Application of modal testing methods in rotating machinery

Al-Khazali, Hisham Ahmad Humadi (2012) Application of modal testing methods in rotating machinery. (PhD thesis), Kingston University, .


The experimental and analytical modal analysis is used to establish a system modelling methodology in rotating structures, which subsequently can help in design and development of rotating machinery. The purpose of the study is to develop and use modal testing and vibration analysis which would involve obtaining the mathematical model of the system from the test data and subsequently obtaining the unbalanced parameters. The research work includes the application of modal testing method in rotor rig to investigate different modal parameters and detect the behaviour and performance of rotating machinery. This method would be capable of solving many of the related rotating machine problems, such as in turbine and compressor. Unbalance is one of the problems which exist in rotating machinery. Balancing is usually an expensive and laborious procedure and a balancing system would be beneficial for rotor dynamic systems and power generation applications. Excess vibration can cause noise, cyclic stress and wear in machinery. It is important to identify all the critical speeds within the range of operation and analyse the damping effect, mass unbalance and other phenomena in rotating machinery and their effects in their safe operation. These will be investigated in this study. There are several phenomena associated with rotating machinery such as centrifugal and gyroscopic forces which would create complexity in the mathematical procedures in modal analysis that they need to be addressed and interpreted appropriately before they could be used in modal testing of rotating machinery. The experimental technique used in this thesis to obtain the modal and dynamic response properties of structures. This technique has been applied to rotating structures, however the full implementation of modal testing in rotating structures and the implications are not fully understood. and are therefore in need of further investigations. In this study the Frequency Response Function (FRF) data obtained from the specific experimental results are curve-fitted by theoretical data regenerated from overall statistical analysis of measured data. Different excitation methods are used in experiment (hammer and shaker). For hammer test, transient signal is produced. While for shaker test, different vibration signals are produced (Sine, Random and Burst Random). In shaker test, a special frame was designed and used around a plain bearing and the accelerometers were attached to the outer surface of the bearing to measure the response of the lateral motion on several points of the shaft. The excitation force with help of push' rod was generated and applied to the shaft. This method can help to solve the problem in the attachment of shaker and force transducers to the rotor system. The analysis of vibration suppression with different locations and configurations of the unbalanced masses and effect of the adding of balance masses to suppress the vibration amplitude has been studied properly. The experimental results were used for verification of Finite Element (FE) models, since it has good capability for eigen analysis and also good graphical facility. 3-D models result in large number of nodes and elements. This project demonstrates how to extract a plane 2-D model from the 3-D model that can be used with fewer nodes and elements with no loss in accuracy of the results. Transient orbit analysis in the literature indicates that the bearing stiffness and damping affects the vibration amplitude. In this project the study of the effects on the bearing reaction forces and cyclic. bending stress will be investigated. It is envisaged that the approach is not limited to the condition diagnosis and predictive failure but could help the designers to have better understanding of rotor performance at the system design stage. The experimental data are used to characterise the dynamic behaviour of the system and introduce to the correction unbalance to suppress the excess vibration. The experimental data are also used to generate the FE models and subsequently calculate the dynamic reaction forces in the bearings and the cyclic bending stress.

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