Determination of Global Damping and Stiffness Coefficients of Journal Foil Bearing

For the few last years, in modern low-power generation systems, a demand for oil-free compressors has appeared. This high speed turbomachinery with a nominal speed of tens of thousands rpm strongly depends on the proper rotordynamic design. It is especially important when the foil bearings are taken to consideration. These compliant surface gas bearings are a class of hydrodynamic bearings that use the ambient gas as their working fluid and, thus, require no dedicated lubrication system. On the other hand, due to their relatively low damping, a designer should analyze thoroughly described in the dynamics of the rotor-bearing-casing system in the whole operating rpm range. The foil bearings, although simple in design, indicate complex behaviour resulting from Coulomb friction between their elements. This Coulomb friction affects the damping and stiffness of a given bearing support. So far, many more or less reliable numerical models of this phenomenon have been built and described in the literature. The research approach presented in this paper is different. The authors suggested obtaining the data experimentally from the bearing isolated on a test bench, where, the shaft is stationary (fixed), and the gas film is not present. The shaker excites the bearing sleeve while the damping and stiffness are provided to the system by the foil structure. The information collected about the bearing’s global coefficients can be implemented afterwards to the rotordynamic software as a tabular data. This will allow to prepare reliable models that will shorten the design process of newly developed compressors with these oil-free supports.