Abstract:
A bucket to wheel dovetail attachment includes axially spaced radially extending wheel fingers having discrete sections of decreased axial thickness in a radial outward direction with transition fillets between sections of different thicknesses. The wheel fingers define wheel finger slots for receiving correspondingly shaped bucket dovetail fingers. Pins interconnect the buckets and rotor wheel. The slot bottoms likewise have fillets. The fillets on the wheel fingers and slot bottoms have a blend of different radii with the larger radii outward of the smaller radii to reduce stress concentrations and to avoid stress corrosion cracking in steam turbine applications.

Description:
The present invention relates to an attachment between a rotor wheel dovetail and a dovetail on buckets for minimizing the concentrated stress caused by the centrifugal force of the buckets in the wheel fingers and particularly relates to a compound fillet at the wheel finger transition between sections of different radial thicknesses and at wheel finger slot bottom locations for stress reduction for a given set of radii. 
   BACKGROUND OF THE INVENTION 
   In turbines, particularly steam turbines, attachment between the plurality of buckets and the rotor wheel is typically accomplished using radial entry bucket dovetail fingers and radial fingers about the margins of the rotor wheel. For example, the finger dovetails on the buckets include a plurality of axially spaced fingers having sections which decrease in thickness in a radial inward direction for reception in slots defined between axially adjacent radially outwardly projecting fingers having sections which decrease in thicknesses in a radial outward direction about the margin of the rotor wheel. Axially extending pins secure the fingers of the wheel and bucket to one another. Single radius fillets are conventionally provided in the wheel fingers at the transitions between the sections of different thicknesses. Similar fillets have been used at the bottom of the finger slots in the wheel. 
   In many steam turbine applications, the finger dovetails operate in an environment that is conducive to stress corrosion cracking (SCC). SCC is accelerated by the stress levels that are present in the wheel transition fillets and slot bottoms. These stresses are normally acceptable. However in steam turbines having contaminated steam, cracks can initiate and if left undetected, may grow to a depth that will cause failure of the wheel fingers. Experience has shown that wheel dovetail fingers crack while bucket dovetails typically do not crack. This is because the materials used for the rotors are much less resistant to SCC than are the materials used for the buckets. For example, NiCrMoV and similar low alloy steels are typically used in rotors whereas 12 Cr steels are typically used for buckets as those materials afford an optimum combination of properties available for overall low pressure design. Typically a single radius at the wheel transition fillets and slot bottom have been used and these have experienced SCC cracking in the field. Accordingly, there is a need to provide an effective means of avoiding SCC in wheel dovetails which is compatible with existing steam paths, does not affect bucket dovetail geometry and will reduce the stress concentration. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In a preferred embodiment of the present invention there is provided a rotor wheel for a turbine comprising: a plurality of fingers spaced axially one from another and extending circumferentially about a margin of the wheel for receiving fingers of buckets; the wheel finger including a plurality of circumferentially extending sections of reduced axial extent in a radial outward direction; each of the wheel fingers having fillets at transitions between radially adjacent sections; each fillet being comprised of first and second radii with the first radii having a larger radius than the second radius. 
   In a further preferred embodiment of the present invention there is provided a rotor wheel for a turbine comprising: a plurality of fingers spaced axially from one another and extending circumferentially about a margin of the wheel for receiving fingers of buckets; the wheel fingers including a series of circumferentially extending sections of reduced axial extent in a radial outward direction; axially adjacent wheel fingers at radially innermost locations along the wheel margin defining a bottom of a generally dovetail-shaped slot between the adjacent wheel fingers; the bottom of the slot having a pair of fillets with each bottom slot fillet comprised of first and second radii with the first radius of each bottom slot fillet having a larger radius than the second radius thereof. 
   In another embodiment of the present invention there is provided a rotor wheel and bucket attachment for a turbine comprising: a plurality of fingers carried by the rotor wheel spaced axially from one another and extending circumferentially about a margin of the wheel, the wheel fingers defining wheel finger slots therebetween having a bottom; a plurality of buckets each having a plurality of bucket fingers extending radially into the wheel finger slots; pins extending generally axially through the wheel and bucket fingers to secure the buckets and wheels to one another; the wheel fingers including a plurality of circumferentially extending sections of reduced axial thickness in a radially outward direction; each of the wheel fingers having fillets at transitions between radially adjacent sections and at the bottom of the finger wheel slots, at least one of the fillets for each wheel finger being comprised of first and second radii with the first radius being larger than the second radius. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a fragmentary perspective view of portions of a bucket and its attachment to the margin of a rotor wheel of a turbine; 
       FIG. 2  is an enlarged fragmentary tangential view of the wheel finger dovetails; and 
       FIGS. 3 AND 4  are enlarged fragmentary views of the fillets at the transitions of the wheel finger sections of different thicknesses and at the bottom of the slots, respectively. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings, particularly to  FIG. 1 , there is illustrated a fragmentary portion of the margin of a rotor wheel  10  illustrating the attachment of a plurality of buckets, only one bucket  12  being illustrated, to the rotor wheel. Rotor wheel  10  includes a plurality of radially outwardly extending fingers  14 , two end fingers  16  being located along axial opposite sides of the wheel  10 . The fingers  14  and  16  form a wheel dovetail. Four intermediate wheel fingers  18  extend between the end wheel fingers  16 . The wheel fingers  16  and  18  extend continuously in a circumferential direction about the margin of the wheel and project radially outwardly defining wheel finger slots  20  therebetween. Each wheel finger  18 , as best illustrated in  FIG. 2 , includes a series of circumferentially extending sections or steps of reduced axial extent in a radial outward direction. For example, the wheel finger  18  illustrated in  FIG. 2  includes reduced thickness sections  22 ,  24  and  26  with the reductions in thickness being stepped radially outwardly. A fillet  28  forms a transition between the thickest section  22  and the intermediate section  24  on each of the opposite sides of each wheel finger. A fillet  30  also forms a transition between the intermediate thickness section  24  and the final radial outwardly thinnest section  26  on each of the opposite sides of each wheel finger. Referring back to  FIG. 1 , it will be appreciated that the end wheel fingers  16  similarly have fillets  28  and  30  along their axially inside surfaces at the transitions between the sections of different axial thickness. 
   Also as best illustrated in  FIG. 2 , the slots  20  between the adjacent wheel fingers  18  and between the end wheel fingers  16  and axially outermost intermediate wheel fingers  18  terminate at their radial inner ends in bottom slots  32 . Bottom slots  32  define fillets  34  with the side wall surfaces of the adjacent wheel fingers. 
   Each bucket  12  includes an airfoil  40  having a root or base  42  from which project radially inwardly a plurality of dovetail-shaped fingers  44 . The fingers  44  are generally complementary in shape to the finger slots between adjacent intermediate wheel fingers  18 . The base  42  of each bucket  12  has a recess, not shown, on one side and a tangential projection  46  along its opposite side. The intermediate fingers  44  lie flush with the surfaces of the recess and projection  46 . 
   Bucket  12  also includes end fingers  48  on adjacent axially opposite sides of the bucket. The end fingers  48  are thus tangentially offset from the intermediate bucket fingers  44 . The end fingers  48  have a plurality, three being preferred, of semi-cylindrical openings  50 . Each of the intermediate fingers  44  of the bucket  12  has a full circular opening coaxial with the semi-circular openings  50 . Additionally, the wheel fingers  16  and  18  have circular openings aligned with the semi-circular end bucket fingers  48  and the circular openings of the intermediate bucket fingers  44 . Thus, when the buckets are inserted radially onto the rotor wheel  10 , the bucket fingers and the wheel fingers interdigitate with the openings aligned axially relative to one another. Pins  49  may thus be received within the aligned openings and secured to maintain the attachment between the buckets and the rotor wheel. It will be appreciated that adjacent buckets have end fingers  48  with semi-cylindrical openings and the adjacent buckets therefore share the pins with one another in that region. 
   As noted previously, the wheel finger dovetails of conventional turbines have a single radius at each of the transitions between the sections of the fingers of different thicknesses and at the bottoms  32  of the finger slots. In a preferred embodiment of the present invention, there is provided at each wheel finger transition area location a compound fillet to reduce the stress. It will be appreciated that the airfoil and bucket dovetail carry a centrifugal loading through the pins which secure the buckets and the wheel to one another. These forces give rise to stress in the wheel dovetail and peak stresses in the fillets and slot bottom region of the wheel fingers. In a preferred aspect of the present invention, a compound fillet  61  is used, i.e. a fillet having a first large radius  60  and a second smaller radius  62 . For example as illustrated in  FIG. 3  showing transition fillets  28  between wheel finger sections  22 ,  24  and  24 ,  26 , the large radius  60  blends into the side surface of the adjacent finger and blends into the smaller radius  62 . As a representative example, the large radius may be 0.225 inches while the small radius may be 0.080 inches. The distance d from the large radius center  64  is 0.130 inches. Thus the larger radius  60  lies radially outwardly of the smaller radius  62 . From a stress concentration standpoint, the larger radius is accordingly more resistant to stress/ 
   Additionally, referring to  FIG. 4 , each fillet forming the bottom  32  of each slot similarly has a compound fillet. For example the fillet  34  includes a large radius  68  and a smaller radius  70 . Each large radius section  68  transitions into the side wall of the base of the wheel fingers  18  or  16  while each small radius  70  transitions from the larger radius  68  into a small flat  72  at the base of the slot bottom  32 . The large radius  68  may, for example, be 0.225 inches and the small radius  70  0.080 Inches. The center of the large radius may be 0.146 inches in a radial direction from the bottom of the slot. The flat may extend axially a distance of about 0.16 inch. 
   By the foregoing geometry, the bucket to wheel finger dovetail configuration has peak stresses at the wheel finger transition locations and bottom slots sufficiently low to avoid stress corrosion cracking of the wheel fingers. Also and significant from manufacturing and operational standpoints, the compound fillets at the transition areas and bottom slot locations are compatible with existing steam paths and do not affect bucket dovetail geometry thus enabling a reduction in stress in the wheel fingers without changes to the configuration of the buckets. 
   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.