Scully, Scott, Murphy & Presser
Automatic balancing mechanism for disk driver free from vibrations due to characteristic angular velocity
 A weight member is attached to a rotor so as to forcibly offset a center of gravity of a rotor from a rotating axis, centrifugal force moves two movable weight members to appropriate positions for canceling the unbalance due to the center of gravity offset from the rotating axis, and stoppers restrict the movement of the two movable weight members in a sectoral area opposite to the center of gravity so that the rotor is free from vibrations due to the characteristic angular velocity. 
NEC Corporation
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Kazuhiko Makoto
Kitamura Takeuchi