Shrink fit effects on rotordynamic stability: experimental and theoretical study

Abstract

This dissertation presents an experimental and theoretical study of subsynchronous rotordynamic instability in rotors caused by interference and shrink fit interfaces. The experimental studies show the presence of strong unstable subsynchronous vibrations in two different rotor setups with interference and shrink fit interfaces that were operated above their first critical speeds. The unstable vibrations occur at the first natural frequency of the rotor-bearing system. The instability caused complete wreckage of the test rig in one of the setups showing that these vibrations are potentially dangerous to the safe operation of rotating machines. The two different rotor setups that are studied are a single-disk rotor mounted on a uniform diameter shaft and a two-disk rotor with an aluminum sleeve shrink fitted to it at the outer surface of the two disks. In the single-disk rotor, an adjustable interference arrangement between the disk and the shaft is obtained through a tapered sleeve arrangement, which acts as the interference fit joint. The unstable sub-synchronous vibrations originate from slippage in the shrink fit and the interference fit interfaces that develop friction forces, which act as destabilizing cross-coupled moments when the rotor is operated above its first critical speed. The unique contribution offered through this work is the experimental validation of a physically correct model of internal friction which models the destabilizing mechanism as a system of cross-coupled internal moments at the shrink fit interface. The dissertation describes stability simulations of various test rotor setups using the correct internal moments model. A commercial finite-element based software called XLTRCTM is used to perform rotordynamic simulations for stability studies. The method of stability study is the computation of eigenvalues of the rotor-bearing system. A negative real part of the eigenvalue indicates instability. The simulations include the test rotors that were experimentally observed as stable and unstable with shrink and interference fit interfaces in their assemblies. The dissertation also describes the simulations of various imagined rotor configurations with shrink fit interfaces, and seeks to explain how configurations differ on rotordynamic stability depending upon several rotor-bearing parameters such as geometry and elastic properties, as well as upon the amount of internal friction parameters, which differ from configuration to configuration.