The spindle motor in a computer hard disk drive can be modeled as a rotor-bearing system supported by the base plate. Ball bearings are the crucial element to determine the stiffness of the spindle motor, and their design parameters and operating conditions determine the dynamic characteristics of the spindle motor. In the analysis of a rotor-bearing system with a short shaft like a spindle motor, the stiffness of the shaft and the base plate as well as ball bearings must be considered to analyze the dynamic characteristics of a spindle motor accurately. In this paper, the lateral and the axial vibration of the spindle motor were analyzed by the transfer matrix method for the dual-shaft rotor-bearing model and by 2 d.o.f lumped parameter model, respectively. The simulation results had good agreements with the experimental modal testing. The dynamic characteristics were fully investigated for the change of the major design parameters of the spindle motor, i.e. the preload of ball bearing and the rotational speed.
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