Worm wheel and method of manufacturing worm wheel

Abstract

To improve performance of a worm wheel.SOLUTION: A worm wheel 100 is equipped with wheel teeth 20 that are tooth parts formed on an outer periphery and to be engaged with a worm shaft 6, and a tooth bottom 30 that is formed between adjacent wheel teeth 20. The tooth bottom 30 has a center part 31 that includes a center in an axial direction of the worm wheel 100, formed in a predetermined range in the axial direction, and has a constant distance in a diametrical direction from the center of the worm wheel 100, and curved surface parts 36 that are respectively formed at both end portions of the center part 31 in the axial direction, extend to both end portions of the tooth bottom 30, and increase a distance in the diametrical direction from the center of the worm wheel 100 toward both end portions of the tooth bottom 30. The wheel teeth 20 have tooth portions 21 ranging to the center part 31, and worm wheel tooth parts 26 ranging to the curved surface parts 36.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a worm wheel and a method for manufacturing a worm wheel. [Background technology]

[0002] Patent Document 1 discloses a reduction mechanism including a worm and a metallic reduction gear that meshes with the worm. The tooth surface of the reduction gear has a worm wheel tooth shape, and the tooth bottom is formed into a curved surface. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2012-163179 A Summary of the Invention [Problem to be solved by the invention]

[0004] The reduction gear in Patent Document 1 has a worm wheel tooth shape with a curved tooth bottom. Therefore, since the area where the worm and reduction gear mesh is large, poor machining accuracy of the teeth and tooth bottom may cause poor meshing between the worm and reduction gear, which may result in large friction between them. Furthermore, it is difficult to measure the dimensional accuracy of the teeth and tooth bottom of the reduction gear during manufacturing, etc., because the worm wheel tooth shape has a curved tooth bottom.

[0005] In contrast, when the reduction gear has a helical tooth shape with a flat tooth bottom, the area where the worm and reduction gear mesh is small, reducing friction between them, and the flat tooth bottom makes it easy to measure the dimensional accuracy of the teeth and tooth bottom of the reduction gear. However, the teeth are taller than those of a worm wheel tooth shape with a curved tooth bottom, reducing the strength of the teeth, and the machining time is long because it takes time to machine the flat parts of the teeth and tooth bottom. Thus, the worm wheel tooth shape with a curved tooth bottom and the helical tooth shape with a flat tooth bottom each have their advantages and disadvantages.

[0006] The present invention has been made in consideration of the above problems, and has an object to improve the performance of a worm wheel. [Means for solving the problem]

[0007] The present invention is a worm wheel comprising teeth formed on an outer periphery and meshing with a worm shaft, and tooth bottoms formed between adjacent tooth portions, the tooth bottoms being formed over a predetermined range in the axial direction including the axial center of the worm wheel, and having a central portion having the same radial distance from the center of the worm wheel, and curved surface portions formed at both ends of the central portion in the axial direction and extending to both ends of the tooth bottom, with the radial distance from the center of the worm wheel increasing as the tooth portions approach both ends of the tooth bottom, and the tooth portions having helical tooth portions connected to the central portion and worm wheel tooth portions connected to the curved surface portions.

[0008] In this invention, the tooth bottom of the worm wheel has a central portion formed over a predetermined range in the axial direction, and the tooth portion has a helical tooth portion connected to the central portion. Therefore, the helical tooth portion and central portion reduce the meshing area between the worm shaft and the worm wheel, reducing friction between them and facilitating measurement of the dimensional accuracy of the tooth portion and tooth bottom of the worm wheel. Furthermore, the tooth bottom of the worm wheel has curved surface portions formed on both ends of the central portion in the axial direction. Therefore, the curved surface portions can improve the strength of the tooth portion and shorten the processing time of the worm wheel compared to when the entire tooth bottom is the central portion.

[0009] The present invention provides a method for manufacturing a worm wheel having teeth formed on its outer periphery and meshing with a worm shaft, and tooth bottoms formed between adjacent tooth portions, comprising a first step of rotating a tool having a blade on its outer periphery and bringing it close to the worm wheel in the axial direction to perform cutting, and a second step of moving the tool brought close to the worm wheel axially a distance smaller than the axial length of the worm wheel to perform cutting.The present invention is characterized in that the first and second steps form a tooth bottom having a central portion formed over a predetermined range in the axial direction including the axial center, and curved surface portions formed at both ends of the central portion in the axial direction and extending to both ends of the tooth bottom, and also form a tooth portion having a helical tooth portion connected to the central portion and a worm wheel tooth portion connected to the curved surface portion.

[0010] In this invention, the helical teeth and the central portion reduce the meshing area between the worm shaft and the worm wheel, reducing friction between them and facilitating measurement of the dimensional accuracy of the teeth and tooth bottom of the worm wheel. Furthermore, the curved surface portion improves the strength of the teeth and shortens the processing time of the worm wheel compared to when the entire tooth bottom is in the central portion. Effect of the Invention

[0011] According to the present invention, the performance of the worm wheel can be improved. [Brief description of the drawings]

[0012] [Figure 1] FIG. 2 is a side view of the worm wheel according to the embodiment of the present invention, showing a state in which the worm wheel is engaged with the worm shaft. [Diagram 2] FIG. 2 is a perspective view showing a tooth portion and a tooth bottom of the worm wheel according to the embodiment of the present invention. [Diagram 3] FIG. 2 is a schematic diagram showing a method for manufacturing a worm wheel according to an embodiment of the present invention, as viewed from the axial direction of a hob. [Figure 4] FIG. 2 is a schematic diagram showing a method for manufacturing a worm wheel according to an embodiment of the present invention, as viewed from a radial direction of a hob. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, a worm wheel 100 according to an embodiment of the present invention will be described with reference to the drawings.

[0014] 1, the worm wheel 100 is used in the worm reducer 1 and meshes with the worm shaft 6. For example, the worm shaft 6 is connected to a rotating shaft (not shown) of a motor, and the worm wheel 100 has a pinion shaft (not shown) connected to its center. The rotational force output from the motor is transmitted through the worm wheel 100 and the pinion shaft to a rack shaft that rotates the wheels, and assists the driver in steering the wheels.

[0015] The rotation of the motor rotates the worm shaft 6, which in turn rotates the worm wheel 100. The rotation of the worm shaft 6 is decelerated and transmitted to the worm wheel 100, and the pinion rotates in response to the rotation of the worm wheel 100. In this way, the worm reducer 1 decelerates the rotation of the motor via the worm shaft 6 and the worm wheel 100 and transmits it to the pinion shaft.

[0016] The worm shaft 6 has a cylindrical shaft body 6a and shaft teeth 6b formed in a spiral shape on the outer periphery of the shaft body 6a. The worm wheel 100 includes an annular wheel body 10, wheel teeth 20 as tooth portions formed on the outer periphery of the wheel body 10, and tooth bottoms 30 formed between adjacent wheel teeth 20. A plurality of wheel teeth 20 are formed on the outer periphery of the wheel body 10. The pitch of the wheel teeth 20 is approximately equal to the pitch of the shaft teeth 6b, and the wheel teeth 20 mesh with the shaft teeth 6b. The wheel body 10 and the wheel teeth 20 are made of a resin material and are molded integrally. A metal sleeve 40 is provided on the inner periphery of the wheel body 10.

[0017] When the worm shaft 6 rotates in a predetermined direction (direction D1 shown in FIG. 1), the wheel teeth 20 are pushed by the shaft teeth 6b. As a result, the worm wheel 100 rotates in direction D3 shown in FIG. 1. When the worm shaft 6 rotates in the opposite direction (direction D2 shown in FIG. 1), the wheel teeth 20 are pushed in the opposite direction by the shaft teeth 6b. As a result, the worm wheel 100 rotates in direction D4 shown in FIG. 1. Hereinafter, the axial direction of the worm wheel 100 will also be simply referred to as the "axial direction."

[0018] Fig. 2 is a perspective view showing the wheel teeth 20 and the tooth bottoms 30 of the worm wheel 100. As shown in Fig. 2, the tooth bottoms 30 have a central portion 31 formed over a predetermined range in the axial direction including the axial center, and curved surface portions 36 formed on both ends of the central portion 31 in the axial direction. The central portion 31 and the curved surface portions 36 are formed on each of the multiple tooth bottoms 30 of the worm wheel 100.

[0019] The central portion 31 has the same radial distance from the center of the worm wheel 100. The central portion 31 is formed in the range indicated by the arrow L in FIG. 2. Note that the arrow L is greatly exaggerated in FIG. 2. In this embodiment, the axial length of the central portion 31 is about one-ninth (approximately 11%) of the axial length of the worm wheel 100.

[0020] A pair of curved surface portions 36 are formed symmetrically at both ends of the central portion 31 in the axial direction. The curved surface portions 36 are formed to extend to both ends of the tooth bottom 30 in the axial direction. The curved surface portions 36 increase in radial distance from the center of the worm wheel 100 toward both ends of the tooth bottom 30. In other words, the curved surface portions 36 are formed in an arc shape such that the outer diameter of the tooth bottom 30 increases from the central portion 31 toward the ends of the tooth bottom 30 (see FIG. 3(c)). The tooth bottom 30 is composed of the central portion 31 and a pair of curved surface portions 36. In other words, the worm wheel 100 has both features of a helical tooth shape in which the tooth bottom 30 has the central portion 31 and a worm wheel tooth shape in which the tooth bottom 30 has the curved surface portions 36. The curved surface portions 36 are formed in a curved shape corresponding to the shaft teeth 6b of the worm shaft 6.

[0021] The wheel teeth 20 have a helical tooth portion 21 connected to the central portion 31 and a worm wheel tooth portion 26 connected to the curved surface portion 36. A pair of the worm wheel tooth portions 26 are formed symmetrically at both ends of the helical tooth portion 21 in the axial direction. The wheel teeth 20 are formed such that the width dimension (dimension in the circumferential direction) is large at the worm wheel tooth portion 26 (both ends in the axial direction) and small at the helical tooth portion 21 (center portion in the axial direction). In other words, the circumferential interval between the adjacent wheel teeth 20 is large near the helical tooth portion 21. The helical tooth portion 21 is the place where the worm shaft 6 is most fully engaged (the place where the worm wheel 100 and the worm shaft 6 are closest to each other), and the worm shaft 6 is largely inserted between the adjacent wheel teeth 20. Therefore, by forming the helical tooth portion 21 in this manner, the worm shaft 6 can easily enter between the adjacent wheel teeth 20, and the two can easily engage with each other. A chamfered portion 24 is formed between the top surface 22 and the two side surfaces 23 of the wheel tooth 20. The chamfered portion 24 is formed on the beveled tooth portion 21 and a part of the worm wheel tooth portion 26 (in other words, at the axial center of the wheel tooth 20). This prevents the beveled tooth portion 21 between the top surface 22 and the two side surfaces 23 from coming into contact with the worm shaft 6 and being damaged, compared to a case where the chamfered portion 24 is not formed.

[0022] The side surface 23 of the wheel tooth 20 is formed flat between the tooth bottom 30 and the top surface 22. In the worm wheel 100, the shaft teeth 6b of the worm shaft 6 come into contact with the tooth bottom 30 and the side surface 23 and mesh with the worm shaft 6.

[0023] Next, a manufacturing method of the worm wheel 100 (specifically, a method of machining the wheel teeth 20 and the tooth bottom 30) will be described with reference to Fig. 3 and Fig. 4. Fig. 3 is a schematic diagram showing the manufacturing method of the worm wheel 100 as viewed from the axial direction of a hob 60 described later, and Fig. 4 is a schematic diagram showing the manufacturing method of the worm wheel 100 as viewed from the radial direction of the hob 60.

[0024] The wheel teeth 20 and the tooth bottom 30 are machined using a hob 60 as a tool having a blade on the outer periphery. First, as shown by the arrow A in FIG. 3(a), in the first step, the hob 60 is rotated and brought close to the worm wheel 100 for cutting. Specifically, while the worm wheel 100 is not rotating, the position of the center O of the hob 60 in the vertical direction (the axial direction of the worm wheel 100) is adjusted to a position corresponding to one of the two boundaries between the central portion 31 of the tooth bottom 30 and the curved surface portion 36. Then, while rotating the hob 60, the hob 60 is moved in the radial direction of the worm wheel 100 to bring it closer. As a result, as shown in FIG. 3(b), the wheel body 10 of the worm wheel 100 is cut into a curved shape, and one curved surface portion 36 of the tooth bottom 30 and one worm wheel tooth portion 26 of the wheel tooth 20 are formed.

[0025] Next, as shown by the arrow B in FIG. 3(b), in the second step, the hob 60, which has been brought close to the worm wheel 100, is moved in the axial direction of the worm wheel 100 while being rotated to perform cutting. Specifically, the hob 60 is moved downward in FIG. 3(b), and the center O of the hob 60 is moved to a position corresponding to the other of the two boundaries between the central portion 31 and the curved surface portion 36. The moving distance of the hob 60 (the moving distance of the center O) is smaller than the axial length of the worm wheel 100. As a result, as shown in FIG. 3(c), the central portion 31 having a length equal to the moving distance of the hob 60 is formed in the tooth bottom 30, and the other curved surface portion 36 is formed. Also, the helical tooth portion 21 having a length equal to the moving distance of the hob 60 is formed, and the other worm wheel tooth portion 26 is formed. That is, when forming the central portion 31 having an axial length L (see FIG. 2) and the helical tooth portion 21 on the worm wheel 100, the hob 60 is moved by the distance L in the axial direction of the worm wheel 100. Then, as shown by the arrow C in FIG. 3(c), the hob 60 is moved away from the worm wheel 100. As a result, the tooth bottom 30 having the central portion 31 and a pair of curved surface portions 36, and the wheel tooth 20 having the helical tooth portion 21 and a pair of worm wheel tooth portions 26 are formed.

[0026] Then, the worm wheel 100 is rotated and shifted by a predetermined angle, and the steps of Fig. 3(a) to Fig. 3(c) are performed again. By repeating this process, the wheel teeth 20 and the tooth bottoms 30 are formed over the entire circumference of the worm wheel 100, as shown in Fig. 4. In this embodiment, the teeth of the hob 60 are formed at an angle inclined with respect to the circumferential direction of the hob 60. Therefore, the wheel teeth 20 are formed at an angle inclined with respect to the axial direction of the worm wheel 100, and a helical tooth portion 21 continuing to the central portion 31 is formed.

[0027] Here, if the tooth bottom 30 of the worm wheel 100 has only the curved surface portion 36, the shaft teeth 6b of the worm shaft 6 contact the curved surface portion 36 and the worm wheel tooth portion 26 over a large area, and the worm shaft 6 and the worm wheel 100 mesh with each other. Therefore, since the meshing area between the worm shaft 6 and the worm wheel 100 is large, if the machining accuracy of the worm wheel tooth portion 26 and the curved surface portion 36 is poor, the meshing between the worm shaft 6 and the worm wheel 100 will be poor. Since the two cannot move away from each other when assembled in the worm reduction gear 1, if the machining accuracy of the worm wheel tooth portion 26 and the curved surface portion 36 is poor, there is a risk of large friction occurring between the worm shaft 6 and the worm wheel 100. In addition, when the worm reduction gear 1 is manufactured, the dimensional accuracy of the wheel teeth 20 and the tooth bottom 30 is measured. If the tooth bottom 30 has only the curved surface portion 36, the dimensions of the tooth bottom 30 are not uniform, making it difficult to measure the dimensional accuracy.

[0028] In contrast, if the tooth bottom 30 of the worm wheel 100 has only the central portion 31, the shaft teeth 6b of the worm shaft 6 contact the central portion 31 and the helical tooth portion 21 over a small area (at the approximate center of the central portion 31 and the helical tooth portion 21 in the axial direction) to mesh with the worm shaft 6 and the worm wheel 100. Therefore, since the meshing area between the worm shaft 6 and the worm wheel 100 is small, even if the machining accuracy of the helical tooth portion 21 and the central portion 31 is poor, friction generated between the worm shaft 6 and the worm wheel 100 is reduced. In addition, the dimensional accuracy of the wheel teeth 20 and the tooth bottom 30 can be easily measured by using the central portion 31. However, the height of the wheel teeth 20 is higher than that of the tooth bottom 30 having the curved surface portion 36 (particularly, the height of the wheel teeth 20 at both ends in the axial direction is higher), and the strength of the wheel teeth 20 is reduced. Furthermore, in order to machine the helical tooth portion 21 and the central portion 31, it is necessary not only to move the hob 60 closer to the worm wheel 100 but also to move it axially beyond the axial dimension of the worm wheel 100, which lengthens the machining time of the worm wheel 100. Thus, the configuration in which the tooth bottom 30 has only the curved surface portion 36 and the configuration in which the tooth bottom 30 has only the central portion 31 each have advantages and disadvantages.

[0029] However, in the worm wheel 100 of this embodiment, the tooth bottom 30 has both the central portion 31 and the curved surface portion 36, and the wheel tooth 20 has both the helical tooth portion 21 and the worm wheel tooth portion 26. Therefore, the central portion 31 and the helical tooth portion 21 reduce the meshing area between the worm shaft 6 and the worm wheel 100, reducing the friction between them and facilitating the measurement of the dimensional accuracy of the wheel tooth 20 and the tooth bottom 30 of the worm wheel 100. Furthermore, the curved surface portion 36 reduces the height of the wheel tooth 20 at both ends in the axial direction compared to when the entire tooth bottom 30 is the central portion 31, so that the strength of the wheel tooth 20 can be improved. Furthermore, since it is not necessary to form the central portion 31 over the entire tooth bottom 30 and it is not necessary to form the helical tooth portion 21 over the entire wheel tooth 20, the processing time of the worm wheel 100 can be shortened. In other words, the worm wheel 100 of this embodiment can make use of the advantages of a configuration in which the tooth bottom 30 has only the central portion 31 or only the curved surface portion 36, while reducing the disadvantages.

[0030] Furthermore, in the worm wheel 100 of this embodiment, the length of the central portion 31 (helical tooth portion 21) in the axial direction is about one-ninth (about 11%) of the axial length of the worm wheel 100. This reduces friction between the worm shaft 6 and the worm wheel 100, improves the strength of the wheel teeth 20, and shortens the processing time of the worm wheel 100. This makes it possible to more effectively improve the performance of the worm wheel 100. Note that the length of the central portion 31 in the axial direction is not limited to the above, but in order to effectively improve the performance of the worm wheel 100, it is preferably 5% to 17% of the axial length of the worm wheel 100.

[0031] According to the above embodiment, the following advantageous effects are achieved.

[0032] In the worm wheel 100, the central portion 31 and the helical tooth portion 21 reduce friction occurring between the worm shaft 6 and the worm wheel 100, and facilitate measurement of the dimensional accuracy of the wheel teeth 20 and the tooth bottoms 30 of the worm wheel 100. Furthermore, the curved surface portion 36 can improve the strength of the wheel teeth 20, and can shorten the processing time of the worm wheel 100.

[0033] Next, modified examples of the above embodiment will be described. The following modified examples are also within the scope of the present invention, and it is possible to combine the following modified examples with the configuration of the above embodiment, or to combine the following modified examples with each other.

[0034] <Variation 1> In the above embodiment, the wheel teeth 20 are formed at an angle with respect to the axial direction of the worm wheel 100, and the helical tooth portion 21 continuing to the central portion 31 is formed. However, the present invention is not limited to this, and the wheel teeth 20 may be formed along the axial direction of the worm wheel 100. In this case, the helical tooth portion 21 continuing to the central portion 31 becomes a spur tooth portion. In other words, the "helical tooth portion" in the claims includes a spur tooth portion.

[0035] <Variation 2> In the above embodiment, the wheel teeth 20 are formed such that the width dimension (dimension in the circumferential direction) is large at the worm wheel tooth portion 26 and small at the helical tooth portion 21 so as to escape from the worm shaft 6. However, the wheel teeth 20 may be formed to have a uniform width dimension.

[0036] <Variation 3> In the above embodiment, a chamfered portion 24 is formed between the top surface 22 and the two side surfaces 23 of the wheel tooth 20. This prevents the top surface 22 and the two side surfaces 23 of the helical tooth portion 21 from coming into contact with the worm shaft 6 and being damaged. However, the chamfered portion 24 is not an essential component, and in cases where the top surface 22 and the two side surfaces 23 of the helical tooth portion 21 do not come into contact with the worm shaft 6, the chamfered portion 24 does not have to be formed.

[0037] <Modification 4> In the above embodiment, the machining of the wheel teeth 20 and the tooth bottom 30 is performed by adjusting the vertical position of the hob 60 to a position corresponding to one of the two boundaries between the center portion 31 and the curved surface portion 36 of the tooth bottom 30, moving the hob 60 in the radial direction of the worm wheel 100 to approach it, and then moving it to a position corresponding to the other of the two boundaries between the center portion 31 and the curved surface portion 36. In other words, in the above embodiment, the machining of the wheel teeth 20 and the tooth bottom 30 is performed by moving the hob 60 only in one direction in the vertical direction. This is not limited to this, and the machining of the wheel teeth 20 and the tooth bottom 30 may be performed by moving the hob 60 in both the vertical directions. Specifically, the hob 60 may be moved in the radial direction of the worm wheel 100 to approach it, moved to a position corresponding to one of the two boundaries between the center portion 31 and the curved surface portion 36 of the tooth bottom 30, and then moved to a position corresponding to the other.

[0038] <Variation 5> In the above embodiment, the wheel teeth 20 and the tooth roots 30 are machined while the worm wheel 100 is not rotating. In other words, the wheel teeth 20 are formed to extend linearly in the axial direction. However, the present invention is not limited to this, and the wheel teeth 20 and the tooth roots 30 may be machined while the worm wheel 100 is rotating. In this case, the wheel teeth 20 are formed to extend spirally in the axial direction.

[0039] The configuration, operation, and effects of the embodiment of the present invention will be described below.

[0040] The worm wheel 100 comprises wheel teeth 20 formed on the outer periphery as teeth portions that mesh with the worm shaft 6, and tooth bottoms 30 formed between adjacent wheel teeth 20. The tooth bottoms 30 are formed over a predetermined axial range including the axial center of the worm wheel 100 and have a central portion 31 that is the same radial distance from the center of the worm wheel 100, and curved portions 36 that are formed at both ends of the central portion 31 in the axial direction and extend to both ends of the tooth bottoms 30, with the radial distance from the center of the worm wheel 100 increasing as the end of the tooth bottoms 30 is approached. The wheel teeth 20 have helical tooth portions 21 connected to the central portion 31, and worm wheel tooth portions 26 connected to the curved portion 36.

[0041] In this configuration, the tooth bottom 30 of the worm wheel 100 has a central portion 31 formed over a predetermined range in the axial direction, and the wheel teeth 20 have helical tooth portions 21 connected to the central portion 31. Therefore, the central portion 31 and the helical tooth portions 21 reduce the meshing area between the worm shaft 6 and the worm wheel 100, reducing friction between them and facilitating measurement of the dimensional accuracy of the tooth portions 21 and the tooth bottom 30 of the worm wheel 100. Furthermore, the tooth bottom 30 of the worm wheel 100 has curved surface portions 36 formed at both ends of the central portion 31 in the axial direction. Therefore, the curved surface portions 36 can improve the strength of the wheel teeth 20 and shorten the processing time of the worm wheel 100 compared to when the entire tooth bottom 30 is the central portion 31.

[0042] In addition, a manufacturing method for a worm wheel 100 having wheel teeth 20 formed on the outer periphery and meshing with the worm shaft 6, and a tooth bottom 30 formed between adjacent wheel teeth 20, includes a first step of rotating a hob 60 as a tool having a blade on the outer periphery and bringing it close to the worm wheel 100 for cutting, and a second step of axially moving the hob 60 brought close to the worm wheel 100 a distance smaller than the axial length of the worm wheel 100 for cutting.By the first and second steps, a tooth bottom 30 is formed having a central portion 31 formed over a predetermined range in the axial direction including the axial center, and curved portions 36 formed at both ends of the central portion 31 in the axial direction and extending to both ends of the tooth bottom 30, and a wheel tooth 20 is formed having a helical tooth portion 21 connected to the central portion 31 and a worm wheel tooth portion 26 connected to the curved portion 36.

[0043] In this configuration, the central portion 31 and the helical tooth portion 21 reduce the area where the worm shaft 6 and the worm wheel 100 mesh, reducing friction between them and facilitating measurement of the dimensional accuracy of the tooth portion 21 and tooth bottom 30 of the worm wheel 100. Furthermore, the curved surface portion 36 can improve the strength of the wheel teeth 20 and shorten the processing time for the worm wheel 100 compared to when the entire tooth bottom 30 is the central portion 31.

[0044] Although the embodiments of the present invention have been described above, the above-mentioned embodiments merely show some of the application examples of the present invention, and it is not intended that the technical scope of the present invention be limited to the specific examples of the above-mentioned embodiments. [Explanation of symbols]

[0045] 6...worm shaft, 6a...shaft body, 20...wheel tooth (tooth portion), 21...helical tooth portion, 26...worm wheel tooth portion, 30...tooth bottom, 31...center portion, 36...curved surface portion, 60...hob (tool), 100...worm wheel

Claims

1. A worm wheel, A tooth portion formed on an outer periphery thereof and meshing with a worm shaft; and a tooth bottom formed between adjacent tooth portions, The tooth bottom is a central portion that is formed within a predetermined range in the axial direction including the axial center of the worm wheel and that is uniformly spaced from the center of the worm wheel in a radial direction; a curved surface portion formed at each end of the central portion in the axial direction, extending to each end of the tooth bottom, and having a radial distance from the center of the worm wheel increasing as the curved surface portion approaches the each end of the tooth bottom, The tooth portion is a helical tooth portion connected to the central portion; and a worm wheel tooth portion connected to the curved surface portion.

2. A method for manufacturing a worm wheel including teeth formed on an outer periphery thereof and meshing with a worm shaft, and a tooth bottom formed between adjacent teeth, comprising the steps of: A first step of rotating a tool having a blade on an outer periphery and bringing it close to the worm wheel to perform cutting; a second step of cutting the tool by moving the tool close to the worm wheel in the axial direction by a distance smaller than the axial length of the worm wheel, A method for manufacturing a worm wheel, characterized in that, by the first and second steps, the tooth bottom is formed having a central portion formed over a predetermined range in the axial direction including the center in the axial direction, and curved surface portions formed at both ends of the central portion in the axial direction and extending to both ends of the tooth bottom, and the tooth portion is formed having a helical tooth portion connected to the central portion and a worm wheel tooth portion connected to the curved surface portion.

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