Gear, balancer device, and balancer device provided with oil pump

A gear configured to rotate as a unit with a shaft, the gear comprising:

• a teeth portion which are provided in a circumferential direction with respect to the rotation axis of the shaft, and which has a predetermined torsion angle with respect to the rotation axis;
• and
• a first annular groove which is formed on a first side surface of both side surfaces of the gear in a direction of the rotation axis, the first side surface being on a direction of a thrust force received by the teeth portion, the first annular groove including;
• a first inner circumference surface provided on the rotation axis side in a radial direction with respect to the rotation axis,
• a first outer circumference surface which is provided on the teeth portion side in the radial direction with respect to the rotation axis, and which has an axial depth smaller than an axial depth of the first inner circumference surface, and
• a first bottom portion connecting the first inner circumference surface and the first outer circumference surface, and having an angle which is formed by the first bottom portion and the first inner circumference surface, and which is acute.

A balancer device comprising:

• a drive gear engaged with an input gear to which a rotation force is transmitted from a crank shaft;
• a balancer drive shaft to which the rotation force is transmitted from the drive gear, and which includes a first balancer weight;
• a balancer drive gear configured to rotate as a unit with the balancer drive shaft;
• a balancer driven gear engaged with the balancer drive gear;
• a balancer driven shaft which is configured to rotate as a unit with the balancer driven gear, and which includes a second balancer weight;
• and
• a plurality of annular groove which are formed in both side surfaces of at least one of the drive gear, the balancer drive gear, and the balancer driven gear, in a rotation axis of the balancer drive shaft, which are at least partially overlapped when viewed in the direction of the rotation axis, and which are at least partially overlapped in a radial direction with respect to the rotation axis.