Abstract:
A combustion case for a gas turbine engine. A typical combustion case is generally cylindrical or conical. Apertures penetrate the case, from the outer surface, through the case, to the inner surface. The apertures act as concentration points for stress. To dissipate the stress, bosses buttress the apertures, with each aperture having two bosses: one on the outer surface of the case, and another on the inner surface of the case. The invention eliminates the latter bosses. The invention dissipates stress by providing an array of T-slots on the inner surface.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to stress reduction in combustion cases in gas turbine engines.  
         BACKGROUND OF THE INVENTION  
         [0002]    [0002]FIG. 1 illustrates the outer surface of a segment  3  of a combustor case used in a gas turbine engine. The overall case is generally cylindrical, or conic, and the conic/cylinder is formed by extending segment  3  around axis  6 , as indicated by arrows  9 . FIG. 2 illustrates the inner surface  12  of the segment  3  of FIG. 1.  
           [0003]    Apertures or holes  15  are formed within the case, for various purposes, such as delivery of fuel to combustors (not shown) within the case. The apertures penetrate the case in regions where the material of which the case is constructed is dimensionally thin. The thin material provides a less-than-optimal attachment point for external structures, such as a fuel-delivery tube. Further, the apertures themselves act as stress-risers, and increase stress concentrations in the already thin material surrounding them.  
           [0004]    In order to dissipate the stress concentrations, strengthen the region surrounding the apertures  15 , and to provide a convenient flange for attachment of tubing or sensors, bosses  18  are provided. FIG. 3 illustrates a boss  18  in schematic, cross-sectional view.  
           [0005]    Traditionally, as indicated in FIGS. 1 and 2, a separate boss  18  is provided for each individual aperture  15 . Further, for each aperture, two bosses are provided: a boss  18  on the outer surface, as in FIG. 1, and a boss  18  on the inner surface, as in FIG. 2.  
           [0006]    The individual bosses on the inner surface increase manufacturing costs. In one manufacturing approach, a complex milling set-up must be used, partly because the diameter of the case is small compared with the size of an ordinary vertical mill. In another approach, Electro Chemical Machining, ECM, is used.  
           [0007]    It is desired to eliminate, or reduce, the complexity and expense of the traditional approach to manufacturing the case of FIGS. 1 and 2.  
         SUMMARY OF THE INVENTION  
         [0008]    In one form of the invention, individual bosses for individual apertures on the inner surface of a combustion case are eliminated, and replaced by a continuous circumferential band having a thickness similar to that of the eliminated bosses. A circumferential array of T-shaped slots is generated within the band, on the inner surface of the case. These T-shaped slots separate the continuous band into individual areas of reinforcement bosses, each of which surrounds multiple apertures.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a perspective view of the outer surface of a segment of a combustion case for a gas turbine engine.  
         [0010]    [0010]FIG. 2 is a perspective view of the inner surface of the segment of FIG. 1.  
         [0011]    [0011]FIG. 3 illustrates a boss  18  of FIGS. 1 and 2 in schematic, cross-sectional view.  
         [0012]    [0012]FIG. 4 illustrates one form of the invention.  
         [0013]    [0013]FIG. 5 contains a magnified view  44  of a T-slot  25  of FIG. 4, and a cross-sectional view  45  of the T-slot  25 , as cut by plane  47 .  
         [0014]    [0014]FIG. 6 illustrates, in schematic form, a circumferential array of T-slots, according to one form of the invention.  
         [0015]    [0015]FIGS. 7 and 8 illustrate differences in cross-sectional geometries, by comparing the apparatus of FIGS. 1 and 4.  
         [0016]    [0016]FIG. 9 schematically illustrates a gas turbine engine utilizing one form of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    [0017]FIG. 4 illustrates one form of the invention. T-shaped slots, or T-slots,  25  are cut into the inner surface, or inner face,  30  of the casing. As FIG. 5 indicates, the T-slot  25  does not fully penetrate the casing, but the outer surface, or face,  35  remains intact.  
         [0018]    Generalized dimensions of FIG. 5 are the following: dimension  40 , representing the thicker region of the case wall; dimension  46 , representing the thinner region of the case wall dimension  50 , representing the depth of the T-slot. The T-slot  25  need not have uniform depth.  
         [0019]    An array of the T-slots  25  is provided along the inner circumference  51  of the case, as schematically shown in FIG. 6. Preferably, no bosses of the type  18  in FIG. 2 are contained on the inner circumference in FIG. 6. The inner circumference is smooth, in the area of the apertures  15 , with the exception of the T-slots  25  and the apertures  15  and  105  in FIG. 4.  
         [0020]    From one point of view, in one form of the invention, the T-slots  25  in FIG. 4 divide the inner surface of the case into individual bosses, one of which is indicated as  55 . That boss  55  contains three apertures  15 , as opposed to the situation in FIGS. 1 and 2, wherein each individual boss  18  contains its own, single aperture  15 .  
         [0021]    In addition, in FIG. 4, the overall thickness of the material surrounding an aperture  15 , can be the same as that in FIGS. 1 and 2. FIGS. 7 and 8 represent this thickness.  
         [0022]    [0022]FIG. 7 represents the situation of FIG. 1, and shows a boss  18  which is symmetrical about casing  58 . FIG. 8 represents one form of the invention. T-slot  25  is shown in the inner surface, or inner side,  73  of the case, while boss  18  is shown on the outer surface, or side,  74 . Boss  18  lacks the symmetry of FIG. 7  
         [0023]    Definitions will be given for several terms, partly to assist characterizations of the invention which will follow. Other definitions are possible.  
         [0024]    Axis  80  in FIG. 6 defines the axial direction. Arrows  85  represent the circumferential direction. Arrows  90  represent the radial direction. The apertures  15  in FIGS. 1, 5, and  8  can thus be termed radially facing.  
         [0025]    One type of numerical relationship between the number of T-slots and the number of apertures  15  will be examined. In FIG. 4, the two T-slots  25  can be viewed as defining a sector  55 . If this sector is taken as covering 30 degrees, then 12 such sectors would be found in the overall case, to cover 360 degrees. Restated, 12 T-slots  25 , evenly spaced over the case, would divide the case into 12 sectors.  
         [0026]    The sector  55  shown in FIG. 4 contains 3 primary apertures  15 . Secondary apertures or holes  105  are also shown, and they are used to attach threaded fasteners to connect external components such as flanges for tubing, such as fuel lines, or sensors. The 12 sectors as shown in FIG. 6 would contain  36  primary apertures  15 . Thus, if “T” represents the total number of T-slots around the circumference of the inner face  30  of the casing and “N” represents the total number of primary apertures  15  around the circumference of the inner face  30  of the casing, the ratio, T/N, of T-slots  25  to primary apertures  15  is {fraction (12/36)}, or ⅓.  
         [0027]    In another form of the invention, another numerical relationship will be examined. The sector shown in FIG. 4 also contains boss  56 , which is formed by the 2 T-slots  25  and contains one primary aperture  15  and 3 secondary apertures  105 . Using the same methodology as before, this boss  56  can be said to be an 18 degree sector, thus the number of such bosses  56  and bosses  55  would be used around the circumference as appropriate to accommodate the requirement for apertures for the overall case to cover 360 degrees. Restated, the overall number T of T-slots  25 , spaced over the case would divide the case into sectors containing a number N of primary apertures in sectors  55  or  56 , so that the ratio of T/N does not equal 1. The invention contemplates using any number of bosses appropriate to the stress relief requirement for a required number of apertures for any particular application. For example, a boss could be formed around any number of apertures between a pair of adjacent T-slots, and an adjacent boss could be provided for any other number of apertures. The resulting casing could include a combination of T-slots forming bosses each of which contains more than one aperture or any combination of T-slots to provide stress relief for bosses needed to strengthen the region surrounding the apertures. The invention is defined in that at least one of the bosses contains either no aperture or more than one aperture, so that the total number of stress relief slots T around the circumference of the casing is not equal to the total number of apertures through the casing.  
         [0028]    Thus, the number of bosses needed to dissipate the stress due to the 36 primary apertures  15  is less than the number of apertures themselves, compared with the situation of FIGS. 1 and 2.  
         [0029]    In addition, if the sector under consideration is viewed as containing a single boss which serves multiple primary apertures  15 , that single boss also contains multiple sets of secondary apertures, each set corresponding to a primary aperture  15 .  
         [0030]    From another perspective, the single boss can be viewed as cooperating with its neighbor (not fully shown) to form the T-slot  25  in FIG. 4. The edges  94  of the bosses cooperate to form, and define, the T-slot  25 .  
         [0031]    The invention presents the benefit of providing the needed stress dissipation, yet eliminating the need to construct individual bosses for each aperture on the inner surface of the case, as in FIG. 2. Further, each T-slot  25  can be constructed as shown in FIG. 5, using a pair of straight-line milling cuts: one for the stem  95 , or vertical part, of the T, and one for the bar  98 , or horizontal part, of the T.  
         [0032]    Of course, multiple passes can be taken, so that each pass need only take a shallow cut, such as one, or a few, mils in depth. Since the stem  95  of the T is aligned generally axially, one set of passes is taken in the axial direction. Since the bar  98  of the T is aligned generally circumferentially, one set of passes is taken in the circumferential direction.  
         [0033]    In one form of the invention, the stem  95  and bar  98  of the T need not be conjoined to each other, but can be positioned apart from each other. That is, a circumferential array of generally axially aligned stems is provided, and a separate circumferential array of generally circumferentially aligned bars is also provided.  
         [0034]    In one form of the invention, the normal boss structure of FIG. 1 is maintained on the outer surface of the case. However, on the inner surface, as in FIG. 4, no bosses are present, except for those defined by the T-slots  25 . The T-slots  25  in FIGS. 4 and 6 are contained in an annulus  99 , which also contains apertures  15 .  
         [0035]    [0035]FIG. 9 illustrates one form of the invention. A gas turbine engine  100  contains the combustor case  105 , which is configured with T-slots  25  as described above. The engine  100  includes a fan  110 , low pressure turbine  115 , high pressure compressor  120 , and a high pressure turbine  125 .  
         [0036]    Numerous substitutions and modifications can be undertaken without departing from the true spirit and scope of the invention. For example, the embodiments described herein have been framed in the context of a gas turbine aircraft engine. However, the invention can be used in casings used in electrical power generation equipment, and such casings, in many instances, are much thicker than those used in aircraft engines.