Effect of Disk Flexibility on Rocking Mode Frequencies of a Disk Drive Spindle Motor System

The lowest natural frequency of a hard disk drive related to a spindle motor is the so-called “rocking mode,” the frequency at which the rotor of the motor is whirling conically. A traditional rotordynamics theory with thick/rigid disks is not able to predict the rocking mode frequencies correctly. In this paper, the rocking mode frequencies of a single-disk rigid-rotor motor were solved analytically, including the effects of disk flexibility and ball bearing contact angle. In addition, the disk flexibility model was integrated to a finite element-based (FEM) flexible-shaft rotordynamics computer program. The closed-form solution of the analytical model provides design engineers a tool to identify critical design parameters. It was found that the rocking mode frequency, which is mainly affected by the clamped condition at the inner radius of the disk, is the flexible disk one-nodal-diameter mode frequency reduced by the compliance of ball bearings. The results obtained from both the analytical model and the FEM computer program matched the experimental results with an error less than two percent. Neglecting the effect of ball bearing contact angle increases the error three times. The effects of design parameters were also investigated.