Wave gear device

Abstract

To provide a wave gear device capable of shortening an axial length and increasing an allowable torque.SOLUTION: In a wave gear device, a flexible, annular external gear (2) meshes with an internal gear inside the internal gear while deforming in a wave shape. The Young's modulus E [GPa] of the external gear (2) is 2.0<E<120. The external gear (2) has a cylindrical wall (21). The cylindrical wall (21) is composed of a toothed part (21a) with external teeth (2a) provided on its outer surface, and a trunk part (21b) connected to the toothed part (21a). A tooth width a of the toothed part (21a) and a trunk width b (b≥0) of the trunk part (21b) satisfy the relation of b / (a+b)<0.5.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a harmonic gear device.

Background Art

[0002] In recent years, in the robot industry such as industrial or collaborative robots, the demand for small and lightweight robots has been increasing. For this reason, development of a harmonic gear device (harmonic gear reducer) that can be used as a main component of a robot and is small, lightweight, and has a large allowable torque has been carried out.

[0003] As one of such harmonic gear devices, for example, while increasing the outer diameter dimension of a flexible external gear, which is a part of the components of the harmonic gear device, the axial length is shortened, and by flattening the external gear, space saving is realized (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, it has been difficult to flatten the external gear while increasing the allowable torque in the harmonic gear device described in Patent Document 1.

[0006] An object of the present invention is to provide a harmonic gear device in which the axial length is suppressed to be short and the allowable torque is increased.

Means for Solving the Problems

[0007] (1) The wave gear device according to the present invention is a wave gear device in which an annular external gear having flexibility meshes with an internal gear inside the internal gear while being deformed into a waveform. The Young's modulus E [GPa] of the external gear satisfies 2.0 < E < 120. The external gear includes a cylindrical wall, and the cylindrical wall is composed of a tooth portion provided with external teeth on the outer surface of the external gear and a body portion continuous with the tooth portion. The tooth width a (a > 0), which is the axial length of the tooth portion, and the body width b (b ≥ 0), which is the axial length of the body portion, satisfy the relationship b / (a + b) < 0.5. According to the wave gear device of the present invention, the axial length can be suppressed to be short and the allowable torque can be increased.

[0008] (2) In the wave gear device of (1) above, it is preferable that the Young's modulus E [GPa] of the external gear satisfies 2.0 < E < 30. In this case, by using a resin gear for the external gear, the axial length can be suppressed to be shorter and the allowable torque can also be increased. Further, in the wave gear device of (1) above, preferably, b / (a + b) < 0.4.

[0009] (3) In the wave gear device of (1) or (2) above, it is preferable that the tip circle diameter Df of the external gear, the tip circle diameter Dr of the internal gear, and the total tooth height h of the external gear satisfy the relationship 0.1h < (Df - Dr) < h. In this case, manufacturing becomes easy without reducing the allowable torque.

[0010] (4) In the wave gear device of (3) above, it is preferable that the tip circle diameter Df of the external gear, the tip circle diameter Dr of the internal gear, and the total tooth height h of the external gear satisfy the relationship 0.3h < (Df - Dr) < h. In this case, the allowable torque can be improved by increasing the number of meshing teeth.

[0011] (5) In any one of the harmonic gear devices (1) to (4) above, the external gear includes a bottom wall connected to one end of the cylindrical wall, and a plurality of slit holes are intermittently arranged annularly around the axis on the bottom wall, and the plurality of slit holes arranged annularly can be arranged on at least one circle centered on the axis. In this case, efficient torque transmission can be achieved.

[0012] (6) In the harmonic gear device (5) above, each of the plurality of slit holes can have a length along the circumferential direction longer than the length in the radial direction. In this case, efficient torque transmission can be achieved.

[0013] (7) In the harmonic gear device (5) or (6) above, the plurality of slit holes can be arranged in a plurality of concentric circles centered on the axis so that the radial distances from the axis are different. In this case, more efficient torque transmission can be achieved.

[0014] (8) In the harmonic gear device (7) above, the plurality of slit holes arranged in a plurality of concentric circles are preferably arranged in a plurality of concentric circles centered on the axis such that the circumferential end positions of the slit holes in each row are different between adjacent rows. In this case, more efficient torque transmission can be achieved.

Advantages of the Invention

[0015] According to the present invention, it is possible to provide a harmonic gear device with a reduced axial length and an increased allowable torque.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Embodiments for Carrying Out the Invention

[0017] Hereinafter, with reference to the drawings, a harmonic gear device which is an exemplary embodiment of the present invention will be described.

[0018] FIG. 1 schematically shows a harmonic gear device 1A which is a first embodiment of the present invention. The harmonic gear device 1A can transmit power by a flexible annular external gear 2 meshing with an internal gear 3 inside the internal gear 3 while being deformed into a waveform.

[0019] The harmonic gear device 1A includes a flexible external gear 2, an internal gear 3 with the external gear 2 disposed on the inner peripheral side, and a wave generator 4 disposed on the inner peripheral side of the external gear 2. The external gear 2 is an annular gear having a plurality of external teeth 2a. The plurality of external teeth 2a each project radially outward. The internal gear 3 is an annular gear having a plurality of internal teeth 3a. The plurality of internal teeth 3a each project radially inward. In the present disclosure, the internal gear 3 is a rigid gear having high rigidity that is difficult to deform. The internal gear 3 is formed of, for example, an iron-based material such as cast iron, alloy steel, or carbon steel. The wave generator 4 includes a non-circular cam member 41 rotatable about an axis O, and a bearing 42 disposed on the outer peripheral side of the cam member 41. In the present disclosure, as shown in the figure, the cam member 41 has a two-lobe shape having an elliptical shape when viewed in the axial direction.

[0020] FIG. 2 schematically shows the harmonic gear device 1A in an X-X cross section of FIG. 1. In the present disclosure, the bearing 42 includes an outer ring 42a and an inner ring 42b, and a plurality of rolling elements (for example, rollers, balls) 42c interposed therebetween. However, at least one of the outer ring 42a and the inner ring 42b of the bearing 42 can be omitted. Further, the bearing 42 can be provided with a retainer (not shown) for holding the rolling elements 42c.

[0021] The wave generator 4 is assembled to the inner peripheral surface of the external gear 2, thereby deflecting the external gear 2 into a non-circular shape so that the external teeth 2a of the external gear 2 can mesh with the internal teeth 3a of the internal gear 3. In the present disclosure, the external gear 2 and the internal gear 3 mesh at two positions on the major axis portion of the cam member 41. In FIG. 1, the reference symbol P indicates the meshing portion between the external teeth 2a of the external gear 2 and the internal teeth 3a of the internal gear 3. As shown in FIG. 1, in the present disclosure, the meshing portion P is located on the two major axis sides of the cam member 41. However, there is a difference in the number of teeth (for example, a difference in the number of teeth of 2) between the number of teeth Zf of the external teeth 2a of the external gear 2 and the number of teeth Zr of the internal teeth 3a of the internal gear 3.

[0022] The cam member 41 is formed with a mounting hole 43 for mounting an input shaft (not shown) coaxially with the axis O. In the present disclosure, the mounting hole 43 is formed with a key groove 43a for preventing the input shaft from rotating.

[0023] In the wave generator 4, the cam member 41 rotates about the axis О together with the input shaft. Thereby, the cam member 41 can relatively rotate the external gear 2 with respect to the cam member 41 via the rolling elements 42c of the bearing 42. On the other hand, the internal gear 3 is fixed to a housing case or the like. For this reason, when the cam member 41 is rotated, a relative rotation due to the difference in the number of teeth occurs between the external gear 2 and the internal gear 3. As a result, the meshing portion P of the external gear 2 moves in the circumferential direction of the internal gear 3 in a direction opposite to the rotation direction of the cam member 41. In the present disclosure, the meshing portion P moves in a direction opposite to the rotation direction of the wave generator 4 with respect to the internal gear 3 every time the cam member 41 rotates 180 degrees around the axis O. That is, in the present disclosure, the input rotation from the input shaft is reversely output as a decelerated rotation from the external gear 2.

[0024] Here, as described above, since the external gear 2 has flexibility, it can be bent non-circularly along the outer shape of the cam member 41. In the present embodiment, specifically, the Young's modulus E [GPa] of the external gear 2 satisfies 2.0 < E < 120. Examples of materials having such a Young's modulus E include resins, light metals, and graphite (carbon). Specific examples of resins include POM (polyacetal), PEEK (polyetheretherketone), FRP (fiber-reinforced plastic), nylon, and PE (polyurethane). Specific examples of light metals include titanium alloys, copper alloys, and aluminum alloys. Further, preferably, the Young's modulus E [GPa] of the external gear 2 satisfies 2.0 < E < 30. Examples of materials having such a Young's modulus E include resins, light metals, and graphite (carbon). Specific examples of resins include POM (polyacetal), PEEK (polyetheretherketone), FRP (fiber-reinforced plastic), nylon, and PE (polyurethane). Specific examples of light metals include titanium alloys, copper alloys, and aluminum alloys.

[0025] FIG. 3 schematically shows a partial cross-section of the external gear 2 in a half-cut state. As shown in FIG. 3, the external gear 2 has a cup-shaped outer shape with a front-end opening A1 formed at one end in the axial direction. In the present disclosure, the external gear 2 includes a cylindrical wall 21 and a bottom wall 22 connected to the cylindrical wall 21. In the present disclosure, the front-end opening A1 is formed at one end in the axial direction of the cylindrical wall 21. Also, in the present disclosure, the bottom wall 22 is connected to the other end in the axial direction of the cylindrical wall 21. Further, in the present disclosure, a boss (mounting portion) 23 is provided coaxially with the axis O on the bottom wall 22. In the present disclosure, a mounting hole 24 for mounting an output shaft (not shown) coaxially with the axis O is formed in the boss 23. Further, in the present disclosure, a key groove 24a for preventing the output shaft from rotating is formed in the mounting hole 24. In the present disclosure, the input rotation from the input shaft is reversely output as a decelerated rotation from the output shaft connected to the external gear 2.

[0026] The cylindrical wall 21 is composed of a tooth portion 21a provided with external teeth 2a of the external gear 2 on its outer surface, and a body portion 21b continuous with the tooth portion 21a. In the present disclosure, the tooth width a is the axial length of the tooth portion 21a. Specifically, the tooth width a is the width in the axial direction where the external teeth 2a are cut. The range of the tooth width a is a range having the same value as the tooth width of the meshing internal teeth 3a. Also, in the present disclosure, the body width b is the axial length of the body portion 21b. In the present disclosure, the body width b is the axial length between the front end 2e of the external gear 2 and the upper surface 22a of the bottom wall 22 of the external gear 2. Further, in the present disclosure, the axial length (total width) L of the cylindrical wall 21 is L = a + b.

[0027] In the present embodiment, the tooth width a (a > 0), which is the axial length of the tooth portion 21a, and the body width b (b ≧ 0), which is the axial length of the body portion 21b, satisfy the relationship b / (a + b) < 0.5.

[0028] For the purpose of reducing the size and weight of the harmonic gear device, when flattening the external gear 2 by shortening the axial length (total width L) of the cylindrical wall 21 of the external gear 2, in the configuration of the conventional harmonic gear device, there is a problem that the allowable torque of the harmonic gear device decreases.

[0029] On the other hand, the harmonic gear device 1A according to the present embodiment focuses on the meshing between the external teeth 2a of the external gear 2 and the internal teeth 3a of the internal gear 3, and, after recognizing that the meshing is caused by the cornering described later, aims to improve the allowable torque of the harmonic gear device 1A.

[0030] FIG. 4 schematically shows a partial cross-section of the external gear 2 to explain the above-mentioned cornering.

[0031] The external gear 2 is bent non-circularly by the cam member 41 of the wave generator 4. On the other hand, as shown in FIG. 4, along the tooth row direction (axial direction) of the external teeth 2a, as it goes from the side of the bottom wall 22 toward the front end opening A1, the amount of bending increases substantially in proportion to the axial distance from the bottom wall 22. In FIG. 4, the amount of bending is shown as a bending angle θ with the connecting portion between the cylindrical wall 21 and the bottom wall 22 as the reference point. Further, as the wave generator 4 rotates, each part in the circumferential direction of the tooth portion 21a of the external gear 2 repeats bending in the radial direction (a direction orthogonal to the axial direction). Such a bending operation of the external gear 2 caused by the operation of the wave generator 4 is called "corning".

[0032] As shown in FIG. 4, the meshing width between the external teeth 2a of the external gear 2 and the internal teeth 3a of the internal gear 3 decreases due to corning. In particular, when the rigidity of the external gear 2 is high, it becomes difficult for the bending of the cylindrical wall 21 to follow the external shape of the cam member 41 of the wave generator 4. As a result, as shown in FIG. 4, the cylindrical wall 21 bends so as to be bent at an angle θ with the connecting portion between the cylindrical wall 21 and the bottom wall 22 as the reference point. In this case, the meshing width between the external teeth 2a of the external gear 2 and the internal teeth 3a of the internal gear 3 also decreases further. In addition, when the external gear 2 is flattened by shortening the axial length (total width L) of the cylindrical wall 21, the rigidity of the cylindrical wall 21 is further increased. Furthermore, in this case, since the total width L of the cylindrical wall 21 becomes shorter, the meshing width between the external teeth 2a of the external gear 2 and the internal teeth 3a of the internal gear 3 tends to decrease further. Therefore, when the external gear 2 is flattened, the allowable torque of the harmonic gear device decreases.

[0033] In contrast, the harmonic gear device 1A according to the present embodiment makes the outer gear 2 have a Young's modulus E [GPa] in the range of 2.0 < E < 120, making the outer gear 2 more flexible. In this case, for example, as shown by the arrow D in FIG. 4, the cylindrical wall 21 itself of the outer gear 2 becomes more flexible. As a result, according to the harmonic gear device 1A, as shown in FIG. 2, the shape of the outer gear 2 can be made such that the meshing ratio in the tooth width direction between the outer teeth 2a of the outer gear 2 and the inner teeth 3a of the inner gear 3 is increased. Also, according to the outer gear 2 with the Young's modulus E within the above range, for example, as shown by the arrow D in FIG. 4, when the outer gear 2 is deformed, it is less likely to be affected by cornering. Therefore, according to the harmonic gear device 1A, the allowable torque of the harmonic gear device 1A can be improved by the amount by which the meshing width between the outer teeth 2a of the outer gear 2 and the inner teeth 3a of the inner gear 3 increases while suppressing the influence of cornering. In addition, according to the harmonic gear device 1A, since the cylindrical wall 21 itself of the outer gear 2 becomes more flexible, the outer gear 2 can be flattened by shortening the axial length (total width L) of the cylindrical wall 21 while improving the allowable torque of the harmonic gear device 1A.

[0034] In addition, the harmonic gear device 1A is configured such that the tooth width a (a > 0) and the body width b (b ≧ 0) satisfy the relationship b / (a + b) < 0.5. In this case, the ratio of the tooth portion 21a where the outer teeth 2a are arranged to the total width L of the cylindrical wall 21 is more than half of the total width L. That is, if the tooth width a (a > 0) and the body width b (b ≧ 0) satisfy the relationship b / (a + b) < 0.5, a region occupying the meshing portion between the outer teeth 2a of the outer gear 2 and the inner teeth 3a of the inner gear 3 can be ensured to be larger than the body portion 21b of the cylindrical wall 21 of the outer gear 2. Therefore, in this case, the allowable torque of the harmonic gear device 1A is selected within a range where a larger allowable torque can be obtained.

[0035] In addition, the wave gear device 1A is set such that the Young's modulus E of the external gear 2 exceeds 2.0 [GPa]. For example, when the Young's modulus E is 2.0 [GPa] or less, the external gear 2 is likely to be twisted when torque is applied, and the required rigidity as a speed reducer may not be maintained. On the other hand, when the Young's modulus E exceeds 2.0 [GPa], for example, the allowable torque that the external gear 2 can obtain tends to make it difficult to twist the external gear 2 when power is input to the external gear 2. That is, if the Young's modulus E of the external gear 2 is set to exceed 2.0 [GPa], the required rigidity as a wave gear speed reducer can be maintained. In addition, the wave gear device 1A is set such that the Young's modulus E of the external gear 2 is less than 120 [GPa]. When the Young's modulus E is 120 [GPa] or more, it becomes difficult to bend, and it may be difficult to make b / (a + b) less than 0.5. On the other hand, when the Young's modulus E is less than 120 [GPa], the external gear 2 is likely to bend, so that the tooth width a (a > 0) and the body width b (b ≧ 0) can easily satisfy the relationship of b / (a + b) < 0.5.

[0036] Therefore, according to the wave gear device 1A according to the present embodiment, it is possible to provide a wave gear device in which the axial length is suppressed to be short and the allowable torque is increased.

[0037] By the way, in a wave gear device, in many cases, the tip circle diameter Df of the external gear 2 is designed to be equal to or slightly interfere with the tip circle diameter Dr of the internal gear 3. The reason is that, for example, as shown in FIG. 5, when the design is such that Df > Dr, when the external gear 2 is inserted into the internal gear 3, the tips of the tooth tips 2p of the external teeth 2a provided on the external gear 2 and the tooth tips 3p of the internal teeth 3a provided on the internal gear 3 interfere with each other, making it difficult to assemble the external gear 2 to the internal gear 3.

[0038] On the other hand, in the case of a small-sized harmonic gear device, the thickness of the flexible external gear 2 becomes thinner in accordance with the miniaturization of the harmonic gear device. In this case, the stress of the external gear 2 also becomes smaller in accordance with the thinning of the external gear 2. Therefore, the external gear 2 can be bent into a non-circular meshing shape without being attached to the wave generator 4 in advance. Accordingly, when the external gear 2 is thinned, it may be possible to assemble the external gear 2 by bending it.

[0039] Therefore, as an example for improving the meshing ratio between the external gear 2 and the internal gear 3, it is also conceivable to increase the number of teeth of the external teeth 2a of the external gear 2 to prevent ratcheting while reducing the overall thickness of the external gear 2. If the number of teeth of the external teeth 2a is increased, it is also effective in preventing ratcheting that may occur between the external gear 2 and the internal gear 3.

[0040] However, when the external gear 2 is thinned while increasing the number of teeth of the external teeth 2a, the external gear 2 may break due to the load torque received by the external gear 2. In addition, the processing difficulty also increases when further thinning the thin external gear 2. Therefore, as a means for improving the meshing ratio between the external gear 2 and the internal gear 3, it is not realistic to attempt to thin the external gear 2 while increasing the number of teeth of the external teeth 2a.

[0041] In contrast, the harmonic gear device 1A is configured such that the material used for the external gear 2 has a low Young's modulus E, so that the external gear 2 is easily bent when assembling the external gear 2 to the internal gear 3. As a result, according to the harmonic gear device 1A, it is possible to easily manufacture the harmonic gear device without further thinning the external gear 2 after thinning it.

[0042] Furthermore, the Young's modulus E [GPa] of the external gear 2 is preferably 2.0 < E < 30. In this case, since the Young's modulus E of the external gear 2 is E < 30, for example, by using a resin gear for the external gear 2, the axial length can be suppressed to be shorter and the allowable torque can also be increased. As a specific example, by using PEEK (Young's modulus E = 4) as the material of the external gear 2, the tooth width a (a > 0) and the body width b (b ≧ 0) can satisfy the relationship of b / (a + b) < 0.4. In this case, the meshing width between the external teeth 2a of the external gear 2 and the internal teeth 3a of the internal gear 3 can be further increased. Therefore, in this case, the allowable torque of the harmonic gear device 1A can be further improved.

[0043] Also, referring to FIG. 5, in the present embodiment, it is preferable that the tip circle diameter Df of the external gear 2, the tip circle diameter Dr of the internal gear 3, and the total tooth depth h of the external gear 2 satisfy the relationship of 0.1h < (Df - Dr) < h.

[0044] When 0.1h < (Df - Dr), the allowable torque can be increased without reducing the number of meshing teeth. Also, when (Df - Dr) < h, the external gear 2 can be assembled to the internal gear 3 without significantly interfering the external teeth 2a of the external gear 2 with the internal teeth 3 of the internal gear 3. Therefore, when the tip circle diameter Df of the external gear 2, the tip circle diameter Dr of the internal gear 3, and the total tooth depth h of the external gear 2 are set to satisfy the relationship of 0.1h < (Df - Dr) < h, the manufacturing of the harmonic gear device 1A becomes easy without reducing the allowable torque.

[0045] Furthermore, in the present embodiment, it is preferable that the tip circle diameter Df of the external gear 2, the tip circle diameter Dr of the internal gear 3, and the total tooth depth h of the external gear 2 satisfy the relationship of 0.3h < (Df - Dr) < h. In this case, by increasing the number of meshing teeth, the allowable torque can be further increased.

[0046] Furthermore, as a means of improving the allowable torque of the wave gear device flattened by making the external gear 2 flat, for example, as shown in Fig. 6, there is a method of adjusting the deflection amount d which is a value indicating the amount of deflection of the external gear 2. Here, the deflection amount d can be defined as d = (Zr - Zf) × m from the relationship between the module m, the number of teeth Zf of the external gear 2, and the number of teeth Zr of the internal gear 3. As a specific example, in a wave gear device with a module m = 1 and a reduction ratio of 50, when the difference in the number of teeth is 2, the deflection amount d is d = (102 - 100) × 1, so d = 2.

[0047] On the other hand, in order to avoid interference during the assembly of the external gear 2 and the internal gear 3, if the tooth tip of the external gear 2 is made excessively low, the number of meshing teeth or the meshing area becomes small. Therefore, when the tooth tip of the external gear 2 and the internal gear 3 is made excessively low, there is also a concern that the allowable torque of the wave gear device will decrease or rattling will occur. Therefore, there are certain restrictions on the tooth tip in the wave gear device. For example, as an element that can adjust the pitch diameter Df of the tooth tip of the external gear 2, a tooth tip not exceeding d / 2, that is, a tooth tip with a value half of the deflection amount d is required.

[0048] On the contrary, in order to improve the life or gear strength of the wave gear device, the deflection amount d is also intentionally adjusted. As the tooth profile obtained by adjusting the deflection amount d, for example, the one called "offset tooth profile" described in Japanese Patent Publication No. 45-41171 is known. The offset gear is said to be compatible with a material that is easily deformed. The offset tooth profile can be specified, for example, by the deflection amount d expressed by the following formula (1).

[0049] d = (Zr - Zf) × m × Κ ··· (1) (Κ: offset coefficient)

[0050] For example, in the case of a harmonic gear device with a large reduction ratio, the module m of the external gear 2 and the internal gear 3 becomes small, so there is an inevitable problem that the number of teeth becomes small. Therefore, in order to compensate for the reduction in the number of teeth itself, a positive displacement tooth profile that increases the effective height of the teeth by increasing the deflection amount d may be used. Here, the positive displacement tooth profile can be expressed as (d = (Zr - Zf) × m × Κ: (Κ > 1)). That is, the positive displacement tooth profile satisfies Κ > 1 in Equation (1). Fig. 7 schematically and partially shows the meshing model of the positive displacement tooth profile.

[0051] Conversely, in the case of a harmonic gear device with a small reduction ratio, the number of teeth can be increased. However, in this case, the deflection amount d becomes large. For this reason, since the load stress applied to the bearing 42 of the wave generator 4 increases, there is a problem that the life of the speed reducer decreases. Therefore, a negative displacement tooth profile that increases the number of meshing teeth by reducing the deflection amount d may be used. Here, the negative displacement tooth profile can be expressed as (d = (Zr - Zf) × m × Κ: (Κ < 1)). That is, the negative displacement tooth profile satisfies Κ < 1 in Equation (1). Fig. 8 schematically and partially shows the meshing model of the negative displacement tooth profile. When the negative displacement tooth profile is used, the allowable torque can be improved and the stress applied to the wave generator 4 can be relaxed by increasing the number of meshing teeth.

[0052] However, the technology described in the above publication does not define the relationship between the tooth shape of the external gear 2 and the material characteristics optimal for the external gear 2. On the other hand, the harmonic gear device 1A according to the present embodiment is applicable to both the positive displacement tooth profile and the negative displacement tooth profile.

[0053] For example, in the case of a positive offset tooth profile, since the deflection amount d becomes larger than normal, a material with a lower Young's modulus E is considered better. However, if the Young's modulus E is only low, the allowable torque of the harmonic gear device may sometimes become small. Therefore, in the harmonic gear device 1A according to the present embodiment, if the shapes of the external teeth 2a and the internal teeth 3a are positive offset tooth profiles, the meshing in the tooth width direction is improved. Accordingly, in the harmonic gear device 1A, the deflection amount d can be set to a value that satisfies Κ > 1 in Equation (1). Further, when the corning angle of the external gear 2 is θ, if the harmonic gear device 1A is a harmonic gear device that satisfies (b × sin(θ) + Df) > Dr, sufficient allowable torque can be ensured.

[0054] On the other hand, in the case of a negative offset tooth profile, since the deflection amount d becomes smaller than normal, a material with a high Young's modulus E can also be applied. However, as described above, when Df > Dr, it becomes difficult to assemble the external gear 2 of the external gear 2 to the internal gear 3a of the internal gear 3 due to the interference between the external teeth 2a of the external gear 2 and the internal teeth 3a of the internal gear 3. However, in the case of the external gear 2 of the harmonic gear device 1A according to the present embodiment, the external gear 2 is formed of a material that is more easily bent with a smaller Young's modulus E than a general external gear having flexibility. Therefore, when the shapes of the external teeth 2a and the internal teeth 3a are negative offset tooth profiles, the value of the offset coefficient Κ in Equation (1) is Κ < 1, so that the Κ can be made smaller, and the number of meshing teeth can be improved. Accordingly, also in the case of a negative offset tooth profile, if the offset coefficient Κ is made smaller, sufficient allowable torque can be ensured.

[0055] Next, FIG. 9 is a bottom view schematically showing a modified example of the external gear 2.

[0056] The external gear 2 includes a bottom wall 22 connected to one end of the cylindrical wall 21. A plurality of slit holes 25 are intermittently arranged in an annular shape around the axis O on the bottom wall 22. The plurality of slit holes 25 arranged in an annular shape are arranged on at least one circle centered on the axis O. The slit hole 25 is a through hole that penetrates the bottom wall 22.

[0057] Regarding the external gear 2, in the bottom wall 22, it is preferable to have a shape that is easily deformable in the radial direction and difficult to twist in order to transmit torque. As in this embodiment, a plurality of slit holes 25 are formed annularly and intermittently around the axis O in the bottom wall 22 of the external gear 2, and the plurality of slit holes 25 arranged annularly are formed so as to be arranged on at least one circle centered on the axis O. Then, the bottom wall 22 of the external gear 2 has a structure that is easily deformable in the radial direction and strong against twisting in the circumferential direction. That is, according to this embodiment, anisotropy can be imparted to the rigidity of the bottom wall 22. Thereby, the external gear 2 according to this embodiment has a shape that is easily deformable in the radial direction but difficult to twist, so that efficient torque transmission can be performed. Therefore, according to this embodiment, while performing efficient torque transmission, the axial length can be suppressed to be shorter. Furthermore, in this case, the allowable torque can be increased. Therefore, according to this embodiment, while increasing the allowable torque, the axial length can be suppressed to be shorter.

[0058] Also, as shown in FIG. 9, in this embodiment, each of the plurality of slit holes 25 has a length along the circumferential direction longer than the length in the radial direction. In this embodiment, the slit hole 25 is a long hole extending in the circumferential direction. In this case, the external gear 2 has a shape that is more easily deformable in the radial direction but also less likely to twist, so that efficient torque transmission can be performed. Therefore, according to this embodiment, efficient torque transmission can be performed. Furthermore, in this case, the allowable torque can be further increased.

[0059] Also, in the present embodiment, the plurality of slit holes 25 are arranged in a plurality of concentric circles centered on the axis O such that the radial distances from the axis O are different. Referring to FIG. 9, in the present embodiment, the plurality of slit holes 25 are arranged in three concentric circles centered on the axis O such that the radial distances from the axis O are different. In the present embodiment, the twelve slit holes 25 arranged in a ring shape are arranged on each of the three concentric circles centered on the axis O. When the plurality of slit holes 25 are arranged in a plurality of concentric circles centered on the axis O such that the radial distances from the axis O are different as in the present embodiment, the external gear 2 is more likely to be deformed in the radial direction, but the shape is also less likely to be twisted, so that more efficient torque transmission can be performed. Therefore, according to the present embodiment, more efficient torque transmission can be performed. Further, in this case, the allowable torque can be further increased.

[0060] Also, in the present embodiment, the plurality of slit holes 25 arranged in a plurality of rows are arranged in a plurality of concentric circles centered on the axis O such that the circumferential end positions of the slit holes 25 in each row are different between adjacent rows. For example, as shown in FIG. 9, in the present embodiment, the three slit holes 25 arranged in the radial direction are arranged in three rows in the radial direction such that the inner and outer slit holes 25 in the radial direction are aligned in the radial direction, while the slit hole 25 in the center in the radial direction is displaced in the circumferential direction with respect to the inner and outer slit holes 25 in the radial direction. In other words, in the present embodiment, the plurality of slit holes 25 arranged intermittently and annularly around the axis O are aligned in the radial direction on the inner and outer sides in the radial direction, while in the center in the radial direction, they are arranged so as to be displaced in one circumferential direction with respect to the inner and outer sides in the radial direction while leaving a portion overlapping in the circumferential direction with each other in the radial direction.

[0061] In a bottom view as shown in Fig. 9, when the circumferential arrangement angles of a plurality of rows of slit holes 25 arranged on concentric circles around the respective axes O are the same angle, and the plurality of rows of slit holes 25 are arranged so as to overlap in the radial direction, the torsional rigidity of the bottom wall 22 decreases. For this reason, in the present embodiment, it is preferable that each of the slit holes 25 arranged in a plurality of rows in the radial direction is arranged at a position shifted in the circumferential direction. Therefore, as in the present embodiment, if a plurality of slit holes 25 arranged in a plurality of rows are arranged in a plurality of concentric circles around the axis O such that the circumferential end positions of the slit holes 25 within each row are different between adjacent rows, more efficient torque transmission can be performed. Therefore, according to the present embodiment, more efficient torque transmission can be performed. Further, in this case, the allowable torque can be further increased. In addition, in this case, since the axial length can be further suppressed to be shorter, the harmonic gear device 1A can be made flatter. Therefore, in this case, the allowable torque can be further increased while suppressing the axial length to be shorter.

[0062] Next, Fig. 10 is a cross-sectional view schematically showing an external gear 2 applicable to the harmonic gear device 1A corresponding to the X-X cross-section of Fig. 1, and this cross-sectional view shows the states before and after tooth profile modification is performed on the external teeth 2a.

[0063] According to the present embodiment, the body width b of the external gear 2 can be set to 0 (zero). That is, according to the present embodiment, the tooth width a of the external gear 2 can be increased up to the bottom wall 22.

[0064] However, generally, when the tooth width a is increased up to the bottom wall 22, the amount of deflection d is substantially small on the side of the bottom wall 22. For this reason, when the tooth width a is increased up to the bottom wall 22, the tooth width direction end portions on the tooth surfaces of the external teeth 2a and the tooth width direction end portions on the tooth surfaces of the internal teeth 3a may result in excessive interference.

[0065] On the other hand, according to the harmonic drive device 1A according to the present embodiment, when the outer gear 2 and the inner gear 3 are excessively interfered with by increasing the tooth width a of the outer gear 2 up to the bottom wall 22, as shown in FIG. 10, by modifying the end portion in the tooth width direction on the tooth surface of the outer teeth 2a, a taper (inclined surface) or an R (curved surface) can be formed. Examples of such a tooth profile modification method include crowning, end relief, and the like.

[0066] As shown in FIG. 10, when the tooth profile is modified for the end portion in the tooth width direction on the tooth surface of the outer teeth 2a, the tooth thickness at the end portion in the tooth width direction on the tooth surface of the outer teeth 2a becomes smaller. As a result, excessive interference between the outer gear 2 and the inner gear 3 that may occur due to increasing the tooth width a of the outer gear 2 up to the bottom wall 22 is suppressed. In particular, in this case, since the tip circle of the outer gear 2 becomes smaller in a harmonic drive device with a small module, it is effective in avoiding excessive interference between the outer gear 2 and the inner gear 3.

[0067] Next, FIG. 11 is a cross-sectional view schematically showing an outer gear 2 applicable to the harmonic drive device 1A corresponding to the X-X cross section of FIG. 1, and this cross-sectional view shows the states before and after the outer teeth 2a are displaced.

[0068] When the outer gear 2 and the inner gear 3 are excessively interfered with by increasing the tooth width a of the outer gear 2 up to the bottom wall 22, similar to performing tooth profile modification, displacement can be applied to the outer teeth 2a of the outer gear 2 in proportion as it goes toward the bottom wall 22 side. As a result, similar to performing tooth profile modification, excessive interference between the outer gear 2 and the inner gear 3 that may occur due to increasing the tooth width a of the outer gear 2 up to the bottom wall 22 is suppressed. Also, in this case as well, since the tip circle of the outer teeth 2a becomes smaller, it is effective in avoiding excessive interference between the outer gear 2 and the inner gear 3. Note that when the tooth width a is increased, depending on the amount of deflection d, there may be a case where a gap occurs in the meshing between the outer gear 2 and the inner gear 3 in the direction of the tooth width a. In that case, the meshing can be improved and the allowable torque can be increased by applying displacement in the reverse direction to the outer gear 2.

[0069] Furthermore, in order to improve the strength and the ease of flexural deformation of the external gear 2 in the wave gear device 1A according to the present embodiment, it is preferable that the average thickness of the body width b of the body portion 21b is 30 to 90% of the maximum thickness of the tooth width a of the tooth portion 21a. In this case, it is possible to improve the strength of the external gear 2 and the ease of bending of the external gear 2.

[0070] Next, FIG. 12 is a cross-sectional view schematically showing another external gear 2 applicable to the wave gear device 1A, which is equivalent to the cross-section of FIG. 2.

[0071] In the wave gear device 1A, the external gear 2 is a cup-shaped external gear, but the external gear 2 can be a silk hat-shaped external gear as shown in FIG. 12. In the present disclosure, the external gear 2 includes a cylindrical wall 21 and a flange 26 continuous with the cylindrical wall 21. In the present disclosure, the flange 26 is continuous with the lower end of the cylindrical wall 21 and protrudes radially outward in an annular shape in the circumferential direction around the axis O.

[0072] In the present disclosure, the cylindrical wall 21 is composed of a tooth portion 21a provided with external teeth 2a of the external gear 2 on the outer surface and a body portion 21b continuous with the tooth portion 21a. Also in the present disclosure, the tooth width a is the axial length of the tooth portion 21a. Also in the present disclosure, the body width b is the axial length of the body portion 21b. Furthermore, also in the present disclosure, the axial length (total width) L of the cylindrical wall 21 is L = a + b. However, in the present disclosure, the body width b is the axial length between the front end 2e of the external gear 2 and the upper surface 26a of the flange 26 of the external gear 2.

[0073] Furthermore, FIG. 13 schematically shows a wave gear device 1B according to a second embodiment of the present invention. In FIG. 13, matters such as parts or members substantially the same as those in the wave gear device 1A are denoted by the same reference numerals, and the description thereof is omitted.

[0074] The wave gear device according to the present invention can be a multi-lobe type wave gear device having three or more meshing points. The wave gear device 1B in Fig. 13 is a three-lobe type wave gear device in which the meshing portion P between the external teeth 2a of the external gear 2 and the internal teeth 3a of the internal gear 3 has three locations. In the present embodiment, the cam member 41 has a triangular three-lobe shape. Also, in the present embodiment, the bearing 42 includes a cage 42d that holds the rolling elements 42c. The matters described in the wave gear device 1A can also be applied to a multi-lobe type wave gear device having three or more lobes, such as the wave gear device 1B.

[0075] Also, in the wave gear device according to the present invention, when the outer peripheral radius of the body portion 21b of the external gear 2 is rD (when b = 0, the bottom circle radius of the external gear 2 is defined as rD), and the thickness of the body portion 21b is t, the relational expression between the Young's modulus E and b / (a + b) can satisfy the following formula.

[0076] 0.4 < Y < 30 ···(2)

[0077] TIFF2025084048000002.tif15158

[0078] For example, when the Young's modulus E of the generally used external gear 2 exceeds 200 and satisfies b / (a + b) < 0.5, the formula (3) becomes less than 0.4. In this case, it is necessary to make the thickness t of the body portion 21b extremely small. Therefore, in this case, it becomes difficult to establish as a wave gear device.

[0079] On the other hand, there is a method of expanding the outer peripheral radius rD of the body portion 21b. However, in the case of this method, since the outer diameter of the external gear 2 expands, it becomes difficult to achieve the object of the present invention, that is, miniaturization of the external gear 2, i.e., the wave gear device.

[0080] On the other hand, for example, when PEEK (Young's modulus E = 4) is used as the material of the external gear 2, the relationship of the above formula (2) can be satisfied. In this case, the allowable torque of the harmonic gear device 1A (1B) can be improved while keeping it small.

[0081] Furthermore, it is preferable that formula (3) satisfies the relationship of the following formula (4).

[0082] 0.5 < Y < 20 ···(4)

[0083] When formula (3) satisfies the relationship of formula (4), the meshing width between the external teeth 2a of the external gear 2 and the internal teeth 3a of the internal gear 3 can be further increased. Therefore, in this case, the allowable torque of the harmonic gear device 1A (1B) can be further improved.

[0084] What has been described above merely shows exemplary embodiments according to the present invention, and various modifications are possible according to the description of the claims. Also, in the above description, the power transmission path of the harmonic gear device takes the wave generator 4 as the input and the external gear 2 as the output, but it is not limited thereto. For example, the power transmission path of the harmonic gear device may take the external gear 2 as the input and the wave generator 4 as the output.

Explanation of Reference Numerals

[0085] 1A: Wave gear device (first embodiment), 1B: Wave gear device (second embodiment), 2: External gear, 2a: External teeth of the external gear, 2e: Front end of the external gear, 2p: Tooth tip of the external teeth, 21: Cylindrical wall, 21a: Tooth portion of the cylindrical wall, 21b: Body portion of the cylindrical wall, 22: Bottom wall, 22a: Upper surface of the bottom wall, 23: Boss (mounting portion), 24: Mounting hole, 24a: Key groove, 25: Slit hole, 26: Flange, 26a: Upper surface of the flange, 3: Internal gear, 3a: Internal teeth of the internal gear, 3p: Tooth tip of the internal teeth, 4: Wave generator, 41: Cam member, 42: Bearing, 42a: Outer ring, 42b: Inner ring, 42c: Rolling element, 42d: Retainer, 43: Mounting hole, 43a: Key groove, A1: Front end opening of the external gear, a: Tooth width of the external gear, b: Body width of the external gear, Df: Tooth tip circle diameter of the external gear, Dr: Tooth tip circle diameter of the internal gear, L: Total width of the cylindrical wall, P: Meshing portion, Zf: Number of teeth of the external gear, Zr: Number of teeth of the internal gear

Claims

1. A wave gear device in which a flexible annular external gear meshes with an internal gear on the inside of the internal gear while deforming in a wave shape, The Young's modulus E [GPa] of the external gear is 2.0<E<120, the external gear has a cylindrical wall, and the cylindrical wall is configured by a tooth portion on an outer surface of which the external teeth of the external gear are provided, and a body portion connected to the tooth portion, A wave gear device in which a tooth width a (a>0), which is the axial length of the tooth portion, and a body width b (b≧0), which is the axial length of the body portion, satisfy the relationship b / (a+b)<0.

5.

2. 2. The strain wave gear device according to claim 1, wherein the Young's modulus E [GPa] of the external gear is in the range of 2.0<E<30.

3. 3. The wave gear device according to claim 1, wherein a tip circle diameter Df of the external gear, a tip circle diameter Dr of the internal gear, and a total tooth depth h of the external gear satisfy the relationship 0.1h < (Df - Dr) < h.

4. The wave gear device according to claim 3, wherein a tip circle diameter Df of the external gear, a tip circle diameter Dr of the internal gear, and a total tooth depth h of the external gear satisfy the relationship 0.3h < (Df - Dr) < h.

5. the external gear includes a bottom wall connected to one end of the cylindrical wall, 3. The wave gear device according to claim 1 or 2, wherein a plurality of slit holes are intermittently arranged in an annular shape around the axis line in the bottom wall, and the plurality of annularly arranged slit holes are arranged on at least one circle centered on the axis line.

6. 6. The strain wave gear device according to claim 5, wherein each of the plurality of slit holes has a length in a circumferential direction longer than a length in a radial direction.

7. 7. The strain wave gear device according to claim 6, wherein the plurality of slit holes are arranged in a plurality of rows concentrically about the axis so as to have different radial distances from the axis.

8. 8. The strain wave gear device according to claim 7, wherein the plurality of slit holes arranged in the plurality of rows are arranged in a plurality of concentric rows around the axis such that circumferential end positions of the slit holes in each row differ between adjacent rows.

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