Abstract:
A multiple hook dovetail connection for connecting a rotor wheel and a bucket of a turbine rotor that permits the use of wider vanes at the ends of the buckets without changing the size of the wheel and the other existing components of the turbine. The dovetail connection comprises a male dovetail component and a female dovetail component. The male dovetail component includes first and second hooks. Each of the hooks includes a crush surface, a neck and an angle formed between the crush surface and the neck. These hooks are dimensioned in accordance with at least one of the included tables.

Description:
TECHNICAL FIELD 
     The present invention relates to steam turbine rotors, and more particularly to dovetail connections between steam turbine rotor wheels and steam turbine buckets. 
     BACKGROUND OF THE INVENTION 
     Dovetail connections between turbine rotor wheels and turbine buckets include wheel hooks and bucket hooks that cooperate with each other to counter the centrifugal forces that are imposed on the connections. These hooks also prevent the buckets from separating from their wheel during the operation of the steam turbine. However, a major failure potential for conventional dovetail connections is their margin for creep. This is especially true in high pressure (HP) sections of the turbine where high temperatures are produced. For a bucket and wheel dovetail connection in a turbine operating at operational temperature of between about 850 to 1000 degrees Fahrenheit and at given stress levels, the creep strength of the bucket and rotor materials is not equal. Instead, the strength of the wheel is less than that of the bucket. As a consequence of the material strength differences and the load distribution that occurs during operation, the lower strength wheel limits the magnitude of the stresses that the connection can withstand. If the stresses exceed the material strength of the wheel, creeping will occur and the connection may fail. The limitations created by the configuration and dimensions of these conventional connections and the dependence of these connections on the lower material strength of the wheel prevent conventional turbines from reaching optimal levels of performance. 
     Commonly assigned U.S. Pat. No. 5,474,423 to Seeley et al. discloses a bucket and wheel dovetail connection for steam turbine rotors. In the Seeley et al. patent, the dovetail connection includes four hooks on the rotor wheel that have crush surfaces. The crush surfaces each form an angle with a respective neck surface that is greater than ninety degrees. The radially innermost hook includes a crush surface that is at an angle of ninety degrees to its respective neck surface. The slanted crush surfaces result in lower stress concentrations in the wheel hooks. Nevertheless, the need for optimization of a dovetail connection between a wheel and a bucket still existed. 
     These conventional dovetail connections cannot support a larger vane unless the size of the connection is also increased. Applying a larger vane would cause a turbine using conventional dovetail connections to fail. Alternatively, an increase in the overall size of the dovetail connections would result in an undesirable increase in the overall size of the turbine stages and an increase in the spacing of the turbine stages. These increases would require that the overall size of the steam turbine also be increased. 
     BRIEF SUMMARY OF THE INVENTION 
     It is desirable to overcome the drawbacks in the prior art by providing a dovetail connection that optimizes the joint between a rotor wheel and a bucket and permits the use of wider vanes at the ends of the buckets without changing the size of the wheel and the other existing components of the turbine. In an embodiment of the present invention, a dovetail connection for connecting a rotor wheel and bucket for a turbine comprises a male dovetail component and a female dovetail component. The male dovetail component includes first and second hooks. Each of the hooks includes a crush surface, a neck and an angle formed between the crush surface and the neck. These hooks are dimensioned in accordance with at least one of the below listed tables. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a conventional turbine rotor wheel and bucket dovetail joint; 
     FIG. 2 is a partial cross-sectional view of an embodiment of a turbine wheel dovetail connection in accordance with the present invention; 
     FIG. 3 is an enlarged fragmentary cross-sectional view of the hooks of the dovetail connection of FIG. 2; 
     FIG. 4 is a perspective view of the dovetail connection; 
     FIG. 5 is a graph showing the improvement in resisting creep experienced by the dovetail connection described herein; and 
     FIG. 6 is a chart showing the reduction in shear stresses and equivalents in wheel and bucket hooks described herein. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The figures illustrate portions of a steam turbine rotor that include a bucket and wheel dovetail connection. Compared to conventional dovetail connections, the dovetail connection according to the present invention reduces the average and concentrated stresses in the bucket and wheel that result from centrifugal forces created during the operation of the turbine. For example, the geometry and dimensions of the hooks prevent excessive creeping at high temperatures. Also, the dovetail connection discussed below has a higher load carrying capacity when compared to existing dovetail connections with the same external dimensions. The dovetail connection permits at least 30% more load to be distributed to the bucket than to the wheel during the operation of the turbine. This distribution takes advantage of the greater material strength of the bucket material. This is especially advantageous in the portions of the turbine where the operating temperatures reach between about 850 and 1000 degrees Fahrenheit. Additionally, the distribution of the additional stresses to the bucket permits the use of larger blades within the size constraints of a conventional steam turbine. As a result, the performance of the turbine can be increased to an optimum level without increasing its overall size. 
     FIG. 1 illustrates a portion of a steam turbine including a shaft  10  and a rotor wheel  12  secured to the shaft  10  in any well-known manner. Though not illustrated, the shaft  10  also includes additional rotor wheels that are spaced from each other along the length of shaft  10 . Each rotor wheel  12  mates with a plurality of steam turbine buckets  16  that carry a blade as is known in the art. The material used to form the wheel  12  can include, but is not limited to, Chrome-Moly-Vanadium alloy steel. The materials used to form the bucket  16  can include, but are not limited to, stainless steels including a 12% chrome stainless steel. 
     For clarity, only wheel  12  and bucket  16  have been illustrated and described herein. However, the discussions relating to the wheel  12  and the bucket  16  are equally applicable to the other wheels and buckets positioned along the length of the shaft  10 . 
     An embodiment of the present invention includes the wheel  12  and bucket  16  partially illustrated in FIG.  2 . As discussed below, the embodiment shown in FIG. 2 includes two hooks. The geometry and dimensions of these hooks that add to the optimum performance of the turbine are set forth in the tables below. 
     As shown in FIG. 2, the rotor wheel  12  terminates along its outer radius in a male dovetail component  14 . The male dovetail component  14  includes a plurality of hooks  40  and  42 . Component  14  also includes a tang  46 . A turbine bucket  16  including a female dovetail component  18  is shown positioned on the wheel  12 . The female dovetail component  18  includes a plurality of hooks  52  and  54  that extend along the innermost portion of the bucket  16  for mating with the hooks  40  and  42 , respectively, of the male dovetail component  14 . The bucket  16  also includes a blade  20  that extends away from the female dovetail component  18 . In one embodiment, the dovetail connection includes a tangential entry-type dovetail arrangement. 
     As illustrated in FIG. 2, the first hook  40  is furthest away from the centerline of the shaft  10 . Conversely, the second hook  42  is closest to the centerline of the shaft  10 . 
     In the ensuing description, it will be appreciated that the dovetail hooks  40  and  42  of the male component  14  and the hooks  52  and  54  of the female component  18  are symmetric with respect to a radial plane  100  that extends normal to the axis of rotation of the shaft  10 . Also, it is accepted practice to refer only to half the dovetail hooks  40  and  42  and  52  and  54  of the components  14 ,  18 , i.e., the dovetail hooks along one side of the radial plane  100 . Thus, the description of the embodiment illustrated in FIG. 2 refers to the two hooks  40  and  42  and two hooks  52  and  54  along one side of the plane  100  that is parallel to and coextensive with the mid-plane, which includes the axis of symmetry, of the wheel  12  that extends in the direction of the bucket  16 . It is understood by one of skill in the art that the hooks  40 ,  42 ,  52  and  54  form only a portion of the dovetail joint and that each component  14 ,  18  of the dovetail joint includes four hooks, as shown in FIG.  4 . 
     Above the first hook  40 , the wheel  12  includes an end surface  44  having a width of about 0.297 inch. End surface  44  can also be referred to as the wheel rim surface. The width according to the present invention extends in an axial direction. The width extends from plane  100  to an upper surface  60  of the first hook  40 . The upper surface  60  extends between the end surface  44  and an upper hook face  62 . The upper surface  60  is slanted relative to the plane  100  so that it forms an angle A′ with the upper hook face  62  that is equal to 180 degrees minus angle A. As shown in FIG. 2, the upper hook face  62  extends substantially parallel to the plane  100 . 
     A contact surface  64  extends between the face  62  and a neck  66  of the first hook  41 . The contact surface  64  is slanted at an angle of greater than 90 degrees relative to the face  62  and the neck  66 . The neck  66  extends substantially parallel to plane  100  and at an angle B′ to the contact surface  64 . Angle B′ is equal to 180 degrees minus angle B. As shown in FIG. 3, angle B is defined as the angle between the contact surface  64  and a line that extends parallel to the plane  100  and the neck  66  in the direction away from the neck  66  and toward end surface  44 . 
     The second hook  42  includes an upper surface  70  that extends between, and is slanted relative to, the neck  66  and a hook face  72 . Like hook face  62 , the hook face  72  extends substantially parallel to the plane  100 . As a result, the upper surface  70  forms an angle C′ with the hook face  72  as shown in FIG.  3 . Angle C′ is equal to 180 degrees minus angle C. Angle C extends between the upper surface  70  and a line that extends parallel to plane  100  and hook face  72  and toward the end surface  44 . 
     The second hook  42  also includes a contact surface  74  that extends between the hook face  72  and a neck  76 . The contact surface  74  extends at an obtuse angle to hook face  72 . Like the neck  66 , the neck  76  extends substantially parallel to the plane  100  and at an angle D to the contact surface  74 . As shown in FIG. 3, angle D′ is equal to 180 degrees minus the angle between the contact surface  74  and a line that extends parallel to plane  100  and the neck  76  in the direction away from the neck  76  and toward end surface  44 . The second hook further includes a surface  78  that extends between the neck  76  and an upper surface  80  of a shoulder  82  of the wheel  12 . The surface  78  extends at an angle E to the neck  76 . 
     Angle E′ is equal to 180 degrees minus the angle E. Although the figures show the surface  78  being slanted relative to neck  76 , the surface  78  extends substantially parallel to neck  76 . The shoulder  82  includes the tang  46  at its outer edge. The shoulder  82  and tang  46  can provide support for the bucket when the turbine is not operating. Additional support is also provided by the upper surfaces  60  and  70  of each hook. As understood from the figures, when the turbine is at rest, the upper surfaces  60  and  70  each contact a cooperating surface on the bucket  16  to support the bucket  16  on the wheel  12 . 
     During the operation of the turbine, the centrifugal forces generated by the rotation of the wheel  12  causes the contact surfaces  64  and  74  to engage with a contact surface  22  on the cooperating hooks  52  and  54  of the bucket  16  in order to prevent the separation of the wheel  12  and the bucket  16 . These contact surfaces  64 ,  74  and  22  are commonly referred to as crush surfaces. Concentrated stresses result when load paths are forced to change directions abruptly. Accordingly, the slanted crush surfaces  64 ,  74  and  22 , having the configuration described herein, cause a less severe change in direction and hence afford lower stress concentrations in the wheel  12 . Additionally, these slanted crush surfaces  64 ,  74  and  22  transfer the stresses to the bucket  16  that, as discussed above, have a stronger material strength than the wheel  12  when the turbine is operating at temperatures of 850 to 1000 degrees Fahrenheit. By transferring the stresses to the bucket  16 , the elastic shear stresses in the hooks  42  and  44 , creep deformation (due to high temperature environment of high pressure stages) and stress concentrations within the wheel  12  are reduced relative to the prior art as shown by the creep improvement graph of FIG.  5 . Moreover, a percentage of reduction in the shear and equivalent stresses in the wheel and bucket hooks as compared to conventional dovetail connections is shown in FIG.  6 . 
     As discussed above, the connection of the present invention permits the load on the bucket  16  to be increased by about 30% or greater. An increased load can include the application of a larger blade  20  on the end of the bucket  16 . Larger blades  20  can be used with the present invention without the size of the wheel  12 , the bucket  16  or the dovetail connection being increased. The use of a larger blade will improve the performance of the turbine. Larger blades will contribute to optimum engine performance. 
     In order to reduce the stress concentrations at the intersection of adjacent surfaces of the dovetail connection, each crush surface  64  and  74  is spaced from its respective neck  66  and  76  and face  62  and  72  by fillets  92 . The radii of the fillets  92  are listed below in Table VI. The determined radii for the fillets  92  result in a further lowering of the concentrated stresses in the wheel  12  and the bucket  16  at the dovetail connection. As noted previously, slanted crush surfaces cause a component of force in the axial direction which gives rise to the bending of the bucket leg and an axial load on the tang  46  of the wheel dovetail. To minimize this effect, the hook thickness (height) H of all of the hooks is substantially the same as shown in the tables below. 
     As seen in FIG. 2, the height of each hook H is the distance from the beginning of its upper surface to the juncture between the crush surface for the hook and its neck. As also shown, the height L of each wheel neck is the distance between the juncture of its crush surface and the neck and the juncture of the neck and the beginning of an upper surface for an adjacent hook. 
     The magnitudes of the above-discussed angles are listed below in Table I. These angles and the dimensions discussed herein for the parameters of the dovetail connection were found to contribute to the optimum performance of the turbine by, at least, taking advantage of the increased material strength of the bucket  16  and reducing the stresses along the crush surfaces as discussed above. 
     Other dimensions relating to the disclosed exemplary embodiment are also disclosed in the tables below. These dimensions have a tolerance of +/− one ten-thousandths of an inch. These dimensions include the wheel neck width W for each surface of the hook. As seen in FIG. 2, the wheel neck width W for a given surface is the distance that the surface is spaced from the plane  100  on one side of the wheel  12 . 
     The below listed dimensions also include the clearance C between the surfaces of the wheel hooks and the surfaces of the bucket hooks during the operation of the turbine. The clearances C13, C21 and C23 between certain surfaces that do not contact each other during the operation of the turbine are substantially the same. These surfaces include the necks of the wheel hooks and the faces of the bucket hooks. These similar clearances have been referenced collectively in the tables below as Ca. Similarly, the clearances C12, C22 and C35 for the upper surfaces of the wheel hooks that do not contact the lower surfaces of the bucket hooks are substantially the same. As a result, they have been referenced collectively in the tables below as Cb. 
     The heights from the bottom of the bucket  16  to the illustrated intersections of different sections of the bucket are also included in the tables below and shown in FIG.  2 . Similarly, the angles formed by the surfaces of these sections with a plane extending parallel to the plane  100  are included in Table I. 
     Other dimensions include the distance DW/2 from the plane  100  to the outer surface of the bucket  16 . This distance is about 1.1 inches in an embodiment of the present invention. The height S of the bucket  16  from surface  44  to a blade surface  84  is about 0.875 inch. In this same embodiment, the radial distance RW/2 from the plane  100  to the outer edge of the bucket is about 0.674 inch. Other dimensions such as the wheel rim diameter (WRD) can be the same as found with a conventional dovetail connection. As understood in the art, the wheel rim diameter is twice the distance from a point to the axis of rotation of the shaft. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Angle 
                 Degrees 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 A 
                 80 
               
               
                   
                 A′ 
                 100 
               
               
                   
                 B 
                 65 
               
               
                   
                 B′ 
                 115 
               
               
                   
                 C 
                 80 
               
               
                   
                 C′ 
                 100 
               
               
                   
                 D 
                 90 
               
               
                   
                 D′ 
                 90 
               
               
                   
                 E 
                 0 
               
               
                   
                 E′ 
                 180 
               
               
                   
                 AB1 
                 21 
               
               
                   
                 AB2 
                 21 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 Bucket 
                   
               
               
                   
                 Dimensions 
                 Inches 
               
               
                   
                   
               
             
             
               
                   
                 B1 
                 0.75 
               
               
                   
                 B2 
                 0.76 
               
               
                   
                 B3 
                 0.219 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
           
               
                   
                 TABLE III 
               
               
                   
                   
               
               
                   
                 Clearance 
                   
               
               
                   
                 Between the 
                   
               
               
                   
                 Wheel and the 
                   
               
               
                   
                 Bucket 
                 Inches 
               
               
                   
                   
               
             
             
               
                   
                 C11 
                 0.01 
               
               
                   
                 Ca 
                 0.0135 
               
               
                   
                 Cb 
                 0.01 
               
               
                   
                 C34 
                 0.01 
               
               
                   
                 C36 
                 0.031 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
           
               
                   
                 TABLE IV 
               
               
                   
                   
               
               
                   
                 Hook Height 
                 Inches 
               
               
                   
                   
               
             
             
               
                   
                 H1 
                 0.37 
               
               
                   
                 H2 
                 0.65 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
           
               
                   
                 TABLE V 
               
               
                   
                   
               
               
                   
                 Wheel Neck 
                   
               
               
                   
                 Height 
                 Inches 
               
               
                   
                   
               
             
             
               
                   
                 L1 
                 0.165 
               
               
                   
                 L3 
                 0.281 
               
               
                   
                 L4 
                 1.422 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
           
               
                   
                 TABLE VI 
               
               
                   
                   
               
               
                   
                 Fillets 92 
                 Radii (inches) 
               
               
                   
                   
               
             
             
               
                   
                 R1 
                 0.109 
               
               
                   
                 R2 
                 0.109 
               
               
                   
                 R3 
                 0.078 
               
               
                   
                 R4 
                 0.078 
               
               
                   
                 R5 
                 0.109 
               
               
                   
                 R6 
                 0.109 
               
               
                   
                 R7 
                 0.078 
               
               
                   
                 R8 
                 0.078 
               
               
                   
                 R13 
                 0.188 
               
               
                   
                 R14 
                 0.075 
               
               
                   
                 R15 
                 0.094 
               
               
                   
                 R16 
                 0.05 
               
               
                   
                 R17 
                 0.25 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
           
               
                   
                 TABLE VII 
               
               
                   
                   
               
               
                   
                 Distances From 
                   
               
               
                   
                 Plane 100 
                 Inches 
               
               
                   
                   
               
             
             
               
                   
                 W11 
                 0.297 
               
               
                   
                 W12 
                 0.54 
               
               
                   
                 W13 
                 0.365 
               
               
                   
                 W21 
                 0.8 
               
               
                   
                 W22 
                 0.575 
               
               
                   
                   
               
             
          
         
       
     
     With the foregoing dimensions, it will be appreciated that the dovetail shape minimizes concentrated stresses, while maintaining an overall size compatible with existing steam paths. 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. For example, the male hooks could be positioned on the bucket and the female hooks on the wheel.