Foil bearings have been attracting considerable attention for their applications to micro turbomachinery, such as blowers and compressors, because of their excellent stability at high speeds and durability in high-temperature environments. This paper investigates experimentally and numerically the high-speed instability of a rotor supported by small aerodynamic foil journal bearings. Two types of foil journal bearings were prepared: a first-generation bump-type foil bearing and a dimple-type foil bearing; these consist of a top foil and a support foil with bumps or dimples, respectively. The dynamic characteristics of a support foil using the frequency response and the threshold speed of instability at high speeds were measured experimentally. Furthermore, the numerical threshold speed of instability was obtained using the nonlinear orbit method. It was confirmed experimentally and numerically that a 6 mm diameter rotor with a mass of 4.7 g supported by either of the two types of foil journal bearing treated in this paper could rotate stably at speeds of more than 760,000 rpm.

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