A torque limiting mechanism includes a pulley (33) and a hub (38). The pulley (33) has a transmission spring (44). The hub (38) has a groove (41). A transmission ring (59) is engaged with the transmission spring (44) such that power can be transmitted between the pulley (33) and the hub (38). The transmission ring (59) includes a coupler block (51) and a coupler (58). The coupler block (51) is engaged with the transmission spring (44) and the groove (41). The coupler (58) is connected to the coupler block (51). The coupler (58) urges the coupler block (51) in a direction disengaging the coupler block (51) from one of the transmission spring (44) and the groove (41). When the load between the pulley (33) and the hub (38) exceeds a predetermined level, the coupler block (51) is disengaged from the pulley (33) and the hub (38). Thereafter the coupler (58) keeps the coupler block (51) disengaged from the pulley (33) and the hub (38). This positively discontinues power transmission according to an applied load and maintains the discontinuation. <IMAGE>
