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Shaft-Stiffening Effects in Large Turbomachines, 18-9180

Principal Investigators
Anthony J. Smalley
P. Joe Pantermuehl
Justin R. Hollingsworth

Inclusive Dates:    01/11/00 - 05/11/00

Background - Many manufacturers and operators of high-performance turbomachinery need assurance when buying a new machine that its critical speeds will lie safely outside the operating range and that no chance exists for damaging self-excited vibrations to occur. Providing this assurance depends on the ability to predict shaft-bending flexibility. Well-established relationships exist for the flexibility of a straight shaft. In mounting components such as an impeller on a shaft, however, the widely used method of attachment is an interference fit. Conservative practice neglects any stiffening influence of the interference fit, and as a result, can lead to underprediction of critical speeds. Prior to this project, the need existed for a documentable method of quantifying the added bending stiffness provided by an interference fit.

Approach - Using extensive engineering data provided by the manufacturer on a large centrifugal compressor with shrink-fit impellers, a detailed finite element model of the rotor-impeller assembly was built. SwRI's previous conservative predictions had underpredicted the first critical speed on this compressor by more than six percent. The model included contact elements throughout the interference, friction between all interfering surfaces, and a three-dimensional analysis that enabled nonlinear analysis of the response to an applied bending moment. 

Accomplishments - The detailed model was used to compare the amount of bending for the bare shaft, with the amount of bending for the same shaft accounting for the added stiffening, provided by the interference. The approach allowed a study of the geometrical and interference parameters and how they affect the bending behavior. Working charts were developed that could be used with standard rotor flexibility models to provide factors by which to adjust the effective diameter of the shaft. Using these charts, the six-percent discrepancy in critical speed was reduced to zero. Because of SwRI's willingness to develop this improved method of analyzing bending of shrink fit rotors, the compressor manufacturer involved the Institute in a series of design analyses of machines.

Fluid and Machinery Dynamics Program
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