PURPOSE: To reduce production of a fluctuation in torque exerted on an upper shaft by a method wherein planetary gears arranged symmetrically about the upper shaft are revolved and rotated around the upper shaft through rotation of the upper shaft and torque is generated by means of a centrifugal force exerted on an eccentric mass attached to the planetary gear.

CONSTITUTION: A rotary arm 11 is rotated through rotation of an upper shaft 20 and planetary gears 12 and 12' are revolve around an upper shaft 10 through a rotary arm 11. Further, the planetary gears 12 and 12' are arranged concentrically to the shaft 20 and also rotated to be geared with a fixed gear 10 fixed to a machine frame. Since eccentric masses 13 and 13' are arranged symmetrically about the rotation center of the upper shaft 20, torque is exerted on the rotary arm 11 by means of an inertia force generated through acceleration movement of the eccentric mass. By changing a ratio between the radii of the pitch circles of the gears 10 and 12, the frequency of generated torque is changed and the magnitude of torque is regulatable by means of the masses of the eccentric masses 13 and 13' and an eccentric amount of the eccentric mass 13. This mechanism erases torque of the upper shaft 20 generated by an inertia force with torque generated by the mechanism.