Abstract:
A rotor disc, such as one made of a damage intolerant material or other material sensitive to stress concentrations, has at least one balancing assembly which includes a plurality of circumferentially spaced-apart sacrificial protrusions projecting between adjacent stress-relieving slots. Selective material removal is permitted from the rotor disc, while managing stress concentrations in the rotor disc created by such material removal, such that the rotor disc may be balanced without detrimentally affecting its service life.

Description:
TECHNICAL FIELD 
       [0001]    The technical field relates generally to rotor discs and rotor disc balancing for turbomachines. 
       BACKGROUND 
       [0002]    Turbomachines often comprise rotor discs, each configured with a generally radially outer rim to which are connected a row of circumferentially-disposed blades. Rotor discs are designed to withstand the centrifugal loads developed by the blades as the rotor discs rotate at very high speeds about a central axis and also other loads resulting from forces acting on the blades during operation of the turbomachines. The strength of rotor discs is generally calculated so as to be maximized while their weight is minimized. The rotor discs are designed to withstand the various loads during their entire planned service life. 
         [0003]    The balancing of rotor discs must be done before putting them into service and also after a maintenance operation. A balancing operation is generally carried out with the blades mounted on a rotor disc, the rotor disc and the blades forming a rotor disc assembly. Various balancing techniques exist. Some involve a repositioning of the blades around the rotor discs. Others involve adding balancing weights to the rotor disc or removing material from the rotor discs, for example by machining holes therein. However, adding or removing weight on rotor discs can locally increase internal stresses during rotation, especially when high strength alloys developed for high speed rotor discs are used. These alloys have a lower damage tolerance compared to other materials and can be prone to crack propagation, for instance around holes that may be provided for attaching balancing weights or in areas where material is removed for balancing. Room for improvements thus exists. 
       SUMMARY 
       [0004]    In one aspect, the present concept provides a gas turbine rotor disc comprising a plurality of circumferentially sacrificial protrusions delimited circumferentially by stress-relieving slots disposed between and defining the protrusions, the protrusions provided in a circular array coaxially disposed with reference to a central rotation axis of the rotor disc, the protrusions projecting from a bottom end of adjacent slots to a free end, the protrusions configured to permit selective removal of a portion of the free end to thereby balance the rotor. 
         [0005]    In another aspect, the present concept provides a method of manufacturing a turbomachine rotor disc, the method comprising: providing the turbomachine rotor disc with at least one generally annular appendage coaxially disposed with reference to a central rotation axis of the turbomachine rotor disc; and machining a plurality of spaced-apart and substantially radially-extending slots in a free end of the appendage, the slots delimiting a plurality of sacrificial protrusions from which material can be removed during balancing. 
         [0006]    In a further aspect, the present concept provides a method for gas turbine rotor disc balancing comprising the steps of: providing a rotor disc having at least one balancing assembly provided substantially coaxially with reference a rotation axis of the rotor disc, the balancing assembly having a plurality of spaced-apart sacrificial protrusions extending between adjacent stress-relieving slots, bottoms of said adjacent slots defining a base end of the protrusions, each protrusion extending from its base end to a free end, the slots provided with a shape providing a stress concentration below a crack propagation threshold in a region of the slot bottoms; determining an imbalance to the rotor disc; and then remedying the imbalance by permanently removing material from the free end of at least one of the sacrificial protrusions. 
         [0007]    Further details on these and other aspects will be apparent from the detailed description and figures included below. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0008]      FIG. 1  is an isometric view showing an example of a rotor disc assembly with a rotor disc as improved; 
           [0009]      FIG. 2  is a cross-sectional view of the rotor disc alone taken along line  2 - 2  in  FIG. 1 ; and 
           [0010]      FIG. 3  is an enlarged view of the scalloped appendage shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1  is an isometric view showing an example of a turbomachine rotor disc assembly  10  designed for rotation around a central rotation axis  12 . The assembly  10  includes a rotor disc  14  to be mounted around a drive shaft (not shown). The rotor disc  14  includes a hub portion  16  having a central bore  18  through which the drive shaft is inserted. 
         [0012]    As best shown in  FIG. 2 , which is a cross-sectional view of the rotor disc  14  alone according to line  2 - 2  in  FIG. 1 , the rotor disc  14  includes a web portion  20  extending generally radially from the hub portion  16 . The rotor disc  14  also has two opposite faces  22 ,  24 . 
         [0013]    The outer periphery of the rotor disc  14  includes a rim portion  26  encircling the web portion  20 . The hub portion  16 , the web portion  20  and the rim portion  26  in the illustrated example are made integral with each other and form a monolithic piece. The monolithic rotor disc  14  can be made of a single material. Other rotor disc constructions are possible as well. 
         [0014]    The rotor disc assembly  10  shown in  FIG. 1  includes a plurality of circumferentially-disposed and radially extending blades  30  mounted in corresponding blade-receiving slots  32  provided in the rim portion  26  for receiving roots of the blades  30 . The slots  32  are designed to prevent the blades  30  from being ejected radially during rotation. Other components (not shown), such as fixing rivets, spring plates, etc., can also be provided in the rotor disc assembly  10 , depending on the design. It should be noted that blades  30  can be made integral with the rotor disc  14  in some designs, thereby forming a monolithic assembly that is sometimes called a blink. 
         [0015]    The illustrated rotor disc  14  comprises two rotor balancing assemblies  40 , in this example provided by circular and scalloped appendages  40 , one on each face  22 ,  24 . Each appendage  40  is coaxially disposed with reference to the central rotation axis  12 . Although the illustrated example shows two appendages  40 , it is possible to provide only one instead of two. The sole appendage could then be on either face  22  or face  24 . It is also possible to provide two or more appendages on one side and none or a different number on the other side. Still, any appendage on one side does not need to be identical in size and/or in shape compared to any appendage on the other side. 
         [0016]    As best shown in  FIG. 2 , each appendage  40  comprises a base portion  42  that can be integrally connected to the web portion  20 , thereby being part of the monolithic rotor disc  14 . It is also possible to provide an appendage elsewhere on the rotor disc  14 , such as on the rim portion  26  or on the hub portion  16  for instance. 
         [0017]    The base portion  42  of the appendage  40  is circumferentially continuous in the illustrated example but it is also possible to design an appendage with discrete segments individually connected to the web portion  20  or elsewhere on the rotor disc  14 . These segments would be circumferentially disposed to form together an appendage. Still, appendage(s)  40  can be connected to the rest of the rotor disc  14  without being made integral thereto. For example, an appendage could be connected by welding or gluing, by using fasteners, etc. 
         [0018]    Each appendage  40  may be configured and disposed so as to form a generally annular portion of the rotor disc  14  where internal stresses during operation of the turbomachine will be below the crack propagation threshold. In the illustrated example, the appendages  40  do not support any other portion or component and are simply freely hanging on their respective side of the rotor disc  14 . The internal stresses are thus much lower in use than those of the web portion  20 , for instance. 
         [0019]    Each appendage  40  includes a plurality of circumferentially spaced-apart sacrificial protrusions  46  at a free end thereof. These sacrificial protrusions  46  are the locations where weight can be removed from the rotor disc  14  during balancing. The sacrificial protrusions  46  project substantially axially from the base portion  42  of the corresponding appendage  40 . 
         [0020]      FIG. 3  is an enlarged view showing some of the sacrificial protrusions  46  on the scalloped appendage  40  in  FIG. 1 . The sacrificial protrusions  46  are axisymmetrically disposed with reference to the central rotation axis  12 . The sacrificial protrusions  46  are substantially identical when the rotor disc  14  is new. The size and shape of the sacrificial protrusions  46  are chosen so as to provide the possibility of balancing the rotor disc assembly  10  in the worst possible imbalance scenario. They can also be designed to provide the possibility of carrying out one or more additional balancing operations where one or more protrusions  46  will have some of their material removed even if some of it was already removed during a previous balancing. Such additional balancing operations can be required after a maintenance operation, for instance after replacing or repairing one or more blades  30 . Various techniques can be used to define the sacrificial protrusion geometry. A person skilled in the art will know how to proceed and therefore, these techniques need not be discussed in further details. 
         [0021]    The sacrificial protrusions  46  are delimited circumferentially by a plurality of stress-relieving slots  48 , provided in this example by axisymmetrically spaced-apart scallop-shaped slots  48 . These slots  48  are configured to act as stress relieving slots to prevent the internal stresses due to the material removal in the sacrificial protrusions  46  from initiating and propagating cracks to the other portions of the rotor disc  14 , as discussed further below. The slots are provided, in this example, on the radially-extending end face  44   a  at the free end  44  of the appendage  40  illustrated in  FIG. 1 . Each one of the slots  48  has an internal wall with a shape or slope minimizing the stress concentration in the bottom end  48   a  of the slot  48 . The slots  48  are designed so as to reduce the internal stresses (hoop stress) caused by the rotation of the rotor disc  14  in operation, thus allowing material removal by standards means. This arrangement mitigates the risks of crack propagation if the rotor disc  14  is made of a damage intolerant material prone to crack propagation or another material sensitive to stress concentrations. When manufacturing the rotor disc  14 , the slots  48  can be machined in the free end  44  of the appendage  40 , for instance by using a rotating tool or another technique. Each slot  48  of the illustrated example is oriented substantially radially with reference to the central axis  12 , its central axis being somewhat parallel to a radial direction. 
         [0022]    Balancing the rotor disc assembly  10  is made by removing material only from the sacrificial protrusions  46 . Material is permanently removed from one or more of the sacrificial protrusions  46  during a balancing operation using a suitable technique. For instance, one can chose to drill an axially-extending bore through one of the sacrificial protrusions  46  and/or remove surface material entirely or partially from the end face  44   a  thereof. Material removal may involve mechanical machining or non-mechanical techniques, as desired, as will be appreciated by a person skilled in the art, and therefore the material removal step needs not be discussed in further detail. Material removal may be confined to the zone axially delimited by the end face  44   a  of the appendage  40  and by a radially-extending plane coincident with the bottom ends  48   a  of the slots  48  (i.e. the deepest point of each slot  48 ), and further may be confined to a suitable distance away from said plane, indicated in  FIG. 3  by the imaginary line  50  that is closer to the free end  44   a  than from the plane defined by the bottom ends  48   a  of the slots  48 , to provide for a desired safety margin or safety zone. 
         [0023]    Balancing the rotor disc assembly  10  can require that it be rotated at a given minimum speed for evaluating if it is balanced or not. For instance, in some designs used in turbomachines, the blades  30  can be somewhat loosely fixed in their corresponding slot  32  when the assembly  10  is static and be only brought to their proper radial position when the assembly  10  is rotated at high speeds. Various techniques can be used for conducting a balancing assessment and calculate the position and the amount of material to be removed, as will be understood by a person skilled in the art, and therefore these techniques need not be discussed in further detail. Furthermore, a balancing with weight removal as presented herein does not exclude that another balancing technique be used simultaneously to compensate for a portion of the imbalance, for example a blade permutation. 
         [0024]    Overall, the above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to what is described while still remaining within the same concept. For example, the rotor disc can be different in shape from the one that is shown in the figures. The rotor balancing assembly described may be provided in any suitable manner, and need not be provided on an appendage, per se, nor be provided on a single annular device such as the appendage described. The assembly(ies) or appendage(s) may have any suitable configuration and/or shape. The protrusions not need to be a flat, nor axially extending, nor provided in and radially-extending surface. All protrusions and slots need not be configured or shaped identically. Still other modifications will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the scope of the appended claims.