Gear mechanism, speed reducer, and robot arm

A gear mechanism, comprising:

• a first gear having an annular tooth surface comprising:
• multiple tooth tip portions formed on a distal end side with respect to a predetermined height;
• and
• multiple recessed portions formed on a tooth root side with respect to the predetermined height at positions between the multiple tooth tip portions, respectively;
• and
• a second gear having an annular tooth surface comprising:
• multiple tooth tip portions formed on a distal end side with respect to the predetermined height;
• and
• multiple recessed portions formed on a tooth root side with respect to the predetermined height at positions between the multiple tooth tip portions, respectively,
• the multiple tooth tip portions of the second gear and the multiple recessed portions of the second gear being respectively larger in number than the multiple tooth tip portions of the first gear and the multiple recessed portions of the first gear,
• wherein the first gear and the second gear are arranged under a state in which a rotation axis of the first gear and a rotation axis of the second gear are inclined at a predetermined angle so as to set a deepest meshing position at which one of the multiple tooth tip portions of one of the first gear and the second gear and corresponding one of the multiple recessed portions of another of the first gear and the second gear mesh most deeply with each other, and a grazing position at which the one of the multiple tooth tip portions of the one of the first gear and the second gear and corresponding one of the multiple tooth tip portions of the another of the first gear and the second gear graze each other on an opposite side of the deepest meshing position, and to form a first meshing region in which the one of the multiple tooth tip portions of the one of the first gear and the second gear and the corresponding one of the multiple tooth tip portions of the another of the first gear and the second gear are held in contact with each other on each of both sides of the grazing position, and a second meshing region in which the one of the multiple tooth tip portions of the one of the first gear and the second gear and the corresponding one of the multiple recessed portions of the another of the first gear and the second gear are held in contact with each other on the deepest meshing position side with respect to the first meshing region,
• wherein the multiple tooth tip portions of the first gear and the multiple tooth tip portions of the second gear are each formed to be held in contact with each other at one point in the first meshing region, and
• wherein the multiple recessed portions of the first gear and the multiple recessed portions of the second gear are each formed to conform to a passing region extending along movement of one of the multiple mating tooth tip portions at the time of moving in the second meshing region.