Patent Publication Number: US-9404373-B2

Title: Turbine-blade retaining structure and rotary machine having the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on Japanese Patent Application No. 2012-044672, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a turbine-blade retaining structure that retains turbine blades to a rotor disc. 
     2. Description of the Related Art 
     Known turbine-blade retaining structures that secure the turbine blades of a rotary machine to a rotor disc include, for example, that disclosed in PCT International Application, Publication No. 2009-507176. 
     SUMMARY OF THE INVENTION 
     1. Technical Problem 
     In addition, a turbine-blade retaining structure  51  in a steam turbine, such as that shown in  FIGS. 5 to 10 , has been proposed in recent years. 
     As shown in at least one of  FIGS. 5 to 10 , the turbine-blade retaining structure  51  secures turbine blades  52  to a rotor disc  53  and prevents the turbine blades  52  from coming out (restrains movement of the turbine blades  52 ). 
     The turbine blades  52  are each provided with a Christmas-tree-shaped blade root (bottom)  62  that is slotted into a blade groove  61  formed on the circumferential edge portion of the rotor disc  53  to hold (support) the whole body of the turbine blade  52 , a blade portion  63 , a platform  64  that supports the blade portion  63 , and a shroud (not shown) that extends along the circumferential direction from the distal end (tip) of the blade portion  63  to prevent resonance of the turbine blade  52  and to reduce the leakage loss (leakage of steam) at the distal end of the blade portion  63 . 
     The rotor disc  53  is provided with the blade grooves  61  that extend through the rotor disc  53  in the disc-thickness direction (axial direction) so as to receive the blade roots  62  of the turbine blades  52  that are arranged in the circumferential direction (the blade roots  62  are fitted thereto), a protruding portion (thick-plate portion)  65  that protrudes outwards in the disc-thickness direction as a whole such that its outer circumferential end is located at the inner side of the inner circumferential end of the blade groove  61  in the radial direction, and a ring-shaped locking groove  67  that is formed in the circumferential edge portion of the protruding portion  65  along the circumferential direction so as to open towards the outer side in the radial direction such that locking pieces (clamp members)  66 , which are arranged in the circumferential direction, are received therein (the locking pieces  66  are fitted thereto). 
     On the circumferential edge portion of the protruding portion  65 , at least one insertion window portion (cut-out portion)  68 , which is cut out in the disc-thickness direction such that its inner circumferential surface is flush with the inner circumferential surface of the locking groove  67 , is provided along the circumferential direction (in the turbine-blade retaining structure  51  shown in  FIGS. 5 to 10 , a total of two insertion window portions  68  are located at positions separated by 180 degrees in the circumferential direction (one at each position)). 
     The locking pieces  66  are plate-like members that are fitted (arranged) between the locking groove  67  and a step portion  69  that protrudes outwards in the disc-thickness direction in the circumferential edge portion on the inner circumferential side of the blade root  62  so as to face the locking groove  67 . The circumferential end surface located on the inner circumferential side of the locking piece  66  is curved so as to have the same radius of curvature as (so as to be in contact with) the bottom surface forming the locking groove  67 , and the circumferential end surface located on the outer circumferential side of the locking piece  66  is curved so as to have the same radius of curvature as (so as to be in contact with) the circumferential end surface that is located on the inner circumferential side and that forms the step portion  69 . 
     In addition, among the locking pieces  66  arranged so as to be fitted between the locking groove  67  and the step portion  69 , the adjacent locking pieces  66  that are exposed (visible) through the insertion window portion  68  are joined to each other at the end portions thereof by means of spot-welding. 
     Reference numeral  70  in  FIGS. 5, 6, 8, 10, and 11  indicates a portion welded by means of spot-welding. 
     With such a turbine-blade retaining structure  51 , as shown in  FIG. 11 , a load (force) caused by the turbine blades  52 , which have a tendency to come out from the rotor disc in the axial direction, acts on the end portions of the two locking pieces  66  exposed through the insertion window portion  68 . Thus, there has been a problem in that the end portions of the locking pieces  66  are deformed in the axial direction, causing a crack  71  in the welded portion  70 . 
     In addition, with such a turbine-blade retaining structure  51 , as shown in  FIG. 10( b ) , steam drain  72  collides with the welded portion  70 , causing erosion of the welded portion  70 . Thus, there has also been a problem in that the welded portion  70  is weakened, and the crack  71  tends to be caused in the welded portion  70 . 
     The present invention has been conceived in light of the above-described circumstances, and an object thereof is to provide a turbine-blade retaining structure and a rotary machine having the turbine-blade retaining structure that are capable of preventing deformation of end portions of locking pieces exposed through an insertion window portion provided on a protruding portion of a rotor disc in the axial direction of the rotor disc and that are capable of preventing the occurrence of a crack in a welded portion where the end portions of the locking pieces are welded to each other. 
     2. Solution to the Problem 
     In order to solve the problems described above, the present invention employs the following solutions. 
     A turbine-blade retaining structure according to a first aspect of the present invention includes: blade grooves that extend through a rotor disc in a disc-thickness direction so as to receive blade roots of turbine blades arranged in a circumferential direction; a protruding portion that protrudes, as a whole, outwards in the disc-thickness direction such that its outer circumferential end is located at an inner side of an inner circumferential end of the blade grooves in the radial direction; a ring-shaped locking groove that is formed in a circumferential edge portion of the protruding portion along the circumferential direction so as to open towards an outer side in the radial direction to receive a plate-like locking piece, which is arranged in the circumferential direction; a step portion that protrudes outwards in the disc-thickness direction in a circumferential edge portion on an inner circumferential side of the blade root so as to face the locking groove; and the locking piece that is fitted between the locking groove and the step portion; wherein the circumferential edge portion of the protruding portion is provided with, along the circumferential direction, at least one insertion window portion that is cut out in the disc-thickness direction such that an inner circumferential surface thereof is flush with an inner circumferential surface of the locking groove, and end portions of the adjacent locking pieces exposed through the insertion window portion are joined to each other by means of welding; and at least one of the end portions of the adjacent locking pieces exposed through the insertion window portion is provided with a thick-plate portion expanding in the disc-thickness direction so as to be inside the insertion window portion. 
     With the turbine-blade retaining structure according to the above-mentioned aspect, the end portion of the locking piece exposed through the insertion window portion is formed to have a plate thickness greater than that of the locking piece forming the portion other than the thick-plate portion; in other words, the end portion of the locking piece exposed through the insertion window portion is formed such that the rigidity thereof is higher (greater) than that in a conventional structure. 
     By doing so, it is possible to prevent (reduce) deformation of the end portions of the locking pieces exposed through the insertion window portion in the axial direction and to prevent the occurrence of a crack in the welded portion. 
     In the above-mentioned turbine-blade retaining structure, preferably, the thick-plate portion is provided from one end of the locking piece that is joined by means of welding towards a side at the other end. 
     According to such a turbine-blade retaining structure, the thick-plate portion is provided from the one end of the locking piece that is joined by means of welding towards the side at the other end. In other words, the thick-plate portion is provided in wider region in the longitudinal direction than the thick-plate portion in the above-mentioned turbine-blade retaining structure. 
     By doing so, it is possible to prevent (reduce), to an even greater extent, deformation of the end portions of the locking pieces exposed through the insertion window portion in the axial direction, and it is possible to prevent, to an even greater extent, the occurrence of a crack in the welded portion. 
     In the above-mentioned turbine-blade retaining structure, preferably, a recessed groove is provided in a central portion of the thick-plate portion in the width direction so as to extend from the one end of the locking piece, which is joined by means of welding, towards the side at the other end and so as to open at the one end. 
     According to such a turbine-blade retaining structure, at least a part of the welded portion that joins the end portions of the adjacent locking pieces exposed through the insertion window portion to each other is located (accommodated) in the recessed groove. 
     By doing so, it is possible to reduce the problem where a crack tends to be caused in the welded portion that has been eroded and weakened due to collision and adhesion of steam drain on the welded portion. 
     In the above-mentioned turbine-blade retaining structure, preferably, the thick-plate portion is provided in the end portion of the locking piece on a leading side in a rotation direction of the rotor disc. 
     According to such a turbine-blade retaining structure, the welded portion that joins the end portions of the adjacent locking pieces exposed through the insertion window portion to each other is formed on the opposite side of the thick-plate portion from the leading side in the rotation direction of the rotor disc. In other words, the welded portion that joins the end portions of the adjacent locking pieces exposed through the insertion window portion to each other is provided behind the thick-plate portion. 
     By doing so, it is possible to further reduce the problem where a crack tends to be caused in the welded portion that has been eroded and weakened due to collision of the steam drain on the welded portion. 
     In the above-mentioned turbine-blade retaining structure, preferably, a sloped portion is provided so as to be formed to have a plate thickness that increases gradually from one end of the locking piece, which is joined by means of welding, towards a side at the other end. 
     According to such a turbine-blade retaining structure, the last locking piece to be inserted into the insertion window portion is inserted along the sloped portion. 
     By doing so, it is possible to insert the last locking piece to be inserted into the insertion window portion with ease and to improve the ease of assembly. 
     A rotary machine according to a second aspect of the present invention includes any one of the above-mentioned turbine-blade retaining structures. 
     The rotary machine according to the above-mentioned aspect is provided with the turbine-blade retaining structure that is capable of preventing deformation of the end portions of the locking pieces exposed through the insertion window portion in the axial direction and capable of preventing the occurrence of a crack in the welded portions. 
     By doing so, it is possible to prevent fragments of the locking pieces and the welded portion from scattering downstream, thereby improving the reliability of the rotary machine. 
     3. Advantageous Effects of the Invention 
     According to the present invention, advantages are afforded in that it is possible to prevent deformation, in the axial direction, of end portions of locking pieces exposed through an insertion window portion and to prevent the occurrence of a crack in a welded portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing relevant parts of a turbine-blade retaining structure according to a first embodiment of the present invention, where (a) is a front view, and (b) is a bottom view of (a) viewed from below. 
         FIG. 2  is a view showing relevant parts of a turbine-blade retaining structure according to a second embodiment of the present invention, where (a) is a front view, and (b) is a bottom view of (a) viewed from below. 
         FIG. 3  is a perspective view showing relevant parts of the turbine-blade retaining structure according to the second embodiment of the present invention. 
         FIG. 4  is a perspective view showing, in disassembled form, locking pieces exposed through an insertion window portion. 
         FIG. 5  is a perspective view showing relevant parts of a turbine-blade retaining structure, which has been proposed in recent years. 
         FIG. 6  is a front view showing, in enlarged form, relevant parts of  FIG. 5 . 
         FIG. 7  is a sectional view taken along line X-X in  FIG. 6 . 
         FIG. 8  is a sectional view taken along line Y-Y in  FIG. 6 . 
         FIG. 9  is a perspective view of the locking pieces shown in  FIGS. 5 to 8 . 
         FIG. 10  is a front view showing, in enlarged form, relevant parts of  FIG. 5 . 
         FIG. 11  is a view for explaining problems in the turbine-blade retaining structure proposed in recent years. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
     A turbine-blade retaining structure of a first embodiment of the present invention, which is utilized for a steam turbine, will be described below with reference to  FIG. 1 . 
       FIG. 1  is a view showing relevant parts of the turbine-blade retaining structure according to this embodiment, in which (a) is a front view, and (b) is a bottom view of (a) viewed from below. 
     A turbine-blade retaining structure  11  according to this embodiment secures the turbine blades  52  (see  FIG. 5 ) to the rotor disc  53  (see  FIG. 5 ) and prevents the turbine blades  52  from coming out (restrains movement of the turbine blades  52 ). 
     Because the turbine blades  52  and the rotor disc  53  have been explained in the above-described “Technical Problem”, explanations thereof will be omitted herein. 
     As shown in  FIG. 1 , in the turbine-blade retaining structure  11  according to this embodiment, among two adjacent locking pieces exposed (visible) through the insertion window portion  68 , one of the locking pieces has the configuration shown on the left-hand side in  FIG. 1 , and the other locking piece has the configuration shown on the right-hand side in  FIG. 1  (the locking piece  66  explained in the above-described “Technical Problem”). 
     Because the locking piece  66  has been explained in the above-described “Technical Problem”, an explanation thereof will be omitted herein. 
     One locking piece  12  is, as with the other locking piece  66 , a plate-like member that is fitted (arranged) between the locking groove  67  (see  FIGS. 5 to 8 ) and the step portion  69  (see  FIGS. 5 to 8 ) that protrudes outwards in the disc-thickness direction in the circumferential edge portion on the inner circumferential side of the blade root  62  (see  FIGS. 5 to 8 ) so as to face the locking groove  67 . The circumferential end surface located on the inner circumferential side of the locking piece  12  is curved so as to have the same radius of curvature as (so as to be in contact with) the bottom surface forming the locking groove  67 , and the circumferential end surface located on the outer circumferential side of the locking piece  12  is curved so as to have the same radius of curvature as (so as to be in contact with) the circumferential end surface that is located on the inner circumferential side and that forms the step portion  69 . 
     The end portion of the locking piece  12 , which is exposed through the insertion window portion  68  and which is joined to the end portion of the locking piece  66  exposed through the insertion window portion  68  by means of spot-welding, is provided with a thick-plate portion  13  expanding (protruding) in the disc-thickness direction (a direction perpendicular to the plane of the drawing in  FIG. 1( a ) : the up-down direction in  FIG. 1( b ) ) over the entire width direction (the up-down direction in  FIG. 1( a ) : a direction perpendicular to the plane of the drawing in  FIG. 1( b ) ) so as to be inside the insertion window portion  68  (so as to be directed outwards in the axial direction). 
     The thick-plate portion  13  is a portion that is formed to have a plate thickness greater than that of the locking piece  12  forming the portion other than the thick-plate portion  13 , and is provided with a plate-like portion (constant plate thickness portion)  14  and a sloped portion (varying plate thickness portion)  15 . 
     The plate-like portion  14  is a plate-like portion having a constant plate thickness (thickness) over the entire width direction and longitudinal direction (the left-right direction in  FIGS. 1( a ) and 1( b ) ). 
     The sloped portion  15  continuously connects one end (base end) of the plate-like portion  14  in the longitudinal direction and one end of the locking piece  12  forming the portion other than the thick-plate portion  13 , and the sloped portion  15  is a portion that is formed to have a plate thickness that decreases gradually (at a certain rate) from one end of the plate-like portion  14  in the longitudinal direction to (towards) one end of the locking piece  12  forming the portion other than the thick-plate portion  13 . In other words, the sloped portion  15  continuously connects one end (base end) of the plate-like portion  14  in the longitudinal direction and one end of the locking piece  12  forming the portion other than the thick-plate portion  13 , and the sloped portion  15  is a portion that is formed to have a plate thickness that decreases gradually (at a certain rate) in (towards) the rotation direction of the rotor disc  53  (direction in which the rotor disc  53  rotates). 
     In addition, in the central portion of the plate-like portion  14  in the width direction, a recessed groove (recessed portion)  16  having a (substantially) rectangular shape in front view is provided so as to extend from the vicinity of one end to the other end of the plate-like portion  14  in the longitudinal direction and so as to open at the other end of the plate-like portion  14  in the longitudinal direction. The bottom surface of the recessed groove  16  is formed to have a surface flush with the front surface of the locking piece  66  (in other words, the front surface of the locking piece  12  forming the portion other than the thick-plate portion  13 ). In other words, the plate thickness of the recessed groove  16  is the same as the plate thickness of the locking piece  66  and the same as the plate thickness of the locking piece  12  forming the portion other than the thick-plate portion  13 . 
     With the turbine-blade retaining structure  11  according to this embodiment, the end portion of the locking piece  12  exposed through the insertion window portion  68  is formed to have a plate thickness greater than that of the locking piece  12  forming the portion other than the thick-plate portion  13 ; in other words, the end portion of the locking piece  12  exposed through the insertion window portion  68  is formed such that the rigidity thereof is higher (greater) than that in a conventional structure. 
     By doing so, it is possible to prevent (reduce) deformation of the end portions of the locking pieces  12  exposed through the insertion window portion  68  in the axial direction of the rotor disc and to prevent the occurrence of a crack in the welded portion  70 . 
     In addition, with the turbine-blade retaining structure  11  according to this embodiment, a part of the welded portion  70  that joins the end portions of the adjacent locking pieces  12  and  66  exposed through the insertion window portion  68  to each other is located (accommodated) in the recessed groove  16 . 
     By doing so, it is possible to reduce the problem where a crack tends to be caused in the welded portion  70  that has been eroded and weakened due to collision of the steam drain  72  on the welded portion  70 . 
     Furthermore, with the turbine-blade retaining structure  11  according to this embodiment, the welded portion  70  that joins the end portions of the adjacent locking pieces  12  and  66  exposed through the insertion window portion  68  to each other is formed on the opposite side of the thick-plate portion  13  from the leading side in the rotation direction of the rotor disc  53 . In other words, the welded portion  70  that joins the end portions of the adjacent locking pieces  12  and  66  exposed through the insertion window portion  68  to each other is provided behind the thick-plate portion  13 . 
     By doing so, it is possible to further reduce the problem where a crack tends to be caused in the welded portion  70  that has been eroded and weakened due to collision of the steam drain  72  on the welded portion  70 . 
     On the other hand, with the steam turbine provided with the turbine-blade retaining structure  11  according to this embodiment, it is possible to prevent fragments of the locking pieces  12  and  66  and the welded portion  70  from scattering downstream, thereby improving the reliability of the steam turbine. 
     Second Embodiment 
     The turbine-blade retaining structure of a second embodiment of the present invention, which is utilized for a steam turbine, will be described below with reference to  FIGS. 2 to 4 . 
       FIG. 2  is a view showing relevant parts of the turbine-blade retaining structure according to this embodiment, in which (a) is a front view, and (b) is a bottom view of (a) viewed from below;  FIG. 3  is a perspective view showing relevant parts of the turbine-blade retaining structure according to this embodiment; and  FIG. 4  is a perspective view showing, in disassembled form, the locking pieces exposed through the insertion window portion. 
     A turbine-blade retaining structure  21  according to this embodiment differs from that in the first embodiment described above in that locking pieces  22  are provided instead of the locking pieces  12 . The other constituent elements are the same as those in the first embodiment described above, and therefore, a description of those constituent elements is omitted here. 
     Parts that are identical to those in the first embodiment described above are assigned the same reference numerals. 
     As shown in at least one of  FIGS. 2 to 4 , in the turbine-blade retaining structure  21  according to this embodiment, among the two adjacent locking pieces exposed (visible) through the insertion window portion  68 , one of the locking pieces has the configuration shown on the left-hand side in  FIGS. 2 to 4 , and the other locking piece has the configuration shown on the right-hand side in  FIGS. 2 to 4  (the locking piece  66  explained in above-described “Technical Problem”). 
     Because the locking piece  66  has been explained in the above-described “Technical Problem”, an explanation thereof will be omitted herein. 
     One locking piece  22  is, as with the other locking piece  66 , a plate-like member that is fitted (arranged) between the locking groove  67  (see  FIGS. 5 to 8 ) and the step portion  69  (see  FIGS. 5 to 8 ) that protrudes outwards in the disc-thickness direction in the circumferential edge portion on the inner circumferential side of the blade root  62  (see  FIGS. 5 to 8 ) so as to face the locking groove  67 . The circumferential end surface located on the inner circumferential side of the locking piece  22  is curved so as to have the same radius of curvature as (so as to be in contact with) the bottom surface forming the locking groove  67 , and the circumferential end surface located on the outer circumferential side of the locking piece  22  is curved so as to have the same radius of curvature as (so as to be in contact with) the circumferential end surface that is located on the inner circumferential side and that forms the step portion  69 . 
     The end portion of the locking piece  22 , which is exposed through the insertion window portion  68  and which is joined to the end portion of the locking piece  66  exposed through the insertion window portion  68  by means of spot-welding, is provided with a thick-plate portion  23  expanding (protruding) in the disc-thickness direction (a direction perpendicular to the plane of the drawing in  FIG. 2( a ) : the up-down direction in  FIG. 2( b ) ) over the entire longitudinal direction (the left-right direction in  FIGS. 2 to 4 ) so as to be directed outwards in the axial direction. 
     The thick-plate portion  23  is a portion that is formed to have a plate thickness greater than that of the locking piece  22  forming the portion other than the thick-plate portion  23 , and is provided with a plate-like portion (constant plate thickness portion)  24 , a (first) sloped portion (varying plate thickness portion)  25 , and a (second) sloped portion (varying plate thickness portion)  26 . 
     The plate-like portion  24  is a plate-like portion having a constant plate thickness (thickness) over the entire longitudinal direction and is formed so as also to have, in a region from the vicinity of the center to one end (distal end) in the longitudinal direction, a constant plate thickness (thickness) over the entire width direction (the up-down direction in  FIG. 2( a ) : the direction perpendicular to the plane of the drawing in  FIG. 2( b ) ). 
     In addition, in the region from the vicinity of the center to the other end (base end) of the plate-like portion  24  in the longitudinal direction, a cut-out portion  27  is provided. The cut-out portion  27  is defined by the inner circumferential surface facing (in contact with) the outer circumferential surface of the protruding portion  65 , the side surface that extends along the radial direction and that is located at one side on the inner circumferential surface of the insertion window portion  68  so as to face one of the end surfaces that forms the insertion window portion  68  together with the inner circumferential surface of the insertion window portion  68 , and the bottom surface that is located at the outer side in the axial direction so as to face (be in contact with) the wall surface forming the locking groove  67 . 
     The plate thickness of the cut-out portion  27  is the same as the plate thickness of the locking piece  66  and the plate thickness of the locking piece  22  forming the portion other than the thick-plate portion  23 . 
     The sloped portion  25  continuously connects one end of the plate-like portion  24  in the longitudinal direction and one end (distal end) of the locking piece  22 , and the sloped portion  25  is a portion that is formed to have a plate thickness that decreases gradually (at a certain rate) from one end of the plate-like portion  24  in the longitudinal direction to (towards) one end of the locking piece  22 . In other words, the sloped portion  25  continuously connects one end (distal end) of the plate-like portion  24  in the longitudinal direction and one end of the locking piece  22 , and the sloped portion  25  is a portion that is formed to have a plate thickness that decreases gradually (at a certain rate) in (towards) the direction opposite to the rotation direction of the rotor disc  53  (see  FIG. 5 ) (direction in which the rotor disc  53  rotates). 
     In addition, in the central portion of the sloped portion  25  in the width direction, a recessed groove (recessed portion)  28  having a (substantially) rectangular shape in front view is provided so as to extend from the vicinity of one end (base end) to the other end (distal end) of the sloped portion  25  in the longitudinal direction and so as to open at the other end of the sloped portion  25  in the longitudinal direction. The bottom surface of the recessed groove  28  is formed to have a surface flush with the front surface of the locking piece  66  (in other words, the front surface of the locking piece  22  forming the portion other than the thick-plate portion  23 ). In other words, the plate thickness of the recessed groove  28  is the same as the plate thickness of the locking piece  66  and the same as the plate thickness of the locking piece  22  forming the portion other than the thick-plate portion  23 . 
     The sloped portion  26  continuously connects the other end (base end) of the plate-like portion  24  in the longitudinal direction and one end (distal end) of the locking piece  22  forming the portion other than the thick-plate portion  23 , and the sloped portion  26  is a portion that is formed to have a plate thickness that decreases gradually (at a certain rate) from the other end of the plate-like portion  24  in the longitudinal direction to (towards) one end of the locking piece  22  forming the portion other than the thick-plate portion  23 . In other words, the sloped portion  26  continuously connects the other end (base end) of the plate-like portion  24  in the longitudinal direction and one end of the locking piece  22  forming the portion other than the thick-plate portion  23 , and the sloped portion  26  is a portion that is formed to have a plate thickness that decreases gradually (at a certain rate) in (towards) the rotation direction of the rotor disc  53  (see  FIG. 5 ) (direction in which the rotor disc  53  rotates). 
     In addition, in the region from the one end (distal end) to the other end (base end) of the sloped portion  26  in the longitudinal direction, a cut-out portion  29  is provided. The cut-out portion  29  is formed of the inner circumferential surface facing (in contact with) the outer circumferential surface of the protruding portion  65  and the bottom surface that is located at the outer side in the axial direction so as to face (be in contact with) the wall surface forming the locking groove  67 . 
     The inner circumferential surface of the cut-out portion  29  is formed to have a surface flush with the inner circumferential surface of the cut-out portion  27 , and the bottom surface of the cut-out portion  29  is formed to have a surface flush with the bottom surface of the cut-out portion  27 . In addition, the plate thickness of the cut-out portion  29  is the same as the plate thickness of the locking piece  66  and the plate thickness of the locking piece  22  forming the portion other than the thick-plate portion  23 . 
     With the turbine-blade retaining structure  21  according to this embodiment, the end portion of the locking piece  22  exposed through the insertion window portion  68  is formed to have the plate thickness greater than that of the locking piece  22  forming the portion other than the thick-plate portion  23 ; in other words, the end portion of the locking piece  22  exposed through the insertion window portion  68  is formed such that the rigidity thereof is higher (greater) than that in a conventional structure. 
     By doing so, it is possible to prevent (reduce) deformation of the end portions of the locking pieces  22  and  66  exposed through the insertion window portion  68  in the axial direction and to prevent the occurrence of a crack in the welded portion  70 . 
     In addition, with the turbine-blade retaining structure  21  according to this embodiment, a part of the welded portion  70  that joins the end portions of the adjacent locking pieces  22  and  66  exposed through the insertion window portion  68  to each other is located (accommodated) in the recessed groove  28 . 
     By doing so, it is possible to reduce the problem where a crack tends to be caused in the welded portion  70  that has been eroded and weakened due to collision of the steam drain  72  on the welded portion  70 . 
     Furthermore, with the turbine-blade retaining structure  21  according to this embodiment, the welded portion  70  that joins the end portions of the adjacent locking pieces  22  and  66  exposed through the insertion window portion  68  to each other is formed on the opposite side of the thick-plate portion  23  from the leading side in the rotation direction of the rotor disc  53 . In other words, the welded portion  70  that joins the end portions of the adjacent locking pieces  22  and  66  exposed through the insertion window portion  68  to each other is provided behind the thick-plate portion  23 . 
     By doing so, it is possible to further reduce the problem where a crack tends to be caused in the welded portion  70  that has been eroded and weakened due to collision of the steam drain  72  on the welded portion  70 . 
     In addition, with the turbine-blade retaining structure  21  according to this embodiment, the sloped portion  25  is provided so as to be formed to have a plate thickness that increases gradually from one end of the locking piece  22 , which is joined by means of welding, towards the side at the other end, and the last locking piece  66  to be inserted into the insertion window portion  68  is inserted along the sloped portion  25 . 
     By doing so, it is possible to insert the last locking piece  66  to be inserted into the insertion window portion  68  with ease and to improve the ease of assembly. 
     On the other hand, with the steam turbine provided with the turbine-blade retaining structure  21  according to this embodiment, it is possible to prevent fragments of the locking pieces  22  and  66  and the welded portion  70  from scattering downstream, thereby improving the reliability of the steam turbine. 
     Note that the present invention is not limited to the above-mentioned embodiments, and appropriate modifications/alterations are possible as needed. 
     For example, in the above-mentioned embodiments, the turbine-blade retaining structure according to the present invention has been explained in terms of one that is utilized in steam turbines as a specific example; however, the present invention is not limited thereto, and it is also possible to utilize the turbine-blade retaining structure in rotary machines other than steam turbines (rotary machines such as gas turbines, compressors, and so forth, in which the turbine blades are fixed to a rotor disc). 
     In addition, in the above-mentioned embodiment, an explanation has been given by mentioning the structure in which the thick-plate portion  13  or  23  is provided on only the end portion of one locking piece  12  or  22  among the two adjacent locking pieces exposed through the insertion window portion  68 , in other words, only the end portion of the locking piece  12  or  22  on the leading side in the rotation direction of the rotor disc  53 , as a specific example; however, the present invention is not limited thereto, and the thick-plate portion  13  or  23  may be provided on only the end portion of the other locking piece  66  among the two adjacent locking pieces exposed through the insertion window portion  68 . In other words, only the end portion of the locking piece  66  on the opposite side from the leading side in the rotation direction of the rotor disc  53 , and the thick-plate portion  13  or  23  may be provided on the end portions of both of the locking pieces  12  or  22  and  66 . 
     REFERENCE SIGNS LIST 
     
         
           11  turbine-blade retaining structure 
           12  locking piece 
           13  thick-plate portion 
           16  recessed groove 
           21  turbine-blade retaining structure 
           22  locking piece 
           23  thick-plate portion 
           25  sloped portion 
           28  recessed groove 
           52  turbine blade 
           53  rotor disc 
           61  blade groove 
           62  blade root 
           65  protruding portion 
           66  locking piece 
           67  locking groove 
           68  insertion window portion 
           69  step portion