Patent Publication Number: US-2007122274-A1

Title: Tip shroud attachment for stator vane

Description:
BACKGROUND OF THE INVENTION  
      This invention relates to turbine stator vanes and, particularly, to a mechanical attachment between a tip shroud segment and a group of stator vanes.  
      Tip shrouds on stator vanes provide a deterministic flow path for the particular stator stage, and also serve to increase the stiffness of the vanes. It is customary to assemble arcuate shroud segments over a number of vanes (e.g., 5, 7 or more), with the segments collectively forming a 360° shroud. The tip shroud segments are typically welded to the tips of the vanes.  
     BRIEF SUMMARY OF THE INVENTION  
      In accordance with an exemplary embodiment of this invention, each vane within a group of vanes to be covered by the shroud segment has a projecting tenon at its tip. The shroud segment is formed with corresponding holes that align with the tenons of the respective vanes. A bushing is seated in each of the shroud segment holes, and each bushing is formed with a center opening for receiving a respective one of the tenons. The center opening is counterbored, leaving an internal shoulder or seat for the tenon. The bushing is also formed with an external shoulder that allows the bushing to engage a shoulder surface of the corresponding shroud segment hole. This arrangement permits the bushing to be inserted into the shroud segment hole in a direction opposite the insertion direction of the vane tenons, with the external shoulder providing a stop limit to the bushing insertion. The vane tenons are subsequently welded to the internal bushing seat. Note that the bushings are not otherwise attached directly to the shroud segment but are nevertheless held in place radially by the tenon welds in one direction and the external shoulder engagement in the opposite direction.  
      Accordingly in one aspect, the present invention relates to a stator vane segment and tip shroud segment assembly comprising: a plurality of vanes, each having an airfoil portion and a radially inner tip, each tip formed with at least one radially inwardly projecting tenon; and a tip shroud segment extending over the radially inner tips of the vanes, said tip shroud segment formed with a plurality of holes, each hole fitted with a bushing that receives a respective one of the tenons.  
      In another aspect, the invention relates to a stator vane segment and tip shroud segment assembly comprising: a plurality of vanes, each having an airfoil portion and a radially inner tip, each tip formed with at least one radially inwardly projecting tenon; a tip shroud segment extending over the radially inner tips of the vanes; the tip shroud segment formed with a plurality of holes, each hole fitted with a bushing that receives a respective one of the tenons; wherein the bushing is formed with an internal shoulder on which the respective tenon is seated; and wherein each bushing is formed with an external shoulder engaged by the tip shroud segment.  
      In still another aspect, the invention relates to a stator vane segment and tip shroud segment assembly comprising: a plurality of vanes, each having an airfoil portion and a radially inner tip, each tip formed with at least one radially inwardly projecting tenon; a tip shroud segment extending over the radially inner tips of the vanes, the tip shroud segment formed with a plurality of holes, each hole fitted with a bushing that receives a respective one of the tenons; wherein each bushing is formed with an external shoulder engaged by the tip shroud; and wherein each bushing is formed with a center hole counterbored to provide an internal shoulder that provides a seat for the respective tenon and wherein the tenon is welded to the bushing at the internal shoulder.  
      The invention will now be described in connection with the drawings identified below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       FIG. 1  discloses a perspective view of a shrouded stator vane segment in accordance with an exemplary embodiment of the invention;  
       FIG. 2  is a cross-section taken along the line  2 - 2  of  FIG. 1 ;  
       FIG. 3  is a partial cross-section taken along the line  3 - 3  in  FIG. 1 ;  
       FIG. 4  is a perspective view of another exemplary embodiment of the invention; and  
       FIG. 5  is an enlarged detail similar to  FIG. 3  but taken from the assembly shown in  FIG. 4 ; and  
       FIG. 6  is a perspective view of still another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
      With reference initially to  FIGS. 1-3 , a stator vane and tip shroud segment assembly  10  includes a plurality of stator vanes  12 , each of which includes a dove tail mounting portion  14 , an airfoil portion  16 , and a radially projecting tenon  18  ( FIG. 2 ) that projects from the radially inner tip  20  of the vane. Typically, the tenon  18  is round or oval in cross section and is formed approximately intermediate the forward and trailing edges  22 ,  24  of the airfoil portion.  
      An arcuate tip shroud segment  26  spans and is attached to a selected member of stator vanes  12 , such that the collective stator vane and tip shroud segments will form a 360° stator assembly.  
      The arcuate tip shroud segment  26  is formed with a plurality of holes  28  that are appropriately located to align with the stator tenons  18 . As best seen in  FIG. 3 , the holes  28  are counterbored in a radially outward direction to create a radially inwardly facing shoulder  30  that is adapted to be engaged by a corresponding radially outwardly facing shoulder  32  on a bushing  34 . The bushing  34  is correspondingly shaped to be received in the segment hole  28  (i.e., round or oval), and, as already indicated above, is formed with an external shoulder  32  that engages the corresponding shoulder  30  on the arcuate tip shroud segment  26 . The bushing  34  is also formed with a center opening  36  that is counterbored to provide an annular (or oval)-shaped, radially outwardly facing seat  38  that is adapted to be engaged by the marginal edge of a respective tenon end face  40 .  
      With the bushings  34  inserted in the segment holes  28  in direction A ( FIG. 3 ), the airfoil tenons  18  are inserted into the bushing in direction B until the tenons are fully engaged with the seats  38 . Subsequently, each airfoil tenon  18  is fixed to the bushing at the seat  38  by weld  42 . It is significant that the bushing is not otherwise fixed to the tip shroud segment  26 . As a result, some degree of movement between the airfoil portion  16  and the tip shroud segment  26  is permitted via bushing  34 , thereby reducing stresses during operation. Note that the bushing is recessed relative to the radially inner face of the shroud segment so as not to protrude into the gas flowpath.  
      In the exemplary embodiment, the bushing may be constructed of 316 Stainless Steel while the airfoil portion  16  (including the tenons  18 ) and the tip shroud segment  26  may be constructed of a harder 403 Stainless Steel, but the material compositions may vary.  
       FIGS. 4 and 5  illustrate another embodiment of a vane and tip shroud segment assembly  110  where the shapes of the tip shroud segment, bushing, and airfoil tenon as shown vary somewhat from the embodiment shown in  FIGS. 1-3 . Otherwise, the components and the interaction between the tip shroud segment, airfoil portion and bushing remains as previously described. For convenience, similar reference numerals are used, but with the prefix “1” added, to designate corresponding features, but no further description is required.  
       FIG. 6  illustrates another vane segment configuration that incorporates the subject invention. In this figure, similar reference numerals are used to designate similar components, where applicable, but with the prefix “2” added. The vane and tip shroud segment assembly  210  incorporates bushings  234  as described in connection with  FIGS. 1-3  and this aspect of the assembly need not be repeated here. In this embodiment, the portion (or dovetail mounting portions)  214  of the vanes are ganged together via a strap  244  and a plurality of bolts  246 .  
      In an alternative arrangement that does not require welding, the tenons could be formed with subtenons that project beyond the tenon end face and the extended subtenon may then be cold formed about the bushing end face. In yet another variation, the tenon could be formed with a tapped hole with a bolt passing through the center opening of the bushing and into a threaded hole in the tenon.  
      While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.