Abstract:
A method of assembling a turbine comprises coupling at least one bucket to a rotor wheel. The bucket includes an upstream side, a downstream side, an airfoil extending therebetween and a dovetail extending radially inwardly from the airfoil to the rotor wheel. The method further comprises fixedly securing the bucket to the rotor wheel with a locking pin that extends from the bucket upstream side through the bucket dovetail to the bucket downstream side.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to turbine engines and more particularly to methods and apparatus for securing airfoils used within turbine engines. 
     At least some known turbine rotor assemblies include a rotor wheel to which a plurality of rows of airfoils are coupled. The airfoils are arranged in axially-spaced stages extending circumferentially around the rotor. Each stage includes a set of stationary airfoils or nozzles, and a set of cooperating rotating airfoils, known as buckets. 
     Each bucket includes a dovetail that is used to couple the bucket to a matching dovetail slot defined by the rotor wheel. More specifically, each dovetail includes a recessed portion, known as a hook, that is defined by axial tangs, and that enables each airfoil to be slidably coupled to the machined rotor wheel dovetail. 
     Each rotor wheel dovetail slot is defined by a pair of parallel lands that are machined as part of the slot. During assembly, several bucket dovetails are inserted onto the dovetail lands through a loading slot defined within the rotor wheel dovetail groove lands. The closure bucket is then into the loading to provide a stabile surface to allow coupling of the bucket dovetail to the dovetail lands against the crushing surfaces. Once several buckets are coupled into the proper location, the locking bucket is removed. The coupled buckets now provide the stabile surface for additional buckets. Adjacent buckets are coupled to the rotor wheel through the loading slot and slid circumferentially into position and secured in place with a mounting pin. All of the buckets, with the exception of the closure bucket, are coupled to the rotor wheel by the machined lands. 
     In operation, the buckets are urged in the radial direction by the centrifugal force induced during rotation, and are also urged in the tangential direction by the aerodynamic force exerted on them by fluid flow. The dovetail tangs of the bucket cooperate with the rotor wheel lands by contact at the crushing surfaces to facilitate preventing movement of the buckets in the radial and tangential directions. However, because the closure bucket is positioned in the loading slot, the land portion of the wheel dovetail does not inhibit radial movement of the closure bucket and as such, it is necessary to restrain the closure bucket in the radial direction to prevent the closure bucket from being released from the loading slot during operation. Known closure buckets are coupled in position by a pair of grubscrews, which are inserted between the closure bucket and the circumferentially adjacent buckets. Inserting known grubscrews can be a time-consuming and laborious task that may require a relatively large machining station, such as a horizontal boring mill. During insertion of the grubscrews localized stress may be induced to the rotor assembly. Furthermore, if maintenance is required, removing the closure bucket may also be very time-consuming and requires the removal of a material peaned over the pins. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, a method of assembling a turbine is provided. The method comprises coupling at least one bucket including an upstream side, a downstream side, an airfoil and a dovetail, to a rotor wheel. The method also comprises fixedly securing at least one bucket to the rotor wheel with a locking pin that extends from the bucket upstream side through the bucket dovetail to the downstream side. 
     In another aspect, a rotor assembly for a turbine is provided. The rotor assembly includes a plurality of buckets secured to a rotor wheel. Each bucket includes an upstream side, a downstream side, an airfoil, and a dovetail. Each airfoil extends from each dovetail. The plurality of buckets include at least a first bucket and at least a second bucket. At least one locking pin secures the at least one first bucket to the rotor wheel and extends from the upstream side of the bucket through the bucket dovetail to the downstream side of the bucket. 
     In a further aspect, a turbine including at least one rotor assembly is provided. The rotor assembly includes at least one rotor wheel and a plurality of buckets secured to the rotor wheel. Each bucket includes an upstream side, a downstream side, an airfoil and a dovetail. The airfoil extends radially from the dovetail. The plurality of buckets include at least one first bucket and at least one second bucket. At least one locking pin secures the at least one first bucket to the rotor wheel such that the locking pin extends from the bucket upstream side through the bucket dovetail to the bucket downstream side. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cross-sectional schematic view of a reaction steam turbine; 
     FIG. 2 is a partial perspective view of a bucket coupled within the rotor dovetail shown in FIG. 1; 
     FIG. 3 is a side cross-sectional view of a closure bucket that may be used with the rotor assembly shown in FIG. 1; 
     FIG. 4 is a front view of the closure bucket shown in FIG. 3; and 
     FIG. 5 is a front view of an alternative embodiment of a closure bucket that may be used with the rotor assembly shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a partial cross-sectional schematic illustration of a reaction steam turbine  10  including a drum rotor assembly  12  (hereafter referred to as a rotor) including a plurality of axially spaced wheels  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 . Nozzles  18  are coupled to a carrier dovetail  20  that extends between adjacent wheels  14  of the stages of turbine  10 . 
     In operation, high-pressure steam enters an inlet end (not shown) of turbine  10  and moves through turbine  10  parallel to the axis of rotor  12 . The steam strikes a row of nozzles  18  and is directed against buckets  16 . The steam then passes through the remaining stages, thus forcing buckets  16  and rotor  12  to rotate. 
     FIG. 2 is a perspective view of a bucket  22  coupled to rotor  12  and FIG. 3 is a side cross-sectional view of a closure bucket that may be used with the rotor assembly shown in FIG.  1 . FIG. 4 is a front view of a bucket  22  coupled by locking pin  23 . Bucket  22  includes a platform  24 , an airfoil  26  extending radially outward from platform  24 , and a dovetail  28  extending radially inward from the platform  24 . Airfoil  26  includes a first contoured sidewall  30  and a second contoured sidewall  32 . First sidewall  30  is convex and defines a suction side of airfoil  26 . Second sidewall  32  is concave and defines a pressure side of airfoil  26 . Sidewalls  30  and  32  are joined at a leading edge  34  and at an axially-spaced trailing edge  36  of airfoil  26 . 
     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  22  has a first axial face  40  and an opposite second axial face  41  that each extend between upstream and downstream sides  38  and  39 . 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 upstream side  38  and is substantially parallel to side shoulder  42 , such that an upstream side slot  48  is defined between tang  46  and shoulder  42 . 
     Bucket 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 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 . 
     Rotor  12  includes at least one annular slot  58  that facilitates coupling each bucket dovetail  28  to rotor  12 . Slot  58  is defined by side slot walls  60  and  62  and a radially inward slot wall  64 . Machined dovetail lands  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 tangential entry of buckets  22  into dovetail slot  58 . Loading slot  68  has side slot walls  70  and  72  that do not include machined dovetail lands  66  such that each bucket dovetail  28  may be slidably coupled into dovetail slot  58  without dovetail tangs  46  or  54  contacting machined dovetail lands  66 . 
     A bucket  73 , known as a closure bucket, is inserted into loading slot  68  to facilitate retaining all buckets  22  to rotor  12 . Closure bucket  73  is known in the art and includes dovetail  28  that does not include dovetail tangs  46  or  54 , but rather includes substantially planar upstream sidewall  77  and a substantially planar downstream sidewall  79  that, as described in more detail below, abut substantially flush against loading slot walls  70  and  72  when closure bucket  73  is inserted into loading slot  68 . 
     More specifically, during assembly of rotor  12 , each respective bucket  22  is inserted into loading slot  68 , and then circumferentially slid through slot  58  such that machined dovetail lands  66  are disposed in each respective bucket upstream and downstream side slot  48  and  56 . Additional buckets  22  are then slidably coupled to rotor wheel  12  in a similar fashion about wheel  12 . Closure bucket  73  is then inserted into loading slot  68  to facilitate securing closure buckets  22  to rotor  12 . More specifically, when inserted into slot  68 , a first face (not shown) of closure bucket  73  contacts a first circumferentially-spaced adjacent bucket  22 , and a second face (not shown) of closure bucket  73  contacts an oppositely disposed, second circumferentially-spaced adjacent bucket  22 . 
     Closure bucket  73  is then secured to rotor  12  by inserting a locking pin  23  from bucket upstream side  38  through bucket dovetail  28  to bucket downstream side  39 . FIG. 4 is a front view of a bucket coupled by locking pin  23 . Locking pin  23  is substantially elongate, and is disposed in a channel  76  having a substantially linear cross-sectional profile. Locking pin  23  secures closure bucket  73  to rotor  12 . Channel  76  extends generally from bucket upstream side  38  to bucket downstream side  39 . 
     In the exemplary embodiment, channel  76  extends from a first opening  78  defined in side slot wall  70  through bucket upstream side  38 , and through bucket dovetail  28 , to bucket downstream side  39 , and an opening  80  defined in downstream side slot wall  72 . 
     In an alternative embodiment, channel  76  extends from first opening  78  through bucket upstream side  38 , through bucket dovetail  28 , to bucket downstream side  39 . More specifically, locking pin  23  is inserted into the channel  76  and through bucket dovetail  28  until it contacts downstream side slot wall  72 . Locking pin  23  is then secured within channel  76  by peaning channel opening  78 . 
     FIG. 5 is a front view of an alternative embodiment of a closure bucket  222  that may be used with the rotor assembly shown in FIG.  1 . Closure bucket  222  is substantially similar to closure bucket  73  (shown in FIGS. 3 and 4) and components in closure bucket  222  that are identical to components of closure bucket  73  are identified in FIG. 5 using the same reference numbers used in FIGS. 3 and 4. Specifically, closure bucket  222  is identical to closure bucket  73  with the exception that closure bucket  222  includes a plurality of channels  76  and  224  that extend from bucket upstream side  38  through a portion of bucket dovetail  28  to bucket downstream side  39 . 
     First channel  76  is formed at the interface of the first axial face  40  of closure bucket  222  and the adjacent bucket dovetail  226 . Second channel  224  is formed at the interface of the second axial face  41  of closure bucket  222  and the adjacent bucket dovetail  228 . Thus, channels  76  and  224  are partially reamed in dovetail  28  of closure bucket  222  and partially reamed in each adjacent bucket dovetails  226  and  228 . When locking pins  23  are inserted into channels  76  and  226 , locking pins  23  secure closure bucket  222  to adjacent bucket dovetails  226  and  228 . Since closure bucket  222  is secured to bucket dovetails  226  and  228 , the centrifugal load induced to closure bucket  222  is carried by the two adjacent bucket dovetails  226  and  228 . In one embodiment, locking pin  23  is fabricated from a hardened material. 
     Each channel  76  and  224  extends generally from bucket upstream side  38 , through bucket dovetail  28 , to bucket downstream side  39 . In another embodiment, each channel  76  and  224  extends from bucket upstream side  38  through bucket dovetail  28  to bucket downstream side  39  obliquely relative to either first and/or second axial face  40  and  41 , respectively. 
     If closure bucket  222  needs to be removed, locking pin  23  has a tapped hole on bucket upstream side  38 . A slide hammer screws into the tapped hole in bucket upstream side  38  of locking pin  23  and the slide hammer pulls the locking pin  23  out. When locking pin  23  is removed, closure bucket  222  is released from loading slot  68 . Upon re-insertion of closure bucket  222  into loading slot  68 , the same locking pins  23  may be reinserted into the same channels  76  and  224  such that closure bucket  222  is secured to rotor  12 . 
     The above-described rotor assembly is a cost-effective and time saving device. The rotor assembly includes a re-usable locking pin that facilitates securing a basket assembly to the rotor assembly, thus reducing an amount of time necessary to remove and replace a bucket. Furthermore, the locking pin is more easily removably coupled to the closure bucket than other known locking pins. As a result, the locking pin facilitates extending a useful life of the bucket in a cost-effective and a time-saving manner. 
     Exemplary embodiments of buckets 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 component can also be used in combination with other bucket and rotor components. 
     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.