Patent ID: 7483239

Claim:
A magnetic disk drive, comprising: a motor hub; a clamp board serving as a sheet member mounted on the motor hub and having opening portions passing through the sheet member formed on each of a plurality of directional lines as viewed from a disk rotary axis; a magnetic disk clamped by the motor hub and the clamp board; and a counterweight formed of an elastic material and mounted by being disposed into at least one of the plurality of opening portions; wherein the motor hub has a recessed portion formed at a position opposing the opening portion in the clamp board; wherein the clamp board is disposed by having a gap provided from the motor hub at a location near at least each of the plurality of opening portions; and wherein the counterweight includes a protruded portion on a side surface thereof, the protruded portion being engaged with the gap when mounted in the opening portion; wherein the counter weight includes an upper portion and a lower portion that are symmetrical and formed on opposite sides of the protruded portion, wherein the lower portion of the counter weight corresponds to the depth of the recessed portion of the motor hub.