Estimating bearing coefficients of fluid-dynamic spindle motors using sub-resonant frequency response

In this digest, we present a simple method to extract bearing coefficients of a fluid-dynamic bearing (FDB) spindle motor. The spindle motor is first loaded with a known proof mass. An impact hammer and a laser Doppler vibrometer are used to measured frequency response functions (FRF) of the spindle motor. Since the proof mass is very rigid, the FRF below the first resonance frequency reflects the stiffness of the bearings. This method bas two advantages. First, it can extract both in-line and cross stiffness coefficients. Second, it can extract stiffness coefficients of the upper and lower radial bearings separately by varying the proof mass and thus the centroid of the spindle motor.