A planetary gear system with a sun gear, a ring gear co-axial with the sun gear, at least one planetary gear meshing with both the sun gear and the ring gear, and a planetary gear carrier which rotationally supports the at least one planetary gear. A torque clutch is supported by and connected directly between two of the sun gear, the ring gear, the at least one planetary gear and the planetary gear carrier. The torque clutch prevents relative rotation when an acting torque is below a predetermined torque value and allows relative rotation when the acting torque is above the predetermined torque value. A torque threshold adjustment mechanism is provided for adjusting the predetermined value of the torque at which the torque clutch slips.

This torque transmission mechanism (1) includes a transmission mechanism portion (2) that includes an input shaft (21), an output shaft (22), and an intermediate portion (23) including a connection portion (23b). The torque transmission mechanism includes a limiting mechanism portion (3) that is capable of switching between a connection state where the limiting mechanism portion is connected to the connection portion and limits rotation of the intermediate portion and a disconnection state where the limiting mechanism portion is disconnected from the connection portion and allows the rotation of the intermediate portion, and switches to the disconnection state when torque of at least a predetermined value acts in the connection state.

This torque transmission mechanism (1) includes a transmission mechanism portion (2) that includes an input shaft (21), an output shaft (22), and an intermediate portion (23) including a connection portion (23b). The torque transmission mechanism includes a limiting mechanism portion (3) that is capable of switching between a connection state where the limiting mechanism portion is connected to the connection portion and limits rotation of the intermediate portion and a disconnection state where the limiting mechanism portion is disconnected from the connection portion and allows the rotation of the intermediate portion, and switches to the disconnection state when torque of at least a predetermined value acts in the connection state.