HDD용 스핀들 모터에 사용되는 유체동압베어링의 유체주입 해석방법 개발 및 내부 기포 배출을 위한 유동장 해석

Abstract

Fluid dynamic bearings (FDBs) have replaced ball bearings in hard disk drives (HDDs) since research in the early 1990s first showed that disk-spindle systems supported by FDBs generate less vibration and noise than ball bearings. This is not only because FDBs prevent solid contact between stationary and rotating parts with fluid lubricant, but also because they provide damping to absorb vibration. FDBs generate pressure through the fluid lubricant with pumping and wedge effect in the several micrometers of clearance to support the rotating disk-spindle system. However, one of the weaknesses of FDBs is the instability that arises from air bubbles in the oil lubricant. When the oil is injected inappropriately or when an external shock occurs, air bubbles are formed and trapped in the oil lubricant. Trapped air bubbles decrease the rotational accuracy and the stability of a rotor-bearing system in such a way as to generate non-repeatable run-out (NRRO) and to decrease the stiffness and damping coefficients of FDBs. Firstly, this dissertation investigates the oil injection process of FDBs in a 2.5" HDD, both numerically and experimentally, and discusses the physical phenomena of the oil injection process. Compressible air, which is governed by the ideal gas law, is introduced to model a nearly perfect vacuum. The two-phase flow of compressible air and oil is described by the Navier–Stokes equation, the continuity equation, and the volume of fluid (VOF) method. The numerical method proposed in this paper is verified by an oil injection experiment. A novel oil injection process and FDBs are proposed to reduce the air bubbles. Secondly, this dissertation investigates the motion of a micron-sized air bubble in the operating FDBs in an HDD. The flow field of FDBs is calculated by solving the Navier–Stokes equation and the continuity equation, and the two-phase flow in the air–oil interface is simultaneously solved using the finite volume method and the VOF method. Then, the motion of a micron-sized air bubble is analyzed by applying the discrete phase modeling (DPM) method to the calculated flow field of FDBs. The motion of a micron-sized air bubble determined using the DPM method is verified by a comparison with the trajectory of the micron-sized air bubble determined using the VOF method. Finally, this dissertation investigates the expulsion of air bubbles in the oil lubricant of operating FDBs with a recirculation channel (RC). Experiments are performed to observe the behavior of trapped air bubbles in operating FDBs visually. A numerical study is carried out to investigate the phenomenon of air bubble expulsion from FDBs. A novel RC is proposed to expel the air bubbles from the FDBs efficiently.