Recently, the memory density of HDD (hard disk drives) is quickly growing and it causes that head positioning becomes more difficult. And the improvement of anti-noise performance and crashworthiness is also required in addition to larger memory capacity and high speed read/write performance in HDD. Ball bearings have been used in spindles of hard disk drives, but it has been known that ball bearings generated high frequency vibration and noise. Therefore, the improvement of head positioning accuracy in high frequency region is difficult when ball bearings are used. Then the another type of spindles using fluid dynamic bearings (FDB) that have high-rotational accuracy and small vibration characteristics have been developed and put into practical use. A conventional characteristics evaluation method based on rotational accuracy such as Repeatable Run-Out (RRO) and Non Repeatable Run-Out (NRRO) is still used for fluid dynamic bearing spindles. However, the damping characteristic that is one of the big features of FDB is not fully evaluated by RRO and NRRO measurements. So, we suggest the new evaluation method for FDB. Also we evaluate the dynamic characteristic of FDB at high rotational speed by High-speed rotating air turbine including FDB.
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