In the first part of this study, the dynamic behavior of a cracked rotating shaft with a rigid disk is analyzed, with an ideal and a nonideal motor, passing through its critical speed. The shaft contains a single transverse crack that is assumed to be either completely open or completely closed at any given time, depending on the curvature of the shaft at the cross section containing the crack. Flexible, damped supports and overhangs with a mass at one end are included. The supports are modeled with elastic springs and dashpots. The influence of gyroscopic moments of the disk (with an ideal motor) is investigated. For a nonideal motor, there is an interaction between the shaft and the motor. Eccentricity of the disk, gravitational forces, and internal and external damping are included. The equations of motion and boundary conditions are derived by Hamilton's Principle. To eliminate the spatial dependence, the Extended Galerkin Method is applied. Longitudinal vibration, shear deformation and torsional vibration are neglected. In the second part of this study, the vibration suppression of a cracked, simply supported, rotating shaft with a rigid disk is discussed, with an ideal and a nonideal motor, passing through the critical speed. The use of a flexible internal constraint is introduced to suppress the vibration. By activating this additional internal support, the shaft is prevented from passing its critical speed. Transient motions occur at the time of activation or deactivation of the constraint. The maximum displacement of the shaft during acceleration (run-up) or deceleration (coast-down) can be reduced significantly by appropriate application of this flexible internal support. / Ph. D.

With a new range of water jet assemblies under development, CWF Hamilton & Co. Ltd. highlighted the need to establish a validated model for predicting critical speeds. A review of the relevant literature revealed a significant lack of information concerning the operating properties of a lightly loaded, water lubricated marine bearing. Therefore, an instrumented test rig based on a CWF Hamilton & Co. Ltd. ‘HJ364’ water jet assembly was established to evaluate critical speeds and validate the predictive models. A number of analytical and numerical models for predicting critical speeds were investigated. Geometric modifications were made to the test rig and the changes in critical speeds were observed. The ability of the predictive models to measure these observed critical speeds was examined. Driveline mass and driveline overhang were found to have the most significant effects on critical speeds. Modifications to the thrust bearing housing, the impeller mass, the tailpipe stiffness and the marine bearing resulted in no significant shift in critical speeds. However, a change to the geometry of the thrust bearing resulted in a significant shift. This indicated that the thrust bearing was not performing ideally in the test rig. All three models predicted changes in critical speeds relatively accurately. However, the estimates of the critical speeds themselves were somewhat conservative; approximately 10 to 15 percent lower than those measured. Linearisation of the thrust bearing geometry is recommended if greater accuracy is to be achieved. Of all the predictive methods, the Myklestad-Prohl transfer-matrix and the Isolated-Mainshaft finite-element were deemed to be the most flexible and suitable for CWF Hamilton & Co. Ltd.

This Master thesis describes the design of methodology for calculating the critical speed of rotating electric machines. The aim is to build finite element model of rotor system model, determine the natural frequencies to plot Campbell diagram and use this diagram to obtain the critical speeds. The Matlab software is used for this purpose and the Ansys software is used for verify the calculation.

This Master thesis desires the design of methodology for calculating shaft deflection and critical speed of electric machines. There are two methods used. The first one is transfer matrix method, which is represented by a simple programme for shaft creation and calculating needed results built in the Matlab software. The second one is finite element method, which is presented in the Ansys Workbench software.