On the instability threshold of journal bearing supported rotors

Journal bearing supported rotors present two kinds of self-excited vibrations: oil-whirl and oil-whip. The first one is commonly masked by the rotor unbalance, hence being rarely associated with instability problems. Oil-whip is a severe vibration which occurs when the oil-whirl frequency coincides with the first flexural natural frequency of the shaft. In many cases, oil-whip is the only fluid-induced instability considered during the design stage; however, experimental evidences have shown that the instability threshold may occur much sooner, demanding a better comprehension of the instability mechanism. In this context, numerical simulations were made in order to improve the identification of the instability threshold for two test rig configurations: one on which the instability occurs on the oil-whip frequency, and another which became unstable before this threshold. Therefore, the main contribution of this paper is to present an investigation of two different thresholds of fluid-induced instabilities and their detectability on design stage simulations based on rotordynamic analysis using linear speed dependent coefficients for the bearings2014CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP131658/2010-7PROBRAL 341/102007/54647-

Experimental validation of a bearing wear model using the directional response of the rotor-bearing system

The present work gives continuity in the analysis of the wear influence on cylindrical hydrodynamic bearings by presenting an experimental validation of the wear model previously proposed by the authors. This validation is carried on using the frequency response of the rotor-bearings system in directional coordinates. For this purpose, a test rig was assembled in order to evaluate the behavior of the rotating system when supported by hydrodynamic bearings with different wear patterns. The experimental measurements are used to validate the wear model, comparing the anisotropy influence on the experimental and numerical responses. The simulated directional frequency responses showed a good agreement with the experimental ones, demonstrating the potential of the proposed wear model in satisfactorily represent its influence on the rotor-bearings system response in the frequency range where the numerical model was validated884CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPNão temNão temNão te

On the Instability Threshold of Journal Bearing Supported Rotors

Journal bearing supported rotors present two kinds of self-excited vibrations: oil-whirl and oil-whip. The first one is commonly masked by the rotor unbalance, hence being rarely associated with instability problems. Oil-whip is a severe vibration which occurs when the oil-whirl frequency coincides with the first flexural natural frequency of the shaft. In many cases, oil-whip is the only fluid-induced instability considered during the design stage; however, experimental evidences have shown that the instability threshold may occur much sooner, demanding a better comprehension of the instability mechanism. In this context, numerical simulations were made in order to improve the identification of the instability threshold for two test rig configurations: one on which the instability occurs on the oil-whip frequency, and another which became unstable before this threshold. Therefore, the main contribution of this paper is to present an investigation of two different thresholds of fluid-induced instabilities and their detectability on design stage simulations based on rotordynamic analysis using linear speed dependent coefficients for the bearings

Stability Testing of CO2 Compressors

LectureThis paper presents results from stability analysis and testing on high pressure CO2 centrifugal compressors, beginning by a preliminary analysis conducted on an academic institution. It includes results of stability tests that were carried out in manufacturer facilities. The tests were done on two different compressor designs, both operating with high content of CO2 in super critical condition. Due to the similarities on testing and results, only the results of one compressor type are presented. An important gain from the tests was a clearer understanding of the behavior of this compressor for the stability for different load conditions

Investigation on an experimental approach to evaluate a wear model for hydrodynamic cylindrical bearings

This work aims to investigate and identify the wear parameters in cylindrical hydrodynamic bearings, an inherent problem due to repeated use of rotating machinery, accentuated in starts/stops or during the crossover of resonance. Thereby, a mathematical model to represent the wear in terms of its main parameters is used, namely, the maximum depth, the angular span and the angular position. The pressure distribution and the hydrodynamic forces generated by the oil film are numerically obtained, properly adapted for discontinuous oil film. The rotor model is represented by finite element method and the bearings are approximated by dynamic coefficients of stiffness and damping. The evaluation and identification of the wear parameters is taken from the unbalanced frequency response of the rotor-bearing system in directional coordinates4021-2295469564CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPBEX 9782/11-7141717/2011-42010/20241-

Experimental Analysis Of The Dynamic Behaviour Of A Turbomachine Foundation Structure

The main reason for the development of the present cooperative work was the rising necessity of understanding the dynamic behavior of a specific turbine, rigidly coupled to a compressor, and both connected through hydrodynamic bearings to a concrete supporting structure of foundation. The dynamic response, obtained during the machine run up and run down, involves the mechanical behavior of three subsystems, as follow: the rotor system, the connection elements between rotor and foundation (bearings and seals) and, finally, the supporting pedestals and the foundation structure. An interaction between these subsystems is highly important in the analysis process, whether the concrete foundation structure presents some vibrational natural modes significantly participating into the operational frequency range of the machine. The foundation flexibility reacts to the excitation proceeding from the rotor vibration, which also dynamically reacts to a mass unbalancing external excitation force. The main objective of this work is to estimate the Frequency Response Functions on the bearings of the turbine. This estimation is based on some experimental data and on a finite element model of the foundation structure. These FRFs contain the necessary information to verify the influence of the foundation on the dynamic behavior of the rotating system.196697

Modeling of hydrodynamic bearing wear in rotor-bearing systems

Hydrodynamic bearings are widely used in industry mainly due to their simplicity, efficiency and low cost. However, these components also represent one of the main critical factors with respect to stability and failure analysis of the system. During the operation of rotating machines, these bearings are subjected to a progressive wear process, which can be accentuated in starts and stops or during the passage through the critical speed of the rotor. To prevent catastrophic failure of the rotating system, it is necessary to precisely detect wear, and purpose, it is necessary to accurately model the wear in this kind of bearing. Within this context, the main objective of this paper is to present a numerical model that characterizes both the hydrodynamic bearing as well as the wear present in its wall, and then, to analyze the influence of the wear on the dynamic response of the rotor-bearing system in frequency domain, taking into account the rising of the backward component due to the increase of the bearing anisotropy691523CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPNão temNão temNão te

Experimental validation of a bearing wear model using the directional response of the rotor-bearing system

ABSTRACT The present work gives continuity in the analysis of the wear influence on cylindrical hydrodynamic bearings by presenting an experimental validation of the wear model previously proposed by the authors. This validation is carried on using the frequency response of the rotor-bearings system in directional coordinates. For this purpose, a test rig was assembled in order to evaluate the behavior of the rotating system when supported by hydrodynamic bearings with different wear patterns. The experimental measurements are used to validate the wear model, comparing the anisotropy influence on the experimental and numerical responses. The simulated directional frequency responses showed a good agreement with the experimental ones, demonstrating the potential of the proposed wear model in satisfactorily represent its influence on the rotor-bearings system response in the frequency range where the numerical model was validated