COMPUTING CRITICAL SPEEDS FOR ROTATING MACHINES WITH SPEED DEPENDENT BEARING PROPERTIES

Abstract

In this paper available methods are examined for computing critical speeds for rotating machines whose bearings have speed dependent properties and a modification is proposed which can be incorporated into almost all of the established techniques. The established methods for the calculation of critical speeds are adequately efficient when only a single configuration is analyzed. The trend towards incorporating critical speed calculations into the design optimization process and model-based fault diagnosis systems requires the more efficient calculation of critical speeds. The proposed modification involves replacing the sets of speed-dependent bearing characteristics by fictitious multi-degree-of-freedom systems whose characteristics emulate those of the bearings themselves. The method by which these fictitious systems can be found is explained and the advantages of using this method for computing critical speeds are demonstrated in conjunction with the finite element approach to evaluating critical speeds. The proposed technique is also well suited to computing efficiently the machine response to unbalance by using a truncated modal decomposition.