Air Foil Bearings Research Articles

A computationally efficient nonlinear foil air bearing model for fully coupled, transient rotor dynamic investigations

A novel physical and mathematical model of foil air bearings is developed with emphasis on computational time-efficiency for rotor dynamic investigations. The structural model accounts for the nonlinear deformation behavior, assumes linear viscous damping and is directly coupled to the fluid model. Galerkin's method is applied to the non-dimensional model to reduce its dimension and thus its computational costs. The model is validated via published experimental data and the reduction technique by comparing with the unreduced model. The reduction approach decreases the computation time up to a factor 1000, with a slight loss in accuracy (error of 0.01 in average). Finally, the bearing model is coupled with a rigid rotor model to show the applicability during a run-up and coast-down.

Specialized Coatings for Extreme Environments

Abstract Triboglide coatings offer enhanced wear resistance in applications like air foil bearings, actuators, and wear components where oil-based lubrication is scarce. They exhibit excellent wear and friction properties compared to other commercially available coatings.

Electromagnetic torque and strength analysis of combination rotor in high speed PMSM

For a high-speed permanent magnet synchronous motor supported by air foil bearings, analyzing the influence of the electromagnetic torque on the strength of the combination rotor was necessary. In this study, the electromagnetic torque was obtained using the analytical calculation and the finite element simulation. The contact interface stress analytical equation affected by the electromagnetic torque was elaborated. A three-dimensional finite element model simulating the combination rotor was constructed, and the stress calculations were performed. The strength of the combination rotor met the design requirement. It properly reflected the characteristics of the electromechanical analysis in the combination rotor. The operation experiments were realized to verify the electromagnetic performance and the strength of the combination rotor. The simulation and experimental results were significant for the electromechanical design and analysis of the combination rotor.

수소전기차용 공기압축기 에어포일 스러스트 베어링 2-FSI 해석

A hydrogen-electric car is a fuel-cell vehicle that reacts hydrogen fuel with oxygen in the air to generate electricity to drive the traction motor. An air compressor is a device that compresses air and supplies it to the fuel cell stack. These generally use two types of airfoil bearings: The first is journal bearing, the second a thrust bearing. In this paper, axial thrust bearings are analyzed using 2-Way fluid-structure interactions (FSI). To determine the validity of the 2-Way FSI analysis, a solid thrust bearing was fabricated and the torque was measured. For 60 kr/min 70 kr/min and 80 kr/min rotations, loads were applied at 10 N, 15 N, and 20 N and the resulting torque was measured. For the 2-Way FSI analysis, a 1/6 split model was selected, the turbulence model was Menter’s Shear Stress Trasport (SST), and 10 meshes were made in the air gap using a system coupling condition moving the solid and fluid parts simultaneously. The 2-Way FSI analysis result showed the error within 7.3 %, proving its validity.

Analysis of high speed permanent magnet synchronous motor rotor supported by air foil bearings

Analysis of high speed permanent magnet synchronous motor rotor supported by air foil bearings

Multiphysics analysis of a high speed PMSM for electric turbo charger

The ETC (Electric Turbo Charger) consists of high speed PMSM, ball or air foil bearings, impeller and controller. KERI (Korea Electrotechnology Research Institute) is developing a high speed surface permanent magnet (SPM) type of synchronous motor and a Pulsed Width Modulation (PWM)-driven inverter . This system operates at a power density of 3 kW/kg at 100,000 rpm and is intended to fit the 1,600 cc diesel vehicles to reduce turbo-lag within 0.5 s. The design and analysis of the PMSM for the ETC has been developed successfully considering the several losses and current overload rates using transient computational fluid dynamics (CFD).

Effect of Circumferential Location of Radial Injection on Rotordynamic Performance of Hybrid Air Foil Bearings

Air foil bearings (AFBs) are introduced as promising bearings for oil-free turbomachinery applications. AFBs provide reliable operation at high speed and high temperature with negligible power loss. Hybrid air foil bearing (HAFB) technology utilizes the radial injection of externally pressurized air into the traditional hydrodynamic AFB's film thickness through orifices attached to the top foil. Previous studies have reported enhancement in the rotordynamic stability of HAFBs compared to traditional hydrodynamic AFBs. HAFBs have several orifices distributed in the circumferential direction. In this study, the effect of the circumferential location of radial injection on the rotordynamic performance of the rotor-HAFB is studied. Analytical and experimental evaluations of the rotordynamic performance of a rotor supported by two single-pad HAFBs are presented. Parametric studies are conducted using three sets of single-pad HAFBs. The circumferential locations of orifices are different for each set. The presented simulation analyses consist of time-domain orbit simulation and frequency-domain modal analysis. Imbalance responses of rotor-HAFB were measured with various orifice locations and the results agree well with predictions. Comparison of the rotordynamic performance of HAFBs with different orifice configurations demonstrates substantial improvement in rotordynamic stability as well as enhancement in the stiffness and damping coefficients of HAFBs by choosing the best circumferential location for radial injection to control rotor eccentricity and attitude angle.

Effect of bump structural friction on the performance of bump foil bearing and rotor dynamic behavior: Experimental study

Air foil bearings are a kind of self-acting lubricated bearing, which have potential applications in high-speed turbomachinery. Air foil journal bearings have simple structure that consists of top foil, bump foil, and bearing sleeve, and use gas as working fluid. However, the relative motion of top foil and bump foil, bump foil and bearing sleeve occurs when dynamic pressure is generated as long as there is spinning of shaft. Thus, the friction between each part of bearing should be considered when modeling. Many papers have theoretically shown the effect of Coulomb friction between top foil and top foil, bump foil and bearing sleeve on the static and dynamic performances of air foil journal bearing by developing many bump foil structural models. The results show that this foil structural Coulomb friction can significantly make bump foil stiffer. However, the improvement of the Coulomb friction effect through experiments is quite difficult and there is no study regarding this so far. The purpose of this paper is to certify the Coulomb effect on bearing performance by using experimental method. Two bump foil journal bearings are manufactured with different bearing sleeve surface roughnesses. Foil structure stiffness, bearing lift-off speed, and rotor dynamic behavior supported by two sets of bearings are measured and compared.

Feasibility study of a turbocharger rotor supported by air foil bearings with diameter of 17 mm focusing on rotordynamic performance

This paper presents the feasibility study of an oil-free turbocharger with journal bearing diameter of 17 mm. Rotordynamic performance of a turbocharger rotor, supported by two identical bump-type foil journal bearings and a pair of foil thrust bearings, is predicted and tested. High-pressure cold nitrogen is adopted to drive the turbocharger rotor. In rotordynamic analysis, the critical speeds and logarithmic decrement of the turbocharger rotor are predicted by employing the finite element method, in which the stiffness and damping coefficients of foil journal bearings and aerodynamic cross-coupled stiffness of the turbine are taken into account. Compared with experimental results, the accuracy of the prediction for rotordynamic analysis is verified for 7.82% marginal error of the critical speed. During the experiment, three foil journal bearings with different nominal clearances are manufactured and tested. The maximum stable operating speed reaches 105,000 r/min for this 17-mm-diameter oil-free turbocharger rotor system. Test results indicate the nominal clearance has a negative influence on threshold speed of sub-synchronous motions. When the nominal clearance is relatively small, the foil journal bearing could not lift off due to a large starting torque, while sub-synchronous motions would emerge under a large nominal clearance because of the reduced stiffness and damping coefficients of foil journal bearings.

Performance of Thrust Airfoil Bearing for Oil-Free Turbomachinery

IntroductionDuring operation of any turbomachinery, the pressure distribution on rotor parts, i.e., impellers, causes an occurrence of radial and thrust forces. These forces must be transferred by the bearings to the machine casing. One of the most promising bearing types that can be applied in modern high-speed turbomachinery is the airfoil bearings. They have low losses, they can operate at relative high rotational speed, and they usually are lubricated with machine working medium. On the other hand, they have relative low bearing load capacity (DellaCorte and Valco in Tribol Trans 43:795–801, 2000; Radil and DellaCorte in Tribol Trans 53, 771–778, 2010). This problem especially concerns the airfoil thrust bearing.ObjectiveThe article describes the performance and operational properties of a thrust airfoil bearing on the test stand.MethodThe test stand consisted of vertical axis electric motor with thrust plate attached to the shaft. The test covered a wide range of loads and rotational speeds. At these various conditions, the bearing frictional torque was measured. Indirectly, the power loses were calculated depending on the applied load and the shaft rotational speed. At last, after the test, the wear of top foil coating was inspected visually.

FEM Design of a Cutting-Edge Support System for Micro-GT

The design of the support system (shaft, bearings, and mechanical coupling devices) of the rotor plays a key role in the development of efficient micro-gas turbines (micro-GTs) for distributed power generation. Foil air bearings are the most widespread technical solution well suited to design a reliable support system, although they cannot withstand a large number of start-stop cycles of the units. In order to overcome such limitation, we have recently proposed an innovative support system that takes advantage of spline couplings and two bearing types (e.g., air and rolling-element bearings). The devised support system employs splines as both convenient coupling systems and actuators for the load partition between the two bearing types. In the present work, the helical spline coupling is studied by means of structural FEM analyses including contact simulation in order to design the support system. Numerical results confirm previous findings in that the load transfer through the spline coupling is mainly a function of the helix angle. In addition, friction factor and structural stiffness cannot be neglected in the accurate design of the spline coupling. Such design parameters are now included in the proposed design procedure, which formerly assumed frictionless contact and rigid bodies.

Rotordynamic Performance of Hybrid Air Foil Bearings With Regulated Hydrostatic Injection

Air foil bearing (AFB) technology has made substantial advancement during the past decades and found its applications in various small turbomachinery. However, rotordynamic instability, friction and drag during the start/stop, and thermal management are still challenges for further application of the technology. Hybrid air foil bearing (HAFB), utilizing hydrostatic injection of externally pressurized air into the bearing clearance, is one of the technology advancements to the conventional AFB. Previous studies on HAFBs demonstrate the enhancement in the load capacity at low speeds, reduction or elimination of the friction and wear during starts/stops, and enhanced heat dissipation capability. In this paper, the benefit of the HAFB is further explored to enhance the rotordynamic stability by employing a controlled hydrostatic injection. This paper presents the analytical and experimental evaluation of the rotordynamic performance of a rotor supported by two three-pad HAFBs with the controlled hydrostatic injection, which utilizes the injections at particular locations to control eccentricity and attitude angle. The simulations in both time domain orbit simulations and frequency-domain modal analyses indicate a substantial improvement of the rotor-bearing performance. The simulation results were verified in a high-speed test rig (maximum speed of 70,000 rpm). Experimental results agree with simulations in suppressing the subsynchronous vibrations but with a large discrepancy in the magnitude of the subsynchronous vibrations, which is a result of the limitation of the current modeling approach. However, both simulations and experiments clearly demonstrate the effectiveness of the controlled hydrostatic injection on improving the rotordynamic performance of AFB.

A Contribution to the Thermal Modeling of Bump Type Air Foil Bearings: Analysis of the Thermal Resistance of Bump Foils

A detailed analysis of the effective thermal resistance for the bump foil of air foil bearings (AFBs) is performed. The presented model puts emphasis on the thermal contact resistances between the bump foil and the top foil as well as between the bump foil and the base plate. It is demonstrated that most of the dissipated heat in the lubricating air film of an air foil bearing is not conducted by microcontacts in the contact regions. Instead, the air gaps close to the contact area are found to be thin enough in order to effectively conduct the heat from the top foil into the bump foil. On the basis of these findings, an analytical formula is developed for the effective thermal resistance of a half bump arc. The formula accounts for the geometry of the bump foil as well as for the surface roughness of the top foil, the bump foil, and the base plate. The predictions of the presented model are shown to be in good agreement with measurements from the literature. In particular, the model predicts the effective thermal resistance to be almost independent of the applied pressure. This is a major characteristic property that has been found by measurements but could not be reproduced by previously published models. The presented formula contributes to an accurate thermohydrodynamic (THD) modeling of AFBs.

A fully coupled air foil bearing model considering friction – Theory & experiment

The dynamics of air foil bearings (AFBs) are not yet fully captured by any model. The recent years have, however, seen promising results from nonlinear time domain models, and simultaneously coupled formulations are now available, avoiding the previous requirements for undesirably small time steps and temporal convergence studies.In the present work, an alternative foil structure model is substituted for the simple elastic foundation model to avoid its inherent limitations. The new foil model is based on a truss representation from the literature, but incorporates the foil mass and a dynamic friction model. As a consequence of the friction model's velocity dependency, the foil mass is included to obtain a set of differential equations that can be coupled to the rotor and fluid domains while allowing a simultaneous solution.Considerations leading to a practically applicable implementation are discussed and numerical results are compared with experimental data. The model predicts natural frequencies and mode shapes well, but it is not yet capturing the unbalance response when friction is considered. Possible causes for this discrepancy are discussed and it is suggested that sticking is a more prevalent state than previously assumed.

Transient and steady state behaviour of elasto–aerodynamic air foil bearings, considering bump foil compliance and top foil inertia and flexibility: A numerical investigation

This work gives a theoretical contribution to the problem of modelling air foil bearings considering large sagging effects in the calculation of the non-linear transient and steady state response of a rigid rotor. This paper consists of two parts: the development of a miltiphysics model of the air foil bearing, and a numerical parameter study of a rigid journal supported in an air foil bearing with a partially supported top foil. The mathematical model of the air foil bearing is centred around the finite element models of both the air film and the top foil structure. These finite element models utilise two types of eight-node isoparametric elements. The rotor is modelled as a rigid body without rotational inertia, i.e. as a journal. The bump foil is included via a bilinear version of the simple elastic foundation model. This paper introduces the bilinear simple elastic foundation model, which combined with the top foil structure model, enables a separation of the top foil and the bump foil. A phenomenon associated within areas of the top foil is where the aerodynamic pressure is sub-ambient. The parameter study investigates the performance of three air foil bearings with partially supported top foils and one air foil bearing with a fully supported top foil. The steady state responses of a journal supported by these air foil bearings are investigated for varied rotational speeds and journal unbalances as well as the top foil sagging in the unsupported area. The study reveals that sub-harmonic vibrations associated with a large journal unbalance can be eliminated by a proper design layout of the bump foil, i.e. placement of the unsupported area. The positive effect is attributed to ‘equivalent shallow pockets’ formed by the sagging top foil.

Airfoil Bearing이 장착된 초고속 BLDC 모터 제어

The BLDC motor is used widely in industry due to its controllability and freedom from maintenance because there is no mechanical brush in the BLDC motor. Furthermore, it is suitable for high-speed applications, such as compressors and air blowers. For instance, for a compressor with a small impeller due to miniaturizing, the BLDC motor has to rotate at a very high speed to maintain the compression ratio of the compressor. Typically, to reach an ultra-high speed, airfoil bearings must be used in place of ball bearings because of their friction. Unfortunately, the characteristics of airfoil bearings change drastically depending on the revolution speed. In this paper, a BLDC motor with airfoil bearings is controlled with a PID controller. To analyze and determine the PID coefficients, the relay-feedback method is used. Additionally, for adaptive control, a fuzzy logic controller is used. Furthermore, the auto-tuning and self-tuning techniques are combined to control the BLDC motor. The proposed method is able to control the airfoil-bearing BLDC motor efficiently.

Rotordynamic Performance of a Shaft With Large Overhung Mass Supported by Foil Bearings

This work presents the theoretical and experimental rotordynamic evaluations of a rotor–air foil bearing (AFB) system supporting a large overhung mass for high-speed application. The proposed system highlights the compact design of a single shaft rotor configuration with turbomachine components arranged on one side of the bearing span. In this work, low-speed tests up to 45 krpm are performed to measure lift-off speed and to check bearing manufacturing quality. Rotordynamic performance at high speeds is evaluated both analytically and experimentally. In the analytical approach, simulated imbalance responses are studied using both rigid and flexible shaft models with bearing forces calculated from the transient Reynolds equation along with the rotor motion. The simulation predicts that the system experiences small synchronous rigid mode vibration at 20 krpm and bending mode at 200 krpm. A high-speed test rig is designed to experimentally evaluate the rotor–air foil bearing system. The high-speed tests are operated up to 160 krpm. The vibration spectrum indicates that the rotor–air foil bearing system operates under stable conditions. The experimental waterfall plots also show very small subsynchronous vibrations with frequency locked to the system natural frequency. Overall, this work demonstrates potential capability of the air foil bearings in supporting a shaft with a large overhung mass at high speed.

Effect of Operating Conditions on the Elastohydrodynamic Performance of Foil Thrust Bearings for Supercritical CO2 Cycles

In this paper, a quasi-three-dimensional fluid–structure model using computational fluid dynamics for the fluid phase is presented to study the elastohydrodynamic performance of foil thrust bearings for supercritical CO2 cycles. For the simulation of the gas flows within the thin gap, the computational fluid dynamics solver Eilmer is extended, and a new solver is developed to simulate the bump and top foil within foil thrust bearings. These two solvers are linked using a coupling algorithm that maps pressure and deflection at the fluid structure interface. Results are presented for ambient CO2 conditions varying between 0.1 and 4.0 MPa and 300 and 400 K. It is found that the centrifugal inertia force can play a significant impact on the performance of foil thrust bearings with the highly dense CO2 and that the centrifugal inertia forces create unusual radial velocity profiles. In the ramp region of the foil thrust bearings, they generate an additional inflow close to the rotor inner edge, resulting in a higher peak pressure. Contrary to the flat region, the inertia force creates a rapid mass loss through the bearing outer edge, which reduces pressure in this region. This different flow fields alter bearing performance compared to conventional air foil bearings. In addition, the effect of turbulence in load capacity and torque is investigated. This study provides new insight into the flow physics within foil bearings operating with dense gases and for the selection of optimal operating condition to suit CO2 foil bearings.

Analysis of the operating load of foil-air bearings in the gas generator of the turbine engine during the acceleration and deceleration maneuver

Analysis of the operating load of foil-air bearings in the gas generator of the turbine engine during the acceleration and deceleration maneuver

The Performance of PS400 Subjected to Sliding Contact at Temperatures from 260 to 927°C

ABSTRACTAdequate high-temperature lubrication between loaded surfaces in sliding contact can be one of the most challenging tribological problems confronting today's designers. In an attempt to provide a possible solution a test program was initiated to evaluate PS400, a recently patented, high-temperature solid lubricant coating. Made from nickel–molybdenum–aluminum, chrome oxide, silver, and barium fluoride–calcium fluoride, PS400 is a variant of the earlier coating, PS304, but is formulated for higher density, smoother surface texture, and greater dimensional stability. It was initially developed to minimize the start–stop wear in foil air bearings but is expected to perform well in other high-temperature applications where sliding friction and wear are a concern, such as variable inlet guide vanes and process control valve stems. To better define its operational capabilities, a series of tests was conducted to study the behavior of PS400 under reciprocating sliding contact at temperatures from 260 to 927°C. The tests were performed on stationary, uncoated cobalt-based superalloy bushings loaded against reciprocating PS400-coated shaft specimens in a flat-on-cylinder configuration at Hertz contact pressures from 14.1 to 20.1 MPa. For tests conducted below 927°C, friction coefficients ranged from 0.37 to 0.84 with wear factors on the order of 10−5 and 10−6 at the high temperatures but substantially increased at the lowest temperature. Data collected at 927°C were limited because the coating was found to be dimensionally unstable at this temperature.