Abstract:
Rhombic shrouded tangential entry buckets are circumferentially applied to the rotor wheel rim of a turbine. A fixture is releasably secured to a lug on each bucket and includes a chamfered surface for engaging a complementary surface on the lug. Upon applying a tangential assembly force, the shroud and airfoil of adjacent buckets are pre-twisted in a rotational direction about a generally radial axis enabling dovetail faces to contact one another notwithstanding interference fit shroud contacting surfaces. The removal of the fixture from the lug enables the bias of the airfoil to rotate the shroud in an opposite direction into final assembly with the shroud edges of adjacent buckets in contact with one another and the dovetail faces thereof in contact with one another.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to methods and systems for assembling buckets having a shroud and a tangential entry dovetail onto the rim of a turbine wheel, particularly to assure complete accurate mechanical coupling between adjacent shrouds in final assembly. 
   Shrouded turbine buckets, e.g. for use in steam turbines, require the shroud edges to be in mechanical contact with one another, eliminating any gap between adjacent shrouds. The shrouds typically support application of tip seals to provide improved turbine thermal efficiency. High levels of mechanical reliability are also required under service conditions. A shroud having a predominantly rhombic (i.e., full rhombic or modified rhombi) configuration satisfies these design characteristics. Shrouds having a rhombic configuration, however, are not per se new or new in conjunction with buckets employing a tangential entry dovetail. Buckets having rhombic-shaped shrouds have been previously used in low, intermediate and high pressure turbine applications. Integrally shrouded buckets, however, become increasingly difficult to assemble as airfoil stiffness is increased, as airfoil aspect ratio (i.e., radial height/axial width) is reduced, or as higher pre-twist stresses are required. Problems associated with assembly of buckets having rhombic configured shrouds and tangential entry dovetails include; generating adequately high tangential forces needed to eliminate gaps between (i.e., to pack together) adjacent shrouds and dovetail faces, and to produce an adequate pre-twist of the bucket airfoils. The level of pre-twist must be sufficient to assure that the adjacent shrouds remain in contact, i.e. are mechanically coupled, during all normal phases of turbine operation. Tangential assembly forces required to adequately pack buckets together on a turbine wheel can become very high for buckets of the size employed in large steam turbine applications. Also, as the buckets are packed together, the dovetails undergo rotation, which in turn reduces the level of pre-twist applied to the bucket airfoils. Dovetail rotation must be limited to assure an adequate assembly. Further, the buckets in their packed configuration must be constrained from backing away from each other as additional buckets are installed on the wheel. The tendency to back away is associated with the forces developed at the shroud contact surfaces, and the orientation of these surfaces relative to the tangential direction. Accordingly, there is a need for an assembly method and system which will overcome the aforementioned problems associated with assembly of shrouded buckets on a turbine wheel; and which will in turn permit the buckets to meet all efficiency and reliability objectives. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In a preferred embodiment of the present invention, a method of assembling a plurality of buckets on a rotor wheel wherein each bucket includes an airfoil terminating at opposite ends in a shroud and a dovetail, respectively, comprising the step of pre-twisting the shroud and airfoil of each bucket in a rotational direction about a generally radial axis in response to applying a tangential assembly force to interference fit shroud contacting surfaces thereby imparting a rotational bias to the airfoil enabling subsequent rotation of the shroud and airfoil into final assembly with the shroud edges of adjacent buckets in contact with one another and dovetail faces of adjacent buckets in contact with one another. 
   In a further preferred embodiment of the present invention, a method of assembling a plurality of buckets on a rotor wheel wherein each bucket includes an airfoil terminating at opposite ends in a shroud and a dovetail, respectively, comprising the steps of: providing a lug on the shroud of each bucket; releasably securing a fixture on each lug carried by the shroud of each bucket, the fixture and lug of respective adjacent buckets having generally complementary tapered surfaces at acute angles relative to the tangential direction; and wedging the fixture carried by the shroud of each bucket being installed against the tapered surface of the lug carried by the shroud of the adjacent bucket previously installed on the rotor rim to pre-twist the shroud and airfoil of the bucket. 
   In a further preferred embodiment of the present invention, a turbine wheel and bucket assembly comprising a plurality of buckets each including an airfoil, a shroud adjacent the tip of the airfoil and a dovetail adjacent a root of the airfoil; a lug carried by each shroud; a fixture releasably secured to each lug and having a projection extending in a tangential direction for overlying a portion of a lug of a previously assembled bucket onto the wheel, the adjacent shrouds having interference fit contacting surfaces; at least one of the lug and the fixture projection having a tapered surface in contact with a surface of another of the lug and fixture projection to pre-twist the shroud and airfoil being installed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a fragmentary perspective view illustrating buckets with rhombic-shaped shrouds being installed on the rim of a turbine rotor wheel in accordance with a preferred embodiment of the present invention; 
       FIG. 2  is a plan view of a pair of buckets as viewed looking inwardly towards the radially outer faces of the shrouds in the course of assembly of the buckets onto the wheel; 
       FIG. 3  is an enlarged fragmentary detail of a portion of  FIG. 2 ; 
       FIG. 4  is an enlarged view of a fixture for securement to the lug on the shroud; 
       FIG. 5  is a view similar to  FIG. 2  illustrating a direction of rotation or twist of the shroud and airfoils in the course of the assembly of the buckets on the rotor wheel; 
       FIG. 6  is a view similar to  FIG. 5  with the fixture removed illustrating a counter-rotation of the shroud and airfoil; 
       FIG. 7  is a view similar to  FIG. 6  with the buckets in final assembly; and 
       FIG. 8  is a fragmentary side elevational view of the shroud and airfoil of the bucket upon final securement. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings, particularly to  FIG. 1 , there is illustrated a turbine rotor wheel  10  including a wheel rim  12  having a rib and groove configuration, i.e. a pine tree configuration along opposite axial sides thereof about the circumference of the wheel  10 . Also illustrated in  FIG. 1  are a plurality of buckets generally designated  14 . Each bucket  14  includes an airfoil  16  having a dovetail  18  projecting from the root of the airfoil and a shroud  20  adjacent the tip of the airfoil. It will be appreciated that the dovetail  18  has a mating corresponding rib and groove arrangement, i.e., a pine tree configuration complementary to the pine tree configuration of the wheel dovetail  12 . Thus, the buckets  14  constitute tangential entry buckets whereby the buckets are disposed in a radial slot, not shown, on the wheel  10  and slidably disposed about the turbine wheel with contact faces of the dovetail and contact edges of the shrouds in respective engagement with corresponding parts of adjacent buckets. Also illustrated in  FIG. 1  is an anti-rotation key  22  which extends about the outer periphery of the dovetail  12  of the rotor wheel  10  and which engages in a corresponding slot at the base of the dovetail  18  to minimize or preclude rotation of the dovetail and hence bucket  14  during assembly and operation. A similar anti-rotation key is described and illustrated in U.S. Pat. No. 5,509,784 of common assignee herewith. 
   In  FIG. 1 , each of the buckets  14  is illustrated as including a lug  24  projecting radially outwardly from the forward edge of shroud  20 . The lug  24  is preferably formed integrally with the shroud  20  and is in part removed from each bucket and shroud after final assembly. Also illustrated in  FIG. 1  are fixtures  26  mounted on each of the respective lugs  24  and which fixtures  26  project axially forwardly. Each fixture  26  may be bolted to an associated lug  24  by bolts  28 . 
   As best illustrated in  FIG. 2 , the shrouds  20  have a rhombic configuration. It will be appreciated that in final assembly, the angled margins or tangential edges  30  of the shrouds abut one another as illustrated in  FIG. 7 . However, those adjoining angled edges  30  which typically extend about 40 to 60° relative to the tangential axis or direction have an extant interference condition at their mating shroud contact surfaces  32  when the buckets are assembled to the turbine wheel and the adjacent dovetail faces  34  contact one another. That is, there is an excess amount of material on the contact edges  30  of the shrouds so that the shroud edges would theoretically overlap one another when the dovetail faces  34  of adjacent buckets  14  are in contact one with the other. Because of this shroud interference condition at the shroud contacting surfaces  32 , the adjacent dovetail faces  34  cannot be brought into full flush contact with one another until a rotation or twisting of the shroud  20  occurs. By rotating the shroud about a bucket radial axis, a change in the shroud cover tangential pitch occurs which permits the bucket assembly to accommodate the shroud interference condition. That is, the excess amount of material forming the edges  30  of the adjacent shrouds is taken up by rotation of the shrouds about generally radial axes of the buckets to produce a twisting of the shrouds as well as an elastic pre-twist of the bucket airfoils. Because of the angle of the shroud edges  30 , a twisting of the shroud reduces the tangential width of the shroud as the shroud rotates about the generally radial axis until all of the interference is taken up. By twisting the shroud, the airfoil acts as a torsional spring, which serves to maintain the contact load between adjacent shroud contact surfaces  32  at all normal operating conditions of the buckets. 
   To pre-twist the airfoil during assembly, a substantial tangential assembly force is required to generate the required twisting moment, i.e., torque on the shroud which occurs through the bearing forces on the shroud contact surfaces  32 . The tangential assembly force must also overcome the frictional forces associated with sliding one contact surface  32  relative to the adjacent contact surface  32 . 
   In the above referenced U.S. Pat. No. 5,590,784, there is provided shroud contact surfaces having a shallow angle, i.e., approximately 15° relative to the tangential axis creates a wedging effect as the buckets are tangentially assembled. Large bearing forces are thus generated on the shroud contact surfaces for the steep angle design illustrated in that patent and are oriented principally in the axial direction creating a substantial twisting moment on the shroud. The component of the assembly force in the tangential direction, however, is relatively small compared to the axial component of force which minimizes the required tangential assembly force necessary to overcome the tangential component of the shroud force and frictional forces. 
   A rhombic configured shroud, however, having a substantially larger tangential axis, i.e., on the order of about 40 to 60°, reduces the wedging action between the shroud contact surfaces as the buckets are driven tangentially causing the required tangential assembly force to be substantially greater than for the steep angle design of the prior patent. This places limitations on the size of bucket that can be adequately assembled. 
   In accordance with a preferred embodiment of the present invention, however, the very large interference contact surface angle is accommodated by application of the fixture  26  to the lug  24 . Referring to  FIGS. 2 and 3 , each fixture  26  includes a portion  40  which projects in a tangential direction from the lug  24  to which the fixture  26  is attached. The projection  40  includes, on each axial downstream face, a leading chamfer  42 , having an angle Θ of about 10° to the tangential axis and terminating in a flat  44  oriented at about 0° to the tangential axis, as illustrated in  FIG. 3 . The flat  44  leads to or terminates in a step  46  in the downstream face of the fixture  26 . The size of step  46  is adjusted based on the interference level at the shroud contact surfaces  32 . Additionally, as seen in  FIG. 3 , the axial admission face of the lugs  24  also include a chamfer  48  complementary to the chamfer  42 , the chamfer  48  being located on an adjacent lug to the lug having a registering taper  42 . 
   To assemble the buckets on the rim of the rotor wheel, the fixtures  26  are secured to the lugs  24 , e.g. using the bolts  28 . Each successive bucket to be assembled is slid around the wheel rim to a location where the chamfer  42  contacts the mating chamfer  48  on the lug  24  of the preceding bucket. Once contact is made, a tangential assembly force is applied to the bucket being installed to drive the bucket toward the preceding bucket. The fixture  26  thus initially slides along the wedge angle created by the mating chamfers  42  and  48  causing a substantial twisting movement and corresponding rotation to occur at the shroud as illustrated in  FIG. 5  as well as a twisting action of the airfoil  16 . It will be appreciated that both of the mating buckets will twist with the application of a tangential assembly force. When the axial step between the faces of the two buckets equals the step size in the fixture, the flat surface  44  of the fixture contacts the flat axially forward face of the shroud lug. The magnitude of rotation at the shroud is governed by the fixture step size and is set to slightly exceed the level of rotation that would naturally be created by the interference condition at the shroud contact surfaces  32 . Thus, as the shroud twists and because of the angle of the shroud edges  30 , the tangential width of the cover as the shroud is rotated is taken up to the extent that the faces of the dovetail surfaces of the buckets contact one another. The step size is set, for example, so that approximately a 0.002 to 0.004 inch gap exists between the edges  30 . This enables the adjacent buckets to slide together to enable the dovetail faces  34  to contact one another with only the involved frictional forces resisting motion of the buckets. Because of the small angle between the fixture  26  and lug  24 , i.e., 10° chamfers and the contact between flat  44  and the adjacent lug, the frictional forces at such contact are larger than the forces tending to drive the buckets apart. The buckets will therefore remain in the partially assembled position after being driven together even when the assembly force is removed. This in turn enables additional buckets to be assembled and likewise driven together without interference from the previously assembled buckets. 
   When all of the buckets except for a closure bucket have been applied about the wheel, the closure bucket is inserted into a radial opening in the wheel dovetail and keyed or pinned to adjacent buckets. The assembly fixtures on the shrouds of the closure and adjacent buckets aid in assembly of the closure bucket since a pre-twist of the closure bucket shroud can be applied with the fixtures. Thus, the closure bucket is inserted and driven radially into the notch opening. 
   After assembly of the closure bucket, the assembly fixtures  26  are removed from the shrouds. As the fixtures  26  are removed, a rotation of the shrouds occurs in the opposite direction from the initial pre-twist (i.e., compare  FIGS. 5 and 6 ). This opposite or negative rotation of the shrouds enables the contact surfaces  32  of the shrouds to come into full flush contact with one another. That is, this counter rotation is provided by the bias of the airfoils  16  from the previously applied pre-twist. It will be appreciated that the dovetail anti-rotation key  22  is in place during assembly of the buckets to constrain dovetail rotation. Thus, the level of pre-twist in the bucket airfoil created by the shroud rotation biases the shroud for rotation in the opposite direction into final assembly. Outer portions  54  of the lugs  24  may then be removed, e.g., by machining, leaving the shrouds  20  including remaining portions of the lugs  24  in final position as illustrated in  FIGS. 7 and 8 . 
   Referring to  FIGS. 5-7 , there is provided a relief groove  50  on the shroud pressure side surface. The relief groove  50  provides a low stress transition between the shroud contact and clearance surfaces. The relief groove  50  is also applied to reduce the potential for fretting fatigue by creating a separation between peak shroud bending and bearing stresses. It also creates a separation between the shroud contact and clearance surfaces such that final machine operations on the more critical shroud contact surface can be performed without impacting the finished clearance surface or corner fillet surfaces. 
   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.