We proposed a method to predict the heights of upper and lower air-oil interfaces of the non-operating and operating fluid dynamic bearings (FDBs) in the tied shaft of hard disk drives. To predict the height of upper and lower air-oil interfaces, we formulated the linear pressure and volume equations in both cases of non-operating and operating conditions, respectively. The initial heights of upper and lower air-oil interfaces are determined by pressure and volume equations during non-operating condition. Pressure equation consists of capillary pressure, atmospheric pressure, hydrostatic pressure and applied pressure, and volume equation consists of the volumes of the clearance and the injected oil. In case of operating condition, hydrodynamic pressure generated by grooved bearings is additionally considered in the pressure equation, because heights of air-oil interfaces vary due to hydrodynamic pressure during operating condition. The linear pressure and volume equations were solved simultaneously to calculate the heights of upper and lower air-oil interfaces both in non-operating and operating conditions. In case of non-operating conditions, the height of upper air-oil interface increases with the increase of pressure difference because pressure of lower air-oil interface is higher than that of upper air-oil interface. Since the volume of injected oil is constant, height of lower air-oil interface decreases due to pressure difference. In case of operating conditions, the height of upper air-oil interface decreases according to rotating speed because more oil flows inside the FDBs and recirculation channel than upper seal. On the other hand, the height of the lower air-oil interface increases because volume variation of the oil due to the increase of the flying height is smaller than that due to the decrease of the height of the upper air-oil interface. Finally, we verified the proposed method by measuring the height of upper air-oil interface both in the non-operating and operating conditions.
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