Patent Publication Number: US-9422820-B2

Title: Method and system for self-locking a closure bucket in a rotary machine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and benefit of Italian Patent Application No. CO2013A000002, entitled “METHOD AND SYSTEM FOR SELF-LOCKING A CLOSURE BUCKET IN A ROTARY MACHINE”, filed Jan. 23, 2013, which is herein incorporated by reference in its entirety. 
     BACKGROUND 
     The subject matter disclosed herein relates to methods and systems for self-locking a closure bucket in rotary machines such as turbomachines. 
     Turbomachines or rotary systems, such as axial compressors and turbines (e.g., gas turbine axial compressors, steam turbines, etc.), may generally include a rotor portion that rotates about an axis during the operation of the system. For example, in an axial compressor or steam turbine, the rotor portion (e.g., disk of a stage) may include a number of buckets (e.g., rotary blades) disposed about a shaft. The buckets are circumferentially disposed adjacent each other about the rotor portion. Often these buckets are loaded onto the rotor portion in a tangential direction. The last bucket loaded on the rotor portion is called the closure bucket. The closure bucket is secured to the rotor portion to lock the buckets in place on the rotor and to block circumferential movement of the buckets along the rotor portion (i.e., relative to the rotor portion). However, the mechanisms used to secure the closure bucket to the rotor portion may result in stress concentration the rotor and/or significant remachining of the rotor during reassembly of the stage (e.g., turbine stage of a steam turbine or compressor stage). 
     BRIEF DESCRIPTION 
     Certain embodiments commensurate in scope with the originally claimed invention are summarized below. These embodiments are not intended to limit the scope of the claimed invention, but rather these embodiments are intended only to provide a brief summary of possible forms of the invention. Indeed, the invention may encompass a variety of forms that may be similar to or different from the embodiments set forth below. 
     In accordance with a first embodiment, a system includes a turbomachine. The turbomachine includes at least one rotor wheel that includes a peripheral portion disposed about a rotational axis of the at least one rotor disk or stage. The peripheral portion includes a groove that extends circumferentially about the peripheral portion. The groove has a first groove surface and a second groove surface disposed opposite the first groove surface. The turbomachine also includes at least one bucket disposed within the groove. The turbomachine further includes a closure bucket disposed adjacent the at least one bucket within the groove. The closure bucket has a first surface that interfaces with the first groove surface and a second surface disposed opposite the first surface. The closure bucket blocks circumferential movement of the at least one bucket within the groove relative to the at least one rotor disk or stage. The turbomachine yet further includes a single wedge disposed between and contacting the second surface of the closure bucket and the second groove surface to secure the closure bucket within the groove. 
     In accordance with a second embodiment, a system includes a bucket locking assembly for securing multiple buckets within a groove of a rotor disk or stage of a turbomachine to block circumferential movement of the multiple buckets relative to the rotor disk or stage. The bucket locking assembly includes a closure bucket configured to be disposed between adjacent buckets within the groove. The closure bucket has a first surface configured to interface with a first groove surface of the groove and a second surface disposed opposite the first surface. The closure bucket is configured to block circumferential movement of the multiple buckets within the groove relative to the rotor disk or stage. The bucket locking assembly also includes a single wedge configured to be disposed between and to contact the second surface of the closure bucket and a second groove surface of the groove to secure the closure bucket within the groove. The single wedge is subject to an axial force on the second surface of the closure bucket to secure the closure bucket within the groove. 
     In accordance with a third embodiment, a method for securing buckets within a groove of a rotor disk or stage of a turbomachine is provided. The method includes disposing a single wedge into a closure groove portion of the groove, wherein the closure groove portion includes a first groove surface, a second groove surface, and a third groove surface, the single wedge is disposed between the first and second groove surfaces, the first groove surface includes multiple recesses, and the single wedge includes a first wedge surface and a second wedge surface. The method also includes radially inserting a closure bucket having a first surface and a second surface disposed opposite the second surface into the closure groove portion so that the second surface contacts an outer surface of the rotor adjacent the closure groove, wherein the closure bucket includes multiple protrusions that extend from the first surface. The method further includes axially displacing the closure bucket until the first surface contacts the first groove surface, radially displacing the closure bucket until the multiple protrusions align with the multiple recesses of the first groove surface, and axially displacing the closure bucket until the first surface interfaces with the first groove surface and the multiple protrusions insert into the multiple recesses. The method yet further includes radially displacing the single wedge from the third groove surface until the first wedge surface contacts the second surface of the closure bucket and the second wedge surface contacts the second groove surface to secure the closure bucket within the closure groove portion to block circumferential movement of the buckets relative to the rotor disk or stage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a cross-sectional side view of an embodiment of a turbomachine system (e.g., gas turbine system) including a compressor with a coupled disks rotor having a self-locking closure bucket assembly for each rotor disk or stage; 
         FIG. 2  is a partial side perspective view of an embodiment of a self-locking closure bucket assembly disposed within a groove of the rotor disk or stage; 
         FIG. 3  is partial rear perspective view of an embodiment of the self-locking closure bucket assembly of  FIG. 2  disposed within the groove of the rotor disk or stage between adjacent buckets; 
         FIG. 4  is partial front perspective view of an embodiment of the self-locking closure bucket assembly of  FIG. 2  disposed within the groove of the rotor disk or stage between the adjacent buckets; 
         FIG. 5  is a top view of an embodiment of the self-locking closure bucket assembly of  FIG. 2  disposed within the groove of the rotor disk or stage; 
         FIG. 6  is a cross-sectional side view of an embodiment of a groove portion for buckets taken along line  6 - 6  of  FIG. 5 ; 
         FIG. 7  is a cross-sectional side view of an embodiment of a closure groove portion for the self-locking closure bucket assembly taken along line  7 - 7  of  FIG. 5 ; 
         FIGS. 8A-F  are a series of partial side views illustrating the assembly of the self-locking closure bucket assembly of  FIG. 2  within the closure groove portion in accordance with an embodiment; 
         FIG. 9  is a cross-sectional side view of an embodiment of a turbomachine system (e.g., gas turbine system) including a compressor with a single piece rotor having the self-locking closure bucket assembly of  FIG. 2  for each stage; and 
         FIG. 10  is a partial cross-sectional side view of a turbomachine system (e.g., steam turbine system) including a turbine with a single piece rotor having the self-locking closure bucket assembly of  FIG. 2  for each stage. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     The present disclosure is directed to turbomachines that include self-locking closure bucket assemblies. For example, the turbomachine may be a gas turbine engine, steam turbine engine, compressor, or any other type of rotary machine (e.g., turbomachine). The self-locking closure bucket assembly may be used to block circumferential movement of the other buckets (e.g., tangential entry dovetailed buckets) within a groove of a rotor disk or stage (e.g., same row or stage). In particular, the self-locking closure bucket assembly includes a closure bucket (e.g., rotary blade with a mounting base portion) and only a single wedge disposed within the same portion of the groove (e.g., closure groove portion) to secure the closure bucket within the closure groove. The closure groove portion includes a first groove surface, a second groove surface disposed opposite the first groove surface, and a third surface disposed between the first and second groove surfaces. The closure bucket includes a first surface (e.g., of a male dovetail portion having protrusions) that interfaces with the first groove surface (e.g., having recesses for the protrusions) and a second surface disposed opposite the first surface to contact or interface with the single wedge. The single wedge may be pre-inserted or disposed in the closure groove portion (e.g., against the third groove surface). The closure bucket assembly may include a non-loaded screw (e.g., threaded fastener) that extends along a longitudinal axis of the wedge through the wedge. The screw enables the wedge to be radially displaced from the third groove surface to a location in between the closure bucket and the closure groove portion. For example, the radially displaced wedge may interface or contact both the second surface of the closure bucket and the second groove surface of the closure groove. At operating conditions, along with centrifugal force, an axial force exerted on the single wedge against the second surface of the closure bucket (and second groove surface of the closure groove) secures the closure bucket within the closure groove. The self-locking closure bucket assembly enables the securing of the closure bucket within the closure groove without utilizing a locking screw that extends through the closure bucket (e.g., dovetail portion) into the rotor (e.g., rotor disk or stage). As a result, stress concentrations in the rotor due to a conventional locking screw may be avoided. In addition, the self-locking bucket assembly may enable reassembly of the stage or row without damaging or remachining the rotor (e.g., during maintenance of a turbine or compressor stage). 
     Turning now to the drawings,  FIG. 1  illustrates an embodiment of a turbomachine system  10  (e.g., gas turbine system having an axial compressor  14  with coupled disk rotors) having a self-locking closure bucket assembly (e.g., bucket locking assembly) for each rotor disk or stage  12 . The self-locking closure bucket assembly, described in greater detail below, utilizes the centrifugal moment to which the closure bucket is subject to as a consequence of its asymmetrical shape, to secure the closure bucket itself within a groove of the respective rotor disk or stage  12  and to block circumferential movement of the other buckets within the same row, stage, or groove. The function of the wedge is to react to an axial force which derives from the centrifugal moment of the closure bucket and to transmit axial force to the groove (e.g., downstream groove surface). The self-locking closure bucket assembly avoids the need for a locking screw disposed through a dovetail portion of the closure bucket and into the rotor disk or stage  12 . Thus, the potential stress concentrations due to such a fixing screw may be avoided. Further, the self-locking closure bucket assembly enables reassembly of a stage without damaging or remachining the rotor disk or stage  12 . The self-locking closure bucket assembly may be used in any turbomachine, such as, but not limited to, gas turbine engines, steam turbine engines, hydro turbines, compressors, or any other rotary machines. 
     The system  10  includes a compressor  14  (e.g., rotary machine) and a turbine  20 . In the illustrated embodiment, the compressor  14  includes compressor blades or buckets  32 . The compressor buckets  32  within the compressor  14  are coupled to the rotor disk or stage  12  and rotate as the rotor disk or stage  12  of the compressor  14  (which form a shaft) are driven into rotation by the turbine  20 . Besides the axial compressor  14  of the system  10  of  FIG. 1 , the self-locking closure bucket assembly may also be used in the axial compressor  14  of  FIG. 9 , which illustrates a gas turbine system  150  having the axial compressor  14  with a single piece rotor  152 . Also, the self-locking closure bucket assembly may also be used in a steam turbine system  160  (e.g., axial exhaust steam turbine) having a single piece rotor  162  as illustrated in  FIG. 10 . The steam turbine system of  FIG. 10  includes a turbine section  164  that includes multiple stages  166 . Each stage  166  includes a plurality of blades  168  arranged in rows that extend circumferentially around a shaft  318 . In the following discussion, reference may be made to various directions, such as an axial direction or axis  38 , a radial direction or axis  40 , and a circumferential direction or axis  42 , relative to the rotational or longitudinal axis  28  of the system  10 . 
       FIG. 2  is a partial side perspective view of an embodiment of the self-locking closure bucket assembly  44  disposed within a groove  46  (e.g., closure groove portion  48 ) of the rotor disk or stage  12 . The groove  46  runs in the circumferential direction  42  along a peripheral portion  50  disposed about the rotational axis  28  of the rotor disk or stage  12  (see  FIG. 1 ). The groove  46  includes groove surfaces  52 ,  54 ,  56 . Groove surface  52  is disposed opposite groove surface  54 . Groove surface  56  is disposed at a base or bottom portion  58  of the groove  46  between groove surfaces  52 ,  54 . Groove surface  52  of the closure groove  48  includes a plurality of recesses  60  (e.g., hooks) that extend axially  38  within the groove surface  52  (see  FIG. 7 ). The number of recesses  60  may vary between 1 to 5 or more recesses  60 . As depicted, the groove surface  52  includes two recesses  60 . Groove surface  54  of the closure groove  48  includes a single recess  62  that extends axially  38  within the groove surface  54  (see  FIG. 7 ). Together, the groove surfaces  52 ,  54  form an axial platform  63  that interfaces with the closure bucket assembly  44  to secure the assembly  44  within the closure groove  48 . As described in greater detail below, a cross-sectional area of the closure groove  48  is greater than a cross-sectional area of the groove portion for the other buckets. 
     The closure bucket assembly  44  includes a bucket  64  (e.g., closure bucket  64 ), single wedge  66 , and a threaded fastener or screw  68  (e.g., unloaded fixing screw) disposed within the same closure groove portion  48  (as opposed to axially adjacent groove portions extending in the circumferential direction  42 ). The bucket  64  includes an upper portion  65  (e.g., blade or airfoil  67 ) and a lower portion  69  (e.g., mounting portion or male dovetail configuration  70 ). The lower portion  69  includes surface  71  (e.g., upstream surface) and surface  72  (e.g., downstream surface). The surface  71  includes a plurality of protrusions  74  (e.g., axial projections or hooks) that extend axially  38  from the surface  71 . The number of protrusions  74  may vary between 1 to 5 or more protrusions  74 . As depicted, the groove surface  52  includes three protrusions  74 . At least some of the protrusions  74  are configured to fit within the recesses  60  of the groove surface  52  to block movement of the closure bucket  64  in the radial direction  40 , while other protrusions  74  may abut the groove surface  52  without interacting with the recesses  60 . The surface  72  includes a plurality of recesses  76  that extend axially into the surface  72 . One of the recesses  76  interacts with the single wedge  66 . The closure bucket  64  is configured to be radially  40  inserted and then through a series of axial  38  and radial  40  displacements the bucket  64  is positioned within the closure groove  48  to block circumferential movement  42  of other buckets within the groove  46  relative to the rotor disk or stage  12 . 
     The single wedge  66  includes wedge surfaces  78 ,  80 ,  82 ,  84 . The wedge surface  78  is disposed opposite wedge surface  80 , while wedge surface  82  (e.g., top surface) is disposed opposite wedge surface  84  (e.g., bottom surface). Wedges surfaces  78 ,  80  extend between wedges surfaces  82  and  84 . The screw  68  extends along a longitudinal axis  85  of the wedge  66  through the wedge  66 . The screw  68  is configured to radially  40  displace the wedge  66  via rotation  88  of the screw  68  about the longitudinal axis  85 . In addition, the screw  68  is only needed to avoid the wedge  66  losing operative position when the rotor disk or stage  12  is not rotating. The screw  68  is unloaded (i.e., no forces are exerted against the screw  68 ). Thus, during the rotation  88  of the screw  68 , the screw  58  is free of stress. In certain embodiments, the screw  68  may include a hexagonal socket  81  (or any other suitable tool interface) located at a top end  83  of the screw  68  to enable a tool (e.g., hex key) to rotate the screw  68  to move the wedge  66  up and/or down the screws  68 . The wedge  66  is configured to be inserted within the closure groove portion  48 , prior to the closure bucket  64 , with surface  84  contacting groove surface  56  and the wedge  66  located on a bottom portion  86  of the screw  68  (see  FIG. 8 ). Upon rotation  88  of the screw  68 , the wedge  66  is radially displaced to a top portion  89  of the screw  68  until wedge surface  78  contacts or interfaces with surface  72  (e.g., one of the recesses  68 ) of the closure bucket  64  and wedge surface  80  contacts or interfaces with groove surface  54  (e.g., recess  62 ) as depicted in  FIG. 2 . Both surfaces  54 ,  72  block further radial movement  40  of the wedge  66 . When radially  40  displaced to contact surface  54 ,  72 , the wedge  66  includes an upper portion  90  disposed between and contacting both the bucket  64  and the groove surface  54 . In this position at operating conditions, the upper portion  90  is subject to an axial force (due to the centrifugal moment of the closure bucket) against the groove surface  54 . In conjunction with centrifugal force exerted on the bucket  64  during circumferential  42  rotation of the rotor disk or stage  12  and bucket  64 , the axial force exerted on the wedge  66  secures the closure bucket  64  within the closure groove  48 . This avoids the use of a locking screw disposed through the bucket  64  into the rotor  12  and any associated stress concentration in the rotor  12 . In addition, the stage of buckets may be reassembled without damaging or remachining the rotor  12 . 
     In certain embodiments, the material of the wedge  66  may include a different thermal expansion coefficient than the closure bucket  64 . For example, the wedge  66  may include a higher thermal expansion coefficient than the closure bucket  64 . The higher thermal expansion coefficient of the wedge  66  may enable the wedge  66  (while also giving the wedge  66  a higher friction) to expand more during operation of the turbomachine system  10  to exert an even greater axial  38  force against both the bucket  64  and the closure groove  48 . In some embodiments, the wedge  66  and/or the closure bucket  64  may be frozen (e.g., in liquid nitrogen) prior to assembly of the closure bucket assembly  44  to temporarily shrink the wedge  66  and/or bucket  64  to enable a better interference fit once the wedge  66  and/or bucket  64  warm up and expand. 
       FIGS. 3 and 4  are partial rear (e.g., downstream) and front (e.g., upstream) perspective views of an embodiment of the self-locking closure bucket assembly  44  of  FIG. 2  disposed within the groove  46  of the rotor disk or stage  12  between adjacent buckets  92 . As depicted, the closure bucket  64  abuts against the adjacent buckets  92  blocking the circumferential movement  42  of the buckets  92  relative to the rotor disk or stage  12 . The adjacent buckets  92  include tangential entry dovetail buckets. Similar to the closure bucket  64 , the buckets  92  each include an upper portion  94  (e.g., rotary blade or airfoil  96 ) and lower portion  98  (e.g., mounting portion or male dovetail configuration  100 ). The lower portion  98  is configured to be inserted within or removed from the closure groove  48  of the groove  12  before tangential entry or removal into groove portion  102  of the groove  12 . Groove portion  102  extends circumferentially  42  along the groove  12  from one side  104  of closure groove portion  48  to the other side  106  of closure groove portion  48 . The groove portion  102  includes the groove surfaces  52 ,  54 . Closure groove portion  48  has a larger cross-sectional area than a cross-sectional area of groove portion  102  (see  FIGS. 6 and 7 ). The smaller cross-sectional area of groove portion  102  (as well as arrangement) blocks circumferential  42  movement of the closure bucket  64  from the closure groove portion  48  to the groove portion  102 . 
     The lower portion  98  of each bucket  92  includes surface  108  (e.g., upstream surface) and surface  110  (e.g., downstream surface). Similar to the closure bucket  64 , the lower portion  98  of each bucket  92  includes protrusions  112  (e.g., axial projections) that extend axially  38  outward from both surfaces  108 ,  110 . The number of protrusions  112  extending from each surface  108 ,  110  may vary from 1 to 5 or more. As depicted, surface  108  of each bucket  92  includes an upper axial projection  114  and a lower axial projection  116 , while surface  110  of each bucket  92  also includes an upper axial projection  118  and a lower axial projection  120 . The groove portion  102  includes a plurality of recesses  122  for receiving the protrusions  112  of the buckets  92 . For example, groove surface  52  of the groove portion  102  includes recesses  124 ,  126  and groove surface  54  of the groove portion  102  includes recesses  128 ,  130 . The recesses  124 ,  126 ,  128 ,  130  receive axial projections  114 ,  116 ,  118 ,  120 , respectively. Together, the groove surfaces  52 ,  54  form the axial platform  63  that interfaces with and secures each bucket  92  within the groove portion  102 . For example, the disposition of the lower axial projections  116 ,  120  within the recesses  116 ,  120  blocks the radial movement  40  of each bucket  92 . 
     As depicted, the lower portion  69  of the closure bucket  64  and the wedge  66  are disposed at an angle  132  relative to a centerline  134  of the groove  46  that extends circumferentially  42  about the rotor disk or stage  12  (see  FIG. 5 , a top view of the self-locking closure bucket assembly  44  in the closure groove  48 ) within the closure groove portion  48 . The lower portions  69 ,  98  of the respective buckets  64 ,  92  are disposed at the same angle  132  relative to the centerline  134 . The angle  132  may range from approximately 0 to 60 degrees, 0 to 30 degrees, 30 to 60 degrees, 15 to 45 degrees, and all subranges therebetween. For example, the angle  132  may be approximately 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, or 60 degrees, or any other angle. 
       FIG. 6  is cross-sectional side view of an embodiment of the groove portion  102  for the buckets  92  taken along line  6 - 6  of  FIG. 5 , while  FIG. 7  is a cross-sectional side view of an embodiment of the closure groove portion  48  for the self-locking closure bucket assembly  44  taken along line  7 - 7  of  FIG. 5 . The closure groove portion  48  and the groove portion  102  are as described above in  FIGS. 2-5 . In addition, a depth or height  136  of each groove portion  48 ,  102  are the same from a top portion  138  of the groove portions  48 ,  102  to the bottom portion  58 . As shown in  FIG. 6 , the groove portion  102  includes a width  140  between the surfaces  52 ,  54  at the recesses  124 ,  128  and a width  142  between the surfaces  52 ,  54  at the recesses  126 ,  130 . The width  140  is greater than the width  142 . As shown in  FIG. 7 , the closure groove portion  48  includes the same width  140  between the surfaces  52 ,  54  above the recesses  60 ,  62  adjacent the top portion  138 . In certain embodiments, the width  140  may vary. The closure groove portion  48  includes a width  144  between the surfaces  52 ,  54  beginning with the upper recess  60  of surface  52  and ending with the bottom portion  58 . The width  142  of the groove portion  102  is depicted within the closure groove portion  48 . As illustrated, the width  144  of the closure groove portion  48  from the upper recess  60  to the bottom portion  58  is greater than the width  142  of the groove portion  102 . In addition, as mentioned above, the closure groove portion  48  has a larger cross-sectional area  146  than a cross-sectional area  148  of groove portion  102 . The smaller cross-sectional area  148  of groove portion  102  (as well as arrangement) blocks circumferential  42  movement of the closure bucket  64  from the closure groove portion  48  to the groove portion  102 . In addition, the larger cross-sectional area  146  of the closure groove portion  48  enables the tangential entry and removal of the buckets  92  from the groove portion  102  to the closure groove portion  48 . 
       FIGS. 8A-F  are a series of partial side views illustrating the assembly of the self-locking closure bucket assembly  44  of  FIG. 2  within the closure groove portion  48  of the rotor disk or stage  12 . The closure bucket assembly  44  and the closure groove portion  48  are as described above. As depicted in  FIG. 8A , the wedge  66  is inserted within the closure groove portion  48 , prior to the closure bucket  64 , with surface  84  contacting groove surface  56  within recess  62  and the wedge  66  located on a bottom portion  86  of the screw  68 . In  FIG. 8B , the closure bucket  64  is inserted radially  40  into the closure groove portion  48  until the surface  72  (e.g., upper recess  76 ) contacts or abuts the rotor disk or stage  14 . In  FIG. 8C , the closure bucket  64  is axially  38  displaced or shifted until surface  71  (e.g., middle protrusion  74 ) contacts or abuts the groove surface  52 . In  FIG. 8D , the closure bucket  64  is radially  40  displaced or shifted until the protrusions  74  (e.g., middle and bottom protrusions  74 ) are aligned with the respective recesses  60  within the groove surface  52 . In  FIG. 8E , the closure bucket  64  is axially  38  displaced or shifted until the protrusions  74  (e.g., middle and bottom protrusions  74 ) contact the groove surface  52  and are disposed within the respective recesses  60 . In  FIG. 8F , the screw  68  is rotated  88  (e.g., via a tool such as hex key) about the longitudinal axis  85  to radially  40  displace the top portion  89  of the wedge  66  until the wedge surface  78  contacts or interfaces with surface  72  (e.g., one of the recesses  68 ) of the closure bucket  64  and wedge surface  80  contacts or interfaces with groove surface  54  (e.g., recess  62 ). Both surfaces  54 ,  72  block further radial movement  40  of the wedge  66 . In this position at operating conditions, the upper portion  90  of the wedge  66  is subject to an axial force (due to the centrifugal moment of the closure bucket) against the groove surface  54 . In conjunction with centrifugal force exerted on the bucket  64  during circumferential  42  rotation of the rotor disk or stage  14  and bucket  64 , the axial force exerted on the wedge  66  secures the closure bucket  64  within the closure groove  48 . This avoids the use of a locking screw disposed through the bucket  64  into the rotor  12  and any associated stress concentration in the rotor  12 . In addition, the stage of buckets may be reassembled without damaging or remachining the rotor  12 . The disassembly of the closure bucket assembly  44  may occur via performing some or all of the steps above in reverse order. 
     Also, as mentioned above, the wedge  66  may include a higher thermal expansion coefficient than the closure bucket  64 . Further, in some embodiments, the wedge  66  and/or the closure bucket  64  may be frozen (e.g., in liquid nitrogen) prior to assembly of the closure bucket assembly  44  to temporarily shrink the wedge  66  and/or bucket  64  to enable a better interference fit once the wedge  66  and/or bucket  64  warm up and expand. 
     Technical effects of the disclosed embodiments include providing a self-locking closure bucket assembly  44  to block circumferential movement of buckets  92  within the same groove  42  (e.g., row or stage) of the rotor disk or stage  14 . In particular, the self-locking closure bucket assembly  44  includes the closure bucket  64 , the single wedge  66 , and the screw  68  (e.g., unloaded fixing screw) configured to be disposed within the same closure groove portion  48 . Upon radial  40  displacement of the wedge  66  (e.g., via the screw  68 ) between surface  72  of the closure bucket  64  and the groove surface  54 , the wedge  66  axially  38  exerts force against both the bucket  64  (e.g., surface  72 ) and the rotor surface  54 . In this position at operating conditions, the upper portion  90  of the wedge  66  is subject to an axial force (due to the centrifugal moment of the closure bucket) against the groove surface  54 . This avoids the use of a locking screw disposed through the bucket  64  into the rotor  12  and any associated stress concentrations in the rotor  12 . In addition, the stage of buckets may be reassembled without damaging or remachining the rotor  12 . 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.