Electromagnetic clutch and rotation transmission device

An electromagnetic clutch comprising:

• an inner member;
• an outer member rotatably and concentrically mounted around said inner member;
• a retainer mounted between said inner member and said outer member;
• engaging elements retained by said retainer;
• an armature arranged at one end of said retainer so as to rotate together with said retainer;
• a field core;
• an electromagnetic coil mounted in said field core;
• and
• a rotor fixed to one of said outer member and said inner member for attracting and releasing said armature by energizing and de-energizing said electromagnetic coil, said field core and said rotor each having cylindrical portions on inner and outer peripheral sides thereof, said cylindrical portion on said inner peripheral side of each of said field core and said rotor having a thickness larger than a thickness of said cylindrical portion on said outer peripheral side of each of said field core and said rotor such that magnetic flux density distributions of said field core and said rotor are uniform, a sectional area of said cylindrical portion on said inner peripheral side of said field core being equal to a sectional area of said cylindrical portion on said outer peripheral side of said field core, and a sectional area of said cylindrical portion on said inner peripheral side of said rotor being equal to a sectional area of said cylindrical portion on said outer peripheral side of said rotor.