Abstract:
A damper pin is disposed between adjacent buckets of a turbine rotor. A first bucket has circumferentially extending supports defining a pair of axially spaced surfaces on which a damper pin rests in a cold condition of the turbine. The adjacent bucket is undercut adjacent its platform to provide an angled surface overlying a generally correspondingly angled surface of the damper pin. The damper pin fits loosely within the recess and, upon turbine rotation at speed, the angled surfaces of the damper pin and recess cooperate to bias the damper pin against the first bucket whereby the damper pin engages both buckets and dissipates vibratory action.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to turbines having circumferentially-spaced buckets about a rotor wheel and particularly relates to a bucket damper assembly including a pin disposed between adjacent buckets for damping bucket vibration.  
           [0002]    As well known, turbines generally include a rotor, for example, comprised of a plurality of rotor wheels, each of which mounts a plurality of circumferentially-spaced buckets. The buckets typically include an airfoil, a platform, a shank and a dovetail, the dovetail being received in a slot or opening in the turbine wheel for securing the bucket to the wheel. The airfoils, of course, project into the gas path, e.g., the hot gas path of a gas turbine, and convert kinetic energy of the gases into rotational mechanical energy. During engine operation, vibrations are introduced into the turbine buckets and, if not dissipated, may cause premature failure of the buckets.  
           [0003]    Many different forms of vibration dampers have been proposed and constructed to minimize or eliminate vibratory action of the buckets. For example, see U.S. Pat. Nos. 6,354,803; 5,156,528; 6,390,775; 6,478,544; 5,827,047; and 6,450,769. While the vibration dampers disclosed in these patents may be useful to damp vibrations in certain types of turbine, they do not appear to be completely applicable to or effective with respect to buckets having a short shank and shroudless airfoil tips. It is therefore desirable to provide a damper assembly including a pin specifically useful for this type of turbine bucket, although the assembly and pin have applicability to other turbine buckets.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0004]    It will be appreciated that short shanks on buckets have less platform motion, resulting in less effective damping. The converse is true for shanks of greater length, assuming equal shank width and thickness. Vibratory platform deflection is directly related to shank length relative to the overall bucket length. For buckets in a stage where the ratio of shank length to total length is lower than that of a typical stage 1 bucket ratio, the lower ratio results in a lower magnitude of platform deflection and therefore a lower potential for damper effectiveness. On buckets which have shrouds at their tips, the shrouds afford additional damping, which minimizes the risk of utilizing shorter shanks on shrouded buckets. However, for turbine buckets on stages where the buckets are shroudless and have short shanks, there is increased risk of inadequate damping.  
           [0005]    In accordance with the preferred embodiment of the present invention, there is provided an assembly, for example, pairs of adjacent buckets with a damper pin between the buckets, which reduces the amplitude of vibratory stresses at full-speed full-load, full-speed no-load, and transiently, enables increased bucket life and is particularly useful for short shank shroudless buckets. To accomplish the foregoing, and in a preferred embodiment, each bucket is provided with a configuration along its circumferentially opposite sides, i.e., sides corresponding to the pressure and suction sides of the airfoil, enabling the capture of a damper pin between the adjacent buckets. Particularly, a first bucket includes a support, preferably a generally axially spaced pair of supports, projecting in a generally circumferential direction away from the first bucket beyond a marginal edge of the platform and toward the adjacent second bucket. Preferably, the support extends from the suction side of the buckets. The adjacent second bucket includes an undercut extending in a generally axial direction underlying the platform of the second bucket. The undercut of the second bucket, the support surface and a generally radially extending surface along the first bucket define a generally triangular-shaped, substantially axially extending, recess between the pair of buckets underlying the platform of the second bucket. The recess includes an angled surface formed by the second bucket. An elongated damper pin is disposed in the recess, has a generally triangular cross-sectional shape, and fits slightly loose within the recess.  
           [0006]    In a cold condition of the turbine, the damper pin generally rests on the support of the first bucket. Upon obtaining full-speed operation, the damper pin is displaced generally radially outwardly. The registering angled surfaces of the damper pin and the second bucket bias the damper pin to engage the radial surface of the first bucket. The damper pin thus engages the radial and angled surfaces of the respective first and second buckets. This frictional engagement permits dissipation of the vibratory motion of both buckets. The contact surfaces of the buckets with the damper pin are also preferably machined to provide improved surface fits therebetween and enhance vibration dissipating performance. The recess also opens outwardly in an axial direction, enabling the damper pin to be visible upon installation. This is important when the turbine is assembled to make sure that all damper pins have been installed. Otherwise, higher vibratory amplitudes causing higher stresses may result, causing the buckets to fail due to high-cycle fatigue. Further, the triangular, more particularly, the generally right triangular configuration of the recess and damper pin in cross-section, provides an anti-rotation feature which facilitates correct placement of the damper pin in service without jamming. The configuration of the recess and damper thus improve wear resistance, increase durability, effectively reduce vibratory stresses and inhibit failure due to high-cycle fatigue.  
           [0007]    In a preferred embodiment according to the present invention, there is provided an assembly of buckets for a turbine wheel, comprising a pair of circumferentially adjacent buckets each having a bucket airfoil, a platform, a shank and a dovetail, the dovetails being shaped for securement of the buckets to the turbine wheel, a first bucket of the pair of buckets including at least a first support extending in a generally circumferential direction from a side thereof and beyond a marginal edge of the platform of the first bucket, the support including a support surface, a second bucket of the pair thereof having an undercut opening in a generally circumferential direction toward the first bucket and underlying the platform of the second bucket, the undercut including a surface angled radially outwardly and toward the first bucket, the first bucket including a generally radially extending surface adjacent the support, the radially extending surface and the support surface of the first bucket, together with the angled surface of the undercut of the second bucket, forming a recess between the pair of buckets underlying the second bucket and a damper pin disposed between the adjacent buckets and in the recess, the damper pin being movable within the recess between a first position resting on the support surface and a second position engaging the radially extending surface of the first bucket and the angled surface of the recess of the second bucket for dissipating vibratory motion of the buckets.  
           [0008]    In a further preferred embodiment according to the present invention, there is provided an assembly of buckets for a turbine wheel, comprising a pair of circumferentially adjacent buckets each having a bucket airfoil, a platform, a shank and a dovetail, the dovetails being shaped for securement of the buckets to the turbine wheel, a first bucket of the pair of buckets including first and second supports extending in a generally circumferential direction from a side thereof and beyond a marginal edge of the platform of the first bucket, the supports including first and second support surfaces, a second bucket of the pair thereof having an undercut opening in a generally circumferential direction toward the first bucket and underlying the platform of the second bucket, the undercut including a surface angled radially outwardly and toward the first bucket, the first bucket including a contact surface, the contact surface and the support surface of the first bucket, together with the angled surface of the undercut of the second bucket, forming a recess between the pair of buckets underlying the second bucket and a damper pin disposed between the adjacent buckets and in the recess, at least one boss along the damper pin disposed between the supports for preventing displacement of the damper pin in opposite axial directions, the damper pin being movable within the recess between a first position resting on the support surface and a second position engaging the contact surface of the first bucket and the angled surface of the recess of the second bucket for dissipating vibratory motion of the buckets. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a fragmentary perspective view of a portion of a bucket illustrating the bucket in combination with a damper pin according to a preferred embodiment of the present invention;  
         [0010]    [0010]FIG. 2 is a fragmentary, substantially axial view illustrating the damper pin between adjacent buckets;  
         [0011]    [0011]FIG. 3 is a perspective view of a preferred embodiment of damper pin;  
         [0012]    [0012]FIG. 4 is an enlarged fragmentary cross-sectional view taken generally about on line  4 - 4  in FIG. 1 and illustrating the damper pin in the recess between adjacent buckets in an operable position for dissipating vibration; and  
         [0013]    [0013]FIG. 5 is a view similar to FIG. 3, illustrating a further form of damper pin hereof. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    Referring now to the drawings, particularly to FIG. 1, there is illustrated a bucket, generally designated  10 , including a bucket airfoil  12 , a platform  14 , a shank  16  and a dovetail  18 . It will be appreciated that the bucket  10  as illustrated is one of a plurality of circumferentially spaced buckets secured to and about the rotor of a turbine. For example, gas turbines typically have a plurality of rotor wheels having axial or slightly off-axis dovetail-shaped openings for receiving the dovetail  18  of the bucket  10  whereby an annular array of circumferentially spaced buckets, including the airfoils  12 , is provided about the rotor. From a review of FIGS. 1 and 2, the opposed and adjacent circumferential edges of each of the bucket platforms form slashfaces  20  and  22 . As indicated previously, the airfoils  12  project into the gas stream and enable the kinetic energy of the fluid stream to be converted to mechanical energy through the rotation of the rotor.  
         [0015]    As illustrated in FIG. 2, the assembly hereof includes circumferentially adjacent first and second buckets  12   a  and  12   b , respectively. The slashface  20  of the first bucket  12   a  extends along the side face of platform  14  and into the shank  16 . The shank  16  on the suction side  21  of the bucket airfoil  12  includes a pair of generally axially spaced supports  24  which project beyond the lateral edge of the slashface  20 . Each support  24  includes a generally circumferentially extending shelf or support surface  26  (FIG. 1) adjacent the opposite axial ends of the bucket platform and shank. The region between the supports  24  on the suction side of the bucket  12   a  lies generally open and generally parallel to the slashface  20 . Consequently, the supports  24  provide shelves  26  at generally axially opposite ends of the bucket which project in a circumferential direction from the suction side of the bucket toward the adjacent bucket.  
         [0016]    As illustrated in FIG. 2, the adjacent or second bucket  12   b  includes an undercut  30  generally below the bucket platform  14  on the pressure side of the bucket  12   b  and which may include portions of the platform  14  and the shank  16  of the second bucket. The undercut  30  is formed along the pressure side of the bucket  12   b , the trailing edge  23  of which is illustrated in FIG. 2. It will be appreciated that the undercut includes an angled surface  32  which extends the full axial length of the bucket and is angled radially outwardly and toward the adjacent first bucket. The angled surface  32  terminates in a fillet region  34 . The undercut  30  of the second bucket forms with the support shelves  26  of the first bucket a pair of recesses  36  adjacent axial ends of the adjoining buckets.  
         [0017]    Referring now to FIG. 3, there is illustrated a damper pin, generally designated  40 , for reception in the recess  36  formed between the adjacent buckets. The damper pin  40  includes an elongated, generally triangularly-shaped element in cross-section, as illustrated in FIGS. 2 and 4. A pair of axially spaced bosses  44  are provided along the underside of the damper pin  40 . When installed, the bosses  44  project generally radially inwardly and are spaced one from the other a distance less than the distance between the supports  24  of the turbine bucket. The cross-section of the damper pin between the bosses  44  is generally an equilateral triangle, while the end portions  46  of the damper pin  40  axially outwardly of the bosses  44  have a generally right triangular configuration in cross-section with a hypotenuse and bases  50  and  52  lying generally radially and tangentially, respectively, of the triangle.  
         [0018]    From a review of FIG. 2, it will be appreciated that the end portions  46  of the damper pin  40  reside in the recesses  36  formed between the undercut  32  on the second bucket and the shelves  26  and generally radial slash surface  20  of the first bucket. The configuration of the ends  46  thus is generally conformal to the configuration of the recesses  36 . It will be appreciated, however, that the fit between the end portions  46  and the recesses  36  is slightly loose, permitting generally radial outward movement of the damper pin in the recesses. Additionally, the damper pin  40  may be displaced toward the first bucket upon engagement between the angled surface  32  of the undercut  30  and the surface  48  generally forming the hypotenuse on the damper pin  40 . That is, as the rotor obtains full speed and because of the loose fitting engagement between the end portions  46  and the recesses  36 , the damper pin  40 , upon being displaced radially outwardly by centrifugal force, causes engagement between the surface  48  and angled surface  32  which forces the radial surface  50  of the damper pin into engagement with the slashface  20 . Thus, both buckets  12   a  and  12   b  are engaged by the damper pin at full-speed rotation of the rotor.  
         [0019]    From a review of FIG. 2, it will be appreciated that the damper pin is exposed, i.e., visible, in an axial direction, from at least one and preferably both axial end faces of the buckets. Consequently, it is possible to visually determine that the damper pin has been installed between adjacent buckets upon assembly of the rotor. It will also be appreciated that the bosses  44  preclude removal of the damper pin  40  in an axial direction from the recesses  36  upon installation. While a pair of bosses  44  are illustrated, it will be appreciated that a single boss extending the distance corresponding to the spacing between the bosses  44  may likewise be used if desired.  
         [0020]    Referring to FIG. 5, there is illustrated a further form of damper pin, generally designated  60 . Damper pin  60  includes an identical external surface configuration as described with respect to damper pin  40 . In this form, however, the damper pin  60  may have a hollow core  62 , i.e., a passage  64  extending between opposite ends and through the length of the damper pin. A pair of axially aligned passages may be provided with solid portions of the damper pin positioned between the pair of passages intermediate the opposite ends of the damper pin  60 .  
         [0021]    In operation of the turbine, the damper pin typically rests on the supports  24 , particularly on shelves  26 , when the turbine is not running. At speed, however, the slightly loose fit between the damper pin in the recesses enables the damper pin ends  46  to lift off the support surfaces  26  so that the surface  48  of the damper pin  40  and the angled surface  32  engage one another due to the centrifugal action on the damper pin. That engagement also biases the damper pin for movement in a circumferential direction such that the base  50  of the damper pin engages the slashface  20 . With the damper pin engaging both turbine buckets  12   a  and  12   b  along base  50  and surface  48 , the vibration of the buckets is dissipated by the frictional contact between the damper pin and buckets.  
         [0022]    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.