Kei Somaya

Experimental and Numerical Investigation of the High-Speed Instability of Aerodynamic Foil Journal Bearings for Micro Turbomachinery

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.Copyright

Threshold Speed of Instability of a Herringbone-Grooved Rigid Rotor With a Bearing Bush Flexibly Supported by Straight Spring Wires

In recent years, small-size aerodynamic bearings for turbomachines such as blowers and compressors have attracted considerable attention for increasing rotational speed. These kinds of bearings require excellent stability at high speeds and durability in a high-temperature environment. Foil bearings are one of the most suitable candidates that can satisfy these requirements but their structure is very complicated, and it is difficult to control their manufacturing accuracy. It is well known that flexibly supported herringbone-grooved aerodynamic journal bearings have excellent stability at high speeds and they are relatively easy to manufacture compared with foil bearings. Moreover, their dynamic characteristics can be easily solved numerically. In this paper, a flexibly supported herringbone-grooved aerodynamic journal bearing using straight spring wires made of stainless steel is proposed to provide a simple and reliable support system for a bearing bush. Six straight spring wires were assembled into a hexagonal shape into which the bearing bush was inserted. The threshold speed of instability of the proposed aerodynamic bearing was investigated numerically and experimentally. For this investigation, the nonlinear orbit method was adopted in numerical calculations. This investigation found that straight spring wires could steadily support the bearing bush and provide a simple and reliable support system for the bearing bush and that a 6-mm-diameter rigid rotor with a mass of 4.8 g supported by the proposed aerodynamic journal bearings could stably rotate at speeds of more than 0.7 million rpm.Copyright

Static characteristics of small aerodynamic foil thrust bearings operated up to 350,000 r/min

Foil bearings have been attracting considerable attention for their application in achieving low power consumption in small-sized turbo machines, excellent stability at high speeds, and durability at high temperature. Foil bearings with bump foils are one of the most suitable candidates for these applications as indicated in recent studies. However, it was reported that current small foil thrust bearings had insufficient load capacity to support thrust forces in practical high-speed turbo machines, and were needed to improve the load capacity at high speeds. As a first step in improving the load capacity of small foil thrust bearings, the load capacity and rotational torque of this type of foil thrust bearings that operate at up to 350,000 r/min were investigated experimentally and numerically in this paper. In the numerical calculation, the Reynolds and elasticity equations for the top foil were solved simultaneously by using the finite difference and the finite element method, respectively. It was experimentally and numerically found that the foil thrust bearings treated in this paper have the load capacity coefficient of 5.36 × 10−6 N/(mm)3 kr/min, which was comparable to that of the second-generation foil thrust bearings.