Experimental Analysis of the Dynamic Characteristics of a Hybrid Aerostatic Bearing

The dynamic characteristics of a hybrid aerostatic bearing are experimentally investigated on a test rig consisting of a rigid rotor driven by an impulse turbine. The rotor is horizontally mounted and is supported by two identical aerostatic bearings. Both the impulse turbine and the aerostatic hybrid bearings are fed with air. The feeding pressures in the bearings can be as high as 7 bars and rotation speeds can reach 60 krpm so the dynamic load on the rotor is much larger than the static load engendered by its weight. Excitations are applied either via an impact hammer or via unbalancing masses. The measuring instruments record the bearing feeding pressures, the rotation speed, the impact force, the displacements of the two bearings, and the bearing housing accelerations. The experimental data together with the equations of motion of the rotor enables the identification of the dynamic coefficients of the bearings. A second identification procedure using the same impact hammer is also possible as force transducers are mounted between the bearing housing and its support. The dynamic coefficients of the bearings can then be obtained from the equation of motion of its housing. Unbalance response provide a convenient way for verifying the accuracy of the identified dynamic coefficients. Therefore these coefficients are injected in the equations of motion of a four degrees of freedom rigid rotor and the theoretical results are compared with values measured on the test rig. Comparisons show that predictions are acceptable but become less accurate at high rotation speeds where large dynamic forces are needed for exciting the corresponding synchronous frequencies.