Abstract: A screw engagement portion between a pulley and a shaft is provided with a thread in a direction to increase the tightening torque when the pulley is driven. A breakable part broken by the axial tension produced when the tightening torque acting on the screw engagement portion is above a predetermined value, is also provided. The stress to break the breakable part is mainly a tensile stress. The tensile stress is substantially uniformly distributed in a cross section and, hence, the stress distribution of the breakable part can be relatively precisely predicted and analyzed. Consequently, the necessity of determining aspects of the breakable part, such as dimension, material, and the like, by trial and error is reduced. The breakable part can be broken without being subject to fatigue fracture.

Abstract: In a torque transmission system, plural engagement pieces elastically deformable are formed integrally with an outer peripheral portion of a circular portion of a center hub to be engaged with an inner periphery portion of a pulley. A bridge portion of the center hub is disposed to be broken when a torque large than a predetermined torque is applied there to. Since the engagement protrusion pieces are engaged with a part of the pulley, it can prevent the center hub from being removed and falling when the bridge portion of the center hub is broken.

Abstract: The screw engagement portion (10) between the pulley (9) (the boss (9g)) and the shaft (1a) is provided with a thread in a direction to increase the tightening torque when the pulley (9) is driven. A breakable part (1i) which is broken by the axial tension produced in accordance with the tightening, when the tightening torque acting on the screw engagement portion (10) is above a predetermined value, is also provided. With this structure, the stress to break the breakable part (1i) is not a shearing stress but is mainly a tensile stress. Unlike the shearing stress, the tensile stress is substantially uniformly distributed in a cross section and, hence, the stress distribution of the breakable part (1i) can be relatively precisely predicted and analyzed. Consequently, the necessity of determining the dimension and the material, etc. of the breakable part (1i) by trial and error is reduced, thus leading to facilitation of the design and development.

Abstract: In a torque transmission system, plural engagement pieces elastically deformable are formed integrally with an outer peripheral portion of a circular portion of a center hub to be engaged with an inner periphery portion of a pulley. A bridge portion of the center hub is disposed to be broken when a torque large than a predetermined torque is applied there to. Since the engagement protrusion pieces are engaged with a part of the pulley, it can prevent the center hub from being removed and falling when the bridge portion of the center hub is broken.