Reduction gear

Abstract

In a reduction gear (5), a side part (50a) of a first gear (50) and a side part (51a) of a second gear (51), which fit together in the radial direction, form a recessed part (62).

Description

reducer

[0001] The present invention relates to a reducer.

[0002] Patent Document 1 discloses a reducer in which a recess configured to store a lubricant is provided.

[0003] Japanese Patent Application Laid-Open No. 2020-118200

[0004] In a reducer, it is desirable for the lubricant to remain where the gears fit together.

[0005] Therefore, one of the objects of the present invention is to provide a reducer in which a recess can be formed adjacent to the portion where the gear is fitted.

[0006] In a reducer according to one aspect of the present invention, a side portion of the first gear and a side portion of the second gear that fit together in the radial direction form a recess.

[0007] 1 is a perspective view showing a schematic structure of a rotating device 1 according to a specific example. FIG. 1 is a cross-sectional view taken along line 2-2 in FIG. 1. FIG. 2 is a cross-sectional view taken along line 3-3 in FIG. 2. FIG. 2 is a cross-sectional view taken along line 4-4 in FIG. 2. FIG. 3 is a perspective view showing a state in which a gear cover 22 and a bearing 14 have been removed from the rotating device 1. FIG. 4 is a partial cross-sectional perspective view taken along line 6-6 in FIG.

[0008] An embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a perspective view schematically illustrating the structure of a rotating device 1 according to one specific example. FIG. 2 is a cross-sectional view taken along line 2-2 in FIG. 1. FIG. 2 is a cross-sectional view taken along an imaginary plane including a rotation axis x. This rotating device 1 includes a motor and a reducer that reduces the rotation of the motor, and is incorporated into, for example, an electrically assisted bicycle. The rotating device 1 includes a housing 2. The housing 2 has, for example, a generally cylindrical shape overall. The housing 2 is fixed to, for example, a bicycle frame. Note that bicycles include all types of bicycles, such as road bikes, cross bikes, mountain bikes, and city bikes.

[0009] A crankshaft 3 is supported in the housing 2 so as to be rotatable about a rotation axis x. In this example, the rotation axis x coincides with the central axis of the housing 2. The crankshaft 3 has one end 3a on one end side S1 in a direction along the rotation axis x (hereinafter referred to as the "rotation axis direction") and another end 3b on the other end side S2 opposite to the one end side S1. Both the one end 3a and the other end 3b protrude from the housing 2 in the rotation axis direction. Crank arms (not shown) extending in a direction perpendicular to the rotation axis x are attached to the one end 3a and the other end 3b, respectively. Pedals (not shown) are attached to the tips of the crank arms. A chain ring (not shown) is further attached to the crank arm attached to the one end 3a.

[0010] The housing 2 includes a housing 20 (hereinafter referred to as the "motor housing") that accommodates the motor 4, a housing 21 (hereinafter referred to as the "gear housing") that accommodates the reducer 5 according to one embodiment of the present invention, and a cover 22 (hereinafter referred to as the "gear cover"). The motor housing 20, the gear housing 21, and the gear cover 22 are connected to one another with fastening parts 23, such as bolts extending in the direction of the rotation axis. The motor housing 20 and the gear housing 21 are sealed with an annular member (hereinafter referred to as the "gasket") 24. The gear housing 21 and the gear cover 22 are sealed with an annular member (hereinafter referred to as the "gasket") 25. The housing 2 is formed, for example, from a metal material or a resin material. From the viewpoint of thermal conductivity, the motor housing 20 and the gear housing 21 are preferably formed from a metal material. The gaskets 24, 25 are formed from a metal material or a resin material that has elasticity or restoring force.

[0011] The motor housing 20 has a cylindrical outer portion (hereinafter referred to as the "outer peripheral portion") 20a centered on the rotation axis x, a cylindrical inner portion (hereinafter referred to as the "inner peripheral portion") 20b centered on the rotation axis x, and a wall 20c connecting the outer peripheral portion 20a and the inner peripheral portion 20b. In this example, the wall 20c connects the other end side S2 of the outer peripheral portion 20a and the inner peripheral portion 20b. The outer peripheral portion 20a surrounds the motor 4 from the outer peripheral side in a radial direction perpendicular to the rotation axis x. The inner peripheral portion 20b is disposed inside the motor 4. The inner peripheral surface of the inner peripheral portion 20b faces the outer peripheral surface of the crankshaft 3. In this example, the wall 20c extends along an imaginary plane perpendicular to the rotation axis x. That is, the wall 20c faces the motor 4 in the rotation axis direction and forms a bottom.

[0012] A bearing 10 is disposed between the inner peripheral end of the wall 20c and the outer peripheral surface of the crankshaft 3. The bearing 10 is a bearing having rolling elements, such as a ball bearing. Thus, the motor housing 20 supports the crankshaft 3 at the inner peripheral end of the wall 20c via the bearing 10 so as to be relatively rotatable. The housing 2 further has an annular end cover 26. The end cover 26 covers the other end side S2 of the bearing 10 along the rotational axis direction. The end cover 26 is fixed to the wall 20c with fasteners 27, such as screws. The other end 3b of the crankshaft 3 protrudes from the housing 2 to the other end side S2 through a hole (through-hole) in the end cover 26 formed along the rotational axis x.

[0013] FIG. 3 is a cross-sectional view taken along line 3-3 in FIG. 2. Referring to both FIGS. 2 and 3, the motor 4 includes a stator 40 and a rotor 41 that can rotate relative to the stator 40. The stator 40 includes a magnetic body (hereinafter referred to as a "yoke"), i.e., a stator core 42, coils 43, and insulators 44. The stator core 42 is formed from a laminated body in which multiple annular silicon steel plates or electromagnetic steel plates are stacked in the direction of the rotation axis. The stator core 42 is fixed to the inner circumferential surface of the outer circumferential portion 20a of the motor housing 20. The stator core 42 includes multiple magnetic pole portions, i.e., multiple teeth 42a, arranged circumferentially around the rotation axis x. A coil 43 is wound around each tooth 42a. The insulators 44 insulate the stator core 42 from the coils 43.

[0014] The stator 40 is covered with a predetermined member 45 (see FIG. 2 ). The predetermined member 45 is formed, for example, from a thermally conductive resin material. A substrate (hereinafter referred to as a “printed circuit board”) 46 is fixed to the other end S2 of the predetermined member 45. The printed circuit board 46 is provided with wiring for electrically connecting the coils and multiple electronic components, such as a temperature sensor for detecting temperature and a magnetic sensor. The printed circuit board 46 is electrically connected to another substrate provided, for example, on the outer periphery of the rotating device 1 or externally. The printed circuit board 46 may also be configured with multiple bus bars, and the printed circuit board may be electrically connected to the multiple bus bars. The rotating device 1 incorporates a sensor (not shown) that detects torque acting on the crankshaft 3. A control circuit is mounted on the printed circuit board 46 and is capable of determining the current value supplied to the coil 43 based on the torque detected by the sensor. The coil 43 is supplied with current, for example, from a battery (not shown) installed on the bicycle. In this way, the control circuit can control the rotation speed of the motor 4 in accordance with the torque value of the crankshaft 3 .

[0015] The rotor 41 includes a magnet 47 and a magnetic body (hereinafter referred to as a "yoke") 48. The yoke 48 is formed of, for example, a magnetic material. A plurality of magnets 47 are supported on the yoke 48 and arranged in a circumferential direction. The magnets 47 face the outer peripheral surfaces of the teeth 42a of the stator core 42 with a predetermined magnetic gap therebetween. When a current is supplied to the coil 43, magnetic interaction between the coil 43 and the magnet 47 allows the rotor 41 to rotate relative to the stator 40 around the rotation axis x. The yoke 48 may be, for example, a cylindrical member centered on the rotation axis x. The magnet 47 may be a single cylindrical permanent magnet, or may be a cylindrical shape formed by connecting a plurality of permanent magnets together in the circumferential direction.

[0016] The rotor 41 is attached to a motor shaft 49. The motor shaft 49 has a cylindrical main body 49a centered on the rotation axis x and a flange 49b extending radially and annularly from the outer circumferential surface of the main body 49a. The outer circumferential surface of the main body 49a is defined by a cylindrical surface centered on the rotation axis x, while the outer circumferential surface of the flange 49b is defined by a cylindrical surface having a central axis eccentric from the rotation axis x. The rotor 41 is fixed to the outer circumferential surface of the main body 49a, for example, by press fitting. The motor shaft 49 is rotatably supported on the outer circumferential surface of the inner circumferential portion 20b of the motor housing 20 adjacent to the other end side S2 via a bearing 11. The bearing 11 is a bearing having rolling elements, such as a ball bearing. In the rotation axis direction, the inner circumferential surface of the main body 49a faces the outer circumferential surface of the crankshaft 3. Within the gear housing 21, a reducer 5 (described below) is connected to the end of one end side S1 of the motor shaft 49.

[0017] The gear housing 21 has a cylindrical outer portion (hereinafter referred to as the "outer peripheral portion") 21a centered on the rotation axis x. A plate 28 is fixed to the inner peripheral surface of the outer peripheral portion 21a adjacent to the other end side S2 of the outer peripheral portion 21a. In this example, the plate 28 is directly fixed to the outer peripheral portion 21a, but it may be indirectly fixed via another member. The gear cover 22 has a cylindrical outer portion (hereinafter referred to as the "outer peripheral portion") 22a centered on the rotation axis x and a cylindrical inner portion (hereinafter referred to as the "inner peripheral portion") 22b centered on the rotation axis x, located inside the outer peripheral portion 22a and having a smaller diameter. The reducer 5 is housed within the gear housing 21 and the gear cover 22. The reducer 5 is arranged alongside the motor 4 in the rotation axis direction. The plate 28 is formed in an annular shape extending across the motor 4 and the reducer 5. In other words, the reducer 5 is arranged on the opposite side of the plate 28 from the motor 4 in the rotation axis direction.

[0018] FIG. 4 is a cross-sectional view taken along line 4-4 in FIG. 2. Referring to FIGS. 2 and 4 together, the reducer 5 is, for example, a cycloid reducer. The reducer 5 includes a first gear 50 formed on the outer periphery 21a of the gear housing 21, a second gear 51 disposed on the inner periphery of the first gear 50 and mated with the first gear 50, and a third gear 52 disposed further inner than the first gear 50 and mated with the second gear 51. The first gear 50 has fixed teeth. The second gear 51 is a ring gear used as a planetary gear. The third gear 52 is supported on the outer periphery of the flange 49b of the motor shaft 49 via a bearing 12. The bearing 12 is a bearing having rolling elements, such as a ball bearing. As is clear from FIG. 2, the bearing 12 and the third gear 52 are arranged side by side in the direction of the rotation axis. The third gear 52 is supported on the outer peripheral surface of the main body 49a of the motor shaft 49 via a bearing 13. The bearing 13 is a bearing having rolling elements, such as a ball bearing.

[0019] As shown in FIG. 4 , a plurality of teeth 53 are formed on the side, i.e., inner peripheral portion 50a, of the first gear 50, arranged in the circumferential direction. A plurality of teeth 54 are formed on the side, i.e., outer peripheral portion 51a, of the second gear 51, arranged in the circumferential direction. The outer peripheral portion 51a is surrounded by the first gear 50. A plurality of teeth 55 are formed on the side, i.e., inner peripheral portion 51b, of the second gear 51, arranged in the circumferential direction. A plurality of teeth 56 are formed on the side, i.e., outer peripheral portion 52a of the third gear 52, arranged in the circumferential direction. As described above, the outer peripheral surface of the flange 49b is a cylindrical surface having a central axis eccentric from the rotation axis x, so the central axis x1 of the second gear 51 is eccentric from the rotation axis x. On the other hand, the central axes of the first gear 50 and the third gear 52 coincide with the rotation axis x. In this way, the central axis x1 of the second gear 51 rotates around the rotation axis x, while the second gear 51 rotates eccentrically with respect to the first gear 50 and the third gear 52.

[0020] In this example, the number of teeth 53 of the first gear 50 is greater than the number of teeth 54 on the outer periphery 51 a of the second gear 51. The number of teeth 54 on the outer periphery 51 a of the second gear 51 is greater than the number of teeth 55 on the inner periphery 51 b of the second gear 51. Similarly, the number of teeth 55 on the inner periphery 51 b of the second gear 51 is greater than the number of teeth 56 on the outer periphery 52 a of the third gear 52. When the motor shaft 49 rotates around the rotation axis x, some of the teeth 54 of the second gear 51 mesh with some of the teeth 53 of the first gear 50 due to the eccentricity and the difference in the number of teeth. At the same time, some of the teeth 55 of the second gear 51 mesh with some of the teeth 56 of the third gear 52 on the opposite side of the rotation axis x from the mesh with the first gear 50. As a result, the second gear 51 rotates in the direction opposite to the rotation direction of the motor shaft 49. At this time, the third gear 52 rotates in the same direction as the second gear 51. In this way, the rotation speed of the motor shaft 49 is reduced at a predetermined reduction ratio.

[0021] Returning to FIG. 2 , the third gear 52 has a cylindrical main body 52b centered on the rotation axis x and a cylindrical protrusion 52c centered on the rotation axis x that protrudes from the main body 52b toward one end side S1. The main body 52b has teeth 56 on its outer periphery 52a and is supported on its inner periphery by the motor shaft 49 via a bearing 13. The protrusion 52c is supported on its inner periphery by the outer periphery of the output shaft 58 via a one-way clutch 57, while its outer periphery is supported on its inner periphery by the inner periphery of the outer periphery 22a of the gear cover 22 via a bearing 14. The bearing 14 is a bearing having rolling elements, such as a needle bearing. The output shaft 58 is cylindrically formed around the rotation axis x. The output shaft 58 is arranged alongside the motor shaft 49 in the direction of the rotation axis.

[0022] The output shaft 58 has its outer circumferential surface supported by the inner circumferential surface of the inner circumferential portion 22b of the gear cover 22 via a bearing 15, and its inner circumferential surface supported by the outer circumferential surface of the crankshaft 3 via a bearing 16. The bearings 15 and 16 are bearings having rolling elements, such as ball bearings. The inner circumferential surface of the output shaft 58 and the outer circumferential surface of the crankshaft 3, which face each other, are connected via a one-way clutch 59. At least a portion of the one-way clutches 57, 59 is covered with a fluid, i.e., a lubricant (not shown).

[0023] The one-way clutches 57, 59 are configured to transmit the rotational force of the third gear 52 to the output shaft 58 in one direction about the rotation axis x (i.e., to rotate the third gear 52 and the output shaft 58 in the same direction), while allowing relative rotation between the third gear 52 and the output shaft 58 in the other direction about the rotation axis x. The other end S2 of the output shaft 58 faces the motor shaft 49, while the one end S1 of the output shaft 58 is disposed outside the gear cover 22. The one end S1 of the output shaft 58 is connected to a chain ring fixed to the crank arm on the one end S1. The rotational force generated by the motor 4 is reduced at a predetermined reduction ratio by the reducer 5 and transmitted from the output shaft 58 to the chain ring.

[0024] FIG. 5 is a perspective view showing the rotating device 1 with the gear cover 22 and bearing 14 removed. FIG. 6 is a partial cross-sectional perspective view taken along line 6-6 in FIG. 4. Referring to FIGS. 5 and 6 together, a circumferentially continuous curved surface 60 is formed at the end of one end side S1 of the inner peripheral portion 50a of the first gear 50. The curved surface 60 is formed, for example, on a cylindrical surface about the rotation axis x. On the inner peripheral portion 50a, the teeth 53 and the curved surface 60 are arranged side by side in the rotation axis direction. In this example, the curved surface 60 extending in the rotation axis direction intersects with the surface 53a (hereinafter referred to as the "end surface") of the tooth 53 extending in the radial direction. In this example, the end surface 53a is formed along an imaginary plane perpendicular to the rotation axis x. Meanwhile, in the radial direction, the end of the tooth 53 is arranged on the second gear 51 side with respect to the curved surface 60.

[0025] A circumferentially continuous curved surface 61 is formed on the end of one end side S1 of the outer circumferential portion 51a of the second gear 51. The curved surface 61 is defined, for example, by a cylindrical surface around the rotation axis x. On the outer circumferential portion 51a, the teeth 54 and the curved surface 61 are arranged side by side in the rotation axis direction. In this example, the curved surface 61 is adjacent in the radial direction to the surface 54a (hereinafter referred to as the "end surface") of the tooth 54 in the rotation axis direction. The curved surface 61 is arranged on the opposite side in the radial direction from the curved surface 60 of the first gear 50. The end surface 54a extending in the radial direction intersects with the curved surface 61 extending in the rotation axis direction. In this example, the end surface 54a is defined along an imaginary plane perpendicular to the rotation axis x. For example, the end surface 54a extends along the same imaginary plane as the end surface 53a. Meanwhile, in the radial direction, the end of the tooth 54 is arranged on the first gear 50 side with respect to the curved surface 61.

[0026] A recess 62 is formed by the curved surface 60 and end face 53a of the first gear 50 and the curved surface 61 and end face 54a of the second gear 51. The recess 62 is recessed toward the other end side S2. That is, the recess 62 is formed by the inner peripheral portion 50a of the first gear 50 and the outer peripheral portion 51a of the second gear 51, which are fitted together in the radial direction. The recess 62 forms an area where the lubricant W accumulates (a storage portion for the lubricant W). At least a portion of the area, including the meshing portion between the teeth 53 of the first gear 50 and the teeth 54 of the second gear 51, is covered with the lubricant W. Even if the lubricant W leaks out from between the first gear 50 and the second gear 51 or between the second gear 51 and the third gear 52 due to eccentric rotation of the second gear 51, the recess 62 can retain the lubricant W therein. The lubricant W thus collected in the recess 62 is again drawn between the first gear 50 and the second gear 51 and between the second gear 51 and the third gear 52 .

[0027] An example of how the rotating device 1 described above is used will now be described. When a user of a bicycle incorporating the rotating device 1 pedals to rotate the crank arm, the crankshaft 3 rotates around the rotation axis x. In this example, the crankshaft 3 rotates clockwise when viewed from one end S1. When the torque sensor detects an increase in torque acting on the crankshaft 3 equal to or greater than a predetermined threshold, a control circuit on a printed circuit board 46, which is electrically connected to an external power source via another board, supplies current to the coil 43 of the motor 4. Due to magnetic interaction between the coil 43 and the magnet 47, the rotor 41 rotates around the rotation axis x relative to the stator 40. The rotational force generated by this rotation is transmitted from the motor shaft 49 to the reducer 5. The rotor 41 and the motor shaft 49 rotate counterclockwise when viewed from one end S1.

[0028] In the reducer 5, the rotational force is transmitted from the third gear 52 to the output shaft 58 while reducing the rotational speed due to the engagement of the first gear 50, the second gear 51, and the third gear 52. Note that the rotational direction of the motor shaft 49 is opposite to that of the output shaft 58 due to the action of the reducer 5. That is, when viewed from the one end side S1, the output shaft 58 rotates clockwise. The rotational force with reduced rotational speed is transmitted from the output shaft 58 to the chain ring and the crankshaft 3. As a result, the rotational force of the motor 4 assists in the rotation of the bicycle crank arm while increasing its torque. In this way, the rotational force of the motor 4 allows the bicycle user to pedal easily even in situations where torque is high.

[0029] In this rotating device 1, the inner peripheral portion 50a of the first gear 50 and the outer peripheral portion 51a of the second gear 51, which are fitted together in the radial direction, form a recess 62. The recess 62 is formed, for example, at the end of one end side S1 of the first gear 50 and the second gear 51 in the rotational axis direction. As a result, the lubricant W leaking out in the rotational axis direction from between the teeth 53 of the first gear 50 and the teeth 54 of the second gear 51 when they mesh with each other can be retained in the recess 62. This prevents the lubricant W from leaking out from between the first gear 50 and the second gear 51 into another space. As a result, wear on the first gear 50 and the second gear 51 is reliably reduced, thereby extending the life of the reducer 5.

[0030] In the reducer 5 of the rotating device 1 described above, the recess 62 is formed continuously in the circumferential direction around the rotation axis x, but it may also be formed partially in a partial region in the circumferential direction. Furthermore, while the recess 62 is formed by the first gear 50 and the second gear 51 that are fitted together in the radial direction, an additional recess may be formed by, for example, the second gear 51 and the third gear 52 that are also fitted together in the radial direction. This additional recess may also be formed continuously in the circumferential direction around the rotation axis x, or may also be formed partially in a partial region in the circumferential direction. When the additional recess is formed, a cylindrical wall may be formed between the recess 62 and the additional recess. Furthermore, the formation of the recess 62 may be omitted, and only the additional recess may be provided.

[0031] Furthermore, in the above-described reducer 5, the end faces 53 a of the teeth 53 of the first gear 50 and the end faces 54 a of the teeth 54 of the second gear 51, which form the recesses 62, are defined along the same imaginary plane, but they may be defined along different imaginary planes. That is, the distance from the end of the first gear 50 at one end side S1 in the rotational axis direction to the end face 53 a may be different from the distance from the end of the second gear 51 at one end side S1 to the end face 54 a. Furthermore, although the recesses 62 are formed by the curved surface 60 and end face 53 a of the first gear 50 and the curved surface 61 and end face 54 a of the second gear 51, they may be formed by the curved surface 60 and end face 53 a of the first gear 50 and the teeth 54 of the second gear 51, or by the teeth 53 of the first gear 50 and the curved surface 61 and end face 53 a of the second gear 51.

[0032] Although the reducer 5 according to one embodiment of the present invention is incorporated into the rotating device 1 attached to an electrically assisted bicycle, the reducer 5 may be used for other purposes, such as industrial robots, machine tools, etc. Furthermore, the reducer 5 described above is configured as a cycloid reducer as an example, but it may also be another type of reducer, such as a strain wave gear reducer.

[0033] The above-described embodiments are intended to facilitate understanding of the present invention and are not intended to limit the scope of the present invention. Furthermore, the above-described embodiments do not limit the scope of the present invention, and the present invention may include any and all applications. The components of the above-described embodiments, as well as their arrangement, materials, conditions, shape, size, etc., are not limited to those illustrated and may be modified as appropriate. For example, the present invention includes differences that arise during implementation due to manufacturing tolerances, etc. Furthermore, components illustrated in different embodiments may be partially substituted or combined within the scope of technical inconsistency. Furthermore, the various configurations may be selectively combined as appropriate to achieve at least some of the above-described problems and effects.

[0034] REFERENCE SIGNS LIST 1 Rotating device, 10 Bearing, 11 Bearing, 12 Bearing, 13 Bearing, 14 Bearing, 15 Bearing, 16 Bearing, 2 Housing, 20 Motor housing, 20a Outer part (outer periphery), 20b Inner part (inner periphery), 20c Wall, 21 Gear housing, 21a Outer part (outer periphery), 22 Gear cover, 22a Outer part (outer periphery), 22b Inner part (inner periphery), 23 Fastening part, 24 Annular member (gasket), 25 Annular member (gasket), 26 End cover, 27 Fastening part, 28 Plate, 3 Crankshaft, 3a One end, 3b Other end, 4 Motor, 40 Stator, 41 Rotor, 42 Magnetic material (stator core), 42a Magnetic pole part (teeth), 43 Coil, 44 Insulator, 45 Prescribed member, 46 Board (printed circuit board), 47 Magnet, 48 Magnetic body (yoke), 49 Motor shaft (rotating shaft), 49a Main body, 49b Flange, 5 Reducer, 50 First gear, 50a Inner circumference (side portion), 51 Second gear, 51a Outer circumference (side portion), 51b Inner circumference (side portion), 52 Third gear, 52a Outer circumference (side portion), 52b Main body, 53 Teeth, 53a Surface (end surface), 54 Teeth, 54a Surface (end surface), 55 Teeth, 56 Teeth, 57 One-way clutch, 58 Output shaft, 59 One-way clutch, 60 Curved surface, 61 Curved surface, S1 One end side, S2 Other end side, W Lubricant, x Rotation axis, x1 Central axis

Claims

1. A speed reducer in which side portions of a first gear and a second gear that fit together in the radial direction form recesses.

2. The side portion of the first gear includes a circumferentially continuous curved surface and a plurality of teeth arranged in the circumferential direction, the side portion of the second gear includes a circumferentially continuous curved surface and a plurality of teeth arranged in the circumferential direction, and the curved surface of the first gear and the curved surface of the second gear face each other in the radial direction. The speed reducer according to claim 1.

3. In the radial direction, the ends of the plurality of teeth of the first gear are on the second gear side with respect to the curved surface of the first gear, and in the radial direction, the ends of the plurality of teeth of the second gear are on the first gear side with respect to the curved surface of the second gear. The speed reducer according to claim 2.

4. The speed reducer according to any one of claims 1 to 3, in which fluid accumulates in the recesses.

5. In the radial direction, the curved surface of the first gear and the plurality of teeth are aligned, and in the radial direction, the curved surface of the second gear and the plurality of teeth are aligned. The speed reducer according to any one of claims 1 to 4.

6. In the radial direction, a third gear that fits with the second gear is provided, the side portion of the first gear is an inner peripheral portion that surrounds the second gear, the side portion of the second gear is an outer peripheral portion surrounded by the first gear, in the radial direction, the outer peripheral portion of the third gear and the inner peripheral portion of the second gear fit together, and the second gear rotates eccentrically with respect to the first gear and the third gear. The speed reducer according to any one of claims 1 to 5.

7. The speed reducer according to any one of claims 1 to 6, in which the first gear is a fixed gear.

8. Having a rotating shaft, and the third gear is supported by the rotating shaft. The speed reducer according to claim 7.

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