Patent Publication Number: US-9410429-B2

Title: Air cooling shaft at bearing interface

Description:
TECHNICAL FIELD 
     The present disclosure relates to gas turbine engines and more particularly to improvements in the cooling of coupled shafts. 
     BACKGROUND OF THE ART 
     Shaft and bearing deformation may occur at the interface of a bearing inner race and the shaft to which it is coupled, because of the heat generated by the turbine rotor and conducted by the shaft supporting the turbine rotor, especially when the bearing is close to the turbine rotor. This phenomenon of coning has been found to be especially problematic in gas turbine engines where the main shaft bearing is between the compressor module and the turbine module and in close proximity to the turbine module. The thermal conduction from the turbine rotor has resulted in coning of the shaft as well as of the bearing, leading to premature bearing distress. 
     SUMMARY 
     In one aspect, there is provided a gas turbine engine having at least a spool assembly including at least a compressor rotor and a turbine rotor connected by a shaft assembly, the shaft assembly comprising: a compressor shaft portion connected to the compressor rotor and a turbine shaft portion connected to the turbine rotor; the compressor shaft portion and the turbine shaft portion connected axially together by a shaft coupling between the compressor rotor and the turbine rotor and at least a bearing rotatably coupled to the shaft assembly adjacent the shaft coupling; at least one of the compressor shaft and the turbine shaft being provided with openings between the bearing and the shaft coupling to permit cooling air to enter air passages in the area of the shaft coupling; and a source of pressurized cooling air in communication with the openings provided in the shaft assembly to direct such cooling air to the shaft coupling. 
     In a second aspect, there is provided a shaft assembly for a gas turbine engine of the type including at least a compressor rotor and a turbine rotor connected by the shaft assembly; the shaft assembly comprising a compressor shaft portion adapted to be connected to the compressor rotor and a turbine shaft portion adapted to be connected to the turbine rotor; the compressor shaft portion and the turbine shaft portion connected axially together by a shaft coupling arranged to be between the compressor rotor and the turbine rotor and the shaft assembly adapted to be rotatably coupled to at least a bearing adjacent the shaft coupling; at least one of the compressor shaft and the turbine shaft being provided with openings between the bearing and the shaft coupling to permit cooling air to enter air passages in the area of the shaft coupling. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures in which: 
         FIG. 1  is a schematic cross-sectional view of a gas turbine engine illustrating a multishaft configuration; 
         FIG. 2  is a partly fragmented axial cross-sectional view showing a detail of a preferred embodiment; and 
         FIG. 3  is an enlarged axial cross-section view of the detail similar to that shown in  FIG. 2 . 
     
    
    
     Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below. 
     DETAILED DESCRIPTION 
       FIG. 1  schematically depicts a turbofan engine A which, as an example, illustrates the application of the described subject matter. The turbofan engine A includes a nacelle  10 , a low pressure spool assembly which includes at least a fan  12  and a low pressure turbine  14  connected by a low pressure shaft  16 , and a high pressure spool which includes a high pressure compressor  18  and a high pressure turbine  20  and a high pressure shaft  24 . The engine further comprises a combustor  26 . 
     Referring to  FIG. 2 , the high pressure shaft  24  includes a compressor stub shaft  28  coupled to a turbine stub shaft  30  at spline  34 . The stub shaft  28  typically has an inner diameter. The shield  32  may be within the inner diameter of the stub shaft  28 . Other coupling configurations may be used for the interconnection between the stub shafts  28  and  30 , such as a curvic coupling among other possibilities. 
       FIG. 2  shows a bearing housing  22  isolating a main bearing  23 , the main bearing  23  supporting the shaft  24  and more particularly, compressor shaft segment  28 . In  FIG. 2 , an inner race of the bearing  23  is mounted directly onto the shaft  24 . The bearing housing  22  also includes a pair of oil-air seals  42  and  44  operatively engaging seal runners  38  and  40  mounted to the compressor stub shaft  28 . A cooling air plenum  46  is also defined within the bearing housing  22 . 
     Turbine shaft  30 , which may be at a relatively high temperature due to its direct connection with the turbine rotor (not shown), may thus create thermal stresses within the compressor shaft  28 , thus resulting in coning in the area of the interface of shaft  28  with the inner race  23   a  of bearing  23 . This coning may result from the fact that the compressor stub shaft  28  is relatively cooler than the portion of the compressor shaft coupled to the hotter turbine stub shaft  30 , especially since the bearing  23  is located in a very hot environment between the high pressure compressor  18  and the turbine  20 . 
     As shown in more detail in  FIG. 3 , in an embodiment slightly modified from  FIG. 2 , relatively cooler, pressurized air from the plenum  46  passes through an opening  48 , then through opening  50  in the seal runner  40 , and then through passage  54  in the end of the stub shaft  30 . This pressurized air is then forced through the spline interface at the spline  34 . In this manner, the forward end of the stub shaft  30  which is now surrounded by cooler air, is cooled towards a thermal equilibrium with compressor shaft  28 . 
     Alternatively, or additionally, cooling air may be brought to the spline  34  and thus to further surround stub shaft  30  with cool air, by allowing the bleeding of compressor air or externally cooled air to enter through a passage  56  in compressor shaft  28 , on the forward side of the bearing housing  22 . This pressurized cooling air can then follow a conduit defined between the shield  32  and the inner diameter of the high pressure compressor stub shaft  28  to then exit into this spline interface  34  by means of a passage  58  in the stub shaft  28 . 
     It is pointed out that many of the components described above as being about the shafts  28  and  30  are annular. Accordingly, the various passages such as opening  48 , opening  50 , passage  56  and passage  58  may or many not be circumferentially distributed on the structural components in which they are defined. 
     The provision of pressurized cooling air through the shaft  24 , particularly around the end of the turbine stub shaft  30  by way of the shaft coupling, such as the spline coupling  34 , may contribute to the reduction of the thermal gradient at the compressor stub shaft  28  in the area of the bearing  23 . This arrangement may reduce the occurrence of shaft or bearing race coning. 
     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. 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.