The mechanism of disk-flutter excitation in hard disk drives was experimentally examined in terms of the fluid dynamics. Small pressure sensors for measuring the pressure between the rotating disks were placed in the shroud in the disk-radius direction. It was found that there is the strong correlation between pressure-fluctuation amplitude and disk-flutter amplitude. Therefore, the main force of flutter excitation is considered to be the pressure fluctuation caused by "winded" airflow in the disk-rotational direction. This winding phenomenon is caused by the centrifugal force acting to the airflow and its subsequent reflection from the shroud surface. It is thus concluded the flutter amplitude and pressure fluctuation can be decreased by reducing the disk-shroud spacing.
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