Stability Of Small-Sized Grooved Bearings-Spindle System In High-Speed Hard Disk Drive Considering Thermal Effect

A micro-rotating machinery such as hard disk drive (HDD) requires high efficiency and robust dynamics in high operating speed. Small-sized herringbone grooved journal bearings (HGJBs) are widely used for this application thanks to their excellent dynamic characteristics.The analysis herein on small-sized herringbone grooved journal bearings in hard disk drive is expected to distill design guidelines with main goals of reaching larger stability margin. To fulfill the aforementioned goal, the present study starts with proposing a method to calculate the stiffness and the damping coefficients of the coupled journal and thrust bearings. Reynolds equations and their perturbation equations of journal and thrust bearings are transformed to the finite element equations by considering the continuity of pressure and flow at the interface between the journal and the thrust bearings. It also includes the Reynolds boundary condition in the numerical analysis to simulate the cavitations phenomenon. The stiffness and damping coefficients of the proposed mathematical method are compared with those of the numerical differentiation of the bearing force with respect to finite displacements and finite velocities of bearing center. It shows that the proposed method can calculate the dynamic coefficients of a coupled journal and thrust bearing more numerically stable and computationally efficient than the differentiation method. It also investigates the coupling effect of the coupled journal and thrust bearing and it shows that the proposed method makes it possible to calculate the cross-coupled dynamic coefficients in the radial-axial direction of the coupled journal and thrust bearing. After the linearized stiffness and damping coefficients are evaluated, The threshold speed parameter and whirl frequency ratio are computed using the linearized stability analysis.This paper investigates the dynamic behavior of a HDD spindle system with fluid dynamic bearings (FDBs) by solving the Reynolds equation and the equations of a motion of a HDD spindle system in five degrees of freedom. FEM is used to solve the Reynolds equation in order to calculate the pressure distribution in fluid film. Reaction forces and friction torque are obtained by integrating the pressure and shear stress along the fluid film, respectively. Dynamic behaviors of a HDD spindle system, such as the whirling and the tilting motion, are determined by solving its nonlinear equations of motion with the Runge-Kutta method. Finally, this paper presents a method to investigate the characteristics of hydrodynamic bearings of a HDD spindle motor considering the variation of the clearance as well as the lubricant viscosity due to elevated temperature. This research shows that elevated temperature changes the clearance as well as the lubricant viscosity of the hydrodynamic bearings of a HDD spindle motor. In this article, the thermal expansion of the sleeve's inner radius was determined by Luo Zai's method. Once the viscosity and clearance of hydrodynamic bearings of a HDD spindle motor are determined, finite element analysis of the Reynolds equation is performed to investigate the static and dynamic characteristics of hydrodynamic bearings of a HDD spindle motor at elevated temperature. It shows that elevated temperature is an important design consideration which affects the dynamic characteristics of the hydrodynamic bearing of a HDD spindle motor.