Shock Fragility of Spindle Motors in Hard Drives

The susceptibility of spindle motor ball bearings in disk drives to axial shock is considered in this paper. When a hard drive is subjected to an axial shock, the spindle disk pack is excited axially and the ensuing vibrations lead to dynamic forces. Should the maximum force endured by the bearings exceed their rated carrying capacity, permanent bearing damage will take place at the ball/race interface which manifests itself in higher vibration and acoustic levels. To estimate the dynamic bearing force and the shock fragility of a given drive, it is imperative to solve for the vibration of the drive when subjected to shock. This was achieved in the present work by simplifying the dynamics of a hard drive to that of a two degree of freedom mass-spring system. The equivalent mass and stiffness values were obtained by matching the modal frequencies of the 2 DOF model to those of the disk pack. The response of the model to axial shock was next obtained by numerical integration using the Runge-Kutta method. The non-linear bearing stiffness was incorporated using the Hertzian equations in the model. To validate the model, the response of a hard drive to axial shock was measured using an accelerometer mounted on the motor hub. Excellent agreement was obtained between calculated response and experimentally measured motor hub vibration. Having verified the model, the calculated maximum bearing force was used to estimate the drive’s shock threshold. It was found that base casting stiffness and damping play an important role in the dynamic amplification of the bearing force.