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Papers by Athanasios Chasalevris

A journal bearing with variable geometry (VGJB) was proposed and its operating principle was inve... more A journal bearing with variable geometry (VGJB) was proposed and its operating principle was investigated theoretically in previous works showing a promising impact on the vibration quenching of simple and more complicated rotor bearing system during the passage through the first critical speed. The journal bearing with variable geometry is presented in this paper in its final form with the detailed design and manufacturing procedure. The current journal bearing was constructed in order to be applied in a simple experimental rotor bearing system composed by a Jeffcott rotor mounted on one plain cylindrical journal bearing and one VGJB.

An experimental rotor bearing system with an elastic rotor mounted in worn journal bearings is de... more An experimental rotor bearing system with an elastic rotor mounted in worn journal bearings is designed and investigated. The system is operated at transient run-up and run-down conditions and the response is analyzed with emphasis on passage through the first critical speed. The wear introduces additional sub-and super-harmonics in the response signal compared to the intact system. The time histories are analyzed in the time and frequency domain by applying the short time Fourier transform in order to extract information on the influence of the wear depth on the additional super-and sub-harmonic vibration components. The defect of wear in one journal bearing is produced artificially and two cases of wear depths are investigated: a wear depth of 25% as well as 50% of radial bearing clearance. The resulting scalograms highlight that the defect of wear in a journal bearing affects the system's response, especially during the passage through resonance, with symptoms that do not appear at other defects in journal bearings (e.g. misalignment). The chosen experimental design has the ability to excite the rotor bearing system with an electromagnetic actuator that is realized by an active magnetic bearing (AMB). This specific exciter has the ability to introduce resonance conditions in the system which is operating at a constant rotational speed and, consequently, to observe and extract the symptoms of the worn journal bearing during normal operation of the system.

The floating ring bearings widely used in turbosystems are usually incorporated in rotordynamic a... more The floating ring bearings widely used in turbosystems are usually incorporated in rotordynamic algorithms under short bearing approximation or by using numerical solutions. As an effort to improve both evaluation time and accuracy of the pressure distribution in a floating ring, a recent achievement of an analytical solution for the plain cylindrical journal bearing is applied to the floating ring bearing model. After the procedure of exact and approximate analytical solution of the Reynolds equation for the floating ring bearing, the static characteristics are evaluated and compared with the corresponding results under short bearing approximation and numerical methods. Design characteristics and operational parameters such as ring seed ratio, inner and outer eccentricity, and friction coefficient are presented and compared.

A recent analytical solution for the lubrication problem of the plain cylindrical journal bearing... more A recent analytical solution for the lubrication problem of the plain cylindrical journal bearing is applied in this paper in the three-lobe journal bearing geometry that is commonly applied in the large scale rotational machinery. The aim of this paper is to present the procedure of the analytical solution of the Reynolds equation for each one of the lobes of the bearing that is considered to be of finite length and to present results for static and dynamic characteristics of the journal bearing. Charts for eccentricity ratio and equilibrium locus are presented comparing analytical numerical and experimental results. Additionally, stiffness and damping coefficients are evaluated under the analytical solution and compared with the literature.

Tribology International, 2016

Journal of Tribology, 2015

A thesis submitted to the department of mechanical engineering and aeronautics

Journal bearings are subjected to wear when rub effect happens during the start up or the operati... more Journal bearings are subjected to wear when rub effect happens during the start up or the operation of the rotating system. The wear pattern in many cases is uniformly extended along the width of the bearing. In some other cases the wear pattern is non-uniformly distributed but misaligned along the width, because the rotating shaft inside the bearing operates in misaligned conditions.

Strong nonlinearities in mechanical systems usually lead to the formulation of dynamical systems ... more Strong nonlinearities in mechanical systems usually lead to the formulation of dynamical systems by ways expressing the physical phenomena in an adequate approximation. In this paper the mechanical system is a rotating shaft mounted on finite fluid film bearings. The fact that the rotor model follows the Rayleigh equations of motion and that the fluid film forces are calculated using the Reynolds equation keep the model presented in this paper far away from any approximation usually made towards simplification. The current dynamical system is obtained due to the variable boundary conditions that are functions of time, or more generally are functions of the response that the rotor obtains during operation. So the system of equations of motion is reformed at any time step. The strong non-linearity in this model is due to the fluid film forces that are confronted as a feedback in the journal (rotor) response. The fluid film forces are obtained for each journal displacement and velocity in the bearing. This makes the non-linear system response computable, even in the major resonances that critical speeds provoke. In order to achieve solutions in major resonances, the rotor internal damping is taken into account in the analysis, introducing a higher complexity in the model since in current procedure the damped model is confronted using only real numbers and not complex as usual, due to the coupled mobility equations of the fluid film forces. The fact of "real" confrontment yields the duplication of equations of motion as long as they have to be expressed for real and imaginary part simultaneously. The solution of this dynamical system is achieved using a modified Newton-Raphson method and the resulting time histories of the system response are analyzed regarding time-frequency analysis in order to inspect the development of additional harmonics.

Mechanism and Machine Theory, Jun 30, 2009

In this paper, the cross-coupled bending vibrations of a rotating shaft, with a breathing crack, ... more In this paper, the cross-coupled bending vibrations of a rotating shaft, with a breathing crack, mounted in resilient bearings are investigated. The equations of motion of the continuum and isotropic rotating model of the shaft follow the theory of Rayleigh. The governing equations are coupled in the two main directions, and the partial solution is obtained by solving a linear system of equations, for each time step, taking into account the non-linearity due to the breathing crack. The coupling is introduced in three different ways: the equations of motion, the resilient bearings and the crack.

The exact analytical solution of the Reynolds equation for the finite journal bearing lubrication... more The exact analytical solution of the Reynolds equation for the finite journal bearing lubrication is used in this work in order to evaluate the finite journal bearing impedance forces. A path for obtaining the exact analytical solution is given and the closed form expression of the pressure distribution is integrated through the lubricants volume domain in order to obtain the closed form expressions about the finite journal bearing impedance forces. The pressure profile under the exact analytical solution is compared with other pressure profiles yielded by numerical methods. The main operational journal bearing characteristics such as eccentricity ratio, minimum fluid film thickness location and maximum lubricant pressure location are evaluated and presented in comparison with other correspondent results from the literature. Also the stiffness and damping coefficients of the finite journal bearing are evaluated using the exact analytical solution and they are presented for variable length to diameter ratios of the finite journal bearing.

Mechanical Systems and Signal Processing, 2015

The combination of a continuously modeled rotor supported by two worn nonlinear finite fluid film... more The combination of a continuously modeled rotor supported by two worn nonlinear finite fluid film bearings is the main objective of this work. The rotor was modeled using the Rayleigh equation of motion, and the bearing forces are computed by solving the Reynolds equation using the finite difference method. Additionally the internal damping effect of the rotor is incorporated into the current approach and the wear is introduced using the well known Dufrane wear model. In current work the system appears strongly nonlinear due to both the fluid film forces and the presence of wear. Time histories for system start up and steady state operations under external excitation are analyzed simultaneously in time and frequency domain in order to invest the properties that the wear introduces in the system behavior. Specific components are judged very sensitive to wear development and are used in order to identify the wear extension through the bearing surface.

Cracks are the most serious defect menacing the structural integrity of a rotating system. Monito... more Cracks are the most serious defect menacing the structural integrity of a rotating system. Monitoring, detection and identification of cracks in rotors is a serious safety task for every rotating machine. Crack identification in rotating shafts could be realized by applying either vibration or model-based methods. Vibration methods are based on direct signal measurements such as response or eigenfrequencies; the alteration of these measurements could be assigned to the crack. Model-based methods consider in the place of the crack, or other defects, equivalent loads that give the same result as the crack or the defects do. In this paper a review of recent development on structural integrity assessment methods of rotating shafts is presented along with a new method of identifying cracks, based on the fact that a crack generates a coupled form of lateral vibrations, is presented in this paper. During normal operation and with the use of external electromagnetic harmonic excitation, an ...

In the present paper the free bending vibration of a rotating shaft having a uniform circular cro... more In the present paper the free bending vibration of a rotating shaft having a uniform circular cross-section and a transverse crack is calculated. A disk is adopted in the midspan of the rotor and the crack, which is positioned also in the midspan, is supposed to open and close under the domination of the gravity on the vibration amplitude. The equations of motion of the continuous and isotropic model of the shaft follow the Timoshenko theory and take into account the gyroscopic effect, the shear deformation as long as the torsion that the shaft takes over due to the supposed power transmission. The governing equations are coupled and the partial solution is obtained by solving a linear system of equations. The rotation of the transverse crack changes the shaft stiffness, which is calculated at each angle of rotation analytically in a new way. The existence of the crack introduces the coupling between vibrations in the vertical and horizontal planes and the cross coupling terms of th...

Continuous mechanics approach is used to simulate the rotating cracked shaft. Since in previous a... more Continuous mechanics approach is used to simulate the rotating cracked shaft. Since in previous attempts, due to complexity of the mathematical simulation, low number of elements was used, in the present modelling multiple steps, cracks, discs and bearings can be used. Thus, a way of simulation of a multi-step cracked rotor in combination with various fluid film bearings is presented in this paper, using shaft elements that obey the continuous medium theory and incorporating internal (hysteretic) damping assuming variable loss factor as a function of developed stresses during whirling. The method aims to simulate cracked rotors of complex geometry that are mounted in nonlinear or linear fluid film bearings. The crack model follows previous breathing simulation or any other different form while rotation, yielding the model an additional nonlinearity, besides that bearings introduce. The assembly method uses predefined matrices for any element of the system, including shaft, disc, bea...

Continuous mechanics approach, instead of FEM, is used to simulate the rotating shaft. Since in p... more Continuous mechanics approach, instead of FEM, is used to simulate the rotating shaft. Since in previous attempts, due to complexity of the mathematical simulation, low number of elements was used, in the present modeling multiple steps, discs and bearings are used in order to emphasize the applicability of the method. Thus, a way of simulation of a multi-step rotor in combination with various fluid film bearings is used here. The shaft elements obey the continuous medium theory and incorporating internal (hysteretic) damping assuming variable loss factor as a function of developed stresses during whirling. The method aims to simulate rotors of complex geometry that are mounted in nonlinear or linear fluid film bearings depending on the response of the journal inside the bearing. The assembly method uses predefined matrices for any element of the system, including shaft, disc and bearing no matter how complex or extended is the system, and then the global equations of the system can...

1. Erfinder: • Chasalevris, Athanasios 60598 Frankfurt a.M. (DE) • Dohnal, Fadi 5430 Wettingen (CH)

A journal bearing with variable geometry (VGJB) was proposed and its operating principle was inve... more A journal bearing with variable geometry (VGJB) was proposed and its operating principle was investigated theoretically in previous works showing a promising impact on the vibration quenching of simple and more complicated rotor bearing system during the passage through the first critical speed. The journal bearing with variable geometry is presented in this paper in its final form with the detailed design and manufacturing procedure. The current journal bearing was constructed in order to be applied in a simple experimental rotor bearing system composed by a Jeffcott rotor mounted on one plain cylindrical journal bearing and one VGJB.

An experimental rotor bearing system with an elastic rotor mounted in worn journal bearings is de... more An experimental rotor bearing system with an elastic rotor mounted in worn journal bearings is designed and investigated. The system is operated at transient run-up and run-down conditions and the response is analyzed with emphasis on passage through the first critical speed. The wear introduces additional sub-and super-harmonics in the response signal compared to the intact system. The time histories are analyzed in the time and frequency domain by applying the short time Fourier transform in order to extract information on the influence of the wear depth on the additional super-and sub-harmonic vibration components. The defect of wear in one journal bearing is produced artificially and two cases of wear depths are investigated: a wear depth of 25% as well as 50% of radial bearing clearance. The resulting scalograms highlight that the defect of wear in a journal bearing affects the system's response, especially during the passage through resonance, with symptoms that do not appear at other defects in journal bearings (e.g. misalignment). The chosen experimental design has the ability to excite the rotor bearing system with an electromagnetic actuator that is realized by an active magnetic bearing (AMB). This specific exciter has the ability to introduce resonance conditions in the system which is operating at a constant rotational speed and, consequently, to observe and extract the symptoms of the worn journal bearing during normal operation of the system.

The floating ring bearings widely used in turbosystems are usually incorporated in rotordynamic a... more The floating ring bearings widely used in turbosystems are usually incorporated in rotordynamic algorithms under short bearing approximation or by using numerical solutions. As an effort to improve both evaluation time and accuracy of the pressure distribution in a floating ring, a recent achievement of an analytical solution for the plain cylindrical journal bearing is applied to the floating ring bearing model. After the procedure of exact and approximate analytical solution of the Reynolds equation for the floating ring bearing, the static characteristics are evaluated and compared with the corresponding results under short bearing approximation and numerical methods. Design characteristics and operational parameters such as ring seed ratio, inner and outer eccentricity, and friction coefficient are presented and compared.

A recent analytical solution for the lubrication problem of the plain cylindrical journal bearing... more A recent analytical solution for the lubrication problem of the plain cylindrical journal bearing is applied in this paper in the three-lobe journal bearing geometry that is commonly applied in the large scale rotational machinery. The aim of this paper is to present the procedure of the analytical solution of the Reynolds equation for each one of the lobes of the bearing that is considered to be of finite length and to present results for static and dynamic characteristics of the journal bearing. Charts for eccentricity ratio and equilibrium locus are presented comparing analytical numerical and experimental results. Additionally, stiffness and damping coefficients are evaluated under the analytical solution and compared with the literature.

Tribology International, 2016

Journal of Tribology, 2015

A thesis submitted to the department of mechanical engineering and aeronautics

Journal bearings are subjected to wear when rub effect happens during the start up or the operati... more Journal bearings are subjected to wear when rub effect happens during the start up or the operation of the rotating system. The wear pattern in many cases is uniformly extended along the width of the bearing. In some other cases the wear pattern is non-uniformly distributed but misaligned along the width, because the rotating shaft inside the bearing operates in misaligned conditions.

Strong nonlinearities in mechanical systems usually lead to the formulation of dynamical systems ... more Strong nonlinearities in mechanical systems usually lead to the formulation of dynamical systems by ways expressing the physical phenomena in an adequate approximation. In this paper the mechanical system is a rotating shaft mounted on finite fluid film bearings. The fact that the rotor model follows the Rayleigh equations of motion and that the fluid film forces are calculated using the Reynolds equation keep the model presented in this paper far away from any approximation usually made towards simplification. The current dynamical system is obtained due to the variable boundary conditions that are functions of time, or more generally are functions of the response that the rotor obtains during operation. So the system of equations of motion is reformed at any time step. The strong non-linearity in this model is due to the fluid film forces that are confronted as a feedback in the journal (rotor) response. The fluid film forces are obtained for each journal displacement and velocity in the bearing. This makes the non-linear system response computable, even in the major resonances that critical speeds provoke. In order to achieve solutions in major resonances, the rotor internal damping is taken into account in the analysis, introducing a higher complexity in the model since in current procedure the damped model is confronted using only real numbers and not complex as usual, due to the coupled mobility equations of the fluid film forces. The fact of "real" confrontment yields the duplication of equations of motion as long as they have to be expressed for real and imaginary part simultaneously. The solution of this dynamical system is achieved using a modified Newton-Raphson method and the resulting time histories of the system response are analyzed regarding time-frequency analysis in order to inspect the development of additional harmonics.

Mechanism and Machine Theory, Jun 30, 2009

In this paper, the cross-coupled bending vibrations of a rotating shaft, with a breathing crack, ... more In this paper, the cross-coupled bending vibrations of a rotating shaft, with a breathing crack, mounted in resilient bearings are investigated. The equations of motion of the continuum and isotropic rotating model of the shaft follow the theory of Rayleigh. The governing equations are coupled in the two main directions, and the partial solution is obtained by solving a linear system of equations, for each time step, taking into account the non-linearity due to the breathing crack. The coupling is introduced in three different ways: the equations of motion, the resilient bearings and the crack.

The exact analytical solution of the Reynolds equation for the finite journal bearing lubrication... more The exact analytical solution of the Reynolds equation for the finite journal bearing lubrication is used in this work in order to evaluate the finite journal bearing impedance forces. A path for obtaining the exact analytical solution is given and the closed form expression of the pressure distribution is integrated through the lubricants volume domain in order to obtain the closed form expressions about the finite journal bearing impedance forces. The pressure profile under the exact analytical solution is compared with other pressure profiles yielded by numerical methods. The main operational journal bearing characteristics such as eccentricity ratio, minimum fluid film thickness location and maximum lubricant pressure location are evaluated and presented in comparison with other correspondent results from the literature. Also the stiffness and damping coefficients of the finite journal bearing are evaluated using the exact analytical solution and they are presented for variable length to diameter ratios of the finite journal bearing.

Mechanical Systems and Signal Processing, 2015

The combination of a continuously modeled rotor supported by two worn nonlinear finite fluid film... more The combination of a continuously modeled rotor supported by two worn nonlinear finite fluid film bearings is the main objective of this work. The rotor was modeled using the Rayleigh equation of motion, and the bearing forces are computed by solving the Reynolds equation using the finite difference method. Additionally the internal damping effect of the rotor is incorporated into the current approach and the wear is introduced using the well known Dufrane wear model. In current work the system appears strongly nonlinear due to both the fluid film forces and the presence of wear. Time histories for system start up and steady state operations under external excitation are analyzed simultaneously in time and frequency domain in order to invest the properties that the wear introduces in the system behavior. Specific components are judged very sensitive to wear development and are used in order to identify the wear extension through the bearing surface.

Cracks are the most serious defect menacing the structural integrity of a rotating system. Monito... more Cracks are the most serious defect menacing the structural integrity of a rotating system. Monitoring, detection and identification of cracks in rotors is a serious safety task for every rotating machine. Crack identification in rotating shafts could be realized by applying either vibration or model-based methods. Vibration methods are based on direct signal measurements such as response or eigenfrequencies; the alteration of these measurements could be assigned to the crack. Model-based methods consider in the place of the crack, or other defects, equivalent loads that give the same result as the crack or the defects do. In this paper a review of recent development on structural integrity assessment methods of rotating shafts is presented along with a new method of identifying cracks, based on the fact that a crack generates a coupled form of lateral vibrations, is presented in this paper. During normal operation and with the use of external electromagnetic harmonic excitation, an ...

In the present paper the free bending vibration of a rotating shaft having a uniform circular cro... more In the present paper the free bending vibration of a rotating shaft having a uniform circular cross-section and a transverse crack is calculated. A disk is adopted in the midspan of the rotor and the crack, which is positioned also in the midspan, is supposed to open and close under the domination of the gravity on the vibration amplitude. The equations of motion of the continuous and isotropic model of the shaft follow the Timoshenko theory and take into account the gyroscopic effect, the shear deformation as long as the torsion that the shaft takes over due to the supposed power transmission. The governing equations are coupled and the partial solution is obtained by solving a linear system of equations. The rotation of the transverse crack changes the shaft stiffness, which is calculated at each angle of rotation analytically in a new way. The existence of the crack introduces the coupling between vibrations in the vertical and horizontal planes and the cross coupling terms of th...

Continuous mechanics approach is used to simulate the rotating cracked shaft. Since in previous a... more Continuous mechanics approach is used to simulate the rotating cracked shaft. Since in previous attempts, due to complexity of the mathematical simulation, low number of elements was used, in the present modelling multiple steps, cracks, discs and bearings can be used. Thus, a way of simulation of a multi-step cracked rotor in combination with various fluid film bearings is presented in this paper, using shaft elements that obey the continuous medium theory and incorporating internal (hysteretic) damping assuming variable loss factor as a function of developed stresses during whirling. The method aims to simulate cracked rotors of complex geometry that are mounted in nonlinear or linear fluid film bearings. The crack model follows previous breathing simulation or any other different form while rotation, yielding the model an additional nonlinearity, besides that bearings introduce. The assembly method uses predefined matrices for any element of the system, including shaft, disc, bea...

Continuous mechanics approach, instead of FEM, is used to simulate the rotating shaft. Since in p... more Continuous mechanics approach, instead of FEM, is used to simulate the rotating shaft. Since in previous attempts, due to complexity of the mathematical simulation, low number of elements was used, in the present modeling multiple steps, discs and bearings are used in order to emphasize the applicability of the method. Thus, a way of simulation of a multi-step rotor in combination with various fluid film bearings is used here. The shaft elements obey the continuous medium theory and incorporating internal (hysteretic) damping assuming variable loss factor as a function of developed stresses during whirling. The method aims to simulate rotors of complex geometry that are mounted in nonlinear or linear fluid film bearings depending on the response of the journal inside the bearing. The assembly method uses predefined matrices for any element of the system, including shaft, disc and bearing no matter how complex or extended is the system, and then the global equations of the system can...

1. Erfinder: • Chasalevris, Athanasios 60598 Frankfurt a.M. (DE) • Dohnal, Fadi 5430 Wettingen (CH)