Patent Publication Number: US-7901187-B2

Title: Turbine rotor blade groove entry slot lock structure

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/816,008, filed Jun. 23, 2006, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a blade lock for use in blading an axial flow turbine and, more particularly, to a blade lock including a removable insert piece cooperating with a blade through a lock screw. 
     BACKGROUND OF THE INVENTION 
     During a known blading operation for axial flow turbines, blades are successively mounted to a circumferential groove formed in a rotor by inserting the blade roots radially into an entry slot and then circumferentially moving the blades through the circumferential groove. The circumferential groove is formed to define an undercut blade groove to receive an inverted T-shaped, i.e., a hammerhead or double hammerhead, portion of the blade root in a positive-locking manner. The entry slot comprises an area of the circumferential groove that is formed without the undercut, such that the last blade inserted and remaining in the location of the entry slot must be retained by a locking means. 
     One known means for retaining the last blade, or closing blade, comprises drilling and tapping a locking hole at the junction between the rotor surface defining the groove and the closing blade, such that a half-hole is defined in each of the rotor and the closing blade to receive a lock screw. Threaded engagement of the screw with the threads of the half-holes prevents radial movement of the closing blade out of the circumferential groove. In the event that repairs must be performed, such as replacement of the blades, the locking hole may be re-tapped with the next largest lock screw size, or an even larger screw size, as part of the operation of locking the replacement blades in place. Eventually, with multiple blade replacements, the threads of the locking hole will reach a maximum allowable size and require more extensive repair. 
     Accordingly, there continues to be a need for a blade lock structure that operates to securely lock the blades in place and that provides efficiencies during multiple blade replacement procedures. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a blade lock structure is provided for blading an axial flow turbine having a rotor and blades, wherein the blades are inserted into an undercut blade groove of the rotor providing a positive lock between the blades and the rotor. The lock structure comprises an entry slot located in the blade groove for receiving the blades for insertion to the undercut blade groove, and an insert space axially extending from the entry slot, the insert space defining a radially extending longitudinal axis. An insert piece is located in positive-locking relationship with the insert space, and a lock screw threadably engages between the insert piece and one of the blades. 
     In accordance with another aspect of the invention, a blade lock structure is provided comprising a rotor defining a circumferential groove, the rotor also defining an insert space extending from the circumferential groove. An insert piece is insertable into the insert space, and a lock screw is threadably engagable between the insert piece and a blade located within the circumferential groove. 
     In accordance with a further aspect of the invention, a process is provided for repairing a blade lock structure for blading an axial flow turbine having a rotor defining a circumferential groove and blades, where the rotor is also provided with an insert space extending from the circumferential groove, the insert space defining a radially extending longitudinal axis. The process comprises the steps of inserting an insert piece into the insert space, inserting a blade in the circumferential groove adjacent the insert piece, drilling and tapping a hole between the insert piece and the blade, and threadably engaging a lock screw in the hole. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed that the present invention will be better understood from the following description in conjunction with the accompanying Drawing Figures, in which like reference numerals identify like elements, and wherein: 
         FIG. 1  is cross-sectional view of a detail of a turbine rotor taken at a blade entry slot of a blade groove, along a line depicted by line  1 - 1  in  FIG. 4 ; 
         FIG. 2  is a perspective view of a portion of a turbine rotor including an insert piece; 
         FIG. 3  is cross-sectional view of a blade entry slot and inlet and exit insert spaces with an insert piece located in the exit insert space; 
         FIG. 4  is plan view of the entry slot of the blade groove with the exit insert piece located in position and showing the inlet insert piece prior to insertion to the inlet insert space; 
         FIG. 5  is an elevation view of an insert piece taken from the blade engaging side of the insert piece; and 
         FIG. 6  is an elevation view of the insert piece, taken at 90 degrees to the view of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description of the preferred embodiment, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, a specific preferred embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention. 
     Referring to  FIGS. 1-4 , the present invention relates to providing a blade lock structure  10  for use in a blading operation for installing blades, as depicted for example by a plurality of row one blades  12 , into an undercut circumferential groove  14  defined in a rotor disc  16 . The undercut of the groove  14  is formed by one or more opposing pairs of shoulders, depicted here as shoulder pairs  18  and  20  ( FIG. 3 ), which is shown in this exemplary embodiment configured for receiving a blade root of double hammerhead configuration (not shown). The groove  14  receives a set of blades comprising a plurality of similar blades  12  having the double hammerhead configuration conforming to the undercut shape of the groove  14 . 
     As seen in  FIG. 1 , a last inserted blade or closing blade  12 , particularly identified as  12   a , is provided with a blade root  22  having a different configuration than the other blades  12  of the blade set. In particular, the root  22  of blade  12   a  comprises a generally planar axially facing inlet face  24  and a generally planar axially facing exit face  26 . In addition, a pair of parallel generally planar axially extending sides (not shown) connect the inlet and exit faces  24 ,  26  to define a parallelogram configuration for the blade root  22 . 
     As seen in  FIG. 4 , the shoulder pairs  18 ,  20  are discontinued at least at one point comprising an entry slot  28  located in the groove  14 . The entry slot  28  provides an area where the blades  12  with the double hammerhead root configuration may be initially inserted to the groove  14 , and subsequently slide circumferentially within the undercut area defined by the shoulder pairs  18 ,  20  to provide a positive lock between the rotor disc  16  and the blades  12 . The root  22  of the closing blade  12   a  is also sized to fit within and substantially fill the space provided by the entry slot  28 . 
     An inlet insert space  30   a  axially extends from an inlet side  32  of the entry slot  28 , and an exit insert space  30   b  axially extends from an exit side  34  of the entry slot  28  at a side of the groove  14  opposite the inlet insert space  30   a . The insert spaces  30   a ,  30   b  each define a respective radially extending longitudinal axis  31   a ,  31   b , and have a substantially U-shaped configuration. The insert spaces  30   a ,  30   b  may be circumferentially displaced from each other to correspond to the parallelogram configuration of the root  22 , where it may be understood that the inlet face  24  of the blade root  22  is circumferentially offset from the exit face  26 , and the axially extending sides of the blade root  22  extend at an angle to the longitudinal axis of the rotor disc  16 , as depicted diagrammatically by the parallel lines  36 ,  38 . 
     As seen in  FIG. 3 , the inlet spaces  30   a ,  30   b  are defined by axially and radially extending side walls  40 . The side walls  40  are machined or otherwise formed with generally V-shaped thread features  42  to provide the side walls  40  with a serrated surface. The inlet and exit insert spaces  30   a ,  30   b  are configured to receive inlet and exit insert pieces  44   a  and  44   b , respectively. Specifically, the inlet and exit insert pieces  44   a ,  44   b  each comprise unitary members including an outer wall  46  having opposing axially extending sides, and a blade engaging face  48  extending between the axially extending sides, as depicted by insert piece  44   a  in  FIGS. 5 and 6 . The outer wall  46  has a height, in the radial direction, that is equal to or slightly less than the radial depth of the insert spaces  30   a ,  30   b . In addition, the outer wall  46  is formed with generally V-shaped thread features  50  extending circumferentially about the outer wall  46 , extending along the sides of the insert pieces  44   a ,  44   b  and around a portion of the outer wall  46  extending between the opposing sides of the outer wall  46 , to provide the outer wall  46  with a serrated surface matching the serrated surface of the insert space side walls  40 . 
     The insert pieces  44   a ,  44   b  are configured as U-shaped members and have circumferentially and radially extending dimensions that are substantially the same as the corresponding dimensions of the insert spaces  30   a ,  30   b . Each insert piece  44   a ,  44   b  may be inserted into the corresponding insert space  30   a ,  30   b  by first inserting the insert piece  44   a ,  44   b  radially into the entry slot  28 , as illustrated by insert piece  44   a  in  FIG. 4 , and then moving the insert piece  44   a ,  44   b  axially, i.e., transverse to the longitudinal axes  31   a ,  31   b , into its insert space  30   a ,  30   b . Axial movement of the insert pieces  44   a ,  44   b  into the insert spaces  30   a ,  30   b  places the V-shaped features  42  and  50  into engagement with one another to lock the insert pieces  44   a ,  44   b  against radial movement out of the spaces  30   a ,  30   b . When the insert pieces  44   a ,  44   b  have been located in position within the insert spaces  30   a ,  30   b , the blade engaging face  48  of each of the insert pieces  44   a ,  44   b  defines a circumferentially extending surface comprising a generally planar wall within the circumferential groove  14 . 
     It should be noted that at the openings to the inlet and exit insert spaces  30   a ,  30   b , respective angled inlet cutout portions  52  and exit cutout portions  54  may be provided in the shoulder pairs  18 ,  20  to facilitate clearance of the insert pieces  44   a ,  44   b  as they are inserted to the entry slot  28 , as seen in  FIG. 4 . 
     Referring to  FIGS. 3 and 4 , each insert space  30   a ,  30   b  may be provided with one or more half-holes  56 . After the insert piece  44   a ,  44   b  is positioned within the insert space  30   a ,  30   b , the insert piece  44   a ,  44   b  may be drilled at the location of each half-hole  56  to form a matching half-hole in the insert piece  44   a ,  44   b  which may then be tapped to define an insert locking passage extending generally parallel to the longitudinal axes  31   a ,  31   b  for receiving a locking member comprising a stud screw  58 . The stud screw  58  prevents the insert piece  44   a ,  44   b  from moving axially out of the insert space  30   a ,  30   b . The stud screw  58  also further locks the insert piece  44   a ,  44   b  against radial movement out of the space  30   a ,  30   b.    
     In an installation procedure, after both of the insert pieces  44   a ,  44   b  are located in their respective insert spaces  30   a ,  30   b , the closing blade  12   a  is inserted into the entry slot  28  where the inlet and exit faces  24 ,  26  of the blade root  22  are positioned in engagement with the blade engaging faces  48  of the insert pieces  44   a ,  44   b . A hole  60  is then drilled and tapped at the junctions between the insert pieces  44   a ,  44   b  and the blade  12   a , and a lock screw  62  is threaded into each of the holes  60 , where the threads of the lock screws  62  engage adjacent half-holes in the insert pieces  44   a ,  44   b  and the blade  12   a  to lock the blade  12   a  against radial movement out of the circumferential groove  14 . 
     Further, the V-shaped features  42 ,  50  are designed such that they are able to withstand the centrifugal force loads applied from the insert piece  44   a ,  44   b  as well as loads applied by the closing blade  12   a  and lock screw  62 . For example, in a particular design of the features  42 ,  50 , as illustrated on the features  50  in  FIG. 5 , the features  50  may be formed with a thread height, H, of 2.15 mm, a pitch, P, of 4.23 mm and a basic angle of thread, φ, of 60°. 
     It should be understood that within the scope of the invention, the insert pieces  44   a ,  44   b  may be retained in position by either the stud screws  58  or the features  42 ,  50 , or may be retained by both structures, as is illustrated herein. Alternatively, the stud screws  58  could be replaced by unthreaded dowel members to retain the insert pieces  44   a ,  44   b  against axial movement transverse to the longitudinal axes  31   a ,  31   b , and retention of the insert pieces  44   a ,  44   b  against radial movement may be effected through the cooperating features  42 ,  50 . 
     The present invention may be provided on new turbine rotors or may be provided as a repair for existing turbine rotors in which the original blade lock structure comprises a lock screw provided as a locking element directly between a blade surface and a cooperating surface integral with the turbine rotor. For example, in a repair operation, the rotor disc of a turbine may be machined to form the described inlet and exit insert spaces  30   a ,  30   b  for receiving the insert pieces  44   a ,  44   b  in the installation procedure described above. 
     Alternatively, the present invention may be applied to replace lock structures incorporating multiple locking pieces for holding the last inserted blade in place, such as lock structures including wedged filler pieces provided adjacent the blade root. An example of such a structure is disclosed in U.S. Pat. No. 6,431,836, which patent is incorporated herein by reference. In accordance with the present invention, the prior art structure comprising a mounting region for receiving multiple components, such as a filler piece and a wedge, may be re-machined to conform to the presently described insert spaces  30   a ,  30   b  for mounting a closing blade  12   a  using the insert pieces  44   a ,  44   b.    
     It may be noted that in a typical installation procedure performed as part of a re-blading process for the rotor disc  16 , the closing blade  12   a  is provided without a hole or half-hole for cooperating with an adjacent insert piece  44   a ,  44   b , and the hole  60  is drilled and tapped as a new hole for receiving the lock screw  62 . In subsequent re-blading operations, the hole  60  may be redrilled and tapped to larger screw sizes. Subsequently, for example after several re-blading operations, when the hole  60  has been drilled and tapped to a maximum allowable size, the insert pieces  44   a ,  44   b  may be replaced to again drill and tap a new hole  60 , returning to a smaller screw size. Accordingly, the present invention permits the life of the rotor disc  16  to be extended, facilitating repair of the portion of the disc  16  forming the blade lock structure  10 . 
     While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.