear side recess 15.

[0053] After the synthetic resin has been filled entirely inside the cavity 22, the synthetic resin is cooled. Cooling can be performed, for example, by natural cooling or liquid cooling in which a cooling liquid such as water is circulated inside the mold device 19. As the synthetic resin filled in the cavity 22 cools, it solidifies while shrinking (molding shrinkage), thereby molding the gear portion 5, and at the same time, the gear portion 5 is joined and fixed to the hub 4.

[0054] Here, the radial thickness of the portion of the gear portion 5 located radially outside the outer diameter side cylinder portion 7 and radially outside the hub side recess 9 is thicker than the radial thickness of the portion of the gear portion 5 located radially outside the outer diameter side cylinder portion 7 and adjacent to both axial sides of the hub side recess 9 by the amount of the gear side protrusion 16. In other words, the volume of the portion of the gear portion 5 located radially outside the outer diameter side cylinder portion 7 and radially outside the hub side recess 9 is larger than the volume of the portion of the gear portion 5 located radially outside the outer diameter side cylinder portion 7 and adjacent to both axial sides of the hub side recess 9. For this reason, the amount of contraction accompanying cooling of the synthetic resin is greater in the portion of the gear portion 5 located radially outside the outer diameter side cylinder portion 7 and radially outside the hub side recess 9 than in the portion of the gear portion 5 located radially outside the outer diameter side cylinder portion 7 and adjacent to both axial sides of the hub side recess 9.

[0055] Therefore, when the synthetic resin filled in the cavity 22 is cooled to shrink and solidify, a depression called a sink mark is formed on the outer peripheral surface of the axially middle part of the gear portion 5. As a result, a gear-side recess 15 is formed in the axially middle part of the outer peripheral surface of the gear portion 5, including a part located radially outside the hub-side recess 9, which is more recessed than the parts adjacent to both sides in the axial direction. In particular, in this example, since the bottom surface 10 of the hub-side recess 9 is formed by a cylindrical surface, a part with a constant or almost constant depth is formed in the axially middle part of the gear-side recess 15.

[0056] After the synthetic resin filled in the cavity 22 is cooled and solidified, the movable mold 21 is moved axially away from the fixed mold 20 to open the mold device 19 and remove the worm wheel 1. Then, the synthetic resin solidified in the runner 23 and disc gate 29 is removed. Thereafter, the surface and corners of the gear portion 5 are finished as necessary to obtain the worm wheel 1.

[0057] The worm wheel 1 of this example has a gear side recess 15 recessed more than adjacent parts on both axial sides on the outer peripheral surface of the axial middle part of the gear part 5 including the tooth tip surface and tooth surface of the multiple worm wheel teeth 14. Therefore, when the worm wheel teeth 14 of the worm wheel 1 are meshed with the worm teeth provided on the worm 2, the tooth surface of the worm tooth contacts the part of the tooth surface of the worm wheel teeth 14 where the gear side recess 15 is provided, that is, the tooth surface recess 15a. Therefore, the contact state between the tooth surface of the worm wheel teeth 14 and the tooth surface of the threaded worm teeth provided on the worm 2 can be line contact or surface contact, and the contact surface pressure between the tooth surface of the worm wheel teeth 14 and the tooth surface of the worm tooth can be kept low compared to the structure described in the pamphlet of International Publication No. 2017 / 135140. As a result, the wear of the worm wheel teeth 14 can be kept low, and the durability of the worm wheel 1 can be well ensured.

[0058] In addition, since the gear portion 5 is made of synthetic resin, when the torque transmitted by the worm reducer 3 increases and the force (surface pressure) applied to the tooth surface of the worm wheel teeth 14 increases, the worm wheel teeth 14 elastically deform, and the contact area between the tooth surface of the worm wheel teeth 14 and the tooth surface of the worm teeth increases so as to expand in the width direction (axial direction). In this example, the depth of both axial side portions of the gear side recess 15 becomes shallower toward both axial sides. Therefore, even when the torque transmitted by the worm reducer 3 is relatively small, the contact area between the tooth surface of the worm wheel teeth 14 and the tooth surface of the worm teeth can be secured to be large. From this viewpoint, the contact surface pressure between the tooth surface of the worm wheel teeth 14 and the tooth surface of the worm teeth can be kept low, and the durability of the worm wheel 1 can be secured.

[0059] In this example, the gear-side recess 15 has a portion with a constant or nearly constant depth in the axial middle portion. Therefore, even if the contact position between the tooth surface of the worm wheel tooth 14 and the tooth surface of the worm tooth shifts slightly in the axial direction of the worm wheel 1, the meshing state between the worm wheel tooth 14 and the worm tooth can be maintained in an appropriate state.

[0060] In the worm wheel 1 of this embodiment, the hub side recess 9 provided on the outer peripheral surface of the axially intermediate portion of the inner diameter side cylinder portion 6 of the hub 4 over the entire circumference is engaged with the gear side protrusion 16 of the gear portion 5. Therefore, even if an axial force is applied to the gear portion 5 from the meshing portion between the worm wheel teeth 14 and the worm teeth, the axial engagement between the hub side recess 9 and the gear side protrusion 16 can prevent the gear portion 5 from being displaced relative to the hub 4 in the axial direction. In addition, the contact area between the hub 4 and the gear portion 5 can be increased by the contact portion between the pair of side surfaces 11 of the hub side recess 9 and the side surfaces on both axial sides of the gear side protrusion 16, and the friction against the relative rotation between the hub 4 and the gear portion 5 can be increased. In short, the engagement between the hub side recess 9 and the gear side protrusion 16 can improve the holding force of the gear portion 5 to the hub 4.

[0061] In this embodiment, the axial direction of the outer diameter side cylindrical portion 7 of the hub 4 othersA groove 12 provided around the entire inner circumferential surface of the side portion is engaged with a protrusion 17 of the gear portion 5. Therefore, even if an axial force is applied to the gear portion 5 from the meshing portion between the worm wheel teeth 14 and the worm teeth, the axial engagement between the groove 12 and the protrusion 17 can also prevent the gear portion 5 from being displaced axially relative to the hub 4.

[0062] Furthermore, in this example, a gear-shaped hub-side uneven portion 13 provided on the inner peripheral surface of one axial side portion of the outer diameter side cylindrical portion 7 of the hub 4 is engaged with a gear-side uneven portion 18 of the gear portion 5. Therefore, the hub 4 is also engaged in the circumferential direction between the hub-side uneven portion 13 and the gear-side uneven portion 18. of Rotation (creep) relative to the gear portion 5 can be prevented.

[0063] However, so long as the force for holding the gear portion 5 on the hub 4 can be sufficiently ensured, the engagement between the recessed groove and the protrusion and / or the engagement between the hub side uneven portion and the gear side uneven portion may be omitted.

[0064] According to the manufacturing method of this example, as described above, the worm wheel 1 can be manufactured efficiently at low cost, while keeping the contact pressure of the worm wheel teeth 14 against the worm teeth low, thereby reducing wear on the worm wheel teeth 14.

[0065] In this example, molten synthetic resin is filled into cavity 22, which has tooth-forming unevenness 28 on its inner peripheral surface, and then cooled and solidified to form gear portion 5 having a plurality of worm wheel teeth 14 on its outer peripheral surface, and at the same time, gear portion 5 is fixedly joined to hub 4. In other words, after synthetic resin is injection molded around the hub, there is no need to perform gear cutting to form the worm wheel teeth. This allows worm wheel 1 to be manufactured at low cost.

[0066] In particular, in this example, when the synthetic resin constituting the gear portion 5 is injection molded and then cooled, a recess called a sink is generated on the outer peripheral surface of the axial middle portion of the gear portion 5 due to molding shrinkage, thereby forming the gear side recess 15. That is, the mold device 19 does not have a convex portion having a surface shape that conforms to the surface shape of the gear side recess 15 in the axial middle portion of the teeth forming uneven portion 28 of the inner peripheral surface of the cavity 22. In other words, the teeth forming uneven portion 28 does not have unevenness in the axial direction. Therefore, the mold device 19, and in particular, the third mold part 24c having the teeth forming uneven portion 28 on its inner peripheral surface, can be manufactured at low cost. In addition, when the worm wheel 1 is removed from the mold device 19 after the gear portion 5 is molded, the movable mold 21 can be easily removed from the periphery of the worm wheel 1 by simply displacing the movable mold 21 in the axial direction. Therefore, the worm wheel 1 can be manufactured efficiently at low cost.

[0067] In this example, the hub side recess 9 provided on the outer peripheral surface of the outer diameter side cylinder portion 7 of the hub 4 has the same cross-sectional shape over the entire circumference. In other words, the hub side recess 9 has no unevenness in the circumferential direction on either the bottom surface 10 or the side surfaces 11 on both axial sides. Therefore, even if the gear side recess 15 is formed by generating a depression called a sink mark on the outer peripheral surface of the axial middle portion of the gear portion 5 during molding, the sink mark can be generated uniformly (the amount of molding shrinkage can be made uniform). In short, the shape of the worm wheel teeth 14 can be stabilized, and the pitch of the worm wheel teeth 14 can be stabilized. As a result, the meshing state between the worm wheel teeth 14 and the worm teeth can be stabilized.

[0068] [Second Example of the Implementation Form] A second embodiment of the present invention will be described with reference to Figures 10 to 13. In a worm wheel 1a of this embodiment, the cross-sectional shape of a hub side recess 9a provided on the outer peripheral surface of the axially intermediate portion of an outer diameter side cylindrical portion 7a of a hub 4a is made different from that of the first embodiment. In this embodiment, the hub side recess 9a is substantially trapezoidal and has a cross-sectional shape that is asymmetric in the axial direction. The cross-sectional shape of the hub side recess 9a is the same over the entire circumference.

[0069] That is, in this example, the bottom surface 10a of the hub side recess 9a is formed of a cylindrical surface centered on the central axis of the hub 4a, except for the ends on both axial sides connected to the side surfaces 11a1 and 11a2 on both axial sides. The bottom surface 10a is provided in a portion closer to one axial side than the axial center position of the outer diameter side cylindrical portion 7a.

[0070] The side surface 11a1 on one axial side of the hub-side recess 9a is formed of a truncated cone surface having a linear generatrix shape inclined toward one axial side as it moves radially outward, except for both radial end portions, and the side surface 11a2 on the other axial side is formed of a truncated cone surface having a linear generatrix shape inclined toward the other axial side as it moves radially outward, except for both radial end portions. In this example, the inclination angle θ1 of the side surface 11a1 on one axial side with respect to the bottom surface 10a is larger than the inclination angle θ2 of the side surface 11a2 on the other axial side with respect to the bottom surface 10a (θ1>θ2).

[0071] In this example, too, the gear portion 5 has a gear side recess 15 around the entire circumference in the axially middle portion, including the portion of its outer surface that is located radially outside the hub side recess 9a, which is recessed more than the adjacent portions on both axial sides.

[0072] According to this embodiment, the cross-sectional shape of the gear-side recess 15 can be easily restricted to a desired shape.

[0073] That is, when the gear portion 5 is molded, the synthetic resin filled in the cavity 22, which is filled in the radially outer portion of the outer diameter side tubular portion 7a, varies in the degree of occurrence of sink marks between one axial side portion far from the disk gate 29 and the other axial side portion close to the disk gate 29, depending on the volume of the synthetic resin including the synthetic resin filled in the disk gate 29, the temperature distribution in the cavity 22, and the like. Specifically, the degree of occurrence of sink marks tends to be greater in the other axial side portion close to the disk gate 29 than in the one axial side portion far from the disk gate 29, based on the presence of the synthetic resin filled in the disk gate 29. Therefore, in this example, the cross-sectional shape of the hub side recess 9a is made asymmetrical with respect to the axial direction, specifically, the bottom surface 10a is disposed in a portion closer to one axial side than the axial center position of the outer diameter side tubular portion 7a, and the inclination angle θ1 of the side surface 11a1 on one axial side is made larger than the inclination angle θ2 of the side surface 11a2 on the other axial side, thereby adjusting the degree of occurrence of sink marks. That is, the occurrence of sink marks is made more uniform between one axial side portion far from the disk gate 29 and the other axial side portion close to the disk gate 29. This makes it possible to form the gear-side recess 15 having a desired cross-sectional shape based on molding shrinkage.

[0074] If the cross-sectional shape of the gear-side recess 15 can be formed into a desired shape, the cross-sectional shape of the hub-side recess 9a can also be made symmetrical with respect to the axial direction.

[0075] In this example, the side surfaces 11a1, 11a2 on both axial sides of the hub side recess 9a are formed of truncated cone surfaces having a straight generatrix shape. Therefore, when the hub 4 is set inside the mold device 19 and molten synthetic resin is filled into the cavity 22, the synthetic resin can flow smoothly along the hub side recess 9a. This makes it possible to prevent the occurrence of weld lines and air bubbles, and makes it easy to ensure good strength of the worm wheel 1a. The configuration and effects of the other parts are the same as those of the first example of the embodiment.

[0076] [Third Example of the Implementation Form] A third embodiment of the present invention will be described with reference to Figures 14 and 15. In a worm wheel 1b of this embodiment, a hub-side recess 9b provided on the outer peripheral surface of the axially intermediate portion of the outer diameter side cylindrical portion 7b of a hub 4b has an arc-shaped cross-sectional shape. The cross-sectional shape of the hub-side recess 9b is the same over the entire circumference.

[0077] In this example, the gear portion 5 also has a gear side recess 15 around the entire circumference in the axially middle portion, including the portion of the outer surface that is located radially outside the hub side recess 9b, which is recessed more than the adjacent portions on both axial sides.

[0078] According to this embodiment, the formation of corners on the inner surface of the gear-side recess 15 can be reliably prevented.

[0079] In the first example of the embodiment, the inner surface of the hub side recess 9 has a bottom surface 10 formed by a cylindrical surface centered on the central axis of the hub 4, and a pair of side surfaces 11 formed by flat surfaces perpendicular to the central axis of the hub 4. For this reason, when the gear portion 5 is molded, if a sink mark is generated in an axial middle portion including a portion located radially outward of the hub side recess 9 on the outer circumferential surface of the gear portion 5 to form the gear side recess 15, a corner portion may be formed on the inner surface of the gear side recess 15.

[0080] In this example, the cross-sectional shape of the hub side recess 9b is an arc shape, so that when the gear portion 5 is molded, a sink mark is generated in an axially intermediate portion including a portion located radially outward of the hub side recess 9b on the outer circumferential surface of the gear portion 5, and the gear side recess 15 can be smoothly recessed along the hub side recess 9b when forming the gear side recess 15. In short, it is possible to reliably prevent corners from being formed on the inner surface of the gear side recess 15, and when the worm wheel teeth 14 are meshed with the worm teeth, it is possible to reliably prevent local contact between the tooth surfaces of the worm wheel teeth 14 and the worm teeth, and it is possible to reliably prevent local contact between the tooth surfaces of the worm wheel teeth 14 and the worm teeth. And Surface contact can be achieved.

[0081] In addition, because the cross-sectional shape of the hub side recess 9b is an arc shape, when the hub 4 is set inside the mold device 19 (see FIG. 9) and molten synthetic resin is filled into the cavity 22, the synthetic resin can flow smoothly along the hub side recess 9b. This makes it possible to prevent the occurrence of weld lines and air bubbles, making it easier to ensure good strength of the worm wheel 1b. The configuration and effects of the other parts are the same as those of the first embodiment.

[0082] [Fourth Example of the Implementation Form] A fourth embodiment of the present invention will be described with reference to Figures 16 and 17. In a worm wheel 1c of this embodiment, the cross-sectional shape of a hub side recess 9c provided on the outer peripheral surface of the axially middle part of an outer diameter side cylindrical portion 7c of a hub 4c is made different from that of the first embodiment.

[0083] The bottom surface 10 constituting the inner surface of the hub side recess 9c is formed of a cylindrical surface centered on the central axis of the hub 4, except for both axial ends connected to the side surfaces 11b1, 11b2 on both axial sides.

[0084] Each of the side surfaces 11b1 and 11b2 on both axial sides constituting the inner surface of the hub-side recess 9c has a flat surface portion 30a, 30b on the radially inner side and an inclined surface portion 31a, 31b on the radially outer side. That is, the side surface 11b1 on one axial side has a flat surface portion 30a provided on the radially inner side and perpendicular to the central axis of the hub 4c, and an inclined surface portion 31a provided on the radially outer side and constituted by a truncated cone surface having a linear generatrix shape inclined toward one axial side as it moves radially outward. The side surface 11b2 on the other axial side has a flat surface portion 30b provided on the radially inner side and perpendicular to the central axis of the hub 4c, and an inclined surface portion 31b provided on the radially outer side and constituted by a truncated cone surface having a linear generatrix shape inclined toward the other axial side as it moves radially outward.

[0085] In this example, the gear portion 5 also has a gear side recess 15 around the entire circumference in the axially middle portion of its outer surface, including the portion located radially outside the hub side recess 9c, which is recessed more than the adjacent portions on both axial sides.

[0086] The gear portion 5 has a gear side protrusion 16a that engages with the hub side recess 9c over the entire circumference in a portion that covers the outer peripheral surface of the outer diameter side cylindrical portion 7c of the hub 4c. Side surfaces 32a, 32b on both axial sides of the gear side protrusion 16a abut against side surfaces 11b1, 11b2 on both axial sides of the hub side recess 9c without any gap. That is, the side surfaces 32a, 32b on both axial sides have flat surface portions 33a, 33b on the radially inner side and inclined surface portions 34a, 34b on the radially outer side.

[0087] The side surface 32a on one axial side is provided on the radially inner side and has a flat surface portion 33a that is perpendicular to the central axis of the hub 4c, and an inclined surface portion 34a that is provided on the radially outer side and is constituted by a truncated cone surface having a linear generatrix shape that inclines in a direction toward one axial side as it moves radially outward. The side surface 32b on the other axial side is provided on the radially inner side and has a flat surface portion 33b that is perpendicular to the central axis of the hub 4c, and an inclined surface portion 34b that is provided on the radially outer side and is constituted by a truncated cone surface having a linear generatrix shape that inclines in a direction toward the other axial side as it moves radially outward.

[0088] According to the worm wheel 1c of this embodiment, it is possible to ensure both the force for holding the gear portion 5 to the hub 4c and the strength of the gear portion 5 at a high level.

[0089] That is, even if an axial force is applied to the gear portion 5 from the meshing portion between the worm wheel teeth 14 and the worm teeth, the axial engagement between the hub side recess 9c and the gear side protrusion 16a, particularly the abutment portion between the flat surface portion 30a (or 30b) of the side surface 11b1 (or 11b2) of the hub side recess 9c and the flat surface portion 33a (or 33b) of the side surface 32a (or 32b) of the gear side protrusion 16a, can prevent relative axial displacement of the gear portion 5 with respect to the hub 4c. This can improve the holding force of the gear portion 5 with respect to the hub 4c.

[0090] Also, inclined surface portions 31a, 31b formed of a truncated cone surface having a linear generatrix shape are arranged on the radially outer side of each of side surfaces 11b1, 11b2 on both axial sides of the hub side recess 9c. Therefore, when the hub 4 is set inside the mold device 19 (see FIG. 9) and molten synthetic resin is filled into the cavity 22, the synthetic resin can flow smoothly along the hub side recess 9c. This makes it possible to suppress the occurrence of weld lines and air bubbles, and makes it easy to ensure the strength of the worm wheel 1c. The configuration and effects of the other portions are the same as those of the first embodiment.

[0091] [Fifth Example of the Implementation Form] A fifth embodiment of the present invention will be described with reference to Figures 18 and 19. In a worm wheel 1d of this embodiment, the outer diameter of a portion of the outer peripheral surface of an outer diameter side cylinder portion 7d of a hub 4d that is located on one axial side of the hub side recess 9d is different from the outer diameter of a portion of the outer peripheral surface of the outer diameter side cylinder portion 7d that is located on the other axial side of the hub side recess 9d. Specifically, the outer diameter D 35 However, the outer diameter D of the second cylindrical surface portion 36 provided on the other axial side of the hub side recess 9d on the outer peripheral surface of the outer diameter side cylindrical portion 7d is 36 Smaller than (D 35 <D 36 ).

[0092] The side surfaces 11c1, 11c2 on both axial sides constituting the inner surface of the hub-side recess 9d are configured as truncated cone surfaces having a linear generatricle shape that inclines in directions that move away from each other in the axial direction as they move radially outward. In this example, the inclination angle of the side surface 11c1 on one axial side relative to the bottom surface 10 is equal to the inclination angle of the side surface 11c2 on the other axial side relative to the bottom surface 10. Meanwhile, the radial height of the side surface 11c1 on one axial side is made lower than the radial height of the side surface 11c2 on the other axial side. As a result, the outer diameter D of the first cylindrical surface portion 35 on one axial side 35 The outer diameter D of the second cylindrical surface portion 36 on the other axial side 36 It is smaller than that.

[0093] In this example, the gear portion 5 also has a gear side recess 15 around the entire circumference in the axially middle portion of its outer surface, including a portion located radially outside the hub side recess 9d, which is recessed more than the adjacent portions on both axial sides.

[0094] According to this embodiment, the cross-sectional shape of the gear-side recess 15 can be easily restricted to a desired shape.

[0095] That is, as explained in the second embodiment, the synthetic resin filled in the cavity 22, which is filled in the radially outer portion of the outer diameter side cylinder portion 7d, has a sink mark occurrence rate that varies between one axial side portion far from the disk gate 29 and the other axial side portion close to the disk gate 29. In this example, the outer diameter D 35 However, the outer diameter D of the second cylindrical surface portion 36 provided on the other axial side of the hub side recess 9d on the outer peripheral surface of the outer diameter side cylindrical portion 7d is 36 By making the thickness smaller than the thickness of the molded product, the occurrence of sink marks is adjusted. This makes it possible to form gear-side recesses 15 having a desired cross-sectional shape based on molding shrinkage. The configurations and effects of other parts are the same as those of the first and second embodiments.

[0096] Although the embodiment of the present invention has been described above, the present invention is not limited thereto and can be modified as appropriate without departing from the technical concept of the invention. Moreover, the first to fifth embodiments can be combined as appropriate within a range that does not cause contradictions. That is, the cross-sectional shape of the hub-side recess can be modified as appropriate so that a gear-side recess having a desired cross-sectional shape can be obtained during molding of the gear portion, and the holding force of the gear portion relative to the hub and / or the strength of the gear portion can be improved as necessary.

[0097] Furthermore, the worm wheel of the present invention can be incorporated into worm reduction gears constituting reduction gears of various machines, and can be used in addition to worm reduction gears constituting reduction gears of electric power steering devices. [Explanation of symbols]

[0098] 1, 1a, 1b, 1c, 1d Worm Wheel 2. Warm 3 Worm reducer 4, 4a, 4b, 4c, 4d hubs 5 Gear section 5a Outer diameter side cover part 5b First inner diameter side cover part 5c Second inner diameter side cover part 6 Inner cylinder part 7, 7a, 7b, 7c, 7d Outer diameter side cylinder part 8 Connections 9, 9a, 9b, 9c, 9d Hub side recess 10, 10a Bottom 11, 11a1, 11a2, 11b1, 11b2, 11c1, 11c2 side 12 Groove 13 Hub side unevenness 14 worm wheel teeth 15 Gear side recess 15a Tooth surface recess 15b Tooth tip recess 15c Tooth bottom recess 16, 16a Gear side protrusion 17 protrusion 18 Gear side unevenness 19. Mold Equipment 20 Fixed mold 21 Movable mold 22 Cavity 23 Runner 24a First mold part 24b Second mold part 24c 3rd mold part 25 Large diameter section 26 Small diameter section 27 Convex 28 Tooth forming concave and convex part 29 Disc Gate 30a, 30b flat surface section 31a, 31b Slope section 32a, 32b side 33a, 33b flat surface section 34a, 34b Slope section 35 First cylindrical surface part 36 Second cylindrical surface part 37 Stepped section 100 Worm Wheel 101 Hub 102 Gear section 103 Inner cylinder part 104 Outer cylinder part 105 Connection 106 Worm Wheel Teeth

Claims

1. a hub including an inner diameter side cylindrical portion, an outer diameter side cylindrical portion having a hub side recess having the same cross-sectional shape over the entire circumference on an outer peripheral surface thereof and arranged coaxially with the inner diameter side cylindrical portion around the inner diameter side cylindrical portion, and a connecting portion connecting the inner diameter side cylindrical portion and the outer diameter side cylindrical portion; a gear portion made of synthetic resin, the gear portion having a plurality of worm wheel teeth arranged at equal intervals in the circumferential direction and coupled and fixed to the hub so as to cover the radially outer end portion of the hub; Equipped with the gear portion has a gear-side recess in an axial middle portion including a portion of an outer peripheral surface of at least a tooth surface of each of the worm wheel teeth that is positioned radially outward of the hub-side recess, the gear portion being recessed more than adjacent portions on both axial sides; an outer diameter of a portion of the outer peripheral surface of the outer diameter side cylindrical portion that is located on one axial side of the hub side recess and an outer diameter of a portion of the outer peripheral surface of the outer diameter side cylindrical portion that is located on the other axial side of the hub side recess are different from each other; Worm wheel.

2. At least a radially outer portion of at least one of the side surfaces on both axial sides of the hub side recess is configured with a truncated cone surface having a linear generatrix shape inclined in a direction in which the axial width of the hub side recess becomes wider as it goes radially outward with respect to the central axis of the hub.

2. The worm wheel according to claim 1.

3. The bottom surface of the hub-side recess is formed of a cylindrical surface centered on the central axis of the hub.

3. The worm wheel according to claim 1 or 2.

4. a hub including an inner diameter side cylindrical portion, an outer diameter side cylindrical portion having a hub side recess having the same cross-sectional shape over the entire circumference on an outer peripheral surface thereof and arranged coaxially with the inner diameter side cylindrical portion around the inner diameter side cylindrical portion, and a connecting portion connecting the inner diameter side cylindrical portion and the outer diameter side cylindrical portion; a gear portion made of synthetic resin, the gear portion having a plurality of worm wheel teeth arranged at equal intervals in the circumferential direction and coupled and fixed to the hub so as to cover the radially outer end portion of the hub; Equipped with the gear portion has a gear-side recess in an axial middle portion including a portion of an outer peripheral surface of at least a tooth surface of each of the worm wheel teeth that is positioned radially outward of the hub-side recess, the gear portion being recessed more than adjacent portions on both axial sides; The hub-side recess has an arc-shaped cross-sectional shape. Worm wheel.

5. a hub including an inner diameter side cylindrical portion, an outer diameter side cylindrical portion having a hub side recess having the same cross-sectional shape over the entire circumference on an outer peripheral surface thereof and arranged coaxially with the inner diameter side cylindrical portion around the inner diameter side cylindrical portion, and a connecting portion connecting the inner diameter side cylindrical portion and the outer diameter side cylindrical portion; a gear portion made of synthetic resin, the gear portion having a plurality of worm wheel teeth arranged at equal intervals in the circumferential direction and coupled and fixed to the hub so as to cover the radially outer end portion of the hub; Equipped with The gear portion has a gear-side recess in an axial middle portion including a portion of an outer peripheral surface of at least each tooth surface of the worm wheel teeth that is located radially outward of the hub-side recess, the gear portion being recessed more than adjacent portions on both axial sides. A method for manufacturing a worm wheel, comprising the steps of: a step of setting a radially outer end of the hub in a cavity of a mold device having a tooth forming concave and convex portion on an inner peripheral surface thereof, and filling the cavity with molten synthetic resin; a step of forming the gear-side recess by cooling the synthetic resin to cause the synthetic resin to shrink and solidify, thereby generating a sink mark on an outer peripheral surface of an axially intermediate portion of the gear portion; A method for manufacturing a worm wheel comprising the steps of:

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