Abstract:
A balance weight for a turbine rotor includes: a block-like centerbody; a pair of resilient spring arms extending laterally from opposite sides of the centerbody, the centerbody and the spring arms collectively defining an arcuate shape; at least one locating structure extending from a radially outer surface of the balance weight; and a limit tab extending radially inward from a distal end of each of the spring arms.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation-In-Part of application Ser. No. 12/485,122 filed Jun. 16, 2009, which is currently pending. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates generally to rotating machinery and more particularly to apparatus for balancing rotors. 
         [0003]    Gas turbine engines typically include several rotor stages, each having a rotor disk carrying an array of airfoils, i.e., compressor or turbine blades. Turbine rotors must be balanced to prevent damage and excessive loads on bearings and supporting structures, as well as efficiency losses caused by loss of clearance between the airfoils and the surrounding structure (caused by, e.g., shroud rubs). 
         [0004]    Despite efforts to first balance their constituent components, turbine rotors still require dynamic balancing following assembly. For this purpose, it is desirable to use balance weights that can be re-positioned to redistribute the mass of the rotor as needed and allow the system unbalance to be fine-tuned to meet precise requirements. Separable balance weights are a common practice in larger gas turbine engines. These include bolts, washers, nuts and other fasteners of varying sizes. 
         [0005]    In some gas turbine rotors, notably those in smaller engines, CURVIC couplings and friction joints are assembled using a single bolt or a group of bolts (referred to as a “tie rod” or “tie bolts”) spanning the length of the assembly. A tie bolt configuration weighs less than a conventional bolted joint, but the absence of bolt holes eliminates convenient features on the rotor disk which could otherwise be used to attach separable balance weights. Accordingly, the current state of the art for smaller turbine engines is to balance the assembly by selectively machining a sacrificial surface on the rotating part. Material is removed at the location of peak unbalance to redistribute the mass of the rotor about the axis of rotation. This process is irreversible and risks damaging a component such as an integrally-bladed rotor or “blisk”, which is both safety-critical and expensive. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    These and other shortcomings of the prior art are addressed by the present invention, which provides a trapped spring balance weight for a turbine rotor. 
         [0007]    According to one aspect of the invention, a balance weight for a turbine rotor includes: a block-like centerbody pair of resilient spring arms extending laterally from opposite sides of the centerbody, the centerbody and the spring arms collectively defining an arcuate shape; at least one locating structure extending from a radially outer surface of the balance weight; and a limit tab extending radially inward from a distal end of each of the spring arms. 
         [0008]    According to another aspect of the invention a turbine rotor assembly includes: a rotor element including an annular hub surface and an annular flange surrounding the hub surface, spaced away from the hub surface so as to define a pocket; and at least one balance weight disposed in the pocket, including: a block-like centerbody; a pair of resilient spring arms extending laterally from opposite sides of the centerbody, the centerbody and the spring arms collectively defining an arcuate shape; at least one locating feature extending radially outward from the balance weight; and a limit tab extending radially inward from a distal end of each of the spring arms; wherein the spring arms and the centerbody resiliently bear against the flange and the hub surface, respectively, so as to retain the balance weight in the pocket. A radial height of the limit tabs is selected so as to prevent insertion of the balance weight into the pocket if the spring arms are deflected beyond a predetermined limit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0010]      FIG. 1  is a cross-sectional view of a gas turbine engine constructed in accordance with an aspect of the present invention; 
           [0011]      FIG. 2  is an enlarged view of the forward portion of the compressor of the engine shown in  FIG. 1 ; 
           [0012]      FIG. 3  is an enlarged view of the aft portion of the compressor of the engine shown in  FIG. 1 ; 
           [0013]      FIG. 4  is a perspective view of a balance weight constructed according to an aspect of the present invention; 
           [0014]      FIG. 5  is a rear elevational view of the balance weight of  FIG. 4 ; 
           [0015]      FIG. 6  is a perspective view of the balance weight of  FIG. 4  installed in a rotor disk of the engine of  FIG. 1 ; 
           [0016]      FIG. 7  is a front view of a spanner tool for use with a balance weight; 
           [0017]      FIG. 8  is a side view of the spanner tool of  FIG. 7 ; 
           [0018]      FIG. 9  is a rear view of the spanner tool of  FIG. 7 ; 
           [0019]      FIG. 10  is a view of the spanner tool of  FIG. 7  in use; 
           [0020]      FIG. 11  is a perspective view of a balance weight constructed according to another aspect of the present invention; 
           [0021]      FIG. 12  is a rear elevational view of the balance weight of  FIG. 11 ; 
           [0022]      FIG. 13  is a perspective view of the balance weight of  FIG. 11  installed in the engine of  FIG. 1 ; 
           [0023]      FIG. 14  is a cross-sectional view of portion of a compressor of a gas turbine engine, with a balance weight installed therein; 
           [0024]      FIG. 15  is a perspective view of the balance weight of  FIG. 14 ; 
           [0025]      FIG. 16  is a rear elevational view of the balance weight of  FIG. 15 ; and 
           [0026]      FIG. 17  is a perspective view of the balance weight of  FIG. 15  in an installed condition. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  depicts an exemplary gas turbine engine  10  having a compressor  12 , a combustor  14 , a high pressure or gas generator turbine  16 , and a work turbine  18 , all arranged in a serial flow relationship. Collectively the compressor  12 , the combustor  14 , and the gas generator turbine  16  are referred to as a “core”. The compressor  12  provides compressed air that passes into the combustor  14  where fuel is introduced and burned, generating hot combustion gases. The hot combustion gases are discharged to the gas generator turbine  16  where they are expanded to extract energy therefrom. The gas generator turbine  16  drives the compressor  12  through an impeller shaft  20 . Pressurized air exiting from the gas generator turbine  16  is discharged to the work turbine  18  where it is further expanded to extract energy. The work turbine  18  drives an inner shaft  22 . 
         [0028]    In the illustrated example, the engine is a turboshaft engine, and the inner shaft  22  would be coupled to an external load such as a reduction gearbox or propeller. However, the principles described herein are equally applicable to turboprop, turbojet, and turbofan engines, as well as turbine engines used for other vehicles or in stationary applications. These principles are also applicable to any other type of rotating machinery (e.g. wheels, gears, shafts, etc.) which require balancing. 
         [0029]    In the illustrated example, the compressor  12  includes five axial-flow rotor stages and one mixed-flow stage which is positioned immediately upstream of the combustor  14 . As best seen in  FIG. 2 , the first stage rotor  24  of the compressor  12  is an integrally-bladed rotor or “blisk” in which a rotor disk  26  and a plurality of airfoil-shaped compressor blades  28  are formed as one integral component. The aft end of the rotor disk  26  includes an annular hub surface  30  and an annular flange  32  extending over the hub surface  30 . Together, the hub surface  30  and the flange  32  define a pocket  34  (best seen in  FIG. 6 ). An inner surface  36  of the flange  32  has an array of grooves  38  formed therein (again, see  FIG. 6 ). 
         [0030]    As seen in  FIG. 3 , the final stage of the compressor  12  includes a rotor disk  40  which carries a plurality of blades  42 . The annular impeller shaft  20  extends axially aft from the rotor disk  40 . The intermediate section of the impeller shaft  20  includes an annular hub surface  46  and an annular flange  48  extending over the hub surface  46 . Together, the hub surface  46  and the flange  48  define a pocket  50  (best seen in  FIG. 13 ). The flange  48  includes an annular array of apertures formed therein. In the illustrated example, as seen in  FIG. 13 , this array comprises open-ended slots  52  alternating with holes  54 . 
         [0031]    One or more forward balance weights  60  are installed in the pocket  34  of the first stage rotor  24 , and one or more aft balance weights  160  are installed in the pocket  50  of the impeller shaft  20 . The exact number, position, and distribution of weights will vary by individual engine. In the particular engine illustrated, only two balance weights are used. Correction of rotor imbalance is accomplished by re-positioning the weights as needed. 
         [0032]      FIGS. 4 and 5  illustrate one of the forward balance weights  60  in more detail. It is generally arcuate in shape and comprises a block-like centerbody  62  with resilient spring arms  64  extending laterally outward therefrom. A notch  66  is formed in the radially inner end of the centerbody  62 . At the distal end of each spring arm  64 , an axially-elongated rail  68  extends radially outward. Opposite each rail  68 , a stop block  70  extends radially inward. The forward balance weights  60  may be constructed from any material with an appropriate density and the ability to form the spring arms which can deflect elastically. For example, metal alloys may be used. 
         [0033]    With reference to  FIG. 6 , the forward balance weights  60  are installed into the first stage rotor  24  as follows. The spring arms  64  are deflected radially inward relative to the centerbody  62 . They may be held in this position by an appropriate tool or jig. Then the forward balance weight  60  is slid axially into the pocket  34 , at the appropriate position. The spring arms  64  are then released. After release, the residual spring force urges the spring arms  64  radially outward against the flange  32  and urges the centerbody  62  against the hub surface  30 . The rails  68  engage the grooves  38  in the inner surface of the flange  32  to prevent tangential movement. A mating component (in this case the forward end of an annular shaft  72 , seen in  FIG. 2 ) abuts the notch  66  to prevent axial movement of the forward balance weight  60 .  FIG. 6  shows one of the forward balance weights  60  in an installed condition. During engine operation, centrifugal loading reseats the forward balance weights  60  against the flange  32 . 
         [0034]    If necessary as indicated by a balancing operation, the forward balance weights  60  can be repositioned circumferentially while the compressor  12  is assembled, for example through use of a spanner-wrench tool. For example,  FIGS. 7-9  illustrate a suitable tool  74  which has an elongated handle  76  and a curved head  78  with spanner fingers  80  extending radially inward and laterally outward from its distal ends. As shown in  FIG. 10 , the tool  74  is inserted into the pocket  34  and used to deflect the spring arms  64  radially inward, disengaging the rails  68  from the grooves  38 . The tool  74  may then be moved tangentially in the direction of the arrows, causing the spanner fingers  80  to contact the forward balance weight  60  and push it to a new position. Once the tool  74  is removed, the rails  68  re-engage grooves  38  at the new location. During this operation, the stop blocks  70  contact the annular shaft  72  if an attempt is made to deflect the spring arms  64  too far. This prevents permanent deformation of the spring arms  64 . 
         [0035]      FIGS. 11 and 12  illustrate one of the aft balance weights  160  in more detail. It is generally arcuate in shape and comprises a block-like centerbody  162  with resilient spring arms  164  extending laterally outward therefrom. An anti-rotation lug  166  extends radially outward from the centerbody  162 . At the distal end of each spring arm  164 , a shear pin  168  extends radially outward. Opposite each shear pin  168 , a stop block  170  extends radially inward. A forward face  172  of the aft balance weight  160  has a convex contour complementary to the cross-sectional profile of the pocket  50  in the impeller shaft  20 . The aft balance weights  160  may be constructed from any material with an appropriate density and the ability to form the spring arms which can deflect elastically. For example, metal alloys may be used. 
         [0036]    As seen in  FIG. 13 , the aft balance weights  160  are installed using a method similar to that for the forward balance weights  60 , as follows. The spring arms  164  are deflected radially inward relative to the centerbody  162 , as shown by the arrows in  FIG. 12 . They may be held in this position by an appropriate tool or jig. Then the aft balance weight  160  is slid axially into the pocket  50 , at the appropriate position. The stop blocks  170  are sized and shaped so as to prevent insertion into the pocket  50  if the spring arms  164  are deflected too far, and thus prevent permanent deformation of the spring arms  164 . The spring arms  164  are then released. After release, the residual spring force urges the spring arms  164  radially outward against the flange  48  and urges the centerbody  162  against the hub surface  46 . The anti-rotation lug  166  engages one of the slots  52  in the flange  48 . The shear pins  168  engage the holes  54  in the flange  48  to prevent axial movement.  FIG. 13  shows one of the aft balance weights  160  in an installed condition. During engine operation, centrifugal loading reseats the aft balance weights  160  against the flange  48 . If necessary, the aft balance weights  160  can be removed and re-positioned while the compressor rotor is assembled, without any unique jigs or tools. 
         [0037]    While the balance weights  60  and  160  have described as “forward” and “aft” weights, it will be understood that these terms are used merely for convenience in description of a particular embodiment. Depending upon the specific engine application and the mating hardware, either design could be used on the forward or aft face of a turbine rotor disk or shaft. Furthermore, the anti-rotation and axial restraint features could be modified or used in different combinations to produce a balance weight suitable for a particular application. 
         [0038]      FIG. 14  illustrates a portion of a compressor section of a gas turbine engine, similar in operating principle to the engine  10  described above. A first stage rotor  224  in the compressor section is an integrally-bladed rotor or “blisk” in which a rotor disk  226  and a plurality of airfoil-shaped compressor blades  228  are formed as one integral component. The aft end of the rotor disk  226  includes an annular hub surface  230  and an annular flange  232  extending over the hub surface  230 . Together, the hub surface  230  and the flange  232  define a pocket  234  (best seen in  FIG. 17 ). An inner surface  236  of the flange  232  has an array of grooves  238  formed therein (again, see  FIG. 17 ). 
         [0039]    One or more balance weights  260  are installed in the pocket  234  of the first stage rotor  224 . The exact number, position, and distribution of weights will vary by individual engine. Correction of rotor imbalance is accomplished by re-positioning the weights as needed. 
         [0040]      FIGS. 15 and 16  illustrate one of the balance weights  260  in more detail. It is generally arcuate in shape and comprises a block-like centerbody  262  with resilient spring arms  264  extending laterally outward therefrom. A notch  266  is formed in the radially inner end of the centerbody  262 . At the distal end of each spring arm  264 , an axially-elongated rail  268  extends radially outward. Opposite each rail  268 , a stop block  270  extends radially inward. A limit tab  271  extends radially inward from each stop block  270 . The balance weights  260  may be constructed from any material with an appropriate density and the ability to form the spring arms which can deflect elastically. For example, metal alloys may be used. 
         [0041]    With reference to  FIG. 17 , the balance weights  260  are installed into the first stage rotor  224  as follows. The spring arms  264  are deflected radially inward relative to the centerbody  262 . They may be held in this position by an appropriate tool or jig. Then the balance weight  260  is slid axially into the pocket  234 , at the appropriate position. The radial height “H” of each limit tab  271  relative to the stop block  270  (see  FIG. 15 ) is selected to prevent deflection of the spring arms  264  beyond a predetermined limit. More specifically, the height H is set such that the limit tab  271  will interfere with the hub surface  230  before the spring arm  264  can be deflected enough to cause plastic deformation thereof After insertion, the spring arms  264  are released. After release, the residual spring force urges the spring arms  264  radially outward against the flange  232  and urges the centerbody  262  against the hub surface  230 . The rails  268  engage the grooves  238  in the inner surface of the flange  232  to prevent tangential movement. A mating component (in this case the forward end of an annular shaft  272 , seen in  FIG. 14 ) abuts the notch  266  to prevent axial movement of the balance weight  260 .  FIG. 17  shows one of the balance weights  260  in an installed condition. During engine operation, centrifugal loading reseats the forward balance weights  260  against the flange  232 . The balance weights  260  may be repositioned as described above for the balance weights  60  and  160 . It is also noted that the limit tab feature described with respect to the balance weights  260  may be incorporated in the balance weights  60  or  160 . 
         [0042]    The balance weight design described herein has several advantages over the current state-of-the-art for small engines. Process control is improved compared to material removal directly from the first stage rotor  24 , which introduces local stress concentrations on highly stressed critical rotating parts. Any stress concentration features present on the balance weights 60, 160, or 260 would be generated using precision machining techniques and are therefore more well controlled. Engine cleanliness is also enhanced, as the balance weights do not require any machining at engine assembly and therefore do not create dust or grit that could contaminate the engine system. Finally, cycle time for the balancing process is reduced, because the balance weights can be easily re-positioned while the rotor is loaded in a balance machine, eliminating the re-work loop associated with a material removal balancing process. 
         [0043]    The foregoing has described balance weights for a turbine rotor and a balanced rotor assembly. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.