Compliant Foil Bearings Research Articles

Design guidelines for bump-type compliant conical foil bearing

The integral bump foil strip cannot optimize the performance for the compliant conical foil bearing (CFB) as the uneven distribution of structural stiffness. To maximize the bearing characteristics, this paper proposed different bump foil schemes. Firstly, the anisotropy of CFB was studied based on the nonlinear bump stiffness model, and the circumferentially separated foil structure was proposed. Moreover, an axially separated bump foil structure with the variable bump length was introduced to make the axial stiffness distribution more compliant with the gas pressure. In addition, the effect of foil thickness was also discussed. The results show that CFB with integral bump foil exhibits obvious anisotropy, and the suggested installation angle for largest load capacity and best dynamic stability are in the opposite position. Fortunately, a circumferential separated bump foil can improve this defect. The characteristics of CFB with axial separated foil structure can be improved significantly, especially for that with more strips and the variable bump half-length design. The suitable bump and top foil thickness should be set considering the improved supporting performance and proper flexibility. The results can give some guidelines for the design of CFB.

On the Design, Manufacture, and Premature Failure of a Metal Mesh Foil Thrust Bearing—How Concepts That Work on Paper, Actually Do Not

Oil-free microturbomachinery (OFT) implements compliant foil bearings because of their minute drag and ability to operate in extreme (high or low) temperature. Prominent to date, bump-foil thrust bearings integrate an underspring thin metal structure that provides resilience and material damping, and while the rotor is airborne, it acts in series with the stiffness and damping of the gas film. The design and manufacturing of foil bearings remain costly as it demands extensive engineering and actual experience. Alternative foil bearing configurations, less costly and easier to manufacture, are highly desirable to enable widespread usage of OFT. This paper details the design and manufacturing of a novel Rayleigh-step metal mesh foil thrust bearing (MMFTB) as well as its testing on a dedicated rig. Metal mesh structures offer significant material structural damping and can be easily procured at a fraction of the cost of a typical bump-foil strip layer. The MMFTB consists of a solid carrier, a number of stacked annular copper mesh sheets (wire diameter = 0.25, 0.3, and 0.41 mm), and a steel top foil (0.127 mm thick) that makes six pads (ID = 50.8 mm, OD =2 ID), each 45 deg in extent. A 3 μm polymer coats each pad, and a photochemical process etches a step 20 μm in height. Static and dynamic load measurements (without rotor speed) demonstrate that the MMFTB has structural stiffness and material damping similar to that of a publicized bump-type foil thrust bearing. A maiden test of the MMFTB with rotor speed of Ω = 15 krpm (∼80 m/s at bearing outer diameter (OD)) proved briefly the bearing operation when applying a tiny thrust load. Further tests with ambient air, a rotor speed of 40 krpm (∼212 m/s at bearing OD), and a very light load/area <7 kPa failed several of the prototype bearings, all exhibiting significant wear on one or more pads. The source of the failure is the inherent unevenness of the metal mesh stacked substructures, thus causing the pads to bulge toward the rotor collar surface before a load applies. A deficient anchoring method exacerbates the unevenness. Incidentally, a high rotor speed induced large windage that lifted the top foils pushing them against the spinning collar. As the bearing moved toward the rotating collar to begin applying thrust, the local high spots rubbed against the collar before a hydrodynamic wedge could form to separate the surfaces. Without a robust sacrificial coating, metal-to-metal contact quickly disfigured the contacting top foil pads, erasing the etched step, and leading to failure. In concept, and on paper, the mesh sheets and the top foil lay flat against the bearing carrier, giving a false sense of uniformity in the design process. In actuality, a designer must consider the manufactured states of the individual components and how they assemble. A redesign of the bearing intends to overcome the existing flaws (highlighted herein) by incorporating a thicker top foil that is well anchored (to better withstand the effects of windage), a robust sacrificial coating, and a hydrodynamic wedge accomplished via a circumferential taper on each pad.

The Wing Foil: A Novel Compliant Radial Foil Bearing Design

To meet the challenging demands for high performance, affordable compliant foil bearings (CFBs), a novel compliant support element has been developed. This recently patented, novel support element uses a multidimensional array of multiple, formed, cantilever “wing foil” tabs. The wing foil bearing has all the features required to achieve state-of-the-art performance (Gen III for radial bearings). This paper describes two radial foil bearings using the wing foil and the unique design features. Test data for a 31.75 mm diameter bearing operating in air and in steam up to 42 krpm are presented to demonstrate the performance of this bearing. It is shown to have low subsynchronous vibration and reasonable damping through rigid shaft critical speeds.

On the manufacturing of compliant foil bearings

The fabrication of foil bearings is challenging due to the dependence on sheet metal forming to produce the compliant structure. This paper is an attempt to shed light into the foil bearing manufacturing know-how. Design of experiments techniques are used to quantify the effects of the different manufacturing parameters, as well as defining an optimum manufacturing procedure. The effect of manufacturing noise on the static performance of foil bearings is quantified using a Monte Carlo simulation of a bump foil stiffness model. A non-intrusive optical measurement technique which has been developed to measure the formed bump foils is also presented. An uncertainty quantification was performed for the produced foils, showing large uncertainty in the bump dimensions, which significantly affect both nominal bearing clearance and compliance. Finite element simulations are used to model the bump foil forming process that would present potential problems during fabrication, suggesting sharp bends in the bump foil as the main driver for manufacturing deviations. Based on this outcome an improved design for the compliant bump foil with reduced curvature is proposed, manufactured and measured. The novel design allows to reduce springback error by 69% compared to classical bump foils and thus offers an equivalent yet more robust foil bearing design.

Structural Stiffness of X-750 Alloy Bump Foil Strips for Compliant Foil Bearings With Different Heat Treatments

This paper focuses on the structural stiffness of bump foils which are used for compliant foil bearings with different heat treatments. After heat treatments in vacuum environments, the mechanical properties of the foil strips were tested, and the structural stiffness was estimated from the static load versus displacement curves obtained from the experiments. High cycle dynamic load tests were also applied to the bump foil under different cycle loads, and the shape of the foil was scanned after the tests to measure the height variation of the bumps. The results show that the modulus of elasticity and strength of Inconel X-750 strip with thickness of 0.1 mm after different treatments are lower than that with the thickness of 0.18 mm at room temperature. Moreover, the sample foil strips which have been treated with a lower solution anneal temperature at 980 °C (2 hrs) and precipitation heat treatment at 732 °C (16 hrs) have the largest modulus of elasticity and strength at room temperature. Therefore, heat treatments have a great influence on the structure stiffness of the bump foil. At last, the results of the high cycle dynamic load tests show that the bump foil with suitable heat treatment will have a good load capacity and stress-relaxation property.

Modeling of Gas Lubricated Compliant Foil Bearings Using Pseudo Spectral Scheme

Modeling of Gas Lubricated Compliant Foil Bearings Using Pseudo Spectral Scheme

Experimental Identification of Dynamic Force Coefficients for a 110 MM Compliantly Damped Hybrid Gas Bearing

Compliant hybrid gas bearings (HGBs) combine key enabling features from both fixed geometry externally pressurized gas bearings and compliant foil bearings. The compliant hybrid bearing relies on both hydrostatic and hydrodynamic film pressures to generate load capacity and stiffness to the rotor system, while providing damping through integrally mounted metal mesh bearing support dampers. This paper presents experimentally identified force coefficients for a 110 mm compliantly damped gas bearing using a controlled-motion test rig. Test parameters include hydrostatic inlet pressure, excitation frequency, and rotor speed. The experiments were structured to evaluate the feasibility of implementing these bearings in large size turbomachinery. Dynamic test results indicate weak dependency of equivalent direct stiffness coefficients to most test parameters except for frequency and speed, where higher speeds and excitation frequency decreased equivalent bearing stiffness values. The bearing system equivalent direct damping was negatively impacted by increased inlet pressure and excitation frequency, while the cross-coupled force coefficients showed values an order of magnitude lower than the direct coefficients. The experiments also include orbital excitations to simulate unbalance response representative of a target machine while synchronously traversing a critical speed. The results indicate the gas bearing can accommodate vibration levels larger than the set bore clearance while maintaining satisfactory damping levels.

Simulation of Compliant Bump Foil Journal Bearing Using Coupled Reynolds Equation and Finite Element Model Method

Based on a “1st generation” compliant foil journal bearing, the performance of foil bearing was simulated according to the model base on Reynolds equation coupled the finite element method. Model considers the bending of top foil, elastic deformation of bump foil, the friction between top foil and bump foil and the interaction between bumps. After compared analysis results to test data, the result of simulation was more accurate than other models, thus this model establish a basis for further study of the compliant foil bearing.

Experimental Investigation of Prototype Water-Lubricated Compliant Foil Bearings

First gas-lubricated compliant foil bearings (CFBs) were built in the 1950s. Due to their significant advantages, such as oil-free operation, good tolerance to bearing misalignment and very low maintenance, they have been penetrating the bearing applications for high speed compressors, air-cycle machines and gas turbines. The work presented here investigates a novel idea of water-lubricated compliant foil bearings, which could be used in applications where environmentally friendly lubrication is desired, for example in hydroelectric turbines or water pumps. Experimental results collected for three prototype water-lubricated foil journal bearings are presented. The tests were conducted under steady radial load and with the sliding speed varied incrementally. A sequence of design improvements is presented, with the best bearing demonstrating friction coefficient of about 0.01 at the sliding speed of about 4 m/s and the radial load of about 300 kPa. Encountered difficulties, research methodology and the testing equipment are also described.

Structural Properties of Foil Bearings: A Closed-Form Solution Validated with Finite Element Analysis

Fluid film thickness in a compliant foil bearing is greatly influenced by the deflection of the bearing structure. Therefore, in order to properly model performance of a foil bearing, it is mandatory that the deflection of the compliant bearing structure due to the generated hydrodynamic pressure is determined accurately. This article proposes an easy-to-use two-dimensional model, which takes into account detailed geometry of the bump foil-top foil assembly and the interaction between bumps and which can predict the bearing deflection and stresses due to an arbitrary pressure load. The proposed model is first validated using a finite element analysis and the results available in the literature and then used to conduct a parametric study investigating the influence of bump foil geometry, the coefficient of friction between the bearing components, and the type of loading on the structural properties of the bearing. The most important parameters are also identified. The proposed analytical model is completely algebraic and can be easily implemented using any programming language, a spreadsheet, or even a calculator. The resulting solution can also be coupled with the appropriate hydrodynamic model to predict static performance of compliant foil bearings.

Dynamic stability experiments of compliant foil thrust bearing with viscoelastic support

In order to reduce the complexity of machine construction and improve the mechanical efficiency, high speed rotation machineries usually implement self-acting gas bearings to substitute the traditional oil-lubricated bearings. This paper presents test results of a gas thrust bearing with viscoelastic support which is designed for high speed turbo-machinery. The gas bearing, which belongs to compliant foil bearings, consists of a top thin metal foil and a bottom thin rubber foil. Static and stability experiments are conducted on a high speed gas turbine test rig. The static results indicate that the structural stiffness of test bearing generally increases with the increase in axial load and the decrease in thickness of bottom foil. In the rotation tests, rotor runs stably with small vibration amplitude, which is dominant in waterfall plot during whole speed up procedure. It shows that test bearing has preferable stability characteristics for high speed gas turbines.

Testing of a Small Turbocharger/Turbojet Sized Simulator Rotor Supported on Foil Bearings

A small rotor designed to simulate a miniature turbojet engine or turbocharger rotor mounted on compliant foil bearings was tested at speeds in excess of 150,000rpm and temperatures above 260°C(500°F). The simulator rotor-bearing system was operated while positioned in various orientations and was subjected to transient shock impacts exceeding 35g. Subsequent testing was completed to demonstrate the capabilities of miniature thrust foil bearings as well. The tested rotor weighed approximately 400g and used 15mm diameter foil journal bearings and a multipad 25mm diameter double acting thrust foil bearing. Results of the rotor-bearing system dynamics are presented along with experimentally measured natural frequencies, rotor displacements, and thrust load carrying ability. A good correlation between measurement and analysis is observed. Very short rotor acceleration times from rest to maximum speed were also measured. A parallel test simulator has been used to accumulate over 1000 start-stop cycles to demonstrate the life of the bearing and coating. Based on this successful testing, it is expected that the goal of developing oil-free turbochargers and small turbojet engines that operate at high speeds with long life will be achieved.

Durability and Stability Analysis of Compliant Foil Journal Bearings with Elastic Support Using Different Surface Treatments

The durability and stability characteristics of compliant foil journal bearings with elastic support are discussed in this paper. Using different choices for surface treatment of foil materials and foil element geometry parameters, the start-stop cycle and the rotor-bearing system dynamic characteristics of a compliant foil bearing (CFB) were experimentally studied. This CFB with elastic support can steadily run the turbo expander with a 25.0-mm-diameter rotor revolving at 14.8 × 10 4 rpm and has excellent performance in terms of stability and durability. The test results indicate that with suitable bearing structure parameters, foil materials and application of the surface treatment of this type of CFB is appropriate for high-speed turbo machinery.

Analytical Study of Aerodynamic Foil Journal Bearing with Elastic Support

A new type of compliant foil bearing (CFB)-Aerodynamic Foil Journal Bearing with Elastic Support was developed and analyzed in this paper. Sets of physical models were proposed to solve the Elastohydrodynamic (EHD) coupling problem for this new type of CFB. The characteristics of compliant foil supported by elastic foundation were studied. The transient compressible Reynolds' equation was adopted while considering the nonlinear dynamic force of the gas film. The theoretical methods of obtaining the static and dynamic parameters of the bearing-rotor system were deduced. Finite Element Method (FEM) was used in order to solve the elastic foundation deformation and CFB's governing equations.

Low-friction wear-resistant coatings for high-temperature foil bearings

Compliant foil bearings offer many advantages over rolling element bearings in high-speed and high-temperature applications. However, implementation of foil bearings in these applications requires development of solid lubricant coatings that can survive the severe operating conditions encountered at high speeds and high temperatures. The objective of this paper is to present results on development of an advanced coating system for use with compliant foil bearings that permits higher operating speeds and temperatures. In order to evaluate the coating performance and to select the best coating combination for implementation, tests were conducted using a high-temperature, high-speed tribometer. In these tests, Inconel test substrates, representative of a portion of a foil bearing, were coated with several different Korolon TM coatings. The counterface disks were coated with a dense chrome, plasma sprayed PS304, hard chrome and Korolon TM 1350B. Each test was conducted for 500 start–stop cycles up to 810 °C foil pad temperature under 13.8 kPa normal loading. The test results confirmed the excellent tribological behavior of Korolon TM coatings for high-speed, high-temperature foil bearing applications. While the tribological behavior of Korolon TM coatings were determined to be a function of temperature, in most cases a minimum coefficient of friction less than 0.1 was observed during startup/shutdown periods. Based on the measured coefficient of friction and post-test visual inspection of the mating surfaces, the hard chrome coating proved unacceptable for high-temperature applications due to extensive surface cracking. The other disk coatings exhibited excellent tribological performance. Following these tests, a foil journal bearing was designed and a composite coating consisting of Korolon TM 1350A with an overcoat of Korolon TM 800 was applied to the bearing top foil; and a dense chrome coating was applied to the journal surface. The foil bearing and journal were installed in a 240-lb thrust turbojet engine and operated successfully to 54,000 rpm for over 70 start–stop cycles and 14 h.

A generalized solution of elasto-aerodynamic lubrication for aerodynamic compliant foil bearings

Although aerodynamic compliant foil bearings are successfully applied in a number of turbo-machineries, theoretical researches on the modeling, performance prediction of compliant foil bearings and the dynamic analysis of the related rotor system seem still far behind the experimental investigation because of structural complexity of the foil bearings. A generalized solution of the elasto-aerodynamic lubrication is presented in this paper by introducing both static and dynamic deformations of foils and solving gas-lubricated Reynolds equations with deformation equations simultaneously. The solution can be used for the calculation of dynamic stiffness and damping, as well as the prediction of static performances of foil bearings. Systematical theories and methods are also presented for the purpose of the prediction of dynamic behavior of a rotor system equipped with foil bearings.

Hydrodynamic Analysis of Compliant Foil Bearings With Compressible Air Flow

A model is developed to predict the hydrodynamic performance of a foil journal bearing. The model accounts for both the compressibility of air and the compliance of the bearing surface. A series of predictions of the load-carrying capacity based on the numerical solution for pressure is presented that cover a wide range of operating speeds. The results show good agreement with existing experimental data.

Experimental Study of a New Compliant Foil Air Bearing with Elastic Support

A new air-lubricated compliant foil journal bearing with elastic support, which has uniform surface stiffness and is much simpler in structure than previous compliant foil bearings (CFBs), is introduced in this article. Experiments have been conducted on the application of this type of CFB to a high-speed test rig, and this CFB can operate stably at 151,000 rpm. From the tests it is clear that the radial clearance C has a direct impact on the performance of this CFB, so the numerical relationship of structural parameters is listed in this article. Experimental results indicate that the CFB presented here offers preferable system dynamic and stability performance and has adequate damping to effectively reduce the possibility of self-excited and fractional frequency whirl.

Advancements in the Structural Stiffness and Damping of a Large Compliant Foil Journal Bearing: An Experimental Study

This paper presents the results of an experimental investigation into the dynamic structural stiffness and damping characteristics of a 21.6‐cm(8.5in.)-diameter compliant surface foil journal bearing. The goal of this development was to achieve high levels of damping without the use of oil, as is used in squeeze film dampers, while maintaining a nearly constant dynamic stiffness over a range of frequencies and amplitudes of motion. In the experimental work described herein, a full compliant foil bearing was designed, fabricated, and tested. The test facility included a non-rotating journal located inside the bearing. The journal was connected to an electrodynamic shaker so that dynamic forces simulating expected operating conditions could be applied to the structurally compliant bump foil elements. Excitation test frequencies to a maximum of 400Hz at amplitudes of motion between 25.4 and 102μm were applied to the damper assembly. During testing, both compressive preload and unidirectional static loads of up to 1335 and 445N, respectively, were applied to the damper assembly. The experimental data from these tests were analyzed using both a single degree of freedom model and an energy method. These methods of data analysis are reviewed here and results are compared. Excellent agreement in results obtained from the two methods was achieved. Equivalent viscous damping coefficients as high as 1050N.s∕cm(600lbf.s∕in) were obtained at low frequencies. Dynamic stiffness was shown to be fairly constant with frequency.

Comparative test on two kinds of new compliant foil bearing for small cryogenic turbo-expander

Two kinds of new compliant foil journal bearing for a small cryogenic turbo-expander used in the Brayton cycle cryocooler are presented in this paper. The foil of bearings is supported with copper wires and elastic material respectively, so the bearings are much simpler in structure than previous compliant foil bearing. The performances of the bearings are studied under the same experiment conditions. The results indicate that both bearings have better damp performance and the foil bearing supported with elastic material has preferable stability to the one supported with copper wires.