Prototyping of palm top size micro gas turbine shaft system supported by foil bearings and the evaluation of dynamic characteristics

In recent years, micro gas turbines (MGT) of 100kW or less are drawing close attention as power generators equipped with cogeneration features. Aerodynamic foil bearings are considered to be the best candidate for the MGT due to its ease of maintenance. However, the details of bearing design guidelines have not been clarified though this technology is used in auxiliary power units (APU) for aircraft units. Firstly, we designed and manufactured radial foil bearings for the rotor manufactured from commercially available turbochargers. The diameter of the rotor is 20mm and the surface of the rotor is coated by ceramic. The radial bearing used here is composed of two elements; i.e. a housing and a multi wounded foil with projections of hemisphere on one side in the adequate interval. The bearing radial clearance between the rotor surface and the foil is 20m. In the experiment, we measured rotational speed and vibrations of the rotor in the radial and axial directions. Subsequently, analyses for the evaluation of first natural frequencies corresponding to lateral, conical and the first bending mode were introduced. Finally, the method for predicting the onset whirl rotational speed based on simple measurement is explained.