Abstract:
A gas turbine engine disk that includes a centrally disposed disk hub having an integrally-formed, radially outwardly extending web terminating at an outer end. The disk hub has a radially-displaced annular hub surface exposed to high pressure, high temperature discharge gases during engine operation. The radially-displaced annular hub surface acts as an axial free surface mitigating the formation undesirable axial stress in the disk hub.

Description:
TECHNICAL FIELD AND BACKGROUND OF THE INVENTION 
     This invention relates to gas turbine engines, and more specifically to the reduction of axial stress in disk hubs of gas turbine aircraft engines. The invention is disclosed and explained in this application with specific reference to high pressure turbine (“HPT”) disk hubs of gas turbine aircraft engines. Severe thermal gradients at the hub of HPT disks during takeoff can lead to high compressive axial stresses at the center of the hub surface. This high axial stress can lead to calculated life values well below engine program requirements. Prior art solutions have included large reductions in thermal gradients and/or the disk rim loading, or a large increase in hub size. These solutions negatively impact engine performance. 
     More particularly, current practices to reduce axial stress include adjusting the disk rim load, hub size, or idle hub flow to get adequate life from the disk hub. The approach of adjusting the disk rim load is indirect. The weight of the blades is reduced in order to reduce the hoop stress in the disk to the point that it meets life requirements even with the high axial hub stress. This approach has negative life and performance implications for the blade. Adjusting the hub size is indirect as well. This practice also reduces the hoop stress so that the disk will accommodate the large axial stress with acceptable life. This approach has negative weight and thermal performance impacts for the disk. Increasing the engine idle hub flow directly reduces the axial stress on the hub by warming the disk prior to takeoff. This, in turn, reduces the thermal gradient that causes the high axial stress. However, the high hub flow has negative system performance implications. 
     The invention disclosed and claimed in this application addresses this problem in a novel manner and thereby reduces axial stress on the HPT disk hub without disadvantageous tradeoffs incurred with prior art solutions. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, geometric features are introduced into the disk hub to mitigate high axial stress in HPT disk hubs. 
     According to another aspect of the invention, a chamfer is formed into the inner diameter of the disk hub 
     According to another aspect of the invention, radial grooves are formed in the hub surface. 
     According to yet another aspect of the invention, hub material is removed along a line parallel to an axial stress isoline. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial vertical cross-sectional view of a HPT disk hub with indicated axial thermal gradients according to a prior art HPT disk hub design; 
         FIG. 2  is a fragmentary cross-section, taken along a longitudinal axis, of the HPT section of a gas turbine engine; 
         FIG. 3  is a partial vertical cross-sectional view of a HPT disk hub with indicated axial thermal gradients according to a HPT disk hub design in accordance with one aspect of the invention; 
         FIG. 4  is a partial vertical cross-sectional view of a HPT disk hub in accordance with another aspect of the invention; 
         FIG. 5  is a partial vertical cross-sectional view of a HPT disk hub in accordance with one aspect of the invention; and 
         FIG. 6  is a partial vertical cross-sectional view of a HPT disk hub in accordance with one aspect of the invention; 
     
    
    
     DETAILED DESCRIPTION 
     A typical prior art disk hub is shown in  FIG. 1  at reference numeral  1 , and includes a hub surface  2 . Stress gradient lines  7 - 3  indicate progressively higher stresses towards the hub surface  2 . This occurs as the material at the hub surface  2  increases in temperature and thermally expands. The interior material of the disk hub  1  is cooler, as indicated by a relatively cool interior area  8  of reduced stress that restrains the thermal expansion of the hotter material closer to the surface  2  of the hub  1 . The stress peaks in the center, as shown, and falls away at the opposing ends due to the axial free surfaces that permit the expansion. As described below, reducing the distance from the center of the hub to a free axial surface has been shown to reduce the magnitude of the central axial stress. 
       FIG. 2  illustrates a portion of a HPT section  10  of an aircraft high bypass ratio gas turbine engine. The HPT section  10  includes first and second stage disks  14 ,  16 , having respective webs  18 ,  20  extending outwardly from respective hubs  21 ,  24 . The first stage disk hub  21  includes a hub surface and a chamfer  23 , as described in further detail below. Dovetail slots  26 ,  28  are formed on the outer ends of the webs  18 ,  20 , respectively. 
     The first stage disk  14  includes a forward shaft  30  that is integral with the web  18 . Hub  21  of the first stage disk  14  includes a rearwardly-extending aft shaft  42  that is press-fitted into engagement with a bearing  44 . The shaft  42  includes a plurality of openings  46  that allow cooling air to enter the interstage volume  48 . An interstage seal  50  is positioned between the first stage disk  14  and the second stage disk  16 , and includes an outer shell  52  and a central disk  54  having a hub  56 . Shell  52  is generally cylindrical with forward and aft-extending curved arms  58  and  60  that extend from a mid-portion  62  that supports seal teeth  64  and attach to the respective disks  14 ,  16 . 
     Referring now to  FIG. 3 , the surface  22  of the disk hub  21  is provided with a radially-displaced chamfer  23  on the forward end of the hub surface  22 . This places a free surface, i.e., a “corner”, of the chamfer  23  immediately below the coolest portion of the hub  21 , thereby forcing the axial stress to be the greatest at an off-center position, thereby decreasing its magnitude. This is shown in  FIG. 3 , where the area of greatest stress, indicated at “X” is shifted to a forwardly off-center position. 
     Optimum shape, angle, size and dimensions of the chamfer are determined empirically by implementing a design change and then reviewing the effect of the change through computer analysis to observe the resulting stresses, rather than by a purely analytical method. The design process is adapted to balance the decrease in axial stress with an accompanying increase in hoop stress caused by lowering the cross-sectional area of the disk hub  21 . 
     In a preferred version, the chamfer  23  intersects the non-chamfered portion of the hub surface  22  at the same axial location as the center of maximum axial tensile stress. The chamfer  23  is preferably planar, as shown, with radiused fore and aft transitions and may be between about 0 and 50 degrees. 
     Prior art disk hubs in a specific General Electric gas turbine engine were rated at 11,000 cold start cycles. Incorporation of the chamfer as shown and described above into a computer simulation resulted in an improvement to 15,300 cold start cycles, enabling the engine to meet program life requirements. 
     Similar improvements may be obtained with a variety of techniques. As is shown in  FIG. 4 , a disk  70  includes an integrally-formed web  72  and a disk hub  74  with a hub surface  76 . The disk  70  includes an integrally-formed forward shaft  78  and a rearwardly-extending aft shaft  80 . The hub surface  76  is provided with radial grooves  82  and  84 , the shape of which is defined by the nine indicated variables. Thermostructural DOE is used to determine the appropriate design space to achieve minimum stress in the hub  74 . Average hoop stress, burst margin and selected rim stress are other variables that must be taken into account. The grooves  82  and  84  effectively cut the axial stress path at the hub surface  76 . Somewhat less material is removed from the hub  74  for a given amount of stress reduction in comparison with the chamfered hub surface  22  shown in  FIG. 3 , thereby minimizing the increase in disk hoop stress resulting from the reduction in disk cross-sectional area. 
     Another alternative is shown in  FIG. 5 , where a disk  90  includes an integrally-formed web  92  and a disk hub  94  with a hub surface  96 . The disk  90  includes an integrally-formed forward shaft  98  and a rearwardly-extending aft shaft  100 . The hub surface  96  is provided with a concave annular recess, the shape of which is defined by variable A, R 1  and R 2 . Thermostructural DOE is used to determine the appropriate design space to achieve minimum stress in the hub  94 . While the impact on the disk temperature may be moderate, this design may significantly reduce axial stress by decreasing the thermal gradient within the hub  94 . 
     Referring now to  FIG. 6 , a further modified design is illustrated. A disk  110  includes an integrally-formed web  112  and a disk hub  114  with a hub surface  116 . The disk  110  includes an integrally-formed forward shaft  118  and a rearwardly-extending aft shaft  120 . The hub surface  116  is provided with a radially-extending annular convex ring, the shape of which is defined by variables essentially as with  FIG. 5 . Thermostructural DOE is used to determine the appropriate design space to achieve minimum stress in the hub  114 . While the impact on the disk temperature may be moderate, this design may significantly reduce axial stress by increasing the distance over which the thermal gradient is formed within the hub  114 . This design illustrates the principle that any surface other than a planar axial cylindrical surface will achieve a reduction in peak axial stress. The objective is to reduce peak axial stress while minimizing compensating variations in other undesirable conditions. For example, cylindrical grooves in the hub surface would reduce peak axial stress, but would also introduce very high stress points at the sharp corners that would be highly detrimental to the operational life of the disk. As is evident from the foregoing, the radially-displaced portion of the disk hub surface may be planar, e.g.,  FIGS. 2 and 3 , or non-planar, e.g., FIGS.  4 - 6 —the principal determining factor being the results achieved by DOE studies and the effect of the radially-displaced portion of the disk hub surface on axial stress, hoop stress, burst margin and rim stress. 
     A disk hub with reduced axial stress, and methods of reducing axial stress in a disk hub are disclosed above. Various details of the invention may be changed without departing from its scope. Furthermore, the foregoing description of the preferred aspect 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.