Abstract:
A damper pin for a turbine bucket includes an elongated main body portion having a first substantially uniform cross-sectional shape and axially-aligned, leading and trailing end portions having a second relatively smaller cross-sectional shape at opposite ends of the main body portion. A seal element is provided on one or both of the opposite leading and trailing end portions projecting radially outwardly beyond the main body portion.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to turbo machines and particularly, to damper pins disposed between adjacent buckets on a rotor wheel for damping bucket vibrations. 
     As is well known, turbines generally include a rotor comprised of a plurality of rotor wheels, each of which mounts a plurality of circumferentially-spaced buckets. The buckets each typically include an airfoil, a platform, a shank and a dovetail, the dovetail being received in mating dovetail slot in the turbine wheel. The airfoils project into a hot gas path downstream of the turbine combustors and convert kinetic energy into rotational, mechanical energy. During engine operation, vibrations are introduced into the turbine buckets and if not dissipated, can cause premature failure of the buckets. 
     Many different forms of vibration dampers have been proposed to minimize or eliminate vibrations. Vibration dampers are often in the form of an elongated damper pin that fits between adjacent buckets and provides the damping function by absorbing harmonic stimuli energy produced as a result of changing aerodynamic loading. The damper pin is typically retained in a groove formed along one circumferentially-oriented “slash face” in the turbine blade shank region of one of each pair of adjacent buckets. The damping pin is centrifugally loaded during operation and, in order to prevent bucket-to-bucket binding, the groove must be machined so as to allow the pin to float relatively freely within the groove. 
     At the same time, highly-compressed air is often extracted from the compressor of an axial turbine for the purpose of cooling turbine components. This cooling air is required to maintain the temperature of the turbine components at an acceptable level for operation, but comes at a cost to overall turbine efficiency and output. Any of the cooling flow that leaks out of the turbine components is essentially wasted. The pocket created by a damper pin groove provides a large leakage path for cooling flow to escape from the bucket shank region. The cooling efficiency can also be impaired by ingress of hot gas from the hot gas path into the bucket shank region. 
     It would therefore be desirable to add a sealing feature to otherwise conventional damper pins in order to prevent, minimize or control the escape of cooling flow from a pressurized shank cavity, prevent or minimize flow from leaking across the turbine blade from the forward wheel space to the aft wheel space in the case of a non-pressurized shank cavity, and/or to prevent ingress of hot gas path air into the shank region. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with a first exemplary but non-limiting embodiment, the present invention provides a damper pin for a turbine bucket comprising: an elongated main body portion having a first substantially uniform cross-sectional shape and axially-aligned, leading and trailing end portions having a second relatively smaller cross-sectional shape at opposite ends of the main body portion, the leading and trailing end portions joining the elongated main body portion at respective shoulders; and a seal element on one or both of the opposite leading and trailing end portions projecting radially outwardly beyond the main body portion. 
     In another exemplary but non-limiting embodiment, the present invention provides a turbine rotor wheel comprising a plurality of circumferentially arranged buckets, each adjacent pair of buckets having a damper pin inserted therebetween, the damper pin comprising an elongated main body portion of a first substantially uniform cross-sectional shape, with opposite leading and trailing ends of a different cross-sectional shape, and a seal element on one or both of the opposite leading and trailing end portions projecting radially outwardly beyond the main body portion. 
     In still another exemplary but non-limiting aspect, the present invention provides in a turbine rotor wheel comprising a plurality of circumferentially arranged buckets, each adjacent pair of buckets having a damper pin inserted therebetween, the damper pin comprising an elongated main body portion of a first substantially uniform cross-sectional shape, with opposite leading and trailing ends of a different cross-sectional shape, a method for controlling escape of cooling air from a cavity in a shank portion of the bucket along the damper pin, the method comprising providing at least one seal element on the damper pin; and providing at least one recess or slot in the shank portion of the bucket; and locating the damper pin such that the at least one seal element is seated in the at least one recess. 
     The invention will now be described in detail it connection with the drawings identified below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a gas turbine bucket and damper pin assembly; 
         FIG. 2  is a partial side elevation showing a pair of circumferentially adjacent buckets with a damper pin located therebetween; 
         FIG. 3  is a perspective view of a damper pin with attached seal elements in accordance with a first exemplary but non-limiting embodiment of the invention; 
         FIG. 4  is a perspective view of one seal element shown in  FIG. 3 , removed from the damper pin; 
         FIG. 5  is a side elevation view of a damper pin similar to that shown in  FIG. 3 , but wherein the seal element has been made integral with the damper pin; 
         FIG. 6  is an end view of the pin shown in  FIG. 5 ; 
         FIG. 7  is a plan view of the pin shown in  FIG. 5 ; 
         FIG. 8  is a partial perspective view of a bucket shank, illustrating a groove adapted to receive the damper pins of  FIG. 3  or  5 ; 
         FIG. 9  is a top plan view of the damper pin of  FIGS. 4-6  (shown in phantom) installed between two adjacent buckets; 
         FIG. 10  is a side elevation of an alternative damper pin incorporating seal elements in accordance with another exemplary but non-limiting embodiment of the invention; 
         FIG. 11  is an end view of the damper pin shown in  FIG. 10 ; and 
         FIG. 12  is a top plan view of the pin shown in  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a conventional bucket  10  including an airfoil  12 , a platform  14 , a shank  16  and a dovetail  18 . The dovetail  18  is utilized to secure the bucket  10  about the periphery of the rotor wheel (not shown), as is well understood in the art. A damper pin  20  is located along one axial edge (or slash face)  22  adjacent (i.e., radially inward of) the bucket platform  14  with the leading end  24  of the damper pin  20  located nearer the leading edge of the bucket, and the trailing end  26  of the damper pin located nearer the trailing edge of the bucket. 
     It will be appreciated that a similar pin  20  is located between each adjacent pair of buckets  18 ,  118  on the turbine wheel, as apparent from  FIG. 2 . Specifically, the damper pin  20  is located in a groove  28  extending along the entire slash face  22  of the bucket  118 . The damper pin includes a substantially cylindrical body portion  30  between a pair of substantially semi-cylindrical, opposite ends  24 ,  26  interfacing at shoulders  39 . This configuration creates flat support surfaces  32 ,  34  (best seen in  FIG. 1 ) that are adapted to rest on the machined bucket platform surfaces or shoulders (one shown at  36  in  FIG. 2 ) at opposite ends of the groove  28  formed in the bucket slash face, thereby providing good support for the pin while preventing undesirable excessive rotation during machine operation. 
     With reference now to  FIGS. 3 and 4 , a damper pin  33  in accordance with a first exemplary but nonlimiting embodiment is illustrated. Like the conventional pin described above, the pin  38  includes a substantially cylindrical body portion  40  and a pair of reduced cross-section, substantially semi-cylindrical, opposite ends  42 ,  44 . The damper pin  38  is now provided with a sealing feature in the form of a pair of attached seal elements  46 ,  48  provided, respectively, at the interfaces (or shoulders  39 ) between the cylindrical body portion  40  and the opposite ends  42 ,  44 . Each of the mirror image seal elements is formed as a substantially circular disc  50  having an annular peripheral rim  52 . The circular disc  50  includes a solid half-section  54  and an open half section, the latter defined by a substantially semi-circular opening or aperture  56  that allows the seal element to be received over a respective end  42  or  44  of the damper pin  38 . 
     The seal elements  46 ,  48  may be composed of the same or different alloy material as the damper pin  38 . For example, either or both of the damper pin  46  and seal elements  46 ,  48  may be a Nickel-based alloy such as X-750, or a cobalt-based alloy such as L-605. It will be understood, however, that the invention is not limited by the choice of alloy materials for either the pin  38  or the seal elements  46 ,  48 . 
     The seal elements  46 ,  48  are sized to slide somewhat loosely over the ends  42 ,  44  of the damper pin  42 , so that the seal elements can easily seat within mating, machined slots or recesses  58 ,  60  formed in the bucket shank  62  at the slash face ( FIG. 8 ) as described further below. It is intended that most if not all radial loading during machine operation be taken up by the damper pin  38  as it moves radially within its slash groove  64  ( FIG. 8 ) due to centrifugal forces and as permitted by the loose tolerances. The tolerances relating to the slots or recesses  58 ,  60  and groove  64  and damper pin/sealing elements  46 ,  48  also allow for some minimal rotation of the damper pin during machine operation, but centrifugal forces are not taken up by the seal elements themselves. 
     The damper pin  38  provides the desired damping between adjacent buckets as in prior designs, while the seal elements  42 ,  44  provide a barrier that prevents, minimizes or controls escape of cooling air from of pressurized cavity in the shank portion along the pin, especially nearer the leading edge of the bucket. In the case of a non-pressurized cavity, the seal elements also prevent, minimize or control leakage across the bucket from the forward wheel space to the aft wheel space. In addition, the seal elements also serve to prevent or minimize any ingress of hot combustion gases into the shank portion along the pin, especially nearer the trailing edge of the bucket. 
       FIGS. 5-7  illustrate a variation of the damper pin  38  shown in  FIGS. 3 and 4 . Here, the damper pin  68  and seal elements  70 ,  72  are formed integrally as one piece. The damper pin otherwise is substantially the same as damper pin  38 . Here again, the seal elements  70 ,  72  are seated relatively loosely in the machined slots or recesses  58 ,  60 . 
     Edge faces  74 ,  76  of the reduced cross-sectional ends  78 ,  80  are angled to match the adjacent bucket edges as best seen in  FIG. 9 . 
       FIG. 8  illustrates the groove  64  formed in the slash face  82  of a bucket. The groove  64  is shaped to include the machined support surfaces  84 ,  86  adapted to engage the flat surfaces  88 ,  90  of the semi-circular ends  42 ,  44  of the damper pin  38 , or the flat surfaces  92 ,  94  of the semi-cylindrical ends  96 ,  98  of the pin  68 . 
       FIG. 9  illustrates the damper pin  68  seated between two adjacent buckets  100 ,  102 . The damper pin extends substantially parallel to the opposed bucket slash faces, requiring the pin end faces  74 ,  76  (as mentioned above) to be cut at a non-perpendicular angle relative to a longitudinal axis of the damper pin to remain flush with the end faces of the buckets. 
       FIGS. 10-12  illustrate another exemplary but nonlimiting embodiment of a damper pin provided with a sealing feature. The damper pin  104  is formed to include a cylindrical body portion  106  and opposite ends opposite ends  108 ,  110  of substantially semi-circular cross-section as in the previous embodiments. In this embodiment, however, the integral seal elements  112 ,  114  are formed at the outer tips of the opposite ends  108 ,  110 . Because the seal elements are identical, only one need be described. The seal element  112  extends radially beyond the arcuate segment of the semi-circular end  108  of the pin in all directions, including beyond the flat support surface  116 , creating a flat  118  that extends substantially parallel to the support surface  116 . Thus, the arcuate extent of the seal element is greater than 180°, as best seen in  FIGS. 30 and 11 . Note that as in the previously described embodiments, edge surface  120  of the seal is machined at an angle to the longitudinal axis of the damper pin  104  so that the seal edge surface will remain flush with adjacent bucket surfaces when installed, similar to the end edges  74 ,  76  of the damper pin  68  described above. A further beveled edge portion  122  ( FIG. 11 ) is formed on the end face  120  of the seal element, extending at about a 60° angle relative to the flat  118  and intersecting the outer or peripheral surface of the seal. The shape of the seal element is again designed to engage or nest loosely within correspondingly shaped slots or recesses formed in the bucket shank to provide the required sealing feature. 
     It will be appreciated that the location and shape of the seal (and the corresponding pin groove and seal receiving recesses) may vary as dictated by specific applications. 
     It will also be appreciated that the cross-sectional shape of the damper pins need not be cylindrical and the cross-sectional shape of the reduced-diameter ends need not be semi-cylindrical as described above. Other non-round or non-uniformly shaped damper pins are within the scope of the invention. In addition, the damper pin may receive, or be formed with, only one seal element. For pressurized shanks, two seals are preferred while, for non-pressurized shanks, one seal would be sufficient. 
     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.