Abstract:
A method for providing a disc of a bladed rotor for use in a gas turbine engine including the steps of determining the location of blade fixing slots to be formed on the disc to receive the roots of respective blades, determining a virtual profile of each slot on the disc, and forming, prior to the slots being formed in the disc, a narrow, axial pilot slot centrally of the virtual profile of each slot.

Description:
TECHNICAL FIELD 
     The application relates generally to gas turbine engines and, more particularly, to a method and apparatus for providing rotor discs designed to mount blades thereto. 
     BACKGROUND 
     Conventional gas turbine engines include rotor blades which are removably mounted to respective rotor discs. The disc and blade fixings of a rotor assembly of gas turbine engines, particularly of the high pressure turbine rotor assembly, conventionally comprise an undulating or “firtree” shaped profile in order to meet the requirements of engine performance, weight reduction, secondary air consumption, disc/blade life considerations, etc. 
     The prior art is replete with attempts to reduce or compensate for the inherent stresses in the discs, created during the disc&#39;s manufacture, the formation of the blade fixing slots and/or the placement of the blade roots in the dovetail slots. However these solutions remain complicated and somewhat lacking. 
     Accordingly, there is a need to provide an improved method and apparatus for providing such discs. 
     SUMMARY 
     There is provided a method for reducing residual stress in a disc of a bladed rotor for use in a gas turbine engine, the method comprising: determining circumferential locations of blade fixing slots to be formed on the disc, the slots being adapted to receive the roots of respective airfoil blades; determining a virtual profile of each of said blade fixing slots on the disc; and forming, prior to the blade fixing slots being formed in the disc, a narrow, axial pilot slot centrally of the virtual profile of each said blade fixing slot, the pilot slot extending radially inwardly from an outer periphery of the disc. 
     There is also provided a process for providing an annular disc utilized as a support for mounting blades in a rotor assembly for a gas turbine engine, comprising the steps of: a) determining the location of blade fixing slots to be formed in the disc; b) forming a pilot slot within the confines of each said blade fixing slot to be formed by selecting a cutting tool and passing it axially and radially through a portion of the disc circumferentially centered at each location of the blade fixing slots to be formed; c) limiting the width of each pilot slot to a relatively narrow dimension compared to the width of the blade fixing slot; and then d) cutting the blade fixing slots in the disc. 
     There is further provided a preform for an annular disc utilized as a support for mounting blades in a rotor assembly for a gas turbine engine, the preform comprising a plurality of equally spaced pilot slots extending axially and radially inwards from a periphery of the preform at circumferential locations on the preform corresponding to blade fixing slots to be formed in the disc, wherein the preform has reduced inherent stresses compared to discs of rotor assemblies in which dovetail slots have been formed without pilot slots. 
     Further details will be apparent from the detailed description and figures included below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures, in which: 
         FIG. 1  is a schematic cross-sectional view of a turbofan gas turbine engine; 
         FIG. 2  is a fragmentary cross-sectional view taken through the rotational axis of a bladed rotor including a disc and radially extending blades mounted to the disc; 
         FIG. 3  is a fragmentary front elevational view of the disc and the blade of  FIG. 2  showing a detail thereof; 
         FIG. 4  is a schematic fragmentary view in front elevation of one embodiment of a disc showing an embodiment of a pilot slot; and 
         FIG. 5  is a schematic view similar to  FIG. 4  of another embodiment of the pilot slot on a disc. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a turbofan gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan  14  mounted to a shaft  12  through which ambient air is propelled, a multistage low pressure compressor assembly  16 , a low pressure turbine section  18  with a spool assembly  20  that includes a high pressure compressor assembly  22  and a high pressure turbine assembly  24 . The air compressed by the compressor is mixed with fuel and ignited in an annular reverse flow combustor  28 , which is part of the gas generator section  26 , for generating an annular stream of hot combustion gases from which the turbine sections extract energy. 
     Referring to  FIGS. 1-3 , a rotor assembly, for example a turbine rotor assembly  30  in one rotor stage of the high pressure turbine assembly  24 , is depicted. The turbine rotor assembly  30  includes a turbine rotor disc  32  mounted on a rotating shaft of the high pressure spool assembly  20  and is rotatable about a longitudinal axis  29  of the engine, which is also the longitudinal axis of the turbine rotor assembly  30 . An array of rotor blades  34  (only one is shown in  FIG. 2 ) extend radially outwardly from the periphery of the turbine rotor disc  32 . Each of the rotor blades  34  includes an airfoil section  36 , a root section  38  and platform segments  40  extending laterally from opposed sides of the airfoil section  36  into opposing relationship with corresponding platform segments  40  of adjacent rotor blades  34 . 
     The rotor assembly  30  will now be described in greater detail with reference, in particular, to  FIGS. 2-5 . The root section  38  of each turbine rotor blade  34  includes a series of smoothly curved lateral projections preferably referred to as lobes  42 ,  44  and  46  in pairs on opposite sides thereof, extending along the length of the blade root  38 . The root section  38  of such a multi-lobed type is often referred to as a firtree, because of this characteristic shape. 
     The turbine rotor disc  32  includes a web section  33  extending radially outwardly from a hub (not shown) which is mounted to the rotating shaft of the high pressure spool assembly  20  of  FIG. 1 , and a rim section  50  extending radially outwardly from the web section  33 . Rim section  50  has an axial thickness defined by respective front and rear sides thereof (not indicated), and also defines an outer periphery  55 . 
     The turbine rotor disc  32  further includes a plurality of dovetail slots  48  (only one shown in  FIG. 3 ), circumferentially spaced apart one from another and axially extending through the periphery  55  of the turbine rotor disc  32  which in this embodiment, is the entire axial thickness of the rim section  50 . Each of the axial dovetail slots  48  includes a pair of opposed side walls each being defined in an undulating profile having substantially smoothly curved to thereby provide a profiled space defined between the opposed side walls, substantially in accordance with the firtree profile of the root section  38  of the respective turbine rotor blades  34  that are received within these dovetail slots  48 . 
     Prior to assembling the rotor assembly  30 , the disc  32  is formed by forging metal, such as forged or powder metallurgy nickel alloys for example, into an annulus with a peripheral surface  55  and a rim section  55 . The thermal treatment as well as the spinning step may create stresses within the metal that can sometimes complicate the formation of the dovetail slots  48  as well as the retention relationship with the root section  38  of the blades  34 . 
     The location and profile of each dovetail slot  48  is first virtually determined on the disc by a CNC program, and may be in practice physically marked on a surface of the disc  32 . For instance, in  FIG. 4  the firtree profile of the dovetail slot  48  is marked at  58 . A narrow slit or pilot slot  60  is then cut into the metal of the outer rim section  55 , at a point circumferentially centered within the dovetail slot  58 . A wire EDM  62  may be utilized to cut the pilot slot  60 , although other suitable cutting devises may also be used. The pilot slot  60 , so formed, is planar, is radial and the rotational axis of the disc  32  lies in the projection of the plane. The width of the pilot slot  62  is, in one particular embodiment, 0.010 inches (0.0254 cm), which is the thickness of the EDM wire used to create the pilot slot. The pilot slot  62  clearly must not extend radially inwardly beyond a bottom of the virtual firtree slot profile  58  to be formed, and in one particular embodiment does not extend radially inwards beyond a distance of 0.100 inches (0.254 cm) from the bottom of the virtual firtree profile  58 . In the embodiment depicted in  FIG. 4 , the pilot slot  62  extends radially inwardly, from the exterior surface  55  of the disc, to a point substantially near the center of the radius of curvature of the lowermost lobe of the dovetail slot  48  having the firtree profile  58 . 
     A further embodiment of the pilot slot  160  is shown in  FIG. 5  where the pilot slot  160  is in the form of a V-shaped slot. In this embodiment the virtual dovetail slot/firtree profile is shown at  158 . The V-shaped pilot slot  160  may also be cut using the wire EDM  162 . 
     It has been found that by providing pilot slots  62 ,  162  at the location of a dovetail slot  48  to be formed in the disc, that the stresses in the disc may be reduced. Once all of the pilot slots  62 ,  162  have been formed, circumferentially about the perimeter of the disc, the actual dovetail slots  48  may be cut along the outlines of the virtual firtree profiles  58 ,  158 . The roots  38  of the blades  36  can then be inserted axially into the dovetail slots  48 . In some examples, further treatment of the dovetail slots (e.g. for stress relief, etc.) may not be required. 
     Once all the pilot slots  60 ,  160  have been cut into the disc, the dovetail slots  48  can be cut or otherwise machines into the disc in the regions identified by the virtual firtree profiles. A wire EDM process may also be used to cut the dovetail slots  48  in the disc. 
     The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. For example, the pilot slots  60 ,  160  may be of different profiles. However there is no co-relationship between the shape of the pilot slots  60 ,  160  and the profiles of the dovetail slots  48 . Although wire EDM is described as suitable for providing the slot(s) in the disc, any suitable approach may be used to achieve this step. Still other modifications which fall within the scope of the present application 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 appended claims.