Patent ID: 7849988

Claim:
A power transmitting device comprising: a hollow cylindrical first rotor that receives driving force and rotates; a shaft-like second rotor that is rotatably arranged in and coaxial with the first rotor; a first clutch that selectively permits and shuts off torque transmission between the first rotor and the second rotor, wherein, when pressed along an axial direction of the first rotor, the first clutch permits torque transmission between the first rotor and the second rotor; a second clutch arranged adjacent to the first clutch along the axial direction of the first rotor, wherein the second clutch includes an electromagnet, a plurality clutch plates alternately arranged along the axial direction of the first rotor, and an armature movable along the axial direction of the first rotor, the clutch plates being located between the electromagnet and the armature, wherein, when the clutch plates are pressed by the armature that is attracted by and moved toward the electromagnet, each adjacent pair of the clutch plates are frictionally engaged with each other; a cam mechanism having a cam member arranged adjacent to the armature along the axial direction of the first rotor, the cam mechanism linking the first clutch and the second clutch with each other, wherein the cam mechanism converts a torque based on a rotational difference between the first rotor and the second rotor, which torque is transmitted through the second clutch, into a linear motion of the cam member along the axial direction of the first rotor, and wherein the first clutch is pressed by the cam member moving linearly along the axial direction of the first rotor; a plurality of projections provided either on a surface of the cam member that faces the armature or on a surface of the armature that faces the cam member, the plurality of projections being arranged symmetrical with respect to an axis of the respective one of the cam member and the armature on which the projections are provided, wherein the cam member and the armature can contact each other at the projections, wherein the area of a portion of the cam member that can contact the armature corresponds to the area of said projections and is set in a range where a magnetic flux density generated in the portion by the electromagnet is saturated.