Light-weight gear and manufacturing method thereof, manufacturing method of gear train, and robot

A robot comprising:

• a gear train comprising a plurality of gears;
• wherein the plurality of gears includes at least one light-weight gear, the at least one light-weight gear comprising an annular tooth portion made of metal;a shaft made of metal extending along a central axis of the annular tooth portion;
• and
• a coupling element configured to couple the shaft to the annular tooth portion, the coupling element made of a resin,
• wherein a first joining part between the shaft and the coupling element and a second joining part between the annular tooth portion and the coupling element are provided with irregularities configured to be engaged with one another in a circumferential direction, and corners of the irregularities are rounded to release a stress,
• wherein the at least one light-weight gear is manufactured by:
• provision of the annular tooth portion and the shaft in a mold;
• and
• injection of a molten resin into a cavity of the mold, thereby simultaneously performing injection molding of the coupling element and joining of the coupling element to the annular tooth portion and the shaft;
• and
• wherein amounts of backlash at all meshing parts of the plurality of gears is brought to roughly zero by setting a second inter-shaft distance between the plurality of gears to be shorter than a first inter-shaft distances by a predetermined amount utilizing an elastically deformable feature of the resin of the coupling element when the plurality of gears are in a gear-fixed state.