Abstract:
A high load capacity hydrodynamic journal foil bearing system is disclosed, which comprises a top foil and a plurality of undersprings. Preload forces are transferred from the undersprings to internal circumferential compressive forces within a top foil, resulting in low preload forces against the shaft, allowing the shaft to expand at high speeds without increasing the preload forces or overloading the fluid film. One underspring may have a different spring rate than another underspring. The top foil may be normalized to shaft shape and dimensions. These features may be accomplished with using less mechanical parts than other journal foil bearing system designs.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This present invention relates generally to radial-type dynamic pressure fluid bearing systems and, in particular, to foil-type fluid bearing systems comprising a stationary retaining member that surrounds the outer circumference of a rotating journal shaft thereby forming an annular cavity. A foil assembly located in the cavity supports the journal.  
         [0002]     Fluid bearing systems are used in many diverse applications requiring high speed rotating machinery. Fluid bearing systems generally comprise two relatively movable elements with a predetermined gap therebetween filled with a fluid, such as air. For example, a fluid bearing system may comprise a stationary bearing housing that surrounds a rotating shaft. Under dynamic conditions, gaps form between the relatively moving surfaces supporting a fluid pressure sufficient to prevent contact between the two relatively movable bearing elements.  
         [0003]     Hydrodynamic fluid bearings have been developed by using foils in the gap between the relatively movable bearing elements. The hydrodynamic film forces between adjacent bearing surfaces deflect these foils, which are generally thin, pliable sheets of a compliant material. The foils enhance the hydrodynamic characteristics of the fluid bearing systems and provide improved operation under extreme loads. These foils also function to accommodate eccentricity, runout, and other non-uniformities in the motion of the relatively movable elements. The foils also provide a cushioning and damping effect.  
         [0004]     The motion of a rotating element applies viscous drag forces to the fluid in a converging channel. This may result in fluid pressure increases throughout most of the channel. If a rotating element (for example, a shaft) moves toward a non-rotating element (for example, a foil), the fluid pressure increases along the channel. Conversely, if a rotating element moves away, the fluid pressure decreases along the channel.  
         [0005]     Consequently, the fluid in the fluid bearing system exerts damping forces on the rotating element that vary with running clearances between the shaft surface and the top foil surface. Higher pressure along the channel provides more fluid film damping forces. These damping forces may stabilize non-synchronous shaft motion and prevent contact between the rotating and non-rotating elements. Any flexing or sliding of the foils may cause coulomb damping which also adds to the radial stability.  
         [0006]     Due to preload spring forces or gravity forces, a rotating element of the bearing is typically in contact with the fluid foil members of the bearing at zero or low rotational speeds. This contact may result in bearing wear. Only when the rotor speed is above what is termed the lift-off/touch-down speed will the fluid dynamic forces generated in the channel assure a gap between the rotating and non-rotating elements.  
         [0007]     Compliant fluid foil bearing systems typically rely on backing springs and top foils for preload, stiffness, and damping. The foils are preloaded against the relatively movable rotating element to control foil position/nesting and to establish dynamic stability. The bearing starting torque (which should ideally be zero) is proportional to the preload forces. These preload forces also significantly increase the rotational speed at which the hydrodynamic effects in the channel are strong enough to lift the rotating element of the bearing away from the non-rotating members of the bearing. These preload forces and high liftoff/touch-down speeds may result in significant bearing wear each time the rotor is started or stopped.  
         [0008]     Conventional foil bearing systems obtain damping from the fluid film between the foil surface and the shaft, and from coulomb friction forces between the foils and undersprings. To increase damping, the typical design increases bearing preload forces that increase both the fluid damping and the coulomb damping. However, this design also increases the contact force between the shaft and foils, resulting in higher start torque before development of the hydrodynamic fluid film.  
         [0009]     Conventional foil bearing systems may experience wrapping failure, which may occur when a top foil sticks to a rotating shaft, causing the top foil to undergo tension and tighten around the shaft, in effect, wrapping around the shaft. This wrapping effect dramatically increases the torque required to turn the shaft, which can prohibit turning or damage the bearing by pulling them out of its anchoring mechanism.  
         [0010]     One design that attempts to effectively prevent wrapping failure is disclosed in U.S. Pat. No. 5,427,455 to Bosley. A compliant foil hydrodynamic fluid film radial bearing is disclosed, comprising a shaft, a top foil, a spring foil, and a foil-retaining cartridge. The cartridge is located within a bore and has circumferentially undulating cam shaped lobes, or circumferential ramps and joggles, that induce the spring and top foils to form converging fluid-dynamic channels that compress and pressurize the process fluid and diverging channels that draw in makeup fluid. A spring foil is formed as a thin, flat sheet having chemically etched slots of a pattern that cause cantilever beams to stand erect and function as springs when the foil is bent to install in the cartridge.  
         [0011]     The Bosley design seeks to lower start torque and stall speed through minimizing radial force transmitted to the shaft. The Bosley design seeks to accomplish this by pushing the top foil circumferentially away from the shaft by using either only a preload bar or a flat circumferential preload spring at the ends of the top foil. Joggles on the top foil are used to ensure fluid film generation.  
         [0012]     However, manufacturing difficulties, including costs for additional parts, make the use of preload bars or flat circumferential preload springs costly. Additionally, the level of distributed forces, or preload, between the outer circumference of the shaft and the top foil is very sensitive to the manufacturing variations in the shaft and the bore diameters and the bearing stack-up. Also, the circumferential spring and/or preload bar in the Bosley design and other prior art may keep the top foil from collapsing to the shaft; but the control of radial space between the top foil and the shaft is susceptible to variations in bore diameter and the underspring height. In Bosley&#39;s design, if the bore is smaller or if the underspring is taller, the space between the top foil and the shaft will become smaller (and vice versa for short undersprings or larger bore). When the space between the top foil and the shaft becomes too small, too much of the preload from the springs transfers to the shaft through the top foil, dramatically increasing the start torque. If the space between the top foil and the shaft becomes too large, the fluid film damping will decrease dramatically and the rotor will be susceptible to rotor instability.  
         [0013]     The prior art is intended for allowing higher preload forces and higher coulomb damping without higher start torque, but does not improve fluid film damping and some suffer from one or more of the following disadvantages: 
        a) excessive start torque;     b) lower preload forces between the foils and the bore, which may cause lower damping forces;     c) lower tolerances for manufacturing variations;     d) wrapping;     e) higher parts costs.        
 
         [0019]     As can be seen, there is a need for an improved apparatus for hydrodynamic fluid bearing systems wherein preload forces are transferred from the undersprings to internal circumferential compressive forces within a top foil, resulting in high pre-load between the bore and the top foil, while prohibiting the pre-load to be transferred to the shaft. The top foil should be allowed to expand at high shaft speeds to allow some growth in the film thickness at high shaft speeds, but restricting the film thickness from growing too thick and losing fluid film damping. There is also a need for bearing systems that can accommodate high manufacturing tolerances.  
       SUMMARY OF THE INVENTION  
       [0020]     In one aspect of the present invention, a journal foil bearing system comprises a journal member; a shaft arranged for relative coaxial rotation with respect to the journal member; a top foil disposed between the shaft and journal member; the top foil comprising a leading edge and a trailing edge; wherein the leading edge and the trailing edge are pushed against each other; and wherein the trailing edge is disposed upstream, from the leading edge, in the direction of the relative coaxial rotation of the shaft.  
         [0021]     In another aspect of the present invention, a journal foil bearing system comprises a journal member; a shaft arranged for relative coaxial rotation with respect to the journal member; a top foil disposed between the shaft and journal member; the top foil comprising a leading edge and a trailing edge; wherein the leading edge and the trailing edge are pushed against each other; wherein the trailing edge is disposed upstream, from the leading edge, in the direction of the relative coaxial rotation of the shaft; a first underspring layer disposed between the top foil and the journal member; and a second underspring layer disposed between the first underspring layer and the journal member.  
         [0022]     In yet another aspect of the present invention, a journal foil bearing system comprises a journal member, a shaft arranged for relative coaxial rotation with respect to the journal member, a top foil disposed between the shaft and journal member, the top foil comprising a leading edge and a trailing edge; wherein a distance between the trailing edge and the shaft is shorter than a distance between the leading edge and the shaft; wherein the trailing edge is disposed upstream, from the leading edge, in the direction of the relative coaxial rotation of the shaft; and a first underspring layer disposed between the top foil and the journal member, wherein a spring rate of a portion of the first underspring layer under the trailing edge or the top foil is higher than a spring rate of a portion of the first underspring layer under the leading edge of the top foil.  
         [0023]     In an alternative aspect of the present invention, a journal foil bearing system comprises a journal member with a bore; a shaft arranged within the bore for relative coaxial rotation with respect to the journal member; a top foil disposed between the shaft and journal member; the top foil comprising a leading edge and a trailing edge; wherein the trailing edge is disposed upstream, from the leading edge, in the direction of the relative coaxial rotation of the shaft; wherein the leading edge and the trailing edge are pushed against each other; a first underspring layer disposed between the top foil and the journal member; a second underspring layer disposed between the first underspring layer and the journal member; a foil retention slot in communication with the bore; and tabs in the top foil, the first underspring layer, and the second underspring layer, wherein the tabs are fit into the foil retention slot to secure the top foil against wrapping.  
         [0024]     In yet another aspect of the present invention, a journal foil bearing system comprises a journal member with a bore; a shaft arranged within the bore for relative coaxial rotation with respect to the journal member; a top foil disposed between the shaft and journal member; the top foil comprising a leading edge and a trailing edge; wherein a distance between the trailing edge and the shaft is shorter than a distance between the leading edge and the shaft; wherein the trailing edge is disposed upstream, from the leading edge, in the direction of the relative coaxial rotation of the shaft; a plurality of first undersprings disposed between the top foil and the journal member; wherein the plurality of first undersprings are circumferentially separated from one another a plurality of second undersprings disposed between the first undersprings and the journal member, a plurality of foil retention slots in communication with the bore; and tabs in the top foil, the first undersprings, and the second undersprings, with the tabs allowing the top foil, the first undersprings, and the second undersprings to be fitted into the foil retention slots and secured against wrapping.  
         [0025]     In a further aspect of the present invention, a journal foil bearing system comprises a journal member with a bore; a shaft arranged within the bore for relative coaxial rotation with respect to the journal member; a top foil disposed between the shaft and journal member; the top foil comprising a leading edge and a trailing edge; wherein a distance between the trailing edge and the shaft is shorter than a distance between the leading edge and the shaft; wherein the trailing edge is disposed upstream, from the leading edge, in the direction of the relative coaxial rotation of the shaft; wherein the leading edge and the trailing edge are pushed against each other; an underspring disposed between the top foil and the journal member; a foil retention slot in communication with the bore; and tabs in the top foil and the underspring, with the tabs allowing the top foil and the underspring to be fitted into the foil retention slot and secured against wrapping, wherein the underspring is wound at least twice around the circumference of the top foil.  
         [0026]     In still yet another aspect of the present invention, a journal foil bearing system comprises a journal member; a shaft arranged for relative coaxial rotation with respect to the journal member; a top foil disposed between the shaft and journal member; a first underspring layer disposed between the top foil and the journal member; a second underspring layer disposed between the first underspring layer and the journal member; wherein the first underspring layer provides a variable underspring force for supporting the top foil and maintaining an approximately wedge shaped uniform spacing between the top foil and the shaft; wherein the spacing is matched to the changing pressure force along a circumferential length of the top foil; a first anti-telescoping tab located at a leading edge of the top foil; a second anti-telescoping tab located at a trailing edge of the top foil; the first anti-telescoping tab shorter than the second anti-telescoping tab; an anti-wrapping tab located at the distal end of the second anti-telescoping tab; wherein a distance between the trailing edge and the shaft is shorter than a distance between the leading edge and the shaft; wherein the trailing edge is disposed upstream, from the leading edge, in the direction of the relative coaxial rotation of the shaft; and wherein the leading edge and the trailing edge are pushed against each other;.  
         [0027]     These and other aspects, objects, features and advantages of the present invention, are specifically set forth in, or will become apparent from, the following detailed description of the invention when read in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]      FIG. 1  is a perspective view of an exemplary foil journal bearing, according to an embodiment of the present invention;  
         [0029]      FIG. 2  is a perspective view of a top foil, according to an embodiment of the present invention;  
         [0030]      FIG. 3  is a perspective view of an underspring, according to an embodiment of the present invention;  
         [0031]      FIG. 4A  is a side view in section of a foil journal bearing, as seen along line  4 - 4  in  FIG. 1 , according to an embodiment of the present invention;  
         [0032]      FIG. 4B  is an enlarged view of the foil retention slot area H depicted in  FIG. 4A , according to an embodiment of the present invention;  
         [0033]      FIG. 5  is a side view in section of a foil journal bearing, as seen along line  4 - 4  in  FIG. 1 , according to another embodiment of the present invention using an etched spring foil;  
         [0034]      FIG. 6A  is an enlarged view of a portion of an etched spring foil with cantilever beams, according to an embodiment of the present invention;  
         [0035]      FIG. 6B  is an enlarged view illustrating an alternate mounting arrangement for the undersprings of  FIG. 4A ;  
         [0036]      FIGS. 6C-6J  are end views of corrugations of the undersprings in  FIG. 4A ;  
         [0037]      FIG. 7  is a side view in section of a foil journal bearing, as seen along line  4 - 4  in  FIG. 1 , according to an alternative embodiment of the present invention;  
         [0038]      FIG. 8  is a side view in section of a foil journal bearing, as seen along line  4 - 4  in  FIG. 1 , according to still another alternative embodiment of the present invention  
         [0039]      FIG. 9  is a side view in section of a top foil, as seen along line  4 - 4  in  FIG. 1 , according to yet another embodiment of the present invention; and  
         [0040]      FIG. 10  is an enlarged view of the foil retention slot area H depicted in  FIG. 4A , according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0041]     The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims.  
         [0042]     The invention is useful for high speed rotating machinery. The present invention relates to pneumatic journal bearings supporting a rotating shaft of a variety of high speed rotating systems, such as auxiliary power units for aircraft or air conditioning machines and, more particularly, to a gas foil journal bearing having a foil with both a top foil and plurality of undersprings which have a high supporting capacity of the shaft when highly loaded and a high damping capacity. Additionally, the top foil has a leading edge and a trailing edge that push against each other to maintain the top foil shape when starting or stopping high speed rotating machinery.  
         [0043]     Also, foil bearing systems of the present invention are suitable for high-speed machines such as cryogenic turbo-rotors with both expander and compressor wheels running at tens of thousands of rpm or more. These bearings may also be used in the presence of liquid or cryogenic substances or mixed-phase lubrication. Foil bearings may achieve long service life with no scheduled maintenance as well as avoid air cabin contamination by eliminating the oil lubrication system required by conventional ball bearings. The foil bearing system of the present invention accommodates position fluctuations relative to the rotating element in the bearing to minimize damage to aerodynamic components in the event of a system malfunction.  
         [0044]     Bearings in certain military aircraft, such as fighters, must meet the additional requirements of very high speed and severe gyroscopic moments with compact construction (for example, light weight, small rotor, and high ambient temperatures). Furthermore, optimal output power and efficiency of brushless electric motors/generators are realized at higher speeds, in the range beyond 60,000 rpm. Conventional foil bearing systems, containing only one layer of underspring, are considered incapable of meeting these speeds and operating conditions. Furthermore, motor-driven compressor systems, turbo-alternators, and turbochargers put stringent demands on the application of these bearings. Foil bearing systems in these motor-driven compressor systems and turbo-alternators must have the ability to accommodate misalignment, rotor vibrations, shock loading, centrifugal growth, and elastic and thermal distortions, as well as the ability to provide sufficient damping and stiffness for stability.  
         [0045]     Radial displacement of a journal member, supported by the fluid pressure within a foil assembly, generates frictional damping forces on the sliding faces of a top foil and undersprings, thereby suppressing vibration of a journal member. However, since some conventional arrangements employ only a top foil and one flat spring with joggles and cam lobes, it is difficult to generate a sufficient level of frictional damping force, leading to a possibility that the journal member might undergo a damaging resonance phenomenon. Increase in the preload is necessary to increase damping. The increase in the preload may directly increase the start torque because the shaft may absorb all of the preload generated by the foils and springs.  
         [0046]     In contrast to past designs, the present invention provides a top foil positioned in the innermost layer of the foil assembly to receive a radially inward preload that is present between the top foil and the journal member. Most of this preload is not transferred to the shaft, which decreases the start torque. This foil will also receive a radially outward load from the fluid film that is present between the top foil and the shaft and this load is transmitted from the top foil to a stationary retaining member via a first underspring layer and a second underspring layer. One underspring layer may serve to control preload contact pressure while the other underspring layer may serve to optimize the fluid pressure between the top foil and the shaft. Thereby, the present invention eliminates the need for a pre-load bar as in the &#39;455 patent described above. Additionally, the impinging leading edge and trailing edge of the top foil maintain the top foil in an open position.  
         [0047]     An exemplary journal foil bearing system  10  of the present invention is shown in  FIG. 1 . A journal member  12  may house a shaft  14  within a bore  30 . The bore  30  may be of circular cross-section. The shaft  14  may be arranged for relative coaxial rotation with respect to the journal member  12  with a foil bearing  16  in between the shaft  14  and the journal member  12 .  
         [0048]     A top foil  18  is shown in  FIG. 2  before being bent around the shaft  14  to form part of the foil bearing  16 . The top foil  18  may be of a thin compliant metal strip, having a curvature that is larger than the curvature of the journal member  12 , with a tab  24  at one end or, both ends, for prevention of rotating or telescoping, as further described below. These tabs  24  may provide radial rigidity by securing the top foil  18  around the inner diameter of the journal member  12 . After bending, the top foil  18  may be disposed between the shaft  14  and the journal member  12 . The top foil  18  may be made from any material suitable for extreme temperatures, resistance to corrosion, and other extreme conditions. Suitable materials include nickel alloy, beryllium-copper, carbon fiber, and stainless steel.  
         [0049]     An underspring  22  is shown in  FIG. 3 . The underspring  22  may be of a thin compliant metal strip, having a curvature that is larger or smaller than the curvature of the journal member  12 , with a tab  24  at one end or both ends for prevention of rotating or telescoping, as further described below. Optionally, the underspring  22  may have corrugations  26  to accommodate expansion, excursions, and any misalignment. The underspring  22  can be made from the same material as the top foil  18 , or from any material suitable for extreme conditions, such as increased load capacity, e.g., 100 psi or more, at high speeds of perhaps 60,000 rpm or more while being subjected to high temperatures of perhaps 650 degrees C. or higher and resistance to corrosion.  
         [0050]      FIG. 4A  illustrates an embodiment of the present invention, that may comprise a first underspring  22 A (such as underspring  22  in  FIG. 3 ) that may be disposed as a first layer between the top foil  18  (such as in  FIG. 2 ) and the journal member  12  (such as in  FIG. 1 ), and a second underspring  22 B (such as underspring  22  in  FIG. 3 ) may be disposed as a second layer between the first underspring  22 A and the journal member  12 . The present invention may comprise two or more layers of undersprings, which exert higher pressure between undersprings  22 A,  22 B and the top foil  18 . The plurality of layers of undersprings  22 A,  22 B of the present invention more easily provides the preload against the top foil  18 , helps control the size of a fluid film gap  74 , between the shaft  14  and the top foil  18 , and maintains the top foil  18  location. The fluid film gap  74  may be thin for damping. The length of the top foil  18  controls the fluid film gap  74  between the shaft  14  and the top foil  18 . If the fluid film gap  74  is too large (e.g., when the top foil  18  length is too long), then low fluid film damping occurs. If the fluid film gap  74  is too small (e.g., when the top foil  18  length is too short) then preload may be transferred to the shaft  14 , from the top foil  18  contacting the shaft  14 . The top foil  18  working length  80  and the shaft  14  diameter may be the only factors that will determine the spacing between the top foil  18  and the shaft  14 . If the spacing between the shaft  14  and the top foil  18  becomes too large, loss of damping and stiffness may occur, causing the shaft  14  to become unstable. Additionally, the present invention is designed so that most of the preload may not be transferred onto the shaft  14 , but retained by the top foil  18  with ends  50 ,  60  that abut each other.  
         [0051]     If temperature increases during high performance conditions, the shaft  14  may increase in radius R (high speed may also result in increase in the shaft  14  radius R due to centripetal force). As the shaft  14  radius R increases, the top foil  18  and the undersprings  22 A,  22 B get pushed radially outward, keeping the fluid film thickness relatively constant. Radial displacement of the shaft  14 , supported by the fluid pressure on the top foil  18 , can generate large frictional damping forces between the outer circumference  90  of the top foil  18  and undersprings  22 A,  22 B, thereby suppressing vibration of the journal member  12 .  
         [0052]     The first underspring layer  22 A and the second underspring layer  22 B may have a non-linear behavior, with radial forces that vary in the circumferential direction. Also, providing longer cantilever beams  40  (ε 3 ) near the leading edge  60  may make the spring rate (in the radial direction) decrease at the leading edge  60 . With a lesser spring rate at the leading edge  60 , the wedge-shaped gap  72  may be formed as the shorter cantilever beams  40  (ε 1 ) at the trailing edge  50  have a higher spring rate (in the radial direction), such that trailing edge  50  is closer to the shaft  14 , than the leading edge  60 .  
         [0053]     Also, unlike the prior art, the present invention prevents the top foil  18  from collapsing on the shaft  14  while the outer circumference  90  of the top foil  18 , upon starting rotation, is preloaded radially. This may be achieved by using the top foil  18  structure and by using the first underspring layer  22 A with a low spring rate that is lower (i.e. “softer” or “less stiff”) than the second underspring layer  22 B which may have a high spring rate (i.e. “harder” or “stiffer”). A soft spring  22 A may serve to moderate contact pressure between a hard spring  22 B and the top foil  18 . The low stiffness of the soft spring  22 A also allows more even distribution of the force from the harder spring  22 B over the outer circumference  90  of the top foil  18 . The top foil  18  with tabs  20 ,  24  at both ends may provide radial rigidity that will keep the top foil  18  from collapsing on to the shaft  14  when distributed radial forces are applied from the springs  22 A,  22 B. Therefore, we may obtain high preload between the top foil  18  and the journal member  12  without transferring the same preload to the shaft  14 .  
         [0054]     An anti-wrapping tab  20  may be dimensioned to secure the top foil  18  from wrapping, as described below. The leading edge  60  and the trailing edge  50  meet in normal operation. In contrast, the ends of top foils in the prior art do not typically meet. A distance between the trailing edge  50  and the shaft  14  is shorter than a distance between the leading edge  60  and the shaft  14 . This relationship may be accomplished by having the spring rate at a portion of undersprings  22 A,  22 B under the trailing edge  50  be higher (i.e., stiffer spring) than the spring rate at a portion of undersprings  22 A,  22 B under the leading edge  60 . The top foil  18  ends may be disposed such that the trailing edge  50  is disposed upstream, from the leading edge  60 , in the direction G of the relative coaxial rotation of the shaft  14 . The difference in distances from the shaft  14  between the trailing edge  50  and the leading edge  60  (absolute value of distance between the trailing edge  50  and the leading edge  60 ) is a wedge-shaped gap  72 .  
         [0055]     An underspring, for example second underspring layer  22 B, may be formed of a material thicker than another underspring, for example, first underspring layer  22 A. In this situation, the thicker underspring  22 B would be “stiffer” or have a higher spring rate than the thinner underspring  22 A. Relative spring rates are interchangeable; in that second underspring layer  22 B may have the lower spring rate while the first underspring layer  22 A may have a higher spring rate. Likewise, first underspring layer  22 A may have a lower spring rate than the spring rate of the second underspring layer  22 A. An underspring, for example second underspring layer  22 B, may be formed of a material that is about the same thickness as another underspring, for example, first underspring layer  22 A.  
         [0056]     In  FIG. 4A , a foil retention slot  28 , in communication with the bore  30 , may be used for maintaining the installed position of the top foil  18  and the undersprings  22 A,  22 B by securing tabs  20 ,  24  within the foil retention slot  28 . The undersprings  22 A,  22 B do not necessarily have to be synchronized such that peaks and valleys match. An anti-wrapping tab  20  may be affixed to an end of the top foil  18 , the first underspring layer  22 A, or the second underspring layer  22 B, for example, by spot welding. Also, the anti-wrapping tab  20  may be an integral portion of the top foil  18 , the first underspring layer  22 A, or the second underspring layer  22 B, bent at an angle and adapted to be held into foil retention slot  28 . Likewise, an anti-telescoping tab  24  may be affixed to an end of the top foil  18 , the first underspring layer  22 A, or the second underspring layer  22 B, for example, by spot welding. Also, the anti-telescoping tab  24  may be an integral portion of the top foil  18 , the first underspring layer  22 A, or the second underspring layer  22 B, bent at an angle and adapted to be held into foil retention slot  28 . The slot  28  and the tabs  20 ,  24  may be of a shape suitable to secure the foils (for example, top foil  18 , and undersprings  22 A,  22 B), for example, an L- or Z-shaped slot  28 .  
         [0057]     As shown in  FIG. 4B , an approximately wedge-shaped gap  72  may be located between the top foil  18  and the shaft  14  at the leading edge. This assures that a fluid film may be developed within the wedge-shaped gap  72 . The film pressure from the fluid film can provide a bearing effect for the shaft  14  floating in the fluid, enabling rotation of the shaft  14  at a lower speed than otherwise obtainable.  
         [0058]     With reference to  FIG. 4A , the journal bearing  10  can be adapted to prevent failure of the top foil  18 . Such failure may be manifested, for example, by wrapping. This wrapping effect may occur when the shaft  14  rotation induces circumferential tensile stresses, shown by arrow E, in the top foil  18  Only circumferential tensile stresses, shown by arrow E, can cause the top foil  18  to tighten around the shaft  14  and potentially lead to failure of the top foil  18 . Preventing such failure may be achieved through using anti-wrapping tab  20 . The trailing edge  50  may be pushed towards the leading edge  60 . Pushing the trailing edge  50  and the leading edge  60  against each other may prevent the top foil  18  from collapsing against the shaft. The anti-wrapping tab  20  may be fixedly held by inserting into foil retention slot  28 , which may be dimensioned to snugly retain the anti-wrapping tab  20  within the confines of the foil retention slot  28 . The anti-wrapping tab  20  may serve to prevent wrapping, which is failure (for example, the shaft  14  may lock up and cease rotation), in the circumferential direction, of the top foil  18 . Anti-telescoping tab  24  may be fixedly held by insertion into foil retention slot  28 , which may also be dimensioned to snugly retain the anti-telescoping tab  24  within the confines of the foil retention slot  28 . The anti-telescoping tab  24  may serve to prevent the top foil  18  or the undersprings  22 A,  22 B from telescoping, which is failure in the axial direction wherein the top foil  18  or undersprings  22 A,  22 B move out the axial ends of the bore  30 . The anti-telescoping tab  24  may prevent axial movement of the top foil  18 . Additionally, the anti-telescoping tab  24  may provide a surface where ends  50 ,  60  may abut each other. Retaining rings or other features that block the slot  28  at the axial ends of the bearing could be used to prevent the top foils and undersprings from moving axially or telescoping in the housing. Another embodiment of the present invention is shown in  FIG. 5 . In contrast to the embodiment shown in  FIG. 4A , at least one of the undersprings  22 A,  22 B may be in the form of a chemically etched spring foil  32 . A shim  34  may be placed in between the etched spring foil  32  and the underspring  22 B to aid in circumferential force distribution.  
         [0059]      FIG. 6A  shows an enlarged view of a portion of the chemically etched spring foil  32 , which may comprise a plurality of cantilever beams  40 . The etched spring foil  32  may be formed as a thin, flat sheet having chemically etched slots  44  of a spring pattern  42  that causes cantilever beams  40  to stand erect, as shown installed in  FIG. 5 , and function as springs for radial forces when the foil  32  is bent to install inside the bore  30  of the journal member  10 . The cantilever beams  40  may have heights and spring rates that vary along the length of converging fluid channel. As shown in  FIG. 6A , the spring pattern  42  may comprise cantilever beams  40  that are not uniformly shaped or dimensioned. Cantilever beams  40  of different sizes or shapes may have different spring rates. For example, a perimeter row  44  of cantilever beams  40  may be designed to have a different spring rate than an adjacent row  46  of cantilever beams  40 . Furthermore, other rows, such as interior row  48  of cantilever beams  40 , may have a different size and shape than either the perimeter row  44  or adjacent row  46 .  
         [0060]     Cantilever beams  40  may vary in pitch P and width W to optimize the spring force by providing different amounts of resilient material to support the top foil  18 . For example, pitch P 1  may be less in magnitude than pitch P 2 , which, in turn, may be less in magnitude than pitch P 3 . Likewise, cantilever beam  40  width W 1  may be less in magnitude than width W 2 , which may be less in magnitude than width W 3 .  
         [0061]     In  FIG. 6B , underspring  22 A may include a number of corrugations  26  which are varied in pitch P 1 , P 2 , P 3  to vary the force distribution while supporting the top foil  18 . The undersprings  22 A,  22 B may be in the shape of a periodic wave, several forms of which are illustrated in  FIGS. 6C-6J . The undersprings  22 A,  22 B may also be in the shape of a periodic wave. Almost an infinite variety of forms may be made for the corrugations  26  by changing the wavelength W and/or the peak-to-peak wave amplitude β. By changing W and β, one can change implicitly the stiffness of the undersprings  22 A,  22 B and also the damping, which partly depends upon the frictional dissipation of energy due to tangential motion of the top foil  18  relative to the undersprings  22 A,  22 B. Alternating wave heights are shown in  FIG. 6H , such that two or more alternating peak-to-peak wave amplitudes β 1  and β 2  may exist. β 1  and β 2  are not necessarily equal and using only one underspring  22 A is optional. Similarly two different wave designs can be superimposed into one spring as shown in  FIG. 6J . Nested corrugations are shown in  FIG. 61 , such that undersprings  22 A,  22 B may exist in a nested relationship, wherein β 1  and β 2  are not necessarily equal. Furthermore, changing the wave amplitude can vary the local bearing characteristics along its working length  80 , which is the circumferential distance along the top foil  18  surface within the bore  30 , excluding the tabs  20 ,  24  within the foil retention slot  28 . Such variations can provide non-linear behavior to the undersprings  22 A,  22 B such that higher than normal loads are accommodated.  
         [0062]     The fluid film gap  74  between the top foil  18  and the shaft  14  may remain constant (since the top foil  18  leading edge  60  and the trailing edge  50  are pushed against each other) regardless of the variations in spring  22  height and bore  30  size. The variations in spring  22  height and bore  30  size will change the preload only and not the fluid film gap  74  between the top foil  18  and the shaft  14 . The top foil  18  working length  80  and the shaft  14  diameter may be the only factors that will determine the spacing between the top foil  18  and the shaft  14 . If the spacing between the shaft  14  and the top foil  18  becomes too large, loss of damping and stiffness may occur, causing the shaft  14  to become unstable. In  FIG. 7 , another alternative embodiment is shown using a plurality of undersprings  22 A,  22 B and a plurality of foil retention slots  28 A,  28 B, and  28 C instead of only one foil retention slot  28  as in  FIG. 4A . A journal foil bearing system  10  may include a journal member  12  with a bore  30 , and a shaft  14  arranged within the bore  30  for relative coaxial rotation with respect to the journal member  12 . A top foil  18  may be disposed between the shaft  14  and the journal member  12 . A plurality of first undersprings  22 A may be disposed between the top foil  18  and the journal member  12 , and a plurality of second undersprings  22 B may be disposed between the first undersprings  22 A and the journal member  12 . The plurality of first undersprings  22 A may be circumferentially separated and may be secured within a plurality of foil retention slots  28 A,  28 B, and  28 C. The slots  28 A-C may be in communication with or integral with the bore  30  and tabs  20 ,  24  in the top foil  18 , the first undersprings  22 A, and the second undersprings  22 B. The tabs  20 ,  24  can allow the top foil  18 , the first undersprings  22 A, and the second undersprings  22 B to be held in the foil retention slots  28 A,  28 B,  28 C and secured against wrapping and telescoping.  
         [0063]     Still another embodiment of the present invention is shown in  FIG. 8 . In this embodiment, one underspring  22  may be used, instead of a plurality of undersprings  22 A,  22 B as in the above embodiments. A journal foil bearing system  10  may comprise a journal member  12  with a bore  30 , and a shaft  14  arranged within the bore  30  for relative coaxial rotation with respect to the journal member  12 . As described above in reference to  FIG. 4A , a top foil  18  may be disposed between the shaft  14  and the journal member  12 . However, instead of using two separate undersprings, one underspring  22 , longer than the working length  80  of the top foil  18 , is used. The underspring  22  may be wound at least twice around the circumference of the top foil  18 . In other words, the underspring  22  is wound at least two times around the circumference  90  of the top foil  18 . The underspring  22  may have two different spring rates, with one spring rate for the first winding around the top foil  18  and another spring rate for a subsequent winding around the top foil  18 . The different spring rates may be accomplished by varying the corrugation  26  wave lengths W, peak-to-peak wave amplitudes β, cantilever beam  40  pitch ε, or cantilever beam  40  widths δ, as described above regarding  FIGS. 6A-6J .  
         [0064]      FIG. 9  may be referenced to better appreciate how the top foil  18  may be dimensioned to accommodate bore  30  shape and dimensions and shaft  14  shape and dimensions. A wedge-shaped gap  72  may be formed by the combination of the top foil  18  radius as well as the spring rate difference along the circumference under the top foil  18  from the undersprings  22 A,  22 B.  FIG. 9  shows the top foil  18  bent and inserted into bore  30 , as described above regarding  FIGS. 4A, 4B ,  5 ,  7 , and  8 . Only the top foil  18  is shown in  FIG. 9  for illustration purposes. To promote wedge-shaped gaps  72  and to optimize the pre-load distribution along the circumference, different radii of curvature may be present at different locations A, B, C along the working length  80  of the top foil  18 . The top foil  18  working length may also be considered to be divided into various portions, for example, sector arc lengths along the inner circumference of top foil  18 , in a clockwise direction. For example, a first arc sector length A-B may be measured between points A and B, a second arc sector length B-C may be measured between points B and C, and a third arc sector length C-A may be measured between points C and A.  
         [0065]     To normalize the top foil dimensions to the shaft  14  radius, R (as shown in  FIG. 4A ), the total working length (sum of arc sector lengths A-B, B-C, and C-A) of the top foil  18  may be selected to be between about 1.0003(2πR) to about 1.010(2πR) in length along the inner circumference of the top foil  18 , preferably between about 1.003(2πR) to about 1.010(2πR) in length along the inner circumference of the top foil  18 . First arc sector length A-B and third arc sector length C-A may each be designed to be in the range from about 0.20(2πR) to about 0.40(2πR). Second arc sector length B-C may be designed to have a different length, for example, by subtracting A-B and C-A from the total working length (inner circumference of top foil  18 ).  
         [0066]     Radii of curvature for the different lengths may also be designed to normalize the top foil dimensions to the shaft  14  radius R. The radius for first arc sector length A-B and the radius for third arc sector length C-A may each be in the range from about 1.05 R to about 1.10 R. The radius for second arc sector length B-C may be in the range from about 1.05 R to about 5 R, preferably from about 1.05 R to about 1.5 R {Alan, this is to provide a fall-back position, since you expanded this range from 1.5 R to 2 R.], where R is the shaft  14  radius. The radii of curvature are measured before insertion of the top foil  18  into the journal member  12 .  
         [0067]     In  FIG. 10 , the foil retention slot  28  is shown in an enlarged view. The design may serve to mitigate crowding in the foil retention slot  28 , which may occur when using a top foil  18 , a first underspring  22 A, and a second underspring  22 B. As an example, seven tabs may be located within the foil retention slot  28 . The top foil  18  may have the first tab, an anti-telescoping tab  24  at the trailing edge  50  of the top foil  18  and a second, anti-telescoping tab  24  at the leading edge  60  of the top foil  18 . A third tab, an anti-wrapping tab  20 , may be located at the distal end of the longer anti-telescoping tab  24 , which may be attached to the trailing edge  50 . A fourth tab, an anti-telescoping tab  24  may be located at the trailing edge  52  of the first underspring  22 A and a fifth anti-telescoping tab  24  may be at the leading edge  62  of the first underspring  22 A. The sixth tab, an anti-telescoping tab  24 , may be located at the trailing edge  54  of the second underspring  22 B and the seventh, an anti-telescoping tab  24  may be located at the leading edge  64  of the second underspring  22 B.  
         [0068]     Situating all of the seven tabs  20 ,  24  into the foil retention slot  28  may become difficult, especially if the foil retention slot is narrow. If, however, the width of foil retention slot  28  is increased, then the top foil  18  may lose its circularity. The trailing edges  50 ,  52 ,  54  and the leading edges  60 ,  62 ,  64  may potentially push radially outward if not well supported. If the top foil  18  loses circularity, then the top foil  18  may form a teardrop shape, where the flatter portions near the foil retention slot  28  may transmit excessive pre-load forces to the shaft  14  (shown in  FIG. 4A ).  
         [0069]     The tab-support design shown in  FIG. 10  may resolve this problem through controlled anti-telescoping tab  24  lengths and controlling the anti-wrapping tab  20  length. When the trailing edge  60  is pushed to the far right side of the foil retention slot  28 , the trailing edge  60  will be supported by the spring bumps. Adequate length of the anti-wrapping tab  20  may ensure that the trailing edge  60  is pushed to the right. The leading edge  50  may be supported as the anti-telescoping tab  24  bent from the leading edge  50  rests on the anti wrapping tab  20 . The anti-wrapping tab  20  may support the first anti-telescoping tab  24 , which is at the leading edge  50  of the top foil  18 .—Controlling the top foil  18  anti-telescoping tabs  24 —on the leading edge  50 , making it slightly shorter than the top foil  18  anti-telescoping tab  24  on the trailing edge  60 , may help to maintain a wedge-shaped gap  72 , as seen in  FIGS. 5, 7 , and  8 .  
         [0070]     Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained therein.