Optimal Design of Fluid Dynamic Bearings to Support Disk-Spindle System in HDD

Abstract

Fluid dynamic bearings (FDBs) have replaced ball bearings in hard disk drives (HDD) since research in the early 1990s first showed that disk-spindle systems supported by FDBs generate smaller non-repeatable run-out (NRRO) 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, fluid lubricant generates friction torque and that accounts for approximately 33 percent of the total electric power applied to the HDD spindle motor. Also, disk-spindle systems supported by FDBs have intrinsic instability, or half-speed whirl, in which zero pressure is generated at the frequency near half of rotating speed. Disk-spindle systems are vulnerable to the excitation frequency with half of the rotating frequency, and most components of NRRO are populated near that frequency. Firstly, this dissertation proposes the Monte Carlo simulation for manufacturing tolerances in major design variables of FDBs in a 2.5" HDD to identify the sensitive design variables relative to the performance of FDBs and the dynamic performance of a disk-spindle system. Friction torque and critical mass are chosen as the performance indexes of the FDBs and the disk-spindle system. Secondly, this dissertation develops an optimal design methodology for the FDBs of a HDD in such a way as to minimize friction torque while satisfying the constraints of robust dynamic performance. The objective function was defined as the friction torque of the FDBs. Two kinds of constraints were used to satisfy the robust dynamic performance of a rotating disk-spindle system. For under-damped vibration modes of a rotating disk-spindle system, the critical mass (which is determined from the solution of the linear equations of motion of the rotating disk-spindle system) was set to be greater than that of the conventional disk-spindle system; for over-damped vibration modes, the related direct stiffness and damping coefficients were set to be greater than those of conventional FDBs. HDD spindle motors with optimal designed FDBs were prototyped, and their power consumptions and dynamic characteristics were measured and compared with those of conventional and simulated motors at various rotating speeds. Finally, this dissertation proposes a method to develop robust FDBs in an HDD utilizing the modal analysis and optimization to improve both the steady state response and the shock response of a disk-spindle system which supported by FDBs in all five degrees of freedom. The design of the FDBs is optimized to maximize every modal damping ratio of the rotating rigid disk-spindle system supported by FDBs in five degrees of freedom. The results are verified by evaluating the steady-state and shock response of the rotating rigid disk-spindle system supported by FDBs.