Lightweight gear and manufacturing method thereof, manufacturing method of gear train, and robot

Abstract

Provided is a lightweight gear (1), a manufacturing method thereof, a manufacturing method of a gear train, and a robot, capable of preventing a resin-made connecting body from generating excessive stress due to torque transmitted in a joint portion with a metal. The lightweight gear (1) includes a tooth portion (2) of a circular ring shape made of metal, a shaft (3) made of metal extending along a center axis (A) of the tooth portion, and a connecting body (4) made of resin connecting the shaft and the tooth portion. A joint portion of the shaft and the connecting body (4) and a joint portion of the tooth portion and the connecting body (4) are provided with a concave-convex (5, 6, 11, 12) engaged in a circumferential direction, and a corner portion of the concave-convex has a rounded corner for stress relaxation. In addition, the manufacturing method of the lightweight gear (1) includes the steps of arranging the tooth portion and the shaft in a molding mold, and simultaneously performing injection molding of the connecting body (4) and joining of the connecting body (4) and the tooth portion and the shaft by injecting molten resin into a cavity of the molding mold.

Description

TECHNICAL FIELD

[0001] The present application relates to a lightweight gear, a manufacturing method thereof, a manufacturing method of a gear train, and a robot. BACKGROUND

[0002] In order to achieve weight reduction, a lightweight gear is known in which a metal shaft is connected to a metal tooth portion with a resin connecting body (see, for example, Patent Literature 1).

[0003] PRIOR ART DOCUMENTS

[0004] PATENT LITERATURE

[0005] Patent Literature 1: Japanese Patent Application Publication No. 2015-203477 SUMMARY

[0006] PROBLEMS TO BE SOLVED BY THE INVENTION

[0007] Since the resin connecting body has lower strength than the metal shaft and the tooth portion, it is desirable to prevent excessive stress from being generated due to torque transmitted in the joint portion with the metal. In the gear structure of Patent Literature 1, a large repeated stress acts on the corner portion of the resin connecting body, and there is a high possibility that early fatigue failure will occur from this as a starting point.

[0008] SOLUTION TO THE PROBLEM

[0009] One aspect of the present application is a lightweight gear including a metal tooth portion in a circular ring shape, a metal shaft extending along a central axis of the tooth portion, and a resin connecting body connecting the shaft and the tooth portion, a joint portion of the shaft and the connecting body and a joint portion of the tooth portion and the connecting body being provided with a protrusion and a recess engaged in a circumferential direction, a corner portion of the protrusion and the recess having a rounded corner for stress relaxation. BRIEF DESCRIPTION OF DRAWINGS

[0010] Figure 1 is a perspective view showing a lightweight gear according to a first embodiment of the present application.

[0011] Figure 2 is a front view showing the lightweight gear of Figure 1

[0012] Figure 3 is a perspective view showing a tooth portion of the lightweight gear of Figure 1

[0013] Figure 4 is a front view showing the tooth portion of Figure 3

[0014] Figure 5 is a front view showing the tooth portion of Figure 1 ​​​Fig. 1 is a longitudinal sectional view of a lightweight gear according to the present application.

[0015] Figure 6 Fig. 2 is a perspective view of a shaft of the lightweight gear of Fig. 1. Figure 1

[0016] Figure 7 Fig. 3 is a perspective view of a modification of the shaft of the lightweight gear of Fig. 1. Figure 6

[0017] Figure 8 Fig. 4 is an enlarged view of a tooth portion of the lightweight gear of Fig. 1. Figure 3

[0018] Figure 9 Fig. 5 is an enlarged longitudinal sectional view of a joint portion of a tooth portion, a connecting body, and a shaft of the lightweight gear of Fig. 1. Figure 5

[0019] Figure 10 Fig. 6 is a front view of the connecting body of the lightweight gear of Fig. 1. Figure 1

[0020] Figure 11 Fig. 7 is a front view of a modification of the lightweight gear of Fig. 1. Figure 1

[0021] Figure 12 Fig. 8 is a front view of another modification of the lightweight gear of Fig. 1. Figure 1

[0022] Figure 13 Fig. 9 is a front view of one example of a gear train provided with the lightweight gear of Fig. 1. Figure 1

[0023] BRIEF DESCRIPTION OF THE DRAWINGS

[0024] 1: lightweight gear

[0025] 2: tooth portion

[0026] 3: shaft

[0027] 4: connecting body

[0028] 5: first recess (concavo-convex)

[0029] 6: first protrusion (concavo-convex)

[0030] 7: second recess (concavo-convex)

[0031] 8: second protrusion (concavo-convex)

[0032] 11: third recess (concavo-convex)

[0033] 12: third protrusion (concavo-convex)

[0034] 13: fourth recess (concavo-convex)​​​​​​​​

[0035] 14: The fourth convex part (concave-convex)

[0036] 20: Another Gear

[0037] A: Central axis

[0038] Hg, Hs: altitude Detailed Implementation

[0039] The following description, with reference to the accompanying drawings, describes the lightweight gear 1 of the first embodiment of the present invention, its manufacturing method, the manufacturing method of the gear system, and the robot.

[0040] The lightweight gear 1 of this embodiment is used in the robot of this embodiment, such as a multi-joint robot.

[0041] like Figure 1 as well as Figure 2 As shown, the lightweight gear 1 of this embodiment includes: a tooth 2, which is composed of a circular metal plate; a metal shaft 3, which extends along the central axis A of the tooth 2; and a resin connector 4, which connects the tooth 2 and the shaft 3.

[0042] like Figure 3 as well as Figure 4 As shown, the tooth portion 2 has a plurality of teeth arranged circumferentially on its outer peripheral surface, and a plurality of first recesses (convexities / concaves) 5 and first convexities (convexities / concaves) 6 arranged circumferentially and radially convex and concave on its inner peripheral surface. Additionally, as... Figure 5 As shown, on each first protrusion 6, a second recess (concave-convex) 7 is provided at the center of the tooth 2 in the thickness direction, recessed radially outward. Consequently, second protrusions (concave-convex) 8 extending radially inward relative to the second recess 7 are formed on both sides of the second recess 7 in the thickness direction. Furthermore, the second recess 7 is not limited to the shape shown in the figure; it can have a shape with opposite concave and convex features, or it can have a shape composed of multiple segments of concave and convex features.

[0043] like Figure 5 as well as Figure 6 As shown, shaft 3, for example, includes: a columnar portion 9 supported by a bearing or retaining ring (not shown); and a engaging portion 10 disposed on a portion of the axial direction of the columnar portion 9, and having a plurality of third recesses (convexities / concaves) 11 and third protrusions (convexities / concaves) 12 arranged circumferentially and radially. Additionally, as... Figure 5 As shown, on each third protrusion 12, a fourth recess (concave-convex) 13 is provided at the center of the axial direction of the third protrusion 12, which is recessed radially inward. Consequently, fourth protrusions (concave-convex) 14 extending radially outward relative to the fourth recess 13 are formed on both sides of the fourth recess 13 in the axial direction. Alternatively, instead of using a cylindrical component, a shaft 3 can be used as shown in the diagram. Figure 7As shown, a shaft with a concave-convex shape is formed by machining in the direction of a solid cylindrical component.

[0044] like Figure 8 As shown, on the inner circumferential surface of the tooth 2, all corners of the concave-convex structure formed by the first concave portion 5, the first convex portion 6, the second concave portion 7, and the second convex portion 8 are provided with fillets of R2 (radius of curvature 2 mm or more). Additionally, as... Figure 9 As shown, on the outer peripheral surface of the engaging portion 10 of shaft 3, all corners of the concave and convex parts formed by the third concave portion 11, the third convex portion 12, the fourth concave portion 13, and the fourth convex portion 14 are also provided with fillets of R2 or more. As with existing gears, in the case of machining end mills where the R-shape of the corners remains unchanged, even using a strong resin material, fatigue failure due to stress concentration during long-term operation cannot be avoided. The larger the R-size, the more stress relief can be expected.

[0045] like Figure 2 As shown, the connecting body 4 is composed of an annular plate disposed between the shaft 3 and the toothed part 2. Figure 10 As shown, the connector 4 includes: an outer peripheral surface having irregularities that complement the shape of the inner peripheral surface of the toothed portion 2; and an inner peripheral surface having irregularities that complement the shape of the engaging portion 10 of the shaft 3. Figure 5 As shown, the thickness of the connector 4 is set to be less than or equal to the axial dimension of the engaging portion 10 of the shaft 3 and the thickness of the tooth portion 2.

[0046] That is, the connecting body 4 has an outer peripheral surface that fits tightly against the first recess 5, the first protrusion 6, the second recess 7, and the second protrusion 8 of the inner peripheral surface of the tooth 2 without gap. Additionally, the connecting body 4 has an inner peripheral surface that fits tightly against the third recess 11, the third protrusion 12, the fourth recess 13, and the fourth protrusion 14 of the engaging portion 10 of the shaft 3 without gap. Therefore, rounded corners that complement the rounded corners of the tooth 2 and the shaft 3 are also provided at the concave and convex corners of the outer and inner peripheral surfaces of the connecting body 4. In the figure, reference numeral 15 indicates a through hole provided in the connecting body 4 and extending along the plate thickness direction.

[0047] The tooth 2 and shaft 3 are made of sintered metal. Alternatively, the tooth 2 and shaft 3 can also be machined from metal. The tooth 2 or shaft 3 undergoes heat treatment to improve surface hardness.

[0048] Furthermore, the resin matrix of the connector 4 is composed of a thermoplastic resin. Alternatively, the resin of the connector 4 may also be composed of a fiber-reinforced thermoplastic resin.

[0049] The manufacturing method of the lightweight gear 1 thus configured according to the present embodiment will be described below. The lightweight gear 1 according to the present embodiment is first formed by sintering the shaft 3 and by sintering the tooth portion 2. The order of forming the shaft 3 and the tooth portion 2 can also be reversed, or they can be formed simultaneously. In addition, the shaft 3 and the tooth portion 2 thus formed can also be subjected to heat treatment for increasing the surface hardness.

[0050] Then, the shaft 3 and the tooth portion 2 thus formed are accommodated as an insert metal in the cavity formed on the molding die, and the molding die is closed. Thereby, a closed cavity with the inner peripheral surface of the tooth portion 2 and the outer peripheral surface of the engaging portion 10 of the shaft 3 as part of the inner wall is formed, and molten resin is injected into the cavity.

[0051] As a result, the connecting body 4 composed of the circular plate is molded between the shaft 3 and the tooth portion 2, and the gaps between the engaging portion 10 of the shaft 3 and the inner peripheral surface of the tooth portion 2 are filled, thereby manufacturing the lightweight gear 1 in which the connecting body 4 is fixed to the shaft 3 and the tooth portion 2 as one body.

[0052] According to the lightweight gear 1 according to the present embodiment, by making only the shaft 3 and the tooth portion 2 of metal and making the connecting body 4 of resin, the weight of the lightweight gear 1 can be made lightweight as compared with the case where the entire gear is made of metal.

[0053] If torque around the central axis A is applied due to the engagement of the tooth portion 2 with another gear, the torque is transmitted to the shaft 3 via the engagement portions of the tooth portion 2 and the connecting body 4 and the engagement portions of the connecting body 4 and the shaft 3. The same applies to the case where torque is transmitted from the shaft 3 to the tooth portion 2.

[0054] The first recesses 5 and the first protrusions 6 are arranged in the circumferential direction on the inner peripheral surface of the tooth portion 2, and the concavo-convexes of shapes that fit these first recesses 5 and the first protrusions 6 are formed on the connecting body 4. Therefore, the torque applied to the tooth portion 2 is transmitted from the tooth portion 2 to the connecting body 4 in the faces that extend across the circumferential direction between all the first recesses 5 and the first protrusions 6.

[0055] On the other hand, the third recesses 11 and the third protrusions 12 are also arranged in the circumferential direction on the engaging portion 10 of the shaft 3, and the concavo-convexes of shapes that fit these third recesses 11 and the third protrusions 12 are formed on the connecting body 4. Therefore, the torque transmitted to the connecting body 4 is transmitted from the connecting body 4 to the shaft 3 in the faces that extend across the circumferential direction between all the third recesses 11 and the third protrusions 12.

[0056] In this case, according to the lightweight gear 1 of the present embodiment, R2 or more rounded corners are formed at all of the corners of the concave-convex formed by the first recess 5 and the first protrusion 6, and at all of the corners of the concave-convex formed by the third recess 11 and the third protrusion 12. Therefore, there is an advantage that, regardless of whether the torque is transmitted from the tooth portion 2 to the connecting body 4 or the torque is transmitted from the connecting body 4 to the shaft 3, excessive stress concentration can be prevented at the corners of the concave-convex of the connecting body 4.

[0057] In addition, if an axial force is applied to another gear engaged with the tooth portion 2, the force is transmitted to the shaft 3 via the engagement portions of the tooth portion 2 and the connecting body 4 and the engagement portions of the connecting body 4 and the shaft 3. The same is true in the case where the force is transmitted from the shaft 3 to the tooth portion 2.

[0058] The second recess 7 and the second protrusion 8 are arranged in the axial direction on the inner peripheral surface of the tooth portion 2, and the concave-convex of a shape that closely adheres to these second recess 7 and second protrusion 8 is formed on the connecting body 4. Therefore, the force applied to the tooth portion 2 is transmitted from the tooth portion 2 to the connecting body 4 in a surface extending between the second recess 7 and the second protrusion 8 while intersecting the circumferential direction.

[0059] On the other hand, a plurality of fourth recesses 13 and fourth protrusions 14 arranged in the axial direction are also provided on the engagement portion 10 of the shaft 3, and the concave-convex of a shape that closely adheres to these fourth recesses 13 and fourth protrusions 14 is formed on the connecting body 4. Therefore, the force transmitted to the connecting body 4 is transmitted from the connecting body 4 to the shaft 3 in a surface extending between the fourth recess 13 and the fourth protrusion 14 while intersecting the circumferential direction.

[0060] In this case, according to the lightweight gear 1 of the present embodiment, R2 or more rounded corners are formed at all of the corners of the concave-convex formed by the second recess 7 and the second protrusion 8, and at all of the corners of the concave-convex formed by the fourth recess 13 and the fourth protrusion 14. Therefore, there is an advantage that, regardless of whether the axial force is transmitted from the tooth portion 2 to the connecting body 4 or the axial force is transmitted from the connecting body 4 to the shaft 3, excessive stress concentration can be prevented at the corners of the concave-convex of the connecting body 4.

[0061] Furthermore, according to the lightweight gear 1 of the present embodiment, since the stress concentration at all of the corners is reduced, there is also an advantage that the thickness dimension of the connecting body 4 can be made thin, and the lightweight can be effectively achieved.

[0062] Further, according to the lightweight gear 1 of the present embodiment, since the connecting body 4 is configured to have a thickness dimension below the tooth portion 2, sufficient weight reduction can be achieved as compared to a case where the connecting body 4 is composed of metal. In the prior art gear, the thickness of the connecting body 4 made of resin is thick enough to cover the metal tooth portion 2, and thus the connecting body 4 does not have much advantage in weight reduction as compared to the metal connecting body which is commonly thin in the integrated metal gear.

[0063] Further, since the connecting body 4 is composed of a thermoplastic resin, the connecting body 4 can be easily molded by injection molding.

[0064] Further, since the plurality of through holes 15 are provided in the connecting body 4, sufficient weight reduction can be achieved.

[0065] Further, by providing the through holes 15, the fastening work of the bolts at the portions covered by the connecting body 4 can be directly performed in a state where the lightweight gear 1 is installed.

[0066] Further, since the shaft 3 and the tooth portion 2 are composed of sintered metal, the number of machining sites can be reduced to achieve cost reduction. Further, by the anchoring effect of the resin of the connecting body 4 into the fine pores of the sintered metal, the joint strength of the connecting body 4 with the shaft 3 and the tooth portion 2 can be improved.

[0067] By performing heat treatment on the tooth portion 2 and the shaft 3, the surface hardness is improved. Thus, the connecting body 4 can also have the same surface pressure strength as the metal tooth portion 2.

[0068] Further, in the present embodiment, the through holes 15 penetrating in the plate thickness direction are provided in the connecting body 4 for weight reduction, but instead, recessed portions recessed in the axial direction can be provided. Thus, weight reduction can also be achieved.

[0069] Further, as the resin constituting the connecting body, a resin having a high thermal conductivity is preferably used. Thus, heat generated when the gears are engaged can be easily conducted from the tooth portion 2 to the shaft 3 side, and thus the durability of the connecting body 4 can be improved.

[0070] Further, as the resin constituting the connecting body 4, a resin having excellent oil resistance is preferably used. Thus, the connecting body 4 can be prevented from swelling or deteriorating due to the influence of lubricating oil.

[0071] Further, in the present embodiment, the one-stage lightweight gear 1 is used, but instead, a two-stage gear having two gears arranged in the axial direction can be used. In particular, by using the lightweight gear 1 as a large gear, great weight reduction can be achieved. Further, a lightweight reduction mechanism combining the two-stage gear can be configured.

[0072] In addition, as the type of the lightweight gear 1, a spur gear is exemplified in the present embodiment, but is not limited thereto, and a helical gear, a bevel gear, a worm gear, or a hypoid gear, or the like, to which a load also acts in the axial direction, can also be adopted. In the pinion of the worm gear and the hypoid gear, the tooth portion 2 can be made of metal only, and the shaft 3 can be connected to the tooth portion 2 with resin to achieve lightweight. In this case, the joint portion of the shaft 3 to the tooth portion 2 has a concave-convex shape with respect to the rotation direction of the shaft 3 to transmit torque, and an R for stress relaxation is preferably provided at the corner portion thereof. In addition, in the example of the spur gear, there is a possibility that a slight thrust force is generated due to the skew of the gear caused by the force acting on the teeth, and therefore a concave-convex shape with a rounded corner for stress relaxation is preferably provided in the axial direction as well.

[0073] In addition, by using the lightweight gear 1 of the present embodiment more frequently, the lightweight of the robot can be achieved as the gear used for the robot.

[0074] In addition, in the present embodiment, the number of concave-convex shapes engaged in the circumferential direction at the joint portion of the shaft 3 to the connecting body 4 can be set to be larger than the number of concave-convex shapes engaged in the circumferential direction at the joint portion of the tooth portion 2 to the connecting body 4. For example, as shown in FIG. 6, the number of concave-convex shapes composed of the first concave portion 5 and the first convex portion 6 is four, which is smaller than the number of eight of concave-convex shapes composed of the third concave portion 11 and the third convex portion 12. Figure 11

[0075] In the case where torque is transmitted from the tooth portion 2 to the shaft 3 via the connecting body 4, or in the case where the torque is transmitted in the opposite direction, the torque transmitted at the joint portion of the shaft 3 to the connecting body 4 is the same as the torque transmitted at the joint portion of the tooth portion 2 to the connecting body 4. Since the distance Rs from the center axis A to the joint portion of the shaft 3 to the connecting body 4 is smaller than the distance Rg from the center axis A to the joint portion of the tooth portion 2 to the connecting body 4, the force acting in the tangential direction of each concave-convex shape is smaller in the case where the number of concave-convex shapes is the same.

[0076] Therefore, the number of concave-convex shapes composed of the first concave portion 5 and the first convex portion 6 is set to be smaller than the number of concave-convex shapes composed of the third concave portion 11 and the third convex portion 12. Thus, even if the shapes of the concave-convex shapes are made the same, the difference in the surface pressure of the surface subjected to the force in the tangential direction in each concave-convex shape can be reduced.

[0077] Specifically, in the case where the size of the surface subjected to the force in the tangential direction, that is, the size of the surface extending in the direction intersecting the circumferential direction, is the same in each concave-convex shape, the surface pressure can be made the same by satisfying the following conditional expression.

[0078] Pg = Ps x Rs / Rg

[0079] wherein,

[0080] Pg: the number of concave-convex shapes composed of the first concave portion 5 and the first convex portion 6,​

[0081] Ps: number of concave-convexes constituted by the third concave portion 11 and the third convex portion 12,

[0082] Rs: distance from the center axis A to the joint portion of the shaft 3 and the connecting body 4,

[0083] Rg: distance from the center axis A to the joint portion of the tooth portion 2 and the connecting body 4.

[0084] The surface pressure can not necessarily be strictly equal.

[0085] In addition, instead of or in addition to making the number of concave-convexes different, the size of the surface that receives the force in the tangential direction in each concave-convex, that is, the surface that extends in the direction intersecting the circumferential direction, can be made different. For example, as shown in Figure 12 the height Hg of the concave-convex constituted by the first concave portion 5 and the first convex portion 6 can be set to be smaller than the height Hs of the concave-convex constituted by the third concave portion 11 and the third convex portion 12. Or, the axial length of the concave-convex constituted by the first concave portion 5 and the first convex portion 6 can be set to be smaller than the axial length of the concave-convex constituted by the third concave portion 11 and the third convex portion 12. In addition, the type of the concave-convex, the number of segments is not limited to the illustration, and any type can be used as long as the same function is obtained.

[0086] Next, the manufacturing method of the gear train of one embodiment of the present application will be described below with reference to the drawings.

[0087] The manufacturing method of the gear train of the present embodiment is a method of manufacturing a gear train by engaging the above-described lightweight gear 1 with another gear 20.

[0088] The manufacturing method first investigates the backlash amount of the engaging portion of all the teeth of the lightweight gear 1 to be engaged and another gear 20, and sets the first interaxle distance with the minimum value of the backlash being zero, as shown in Figure 13 At this time, the support portion of the other gear 20 is preferably temporarily fixed by the support portion fixing bolt axial force to the extent that the support portion slides away at the time of excessive jamming in order to prevent excessive jamming between the axes at the time of engagement. Next, the interaxle distance between the lightweight gear 1 and the other gear 20 is set to the second interaxle distance that is a predetermined amount smaller than the first interaxle distance. Thereby, the connecting body 4 made of resin is elastically deformed, and a gear train in which the lightweight gear 1 and the other gear 20 are assembled in a state in which the tooth surfaces are in close contact can be manufactured. Thereby, the increase in backlash that occurs due to the amplification of the interaxle distance accompanying the initial wear of the tooth surfaces of the gears 1, 20 and the thermal expansion of the arms can be suppressed to a minimum. In addition, the other gear 20 can also be a lightweight gear.

[0089] Thus, according to the manufacturing method of the gear train of the present embodiment, a gear train in which the tooth surfaces of the lightweight gear 1 and the other gear 20 are always maintained in a close contact state can be easily configured. Thus, the manufactured gear train does not have a backlash, and thus has an advantage in that the operation of the driven-side gear can accurately follow the operation of the driving-side gear.

[0090] Further, the lightweight gear 1 and the other gear 20 are exemplified as the gear train, but the present embodiment is not limited thereto, and two lightweight gears 1 can be used as the gear train, or three or more gears including at least one lightweight gear 1 can be used as the gear train.

[0091] As the robot of the present embodiment, a robot using the lightweight gear 1 or manufactured by the manufacturing method thereof is exemplified, but a robot using a gear train manufactured by the manufacturing method of the gear train of the present embodiment can also be adopted.

[0092] Further, in the present embodiment, a structure in which an R2 or more rounded corner is provided at all corners of the concave-convex formed of the first recess 5, the first protrusion 6, the second recess 7, and the second protrusion 8, and all corners of the concave-convex formed of the third recess 11, the third protrusion 12, the fourth recess 13, and the fourth protrusion 14 is exemplified. The rounded corner for stress relaxation is not limited thereto, and can be an R larger than the R provided at the corner of the machining tool.

[0093] The present application has been described with reference to typical embodiments, but it is understood that those skilled in the art can make the above modifications and various other modifications, omissions, or additions without departing from the scope of the present application.

Claims

1. A manufacturing method of a gear train, characterized by, when a plurality of gears including at least one lightweight gear are engaged, on the basis of investigating the backlash amount of all engagement portions, setting the plurality of inter-shaft distances of the gears to each other to be shorter than the inter-shaft distance in a state where the minimum value of the backlash amount is zero, the lightweight gear includes a tooth portion of a circular ring shape made of metal, a shaft made of metal extending along the center axis of the tooth portion, and a connecting body made of resin connecting the shaft and the tooth portion, and the joint portion of the shaft and the connecting body and the joint portion of the tooth portion and the connecting body are provided with a concave-convex engaged in the circumferential direction, and the corner portion of the concave-convex has a round corner for stress relaxation.

2. A robot, characterized in that characterized by, the lightweight gear includes a tooth portion of a circular ring shape made of metal, a shaft made of metal extending along the center axis of the tooth portion, and a connecting body made of resin connecting the shaft and the tooth portion, and the joint portion of the shaft and the connecting body and the joint portion of the tooth portion and the connecting body are provided with a concave-convex engaged in the circumferential direction, and the corner portion of the concave-convex has a round corner for stress relaxation, the lightweight gear includes a tooth portion of a circular ring shape made of metal, a shaft made of metal extending along the center axis of the tooth portion, and a connecting body made of resin connecting the shaft and the tooth portion, and the joint portion of the shaft and the connecting body and the joint portion of the tooth portion and the connecting body are provided with a concave-convex engaged in the circumferential direction, and the corner portion of the concave-convex has a round corner for stress relaxation, and the lightweight gear is manufactured by a manufacturing method of, arranging the tooth portion and the shaft in a molding die, performing injection molding of the connecting body and joining the connecting body and the tooth portion and the shaft at the same time by injecting molten resin into the cavity of the molding die, or a gear train is manufactured by a manufacturing method of, when a plurality of gears including at least one lightweight gear are engaged, on the basis of investigating the backlash amount of all engagement portions, setting the plurality of inter-shaft distances of the gears to each other to be shorter than the inter-shaft distance in a state where the minimum value of the backlash amount is zero, the lightweight gear includes a tooth portion of a circular ring shape made of metal, a shaft made of metal extending along the center axis of the tooth portion, and a connecting body made of resin connecting the shaft and the tooth portion, and the joint portion of the shaft and the connecting body and the joint portion of the tooth portion and the connecting body are provided with a concave-convex engaged in the circumferential direction, and the corner portion of the concave-convex has a round corner for stress relaxation.

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