Foil bearing

In a foil bearing (23) having a plurality of foils (26) disposed in a gap between a rotating member (12) and a stationary mount member (25), a circumferentially extending portion of each foil and a moveable member (27) rotatable with respect to the stationary mount member are provided with magnets (30, 31) so that a magnetic force between these magnets urges the foils toward the rotating member. By rotating the moveable member to vary the relative circumferential position of the moveable member with respect to the foils, it is possible to adjust an amount of magnetic force between the magnets of the moveable member and the magnets of the foils.

TECHNICAL FIELD

The present invention relates to foil bearings. Particularly, the present invention relates to foil bearings suitable for use as a journal bearing or thrust bearing for a rotor shaft of a micro gas turbine generator.

BACKGROUND OF THE INVENTION

It has been conventionally known to use a foil bearing as a bearing for a rotor shaft. A foil bearing typically comprises a housing surrounding the rotor shaft and a plurality of foils disposed in a space or gap between the rotor shaft and the housing and arranged in a circumferential direction in such a manner that each of the foils is attached to the housing in a cantilever fashion with its free end being urged toward the rotor shaft. As the rotor shaft rotates, a fluid such as ambient air is drawn in between the rotor shaft and the foils, creating a fluid film between an outer surface of the rotor shaft and the foils to allow the rotor shaft to rotate with low friction. Such a foil bearing that supports a load of the rotor shaft via the fluid film formed as a result of the rotor shaft rotation may be called a hydrodynamic foil bearing.

In the foil bearing as described above, characteristics of the fluid film formed between the rotor shaft and the foils may vary between a state where the rotor shaft rotation speed is low, e.g., at the start up or shut down, and a state where the rotor shaft rotation speed is high. Thus, in order to achieve a stable rotation in both states, it is desired that a preload of the foils against the rotor shaft is adjustable according to the rotation speed of the rotor shaft.

U.S. Pat. No. 4,445,792 issued to Trippett has disclosed a foil bearing in that the preload of the foils can be adjusted. The foil bearing has a housing surrounding the rotor shaft, and a plurality of foil mounts arranged along the circumferential surface of the rotor shaft and rotatably supported relative to the housing, wherein each mount has a foil attached thereto to cantilever the foils and has a driven portion. Further, a drive mechanism (ring gear) rotatable relative to the housing is provided outside the housing so as to be engageable with the driven portions of the foil mounts to simultaneously rotate the mounts and vary the preload of the foils against the shaft surface.

In such a foil bearing, however, the preload of the foils against the rotor shaft relies upon an elasticity of each foil, and thus it is difficult to vary the preload in a sufficiently large range, and further, an excessive force tends to concentrate on a root portion of each cantilevered foil.

Other known prior art may include U.S. Pat. No. 3,893,733 issued to Silver et al, which has disclosed to use foil supports that slidingly contact the foils to increase the stiffness (or rigidity) of the foils. U.S. Pat. No. 4,178,046 issued to Silver et al has disclosed a foil bearing in that each foil is mounted intermediate the ends thereof. U.S. Pat. No. 4,128,280 issued to Purtschert has disclosed various types of floating gas bearing utilizing a magnet in the bearing. Further, Japanese Patent Application Laid-Open (kokai) No. 10-292818 has disclosed to use a thrust magnetic bearing to assist a hydrodynamic gas bearing at low rotational speed of the rotor shaft.

BRIEF SUMMARY OF THE INVENTION

In view of such problems of the prior art, a primary object of the present invention is to provide a foil bearing that can allow stiffness or preload of foils against a rotating member, such as a rotor shaft, to be varied in a wide range to ensure stable rotation of the rotating member at both low and high rotation speed regions, while avoiding an excessive force applied locally on a part of each foil.

A second object of the present invention is to provide such a foil bearing simple in structure and low at cost.

According to the present invention, such objects can be accomplished by providing a foil bearing for supporting a rotating member that rotates about an axis, comprising: a stationary mount member spaced from the rotating member so that a gap is defined between the stationary mount member and the rotating member; a moveable member rotatable about the axis; and a first foil disposed in the gap between the rotating member and the stationary mount member to support the rotating member via a fluid film when the rotating member rotates, wherein the first foil comprises a substantially circumferentially extending portion to which a first magnet is provided; wherein the moveable member is provided with a second magnet so that a magnetic force between the first magnet and the second magnet can urge the first foil toward the rotating member; and wherein an amplitude of the magnetic force can be adjusted by rotating the moveable member around the axis to vary a relative circumferential position between the first magnet and the second magnet.

Thus, according to the present invention, the interaction between the magnets provided to the moveable member and the magnets provided to the circumferentially extending portions of the foils can create a force urging the foils toward the rotating member, and the urging force can be adjusted by rotating the moveable member and thereby varying the position thereof relative to the foils. Thus, when the present invention is applied to the journal bearing and/or thrust bearing of a gas turbine engine, it is possible to control the angular position of the moveable member according to the rotational speed of the gas turbine engine, thereby creating an urging force (or preload) suitable for the rotational speed so that favorable bearing properties can be achieved for a wide range covering the low to high rotational speed regions. Further, the urging force acts upon the circumferentially extending portion of each foil, and thus concentration of an excessive force on the root portion (or bending portion) of the foil can be avoided.

When the foil bearing comprises a plurality of the first foils arranged in a circumferential direction of the rotating member, at least one of the plurality of first foils may be provided with the first magnet. In the case that a rotor shaft is used as the rotating member, for example, only a part of the first foils positioned on a lower side of the rotor shaft and thus supporting the weight of the rotor shaft may be provided with the first magnet. It is also possible that each of the first foils is provided with the first magnet.

When the foil bearing comprises a plurality of the first foils and the moveable member is provided with a plurality of the second magnets, the second magnets may have varying magnetic strengths. In the case that a rotor shaft is used as the rotating member, for example, those of the second magnets associated with the first foils disposed on the lower side of the rotor shaft may preferably have a greater strength than those of the second magnets associated with the first foils disposed on the upper side of the rotor shaft to whereby compensate the effects of the rotor shaft weight. Alternatively or additionally, the first magnets provided to the first foils may have varying magnetic strengths. It is also possible that spaces between adjacent ones of the plurality of first foils are varied.

In one embodiment of the present invention, the rotating member comprises a shaft having a substantially cylindrical portion, and the stationary mount member surrounds the shaft so that the gap is formed as an annular gap defined between a cylindrical surface of the cylindrical portion of the shaft and the stationary mount member. In such a case, if the shaft consists of a rotor shaft of a gas turbine engine, the foil bearing can constitute a journal bearing for the rotor shaft of the gas turbine engine.

When the foil bearing comprises a plurality of the first foils arranged in a circumferential direction of the shaft serving as the rotating member, a second foil (top foil) may be disposed between the shaft and the plurality of first foils such that the second foil extends in the circumferential direction to have a substantially cylindrical shape. Alternatively, it is possible to use the top foil in place of the plurality of first foils, and provide the top foil with the first magnet (in such a case, the top foil serves as the first foil).

In another embodiment of the present invention, the rotating member may comprise a disk-shaped member and the gap may be defined between the stationary mount member and a planar surface of the disk-shaped member. In such a case, by providing the disk-shaped member as a unitary portion of a rotor shaft of a gas turbine engine, the foil bearing can constitute a thrust bearing of the rotor shaft of the gas turbine engine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a longitudinal cross sectional view for showing a micro gas turbine generator to which the present invention is applied. The micro gas turbine generator includes a gas turbine engine as a power source and an electric generator driven by the gas turbine engine. The gas turbine engine comprises an annular main housing1of which end remote from the generator is closed, an end plate2attached to an open end of the main housing1, a perforated annular inner housing3coaxially received inside the main housing1to define a combustion chamber4therein, and a plurality of fuel injectors5each having a nozzle end projecting into the combustion chamber4.

The generator comprises a cylindrical main housing6and a pair of end plates7and8attached to either axial end of the main housing6. The main housing6coaxially receives a stator coil9therein. The end plate7facing the gas turbine engine is provided with a tubular extension10extending centrally from the end plate7toward the gas turbine engine. Also, the end plate7facing the gas turbine engine is joined to the opposite end plate2of the gas turbine engine by a plurality of stay members11.

The gas turbine engine is additionally provided with an integral rotor shaft12carrying a compressor wheel13and a turbine wheel14. The compressor wheel13and turbine wheel14are composed of a plurality of compressor blades and turbine blades, respectively. The integral rotor shaft12of this embodiment is made of ceramic material, and is integrally formed with the compressor wheel13and turbine wheel14as a single-piece ceramic rotor assembly. Alternatively, the rotor shaft12may be implemented as an assembly comprising a plurality of individual component parts which may be made of a same material or different materials. The material for the turbine blades should be ceramic or other heat resistant material which may be either electroconductive or electrically insulating, and the choice of the material or materials depends on the particular configuration and specifications of the gas turbine engine. The axial length of this micro gas turbine engine is approximately 10 cm.

The compressor wheel13forms a radial compressor section in cooperation with a shroud15formed by a part of the end plate2attached to the gas turbine main housing1. The inlet end of the compressor section opens out in the axial direction to face the generator. The outlet end of the compressor section communicates with a gap defined between the main housing1and the inner housing3via a diffuser17and an array of stator vanes18arranged in a circumferential direction.

The turbine wheel14forms a radial turbine section in cooperation with a turbine casing16formed by a part of the main housing1. The inlet end of the turbine section communicates with an outlet end of the combustion chamber4via an inlet nozzle19. The combustion chamber4in this embodiment extends from the inlet end of the turbine section in a direction away from the generator. The outlet end of the turbine section opens out in the axial direction facing away from the generator.

The integral rotor shaft12further comprises a generator shaft20which is an integral extension of the rotor shaft12, and is passed centrally through the generator. Permanent magnet pieces21are mounted to the generator shaft20to form the main functional part of the generator in cooperation with the stator coil9.

A thrust bearing22is provided at the end portion of the rotor shaft12. Further, a pair of journal bearings23,24according to the present invention are provided in the. end plates7,8to rotatably support the rotor shaft12at two points.

FIG. 2is a cross sectional view taken along the lines II—II in FIG.1and shows the journal bearing23in detail. It should be noted that inFIG. 2, the end plate7is omitted for clarity and the journal bearing24may have the same structure as the journal bearing23. The journal bearing23comprises an annular stationary mount member25which surrounds the rotor shaft12(or generator shaft20) serving as a rotating member such that a gap is defined between an inner surface of the mount member25and an outer surface of the rotor shaft12, a plurality of foils26serving as first foils disposed in the gap between the rotor shaft12and the stationary mount member25, an annular moveable member27surrounding the stationary mount member25and rotatable with respect to the stationary mount member25around an axis of the rotor shaft12, and an operation bar28attached to the moveable member27for rotational operation therefor. The stationary mount member25is unrotatably secured to the end plate7by a suitable means. The operation bar28extends in a radial direction through a slit formed in the end plate7and can be controlled by a suitable control means not shown in the drawings in accordance with the rotational speed of the gas turbine engine, for example.

Each foil26is made of a flexible material such as a metal, and has one end received in a corresponding one of recesses29formed in an inner circumferential surface of the stationary mount member25so that the end is fixedly attached to the mount member25. Each of the foils26is bent at a part near the attachment so that a main portion thereof extends in a circumferential direction substantially along the outer surface of the rotor shaft12. In the embodiment shown in this drawing, a part of one foil26overlaps a part of a circumferentially neighboring foil26, but in another embodiment each foil26may not overlap its neighboring foil26. The foils26thus bent are urged toward the rotor shaft12due to their own elasticity. In the foil bearing constructed as above, as the rotor shaft12rotates in a direction indicated by an arrow inFIG. 2, a fluid such as ambient air is drawn between the rotor shaft12and the foils26whereby the rotor shaft12is supported with low friction.

FIGS. 3aand3bare enlarged partial views of FIG.2and show a detailed structure of an embodiment of the journal bearing23according to the present invention. As also shown inFIG. 4, a magnet (first magnet)30is provided to cover an substantially entire outer surface of the circumferentially extending portion of each foil26in such a manner that the S and N magnetic poles of the magnet30are arranged in the circumferential direction. This can be achieved by making the foil26of a magnetic material and suitably magnetize the same, or alternatively, attaching a plate-shaped magnet to a surface of the foil26by a suitable means. In this embodiment, the direction of magnetic pole arrangement of one foil26is opposite to that of an adjoining foil26whereby the overlapping portions assume the same magnetic polarity. Thus the N pole and S pole are arranged alternately in the circumferential direction for the whole foils26. Further, the moveable member27is provided with a plurality of magnets (second magnets)31so that the N and S poles are arranged alternately in the circumferential direction. This can be also achieved by forming the moveable member27of a magnetic material and suitably magnetizing the same or by attaching plate-shaped magnets onto the inner surface of the moveable member27. Preferably, a circumferential pitch (angle) of the magnetic pole of the magnets31provided to the moveable member27is substantially the same as that of the magnets30provided to the foils26. It should be noted that the magnets30may be provided to an inner surface of each foil26although the interaction of the magnets30with the magnets31of the moveable member27would be weakened due to a larger distance therebetween.

In a state shown inFIG. 3a,the magnetic poles of a same polarity are aligned in the circumferential direction between the magnets30of the foils26and the magnets31of the moveable member27, thus creating a repulsive force therebetween. This causes each foil26to be more strongly urged against the outer circumferential surface of the rotor shaft12. This is practically equivalent to increasing the stiffness of each foil26.

On the other hand, in a state shown inFIG. 3b,the moveable member27has been rotated with respect to the stationary mount member25and hence the foils26(counterclockwise in this drawing), so that each magnetic pole (e.g., S pole) of the magnets31of the moveable member27overlap the both magnetic poles (i.e., N pole and S pole) of the magnets30of the foils26, creating both the attractive and repulsive forces which cancel each other. Thus, totally, no urging force is created from the magnetic interaction between the moveable member27and the foils26in this state.

As described, according to the above embodiment of the present invention, the interaction between the magnets31of the moveable member27and the magnets30provided to the circumferentially extending portions of the foils26can create a force for urging the foils26toward the rotor shaft12, and an amount of the urging force acting upon the foils26can be adjusted by rotating the moveable member27and thereby varying the relative position thereof with respect to the foils26. Thus, by controlling the angular position of the moveable member27according to the rotational speed of the gas turbine engine, for example, it is possible to provide an appropriate urging force (preload) depending on the rotational speed to thereby achieve a preferable bearing property for a wide range from low to high rotational speed regions. The urging force acts upon the circumferentially extending portion of each foil26, and thus it is possible to prevent an excessive force from being applied on the root portion (bending portion) of the foil26.

FIGS. 5aand5bare enlarged partial views similar toFIGS. 3aand3band show another embodiment of a journal bearing embodying the present invention. In these drawings, component parts similar to those inFIGS. 3aand3bare denoted with same reference numerals. The journal bearing23aofFIGS. 5aand5bdiffers from the embodiment ofFIGS. 3aand3bin a sense that the magnet30of each foil26does not occupy an entire part of the circumferentially extending portion of the foil26but takes up only a part (near the root portion) of the same such that the magnets30of adjacent pairs of the foils26do not overlap each other. Accordingly, the magnets30of the adjacent foils26assume the same circumferential orientation. In this embodiment also, the circumferential pitch (angle) of the magnetic poles of the magnets31provided to the moveable member27is substantially the same as that of the magnets30provided to the foils26.

In a state shown inFIG. 5a,similarly to the state shown inFIG. 3a,the magnetic poles of a same polarity are aligned in the circumferential direction between the magnets30of the foils26and the magnets31of the moveable member27, and the resulting repulsive force urges each foil26toward the outer circumferential surface of the rotor shaft12.

In a state shown inFIG. 5b,similarly to the state shown inFIG. 3b,the moveable member27has been rotated so that each magnetic pole of the magnets31of the moveable member27overlap the both magnetic poles of the magnets30of the associated foils26, and the mutually canceling attractive and repulsive forces result in substantially no magnetic force acting between the moveable member27and the foils26. Thus, in the embodiment ofFIGS. 5aand5b,the amount of force urging the foils26toward the rotor shaft12can be varied by changing the position of the moveable member27. Further, the urging force is applied on the circumferentially extending portion of each foil26, and thus it is possible to prevent an excessive force from being concentrated on the root portion (bend portion) of the foil26.

FIG. 6is a cross sectional view similar to FIG.2and shows another embodiment of a journal bearing embodying the present invention. In this drawing, component parts similar to those inFIG. 2are denoted with same reference numerals and detailed explanation thereof is omitted. This journal bearing23bcomprises a top foil (second foil)32having one end held by the stationary mount member25and extending in a circumferential direction to surround the approximately entire outer surface of the rotor shaft12and assume a substantially cylindrical shape. The foils (first foils)26attached to the stationary mount member25support the rotor shaft12via the top foil32. In this embodiment, each of the foils26disposed in a gap between the stationary mount member25and the top foil32extends from its root portion in a direction opposite the direction of rotation of the rotor shaft12. Further, adjacent foils26do not overlap each other. By providing the top foil32, a frictional force is created between the top foil32and the foils26, and the frictional force serves as Coulomb damping force that can contribute to improving rotational stability of the rotor shaft12.

FIGS. 7aand7bare enlarged partial cross sectional views for showing the journal bearing23bofFIG. 6more in detail. Similarly to the embodiments shown inFIGS. 3a,3bandFIGS. 5a,5b,each foil26disposed in a space between the stationary mount member25and the top foil32is provided with a magnet30having N and S magnetic poles arrange in a circumferential direction. Also, the moveable member27rotatably disposed outside the stationary mount member25is provided with a plurality of magnets31so that the N and S magnetic poles are arranged alternately in the circumferential direction. Owing to such a structure, in a similar manner explained with regards to the preceding embodiments, a force urging the foils26toward the top foil32(i.e., a force urging the top foil32into a smaller diameter) is created in a state ofFIG. 7a,while in a state ofFIG. 7b,substantially no such urging force is created from the magnetic interaction. Thus, it should be understood that the present invention can be applied to the embodiment using the top foil32.

FIG. 8is a cross sectional view similar to FIG.2and shows another embodiment of a journal bearing embodying the present invention. In this drawing also, component parts similar to those inFIG. 2are denoted with same reference numerals and detailed explanation thereof is omitted. This journal bearing23calso comprises the top foil32having one end held by the stationary mount member25and extending in the circumferential direction to surround the approximately entire outer surface of the rotor shaft12and assume a substantially cylindrical shape, but is not equipped with the foils26between the top foil32and the stationary mount member25. In other words, in this embodiment, the top foil32functions as a first foil.

FIGS. 9aand9bare enlarged partial cross sectional views for showing the journal bearing23cofFIG. 8more in detail. As shown in the drawings, a plurality of magnets33are provided on the outer surface of the top foil32so that the N and S magnetic poles are alternately arranged in the circumferential direction. The moveable member27is provided with the magnets31so that the angle pitch of the magnetic poles is the same as that of the top foil32. Owing to such a structure, in a state ofFIG. 9a,the (repulsive) magnetic force created between the magnets33of the top foil32and the magnets31of the moveable member27urges the top foil32toward the rotor shaft12, while in a state ofFIG. 9b,substantially no such urging force is created from the magnetic interaction. Thus, it is possible to apply the present invention to the embodiment comprising only the top foil32and vary the position of the moveable member27to adjust the amount of force for urging the top foil32toward the rotor shaft12. The urging force acts upon the entire body of the top foil32and thus local concentration of an excessive force can be avoided.

FIG. 10is a cross sectional view similar to FIG.1. The gas turbine generator ofFIG. 10differs from that ofFIG. 1in a sense that the present invention is applied to the thrust bearing inFIG. 10instead of the journal bearings. As shown the drawing, this thrust bearing22acomprises a disk-shaped portion35unitarily provided to an end of the rotor shaft12, and a pair of stationary mount members36,37axially interposing the disk-shaped portion35therebetween. In other words, the disk-shaped portion35serves as a rotating member in this embodiment. One stationary mount member36is secured to the end plate8while the other stationary mount member37is securely received in a cover38which in turn is secured to the end plate8. Although not shown inFIG. 10, a plurality of foils39are arranged in a circumferential direction between the disk-shaped portion35and each of the stationary mount members36,37, so that the foils39abut planar surfaces (surfaces facing in the axial direction) of the disk-shaped portion35(see FIG.11). Further, a pair of moveable members40,41are disposed on either side of the stationary mount members36,37opposing the disk-shaped portion35in such a manner that the moveable members40,41can rotate about the rotation axis of the rotor shaft12(and hence of the disk-shaped portion35). Operation bars42are provided on outer cylindrical surfaces of the moveable members40,41and extend radially through slots formed in the end plate8and cover38. The operation bars42are controlled by an appropriate controller not shown, whereby the angular position of the moveable members40,41can be varied according to the rotational speed of the gas turbine engine, for example.

FIG. 11is a partial schematic view for showing the thrust bearing22amore in detail and shows a part of the thrust bearing22aalong the circumferential direction. As shown, the plurality of foils39are disposed on a side of each stationary mount members36,37facing the disk-shaped portion35to constitute a foil bearing. Each foil39is made of a flexible material, and one end thereof is fixedly received in an associated one of recesses43formed in the stationary mount members36,37. Each foil39is bent near the root end (or fixed end) thereof and its main portion extends substantially circumferentially in a direction of rotation of the disk-shaped portion35as indicated by an arrow in the drawing. In this embodiment, adjacent foils39do not overlap each other and are spaced from each other in the circumferential direction. As the rotor shaft12rotates together with the disk-shaped portion35, a fluid such as ambient air is drawn in between the foils39and the disk-shaped portion35to form fluid films between the foils39and the opposing surfaces (planar surfaces) of the disk-shaped portion35, which enables the disk-shaped portion35to rotate with low friction.

In this embodiment also, the circumferentially extending portion of each foil39is provided with a magnet (first magnet)44so that the N and S magnetic poles of the magnet44are arranged in the circumferential direction. Further, each of the moveable members40,41is provided with a plurality of magnets (second magnets)45so that the N and S magnetic poles are alternately arranged in the circumferential direction. In this embodiment, the directions of arrangement of N and S magnetic poles of adjoining foils39are opposite to each other so that the circumferential pitch of the magnetic poles of the foils39corresponds to that of the moveable members40,41. Thus, it should be understood that the direction of arrangement of the N and S poles in each foil39can be determined arbitrarily depending on concrete design details such as the space between the adjacent foils39, presence/absence of overlap between the adjacent foils39, etc.

In a state shown inFIG. 11, a repulsive force is generated between the magnets45of the moveable members40,41and the magnets44of the foils39, and the force urges the foils39against the disk-shaped portion35. Although the drawing is omitted, it will be appreciated that by rotating the moveable members40,41in the circumferential direction, the amount of urging force resulting from the magnetic interaction can be varied in a manner similar to that of the above embodiments. This can make it possible to control the moveable members40,41according to the rotational speed of the disk-shaped portion35(or rotor shaft12), for example, to thereby adjust the urging force acting upon the foils39so that favorable bearing characteristics can be achieved in both the low and high rotational speed regions. Again, the urging force acts upon the circumferentially extending portion of each foil39and thus the excessive concentration of the load on the root portion of the foil39can be avoided. As described, the present invention can be favorably applied to the thrust bearing. It should be noted that although in this embodiment a foil bearing is constituted on each side of the disk-shaped portion35, it may be possible to provide a foil bearing only on one side of the disk-shaped portion35when it is known that an axial force is imparted on the disk-shaped portion35in only one direction.

FIGS. 12 and 13are cross sectional views similar to FIG.2and they each show yet another embodiment of the present invention. In these drawings, component parts similar to those inFIG. 2are denoted with same reference numerals and detailed explanation thereof is omitted. In the journal bearing23dofFIG. 12, a lowermost foil26ais provided with a larger thickness than the other foils26and accordingly has a larger stiffness. This is because the lowermost foil26amust support a larger load due to the gravity, and the larger stiffness of the lowermost foil26acan compensate the effect of the gravity to thereby prevent an undesirable shift of the rotational axis of the rotor shaft12. For a similar purpose, in the journal bearing23eshown inFIG. 13, spaces between adjacent foils26are varied. Specifically, the foils26are disposed relatively closely (three foils in the drawing) on a lower side of the rotor shaft12, and sparsely on an upper side of the same (only one foil in the drawing). Though not shown in the drawings, in these embodiments also, the foils26and the moveable member27may be provided with magnets as shown inFIG. 3, so that the urging force acting upon the foils26can be adjusted by varying the relative position between the foils26and the moveable member27.

Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims.

For instance, all the foils may not be provided with magnets, and instead, only part of the foils may be provided with magnets. It may be also possible that in the foils disposed on the upper side of the rotor shaft, the magnet occupies only a part of the circumferentially extending portion of the foils while in the foils disposed on the lower side of the rotor shaft, upon which a larger load is applied, the magnet occupies an entire part of the circumferentially extending portion of the foils. The magnets provided to the foils and/or moveable member may have varying magnetic strengths. Further, each foil may be provided with more than one pair of NS magnetic poles, so that the magnetic poles are arranged in the order of NSNS, for example, in the circumferential direction.

As described above, according to the present invention, the interaction between the magnets provided to the moveable member and the magnets provided to the circumferentially extending portions of the foils can create a force urging the foils toward the rotating member, and the urging force can be adjusted by rotating the moveable member and thereby varying the position thereof relative to the foils. Thus, when the present invention is applied to the journal bearing and/or thrust bearing of a gas turbine engine, it is possible to control the angular position of the moveable member according to the rotational speed of the gas turbine engine, thereby creating an urging force (or preload) suitable for the rotational speed so that favorable bearing properties can be achieved for a wide range covering the low to high rotational speed regions. Further, the urging force acts upon the circumferentially extending portion of each foil, and thus concentration of an excessive force on the root portion (or bending portion) of the foil can be avoided.