Top Foil Research Articles

Performance potential of gas foil journal bearings enhanced with micro taper-grooves on top foil

PurposeTo improve the load capacity and stability of gas foil journal bearings (GFJB), this paper aims to propose a novel GFJB with taper-grooved top foil.Design/methodology/approachA modified bump stiffness model is established considering rounding and friction. By considering the variation of clearance in the circumferential and axial direction, the static and dynamic characteristics of the novel bearing are calculated using the finite difference method, and perturbation method, respectively. The bearing performance under different groove parameters is studied and compared to the traditional bearings.FindingsThe results show that this novel GFJB can bring multi-extra local dynamic pressure and decrease the gas end leakage, which improves the static and dynamic properties. Moreover, as the increment of groove depth, the load capacity and direct stiffness are reinforced. There is an optimal groove width to maximize the load capacity, and the taper-groove is more beneficial to the improvement of bearing performance than other groove shapes. For the novel GFJB (Ng = 6, Hg = 10µm), the load capacity and direct stiffness increase by about 6.67 and 13.5 per cent, respectively. The stability threshold speed (STS) of a rotor supported by the novel bearings is also increased.Originality/valueThe performance of the presented novel GFJB is enhanced immensely compared to the traditional bearings, and the results are expected to be helpful to bearing designers, researchers and academicians concerned.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-08-2019-0307.

Changing the Operation of Small Geometrically Complex EBG-Based Antennas With Micron-Sized Particles That Respond to Magneto-Static Fields

As the usage of wireless technology grows, there are evermore demands on the antennas that support these platforms. This need has led to the design of unique antennas with improved bandwidth, agile frequency capabilities, compact size and greater efficiencies. In part though, the trade-off for such capabilities is antenna complexity. This paper presents a new technique for simplifying the method of changing the operation of a printed antenna using micron-sized silver coated particles that respond to magneto-static fields. More specifically, a structure consisting of a low-loss dielectric material with a cylindrical cavity containing micro-sized particles is developed. The overall size of the dielectric material is 1.5 mm $\times 1.5$ mm $\times 0.5$ mm and the cavity has a diameter of 0.9 mm. Furthermore, the top and bottom of the cavity with the micron-sized particles is capped with copper foil. Then, to manipulate the enclosed particles, a static magnet is placed near the structure. The enclosed particles columnize and orientate in the direction of the field-lines, connecting the top and bottom copper foil plates. To disconnect the plates then, the field is simply removed and the columns collapse. Macroscopically, the structure has the behavior of a switch. The structures presented in this work are denoted as Magneto-static Field Responsive Structures (MRSs). The MRSs have an additional benefit of not requiring a direct connection to a biasing circuit. This is very useful because there are many antenna designs that make it difficult to embed biasing circuitry to reconfigure printed antennas using MEMS and PIN diodes, for example. Finally, a new frequency reconfigurable Electromagnetic Band Gap (EBG) antenna is presented. This design is unique because the complex layout does not allow for traditional biasing circuitry and the operation is changed using the new MRSs presented in this paper.

Effect of packaging oxygen transmission rate on the shelf life of ready‐to‐heat foods susceptible to postcontamination during refrigerated and illuminated storage

Since more and more pressure is exerted to reduce the use of plastic packaging materials, optimizing the use of food packaging is opportune. The aim of this study was to evaluate the combined effect of packaging materials, spanning a range of oxygen transmission rates (OTR), and retail illumination, on the microbial shelf life and safety of refrigerated ready‐to‐heat foods. Cooked potato slices were packaged in OPA/PP bags with a high OTR (28.85 ccO2/m2/d) and OPA‐EVOH/PP bags with a low OTR (6.57 ccO2/m2/d). Cooked composite meals were packaged in tray and foil combinations, also spanning a range of OTR: PP trays (2.09 ccO2/tray/d) with OPA/PP foils (28.85 ccO2/m2/d), PP trays with OPA‐EVOH/PP (6.57 ccO2/m2/d) foils, and PET trays (0.07 ccO2/tray/d) with PET top foil (32.86 ccO2/m2/d) . The packages were stored in a dark environment, or under fluorescent or LED light. Due to the rapid growth of lactic acid bacteria, the microbial shelf life of both food products was largely unaffected by the type of barrier. Illumination at 1000 lux for 12 hours per day led to temperature differences significantly affecting microbial growth. Based on the results, it could be concluded that re‐evaluating packaging material choices for these foods may prove valuable, since the use of high‐barrier multilayer packaging materials may be considered as a case of overpackaging.

Evaluation of Lubrication Performance of Foil Bearings with New Texturing

Foil bearings have excellent characteristics such as a high damping performance and acceptable dimensional changes caused by installation errors and thermal expansion. However, the problem of large friction losses also occurs at the top foil under the high rotational speed required to support a shaft without any contact. In this paper, a new type of foil bearing with a surface texture on the top foil is proposed. The effects of the texture were verified by the rotational motion experimental analysis under a high circumferential speed and dry lubrication. For a bearing with dimples on the contact area, the effect of friction reduction could be observed under boundary lubrication. Moreover, it was also confirmed that the dimples adversely affected the air film formation with the sliding speed increase. For a newly designed F-grooved model, the phenomenon of an early transition from boundary lubrication to fluid lubrication can be seen. However, the friction coefficient increases at the boundary lubrication. In this study, it was also the highest friction reduction effect was confirmed at the maximum sliding speed. Furthermore, an additional friction reduction effect and a small temperature rise were obtained by combining the above textures.

Study of load carrying mechanism of a novel three-pad gas foil bearing with multiple sliding beams

Abstract This paper studies the load carrying mechanism of a novel three-pad gas foil bearing. An elaborate model of the bearing foil structure is developed, in which the multiple supporting beams are able to slide with friction on the bearing sleeve. The area contact between the top and support foils as well as the rotor-top foil contact is also considered. Under different types of contact constraints, the nonlinear foil structural deformation is further coupled with the compressible Reynolds equation to obtain the aeroelastic solutions. The accuracy of the model is verified by the experimental study in which the test results of bearing load capacity and stiffness are in good agreements with the predictions. The simulation results indicate that, (i) the area contact between top foil and sliding beams increases the foil structural stiffness; (ii) thinner support foil and larger rotor preload can increase the frictional energy dissipation of the foil structure; and (iii) bearing load capacity and the aeroelastic characteristics are significantly influenced by the layouts and thickness of support foil.

A New Structural Bump Foil Model With Application From Start-Up to Full Operating Conditions

Abstract This paper presents a new structural bump foil model that can handle all operating conditions from start-up to full speed. The model is based on a nonlinear contact algorithm with friction and gaps. The top foil is modeled as a curved beam while bump foil uses a coupled truss model. The model considers the gaps between the bump foil and the bearing casing, between the bump foil and the top foil and between the rotor and the top foil. Thus, any numerical interference between the rotor and the top foil is avoided. A mixed lubrication model is used for the thin film pressures. Following this algorithm, contact pressures appear if the film thickness is less than three times the equivalent roughness of the rotor and of the top foil. Fluid pressures are calculated from numerical solutions of Reynolds equation while contact pressures, if present, are calculated with the model of Greenwood and Williamson. The model is validated by comparisons with the experimental results obtained for start-up operating conditions of a first-generation foil bearing of 38.1 mm diameter with static loads of 10–50 N. Theoretical predictions of the start-up torque and takeoff speed compare well with experimental results. It is also shown how manufacturing bump height errors can explain the differences between theoretical and experimental predictions. Further validations are presented for the same bearing operating at high speeds (30, 45, and 55 krpm) and heavy static loads (up to 200 N). The calculated minimum film thickness and attitude angle are compared with experimental data from the literature.

Influence of wedge shape on the performance of air foil thrust bearings

PurposeThe purpose of this paper is to investigate the effect of different wedge shapes on the performance of air foil thrust bearing (AFTB).Design/methodology/approachDuring the study, a bump foil stiffness model considering slip deformation and a two-dimensional sheet top foil model is established, and the Reynolds equation and film thickness equation is solved using the finite difference method and finite element method. The static performance such as load carrying capacity, friction torque and power loss of AFTB under different taper parameters is obtained. The influence of different pitch ratio, film thickness ratio and wedge shapes on the bearing characteristics is studied.FindingsThere is an optimal height and a pitch ratio for the taper of AFTB with certain tile number. Compared to the plane and concave wedge shape, the upper convex shape can enhance the convergence effect of the wedge region, increase the effective film pressure distribution area of the bearing and reduce the local concentrated load of the top foil, which is more conductive to the increase of load capacity.Originality/valueThe wedge shape parameters bring a fundamental difference in the static performance of AFTB. The results are expected to be helpful to bearing designers, researchers and academicians concerned.

SHS-Aided Joining of Ceramic Materials with the Ta Substrate

Experiments on evaluating the possibility of joining ceramic materials with a Ta substrate under conditions of self-propagating high-temperature synthesis (SHS) are explored. The sample consists of tantalum foils and pellets Ti + 0.65C and 5T + 3Si, between which a Ti + 2B igniting tape is laid. The sample is installed on a boron nitride support. A chamotte brick plate (SiO2 + Al2O3) with a load of 3360 g placed on its top is placed on a sample to decrease the heat sink. The experiments are performed in a reaction chamber in argon under a pressure of 1 atm. The samples are preheated from the bottom, and the reaction is initiated from their butt. The sample temperature is measured using W–Re thermocouples. The temperature gradient over the sample thickness, depending on the heating rate, is 50–150 K/mm. The samples with strong joining of the tantalum foil with Ti + 0.65C, as well as between two pellets, are formed. The top foil is not welded to the 5Ti + 3Si pellet, which is explained by a rather low temperature (1600°C) at the interface. When studying the interface of the tantalum foil with titanium carbide, the formation of Ti–Ta and (Ta,Ti)C interlayers is found. The possibility of joining the tantalum foil with ceramic materials during SHS is shown by investigations. The main conditions of such joining are the presence of the liquid phase and correspondence of the combustion temperature of the Ti + 0.65C composition to the melting point of the tantalum substrate. These results can be useful when fabricating multilayered functional coatings and functionally graded materials.

Characterization of a controllable stiffness foil bearing with shape memory alloy springs

Abstract This paper develops a shape memory alloy gas foil bearing (SMA-GFB), in which the stiffness of elastic structure and the clearance between the rotor and top foil are adjusted by the temperature of SMA springs which is controlled by electric heating and a designed cooling method. A numerical model of elastic structure composed by a series of SMA springs is conducted, and the experimental results verify that the active elastic structure can improve SMA-GFB performance. Furthermore, the static and dynamic characteristics of SMA-GFBs are predicted. The load capacity and stability of two different designs of SMA-GFB are explored based on the circumferential distribution of SMA springs. These data expand the path for the active application of SMA in gas bearings.

A simplified structural model of bump-type foil bearings based on contact mechanics including gaps and friction

Abstract The paper introduces a structural model for bump-type foil bearings based on contact mechanics including gaps and friction. The starting point is a model introduced in [32] were the bump foil is approximated by a specific structure of interacting springs. Following this model, contacts between the top and bump foils and between the bump foil and the sleeve are always closed. Recent analyses [31] showed that this not always verified: contacts can become loose even for very simple loading situations. A new model is then developed for considering these situations. The new model takes into account the elasticity of the top foil and three kind of gaps: between the rotor and the top foil, between the top and the bump foils and between the bump foils and the sleeve. The former model of the bump foil was extended by considering an additional degree of freedom per bump. Friction was modeled with Coulomb law. An efficient numerical procedure borrowed from contact mechanics ([41], [42]) was implemented for solving the structural problem with friction and close/loose gaps. The procedure uses the augmented Lagrangian algorithm for the normal contact forces and the penalty method for the tangential friction force. The procedure is very robust in dealing with stick-slip situations and close/loose contacts. The model was validated by comparisons with results from the literature ([31], [32]). By considering gaps in the foil structure and close/loose contacts, the model is not only closer to reality but can also handle manufacturing errors of the foil structure. This is shown by considering bump foil height manufacturing errors.

Effect of Analytical Model of Top Foil Displacement on Predicted Maximum Load Carrying Capacity of Bump Foil Journal Bearings (Part 1)

Effect of Analytical Model of Top Foil Displacement on Predicted Maximum Load Carrying Capacity of Bump Foil Journal Bearings (Part 1)

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.

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.

A Feasibility Study of Controllable Gas Foil Bearings With Piezoelectric Materials Via Rotordynamic Model Predictions

This study presents a new concept of controllable gas foil bearings (C-GFBs) with piezoelectric actuators. The C-GFB consists of a laminated top foil, bump foil, and piezo stacks and can simply change the bearing shape or film thickness locally and globally by varying the thickness of the piezo stacks with input voltages. The control schemes are (1) clearance control: the bearing clearance adjusted by changing overall piezo stack thickness, and (2) preload control: the mechanical preload modulated by the thickness expansion of several piezo stacks. Bearing lubrication performance is predicted for four cases of C-GFBs with different bearing clearances and preloads. The piezo stack control generates meaningful differences in the fluid-film thickness and pressure. Clearance control has a great effect on the dynamic force coefficients, but preload control slightly increases. Furthermore, the rotordynamic prediction of a rotor supported on two journal C-GFBs is conducted. As a result, both control modes for C-GFB are found to have a positive effect on rotordynamic amplitudes. Finally, using the orbit simulations, the C-GFB is controlled to have a small bearing clearance and large preload at critical speeds to make it possible to stably pass through the critical speeds. Consequently, it turns out that the C-GFB can improve bearing lubrication and rotordynamic performances by controlling only the input voltage of the piezo stacks. In addition, the C-GFB can be used to form various shapes to meet the operation conditions of an applied system.

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.

Determination of the Lift-Off Speed in Foil Bearings Using Various Measurement Methods

Abstract One of the most important parameters describing the operation of foil bearings is the speed at which—under certain load conditions—a lubricating film is created. Knowing the value of this parameter, one can determine the nature of the operation of a rotor that is supported by these bearings. The lift-off speed is usually determined during the run-up and coast down of the rotor using the measured moment of friction as a function of the rotational speed. In this article, this method has been compared with other measurement methods proposed by the authors. The first method consisted in recording the temperature of the bearing's top foil during its operation, while the second method was based on measuring the current-voltage characteristics of the electro-spindle used in the drive system. As a result of the conducted research, a simple method that allows determining the lift-off speed in foil bearings was proposed. This method can be applied to various types of high-speed machines, such as turbocompressors or microturbines.

EXPERIMENTAL INVESTIGATION OF A FOIL BEARING STRUCTURE WITH A POLYMER COATING UNDER STATIC LOADS

Gas foil bearings can operate at very high temperatures and rotational speeds. The operation under such conditions requires developing an appropriate bearing design, including the use of advanced material solutions. This article presents one of the basic stages of work on a new foil bearing, namely, experimental research on the structural supporting layer of such a bearing regarding its static loads. Tests of the bearing were carried out on a test rig specially prepared for this purpose. Changing the magnitude and direction of the load was possible. The elasto-damping elements of the bearing were made of thin metal foils. In addition, a layer of carefully selected polymer was applied onto one side of the top foil in order to protect the surface and reduce friction. Characteristics of the structure of the foil bearing were determined at various load variants after taking a series of measurements upon it. The conducted research has yielded much information about static characteristics of the structural supporting layer of a new foil bearing in which the top foil’s surface is covered with a layer of polymer. These results can be used, among other things, to optimise the bearing design and to verify numerical models.

The effect of cooling the foil bearing on dynamics of the rotor-bearings system

In order to protect rotors and foil bearings operating at high temperatures from being overheated and damaged, these components are often cooled by air. In addition, such a cooling method is accompanied by an axial temperature gradient that changes the shape of the lubrication gap in a way likely to affect the operation of a foil bearing. This article presents the research on various methods for cooling a foil bearing and discusses the impact of these methods on dynamic parameters of the rotor-bearings system. To be able to assess the temperature inside such a bearing, there is a need for a reliable measurement method. The authors of the article measured the temperature of the top foil using thermocouples and showed that their measurement method does not exert any significant impact on the operation of the rotor-bearings system. The article also describes a novel method for compensating the axial temperature gradient occurring in the bearing bush using Peltier modules.

The shape error evaluation methods of the foils for a multi-leaf foil bearing

The geometric parameters of the elastic foil bearing are important basis for designing the foil bearing. Whether the main shape indices of the pressed foils meet the design requirements is the key to evaluate a manufacturing method. The inconsistent curvature radii of the top foils, the inconsistent bump heights of the bumps on the bump foil, the roundness error of the top foil, and the curve profile error of the bump foil may cause the pre-tightening assembly difference and different bearing capacity of each bearing shell. Aiming at the shape error evaluations of the foils for a multi-leaf foil bearing, first, the algorithm of geometric area optimization for circle roundness error is introduced and an evaluation method of arc roundness error is put forward according to the minimum zone principle in this paper. This method can be used to calculate the roundness error of the top foil for a multi-leaf foil bearing. The results show that the corresponding roundness error of the top foil decreases with the increasing of the pressing pressure. The relative roundness error is small (less than 7%), which changes a little with different pressing pressures. For the measurement of a bottom bump foil, the method of matching feature points is used in pre-location and then fine-positioning for the measured curve is implemented based on the least square method in order to eliminate the position error between the measured curve and the design curve. Thus, the curve profile error evaluation of the bottom bump foil for a multi-leaf foil bearing is implemented and the profile error of each bump can be obtained. According to the shape error evaluation values of the top foil and the bump foil, the quality control strategy and the error compensation by improving the mold structure can be further researched.

Thermohydrodynamic analysis and thermal management of hybrid bump–metal mesh foil bearings: Experimental tests and theoretical predictions

Hybrid bump–metal mesh foil bearings (HB-MFBs) are novel gas foil bearings (GFBs) composed of foil strips and metal mesh blocks (MMBs) in bearing substructure. HB-MFBs provide more advantages than traditional foil bearings, such as high structural damping, assembly accuracy, and ability to work at high temperatures. A thermohydrodynamic model of HB-MFBs was proposed to predict the bearing temperature field with various bearing loads and rotational speeds. The theoretical model considered the complex thermal boundary of bearing substructure in this high-performance foil bearing, including the top foil, bump foil, and MMBs. A detailed thermal-transfer model of hollow rotor was introduced to calculate the heat energy transferring through the rotor shell to the surrounding ambient. Both thermal and centrifugal growth of hollow rotor were considered because of the thin-air film of GFBs to avoid bearing failure. A test rig used to measure the bearing temperature distribution with static load and various rotational speeds was built to validate the proposed thermohydrodynamic model. Different thermal managements with cooling air flow in hollow rotor and bearing substructure were applied to decrease the bearing temperature. The influence of load carry coefficients on bearing peak temperature was investigated with respect to rotational speed. A comparison of heat carry ratios between hollow rotor and bearing substructure was also conducted. The heat energy conducted through the bump foil region and MMB region was analyzed to validate the high-temperature capacity of HB-MFBs.