Patent Abstract:
A retention system for a plurality of turbine buckets located in respective mating slots in a turbine rotor wheel includes a plurality of first circumferentially-oriented retention slots formed in outer peripheral portions of the turbine wheel; a plurality of second circumferentially-oriented retention slots formed in wheel mounting portions of said buckets, the first and second circumferentially-oriented retention slots aligned to form an annular lockwire retention slot; and a lockwire located within the annular lockwire retention slot. A first surface feature on one or both of the turbine rotor wheel and one or more of said plurality of turbine buckets is adapted to engage a second surface feature on the lockwire for preventing rotation of the lockwire beyond predetermined limits.

Full Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a retention system used to prevent axial movement of a turbine bucket dovetail in a corresponding dovetail slot in a turbine rotor wheel, and more specifically, to techniques for preventing circumferential rotation of the axial retention system. This retention system typically takes the form of a lockwire within an annular slot or groove in the turbine rotor wheel. 
     In conventional turbine and/or turbine compressor components, buckets (or blades, or airfoils) are held in a rotor wheel by means of a slotted connection, e.g., a so-called “fir tree” or “Christmas tree” arrangement where an inwardly-tapered male connector portion at the radially inner end of the bucket is received in a complementary female slot in the rotor wheel. Such connections are also generically referred to as “dovetail” connections, embracing various complementary shapes which lock the buckets to the wheel in the radial and circumferential directions so as to accommodate the high centrifugal forces generated by rotation of the turbine rotor. 
     The fit between the blade dovetail and the dovetail slot is sufficiently loose to allow for assembly and tolerances. Centrifugal loading above a certain threshold speed effectively locks up the bucket in the wheel due to the contact forces and friction. However, operation at low speed, during which the blades are able to rock inside the dovetail, can have the tendency to make the blade move along the dovetail in the absence of axial retention. If the blade is not properly retained, the eventual likely outcome is a collision with neighboring stationary components. Before such collision can take place however, the axial movement along the dovetail could effectively block cooling flow into the blade. In the absence of the cooling flow, oxidation erosion will wear away the leading edge of the blade. An additional consequence, therefore, is unplanned machine down-time and maintenance resulting from varying degrees of machine performance deterioration up to blade separation and resulting collateral or domestic object damage. 
     In accordance with usual design practice, the buckets or blades are prevented from moving axially in the dovetail slots provided in the rotor wheel by a retention device, hereafter called a “lockwire”, passing through an annular slot formed in the radially outer periphery of the wheel and passing through circumferentially-aligned slots in the dovetail portions of the respective buckets. The free ends of the wire are shaped so that they come together at an overlapped joint, thus allowing for minor changes in length and diameter of the lockwire as the rotor wheel, rotor wheel slots and buckets expand and contract during transient periods. The lockwire is held in place by the radial spring force stemming from installation of a relatively larger-diameter lockwire in a relatively smaller-diameter annular slot, and pins mounted in the turbine wheel, radially inwardly of the lockwire. It has been discovered that rotation of the lockwire within the annular slot in the rotor wheel (which occurs over time) can cause the free ends of the lockwire to separate at the overlap joint so that one end of the lockwire may engage a pin and bend downwardly (radially inwardly) below the pin and, thus permit the lockwire to escape the annular slot. 
     Without the lockwire, the airfoils are free to travel axially along the dovetail slots, creating the potential for excessive wear and interference as mentioned above. In addition, this is especially consequential in first and second stage buckets that rely on holes in the base of the bucket to provide internal cooling. When these holes are blocked due to axial movement of the bucket, cooling air cannot reach the target area and the bucket can quickly oxidize along the leading edge. 
     There remains a need for a reliable technique for preventing circumferential rotation of the lockwire within its annular slot to thereby prevent escape of the lockwire from the rotor wheel by preventing rotation of the lockwire. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one exemplary but nonlimiting embodiment, the invention relates to a retention system for a plurality of turbine buckets located in respective mating slots in a turbine rotor wheel, the retention system comprising a plurality of first circumferentially-oriented retention slots formed in outer peripheral portions of the turbine wheel; a plurality of second circumferentially-oriented retention slots formed in wheel mounting portions of the buckets, the first and second retention slots aligned to form an annular lockwire retention slot; a lockwire located within the annular lockwire retention slot, the lockwire having free ends; a first surface feature on one or both of the turbine rotor wheel and one or more of the plurality of turbine buckets; and a second surface feature on the lockwire adapted to engage with the first surface feature on one or both of the turbine rotor wheel and one or more of the plurality of turbine buckets for preventing circumferential rotation of the lockwire beyond predetermined limits. 
     In a second exemplary but nonlimiting embodiment, the invention relates to a retention system for a plurality of turbine buckets located in respective mating slots in a turbine rotor wheel, the retention system comprising a plurality of first circumferentially-oriented retention slots formed in outer peripheral portions of the turbine wheel; a plurality of second circumferentially-oriented retention slots formed in wheel mounting portions of the buckets, the first and second retention slots aligned to form an annular lockwire retention slot; a lockwire located within the annular lockwire retention slot, the lockwire having free ends; at least one axially-oriented surface feature provided on the rotor wheel or on one or more of the plurality of buckets for holding the lockwire in the annular retention slot; and at least one radially extending surface feature on the lockwire engageable with the at least one axially-oriented surface feature for preventing circumferential rotation of the lockwire beyond predetermined limits. 
     In still another nonlimiting aspect, the invention relates to a retention system for a plurality of turbine buckets located in respective mating slots in a turbine rotor wheel, the retention system comprising a plurality of first circumferentially-oriented retention slots formed in outer peripheral portions of the turbine wheel; a plurality of second circumferentially-oriented retention slots formed in wheel mounting portions of the buckets, the first and second retention slots aligned to form an annular lockwire retention slot; a lockwire located within the annular lockwire retention slot, the lockwire having free ends; at least one surface feature provided on the rotor wheel or on one or more of the plurality of buckets for holding the lockwire in the annular retention slot; and at least one axially-extending surface feature on the lockwire engageable with the at least one surface feature on the rotor wheel or on one or more of the plurality of buckets for preventing circumferential rotation of the lockwire beyond predetermined limits. 
     The invention will now be described in detail in connection with the drawings identified below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial top perspective view of a known turbine rotor wheel and bucket assembly showing a lockwire in place; 
         FIG. 2  is a partial bottom perspective view of the rotor wheel and bucket assembly shown in  FIG. 1 ; 
         FIG. 3  is a partial elevation view of overlapped free ends of a lockwire; 
         FIG. 3A  is similar to  FIG. 3  but illustrates an alternative lockwire design formed with tapered free ends; 
         FIG. 4  is a schematic representation of separated free ends of a lockwire, with one end trapped below a retaining pin; 
         FIG. 5  is a partial perspective view of an annular lockwire fitted with radially-inwardly extending anti-rotation tabs in accordance with an exemplary but nonlimiting embodiment of the invention; 
         FIG. 5A  is a schematic view of an alternative, nonlimiting lockwire configuration where a local deformation serves as an anti-rotation tab; 
         FIG. 6  is a partial perspective view of a rotor wheel with the lockwire of  FIG. 5  installed; 
         FIG. 6A  is a partial elevation in transparent format, illustrating an alternative but nonlimiting embodiment where the anti-rotation tabs extend radially outwardly of the lockwire; 
         FIG. 7  is a perspective view of a lockwire fitted with axially-extending anti-rotation tabs in accordance with another exemplary but nonlimiting embodiment of the invention; and 
         FIG. 8  is a partial perspective view of the lockwire of  FIG. 7  installed within a bucket lockwire slot. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 and 2  illustrate one technique for preventing axial movement of a turbine bucket received within a slot in a turbine rotor wheel. More specifically, the turbine rotor wheel  10  is formed with a plurality of dovetail slots  12  about the entire outer periphery of the wheel, each dovetail slot  12  receiving a complementary dovetail portion  14  of a bucket or blade  16  (only three complete slots and one bucket shown in the Figures). It will be understood that the bucket or blade  16  is of conventional construction, including a shank portion  18 , an airfoil portion  20  and the dovetail portion (or simply, dovetail)  14 . 
     The radially projecting portions  24  of the wheel which define the slots  12  are formed with first lockwire slots  26 , each closed at its radially outer end  28  and open at its radially inner end  30 . The first lockwire slots  26  are formed adjacent to one side of the wheel, and together, form an annular 360° slot about the periphery of the wheel, interrupted by the dovetail slots  12 . Axially offset portions (or lock tabs)  32  of the bucket dovetails  14  define a plurality of second lockwire slots  34  that are alignable with the first lockwire slots  26  upon introduction of the buckets  16  into the dovetail slots  12 . A lockwire  36  (preferably a suitable metal alloy) may then be introduced into the aligned lockwire slots  26 ,  34  with free ends  38 ,  40  shaped (e.g., reduced to a semi-circular cross section) to smoothly overlap each other along opposed surfaces  39 ,  41  in a normally-installed condition ( FIG. 3 ), recognizing that the opposed surfaces are substantially flat when the lockwire is uncoiled and arcuate when installed in the annular slots  26 ,  34 . The lockwire itself may be a single strand or multiple connected or overlapping segments. Axially-oriented retaining pins  42  inserted through the portions of the rotor wheel  10  are employed to hold the lockwire  36  within the lockwire slots  26  ( FIGS. 1 and 2 ). 
       FIG. 4  illustrates a problem experienced with the lockwire configuration as described above. Specifically, it has been found that the lockwire  36  is prone to circumferential rotation during turbine operation due perhaps to thermal and/or mechanical ratcheting. Resulting separation of the free ends  38 ,  40  of the lockwire can result in one end (the trailing end in the direction of lockwire rotation) travelling below (i.e., radially inwardly) of one of the pins  42  so that during lockwire rotation in the direction shown by arrow  44 , the lockwire  36  may escape the lockwire slots  26 ,  34 , thereby permitting axial movement of the buckets  16  within the dovetail slots  12 . 
       FIGS. 5 and 6  illustrates an exemplary but nonlimiting embodiment of a lockwire  46  (or other equivalent surface feature) provided with radially inwardly extending tabs  48  for substantially preventing excessive circumferential rotation of the lockwire  46  when installed in the lockwire slots  26 ,  34  ( FIG. 6 ), as described further below. The end result is that the inner and outer free ends (similar to free ends  38 ,  40  in  FIG. 3  but not shown in  FIG. 4 ), of the lockwire  46  are prevented from excessive circumferential rotation which might otherwise lead to one free end moving below or radially inward of the retaining pins  42  as shown in  FIG. 4 . 
     The lockwire  46 , like the lockwire  36 , may have a round cross section with an appropriately chosen diameter, and the free ends  36 ,  38  are each also reshaped to a smaller cross section (e.g., semi-circular) than the remaining major length of the lockwire to provide an overlap region of substantially the same profile as the remainder of the lockwire, with the free ends engaged along opposed substantially flat, circumferentially (or horizontally)-oriented surfaces as shown in  FIG. 3 . The opposed surfaces at the overlap may also be wedge-shaped or tapered as shown at  39 A and  41 A in  FIG. 3A . The ends of the lockwire  46  may also be formed on a slightly larger diameter than the remainder of the lockwire, which is otherwise formed to substantially match the diameter of the lockwire slot. This results in a tighter engagement of the overlapped free ends. 
     The lockwire  46  may also be formed with other cross-sectional shapes such as oval, elliptical, semi-circular or other suitable shape. 
     The lockwire  46  is provided with at least one and preferably between 2 and 4 or more of the radially extending tabs  48  having thicknesses less than the diameter of the lockwire. For example, lockwire diameters of 0.188″, 0.250″, and 0.300″ may have tab thicknesses of substantially half the given diameters. The length, width, thickness and shape of the tabs  48  (or other functionally equivalent surface features added to the lockwire) may vary depending on specific applications as dictated by the available space or load carrying capability required by the intended application. In most cases, the size of the tabs  48  (or other surface features) will be the minimum size that performs the desired function, i.e., stopping any undesirable (i.e., excessive) circumferential rotation of the lockwire by engagement of the tabs (or other surface features) with respective, next-adjacent retaining pins. 
     The anti-rotation tabs  48  are preferably welded or brazed to the lockwire, but the invention is not limited to any particular securement or forming technique. For example, the tabs  48  or other surface features may be attached to the lockwire by casting, forging, welding, brazing, or by any other suitable mechanical attachment. The tabs may also be in the form of sheet material bent about the lockwire and secured by any of the above techniques. The tabs may also be machined or otherwise made integral with the wire. The “tab” may also be formed by one or more local deformations in the lockwire. One example is shown in  FIG. 5A , where a bend  47  creates a tab  49  that will engage the pin  42  in a manner similar to the tab  48 . In addition, the number and location of the tabs (or other surface features) relative to the retaining pins may vary. For example,  FIG. 4  shows a retaining pin  42  circumferentially between a pair of radially inwardly extending tabs  48  so that rotation in either direction will be halted when the pin  42  is engaged by one of the tabs  48 . While some rotation of the lockwire is permitted to accommodate, for example thermal growth, circumferential rotation beyond predetermined limits is prevented. It is also possible to mount the tabs  48  such that two tabs  48  lie, respectively, on opposite sides of two adjacent pins  42  (see the dotted line pins  142  to the outside of adjacent tabs  48 ). The number of tabs  48  (or other surface features) on the lockwire may vary between one and more than four, but it is preferable (but not required) that the tabs or other surface features be located substantially mid-way between the free ends of the lockwire. In addition, the pins  42  need not be of the shape illustrated in the drawings. Other axially extending surface features on the rotor wheel or in the buckets may be used to engage one or more of the tabs  48  or other surface features on the lockwire to prevent circumferential rotation of the lockwire. 
     It will be appreciated that the tabs  48  (or other surface features) may also extend radially outwardly of the lockwire, as illustrated, for example, in  FIG. 6A .  FIG. 6A  is a transparency showing a tab  48 A extending radially outwardly of the lockwire  46 A, and received in an opening  50  formed in the dovetail portion  52  of the bucket  54 . 
     It is also within the scope of the invention to have axially extending tabs or other surface features on the lockwire that, upon minimal rotation of the lockwire, will engage a hole or slot or other surface feature formed in the adjacent slot wall of the bucket or turbine wheel. For example,  FIG. 7  illustrates a lockwire  56  provided with one or more axially-extending tabs  58  sized, shaped and located to engage a hole, slot or other surface feature provided in the rotor wheel or bucket.  FIG. 8  shows one example where the lockwire  56  of  FIG. 7  is installed in the annular groove  60  (shown only with respect to the single bucket  62 ) such that the axially-extending tab  58  is loosely received within a radially extending slot  64  formed in the bucket dovetail  66  that opens into the annular groove  60 . In this way, the lockwire  56  is prevented from excessive circumferential rotation that might otherwise allow escape of the lockwire  56  from the annular slot or groove  60 . It will be appreciated that the axially-extending tab (or other surface feature)  58  may also vary in size, shape and number as described above in connection with the tab(s)  48 , and that the tab  58  may extend axially from either side of the lockwire depending on the location of a hole, groove, notch or other surface feature within the annular or circumferential slot or groove  60  in the bucket (or turbine wheel) with which it cooperates to prevent circumferential rotation of the lockwire. 
     In all cases, the amount of lockwire rotation is limited to the extent that separation of the overlapped free ends of the lockwire is precluded. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Technology Classification (CPC): 5