Experimental Techniques for Bearing Impedance Measurement

Abstract

The dynamic characteristics of rotating machinery are almost always influenced by the impedances of the machine bearings (including the bearing supports) and it is particularly important to know of their likely values at the design stage when predicting machine critical speeds, unbalance response, and stability. In many instances the vital data may be obtained from theory which must be confirmed by experimental measurement beforehand, or alternatively the data may be obtained directly from experimental measurement when the appropriate bearing has already been manufactured. The experimental techniques available for use in the measurement of bearing impedances are varied, both in their experimental procedure and in the theoretical treatment of the data that is gathered. The aim of this paper is to provide a concise account of these techniques, and to discuss the potential advantages and disadvantages of each approach. In this document, the procedures described are classified in terms of the type of experimental measuring equipment required. These vary from the simplest of test rigs, which provide only a static load in one direction and where impedances can be evaluated by hand calculation, to sophisticated apparatus capable of providing static and dynamic loads in several directions and which require computer software to extract the bearing impedances from the measured data. The type of experimental procedure selected by the rotor dynamicist will depend upon the experimental facilities available, on the time available to carry out the testing, and on the required reliability of results. Each of these considerations is discussed for each procedure described. It is concluded that measurements made using multifrequency test signals provide more reliable data, although all measurement methods yield coefficient values which agree with theoretical predictions to within about 20 percent in general, and all methods have a significant scatter of results associated with their use.