Experimental study of the dynamic behavior of gas foil bearings and its constructive controllability

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Gas-bearings can be used as an alternative rotor bearing type for small and light turbomachinery such as turbochargers or small gas turbines. Particularly at very high speeds and high process temperatures, this special form of fluid film bearings that uses gas instead of oil as a lubricant. The omission of a conventional lubrication system reduces weight, maintenance and cost and avoids contamination of process media from lubricants.The Gas Foil bearing (GFB) stands for a special type of gas bearings. GFBs minimize some of the major drawbacks of conventional gas bearing using an elastic structure within. That elastic structure creates additional friction and damping and thus improves stability properties. At the same time the load capacity can be increased.One of the main tasks in the optimization of dynamic properties of GFBs is to minimize the occurring large, subharmonic vibrations during operation. A coincidence of the subharmonic whirl frequencies with a system natural frequency can cause a sudden increase in the sub-harmonic amplitudes, which often leads to damage. Design measures for a vibration reduction of GFBs, leading to an increase in the damping and stiffness, already exist. These measures include, among others, so-called shims. Various preliminary studies have shown that the onset of subharmonic vibrations and their amplitudes are affected by shims. Furthermore, it was shown that conservative predictions of the onset of whirl vibration, which is very important for a design of rotors using GFBs, can be made on the basis of numerical simulations.However, an experimental validation of numerical simulations is missing. Within the proposed project a systematic experimental determination of the required linearized bearing parameters of different GFB types in order to assess the whirl frequency is planned. This results in a validation of a numerical model for parameter determination. That valid model will be used for further investigations on rotordynamics, stability and optimization. During the project a rotordynamic test bed is built that allows a systematical investigation of particularly nonlinear vibration behavior of a rotor in GFBs. The experiments will provide information about the onset speed of the subharmonic vibration as well as give hints on the effectiveness of structural measures such as shims. The overall results will provide a high quality of data for the design and research of various types of GFBs and serve future simulations of whole engine models (structure-housing-bearing-rotor) for verification and validation.