Fluid dynamic bearings (FDBs) are applied to most of the spindle motors of computer hard disk drives (HDDs) since FDBs provide better dynamic characteristics, such as lower vibration and noise, than ball bearings. However, a weakness of FBDs is instability arising from air bubbles in the oil lubricant of FDBs. One possible solution to expel the trapped air bubbles out of FDBs is to include recirculation channel (RC). RC is designed to balance the pressures between upper and lower parts of FDBs and to circulate the oil lubricant as well as to expel air bubbles out of FDBs. This paper experimentally and numerically investigates the behavior of the air bubble in oil lubricant of operating FDBs due to the design of the RC. We created the FDBs with transparent cover and performed the experiment to visually observe the behavior of trapped air bubbles. Also, we numerically studied the phenomena of expelling the air bubble. The flow field of FDBs is calculated by the Navier-Stokes equation and the continuity equation. And we numerically explained that large pressure difference between upper and lower regions of RC and fast flow velocity along RC expel the air bubble out of FDBs. This research can be effectively utilized to develop robust FDBs by expelling the air bubbles out of FDBs.

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