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In this paper, vibration of 1.8-in hard disk drive (HDD) spindle motors is studied experimentally and theoretically. In the experimental study, vibration of 1.8-in HDD motors is measured at 5degC, 25degC, and 70degC in a thermal chamber. Experimental results reveal two important phenomena. First, vibration of 1.8-in HDD spindle motors is dominated by a (0,0) unbalanced mode that has a significant bending deformation of the base plate. Second, the natural frequency of the (0,0) unbalanced mode and the static gain of the frequency response function change significantly as the ambient temperature varies. To explain these experimental observations, we apply a mathematical model developed at the University of Washington to analyze the vibration of the 1.8-in HDD spindle motors. The analysis shows that (0,0) unbalanced mode shifts its natural frequency because membrane stresses are developed in the base plate during temperature changes as a result of a significant mismatch of coefficients of thermal expansion between the base plate and the fixture. Moreover, the change of the static gain results from reduction of fluid-dynamic bearing coefficients as the temperature varies.