Electromagnetic clutch with improved frictional surfaces

In an electromagnetic clutch including a first rotatable member of magnetic material rotatably supported on a bearing, an annular electromagnetic device disposed in a stationary position in a hollow portion of said first rotatable member, said annular electromagnetic device including an annular magnetic housing and an annular magnetic coil disposed therein, a second rotatable member, a hub secured on said second rotatable member, a magnetic armature plate supported on and around said hub at a predetermined radial gap, said magnetic armature plate being capable of limited axial movement and facing said first rotatable member at a predetermined axial air gap, said magnetic armature plate further being attracted to said first rotatable member when said annular magnetic coil is energized such that a first frictional surface of said first rotatable member contacts a second frictional surface of said magnetic armature plate, the improvement comprising said first frictional surface having grooves forming a spiral pattern and having a layer of metal plating with a hardness lower than that of said magnetic material.