Experimental Analysis of 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 located at its midlength. The rotor is horizontally mounted and is supported by two identical aerostatic bearings at its ends. Both the impulse turbine and the aerostatic hybrid bearings are fed with air. The actually available resources enable to attain feeding pressures up to 5 bar in the bearings and rotation speeds up to 60 krpm. Under these conditions the dynamic load on the rotor is much larger than the static load engendered by its weight. Dynamic loads consist either of impacts provided by a hammer or of added unbalance masses. The test rig can measure the bearing feeding pressures, the rotation speed, the impact force, the displacements of the two bearings and the bearing housing accelerations. This experimental information 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 enabled by the fact 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.