Strain wave gearing

Abstract

A strain wave gearing (100) comprises: a wave generator (1) including a cam (10); a rigid internal gear (2); a flex section (3) including a flexible external gear (31) and a flexible internal gear (32); and a rigid external gear (4). The cam (10) includes N pole sections (N being an integer greater than or equal to 2) located at equal intervals in the circumferential direction about an axis (AX), and N intermediate sections located between pole sections adjacent in the circumferential direction. The wave generator (1) flexes the flexible external gear (31), thereby meshing the flexible external gear (31) with the rigid internal gear (2) in N portions respectively corresponding to the N pole sections. The flexible internal gear (32) is flexed in response to the flexible external gear (31) being flexed by the wave generator (1) and meshes with the rigid external gear (4) in N portions respectively corresponding to the N intermediate sections.

Description

Strain wave gearing

[0001] The present disclosure relates to a strain wave gear device.

[0002] Patent Document 1 describes a dual-type wave gearing device that generates two different speed ratios. The device described in Patent Document 1 includes a rigid first internal gear, a rigid second internal gear, a flexible external gear formed with first external teeth that can mesh with the first internal gear and second external teeth that can mesh with the second internal gear, and a wave generator that meshes the flexible external gear with each of the first internal gear and the second internal gear.

[0003] In the device described in Patent Document 1, the number of teeth of the first external teeth of the flexible external gear is smaller than the number of teeth of the first internal gear, and the number of teeth of the second external teeth of the flexible external gear is greater than the number of teeth of the second internal gear, which results in different speed ratios between the first external teeth and the first internal gear, and between the second external teeth and the second internal gear.

[0004] In the device described in Patent Document 1, the flexible external gear is located inside a first internal gear and a second internal gear that are arranged in parallel, and the wave generator has two cams that deflect the first external teeth and the second external teeth of the flexible external gear, respectively.

[0005] Patent No. 6218693

[0006] In the device described in Patent Document 1, the second internal gear is located outside the flexible external gear, so the pitch diameter of the second internal gear is larger than the pitch diameter of the second external gear. Meanwhile, the number of teeth of the second internal gear is smaller than the number of teeth of the second external gear. Here, consider the module, which is the value obtained by dividing the pitch diameter by the number of teeth. Based on the relationship between the pitch diameter and the number of teeth of the device described in Patent Document 1, in principle, the module of the second internal gear and the module of the second external gear of the flexible external gear should not be the same, and it is presumed that a special tooth profile is used.

[0007] Furthermore, according to paragraph 0037 of Patent Document 1, in the device described in Patent Document 1, the second external teeth of the flexible external gear mesh with the second internal gear at both ends of the major axis of the elliptical cam (i.e., two locations). However, according to studies by the present inventors, when attempting to mesh the second external teeth of the flexible external gear with the second internal gear, the second external teeth mesh with the second internal gear at a total of four locations: two locations adjacent to each other across one end of the major axis of the elliptical cam, and two locations adjacent to each other across the other end of the major axis. Even taking this study into consideration, it is inferred that the device described in Patent Document 1 uses a special tooth profile.

[0008] If a special tooth profile is used, the way the second internal gear and the second external teeth of the flexible external gear mesh will differ depending on whether the cam is rotating forward or backward, which may make it difficult to operate the device stably.

[0009] An object of the present disclosure is to provide a strain wave gear device that is capable of stable operation.

[0010] In order to achieve the above object, a wave gear device according to the present disclosure comprises: a wave generator having a cam that rotates around an axis and an annular wave bearing provided on the outer peripheral surface of the cam; a rigid internal gear that surrounds the wave bearing and has first internal teeth formed thereon; an annular flexible external gear that has first external teeth that can mesh with the first internal teeth formed thereon and a flex section that has an annular flexible internal gear that is connected to the flexible external gear and has second internal teeth formed thereon; and a rigid external gear that has second external teeth that can mesh with the second internal teeth formed thereon, wherein the cam has N pole portions (N is an integer of 2 or more) positioned at equal intervals in a circumferential direction about the axis, and N intermediate portions positioned midway between adjacent pole portions in the circumferential direction, and faces the flexible external gear across the wave bearing in a radial direction about the axis, and the rigid external gear faces the flexible internal gear in the radial direction, the number of teeth of the first external teeth is smaller than the number of teeth of the first internal teeth; the number of teeth of the second internal teeth is greater than the number of teeth of the second external teeth; the numbers of teeth of the first internal teeth and the second external teeth are equal; the difference in the numbers of teeth of the first internal teeth and the first external teeth is equal to the difference in the numbers of teeth of the second internal teeth and the second external teeth; the pitch circle diameter of the flexible internal gear is smaller than the pitch circle diameter of the flexible external gear; the wave generator flexes the flexible external gear to mesh with the rigid internal gear at N portions corresponding to each of the N pole portions; and the flexible internal gear flexes in response to the flexible external gear being flexed by the wave generator, and meshes with the rigid external gear at N portions corresponding to each of the N intermediate portions.

[0011] According to the present disclosure, it is possible to provide a strain wave gear device capable of stable operation.

[0012] FIG. 1 is a cross-sectional view of a wave gearing device according to an embodiment of the present disclosure. FIG. 2 is a schematic cross-sectional view of a wave gearing device according to the same embodiment taken along line I-I in FIG. 1, where the cam has two poles. FIG. 3 is a schematic cross-sectional view of a wave gearing device according to the same embodiment taken along line II-II in FIG. 1, where the cam has two poles. FIG. 4 is a schematic cross-sectional view corresponding to FIG. 2 of a wave gearing device according to the same embodiment, where the cam has three poles. FIG. 5 is a schematic cross-sectional view corresponding to FIG. 3 of a wave gearing device according to the same embodiment, where the cam has three poles. FIG. 6 is a partial cross-sectional view of a wave gearing device according to a modified example. FIG. 7 is a schematic cross-sectional view of a wave gearing device according to a modified example taken along line III-III in FIG.

[0013] An embodiment of the present disclosure will be described with reference to the drawings.

[0014] As shown in FIG. 1 , the wave gear device 100 according to this embodiment includes a wave generator 1, a rigid internal gear 2, a flex portion 3 having a flexible external gear 31 and a flexible internal gear 32, a rigid external gear 4, a cover 5, and a support portion 6.

[0015] In the drawings, hatching indicating cross sections of some components has been omitted for ease of viewing. In the following description, the right side in Fig. 1 may be referred to as the input side (Si in the drawing), and the left side as the output side (So in the drawing). In addition, in the following description, the circumferential direction centered on the axis line AX may be simply referred to as the "circumferential direction," the radial direction centered on the axis line AX may be simply referred to as the "radial direction," and the direction in which the axis line AX extends (the left-right direction in Fig. 1) may be simply referred to as the "axial direction."

[0016] The wave generator 1 includes a cam 10 , a hollow cylindrical shaft 12 formed integrally with the cam 10 , and a wave bearing 11 .

[0017] The cam 10 and cylindrical shaft 12 rotate about an axis AX in response to a rotational input. Rotational power from a motor (not shown) is transmitted to the cylindrical shaft 12 via a known transmission mechanism. The cam 10 protrudes from the outer circumferential surface of the cylindrical shaft 12 toward the wave bearing 11.

[0018] The cylindrical shaft 12 has a facing portion 12a that faces the rigid external gear 4 in the radial direction. The facing portion 12a is located inside the annular rigid external gear 4. The facing portion 12a is located on the output side of the cam 10 of the cylindrical shaft 12. An annular bearing B2 is interposed between the facing portion 12a and the rigid external gear 4. Meanwhile, an annular bearing B1 is interposed between a portion of the cylindrical shaft 12 that is on the input side of the cam 10 and a cover 5 fixed to the rigid internal gear 2. This allows the cam 10 and the cylindrical shaft 12 to rotate relative to the rigid internal gear 2. The bearings B1 and B2 are, for example, ball bearings.

[0019] The structure for supporting the rigid external gear 4 via the bearing B2 by the opposing portion 12a of the cylindrical shaft 12 can be simplified.

[0020] The cover 5 is a disk-shaped rigid member and has a circular opening 5 a formed in the cover 5 that surrounds a portion of the cylindrical shaft 12 on the input side relative to the cam 10 .

[0021] 2 and 4, the cam 10 has N (N is an integer of 2 or greater) pole portions 10a positioned at equal intervals in the circumferential direction, and N intermediate portions 10b positioned midway between adjacent pole portions 10a in the circumferential direction. Hereinafter, the number of pole portions 10a owned by the cam 10 will be referred to as the "pole number." The number of poles of the cam 10 in the wave gear device 100 can be determined arbitrarily depending on the purpose.

[0022] As shown in Figure 2, when the number of poles is "2," the cam 10 has an elliptical shape when viewed from the axial direction, and has two pole portions 10a and two intermediate portions 10b. In this case, two pole portions 10a adjacent to each other in the circumferential direction are spaced apart by 180° about the axis AX. Also, two intermediate portions 10b adjacent to each other in the circumferential direction are spaced apart by 180° about the axis AX. Also, two pole portions 10a adjacent to each other in the circumferential direction are spaced apart by 90° about the axis AX.

[0023] When the number of poles of the cam 10 is N≧3, the shape of the cam 10 when viewed from the axial direction is a regular N-sided polygon, and, for example, each pole portion 10a and each intermediate portion 10b has a curved surface that bulges gently in the outer diameter direction.

[0024] As shown in Fig. 4, when the number of poles is "3," the cam 10 has three pole portions 10a and three intermediate portions 10b. In this case, two pole portions 10a adjacent to each other in the circumferential direction are spaced apart by 120° about the axis AX. Also, two intermediate portions 10b adjacent to each other in the circumferential direction are spaced apart by 120° about the axis AX. Also, two pole portions 10a adjacent to each other in the circumferential direction are spaced apart by 60° about the axis AX.

[0025] Generalizing to a case where N is any number, the cam 10 has N pole portions 10a and N intermediate portions 10b. In this case, adjacent pole portions 10a in the circumferential direction are spaced apart by an angle (360° / N) around the axis AX. Also, adjacent intermediate portions 10b in the circumferential direction are spaced apart by an angle (360° / N) around the axis AX. Also, adjacent pole portions 10a in the circumferential direction are spaced apart by an angle (180° / N) around the axis AX.

[0026] As shown in Figure 2, the wave bearing 11 is an annular bearing provided on the outer peripheral surface of the cam 10. Specifically, the wave bearing 11 has an inner ring fixed to the outer peripheral surface of the cam 10, a flexible outer ring, and a plurality of rolling elements inserted between the inner ring and the outer ring in a rollable state. The outer ring of the wave bearing 11 elastically deforms via the rolling elements. The rolling elements are balls or rollers. In other words, the wave bearing 11 may be configured as a ball bearing or a roller bearing. The inner ring may be configured as a portion that includes the outer peripheral surface of the cam 10.

[0027] The rigid internal gear 2 is made of a known material such as metal and has rigidity. As shown in Fig. 2, the rigid internal gear 2 is configured in an annular shape surrounding the wave bearing 11, and first internal teeth 2t are formed on its inner circumferential surface. The first internal teeth 2t are made up of a plurality of teeth arranged circumferentially at a constant pitch on the inner circumference of the rigid internal gear 2.

[0028] The rigid internal gear 2 constitutes a part of the outer frame 2a as shown in Fig. 1. The rigid internal gear 2 may be integral with the outer frame 2a, or may be separate from the outer frame 2a as long as it is immovable relative to the outer frame 2a.

[0029] The flex portion 3 is formed to have flexibility from a metal material such as special steel and is formed into a cylindrical shape. Specifically, the flex portion 3 has a flexible external gear 31, a flexible internal gear 32, and a connection portion 33 that connects the flexible external gear 31 and the flexible internal gear 32. The flexible external gear 31, the flexible internal gear 32, and the connection portion 33 are integrally formed from the same material.

[0030] The flexible external gear 31 is annular and is located between the wave bearing 11 and the rigid internal gear 2. The inner peripheral side of the flexible external gear 31 is fitted into the outer ring of the wave bearing 11. This causes the flexible external gear 31 to bend at a position corresponding to the pole portion 10a of the cam 10 in the wave generator 1. First external teeth 31t that can mesh with the first internal teeth 2t are formed on the outer peripheral surface of the flexible external gear 31. The first external teeth 31t are composed of a plurality of teeth arranged circumferentially at a constant pitch on the outer periphery of the flexible external gear 31.

[0031] As shown in Fig. 1, the flexible internal gear 32 is connected to the flexible external gear 31 via a connection portion 33, and is located closer to the output side than the flexible external gear 31. The flexible internal gear 32 bends in response to the flexible external gear 31 being bent by the wave generator 1. As shown in Fig. 3, second internal teeth 32t are formed on the inner circumferential surface of the flexible internal gear 32. The second internal teeth 32t are composed of a plurality of teeth arranged circumferentially at a constant pitch on the inner circumference of the flexible internal gear 32.

[0032] Here, when the flexible external gear 31 of the flex section 3 is not bent by the wave generator 1, the flexible external gear 31 and the flexible internal gear 32 each form an annular shape when viewed in the axial direction. The outer diameter of the flexible internal gear 32 is smaller than the outer diameter of the flexible external gear 31. The pitch circle diameter of the flexible internal gear 32 is also smaller than the pitch circle diameter of the flexible external gear 31.

[0033] 1 , the connection portion 33 is an annular portion that connects the flexible external gear 31 and the flexible internal gear 32, which has an outer diameter smaller than that of the flexible external gear 31. The connection portion 33 is inclined so as to approach the axis line AX from the flexible external gear 31 toward the flexible internal gear 32. In other words, the connection portion 33 is an annular portion whose diameter decreases from the flexible external gear 31 toward the flexible internal gear 32.

[0034] The connecting portion 33 having the above-described shape can secure a sufficient section modulus at the connecting portion between the flexible external gear 31 and the flexible internal gear 32, and can also prevent stress from concentrating at the connecting portion, thereby ensuring the strength of the entire flex portion 3.

[0035] The rigid external gear 4 is made of a known material such as metal and has rigidity. As shown in Fig. 3, the rigid external gear 4 is configured in an annular shape surrounding the opposing portion 12a of the cylindrical shaft 12, and second external teeth 4t that can mesh with the second internal teeth 32t of the flexible internal gear 32 are formed on its outer circumferential surface. The second external teeth 4t are made up of a plurality of teeth arranged circumferentially at a constant pitch on the outer periphery of the rigid external gear 4.

[0036] 1, the rigid external gear 4 is located next to the cam 10 in the axial direction and faces the flexible internal gear 32 in the radial direction. Specifically, the rigid external gear 4 is located adjacent to the cam 10 in the axial direction. This positional relationship prevents the strain wave gearing 100 from becoming too large in the axial and radial directions, allowing the strain wave gearing 100 to be configured compactly.

[0037] The support unit 6 is made up of, for example, a cross roller bearing, and includes an inner ring 61 fixed to the rigid external gear 4, and an outer ring 62 fixed to the outer frame 2a or the rigid internal gear 2. The support unit 6 supports the rigid external gear 4 so that it can rotate about the axis line AX relative to the rigid internal gear 2. The inner ring 61, which rotates together with the rigid external gear 4, is connected to an output target (not shown). This makes it possible to obtain a decelerated output that is slower than the rotational input, as will be described later.

[0038] To facilitate understanding of the explanation, the following symbols are assigned: Number of teeth of first internal teeth 2t: Zi Number of teeth of first external teeth 31t: Z1 Number of teeth of second internal teeth 32t: Z2 Number of teeth of second external teeth 4t: Zo Pitch circle diameter of rigid internal gear 2: Di Pitch circle diameter of flexible external gear 31: D1 Pitch circle diameter of flexible internal gear 32: D2 Pitch circle diameter of rigid external gear 4: Do

[0039] First, the strain wave gearing 100 satisfies Di > D1 and D2 > Do based on the positional relationship between the rigid internal gear 2 and the flexible external gear 31, and the positional relationship between the flexible internal gear 32 and the rigid external gear 4. Furthermore, the strain wave gearing 100 satisfies the following conditions (i) to (v).

[0040] (i) The number of teeth of the first external teeth 31t of the flexible external gear 31 is smaller than the number of teeth of the first internal teeth 2t of the rigid internal gear 2 (Z1 < Zi). (ii) The number of teeth of the second internal teeth 32t of the flexible internal gear 32 is greater than the number of teeth of the second external teeth 4t of the rigid external gear 4 (Z2 > Zo). (iii) The number of teeth of the first internal teeth 2t and the second external teeth 4t are equal (Zi = Zo). (iv) The difference in the number of teeth between the first internal teeth 2t and the first external teeth 31t is equal to the difference in the number of teeth between the second internal teeth 32t and the second external teeth 4t (Zi - Z1 = Z2 - Zo). (v) The pitch circle diameter of the flexible internal gear 32 is smaller than the pitch circle diameter of the flexible external gear 31 (D2 < D1).

[0041] Specifically, the above conditions (i) to (iv) are satisfied when the following equations are established. Note that N is the number of poles of the cam 10, as described above. Z1 = Zi - N Z2 = Zo + N Zi = Zo

[0042] The flexible external gear 31, which is deflected by the cam 10 via the wave bearing 11, meshes with the rigid internal gear 2 at a portion (first meshing portion E1) corresponding to the pole portion 10a of the cam 10 as described below, and moves in the opposite rotational direction to the cam 10 relative to the rigid internal gear 2 according to the difference in the number of teeth between the first internal teeth 2t and the first external teeth 31t (Zi - Z1 = N) as the cam 10 rotates. At this time, the first speed ratio i1 generated between the rigid internal gear 2 and the flexible external gear 31 is expressed by the following equation. Note that the minus sign of the speed ratio indicates that the rotational direction of the flexible external gear 31 is the opposite direction to the rotational input. Furthermore, R1 is the reciprocal of the first speed ratio i1. i1 = 1 / R1 = (Z1 - Zi) / Z1 = -N / Z1

[0043] The flex portion 3 having the flexible external gear 31 rotates as a whole relative to the rigid internal gear 2. Furthermore, the flexible internal gear 32 of the flex portion 3 bends in response to the flexible external gear 31 being bent by the cam 10.

[0044] As will be described later, the rigid external gear 4 meshes with the flexible internal gear 32 at a portion (second meshing portion E2) corresponding to the intermediate portion 10b of the cam 10, and moves in the opposite rotational direction relative to the flexible internal gear 32 (i.e., the same rotational direction as the cam 10) with respect to the flexible internal gear 32, depending on the difference in the number of teeth between the second internal teeth 32t and the second external teeth 4t (Z2-Zo=N) as the flex portion 3 rotates. At this time, the second speed ratio i2 generated between the flexible internal gear 32 and the rigid external gear 4 is expressed by the following equation. Note that R2 is the reciprocal of the second speed ratio i2. i2=1 / R2=(Z2-Zo) / Z2=N / Z2

[0045] Since the amount of rotational movement of the flexible external gear 31 relative to the rigid internal gear 2 is greater than the amount of rotational movement of the rigid external gear 4 relative to the flexible internal gear 32, as a result, the rigid external gear 4 moves in the opposite rotational direction relative to the cam 10 while being decelerated relative to the rotational input.

[0046] As an example, in a wave gear device 100 in which the cam 10 is elliptical (N=2) and Zi=22, Z1=20, Z2=24, and Zo=22, i1=-1 / 10 (i.e., R1=-10) and i2=1 / 12 (i.e., R2=12).

[0047] By combining the first speed ratio i1 and the second speed ratio i2 (synonymous with the combination of R1 and R2), the strain wave gearing 100 can achieve any reduction ratio from a low reduction ratio to a high reduction ratio. In particular, the strain wave gearing 100 is capable of low reduction ratios in the range of 1 / 5 to 1 / 50 (note that low reduction here refers to a reduction ratio where the reciprocal is small).

[0048] Currently, a low-speed reduction range of reduction ratios of 1 / 5 to 1 / 50 falls within the category of planetary reducers. While a reduction ratio of approximately 1 / 50 is possible by configuring a planetary reducer in multiple stages, a planetary reducer configured in this manner has a long shaft length, is heavy, and results in a low-precision reducer. On the other hand, with the strain wave gear device 100 of this embodiment, a reduction ratio in the range of 1 / 5 to 1 / 50 can be easily achieved by combining the first speed ratio i1, which is negative because Z1<Zi (i.e., reverse rotation relative to the rotational input), with the second speed ratio i2, which is positive because Z2>Zo (i.e., forward rotation relative to the rotational input).

[0049] (Regarding Module) First, the modules of the rigid internal gear 2 and the flexible external gear 31 will be described. The module Mi of the rigid internal gear 2 is expressed as Mi = Di / Zi. The module M1 of the flexible external gear 31 is expressed as M1 = D1 / Z1. Due to the magnitude relationships Di > D1 and Zi > Z1, Mi = M1 can be satisfied, and the module of the rigid internal gear 2 and the module of the flexible external gear 31 can be made the same.

[0050] In the strain wave gear device 100, a combination of Di, Zi, D1, and Z1 is set that satisfies Mi=M1, and the module of the rigid internal gear 2 and the module of the flexible external gear 31 are set to be the same.

[0051] Next, the modules of the flexible internal gear 32 and the rigid external gear 4 will be described. The module M2 of the flexible internal gear 32 is expressed as M2 = D2 / Z2. The module Mo of the rigid external gear 4 is expressed as Mo = Do / Zo. Due to the magnitude relationships D2 > Do and Z2 > Zo, M2 = Mo can be satisfied, and the module of the flexible internal gear 32 and the module of the rigid external gear 4 can be made the same. Therefore, the strain wave gearing 100 is capable of stable operation.

[0052] This is a major difference from the device described in Patent Document 1, in which, as mentioned above, the module of the rigid second internal gear and the module of the second external teeth of the flexible external gear cannot be made the same.

[0053] In the strain wave gear device 100, a combination of D2, Z2, Do, and Zo is set that satisfies M2=Mo, and the module of the flexible internal gear 32 and the module of the rigid external gear 4 are set to be the same.

[0054] (Regarding Meshing Portions) The wave generator 1 bends the flexible external gear 31 to mesh the flexible external gear 31 with the rigid internal gear 2 at N portions corresponding to the N pole portions 10a, respectively.

[0055] The flexible internal gear 32 bends in response to the flexible external gear 31 being bent by the wave generator 1, and meshes with the rigid external gear 4 at N portions corresponding to the N intermediate portions 10b, respectively.

[0056] Here, the flexible internal gear 32 is closest to the rigid external gear 4 in the radial direction at positions corresponding to each of the N intermediate portions 10b, and is farthest from the rigid external gear 4 in the radial direction at positions corresponding to each of the N pole portions 10a, and does not mesh with the rigid external gear 4.

[0057] Hereinafter, a case where the cam 10 has two poles will be described with reference to FIGS. 2 and 3, and a case where the cam 10 has three poles will be described with reference to FIGS.

[0058] (When the number of poles is two) As shown in Figure 2, the wave generator 1 bends the flexible external gear 31, thereby meshing the flexible external gear 31 with the rigid internal gear 2 at two first meshing portions E1 corresponding to each of the two pole portions 10a.

[0059] As shown in Figure 3, the flexible internal gear 32 bends in response to the flexible external gear 31 being bent by the wave generator 1, and meshes with the rigid external gear 4 at the two second meshing portions E2 corresponding to each of the two intermediate portions 10b.

[0060] Here, the angle around the axis AX is set to 0° at the 12 o'clock direction in Figures 2 and 3, and the angle increases clockwise. In this case, the pole portions 10a are located at 0° and 180°, and the intermediate portions 10b are located at 90° and 270°.

[0061] As can be seen by comparing with Figure 2 and referring to Figure 3, the flexible internal gear 32 is closest to the rigid external gear 4 in the radial direction at positions corresponding to each of the two intermediate portions 10a (90° and 270° positions), and is farthest from the rigid external gear 4 in the radial direction at positions corresponding to each of the two pole portions 10a (0° and 180° positions), and does not mesh with the rigid external gear 4.

[0062] Considering one intermediate portion 10b and one pole portion 10a that are adjacent in the circumferential direction, the flexible internal gear 32 is closest to the rigid external gear 4 in the radial direction at the position of that one intermediate portion 10b. From that one intermediate portion 10b toward that one pole portion 10a, the flexible internal gear 32 gradually moves away from the rigid external gear 4 in the radial direction. Then, at the position of that one pole portion 10a, the flexible internal gear 32 is farthest from the rigid external gear 4 in the radial direction. This phenomenon occurs because the flexible external gear 31 is deflected in the outer radial direction at the pole portion 10a of the cam 10, and therefore the position of the flexible internal gear 32 that corresponds to the intermediate portion 10b of the cam 10 is deflected relatively in the inner radial direction.

[0063] Due to the above phenomenon, the rigid external gear 4 meshes with a portion of the bent flexible internal gear 32 that has a small curvature about the axis line AX (a portion that is gently curved). This makes it possible to increase the meshing ratio between the second internal teeth 32t of the flexible internal gear 32 and the second external teeth 4t of the rigid external gear 4, thereby improving the efficiency of torque transmission from the flexible internal gear 32 to the rigid external gear 4. This is a major difference from the device described in Patent Document 1, in which, as mentioned above, the second external teeth of the flexible external gear mesh with the second internal gear at both ends of the major axis of the elliptical cam.

[0064] As described above, the flexible internal gear 32 is closest to the rigid external gear 4 in the radial direction at positions corresponding to the two intermediate portions 10a (90° and 270° positions). Therefore, of the second internal teeth 32t and second external teeth 4t that mesh with each other at positions corresponding to the two intermediate portions 10a, both surfaces of one tooth that fits into the tooth groove of the other tooth are in uniform contact. This prevents the meshing between the flexible internal gear 32 and the rigid external gear 4 from differing depending on whether the cam 10 is rotating in the forward or reverse direction, ensuring stable operation of the strain wave gear device 100.

[0065] Furthermore, in the wave gearing 100, the flexible internal gear 32 meshes with the rigid external gear 4 at second meshing portions E2 whose number corresponds to the number of poles (if N = 2, then two second meshing portions E2). In this way, according to the wave gearing 100, the flexible internal gear 32 can mesh with the rigid external gear 4 at the minimum necessary number of second meshing portions E2, making it easy to ensure the accuracy of the wave gearing 100.

[0066] In the wave gearing 100, the first internal teeth 2t and the first external teeth 31t mesh with each other at a plurality of teeth in the first meshing portion E1, and the second internal teeth 32t and the second external teeth 4t mesh with each other at a plurality of teeth in the second meshing portion E2. The number of first internal teeth 2t and first external teeth 31t that mesh with each other at the first meshing portion E1 and the number of second internal teeth 32t and second external teeth 4t that mesh with each other at the second meshing portion E2 change depending on the settings of the number of poles N and the numbers of teeth Zi, Z1, Z2, and Zo.

[0067] (When the number of poles is 3) As shown in Figure 4, the wave generator 1 bends the flexible external gear 31, thereby meshing the flexible external gear 31 with the rigid internal gear 2 at three first meshing portions E1 corresponding to each of the three pole portions 10a.

[0068] As shown in Figure 5, the flexible internal gear 32 bends in response to the flexible external gear 31 being bent by the wave generator 1, and meshes with the rigid external gear 4 at the three second meshing portions E2 corresponding to each of the three intermediate portions 10b.

[0069] Here, the angle around the axis AX is set to 0° at the 12 o'clock direction in Figures 4 and 5, and the angle increases clockwise. In this case, the pole portions 10a are located at 0°, 120°, and 240°, and the intermediate portions 10b are located at 60°, 180°, and 300°.

[0070] As can be seen by comparing with Figure 4 and referring to Figure 5, the flexible internal gear 32 is closest to the rigid external gear 4 in the radial direction at positions corresponding to each of the three intermediate portions 10a (positions of 60°, 180°, and 300°), and is farthest from the rigid external gear 4 in the radial direction at positions corresponding to each of the three pole portions 10a (positions of 0°, 120°, and 240°), and does not mesh with the rigid external gear 4.

[0071] Whether the number of poles is three or an arbitrary number N, the wave gear device 100 experiences the same phenomenon as described for the case where the number of poles is two, and provides the same effects.

[0072] (Modification) Figure 6 shows a modification in which part of the configuration of the strain wave gearing 100 shown in Figure 1 is modified. Configuration other than that necessary for explaining the modification is omitted in Figure 6. Figure 7 is a schematic cross-sectional view of the configuration shown in Figure 6 taken along line III-III.

[0073] The wave gear device 100 according to the modified example further comprises an enclosing portion 7 that encloses the outer periphery of the flexible internal gear 32 , and an annular flex bearing 8 provided between the enclosing portion 7 and the flexible internal gear 32 .

[0074] The surrounding portion 7 is made of a known material such as metal and has rigidity. The surrounding portion 7 is fixed to the rigid internal gear 2 or the outer frame 2a.

[0075] The flex bearing 8 has an outer ring fixed to the inner circumferential surface of the enclosing portion 7, a flexible inner ring, and a plurality of rolling elements inserted between the outer ring and the inner ring in a rollable state. The flexible internal gear 32 is fitted into the inner ring of the flex bearing 8. The inner ring of the flex bearing 8 elastically deforms via the rolling elements. The rolling elements are balls or rollers. In other words, the flex bearing 8 may be configured as a ball bearing or a roller bearing. The outer ring may be configured as a portion including the inner circumferential surface of the enclosing portion 7.

[0076] In this way, by providing the surrounding portion 7 and the flex bearing 8 in the wave gear device 100, even if a relatively large load is applied to the flexible internal gear 32, the flexible internal gear 32 can be supported from the outer diameter side, ensuring high precision and high output.

[0077] 6, the flexible external gear 31 and the flexible internal gear 32 may be directly connected. In other words, the flex portion 3 does not have to have an annular connecting portion 33 that reduces in diameter from the flexible external gear 31 toward the flexible internal gear 32. As long as the condition that the pitch circle diameter of the flexible internal gear 32 is smaller than the pitch circle diameter of the flexible external gear 31 (D2<D1) is satisfied, the outer diameter of the flexible internal gear 32 and the outer diameter of the flexible external gear 31 may be configured to be approximately the same (including exactly the same) by a method such as adjusting the plate thickness of the flex portion 3.

[0078] The present disclosure is not limited to the above-described embodiments, modifications, and drawings. Appropriate modifications (including the deletion of components) are possible within the scope of the present disclosure. For example, the materials of the components constituting the strain wave gearing device 100 are not limited to metals and may be any material, such as engineering plastics, resins, or ceramics, as long as the intended purpose can be achieved. Furthermore, the strain wave gearing device 100 may be used for any purpose, such as robots, automotive components, home appliances, or hobby supplies. While the above example illustrates a configuration in which the rigid external gear 4 is positioned immediately adjacent to the cam 10 in the axial direction, another member may be interposed between the cam 10 and the rigid external gear 4 in the axial direction.

[0079] The strain wave gear device 100 described above has the features described in the following supplementary notes.

[0080] (Notes) (Note 1) A wave generator including a cam that rotates around an axis and an annular wave bearing provided on the outer peripheral surface of the cam; a rigid internal gear that surrounds the wave bearing and has first internal teeth formed thereon; a flex section that has an annular flexible external gear that has first external teeth that can mesh with the first internal teeth and an annular flexible internal gear that is connected to the flexible external gear and has second internal teeth formed thereon; and a rigid external gear that has second external teeth that can mesh with the second internal teeth, wherein the cam has N pole portions (N is an integer of 2 or more) positioned at equal intervals in a circumferential direction about the axis as the center, and N intermediate portions positioned midway between the pole portions adjacent to each other in the circumferential direction, and faces the flexible external gear across the wave bearing in a radial direction about the axis as the center, and the rigid external gear faces the flexible internal gear in the radial direction, a wave gear device in which the number of teeth of the first external teeth is smaller than the number of teeth of the first internal teeth; the number of teeth of the second internal teeth is greater than the number of teeth of the second external teeth; the numbers of teeth of the first internal teeth and the second external teeth are equal; a difference in the number of teeth of the first internal teeth and the first external teeth is equal to a difference in the number of teeth of the second internal teeth and the second external teeth; a pitch circle diameter of the flexible internal gear is smaller than a pitch circle diameter of the flexible external gear; the wave generator flexes the flexible external gear to mesh with the rigid internal gear at N portions corresponding to each of the N pole portions; and the flexible internal gear flexes in response to the flexible external gear being flexed by the wave generator, and meshes with the rigid external gear at N portions corresponding to each of the N intermediate portions.

[0081] (Supplementary Note 2) The wave gear device according to Supplementary Note 1, wherein the flexible internal gear is closest to the rigid external gear in the radial direction at positions corresponding to each of the N intermediate portions, and is farthest from the rigid external gear in the radial direction at positions corresponding to each of the N pole portions, and does not mesh with the rigid external gear.

[0082] (Supplementary Note 3) A wave gear device according to Supplementary Note 1 or 2, wherein the outer diameter of the flexible internal gear is smaller than the outer diameter of the flexible external gear, the flex portion has a connection portion that connects the flexible external gear and the flexible internal gear, and the flexible external gear, the connection portion and the flexible internal gear are integrally formed from the same material.

[0083] (Supplementary Note 4) The wave gear device according to Supplementary Note 1 or 2, wherein the rigid external gear is annular, the wave generator has a hollow cylindrical shaft that rotates together with the cam, the cam is provided so as to protrude from the outer circumferential surface of the cylindrical shaft toward the wave bearing, and the cylindrical shaft is located inside the rigid external gear and has an opposing portion that faces the rigid external gear in the radial direction.

[0084] (Supplementary Note 5) The strain wave gear device according to Supplementary Note 1 or 2, further comprising: a surrounding portion that surrounds the outer periphery of the flexible internal gear; and an annular flex bearing provided between the surrounding portion and the flexible internal gear.

[0085] (Supplementary Note 6) The strain wave gear device according to Supplementary Note 1 or 2, wherein the rigid external gear is located adjacent to the cam in the direction in which the axis extends.

[0086] In the above description, in order to facilitate understanding of the present disclosure, descriptions of well-known technical matters have been omitted as appropriate.

[0087] The present invention allows various embodiments and modifications without departing from the broad spirit and scope of the present invention. Furthermore, the above-described embodiments are intended to illustrate the present invention and do not limit the scope of the present invention. That is, the scope of the present invention is defined by the claims, not by the embodiments. Various modifications made within the scope of the claims and the meaning of the invention equivalent thereto are considered to be within the scope of the present invention.

[0088] REFERENCE SIGNS LIST 100... Wave gear device 1... Wave generator 10... Cam, 10a... Polar portion, 10b... Intermediate portion 11... Wave bearing, 12... Cylindrical shaft 2... Rigid internal gear, 2a... Outer frame, 2t... First internal tooth 3... Flex portion 31... Flexible external gear, 31t... First external tooth 32... Flexible internal gear, 32t... Second internal tooth 33... Connection portion 4... Rigid external gear, 4t... Second external tooth 5... Cover, 5a... Opening 6... Support portion, 61... Inner ring, 62... Outer ring 7... Surrounding portion 8... Flex bearing AX... Axis E1... First meshing portion E2... Second meshing portion

Claims

1. A wave generator comprising a cam that rotates around an axis and an annular wave bearing provided on the outer peripheral surface of the cam; a rigid internal gear surrounding the wave bearing and having first internal teeth; an annular flexible external gear having first external teeth that can mesh with the first internal teeth and a flex section connected to the flexible external gear and having second internal teeth; and a rigid external gear having second external teeth that can mesh with the second internal teeth, wherein the cam has N pole portions (N is an integer of 2 or more) positioned at equal intervals in a circumferential direction centered on the axis, and N intermediate portions positioned midway between adjacent pole portions in the circumferential direction, and faces the flexible external gear across the wave bearing in a radial direction centered on the axis, and the rigid external gear faces the flexible internal gear in the radial direction, a wave gear device in which the number of teeth of the first external teeth is smaller than the number of teeth of the first internal teeth; the number of teeth of the second internal teeth is greater than the number of teeth of the second external teeth; the numbers of teeth of the first internal teeth and the second external teeth are equal; a difference in the number of teeth of the first internal teeth and the first external teeth is equal to a difference in the number of teeth of the second internal teeth and the second external teeth; a pitch circle diameter of the flexible internal gear is smaller than a pitch circle diameter of the flexible external gear; the wave generator flexes the flexible external gear to mesh with the rigid internal gear at N portions corresponding to each of the N pole portions; and the flexible internal gear flexes in response to the flexible external gear being flexed by the wave generator, and meshes with the rigid external gear at N portions corresponding to each of the N intermediate portions.

2. The wave gear device according to claim 1, wherein the flexible internal gear is closest to the rigid external gear in the radial direction at positions corresponding to each of the N intermediate portions, and is farthest from the rigid external gear in the radial direction at positions corresponding to each of the N pole portions, and does not mesh with the rigid external gear.

3. A strain wave gear device according to claim 1 or 2, wherein the outer diameter of the flexible internal gear is smaller than the outer diameter of the flexible external gear, the flex portion has a connection portion that connects the flexible external gear and the flexible internal gear, and the flexible external gear, the connection portion and the flexible internal gear are integrally formed from the same material.

4. A wave gear device according to claim 1 or 2, wherein the rigid external gear is annular, the wave generator has a hollow cylindrical shaft that rotates together with the cam, the cam is provided so as to protrude from the outer circumferential surface of the cylindrical shaft towards the wave bearing, and the cylindrical shaft is located inside the rigid external gear and has an opposing portion that faces the rigid external gear in the radial direction.

5. A wave gear device according to claim 1 or 2, further comprising: a surrounding portion surrounding the outer periphery of the flexible internal gear; and an annular flex bearing provided between the surrounding portion and the flexible internal gear.

6. A wave gear device according to claim 1 or 2, wherein the rigid external gear is located adjacent to the cam in the direction in which the axis extends.

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