Patent Publication Number: US-9903411-B2

Title: Thrust bearing

Description:
This application is a Continuation Application based on International Application No. PCT/JP2014/074508, filed Sep. 17, 2014, which claims priority on Japanese Patent Application No. 2013-194441, filed Sep. 19, 2013, the contents of both of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a thrust bearing. 
     BACKGROUND 
     In the related art, as a bearing used for a high-speed rotating body, a thrust bearing is known which is disposed so as to face a thrust collar provided on a rotary shaft. As such a thrust bearing, a foil-type thrust bearing, namely a thrust foil bearing is well known. The bearing surface of the thrust foil bearing is formed of a flexible foil (thin metal sheet) in order to accept movement of the rotary shaft (movement in the axial direction of or inclination of the thrust collar) which occurs due to vibration or to impact, and the thrust foil bearing includes a foil structure which is provided under the bearing surface and flexibly supports the bearing surface. 
     As an example of the thrust foil bearing, a structure is known in which a circular ring-shaped (annular shaped) bearing surface is formed of a plurality of individual foil pieces (top foil pieces) obtained by dividing an annular sheet in the circumferential direction thereof, and a wave sheet-shaped foil piece (bump foil piece) supports each top foil piece (for example, refer to Patent Document 1). Each top foil piece (the thickness thereof is about 100 μm) is arranged at an inclination angle with respect to the thrust collar, and thereby a bearing clearance between the thrust collar and the top foil piece is formed in a wedge shape in side view. That is, the bearing clearance is formed so as to gradually decrease from the leading side toward the trailing side in the rotation direction of the thrust collar (the rotary shaft). Thus, when the thrust collar rotates from the large side (the leading side) toward the small side (the trailing side) of the bearing clearance, a lubricating fluid flows into a narrow part of the wedge-shaped bearing clearance, and the load capability of the thrust bearing is obtained. 
     Only the edge of the top foil piece on the leading side in the rotation direction of the thrust collar (the rotary shaft) is fixed to a base plate, and the edge is configured as a fixed edge. When a bearing load increases, in a state where the fixed edge (the edge on the leading side) is a fulcrum, the top foil piece moves so as to be horizontal (parallel to the supported surface of the thrust collar), the inclination angle thereof decreases, and at the time the inclination angle becomes about 0.1°, the thrust bearing generates the maximum load capability. In addition, the bump foil piece is arranged so that the ridge line of a peak thereof is parallel to the edge on the trailing side of the top foil piece, and only the edge of the bump foil piece on the trailing side in the rotation direction of the thrust collar (the rotary shaft) is fixed to the base plate. That is, the edge on the leading side of the bump foil piece is set to be a free end. 
     Since the bump foil piece is arranged and fixed in this way, the pressure of a fluid lubrication film formed at the top foil piece becomes high at the small side (the trailing side) of the bearing clearance. Thus, if this part is supported by a high rigidity, the load capability of the thrust bearing can be increased. 
     Patent Documents 2 to 4 disclose thrust bearings which include a top foil and a bump foil. Patent Document 5 discloses a thrust bearing which includes a bearing foil, and the bearing surface of the bearing foil is provided with an introducing groove. 
     DOCUMENT OF RELATED ART 
     Patent Document 
     [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. H10-331847 
     [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2005-155802 
     [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. S63-195412 
     [Patent Document 4] Japanese Unexamined Utility Model Application No. H1-109617 
     [Patent Document 5] Japanese Unexamined Patent Application, First Publication No. 2006-183786 
     SUMMARY 
     Technical Problem 
     In the above-described thrust foil bearing structure, when the pressure of the fluid lubrication film acts on the top foil piece, the flexure (depression) of an intermediate portion in the radial direction between the inner circumferential edge (the inner end in the radial direction) and the outer circumferential edge (the outer end in the radial direction) of the top foil piece toward a back foil may increase, and the pressure of the fluid lubrication film at the intermediate portion may decrease and thus the load capability of the bearing may deteriorate. 
     That is, at the inner circumferential edge or the outer circumferential edge of the top foil piece, the pressure of the fluid lubrication film formed between the top foil piece and the thrust collar is close to the pressure (ambient pressure) of a fluid in the surroundings of the top foil piece. In contrast, at the intermediate portion in the radial direction between the inner circumferential edge and the outer circumferential edge of the top foil piece, the pressure of the fluid lubrication film is higher than the ambient pressure. If the fluid lubrication film has such pressure distribution, the intermediate portion may greatly bend (be depressed) toward the back foil compared to the inner circumferential edge side or the outer circumferential edge side of the top foil piece. At this time, the bearing clearance (the thickness of the fluid lubrication film) increases at the intermediate portion which greatly bends, and thereby the pressure of the fluid lubrication film at the intermediate portion may decrease, and the load capability of the bearing may deteriorate as described above. 
     In order to decrease the amount of flexure (depression) at the intermediate portion in the radial direction to prevent deterioration of the load capability, for example, it is conceivable that the thickness of the top foil piece is increased. However, in this case, the inclination flexibility in the circumferential direction of the top foil piece may be spoiled, and it may be difficult for the inclination angle of the top foil piece to become about 0.1°, which is close to the horizontal direction as described above. Accordingly, the maximum load capability of the bearing is not easily obtained, and thus the load capability may also deteriorate. 
     The present disclosure has been made in view of the above circumstances, and an object thereof is to provide an excellent thrust bearing which can prevent deterioration of the load capability thereof. 
     Solution to Problem 
     A first aspect of the present disclosure is a thrust bearing disposed so as to face a thrust collar provided on a rotary shaft, the thrust bearing including: a top foil disposed so as to face the thrust collar; a back foil disposed so as to face a surface of the top foil opposite to another surface of the top foil facing the thrust collar, and supporting the top foil; and an annular plate-shaped base plate disposed on a side of the back foil opposite to the top foil, and supporting the back foil. The back foil includes a plurality of back foil pieces arranged in a circumferential direction of the base plate. The top foil includes a plurality of top foil pieces disposed on the plurality of back foil pieces. In addition, a leading side of a top foil piece in a rotation direction of the rotary shaft is provided with a fixed part fixed to the base plate, the top foil piece is provided with a thin part in which a part is removed from a surface of the top foil piece facing a back foil piece, and the thin part extends from an outer circumferential edge to an inner circumferential edge or to a side edge of the top foil piece. 
     A second aspect of the present disclosure is that in the thrust bearing of the first aspect, the thin part includes a fixed part-side thin part formed in the fixed part and in a vicinity of the fixed part on a trailing side of the fixed part in the rotation direction of the rotary shaft and extending from the outer circumferential edge to the inner circumferential edge of the top foil piece. 
     A third aspect of the present disclosure is that in the thrust bearing of the first or second aspect, the fixed part includes a linear fixed edge positioned at a trailing side of the fixed part in the rotation direction of the rotary shaft. The back foil piece is formed of a wave sheet-shaped bump foil piece in which peak parts and valley parts are alternately formed, and is disposed so that an arrangement direction of the peak parts crosses with the fixed edge. In addition, the thin part includes a valley part-side thin part formed in a portion of the top foil piece facing a valley part and formed so as to be thinner than a portion of the top foil piece facing a peak part. 
     A fourth aspect of the present disclosure is that in the thrust bearing of the third aspect, the peak parts are formed so that the height of the peak parts gradually increases from the fixed edge side toward a trailing side of the back foil piece in the rotation direction of the rotary shaft. 
     A fifth aspect of the present disclosure is that in the thrust bearing of any one of the first to third aspects, each of support areas of the base plate, the support areas supporting the back foil pieces, is provided with an inclined surface whose height gradually increases from the fixed edge side toward an edge on a trailing side of the top foil piece in the rotation direction of the rotary shaft. 
     A sixth aspect of the present disclosure is that in the thrust bearing of the fifth aspect, the back foil piece is formed of a wave sheet-shaped bump foil piece in which peak parts and valley parts are alternately formed, and is disposed so that an arrangement direction of the peak parts is the same as an inclination direction of the inclined surface. 
     A seventh aspect of the present disclosure is that in the thrust bearing of the third, fourth or sixth aspect, an edge on a trailing side of the bump foil piece in the rotation direction of the rotary shaft is fixed to the base plate. 
     Effects 
     According to a thrust bearing of the present disclosure, since the thin part is formed extending from the outer circumferential edge to the inner circumferential edge or to the side edge of the top foil piece, the thin part of the top foil piece easily bends, and thus the top foil piece can easily bend in a range from the leading side to the trailing side thereof in the rotation direction of the rotary shaft, namely in the circumferential direction. On the other hand, since the top foil piece is almost not influenced in the radial direction by the thin part, the bending difficulty (the bending rigidity) thereof is almost not changed, and can be maintained to be equivalent to that before the thin part is formed. Thus, if a sheet material thicker than that in the related art is used for forming the top foil piece, while the bending easiness thereof in the circumferential direction is maintained to be equivalent to that in the related art, the top foil piece does not easily bend in the radial direction compared to the related art, and the flexure of the intermediate portion of the top foil piece toward the back foil can be limited. Therefore, deterioration of the load capability of the thrust bearing can be prevented. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing an example of a turbo machine in which a thrust bearing of an embodiment of the present disclosure is provided. 
         FIG. 2  is a view showing a thrust bearing of a first embodiment of the present disclosure, and is a side view of the thrust bearing in which a thrust collar is inserted. 
         FIG. 3A  is a view showing the thrust bearing of the first embodiment of the present disclosure, and is a partial cross-sectional plan view of the thrust bearing. 
         FIG. 3B  is a plan view showing the rear surface of a top foil piece. 
         FIG. 3C  is a cross-sectional view taken along  3 C- 3 C line in  FIG. 3A . 
         FIG. 3D  is an explanatory view in which a plan view and a side view of a bump foil piece correspond to each other in order to show the shape of the bump foil piece. 
         FIG. 4A  is a view showing a thrust bearing of a second embodiment of the present disclosure, and is a plan view showing the rear surface of a top foil piece. 
         FIG. 4B  is a cross-sectional view taken along  4 B- 4 B line in  FIG. 4A . 
         FIG. 4C  is an explanatory view of a valley part-side thin part. 
         FIG. 5A  is a view showing a thrust bearing of a third embodiment of the present disclosure, and is an explanatory view in which a plan view showing the rear surface of and a side view of a top foil piece correspond to each other. 
         FIG. 5B  is a cross-sectional view taken along  5 B- 5 B line in  FIG. 5A . 
         FIG. 6A  is a view showing a thrust bearing of a fourth embodiment of the present disclosure, and is a partial cross-sectional plan view of the thrust bearing. 
         FIG. 6B  is a plan view showing the rear surface of a top foil piece. 
         FIG. 6C  is a cross-sectional view taken along  6 C- 6 C line in  FIG. 6A . 
         FIG. 6D  is an explanatory view in which a plan view and a side view of a bump foil piece correspond to each other in order to show the shape of the bump foil piece. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, thrust bearings of the present disclosure are described in detail with reference to the drawings. In the following drawings, the scale of each member is appropriately changed in order to show each member in a recognizable size. 
       FIG. 1  is a side view schematically showing an example of a turbo machine in which a thrust bearing of the present disclosure is provided. In  FIG. 1 , a reference numeral  1  represents a rotary shaft, a reference numeral  2  represents an impeller provided at an end part of the rotary shaft, and a reference numeral  3  represents the thrust bearing of the present disclosure. 
     A thrust collar  4  is fixed to the vicinity of the end of the rotary shaft  1 , and the impeller  2  is formed at the end. A pair of thrust bearings  3  are disposed such that the thrust collar  4  is interposed therebetween. 
     The impeller  2  is disposed inside a housing  5  which is a stationary member, and a tip clearance  6  is formed between the impeller  2  and the housing  5 . 
     A radial bearing  7  is provided on the rotary shaft  1  at a position closer to the middle of the rotary shaft  1  than the thrust collar  4 . 
       FIGS. 2 and 3A to 3D  are views showing a thrust bearing  3  of a first embodiment of the present disclosure provided in the turbo machine having the above configuration.  FIG. 2  is a side view of the thrust bearing  3  in which the thrust collar  4  is inserted.  FIG. 3A  is a partial cross-sectional plan view of the thrust bearing  3 .  FIG. 3B  is a plan view showing the rear surface (the surface facing a back foil) of a top foil piece.  FIG. 3C  is a cross-sectional view taken along  3 C- 3 C line in  FIG. 3A .  FIG. 3D  is an explanatory view in which a plan view and a side view of a bump foil piece correspond to each other in order to show the shape of the bump foil piece. 
     As shown in  FIG. 2 , in the first embodiment, thrust bearings  3 A( 3 ) are disposed on two sides of the thrust collar  4  such that the thrust collar  4  is interposed therebetween. The pair of thrust bearings  3 A( 3 ) have the same structure, and are annular shaped (cylindrical shaped) devices disposed so as to face the circular plate-shaped thrust collar  4  fixed to the rotary shaft  1 . The thrust bearings  3 A( 3 ) are provided encircling the rotary shaft  1 . Two surfaces (two surfaces opposite to each other in the axial direction of the rotary shaft  1 ) of the thrust collar  4  are configured as supported surfaces, and the pair of thrust bearings  3 A( 3 ) are disposed so as to face and support the two supported surfaces of the thrust collar  4 . 
     The thrust bearing  3 A includes a top foil  10  disposed so as to face the thrust collar  4 , a back foil  20  disposed so as to face a surface of the top foil  10  opposite to another surface of the top foil  10  facing the thrust collar  4 , and an annular plate-shaped base plate  30  disposed on a side of the back foil  20  opposite to the top foil  10 . 
     In this embodiment, a cylindrical bearing spacer  40  shown by a dashed double-dotted line is interposed between the base plates  30  of the pair of thrust bearings  3 A, and the base plates  30  are connected to each other via the bearing spacer  40  using fastening bolts  41 . In addition, the outer surface of one base plate  30  is fixed to the housing  5  using the fastening bolts  41 , and thus the pair of thrust bearings  3 A are fixed to the housing  5  using the fastening bolts  41  in a state where the thrust collar  4  is interposed therebetween. 
     As shown in  FIG. 3A , the base plate  30  is an annular plate-shaped metal member having a thickness of several millimeters, and an outer circumferential portion of the base plate  30  is provided with a plurality of through-holes  42  (the number thereof is eight in this embodiment) allowing the fastening bolts  41  to be inserted thereinto. The surface of the base plate  30  facing the thrust collar  4  is provided with a support area which supports the back foil  20  and the top foil  10 . In this embodiment, as described later, the back foil  20  includes a plurality (six) of back foil pieces  21 , and the top foil  10  includes a plurality (six) of top foil pieces  11 . Accordingly, the base plate  30  is provided with six support areas  31  formed by dividing the surface of the base plate  30  into six areas (into equivalent six areas) in the circumferential direction thereof. It is noted that the six support areas  31  in this embodiment are areas on the design, and the surface of the base plate  30  including the support areas  31  is formed in a uniform and flat surface. 
     As shown in  FIG. 2 , a back foil piece  21  and a top foil piece  11  are disposed in this order on each support area  31 , and are supported by the support area  31 . The base plate  30 , the back foil piece  21  and the top foil piece  11  are piled up in the axial direction of the rotary shaft  1 . 
     As shown in  FIGS. 3A, 3C and 3D , the back foil  20  is formed of the six back foil pieces  21  arranged in the circumferential direction of the base plate  30 . The back foil pieces  21  are disposed on the support areas  31  of the base plate  30 , and thus are arranged in the circumferential direction of the base plate  30 . In addition, each of the back foil pieces  21  is formed to be slightly smaller than the top foil piece  11  (described later) in plan view, and therefore as shown in  FIG. 3A , is covered with the top foil piece  11  at a position above the base plate  30  without being exposed to the thrust collar  4 . 
     The back foil  20  including the back foil pieces  21  is formed of foils (thin sheets), and resiliently supports the top foil  10  (the top foil pieces  11 ). For the back foil  20 , for example, it is possible to use a bump foil, a spring foil disclosed in Japanese Unexamined Patent Application, First Publication No. 2006-57652 or in Japanese Unexamined Patent Application, First Publication No. 2004-270904, a back foil disclosed in Japanese Unexamined Patent Application, First Publication No. 2009-299748, or the like. Although the spring foils disclosed in Japanese Unexamined Patent Application, First Publication No. 2006-57652 and in Japanese Unexamined Patent Application, First Publication No. 2004-270904, and the back foil disclosed in Japanese Unexamined Patent Application, First Publication No. 2009-299748 are foils used for a radial bearing, if each foil is developed in a flat shape and is formed in an annular sheet shape, the foil can be used for a thrust bearing. 
     In this embodiment, as shown in  FIG. 3D , the back foil  20  is formed of a bump foil, and thus the back foil piece  21  is formed of a bump foil piece. In the back foil piece  21  (the bump foil piece), a foil (thin metal sheet) having a thickness of about several hundred micrometers is formed in a wave sheet shape through press molding, and as shown in  FIG. 3D , the whole shape thereof is formed in an approximately pentagonal shape in plan view. 
     The back foil piece  21  formed in a wave sheet shape in the above way is formed such that valley parts  22  contacting the base plate  30  and peak parts  23  contacting the top foil piece  11  are alternately disposed. As shown in  FIG. 3A , the valley parts  22  and the peak parts  23  are arranged in a direction orthogonal in plan view to the extending direction in which an edge  11   a  (described later) of the top foil piece  11  extends. In addition, the valley parts  22  and the peak parts  23  are arranged in a direction crossing the extending direction in which a fixed edge  12  of the top foil piece  11  extends. That is, the plurality of peak parts  23  are next to each other in plan view in a direction crossing with the extending direction of the fixed edge  12 . 
     The valley parts  22  and the peak parts  23  are formed at approximately regular intervals. In addition, the height of the peak parts  23  is formed so as to increase at a constant rate from the fixed edge  12  side toward the opposite side of the back foil piece  21  to the fixed edge  12  (refer to  FIG. 3C ), namely toward the trailing side of the back foil piece  21  in the rotation direction of the rotary shaft  1  (the thrust collar  4 ) shown by an arrow R in  FIG. 3A . 
     An edge  21   a  on the trailing side of the back foil piece  21  in the rotation direction of the rotary shaft  1  is disposed at approximately the same position in plan view as the edge  11   a  on the trailing side of the top foil piece  11  (described later) in the rotation direction of the rotary shaft  1 . The back foil piece  21  is spot-welded (welded at points) on the base plate  30  in the forming direction of the valley part  22  serving as the edge  21   a,  and thus is fixed thereto. 
     At this time, since the entire edge  21   a  of the back foil piece  21  is formed of one continuous valley part  22 , this entire valley part  22  can be easily welded on the base plate  30 . Thus, the fixing of the back foil piece  21  through welding can be easily performed. 
     In addition, the fixing of the edge  21   a  to the base plate  30  can also be performed using fastening screws or the like other than spot-welding. 
     As shown in  FIG. 3A , the top foil  10  is formed of six top foil pieces  11  arranged in the circumferential direction of the base plate  30 . Each of the top foil pieces  11  is an arc sheet-shaped member obtained by dividing a circular annular sheet in the circumferential direction thereof, namely an arc sheet-shaped member in which a portion including the apex of a sector shape of a thin metal sheet (foil) having a thickness of several hundred micrometers is removed from the sector shape, and each of the inner circumferential edge (the inner end in the radial direction) and the outer circumferential edge (the outer end in the radial direction) thereof is formed in an arc. 
     The top foil pieces  11  having such a shape are disposed on the support areas  31  of the base plate  30  so as to cover the back foil pieces  21 . The top foil pieces  11  are arranged at regular intervals in the circumferential direction of the base plate  30 , and the top foil pieces  11  as a whole are disposed in an approximately annular sheet shape, thereby forming the top foil  10 . 
     The top foil piece  11  is formed to be slightly smaller than the support area  31  in plan view and is formed to be slightly larger than the back foil piece  21  in plan view. Therefore, the top foil pieces  11  are disposed on the support areas  31  without contacting each other, and are disposed thereon so as to cover the top surfaces of the back foil pieces  21  without exposing the back foil pieces  21  to the thrust collar  4 . However, the present disclosure is not limited thereto, and the top foil piece  11  may be formed in the same size as the back foil piece  21  in plan view, or may be formed to be smaller than the back foil piece  21  in plan view. 
     The top foil piece  11  includes a fixed part  13  on the leading side of the top foil piece  11  in the rotation direction of the rotary shaft  1  (the thrust collar  4 ), and is fixed to the base plate  30  through the fixed part  13 . The fixing of the fixed part  13  to the base plate  30  is performed through spot-welding (welding at points) similarly to the edge  21   a  of the back foil piece  21 . In addition, the fixing of the fixed part  13  to the base plate  30  can also be performed using fastening screws or the like other than spot-welding. 
     Although the fixed part  13  may be formed in a sheet shape or in a linear shape as long as the fixed part  13  is provided on the leading side of the top foil piece  11  in the rotation direction of the rotary shaft  1  (the thrust collar  4 ), in this embodiment, the fixed part  13  is configured as the fixed edge  12  formed in a linear shape having a width. That is, although the fixed part  13  and the fixed edge  12  in this embodiment are disposed in approximately the same position, the fixed edge of the present disclosure is configured as an edge positioned on the trailing side of the fixed part  13  in the rotation direction of the rotary shaft  1 . 
     As shown in  FIG. 3C , bending work is applied to the top foil piece  11  at the vicinity of the fixed edge  12 , and thereby the top foil piece  11  is formed in a stepped shape so as to secure a height which is approximately the same as the height of the peak parts  23  of the back foil piece  21 , and a portion of the top foil piece  11  closer to the edge  11   a  than the fixed edge  12  is placed on the peak parts  23 . 
     On the other hand, the edge  11   a  (the trailing edge) side of the top foil piece  11  is configured as a free end merely supported by the peak parts  23  of the back foil piece  21  without being fixed to the base plate  30  or the like. 
     In this embodiment, as described above, the back foil piece  21  is disposed such that the valley parts  22  and the peak parts  23  of the back foil piece  21  are arranged in a direction crossing with the fixed edge  12  of the top foil piece  11 . Accordingly, the valley parts  22  and the peak parts  23  extend in the longitudinal direction of the fixed edge  12  or the edge  11   a  of the top foil piece  11 , and particularly extend to be parallel to the edge  11   a.  Thus, when the top foil piece  11  is placed on the back foil piece  21 , the top foil piece  11  is disposed to incline at an initial inclination angle determined by the peak parts  23  of the back foil piece  21  such that the separation between the top foil piece  11  and the inner surface (the surface in which the support areas  31  are provided) of the base plate  30  gradually increases from the fixed edge  12  toward the edge  11   a  in the arrangement direction of the peak parts  23 . 
     The initial inclination angle is an inclination angle of the top foil piece  11  to the base plate  30  when a load is zero. In addition, the inclination angle is an angle (inclination) θ determined by an increment of the height of the peak parts  23  of the back foil piece  21  as shown in  FIG. 3D . The inclination angle is an angle between the top foil piece  11  and the inner surface of the base plate  30 . In addition, the inclination angle is equal to the angle between the top foil piece  11  and the supported surface of the thrust collar  4 . Thus, when the load increases, the peak parts  23  of the back foil piece  21  are pushed toward the base plate  30 , and the entire back foil piece  21  is flattened, whereby the top foil piece  11  also moves toward the base plate  30 , and the inclination angle θ becomes less than the initial inclination angle. 
     In this structure, the top foil piece  11  is disposed such that the edge  11   a  thereof extends in the extending direction of the valley part  22  or the peak part  23  of the back foil piece  21 , and the back foil piece  21  is disposed such that the arrangement direction of the peak parts  23  is orthogonal to the edge  11   a.  Therefore, the height of the edge  11   a  of the top foil piece  11  from the base plate  30 , which is configured as a free end, is approximately constant even when the inclination angle θ of the top foil piece  11  is the initial inclination angle or when a load is added to the back foil piece  21  via the top foil piece  11  due to the rotation of the thrust collar  4  and the inclination angle θ decreases. 
     The top foil piece  11  is provided with a fixed part-side thin part  14  (a thin part) which is formed in the fixed edge  12  (the fixed part  13 ) and in the vicinity of the fixed edge  12  of the top foil piece  11  as shown in  FIG. 3B , namely in the fixed edge  12  and in the vicinity of the fixed edge  12  positioned on the trailing side of the fixed edge  12  in the rotation direction of the rotary shaft  1 , in which a part is cut (removed) from the surface of the top foil piece  11  facing the back foil piece  21  through etching or the like. The fixed part-side thin part  14  is formed to be thinner than other parts of the top foil piece  11 . In addition, electrochemical machining or the like may be used for forming the fixed part-side thin part  14  if distortion or the like does not occur in the top foil piece  11 . 
     The fixed part-side thin part  14  is linearly formed in the longitudinal direction of the fixed edge  12 , that is, is formed extending from the outer circumferential edge to the inner circumferential edge in the radial direction of the top foil piece  11 , and is formed through etching or the like. It is preferable that the thickness of the fixed part-side thin part  14  be set to about 50% to 70% of the thickness (several hundred micrometers) of a portion of the top foil piece  11  in which the cutting process using etching is not performed. If the fixed part-side thin part  14  is formed to have this thickness, a portion of the top foil piece  11  on the trailing side of the fixed part-side thin part  14  in the rotation direction of the rotary shaft  1  can be easily moved. 
     The fixed part-side thin part  14  is formed so as not to be positioned right above the top (the ridge line) of the peak part  23  closest to the fixed edge  12  of the peak parts  23  of the back foil piece  21  shown in  FIG. 3C . That is, the width of the fixed part-side thin part  14  is set and formed so that the fixed edge  12  is included within the fixed part-side thin part  14  and the fixed part-side thin part  14  is positioned between the fixed edge  12  and the top (the ridge line) of the peak part  23  closest to the fixed edge  12 . Accordingly, a portion of the top foil piece  11  other than the fixed part-side thin part  14  is placed on all the peak parts  23  and is evenly supported thereby, and thus the inclination angle θ shown in  FIG. 3D  is maintained. In addition, since the fixed part-side thin part  14  is formed, a portion of the top foil piece  11  closer to the edge  11   a  than the fixed part-side thin part  14  can easily and smoothly move (that is, the inclination angle thereof is easily changed). Furthermore, since the fixed part-side thin part  14  is formed and thus the top foil piece  11  can be easily rotated, the thickness of a portion of the top foil piece  11  other than the fixed part-side thin part  14  can be increased compared to that in the related art. 
     Next, the operation of the thrust bearing  3 A( 3 ) having the above configuration is described. 
     In this embodiment, as shown in  FIG. 2 , the thrust bearings  3 A are provided on two sides of the thrust collar  4 . Since the thrust bearings  3 A are provided on two sides of the thrust collar  4  in this way, the movement amount of the thrust collar  4  (the rotary shaft  1 ) in the thrust direction can be minimized. That is, since the thrust movement amount thereof is decreased, it is possible to prevent the impeller  2  or the like from contacting the housing  5  even if the tip clearance  6  shown in  FIG. 1  is narrowed, and to improve the fluid performance of a turbo machine or the like by narrowing the tip clearance  6 . 
     In order to minimize the movement amount in the thrust direction, both thrust bearings  3 A are disposed to be close to the thrust collar  4  without forming a large gap therebetween. Accordingly, the top foil pieces  11  (the top foils  10 ) of both thrust bearings  3 A are brought into a state of being slightly pressed on the thrust collar  4 . In this case, since the top foil piece  11  is provided with the fixed part-side thin part  14  in this embodiment, a portion of the top foil piece  11  close to the edge  11   a  is configured to easily incline (to easily move). Therefore, the pressing force which occurs in proportion to the pressing amount becomes small, and thus the starting torque of the rotary shaft  1  becomes small. 
     In the related art, an inclination angle greater than an optimum angle is applied to a top foil piece in advance in order that the inclination angle of the top foil piece becomes the optimum angle when the load added to the bearing increases. Thus, in a state where the rotation thereof stops, two top foil pieces contact two surfaces of the thrust collar  4 , and are brought into a state of being pressed on the thrust collar  4  (a state where a preload is added thereto). However, since the thickness of the top foil piece is constant in the related art, the pressing force (preload) on the thrust collar  4  may become high, and the starting torque of the rotary shaft  1  may become large. 
     In contrast, in this embodiment, since the top foil piece  11  is provided with the fixed part-side thin part  14  as described above, the starting torque becomes small. 
     When the rotary shaft  1  rotates in this state and the thrust collar  4  starts rotating, while the thrust collar  4  and the top foil piece  11  rub on each other, an ambient fluid is pushed into a wedge-shaped space (a bearing clearance) in side view formed therebetween. When the rotation speed of the thrust collar  4  reaches a constant rotation speed, a fluid lubrication film is formed therebetween. The top foil pieces  11  (the top foil  10 ) are pressed on the back foil pieces  21  (the back foil  20 ) due to the pressure of the fluid lubrication film, and the thrust collar  4  leaves a state of contacting the top foil pieces  11  and rotates in a non-contact state with the top foil pieces  11 . 
     When a thrust load is added to the bearing, the top foil piece  11  is further pressed on the back foil piece  21 , and the inclination angle θ of the top foil piece  11  decreases. At this time, since the top foil piece  11  rotates (bends) around the fixed edge  12  on the leading side of the top foil piece  11  in the rotation direction of the rotary shaft  1 , a portion of the top foil piece  11  on the trailing side of the fixed part-side thin part  14  in the rotation direction of the rotary shaft  1 , namely a portion of the top foil piece  11  close to the edge  11   a,  easily and smoothly moves due to the fixed part-side thin part  14 . Thus, even when the thrust load further increases and the thickness of the fluid lubrication film further decreases, the top foil piece  11  does not easily contact the thrust collar  4 , and the optimum inclination angle thereof can be easily obtained. 
     In the thrust bearing  3 A( 3 ) of this embodiment, since the fixed part-side thin part  14  is formed in the fixed edge  12  and in the vicinity of the fixed edge  12  of the top foil piece  11  on the trailing side of the fixed edge  12  in the rotation direction, a portion of the top foil piece  11  close to the edge  11   a  can easily and smoothly move when a load is added to the bearing, and thus the starting torque of the rotary shaft  1  decreases. In addition, after the rotary shaft  1  starts rotating, since the top foil piece  11  also easily and smoothly rotates around the fixed part-side thin part  14 , the optimum inclination angle of the top foil piece  11  can be easily obtained, and the load capability of the bearing improves. 
     Since the pressure of the fluid lubrication film becomes high when the bearing receives a high thrust load, a portion of the top foil piece  11  which is not directly supported by the back foil piece  21 , namely a portion positioned right above the valley part  22  of the back foil piece  21 , may bend, and the pressure may escape therefrom and the load capability may deteriorate. 
     However, since the fixed part-side thin part  14  is formed in the fixed edge  12  (in the fixed edge  12  and in the vicinity thereof) of the top foil piece  11  in this embodiment, it is possible to increase the thickness of a sheet material (a thin sheet) forming the top foil piece  11  compared to that in the related art without preventing the movement (the bending) of the top foil piece  11 . 
     Thus, if the thickness of the top foil piece  11  is increased in this way, the flexure of a portion of the top foil piece  11  which is not supported by the back foil piece  21  can be decreased, and the deterioration of the load capability can be limited. 
     Next, a thrust bearing of a second embodiment of the present disclosure is described. 
     A main difference of the thrust bearing of the second embodiment from the thrust bearing  3 A( 3 ) of the first embodiment is that the structure shown in  FIGS. 4A to 4C  is used for a top foil piece.  FIG. 4A  is a plan view showing the rear surface of a top foil piece  50 .  FIG. 4B  is a cross-sectional view taken along  4 B- 4 B line in  FIG. 4A .  FIG. 4C  is an explanatory view of a valley part-side thin part. 
     A difference of the top foil piece  50  shown in  FIGS. 4A to 4C  from the top foil piece  11  shown in  FIGS. 3A to 3D  is that a valley part-side thin part  16  (a thin part) is formed at a portion of the top foil piece  50  facing the valley part  22  of the back foil piece  21  shown in  FIG. 3D  and is formed to be thinner than a portion of the top foil piece  50  facing the peak part  23 . In addition, since a plurality of valley parts  22  are provided, a plurality of valley part-side thin parts  16  are formed in the top foil piece  50 . 
     The valley part-side thin part  16  is formed in a groove shape extending from the outer circumferential edge to the inner circumferential edge or to a side edge of the top foil piece  50 , and is formed through etching or the like similarly to the fixed part-side thin part  14 . The side edge is one of two edges in the circumferential direction of the top foil piece  50 . In this embodiment, some (three in this embodiment) of the plurality of valley part-side thin parts  16  extend from the outer circumferential edge to the inner circumferential edge of the top foil piece  50 , and the others (three in this embodiment) thereof extend from the outer circumferential edge to the side edge of the top foil piece  50  (refer to  FIG. 4A ). Additionally, it is preferable that the valley part-side thin part  16  be set to have a thickness equivalent to that of the fixed part-side thin part  14 . Thus, in this embodiment, etching is performed on the sheet material of the top foil piece  50  one time, and the fixed part-side thin part  14  and the valley part-side thin parts  16  are formed at the same time. 
     However, the valley part-side thin part  16  may be formed to have a different thickness from that of the fixed part-side thin part  14 . In this case, the etching for forming the fixed part-side thin part  14  and the etching for forming the valley part-side thin part  16  are performed at different times. 
     The above-described portion of the top foil piece  50  facing the valley part  22  of the back foil piece  21  is shown by, for example, the following definition. 
     As shown in  FIG. 4C , the top of the peak part  23  of the back foil piece  21  is represented by a reference sign H, the position of the peak part  23  contacting the valley part  22 , namely the borderline (the connection part) between the peak part  23  and the valley part  22 , is represented by a reference sign L, and the middle point between the top H and the borderline L on the base plate  30  is represented by a reference sign M. In this case, a portion of the top foil piece  50  corresponding in the up-and-down direction of  FIG. 4C  to the area between two middle points M between which the valley part  22  is interposed is regarded as a portion of the top foil piece  50  facing the valley part  22 . Thus, as shown in  FIG. 4C , the portion of the top foil piece  50  corresponding to the area between the middle points M is provided with the valley part-side thin part  16 . In addition, although  FIG. 4C  shows that the valley part  22  has a width, in a case where the valley part  22  is a mere bent part and has no width, only one borderline L is set between the peak parts  23  next to each other (this borderline L is positioned at the connection part between the peak parts  23  next to each other), and the above definition can also be used. 
     In this embodiment, since the fixed part-side thin part  14  is formed similarly to the first embodiment, it is possible to increase the thickness of the sheet material forming the top foil piece  50  compared to that in the related art. 
     When the pressure of the fluid lubrication film acts on the top foil piece  50 , the intermediate portion in the radial direction between the inner and outer circumferential edges of the top foil piece  50  may easily bend toward the back foil  20 . If this flexure increases, the pressure of the fluid lubrication film at the intermediate portion may decrease, and the load capability of the bearing may deteriorate. 
     Since the valley part-side thin part  16  is formed in this embodiment, the top foil piece  50  easily bends in a range from the leading side to the trailing side thereof in the rotation direction of the rotary shaft  1 , namely in the circumferential direction (that is, the top foil piece  50  easily bends around an axis extending in parallel to the surface of the top foil piece  50  and in the radial direction), and the bending difficulty (the bending rigidity) of the top foil piece  50  in the radial direction (namely, the bending difficulty around an axis which is parallel to the surface of the top foil piece  50  and is orthogonal to the radial direction) is maintained to be equivalent to that before the valley part-side thin part  16  is formed. Therefore, if a sheet material thicker than that in the related art is used for forming the top foil piece  50 , while the bending easiness in the circumferential direction can be maintained to be equivalent to that in the related art, the top foil piece  50  does not easily bend in the radial direction compared to the related art, and the flexure of the intermediate portion of the top foil piece  50  toward the back foil  20  can be limited. 
     Consequently, in this embodiment, the flexure of the intermediate portion in the radial direction is limited, and thereby it is possible to prevent the load capability from deteriorating due to a decrease in the pressure of the fluid lubrication film at the intermediate portion. 
     Next, a thrust bearing of a third embodiment of the present disclosure is described. 
     A main difference of the thrust bearing of the third embodiment from the thrust bearing of the second embodiment is that the structure shown in  FIGS. 5A and 5B  is used for a top foil piece.  FIG. 5A  is an explanatory view in which a plan view showing the rear surface of a top foil piece  60  and a side view (a cross-sectional view taken along A-A line in the plan view) thereof correspond to each other.  FIG. 5B  is a cross-sectional view taken along  5 B- 5 B line in  FIG. 5A . 
     A difference of the top foil piece  60  shown in  FIGS. 5A and 5B  from the top foil piece  50  shown in  FIGS. 4A to 4C  is that an inner circumferential edge-side thin part  15  as shown by hatching in  FIG. 5A  is formed at the inner circumferential edge side of the top foil piece  60  by cutting (removing) a part from the surface of the top foil piece  60  facing the back foil piece  21  through etching or the like, and is formed to be thinner than the outer circumferential edge side thereof. It is noted that in this embodiment, the inner circumferential edge-side thin part  15  is formed at a position different from the valley part-side thin part  16 , namely between the valley part-side thin parts  16  next to each other. In addition, the term “inner circumferential edge side” denotes a portion of the top foil piece  60  close to the inner circumferential edge thereof (or a portion thereof including the inner circumferential edge), and the term “outer circumferential edge side” denotes a portion of the top foil piece  60  close to the outer circumferential edge thereof (or a portion thereof including the outer circumferential edge). 
     Since the inner circumferential edge-side thin part  15  having the above structure is formed therein, the inner circumferential edge side of the top foil piece  60  easily bends compared to the outer circumferential edge side thereof. 
     In this embodiment, a borderline  15   a  between the inner circumferential edge-side thin part  15  and the outer circumferential edge side thereof, namely a stepped part, is formed in an arc shape extending in the circumferential direction of the inner circumferential edge or the outer circumferential edge of the top foil piece  60 . 
     It is preferable that the width W 1  of the inner circumferential edge-side thin part  15  in the radial direction of the top foil piece  60  be set to ½ or less of the width W 2  in the radial direction of the entire top foil piece  60 , because as described later, the outer circumferential edge side of the top foil piece  60  can operate similarly to the related art while rising of the inner circumferential edge side of the top foil piece  60  toward the thrust collar  4  is limited. In addition, it is preferable that the width W 1  be set to ⅛ or less of the width W 2  in order that the rising of the inner circumferential edge side of the top foil piece  60  toward the thrust collar  4  is reliably prevented. 
     It is sufficient that the cut amount of the inner circumferential edge-side thin part  15 , namely the difference in thickness between the inner circumferential edge-side thin part  15  and the outer circumferential edge side thereof, is about 10 to 20 μm. The top foil piece  60  is cut by this amount, and the inner circumferential edge-side thin part  15  is formed to be thinner than a portion of the outer circumferential edge side other than the fixed part-side thin part  14  or the valley part-side thin part  16  by about 10 to 20 μm, whereby a gap corresponding to the difference can be formed between the inner circumferential edge-side thin part  15  and the top of the peak part  23  of the back foil piece  21 . This gap influences the fluid lubrication film formed between the thrust collar  4  and the top foil piece  60 , and the fluid lubrication film thereat is sufficiently thin. Therefore, the gap sufficiently works even when it is about 10 to 20 μm. Additionally, if the cut amount is such a value, it is possible to suitably limit the rising of the inner circumferential edge side of the top foil piece  60  toward the thrust collar  4  without causing the inner circumferential edge side to very easily bend. 
     The inner circumferential edge-side thin part  15  is also formed through etching or the like similarly to the fixed part-side thin part  14  or the valley part-side thin part  16 . However, as described above, since it is preferable that the fixed part-side thin part  14  and the valley part-side thin part  16  have an equivalent thickness and that the cut amount of the inner circumferential edge-side thin part  15  be less than that of the fixed part-side thin part  14  or the valley part-side thin part  16 , in this embodiment, etching is performed on the sheet material of the top foil piece  60  two times. For example, the valley part-side thin parts  16  and the fixed part-side thin part  14  are formed in the same etching process, and thereafter the inner circumferential edge-side thin part  15  is formed in another etching process. Additionally, in the forming of the inner circumferential edge-side thin part  15 , etching is not performed on the portion of the top foil piece  60  in which the valley part-side thin part  16  is formed, but is selectively performed only on a portion thereof different from the valley part-side thin part  16 . 
     When the rotary shaft  1  is rotated in order to operate the thrust bearing having the above configuration and a thrust load is added to the thrust bearing, the top foil piece  60  is pushed onto the back foil piece  21 , and the inclination angle θ of the top foil piece  60  decreases. At this time, in the thrust bearing  3 A( 3 ), since the circumferential speed at the outer circumferential edge side of the thrust collar  4  is greater than that at the inner circumferential edge side thereof, the pressure (film pressure) of the fluid lubrication film at the inner circumferential edge side at which the circumferential speed is low decreases, and the top foil piece  60  receives a reaction force from the back foil piece  21  and may easily rise toward the thrust collar  4 . 
     However, in this embodiment, since the inner circumferential edge-side thin part  15  is formed in the inner circumferential edge side of the top foil piece  60 , the inner circumferential edge side easily bends compared to the outer circumferential edge side thereof. In addition, since a gap is formed between the inner circumferential edge side and the back foil piece  21  (the bump foil piece) due to the inner circumferential edge-side thin part  15 , the inner circumferential edge side is easily pushed onto the back foil piece  21 . At this time, in the inner circumferential edge side, a force by the back foil piece  21  which pushes back the top foil piece  60  onto the thrust collar  4  does not easily occur because the gap is formed therein. Thus, it is possible to prevent the inner circumferential edge side of the top foil piece  60  from easily rising toward the thrust collar  4  due to the lower circumferential speed at the inner circumferential edge side of the thrust collar  4  in this thrust bearing than that at the outer circumferential edge side thereof. That is, the inner circumferential edge side of the top foil piece  60  does not easily rise toward the thrust collar  4 . 
     Even when the inner circumferential edge side of the top foil piece  60  is pushed onto the back foil piece  21  and the above gap is eliminated, since the resilient force (the reaction force which the top foil piece  60  receives from the back foil piece  21 ) of the inner circumferential edge side of the back foil piece  21  is decreased in accordance with the gap compared to that of the outer circumferential edge side thereof, as described above, the inner circumferential edge side does not easily rise toward the thrust collar  4 . 
     Thus, even when the thrust load further increases and the thickness of the fluid lubrication film further decreases, the top foil piece  60  does not easily contact the thrust collar  4 , and as a result, it is possible to support a high thrust load. 
     In the thrust bearing of this embodiment, the inner circumferential edge-side thin part  15  thinner than the outer circumferential edge side of the top foil piece  60  is formed on the inner circumferential edge side thereof, the inner circumferential edge side easily bends, and a gap is formed between the inner circumferential edge side and the back foil piece  21 . Therefore, it is possible to prevent a force by the back foil piece  21  which pushes back the inner circumferential edge side of the top foil piece  60  onto the thrust collar  4  from easily occurring. Thus, it is possible to prevent the inner circumferential edge side of the top foil piece  60  from easily rising toward the thrust collar  4  due to the lower circumferential speed at the inner circumferential edge side of the thrust collar  4  than that at the outer circumferential edge side thereof, to prevent the top foil piece  60  from approaching and contacting the thrust collar  4 , and thereby to prevent a decrease in the lifetime of the bearing and seizure thereof. 
     Additionally, in this embodiment, although each of the fixed part-side thin part  14 , the inner circumferential edge-side thin part  15  and the valley part-side thin part  16  is formed, the present disclosure is not limited thereto, and the top foil piece  11  of the first embodiment may be provided with the inner circumferential edge-side thin part  15  of this embodiment. In addition, only the inner circumferential edge-side thin part  15  of this embodiment and the valley part-side thin part  16  of the second embodiment may be formed in a top foil piece. 
     Next, a thrust bearing of a fourth embodiment of the present disclosure is described. 
     Main differences of a thrust bearing  3 B( 3 ) of the fourth embodiment from the thrust bearing of the second embodiment are that as shown in  FIGS. 6A to 6D , an inclined surface  32  is formed in the support area  31  of the base plate  30 , and the heights of all the peak parts  23  of the back foil piece  21  are the same.  FIG. 6A  is a partial cross-sectional plan view of the thrust bearing  3 .  FIG. 6B  is a plan view showing the rear surface of a top foil piece  50 .  FIG. 6C  is a cross-sectional view taken along  6 C- 6 C line in  FIG. 6A .  FIG. 6D  is an explanatory view in which a plan view and a side view of a bump foil piece correspond to each other in order to show the shape of the bump foil piece. 
     In this embodiment, as shown in  FIG. 6A , the entire area supporting the back foil piece  21  and the top foil piece  50  used for the second embodiment within the support area  31  is provided with the inclined surface  32  whose height gradually increases from the fixed edge  12  side toward an edge  50   a  on the trailing side of the top foil piece  50 . That is, as shown in  FIG. 6C , the inclined surface  32  is formed to be inclined in a direction orthogonal to the edge  50   a.    
     The back foil piece  21  is formed in a wave sheet shape in which valley parts  22  contacting the base plate  30  and peak parts  23  contacting the top foil piece  50  are alternately arranged similarly to the first embodiment. However, in this embodiment, as shown in  FIGS. 6C and 6D , all the peak parts  23  are formed to have the same height. 
     The valley parts  22  and the peak parts  23  are arranged in a direction crossing with the fixed edge  12  of the top foil piece  50  in plan view similarly to the above embodiments. That is, the arrangement direction of the valley parts  22  and the peak parts  23  is set to be a direction crossing with the fixed edge  12 , and is set to be the same as the inclination direction of the inclined surface  32 . Accordingly, the height of the positions of the tops of the peak parts  23  of the back foil piece  21  gradually increases at a constant rate in the inclination direction of the inclined surface  32  of the base plate  30 , namely toward the trailing side of the back foil piece  21  in the rotation direction of the rotary shaft  1 . That is, the positions of the tops of the peak parts  23  are apparently the same as those in the first embodiment or the second embodiment. Thus, the inclination angle θ of the top foil piece  50  placed on the back foil piece  21  is formed to be similar to that in the first embodiment. In this embodiment, as shown in  FIG. 6D , the inclination angle θ of the top foil piece  50  is determined by the inclination angle θ of the inclined surface  32 . 
     In the thrust bearing  3 B( 3 ) of this embodiment, since the fixed part-side thin part  14  is also formed in the top foil piece  50  as shown in  FIG. 6B , when a load is added to the bearing, a portion of the top foil piece  50  close to the edge  50   a  can easily and smoothly move. Thus, the optimum inclination angle of the top foil piece  50  can be easily obtained even after the rotary shaft  1  starts rotating, and the load capability of the bearing improves. 
     Since the valley part-side thin parts  16  are formed, if a sheet material thicker than that in the related art is used for forming the top foil piece  50 , while the bending easiness in the circumferential direction can be maintained to be equivalent to that in the related art, the top foil piece  50  does not easily bend in the radial direction, and the flexure of the intermediate portion thereof toward the back foil  20  can be limited. Thus, it is possible to prevent the load capability from deteriorating due to a decrease in the pressure of the fluid lubrication film at the intermediate portion in the radial direction. 
     In this embodiment, although each of the fixed part-side thin part  14  and the valley part-side thin part  16  is formed, the present disclosure is not limited thereto, and a top foil piece provided with one of the fixed part-side thin part  14  and the valley part-side thin part  16  may be combined with the base plate  30  of this embodiment including the inclined surface  32 . In addition, the top foil piece  50  of this embodiment may be provided with the inner circumferential edge-side thin part  15  of the third embodiment. 
     According to the first aspect of the present disclosure, the thin part extending from the outer circumferential edge to the inner circumferential edge or to the side edge of the top foil piece is formed by removing a part from the surface of the top foil piece facing the back foil piece. Therefore, since the top foil piece easily bends at the thin part, the top foil piece easily bends in a range from the leading side to the trailing side thereof in the rotation direction of the rotary shaft, namely in the circumferential direction. On the other hand, since the top foil piece is almost not influenced in the radial direction by the thin part, the bending difficulty (the bending rigidity) thereof is almost not changed, and is maintained to be equivalent to that before the thin part is formed. Thus, if a sheet material thicker than that in the related art is used for forming the top foil piece, while the bending easiness in the circumferential direction is maintained to be equivalent to that in the related art, the top foil piece does not easily bend in the radial direction compared to the related art, and the flexure of the intermediate portion of the top foil piece toward the back foil can be limited. 
     According to the second aspect of the present disclosure, the thin part includes the fixed part-side thin part which is formed in the fixed part and in the vicinity of the fixed part on the trailing side of the fixed part in the rotation direction of the rotary shaft and which extends from the outer circumferential edge to the inner circumferential edge of the top foil piece. Therefore, if a sheet material thicker than that in the related art is used for forming the top foil piece as described above, the top foil piece does not easily bend particularly in the radial direction compared to the related art, and the flexure of the intermediate portion of the top foil piece toward the back foil can be limited. 
     Since a portion on the trailing side of the top foil piece in the rotation direction of the rotary shaft can easily and smoothly move due to the fixed part-side thin part, for example, in a case where the thrust collar is interposed between a pair of top foils, the starting torque of the rotary shaft decreases. In addition, after the rotary shaft starts rotating, since the top foil piece easily and smoothly moves, the optimum inclination angle of the top foil piece can be easily obtained, and the load capability of the bearing improves. 
     According to the third aspect of the present disclosure, since the thin part includes the valley part-side thin part, if a sheet material thicker than that in the related art is used for forming the top foil piece as described above, the top foil piece does not easily bend particularly in the radial direction compared to the related art, and the flexure of the intermediate portion of the top foil piece toward the back foil can be limited. In addition, since the top foil piece easily bends due to the valley part-side thin part in a range from the leading side to the trailing side of the top foil piece in the rotation direction of the rotary shaft, namely in the circumferential direction, the inclination angle of the top foil piece easily becomes the optimum inclination angle thereof, and the load capability of the bearing improves. 
     According to the fourth aspect, the bump foil piece can resiliently support the top foil piece. In addition, the height of the top foil piece can be changed in accordance with the peak parts of the bump foil piece whose height is changed. 
     According to the fifth aspect, when the top foil piece is arranged on the inclined surface via the back foil piece, the height of the top foil piece can be accurately changed in accordance with the inclined surface. Additionally, in this case, it is sufficient that the back foil piece is formed to have a constant height without changing the height thereof, and thus the machining cost thereof can be limited. 
     According to the sixth aspect, the bump foil piece can resiliently support the top foil piece. In addition, since the ridge line of the peak part closest to the edge on the trailing side of the bump foil piece is disposed to be parallel to the supported surface of the thrust collar, the edge on the trailing side of the top foil piece supported by this peak part always can be easily disposed to be parallel to the thrust collar. 
     According to the seventh aspect, in order to increase the pressure of the fluid lubrication film formed at the top foil piece at the small side of the bearing clearance, namely at the trailing side of the bearing clearance in the rotation direction of the rotary shaft, the trailing side in the rotation direction of the top foil piece can be supported by a high rigidity, and thereby the load capability of the bearing can be improved. 
     Hereinbefore, although embodiments of the present disclosure have been described with reference to the attached drawings, the present disclosure is not limited to the above embodiments. The shape, the combination or the like of each component shown in the above embodiments is an example, and additions, omissions, replacements, and other modifications of a configuration based on a design request or the like can be adopted within the scope of the present disclosure. 
     For example, in the above embodiments, the back foil  20  and the top foil  10  are formed of six back foil pieces  21  (bump foil pieces) and of six top foil pieces  11 , respectively, and thus six support areas  31  are formed (set) in the base plate  30  in order to correspond thereto. However, the number of the back foil pieces  21  (bump foil pieces) or the top foil pieces  11  may be 5 or less, or 7 or more as long as the number is a number greater than one. In this case, the number of the support areas  31  is also adjusted to be the same as that of the back foil pieces  21  (bump foil pieces) or the top foil pieces  11 . 
     A structure in which the fixed part-side thin part  14  is removed from the top foil piece  50  shown in  FIG. 4A , namely a structure in which only the valley part-side thin parts  16  are formed, can also be used for a top foil piece of the present disclosure. 
     Additionally, in the fourth embodiment, although the top foil piece  50  of the second embodiment is used for a top foil piece, instead of this, the top foil piece  11  of the first embodiment or the top foil piece  60  of the third embodiment can be used therefor. 
     In the fourth embodiment, in a case where the inclined surface  32  is formed in the support area  31  of the base plate  30 , instead of a structure in which the entire surface supporting the back foil piece  21  is provided with the inclined surface  32 , the surface may be provided with an inclined surface and a flat surface (a surface parallel to the supported surface of the thrust collar  4 ), and part of the back foil piece  21  may be disposed on the inclined surface and the other thereof may be disposed on the flat surface. In this case, the flat surface may be formed on the upper side (at a position close to the edge  11   a,  at a position close to the thrust collar  4 ) of the inclined surface, or may be formed on the lower side (at a position close to the fixed edge  12 ) thereof. Furthermore, the flat surface may be formed on each of two sides of the inclined surface in the circumferential direction. 
     In addition, various configurations other than the above embodiments can be adopted with respect to the shape of the top foil piece or the bump foil piece, the arrangement of the top foil piece or the bump foil piece on the support area, the inclination direction of the inclined surface, or the like. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure can be used for a thrust bearing which is disposed facing a thrust collar provided on a rotary shaft and supports the thrust collar.