Chris A. Papadopoulos

University of Patras | UP · Department of Mechanical Engineering and Aeronautics

A proportional–integral–derivative (PID) control model of an active hydromagnetic journal bearing (AHJB) is presented here and its design guidelines are explicitly described. The nonlinear rotor equations of motion, is introduced. Then, the linearization of these equations in the bearing specific equilibrium point is performed and presented along w...

Smart machines are defined here as machines with properties adaptable to measurements of some external situations. In this paper a rotor on two or more journal bearings is considered as a rotating system. As external situation the magnitude of the rotor vibrations is considered here, and the smart of the system is the possibility to change the char...

The architecture of the electrical actuation module driving a magnetic-hydraulic bearing system is presented. The bearing is intended to be scaled for use in applications of all sizes in industries like shipboard for support of the engine-propeller shaft or in power-plants for the shaft through which the prime mover, e.g. steam or gas turbine, is d...

Rotor dynamics considerations play an important role in the design of a number of machine elements, especially those that operate at high rotational speeds. During system start-up and shutdown, the rotor and bearing materials are subjected to high mean and alternating stresses that cause fatigue, particularly when the rotor is running at critical s...

Purpose–The purpose of this paper is to present a method for early crack detection in rotating shafts. A rotor-bearing system, consisting of an elastic rotor mounted on fluid film bearings, is used to detect the presence of the crack at a depth of around 5 percent of shaft radius. The fluid film bearings, the shaft and the crack introduce coupled b...

This paper presents a method for early crack detection in rotating shafts. A rotor-bearing system, consisting of an elastic rotor mounted on fluid film bearings, is used to detect the presence of the crack at a depth of around 5% of shaft radius. The fluid film bearings, the shaft and the crack introduce coupled bending vibrations in the horizontal...

Active magnetic bearings (AMBs) support a rotating shaft, inside a stator, contactless. Their major benefit in respect to hydrodynamic journal bearings is the possibility of operating in much higher rotational speeds (above 20.000 RPM). This is due to the fact that, their angular velocity is only limited by the strength of the shaft material and al...

In this paper the effect of the wear of a short journal bearing, on the rotordynamic stiffness and damping coefficients (K and C) is investigated. Wear in journal bearings appears when the operating conditions (high load, high temperature, low angular velocity or low viscosity), downgrade the ability of the bearing to carry load. The depth of wear...

Purpose – The purpose of this paper is to present a methodology for design, analysis and evaluation of cam-follower systems used for the control of valve movement in internal combustion engines. The strategy begins with the inverse engineering of existing motor parts, designing them with the CATIA CAD Software and consequentially the created assemb...

The hysteretic damping is a measure of the dissipating energy during a period of vibration of the structure and it is a function of the induced stress particularly at the resonance. The crack presence in a structure introduces a local flexibility modelled by a compliance matrix with dimension depending on the degrees of freedom considered. Here the...

The hysteretic damping is a measure of the dissipating energy during a period of vibration of the structure and it is a function of the induced stress particularly at the resonance. The crack presence in a structure introduces a local flexibility modelled by a compliance matrix with dimension depending on the degrees of freedom considered. Here the...

A rotor-bearing system is simulated in this study to investigate the effect of worn journal bearings on the system response and to specify the eventual development of additional frequency components. The well-known symmetric Dufrane bearing wear model is used here. The main target here is the investigation of the wear influence on the system respon...

An active hydromagnetic journal bearing is designed and presented in this study, which is a new and innovative type of hybrid journal bearing. The proposed new mechatronic smart device has a common bearing shell that contains the two parts of the bearing, which is an attempt to combine the advantages of both types of bearings to overcome their draw...

A Hybrid journal bearing, which can operate either as hydrodynamic or as electromagnetic, or as hybrid (hydrodynamic and electromagnetic simultaneously), aiming at the exploitation of advantages of both types of bearings, is presented. It is constituted from stator (3), the two lids (2), the internal ring (7), the paramagnetic parts of the internal...

It is well known that the imposition of an electric field on an Electro-Rheological (ER) fluid alters the viscosity and as a consequence the f flow properties of the f fluid. If such a fluid is used to lubricate a journal bearing system, it is expected that the imposition of an electric field between the rotor and the stator will cause the alterati...

Journal bearing bushing wear and shaft misalignment are the common defects observed in rotating systems. Both defects influence their dynamic behaviour, stability and life time and their detection is a very important task. This article introduces a defect diagnosis approach that uses artificial neural networks. Reynolds equation is solved by finite...

This paper aims to present a methodology for design, analysis and evaluation of follower-cam systems used for the control of valve movement in internal combustion engines. The strategy begins with the inverse engineering of the existing parts from a chosen motor, and designing them within the Dassault Catia CAD Software environment. The assembly cr...

The quality of surface of a journal bearing system affects significantly its performance and its dynamic characteristics. Manufacturing errors can lead to a series of defects in the surface of the bearing, such as out-of-roundness, misalignment and others. Out-ofroundness can be caused by spindle error motion. Since the prediction of the actual cha...

The dynamic behavior of cracked rotors continues to attract the interest of both designers and maintenance engineers. In this work, a continuum mechanics approach is used to simulate rotor vibration. The case of the cracked continuous rotor is examined by introducing suitable complex boundary conditions. The shaft rotates on two journal bearings th...

Rotating machinery is very common and widely used in the modern industrial world. Steam turbines, compressors, pumps and jet engines are the most known and commonly used rotating machinery. In the application of rotating machines, one of the key problems to be solved, is the safety and stability concerning rotor dynamics. Active hydromagnetic jour...

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 beari...

During operation of rotating machines, journal and bearing mechanisms are progressively worn down. To prevent catastrophic failure of a rotating system, it is necessary both to detect wear precisely, without shutting down and dismantling the machinery and to predict future replacement needs. In this work, Computational Fluid Dynamics (CFD) analysis...

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 dir...

An Active Hydromagnetic (hydrodynamic and active magnetic) Journal Bearing (AHJB) is designed in this work, giving thus a new kind of hybrid journal bearing. The two separate parts are included in a common bearing shell and operate together as an individual machine element, which is the AHJB. The present design is an attempt to combine the advantag...

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 vari...

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...

The dynamic behavior of cracked rotors continues to attract the interest of both designers and maintenance engineers. In this work, a continuous mechanics approach is used to simulate rotor vibration. The case of the cracked continuous rotor is examined by introducing suitable complex boundary conditions. The shaft rotates on two journal bearings t...

Design of smart journal-bearing systems is an important issue that opens up the possibility for semi-active dynamic control of bearing behavior. Recent studies show that there is an increasing interest in designing hydrodynamically lubricated bearings using electro-rheological fluids (ERFs) or magneto-rheological fluids (MRFs). Both smart fluids be...

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...

Strong nonlinearities in mechanical systems usually lead to the formulation of dynamical systems by ways expressing the physical phenomena in an adequate ap-proximation. 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 f...

This paper considers the dynamic behavior of a shaft with two transverse cracks characterized by three measures: position, depth and relative angle. Both cracks are considered to lie along arbitrary angular positions with respect to the longitudinal axis of the shaft and at some distance from the clamped end. A local compliance matrix of two degree...

Friction occurs in all mechanical systems such as transmissions, valves, piston rings, bearings, machines, etc. It is well known that in journal bearings, friction occurs in all lubrication regimes. However, shaft misalignment in rotating systems is one of the most common causes of wear. In this work, the bearing is assumed to operate in the hydrod...

The strain energy release rate (SERR) theory, combined with Linear Fracture Mechanics and Rotordynamics theories, has been widely used over the last three decades in order to calculate the compliance that causes a transverse surface crack in a rotating shaft. In this paper, the basic theory of this approach is presented, along with some extensions...

Significant wear occurs on the surface of bearings, which support rotating shafts for long periods of time. Therefore, the need for a new clearance identification method is required. Furthermore, having identified these new clearances, the investigation of gauging reliable future operation with regards to stability is also an important task.The rot...

This paper investigates the coupled bending vibrations of a stationary shaft with two cracks. It is known from the literature that, when a crack exists in a shaft, the bending, torsional, and longitudinal vibrations are coupled. This study focuses on the horizontal and vertical planes of a cracked shaft, whose bending vibrations are caused by a ver...

Two of the most common defects in rotating systems are abnormal wear of the bearing bushing and bearing misalignment. The present paper introduces a new fault diagnosis model that uses artificial neural networks (ANN) in order to identify the increase of wear depth and/or the increment of the misalignment angle. Reynolds equation is solved by FEM a...

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...

The combination of a continuously modeled cracked rotor with nonlinear finite fluid film bearings is the main objective of this work. The rotor shaft was modeled using the Rayleigh equation, since the bearings using the Reynolds equation, solved using the finite difference method. Additionally the internal damping effect of the rotor is incorporate...

The main objective of this paper is the examination of the possibility to identify the wear percentage of the bearings that support a step rotor, applying external excitation on the rotor and analyzing its response. The rotor vibration was modeled using the equation of Rayleigh, and the two nonlinear bearing forces were computed by solving the Reyn...

During the operation of the rotating machines, journal and bearing progressively are worn down. To prevent catastrophic failure of the bearing, it is necessary to detect exactly the wear of the bearing predicting its future replacement. Unfortunately, detecting and evaluating the extent of wear upon the bearing members requires dismantling of the m...

In this paper the damping is examined as an engineering property used in analysis and design of structures and machines. The design engineer needs to know not only the stresses of his structure or machine, under steady state conditions but also the stresses under resonance conditions. Then the material damping, as a function of the stress of the st...

Both wear and misalignment are undesirable defects for a rotor bearing system. The wear is a situation caused during a long period of operation with start-ups and shutoffs of a rotor-bearing system when overload are frequent. The misalignment could be linear or angular, due to bad assembly or due to overloaded long shafts during the operation. It i...

The identification of a transverse crack on a beam is the subject of many investigators. Identifying the crack means to find its position and depth. In many cases there are more than one cracks on a beam. Then the solutions, or the combinations of parameters characterising the cracks are more and the problem becomes more complicated particularly wh...

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 equa...

During the last decades, the engineering community has extensively studied crack identification in rotating machine elements. Although the proposed analytical models may be capable of identifying cracks based on modal analysis, response measurements, or other techniques, the required time for performing the underlying computations is restrictive in...

The calculation of the local flexibility of cracked shafts is discussed. It is observed that a cracked shaft can be represented and calculated by a double integral of the strain energy density function over the crack surface. It is also observed that the local flexibility of a crack depth equal to the diameter of the shaft is found to be infinite....

In this paper a new method is applied in rotating cracked shafts to identify the depth and the location of a transverse surface crack. A local compliance matrix of different degrees of freedom is used to model the transverse crack in a shaft of circular cross section, based on available expressions of the stress intensity factors and the associated...

A rotor bearing system is expected to exhibit large vibration amplitudes when subjected to a large seismic excitation. It is possible that these vibrations can lead to large values the eccentricity of the bearings. Then the bearing is operated in highly nonlinear region because the stiffness and the damping coefficients are nonlinear as functions o...

The problem of journal bearings control is of great importance in mechanical engineering. A very recent method for doing this is the creation of ‘smart’ journal bearings using electro-rheological (ER) fluids. If such a fluid is used to lubricate a journal bearing system, it is expected that the imposition of an electric field between the rotor and...

If an electro!rheological "ER# ~uid is used in viscous coupling devices\ such as clutches or brakes\ then the possibility to control the output capacity of this device exists[ The imposition of an electric _eld between the input and output parts alters the viscosity of the ~uid and consequently the output torque for brakes or angular velocity for t...

In this paper a new method for crack identification in beams is presented. The transverse surface crack is considered to remain always open. Using this method the crack location and depth can be determined. The method is based on the basic observation that the eigenmodes of any cracked structure are different from those of the uncracked one. The di...

In this paper the problem of crack depth and position identification in frame structures is examined, using eigenfrequency measurements. It has been established that a crack has an important effect on the dynamic behavior of a structure. This effect depends mainly on the crack size and position. The basic idea is to present in contour graph form th...

The controllable rheological behaviour of Electro-Rheological Fluids (ERF) is useful in engineering systems where variable performance is required. In particular the ERF have properties which are changed by an imposed electric field. In this paper a generalised Reynolds equation is solved using finite element techniques in order to simulate the beh...

One of the most important problems of rotor-bearing systems, the stability problem, especially in the case that the rotor is in misaligned position in respect with the bearing, is examined in this paper. Lyapunov's direct method was preferred in this analysis instead of a classical eigenvalue analysis in order to obtain the stability conditions in...

In this paper a new method for the determination of the depth and the location of a transverse surface crack in a beam is presented. Simulating the crack by a local compliance matrix of six degrees of freedom has been used for crack modelling in the last decade. This matrix contains diagonal and non-diagonal terms, and thus coupling among different...

The stability of nonlinear misaligned rotor-bearing systems is investigated, using the Lyapunov direct method. A finite element formulation is used to determine the journal bearing pressure distribution. Then the linear and nonlinear stiffness, damping, and hybrid coefficients are calculated. A general method of analysis based on Lyapunov's stabili...

An analysis of hydrodynamic Newtonian lubrication for misaligned journal bearings is given. The primary aim of this paper is to model the non-linearities that occur in a hydrodynamic journal bearing in both aligned and misaligned conditions, and to present the non-linear stiffness, damping and hybrid coefficients in the form of diagrams that can be...

The torsional vibrations of a rotor with a transverse crack are investigated. The crack is modelled by way of a local flexibility matrix which is calculated analytically and measured experimentally. Good agreement between the two methods is obtained. The free vibration problem is solved and the three first eigenvalues are plotted versus the crack l...

The coupling of vibration modes of vibration of a clamped-free circular cross-section Timoshenko beam with a transverse crack is investigated in this paper. A 6 x 6 local flexibility matrix is used to simulate the crack. The nondiagonal terms of this matrix cause coupling between the longitudinal, torsional, and bending vibrations. Coupling is appa...

It is well known that in vibrating cracked beams and shafts, coupling of bending and torsional vibration exists, due to the non-diagonal terms of the local compliance matrix. This phenomenon becomes more important as the slenderness ratio decreases. In this paper the influence of the slenderness ratio of a non-rotating shaft (i.e., a beam) with a c...

A transverse surface crack is known to add to a shaft a local flexibility due to the stress-strain singularity in the vicinity of the crack tip. This flexibility can be represented, in the general case by way of a 6 multiplied by 6 compliance matrix describing the local flexibility in a short shaft element which includes the crack. This matrix has...

This paper describes the coupling of bending and longitudinal vibration of a stationary cracked shaft with an open crack. The crack is modeled by way of a 2 multiplied by 2 local flexibility matrix with coupling terms. The elements of this matrix are obtained analytically. One of the elements compares well with experimental data of other investigat...

The coupling of longitudinal and bending vibrations of a rotating shaft, due to an open transverse surface crack is investigated. The assumption of the open crack leads to a system with behaviour similar to that of a rotor with dissimilar moments of inertia along two perpendicular directions. The local flexibility due to the presence of the crack c...

A transverse surface crack is known to add to the shaft a local flexibility due to the stress-strain singularity in the vicinity of the crack tip. This flexibility can be represented by way of a 6 × 6 matrix describing the local flexibility in a short shaft element which includes the crack. This matrix has off-diagonal terms which cause coupling of...

A transverse surface crack is known to add to a shaft a local flexibility due to the stress-strain singularity in the vicinity of the crack tip. This flexibility can be represented, in the general case by way of a 6 multiplied by 6 compliance matrix describing the local flexibility in a short shaft element which includes the crack. This matrix has...

Cracked rotating shafts exhibit a certain particular dynamic response due to the local flexibility of the cracked section. In this response, most of the features of the response of a shaft with dissimilar moments of inertia can be identified. Moreover, the non-linear behavior of the closing crack introduces the characteristics of non-linear systems...

Newest AUVs have been proposed for inspection and maintenance tasks. These tasks imply a certain degree of autonomy also in target approaching and manipulation. This paper deals with this later issue: the manipulation in the frame of a maintenance task performed from an AUV platform. Due to high delay and low bandwidth communication systems, the ro...