Patent Abstract:
A method of assembling a turbine comprises coupling at least one bucket assembly. The bucket assembly including an upstream side, a downstream side, a blade extending therebetween and a dovetail extending radially inwardly from the blade to a rotor. The method further comprises fixedly securing the at least one bucket assembly to the rotor with a shear pin that extends from the bucket assembly upstream side to the bucket assembly downstream side.

Full Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to turbine engines and more particularly to methods and apparatus for securing blades used within turbine engines.  
           [0002]    At least some known turbine rotor assemblies include a rotor to which a plurality of blades are coupled. The blades are arranged in axially-spaced stages extending circumferentially around the rotor. Each stage includes a set of stationary blades or nozzles, and a set of cooperating rotating blades, known as buckets.  
           [0003]    Each bucket includes a dovetail that is used to couple the bucket to an annular slot defined by the rotor. More specifically, each dovetail includes a recessed portion, know as a hook, that is defined by axial tangs and that enables each blade to be slidably coupled to the rotor.  
           [0004]    Each rotor slot is defined by a pair of substantially parallel retaining rings. During assembly, a first bucket dovetail is inserted into the retaining rings through a loading slot defined within the retaining rings. Adjacent buckets are also coupled to the rotor through the loading slot and slid circumferentially into position. The last bucket, known as the closure bucket, is coupled to the rotor and remains within the loading slot. All of the buckets, with the exception of the closure bucket, are coupled to the rotor by the retaining ring. Known closure buckets are coupled in position within the loading slot by a pair of shear pins which are inserted axially between the closure bucket and the circumferentially adjacent buckets. However, some rotors do not permit axial insertion of shear pins due to close stage to stage spacing.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0005]    In one aspect, a method of assembling a turbine is provided. The method comprises coupling at least one bucket assembly including an upstream side, a downstream side, a blade and a dovetail, to a rotor. The method also includes fixedly securing the bucket assembly to the rotor with a shear pin that extends from the bucket assembly upstream side to the downstream side.  
           [0006]    In another aspect, a rotor assembly for a turbine is provided. The rotor assembly comprises a plurality of bucket assemblies secured to a rotor. Each bucket assembly comprises an upstream side, a downstream side, a blade, and a dovetail. Each blade extends from each dovetail. The plurality of bucket assemblies comprise at least a first bucket assembly and at least a second bucket assembly. At least one shear pin secures the at least one first bucket assembly to the rotor such that the shear pin extends from the upstream side to the downstream side of the bucket assembly.  
           [0007]    In a further aspect, a turbine comprising at least one rotor assembly. The rotor assembly comprising at least one rotor and a plurality of bucket assemblies secured to the rotor. Each bucket assembly comprises an upstream side, a downstream side, a blade and a dovetail. The blade extends radially from the dovetail. The plurality of bucket assemblies comprises at least one first bucket assembly and at least one second bucket assembly. At least one shear pin secures the at least one first bucket assembly to the rotor such that the shear pin extends from the bucket assembly upstream side to the bucket assembly downstream side. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a partial cross-sectional schematic view of a rotor assembly;  
         [0009]    [0009]FIG. 2 is a partial perspective view of a bucket assembly coupled within the rotor assembly shown in FIG. 1;  
         [0010]    [0010]FIG. 3 is a side cross-sectional view of a closure bucket assembly that may be used with the rotor assembly shown in FIG. 1; and  
         [0011]    [0011]FIG. 4 is a front view of the rotor shown in FIG. 1, including the closure bucket assembly shown in FIG. 3 coupled in position. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    [0012]FIG. 1 is a partial cross-sectional schematic illustration of a steam turbine  10  including a rotor assembly  12  (hereafter referred to as a rotor) including a plurality of axially spaced stages  14  used to couple buckets  16  to a rotor assembly  12 . A series of nozzles  18  extend in rows between adjacent rows of buckets  16 . Nozzles  18  cooperate with buckets  16  to form a stage and to define a portion of a steam flow path indicated by the arrow that extends through turbine  10 .  
         [0013]    In operation, steam enters an inlet end (not shown) of turbine  10  and moves through turbine  10  parallel to the rotor  12 . The steam strikes a row of nozzle  18  and is directed against buckets  16 . The steam then passes through the remaining stages, thus forcing buckets  16  and rotor  12  to rotate.  
         [0014]    [0014]FIG. 2 is a perspective view of a bucket assembly  22  coupled to rotor  12  and FIG. 3 is a side cross-sectional view of a closure bucket assembly that may be used with the rotor assembly shown in FIG. 1. Bucket assembly  22  includes a platform  24 , a blade  26  extending radially outward from platform  24 , and a dovetail  28  extending radially inward from the platform  24 . Blade  26  includes a first contoured sidewall  30  and a second contoured sidewall  32 . First sidewall  30  is convex and defines a suction side of blade  26 , and second sidewall  32  is concave and defines a pressure side of blade  26 . Sidewalls  30  and  32  are joined at a leading edge  34  and at an axially-spaced trailing edge  36  of blade  26 .  
         [0015]    Platform  24  includes an upstream side  38  and an opposite downstream side  39 . In the exemplary embodiment, upstream side  38  and downstream side  39  are substantially parallel. Bucket assembly  22  has a first tangential face  40  and an opposite second tangential face  41  that each extend between upstream and downstream sides  38  and  39 . In one embodiment, upstream side  38  includes a side shoulder  42 , known as an outer tang, that extends substantially perpendicularly from upstream side  38  and defines an overhang  44 . A dovetail tang  46  also extends substantially perpendicularly from the upstream side  38  and is substantially parallel to the side shoulder  42  such that an upstream side slot  48  is defined between tang  46  and shoulder  42 .  
         [0016]    Bucket assembly downstream side  39  includes a side shoulder  50  that extends substantially perpendicularly from downstream side  39 . In an exemplary embodiment, shoulder  50  is substantially co-axially aligned with respect to upstream shoulder  42 . Side shoulder  50  defines a downstream side overhang  52 . A dovetail tang  54  also extends substantially perpendicularly from the downstream side  39  and is substantially parallel to side shoulder  50  such that a downstream side slot  56  is defined between. In the exemplary embodiment, tang  54  is substantially co-axially aligned with respect to dovetail tang  46 .  
         [0017]    Rotor  12  includes at least one annular slot  58  that facilitates coupling each bucket assembly dovetail  28  to rotor  12 . Slot  58  is defined by side slot walls  60  and  62  and a radially inward slot wall  64 . Substantially annular retaining rings  66  extend from each side slot walls  60  and  62  to retain each dovetail  28  within dovetail slot  58 . Dovetail slot  58  includes loading slot  68  used to enable radial entry of bucket assemblies  22  into dovetail slot  58 . Loading slot  68  has side slot walls  70  and  72  that do not include retaining rings  66  such that each bucket assembly dovetail  28  may be slidably coupled into dovetail slot  58  without dovetail tangs  46  or  54  contacting retaining rings  66 .  
         [0018]    After each respective bucket assembly  22  is inserted with loading slot  68 , that respective bucket assembly  22  is circumferentially slid into dovetail slot  58  such that the retaining rings  66  are disposed in each respective bucket assembly upstream and downstream side slot  48  and  56 . Additional bucket assemblies  22  are then slidably coupled to rotor  12  in a similar fashion, serially about  12 . Bucket assembly is known as a closure bucket assembly, and is inserted into loading slot  68  to facilitate securing all closure bucket assemblies  22  to rotor  12 . The closure bucket assembly is known in the art and includes a dovetail that does not include dovetail tangs  46  or  54 , but rather a substantially planar upstream sidewall and a substantially planar downstream sidewall for abutting against the loading slot walls  70  and  72  when the closure bucket is inserted into loading slot  68 . Thus, a first tangential face of the closure bucket assembly contacts a first circumferentially-spaced adjacent bucket assembly  22 , and a second tangential face of the closure bucket assembly contacts an oppositely disposed second circumferentially-spaced adjacent bucket assembly  22 .  
         [0019]    In operation, the blades  26  are urged in the radial direction by the centrifugal force exerted on them as a result of their rotation and in the tangential direction by the aerodynamic force exerted on them as a result of the fluid flow. However, the close match in the size and shape of the dovetail tangs  46 ,  54  of the bucket assembly  22  and the retaining rings  66  of the dovetail slot  58  of the rotor prevents movement of the bucket assemblies  22  in the radial and tangential directions. The blades  26  are also urged axially backward during operation by a relatively small force exerted on them by the pressure drop across the row. However, the closure bucket assembly (positioned in the loading slot  68 ) needs to be secured in the radial direction. Hence, it is necessary to restrain the closure bucket assembly in the radial direction to prevent the closure bucket  22  from being released from the loading slot  68 .  
         [0020]    The present invention provides an advantage over known shear pins, or radial oriented grub screws, which entails drilling and tapping the assembled stage of bucket assemblies and then peaning material over the screws. Drilling and tapping the grub screw holes would normally require a large machining station, such as a horizontal boring mill, and would result in causing a localized stress riser in the rotor. The insertion of axial oriented shear pins requires large stage to stage spacing and by relatively large upstream and downstream side shoulders.  
         [0021]    Closely spaced stages of bucket assemblies  22  and relatively small upstream and downstream side shoulders  42  and  50 , implementing drilling axially-orientated pins is difficult and time consuming. In addition, removing a closure bucket assembly is time-consuming which requires removing material peaned over the screw, extracting the screw and then later re-drilling the tap with a larger diameter in order to secure the closure bucket again with a different and larger diameter grub screw.  
         [0022]    A bucket assembly  22  is secured to the rotor  12  by inserting a shear pin  74  as shown in FIG. 3. The shear pin  74  having an arcuate cross-sectional profile is disposed in a channel  76 . In one embodiment, channel  76  is formed to extend generally from the upstream side  38  to the downstream side  39 . In another embodiment, channel  76  is formed to extend from the upstream side  38  having a first opening  78  to the downstream side  39  having a second opening  84 , as shown in FIG. 3.  
         [0023]    In one embodiment, a plurality of channels having an arcuate cross-sectional profile extend from the upstream side  38  to the downstream side  39  of the bucket assembly  22 . As shown in FIG. 4, a first channel  76  is formed at the interface of the first tangential face  40  of the closure bucket assembly and the dovetail  28  of the adjacent bucket assembly. A second channel  82  is formed at the interface of the second tangential face  41  of the closure bucket assembly and the dovetail  28  of the adjacent bucket assembly. Thus, the channels  76 ,  82  are partially machined in the dovetail  28  of the closure bucket assembly and partially machined in the dovetail  28  of the adjacent bucket assembly. With shear pins inserted into channel  76 ,  82 , The shear pin thereby secures the bucket assembly  22  to the adjacent bucket assemblies. Since the closure bucket assembly is secured to the adjacent bucket assemblies, the closure bucket assembly centrifugal load is taken out by the two adjacent bucket assembly dovetail tangs.  
         [0024]    In another embodiment, the channel  76  having an arcuate cross-sectional profile extends through a loading slot wall of the dovetail slot  58 , through the upstream side  38  to the downstream side  39  of the bucket assembly  22  and out through the opposing loading slot wall of the dovetail slot  58 . In an alternative embodiment, the channel  76  extends through a portion of the retaining ring  66 .  
         [0025]    In a further embodiment, at least one channel extends from a loading slot wall through the interface of an axial face of the dovetail of the closure bucket assembly and the dovetail of an adjacent bucket assembly and out to the opposing loading slot wall.  
         [0026]    If the closure bucket needs to be removed, the arcuate shear pin  74  is simply tapped on one end at the first opening  78 , thereby thrusting the other end of the shear pin out the second opening  80  of the channel  76 . The arcuate shear pin  74  is then removed thereby allowing the closure bucket assembly to be released from the loading slot  68 . Upon re-insertion of the closure bucket assembly into the loading slot  68 , the same arcuate shear pin  74  is placed into the same channel  76  to once again secure the closure bucket assembly to the rotor  12 .  
         [0027]    The above-described rotor assembly is cost-effective and time saving. The rotor assembly includes an arcuate shear pin that facilitates securing a bucket assembly to the rotor assembly, thus reducing the amount of time to remove and replace a bucket assembly. Because the shear pin may have an arcuate cross-sectional profile, the shear pin is easily removed from the channel and is more easily coupled to the closure bucket than other known shear pins. As a result, the shear pin facilitates extending a useful life of the bucket assembly in a cost-effective and a time-saving manner.  
         [0028]    Exemplary embodiments of bucket assemblies are described above in detail. The systems are not limited to the specific embodiments described herein, but rather, components of each assembly may be utilized independently and separately from other components described herein. Each bucket assembly component can also be used in combination with other bucket assembly and rotor components.  
         [0029]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.

Technology Classification (CPC): 5