Abstract:
A turbine rotor for a gas turbine engine including a disc having a hub defining a central bore for receiving an engine shaft. A nut retains the disc on the shaft. The disc retaining nut has at least one cooling passage defined therein and disposed for directing a flow of cooling air passing through the bore of the disc.

Description:
TECHNICAL FIELD 
     The application relates generally to gas turbine engines and, more particularly, to turbine rotor assemblies. 
     BACKGROUND OF THE ART 
     High temperature resistant materials, such as nickel based superalloys, have been used in the past in the manufacturing of gas turbine discs and the like. While superalloy materials, like IN100 and ME16, have better strength at high temperatures, they are more brittle and, thus, less tolerant to damage than conventional turbine disc materials. Therefore, when designing engine parts made out of such materials care should be taken to minimize stress concentrations, such as provided by holes, sharp corners, etc. 
     Accordingly, there is a continuing need to address the design constrains presented by damage-sensitive materials in the design of gas turbine engine parts, such as turbine discs. 
     SUMMARY 
     In one aspect, there is provided a turbine rotor for a gas turbine engine, comprising a disc including front and rear axially facing sides extending radially outwardly from a hub to a rim, the hub defining a central bore through which an engine shaft extends, and a disc retaining nut mounted around a portion of the engine shaft for mounting the disc on the engine shaft, the disc retaining nut having a plurality of cooling passages defined therein around a periphery of the nut, and the nut disposed in a cooling flow path defined centrally through the disc, the cooling passages communicating with the flow path for directing a flow of cooling air in the cooling path though the nut. 
     In a second aspect, there is provided a turbine rotor for a gas turbine engine, comprising an engine shaft mounted for rotation about a central axis of the gas turbine engine, a turbine disc mounted to the engine shaft for rotation therewith, and a nut threadably engaged on the engine shaft for mounting the disc on the engine shaft, the nut having a series of cooling passages defined therein and in fluid flow communication with a central bore of the disc. 
    
    
     
       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 an enlarged cross-sectional view of a turbine rotor of the engine shown in  FIG. 1 ; and 
         FIG. 3  is an isometric view of a disc retaining nut forming part of the turbine rotor shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       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  12  through which ambient air is propelled, a multistage compressor  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. 
     As shown in  FIG. 2 , the turbine section  18  comprises, among others, a high pressure rotor disc  20  having front and rear axially facing sides  21  and  23  extending radially inwardly from a rim  22  to a thinner web terminating in a larger hub  24 . The hub  24  includes a central bore  26 , and a row of turbine blades (not shown) extends radially outwardly from the rim  22  of the turbine disc  20 . The disc  20  may be made of a high temperature resistant superalloy, such as IN100 and ME16. 
     A hollow engine shaft  30  extends axially through the central bore  26  of the disc  20 . According to the example illustrated in  FIG. 1 , the shaft  30  interconnects the high pressure turbine rotor to a high pressure rotor of the compressor  14 , thereby forming the high pressure spool of the engine  10 . As shown in  FIG. 2 , a disc retaining nut  32  is threadably engaged to the engine shaft—in this example, on a rear tie-shaft portion  34  of the engine shaft  30  for axially loading the disc  20  on the shaft  30 . The nut  32  in this example contributes to axially hold the components of the complete high pressure rotor stack all together. 
     As shown in  FIGS. 2 and 3 , the nut  32  has an axially extending tubular open ended body  36  adapted to be concentrically nested in a rear recess  38  defined in an axially rearwardly projecting shaft portion of the rotor disc hub  24 . The rear recess forms an enlarged rear end portion of the central bore. Inner threads are provided on the inner surface of the tubular body  36  for engagement with corresponding outer threads on the rear tie-shaft portion  34  of the engine shaft  30 . The nut  32  has a flange  40  extending radially outwardly from the front end of the tubular body  36  and having a frontal face  37  for axial abutment against a corresponding annular shoulder  41  defined in the rear recess  38  of the rearwardly projecting shaft portion of the rotor disc hub  24 . As shown in  FIG. 3 , rectangular notches  39  or the like may be circumferentially distributed along the rear edge of the tubular body  36  of the nut  32  for engagement with a tool (not shown) used for tightening the nut  32  on the shaft  30 . 
     As shown in  FIG. 2 , the disc hub  24  has an axially forwardly projecting shaft portion  42 . The forwardly projecting shaft portion  42  offers sufficient material to accommodate a series of cooling holes  44  which can be made large enough to avoid the formation of unacceptable stress concentrations in “damage-intolerant” disc materials, such as IN100 and ME16. Accordingly, the large cooling holes  44  are sized to avoid unacceptable stress concentrations in the rotor disc  20  and are uniformly circumferentially distributed on the forwardly projecting shaft portion  42 . The large cooling holes  44  are in fluid flow communication with the front cavity  46  of the rotor disc  20 . High pressure air may be bled from the compressor  14  and channelled to the front cavity  46  of the rotor disc  20  and the cooling holes  44  to cool down the front area of the disc  20  during engine operation. It is understood that other suitable sources of coolant could be used to provide disc cooling. As indicated by the arrows in  FIG. 2 , the cooling air exiting the holes  44  is directed axially through the central bore  26  of the disc  20  in the annular spaced defined between the engine shaft  30  and the boundary surface of the central bore  26 . The cooling air flowing through the central bore  26  is received in the rear recess  38  defined in the axially rearwardly projecting shaft portion of the rotor disc hub  24 . The cooling air is discharged from the rear recess  38  in the rear cavity  47  of the disc  20  via a series of circumferentially spaced-apart cooling passages  48  defined in the front end portion of the disc retaining nut  32 . The nut  32  thus acts as a cooling air nozzle as well as a fastener. 
     By providing the cooling passages  48  in the nut  32  instead of in the axially rearwardly projecting shaft portion of the disc  20  the overall stress concentration in the disc  20  can be reduced while still providing cooling to the rear area of the disc. The rearwardly projecting shaft portion of the disc may be free from any cooling holes. Also the cooling passages  48  can be made small enough to perform a flow metering role. If the cooling passages  48  were defined in the rearwardly projecting shaft portion of the disc  20 , the passages would have to be large enough to avoid stress concentrations and could thus not be designed as small flow metering holes. The provision of small flow metering holes in a “damage intolerant” material would create stress concentrations which could render the rotor disc prone to crack propagation. The provision of the cooling passages in the nut  32  also allows providing cooling air passages where the space constraints are too restrictive to incorporate large cooling holes sized to avoid stress concentrations in the “intolerant damage” disc. This provides a design option where there is not enough space for large holes and the disc material does not accommodate small holes. In this way, the size of the disc can be minimized. Also by incorporating, the cooling passages  48  in the nut  32 , there is no need to provide separate or additional pieces to convey the cooling air to the rear cavity  47  of the disc  20 . 
     According to the illustrated embodiment, each cooling passages  48  is provided in the form of a metering hole extending thicknesswise through a portion of the lip  40  of the nut  32  overhanging the shaft/thread engagement portion. The overhang may permit, as in this example, the cooling passages  48  to extend from a radially inner surface  28  to a radially outer surface of the nut  32  in a rearwardly inclined direction. The entrances to cooling passages  48  are disposed on a radially inner surface relative to the nut abutment face  37  in this example. However, it is understood that the cooling passages  48  may take any suitable form, including slots or grooves defined in the nut  32  which, at the interface with the disc  20  for example, would thus provide cooling passages  48 . The cooling passages  48  can be made in any suitable manner. The number and size of passages  48  may be adjusted to limit and/or direct the flow as desired. 
     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, any arrangement of cooling passages passing through the nut may be provided. Rather than fully contained holes through the nut, the passages may be provided as exterior slits which cooperate with another surface (such as the disc) to provide the cooling passages. Any suitable cooling scheme may be provided for feeding the cooling passages. Cooling passages may be integrated in a front disc retaining nut. Although demonstrated above in use with a tie-shaft arrangement, the concept may be applied with any suitable arrangement. Still other modifications which fall within the scope of the present invention 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.