Patent ID: 8109742

Claim:
A rotor yoke comprising: a substantially flat cover portion arranged to be substantially coaxial with a rotational axis; a substantially cylindrical side wall portion extending from an outer circumference of the cover portion in a parallel or substantially parallel relationship with the rotational axis; and a substantially annular flange portion extending radially outwards from an open end of the side wall portion; wherein the cover portion and the side wall portion define a cup portion whose center of gravity deviates in a deviation direction from the rotational axis in a plane perpendicular or substantially perpendicular to the rotational axis; and the flange portion includes a continuously extending outer circumference defined by a substantially uniform circle whose center deviates from the rotational axis in a direction opposite to the deviation direction of the center of gravity of the cup portion such that some portions of the flange portion extend farther away from the rotational axis than other portions of the flange portion.