Fluid Film Bearings Research Articles

Optimally controlled anti-swirl injection to eliminate whirl in fluid-film bearings of rotary machinery

The phenomenon of fluid-induced instability existing in fluid-film bearing systems has been coped with for long time. The study aims to soothe and even eliminate the occurrence of whirl in rotary machinery by increasing the threshold of instability through the anti-swirl injection using the linear quadratic regulator based optimal control. An acceptance region was established in order to decide starting up the control process. Three case studies were carried out to illustrate the effectiveness of the control scheme. The research results demonstrate that the control scheme incorporating with the acceptance region enables to avoid the occurrence of fluid induced instability in rotary machinery. Moreover, the developed techniques can also be applied in other fluid-induced instability problems such as whip and rub, etc.

Experimental detection of an early developed crack in rotor-bearing systems using an AMB

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 bending vibrations both in the horizontal and vertical plane. Experimental time series of the rotor composite response under normal steady-state operation are uncoupled, to develop a signal processing procedure able to reveal the presence of the crack.Design/methodology/approach– The variation of the coupling property that a crack (breathing or not) or a cut (always open) introduces into the system and the localization of the coupling in the time domain is a concept proposed as a means to detect transverse surface cracks in rotating shafts. This consideration is combined with the concept of external excitation for the development of an additional crack-sensitive response during system normal operation. Using an external excitation of an active magnetic bearing of specific duration, frequency and amplitude, the method uses this coupling variation during rotation.Findings– The method is simple, quick and effective for early crack detection, being able to detect cracks as shallow as 5 percent of the shaft radius while the system is under normal operation, and can even be applied real-time. Experimental verification uses a simple elastic rotor with a cut mounted on fluid film bearings, with the cut producing similar coupling phenomena as an opened crack. Experimental results are encouraging.Originality/value– The method used is simple, quick and effective for early crack detection, being able to detect cracks as shallow as 5 percent of the shaft radius while the system is under normal operation, and can even be applied real-time.

Friction and Lubrication of Large Tilting-Pad Thrust Bearings

Fluid film bearings have been extensively used in the industry because of their unbeatable durability and extremely low friction coefficient, despite a very low coefficient of friction dissipation of energy being noticeable, especially in large bearings. Lubricating systems of large tilting pad thrust bearings utilized in large, vertical shaft hydrogenerators are presented in this paper. A large amount of heat is generated due to viscous shearing of the lubricant large tilting pad thrust bearings, and this requires systems for forced cooling of the lubricant. In the dominant bath lubrication systems, cooling is realized by internal coolers or external cooling systems, with the latter showing some important advantages at the cost of complexity and also, potentially, lower reliability. Substantial losses in the bearings, reaching 1 MW in extreme cases, are a good motivation for the research and development aimed at reducing them. Some possible methods and their potential efficiency, along with some effects already documented, are also described in the paper.

Experimental Performance Study of a High Speed Oil Lubricated Polymer Thrust Bearing

With the demand for turbomachinery to operate at higher speeds, loads, and power, fluid film bearings that support turbomachinery must be capable of operating in these more demanding applications. Thrust bearings operating at high speeds and loads can experience high surface temperatures and thin fluid film thickness. Typically, babbitt (white metal) is the bearing lining material for most turbomachinery bearings but is limited in operating temperature and allowable film thickness. Polymer based materials are alternative materials that can operate at high temperatures and with thin films and have been in use for many decades in high load applications, such as electric submersible pumps (ESP). Test results of polymer lined thrust bearings subjected to modern turbomachinery speeds and loads are presented and compared to babbitt lined bearings of the same design and under similar conditions. The test results show polymer lined thrust bearings can operate at higher bearing unit loads than babbitt.

Numerical Solution of High-Dimensional Nonlinear Rotor-Bearing Dynamic System with Precise Integration

Through an example of rotor system which has multi-degree of freedom mounted on the nonlinear fluid film bearings, this paper analyzes the precise integration algorithm, a new numerical solution for high–dimensional nonlinear dynamics system. The precise integration method has advantages of long step, high precision and absolute stability for solving nonlinear dynamics equations. To make good use of the method, firstly, the precise integral iterative formula is given and then the mechanism of controlling high precision and efficiency is discussed. The evolution of precise integration method is an algorithm with explicit, simple form, self-start, and fast to solve nonlinear dynamics equations. High power of athwart of Hamiltonian matrix is not needed, so it is convenient in this case. The stability of period response of nonlinear rotor-bearing system is analyzed by employing the precise integration method. The bifurcation rules of the period response of the elastic rotor system with multi-degree of freedom are obtained and the chaos of the system is determined according to the fractal dimension of Poincare mapping of phase space at a certain speed.

Reliability Improvement of Rotor Supports by Combining Rolling-Element Bearings and Fluid-Film Bearings

Reliability of rotating machinery is determined to a considerable degree by the bearing units. For several applications the requirements in rotation speed, bearing load and maximal vibration level are so extreme that neither rolling-element bearings nor fluid-film bearings could provide necessary performance characteristics during all regimes of operation. Hybrid bearings, which are a combination of rolling-element and fluid-film bearings, can improve performance characteristics and reliability of the rotor-bearing systems. The aim of this work is to analyze the advantages and disadvantages of the hybrid bearings. Known real applications of hybrid bearings are discussed. Analysis shows that depending on the application different hybrid bearing types could improve dynamic characteristics and life time of the bearing unit, increase load capacity and DN limit of the rolling-element bearing.

A Review on Active Bearings and Perspectives of Using Them in Rotating Machinery

The paper examines the issues of improving the rotor units by means of using support units with actively changeable characteristics. An overview of the known solutions related to the use of active bearings in various types of turbomachinery is provided. A closer look is given at the design and features of active radial bearings, the main elements of which are fluid film bearings. The results of mathematical modeling of active hybrid bearings are presented. The prospects of the use of this type of supports to improve the dynamic characteristics of rotating machinery, including reducing vibrations caused by various factors, are analyzed. Promising directions of development of active bearings are considered, which primarily involves the modification of system components and rotor motion control system algorithms, including intelligent technologies and artificial intelligence methods.

Prediction of the nonlinear hysteresis loop for fluid-film bearings by numerical continuation

A nonlinear dynamic model of a short journal bearing is used to predict the steady-state motion of the journal and its successive bifurcations in the neighbourhood of the stability critical speed. Numerical continuation is applied to determine the branch of equilibrium point and its bifurcation into stable or unstable limit cycles. It has been found that the unstable limit cycles undergo a single limit point bifurcation whereas the stable limit cycles undergo two successive limit point bifurcations. Thus, the bi-stability domain, the potential jumping from small to large motion and the hysteresis loop motion are predicted.

A novel semi-analytical method for the dynamics of nonlinear rotor-bearing systems

A semi-analytical simulation of a rotor bearing system that consisted of a multi-segment continuous rotor and plain fluid film bearings with finite length is developed in this paper in order to investigate the system's response under the current proposal of simulation. The rotor is simulated using the continuous medium theory and the bearings with finite length follow a very recent analytical simulation that incorporates the analytical solution of the Reynolds equation for the plain, finite journal bearing. The boundary conditions combine the rotor's shearing force and the fluid film forces at the points where the bearings are located, which are expressed analytically with direct integration of the pressure distribution function.A case study of simulating a multi-segment shaft is used in order to compare the current simulation with corresponding simulations using continuous rotor mounted on linear bearings, or finite bearings numerically simulated using the finite difference method. The time response, the amplitude of the response and the phase during passage through resonance are evaluated for these three cases of bearing simulation using numerical procedures and the differences are notified and remarked.

Nonlinear stability analysis of long hydrodynamic journal bearings using numerical continuation

Hydrodynamic bearings are frequently used in applications involving high loads and high speeds. They may however be subjected to oil whirl instability which may cause their failure. For a successful application of fluid film bearings, it is essential to predict the stability boundaries in terms of the bearing characteristics as well as other nonlinear phenomena observed near the stability limits such as stable and unstable limit cycle motion, hysteresis and jumping phenomena.A model of a long balanced hydrodynamic journal bearing is considered in this paper. Numerical continuation is then used to predict the branch of the journal equilibrium point, the Hopf bifurcation point and the emerging stable or unstable limit cycles.Depending on the bearing characteristics, the stability threshold occurs either at a supercritical or at a subcritical Hopf bifurcation. For journal speeds above the supercritical bifurcation, the journal undergoes stable limit cycles. For the stability boundaries due to a subcritical bifurcation, a limit point of cycle bifurcation is found defining the domain of possible journal jumping from the equilibrium position to large limit cycles and hysteresis phenomenon during rotor speed variation near the stability threshold.

On the dynamics of a high-speed coned fluid-lubricated bearing

An incompressible air-flow model for a fluid film bearing is derived using a modified Reynolds equation for the thin-film dynamics of a rapidly rotating rotor and stator. Mathematical and numerical modelling is applied to the coupled processes of the fluid flow through the bearing and the axial motion of the rotor and stator. This work focuses on extending previous studies to incorporate the dynamics of a coned rotor operating at high speeds and an incompressible lubrication approximation. The dynamics of fully coupled, unsteady bearing motion and associated forcing of the rotor with axial periodic oscillations are studied. The axial motion of the stator is modelled as a spring–mass–damper system that responds to the rotor displacement through the film dynamics. In order to solve the modified Reynolds equation and stator equation simultaneously, a new variable is introduced, namely the time-dependent face clearance. This leads to explicit analytical expressions for the pressure and force in terms of the face clearance and the stator equation is transformed to a non-linear, second-order, non-autonomous, ordinary differential equation for the face clearance. Applying a transient solver gives solutions settling down to a stable periodic behaviour which motivates seeking a solver for periodic solutions. A Fourier spectral collocation scheme is derived to compute the periodic time-dependent face clearance. Both solvers have matching periodic solutions of O(1) with an absolute error of order of magnitude 10−5. The dynamics of the unsteady bearing are examined for a range of pressure gradients and configurations including an asymptotic investigation of small face clearance associated with a start-up transient. Results are provided relating to changes in the width of the bearing, strength of the spring holding stator to its housing, damping of the stator and strength of the force coupling and rotor mass. The dynamics of the bearing are also investigated relative to values of key system parameters including the coning of the rotor, rotation speed and value of the bearing squeeze number. A parameter investigation is undertaken to highlight ideal bearing configurations to maximize the load-carrying capacity, fluid stiffness and damping.

J162015 小型HDD用流体軸受スピンドルの揺動応答特性([J162]メカニカルデバイス・システムの知能化:機械の知能化を学際的に考える)

This paper describes the experimental study on swaying impact response of fluid film bearing (FDB) spindle for Hard Disk Drives (HDDs). Recently, the opportunity for the HDD spindle motors exposed to external vibration has been increasing because the HDDs are used for recording device such as slimline laptop, video cameras, car navigation systems and so on. Hence, the rotating shaft contact the bearing from impact, there is a possibility of data loss. In order to avoid the problems, it is extremely important to know how the spindle moves against the large impact on HDDs experimentally. However, as far as the authors know, there are few experimental studies treating the swaying impact response. In this paper, firstly, we propose a new test rig and experimental method for impact response of FDB spindle. Then we conduct the swaying impact test on FDB spindle for 2.5" HDD and show the experimental results of swaying impact response waveform and maximum displacement of disk.

A criterion for mode shape tracking: application to Campbell diagrams

The correlation criterion proposed in this article and called nc2o (normalized cross complex orthogonality) is based on the bi-orthogonality properties between rotor mode shapes calculated at different speeds of rotation. This criterion is proved using an industrial laminated rotor composed of disks and two fluid film bearings, whose characteristics depend on the speed of rotation. The industrial finite element model shows that the nc2o criterion provides a more efficient mode pairing of rotor shapes than those obtained by using classical correlation criteria. Moreover this criterion makes it easier to plot Campbell diagrams for strongly speed-of-rotation-dependent structures.

An active hydrodynamic bearing for controlling self-excited vibrations: theory and simulation

Fluid-film bearings can suffer from flow-induced instabilities known as ‘whirl’ and ‘whip,’ especially when supporting lightly-loaded shafts. This phenomenon can lead to large rotor (self-excited) vibrations, which eventually result in rotor-bearing failure. In this paper, we introduce a new active hydrodynamic bearing to mitigate such vibrations in lightly loaded rotating machines. The system contains a rotating bushing, actuated by a motor, that serves as the control input. This input is used to control the mean flow velocity in the bearing and thereby the journal vibration. A simple feedback control law is proposed for the bushing velocity, and numerical simulations are presented to evaluate the active bearing.

Lateral Vibration Analysis of Flexible Shafts Supported on Elliptical Journal Bearings

This paper presents the development of a finite element procedure for the dynamic analysis of flexible rotors supported on fluid-film elliptical journal bearings operating under several operating conditions. The rotating shaft is modeled by using Timoshenko beam theory and the coupled rotating components, such as disks and impellers, are modeled by using lumped masses. The modeling of the elliptical journal bearings is performed by solving the lubrication equations generated from the application of a linearized perturbation method on the classical Reynolds equation. The bearing carrying-load capacity and the linearized dynamic force coefficients can be predicted for elliptical bearings with different preloads and journal eccentricities. The rotor transient whirling unbalance response is estimated by performing the time integration of the finite element equations using Newmark method. Experimental whirling unbalance response of a rotating shaft supported at two identical journal bearings is used to validate the finite element procedure. A comparative analysis of the dynamic response of flexible rotors supported on both cylindrical and elliptical journal bearings is performed to show that some elliptical bearings are capable of attenuating the rotor unbalance response more efficiently than cylindrical bearings are.

Fuzzy Based Decision Making for Selection of Fluid Film Journal Bearing

This paper presents a theoretical prediction of selection procedure for fluid film journal bearing by incorporating fuzzy approach. In this paper a fuzzy model is proposed for selection of hydrostatic, hydrodynamic and hybrid journal bearing. Selection criteria is formulated for the choice of space requirement, total cost of bearing and load bearing capacity of the related journal bearing. This method provides a third dimension to the existing method of selection of bearing, which is dynamic and may be further refined to address to every individual needs. Therefore a fuzz y logic approach is used for decision making for selection of correct fluid film bearing

Controllable Lubrication for Main Engine Bearings Using Mechanical and Piezoelectric Actuators

Although mechatronic systems are nowadays implemented in a large number of systems in vehicles, active lubrication systems are still incipient in industrial applications. This study is an attempt to extend the active lubrication concept to combustion engines and gives a theoretical contribution to this field. One refers to active lubrication when conventional hydrodynamic lubrication is combined with dynamically modified hydrostatic lubrication. In this study, two different schemes for the oil injection system in actively lubricated main engine bearings are presented. The use of active lubrication in journal bearings helps to enhance the hydrodynamic fluid film by increasing the fluid film thickness and consequently reducing viscous friction losses and vibrations. In this study, the hydrostatic lubrication is modified by injecting oil at controllable pressures through orifices circumferentially located around the bearing surface. The main equations that govern the dynamics of the injection for a piezo-actuated oil injector and a mechanical-actuated oil injector are presented. It is shown how the dynamics of the oil injection system is coupled to the dynamics of the bearing fluid film through equations. The global system is numerically solved using as a case study a single-cylinder combustion engine, where the conventional lubrication of the main bearing is modified by applying radial oil injection using piezo-actuated injection. The performance of such a hybrid bearing is compared to an equivalent conventional lubricated bearing in terms of the maximum fluid film pressures, minimum fluid film thicknesses, and reduction of viscous friction losses.

Nonlinear effects caused by coupling misalignment in rotors equipped with journal bearings

Misalignment is one of the most common sources of trouble of rotating machinery when rigid couplings connect the shafts. Ideal alignment of the shafts is difficult to be obtained and rotors may present angular and/or parallel misalignment (defined also as radial misalignment or offset). During a complete shaft revolution, a periodical change of the bearings load occurs in hyperstatic shaft-lines, if coupling misalignment between the shafts is excessive. If the rotating machine is equipped with fluid-film journal bearings, the change of the loads on the bearing causes also the variation of their instantaneous dynamic characteristics, i.e. damping and stiffness, and the complete system cannot be considered any longer as linear.Despite misalignment is often observed in the practice, there are relatively few studies about this phenomenon in literature and their results are sometimes conflicting. The authors aim at modeling accurately this phenomenon, for the first time in this paper, and giving pertinent diagnostic information. The proposed method is suitable for every type of shaft-line supported by journal bearings. A finite element model is used for the hyperstatic shaft-line, while bearing characteristics are calculated by integrating Reynolds equation as a function of the instantaneous load acting on the bearings, caused also by the coupling misalignment. The results obtained by applying the proposed method are shown by means of the simulation, in the time domain, of the dynamical response of a hyperstatic shaft-line. Nonlinear effects are highlighted and the spectral components of the system response are analyzed, in order to give diagnostic information about the signature of this type of fault.

Rotordynamic Performance of an Oil-Free Turbo Blower Focusing on Load Capacity of Gas Foil Thrust Bearings

Engineered design of modern efficient turbomachinery based on accurate model predictions is of importance as operating speed and rate power increase. Industrial applications use hydrodynamic fluid film bearings as rotor support elements due to their advantages over rolling element bearings in operating speed, system stability (rotordynamic and thermal), and maintenance life. Recently, microturbomachinery (< 250 kW) implement gas foil bearings (GFBs) as its rotor supports due to its compact design without lubricant supply systems and enhanced stability characteristics. To meet the needs from manufacturers, the turbomachinery development procedure includes a rotordynamic design and a gas foil journal bearing (GFJB) analysis in general. The present research focuses on the role of gas foil thrust bearings (GFJBs) supporting axial load (static and dynamic) in an oil-free turbo blower with a 75 kW (100 HP) rate power at 30,000 rpm. The turbo blower provides a compressed air with a pressure ratio of 1.6 at a mass flow rate of 0.92 kg/s, using a centrifugal impeller installed at the rotor end. Two GFJBs with a diameter of 66mm and a length of 50 mm and one pair of GFTB with an outer diameter of 144 mm and an inner diameter of 74 mm support the rotor with an axial length of 493 mm and a weight of 12.7 kg. A finite element rotordynamic model prediction using predicted linearized GFJB force coefficients designs the rotor-GFB system with stability at the rotor speed of 30,000 rpm. Model predictions of the GFTB show axial load carrying performance. Experimental tests on the designed turbo blower; however, demonstrate unexpected large amplitudes of subsynchronous rotor lateral motions. Post-inspection reveals minor rubbing signs on the GFJB top foils and significant wear on the GFTB top foil. Therefore, GFTB is redesigned to have the larger outer diameter of 166 mm for the enhanced load capacity, i.e., 145%, increase in its loading area. The modification improves the rotor-GFB system performance with dominant synchronous motions up to the rate speed of 30,000 rpm. In addition, the paper studies the effect of GFTB tilting angles on the system performance. Insertion of shims between the GFTB brackets changes the bearing tilting angles. Model predictions show the decrease in the thrust load capacity by as large as 86% by increase in the tilting angle to 0.0006 rad (0.03438 deg). Experimental test data verify the computational model predictions.

Model Based Analysis of Coupled Vibrations Due to the Combi-Bearing in Vertical Hydroturbogenerator Rotors

The studies presented in this paper focus on analyzing how the combined thrust-journal bearing (commonly called combi-bearing) influences the dynamics of hydropower rotors. Thrust bearing is a component used in vertical rotating machinery and shafts designed to transmit thrust. The total axial load is carried by the single thrust bearing. Any design, manufacture, or assembly error in this component (thrust bearing) would certainly influence the functionality of the entire machine. The analyzed combi-bearing is an existing machine component used in the hydropower unit Porjus U9 situated in northern Sweden. This combi-bearing is a fluid-film lubricated tilting-pad thrust and journal bearings combined together. Only linear fluid-film stiffness was taken into account in the model while fluid-film damping and pads inertia effects were not taken into account. The linearized model shows that the combi-bearing couples the rotor’s lateral and angular motions. However, if the thrust bearing’s pads arrangement is not symmetrical or if all the pads are not angularly equidistant the rotor axial and angular motions are also coupled. This last case of coupling will also occur if the thrust bearing equivalent total stiffness is not evenly distributed over the thrust bearing. A defective pad or unequal hydrodynamic pressure distribution on the pads’ surfaces may be the cause. The Porjus U9’s simulation results show that the combi-bearing influences the dynamic behavior of the machine. The rotor motions’ coupling due to combi-bearing changes the system’s natural frequencies and vibration modes.