Identification method for rotordynamic coefficients of cylindrical air bearing using an impact hammer

Abstract

This study develops the use of an impact hammer as an excitation source for identifying the dynamic coefficients of cylindrical aerodynamic bearings. The test rig and the identification method were originally developed for being used with two electromagnetic shakers applying non-synchronous excitations. The test rig is of a floating bearing type which means that the bearing is (almost) freely vibrating around a fixed rotating shaft. The impact hammer can then easily replace the two shakers for providing a source of excitation. This design permits to study all kinds of gas bearings operating at rotation speeds up to 60 kr/min, the static load being applied directly on the bearing via a spring. The dynamic coefficients are identified by applying two distinct (linearly independent) impacts. A transfer function approach of the impedances enables to take into account the frequency dependence of the linear dynamic coefficients. Test results are presented for a bearing of diameter 30 mm, L/D = 1 and clearance 22 µm and compared with theoretical predictions.