Abstract
That a narrow gap between a disk and a shroud reduces disk flutter amplitude is one of the most important knowledge on disk flutter of hard disk drives. This has been explained by reduction of an airflow excitation because of blockage of a vertical flow from a side of the disk to the other side. However, we found a different phenomenon in our experiments: damping ratios of the disk flutter were increased but the mode excitations were not reduced by a narrow disk-shroud gap. In the experiments, frequency spectrums of disk flutter were measured for various disk-shroud gaps. The frequency spectrum for the narrow gap showed more rounded peaks than for the wide gap, and the amplitudes of the flat area between the peaks were approximately same in the both cases. We derived its theoretical equations from energy dissipation of a flow induced by disk flutter at a disk-shroud gap, and these equations with reasonable parameters can explain the experimental results very well. So, the mechanism of the aerodynamic damping was confirmed. According to the equations, damping ratios of disk flutter due to the aerodynamic damping are inversely proportional to cube of a disk-shroud spacing normalized by an outer radius of a disk. Considering a constant damping independent on disk-shroud spacing, we can get a relationship between disk flutter amplitude and disk-shroud spacing. This relationship matches the experimental results with various disk sizes that had been reported.
