Abstract:
A CMC wall ( 20 F) may be attached to a metal wall ( 22 F) by a plurality of bolts ( 28 A,  28 B,  28 C) passing through respective holes ( 24 A,  24 B,  24 C) in the CMC wall ( 20 F) and holes in the metal wall ( 22 F), clamping the walls ( 20 F,  22 F) together with a force that allows sliding thermal expansion but does not allow vibrational shifting. Distal ones of the holes ( 24 A,  24 B) in the CMC wall ( 20 F) or in the metal wall ( 22 F) are elongated toward a central one of the bolts ( 24 C) or at alternate angles to guide differential thermal expansion ( 20 T) of the CMC wall ( 20 F) versus the metal wall ( 22 F) between desired cold and hot geometries. A second CMC wall ( 20 R) may be mounted similarly to a second metal wall ( 22 R) by pins ( 39 A,  39 B,  39 C) that allow expansion of the CMC component ( 201 ) in a direction normal to the walls ( 20 F,  20 R).

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to mechanisms for attaching low alpha ceramic components to high alpha metal support structures in high temperature, dynamic environment, in particular in industrial and aero gas turbines. 
       BACKGROUND OF THE INVENTION 
       [0002]    Ceramic matrix composites (CMCs) have a higher temperature capability than metallic alloys, making them potentially very valuable for implementation into gas turbines, which can run at temperatures well in excess of metal capabilities. Metal is stronger and more ductile, making it better for supporting hardware, such as vane carriers, casings, bolting, etc. To combine the advantages of these two materials, at some point they must attach to one another. However, attaching low alpha (low thermal growth) CMCs to higher alpha metals is not a simple procedure. In dynamic environments, the CMC and metal need to be rigidly attached to prevent vibration, which may lead to wear and/or fatigue issues. But, if high temperatures are also present, the metal and CMC will grow at different rates. If they are rigidly attached, the metal, being stiffer and stronger, will take the CMC with it. CMC is by nature more brittle and less strain-tolerant than metal. Such movement could damage, or even destroy, the CMC. The trick is to design an attachment that satisfies both these concerns—vibration and thermal growth. 
         [0003]    In addition, in curved structures, any thermal gradient will cause the metal and CMC to not just linearly grow, but also curl or uncurl, depending on the thermal gradient characteristics. The attachment must therefore allow for such curling, and even take advantage of it. 
         [0004]    There are also cases where the CMC structure is large enough, or carries enough load, that it requires multiple attachment regions but is still one structure. A second attachment region must allow for thermal growth of a CMC part away from a first attachment region, in addition to the metal it is attached to. This invention is a solution to these design considerations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The invention is explained in the following description in view of the drawings that show: 
           [0006]      FIG. 1  is a sectional view of a metal to CMC attachment. 
           [0007]      FIG. 2  is a sectional view taken along line  2 - 2  of  FIG. 1 . 
           [0008]      FIG. 3  is a sectional view taken along line  3 - 3  of  FIG. 1 . 
           [0009]      FIG. 4  is a perspective view of a slotted spring pin for an elongated bolt hole. 
           [0010]      FIG. 5  is a sectional view as in  FIG. 2  of a curved CMC component. 
           [0011]      FIG. 6  shows expansion geometry of a curved CMC component with alternate angles of the distal bolt holes. 
           [0012]      FIG. 7  is a sectional view of a turbine shroud ring segment using embodiments of the present attachment concept on both front and back walls. 
           [0013]      FIG. 8  is a back view of a ring segment of  FIG. 7 . 
           [0014]      FIG. 9  is a sectional view as in  FIG. 7  showing additional design options for bolts and expansion holes. 
           [0015]      FIG. 10  shows a variation of  FIG. 7  in which the back bolts are turned around. 
           [0016]      FIG. 11  shows a geometry of the invention that expands to one side. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The first design consideration in the attachment of a CMC component to a metal component in a dynamic, high temperature environment is how to clamp the CMC hard enough against the metal to prevent relative vibratory motion while still allowing for the two components to grow different amounts due to temperature changes.  FIG. 1  shows such a solution. A CMC component  201  has a wall  20 F that is mounted against a metal wall  22 F by three bolts  28 A,  28 B, and  28 C. A nut  30  on each of the bolts  28 A,  28 B,  28 C clamps the CMC wall  20 F against the metal wall  22 F with a retaining force in a range between a lower limit below which sliding is present between the CMC wall  20 F and the metal wall  22 F due to operational vibrations in the CMC and metal components  201 ,  22 F, and an upper limit above which it prevents relative sliding of the walls  20 F,  22 F due to differential thermal expansion or above which it exceeds a stress limit in the CMC wall  20 F, the range being non-inclusive of the upper and lower limits. As the structures heat up, the bolts  28 A,  28 B,  28 C elongate more than the CMC wall  20 F thickens. Therefore, a spring washer  32  may be used to maintain load between the CMC wall  20 F and metal wall  22 F as the bolts elongate. Also, in each of the holes  24 A,  24 B,  24 C in the CMC wall, either a wear coating or a spring pin  26  can be used to prevent wear of the CMC. A metal washer plate  34  may be provided to distribute clamping stress on the CMC wall  20 F, and to provide a sliding surface between the spring washers  32  and the CMC. 
         [0018]    Three bolts  28 A,  28 B,  28 C are shown for a reason. In either the CMC wall  20 F or the metal wall  22 F, the bolt holes can be elongated to allow for the thermal growth mismatch. For example, as shown in  FIG. 2 , the central CMC hole  24 C can be elongated in a vertical direction to position the CMC wall  20 F about that center point. The distal holes  24 A,  24 B can be elongated in a horizontal direction to allow the metal to grow relative to the CMC in that direction or to allow the CMC to grow relative to the metal, depending on thermal gradients. Relative thermal growth is indicated by arrows  20 T. The arrangement of these elongations can be changed to allow a component to grow from one side instead of out from the center. A CMC component may have a surface  56  exposed to hot combustion gasses, and this surface may be coated with a refractory insulating layer  48  as known in the art. 
         [0019]    If the CMC component is a curved structure  202  as seen in  FIG. 5  or if there is a thermal gradient through the CMC component, it may flatten out or curl even more, depending on the gradient and geometry. As shown in  FIG. 6 , a curved CMC component  203  may have distal holes  24 A,  24 B elongated at modified angles to allow for this distortion so that additional strain is not imparted to the CMC. Depending on the arrangement of the angled holes  24 A,  24 B, and the curling or flattening of the component  203 , the CMC can be allowed to shift in space outward or inward with respect to the center of its radius of curvature, as shown by alternate positions  34 A,  34 B. 
         [0020]    The CMC component may require more than one attachment area, for example front and back attachment areas as shown for a CMC ring segment  204  in  FIG. 7  due to size, loading, geometry, etc. In this case, the attachment mechanism of the second area will differ from that of the first area. The main difference is that these new points must allow for the thermal growth of the CMC component  204 , which means they can&#39;t be rigidly attached, but must have some sort of sliding joint, as seen in a second CMC wall  20 R of  FIG. 7 . Here, a second set of bolts  38 A,  38 B,  38 C, can be attached to either the second CMC wall  20 R or to a second metal wall  22 R using nuts  30  and spring washers  32 . These second bolts have pin ends  39 A,  39 B,  39 C, that can slide through the other component. The holes  44 A,  44 B,  44 C for the pin ends  39 A,  39 B,  39 C, will need to be elongated in these additional attachment points similarly to the holes  24 A,  24 B, and  24 C of the first attachment points to allow for thermal growth and curling or flattening. The holes  44 A,  44 B,  44 C in the CMC may be lined with spring pins  36 . Note that the spring pins and bolt pin ends are illustrated herein as having a round cross-sectional shape; however, other cross-sectional shapes such as elliptical, for example, may be used in other applications. 
         [0021]    An example of the present invention being used is in the case of a ring segment for a gas turbine. A ring segment is shown in various versions  204 ,  205 , and  206  in  FIGS. 7-10 . In  FIGS. 7 ,  9 , and  10  it is shown mounted between adjacent ring segments partially shown. A ring segment is a curved component about the center of the engine. An entire row of these ring segments form a ring around the rotating blades. It utilizes the initial attachment description for a first wall  20 F per  FIGS. 1-6  as applied to a curved component  202  or  203 . Ring segments carry a large pressure load. Therefore a downstream attachment wall  20 R is required, utilizing the additional attachment scheme with pin ends  39 A,  39 B,  39 C in holes  44 A,  44 B,  44 C. 
         [0022]      FIG. 8  shows a back view of a downstream wall  20 R of a ring segment  204 . The elongated distal holes  24 A,  24 B in the first CMC wall  20 F, and the elongated distal holes  44 A,  44 B in the second CMC wall  20 R can be elongated in directions that guide the ring segment from a first relatively cool operational geometry to a second relatively hot operational geometry  34 A,  34 B as in  FIG. 6 , while maintaining a generally constant clearance between an inner surface  56  of the CMC component and a turbine blade tip. 
         [0023]      FIG. 9  shows an alternate ring segment  205  in which a first set of bolts  28 A′,  28 B′,  28 C′ clamp a front CMC wall  20 F′ against a front metal wall  22 F′. The front metal wall  22 F′ in this embodiment has elongated bolt holes  25 A,  25 B,  25 C. A second set of bolts  38 A′,  38 B′,  38 C′ are fastened onto a back CMC wall  20 R′, and the heads of these bolts serve as pins  39 A′,  39 B′,  39 C′ in elongated holes  45 A,  45 B,  45 C in a back metal wall  22 R′. Other modifications are possible. For example, the back metal wall  22 R′ may be disposed forward of the back CMC wall  20 R′, and the back bolts  38 A′,  38 B′,  38 C′ may be turned around accordingly. 
         [0024]      FIG. 10  shows an alternate embodiment of a ring segment  206  based on a modification of  FIG. 7 , in which the back bolts  38 A″,  38 B″,  38 C″ are turned around by comparison to those of  FIG. 7 , and have heads that also serve as pins  39 A″,  39 B″,  39 C″ in pin holes  44 A″,  44 B″,  44 C″ in the back CMC wall  20 R″ of the modified ring segment  206 . This modification allows the back bolts  38 A″,  38 B″,  38 C″ to be shorter than those of  FIG. 7 , and allows the back metal wall  22 R″ to be closer to the back CMC wall  20 R″. 
         [0025]    Central holes of the present attachment mechanism, such as  24 C and  44 C may be circular or they may be elongated in a direction normal to a line drawn between the respective distal holes  24 A- 24 B or  44 A- 44 B. In the case of a ring segment, this central hole elongation will be along a radius from the turbine axis. Such elongation allows the ring segment to flatten as it expands. It can flatten when the radially inner surface  56  the ring segment heats faster and becomes hotter than the outer portions of the segment. Either a circular or radially elongated shape of the central hole  24 C,  44 C, maintains the ring segment centered about a circumferential position. 
         [0026]    The present CMC-to-Metal attachment mechanism allows the use of longer ring segments because longer expansion geometries can be controlled. This can reduce the number of parts in a gas turbine, reducing manufacturing expense and maintenance, and increasing reliability. Additional bolts (not shown) and respective elongated bolt holes (not shown) may be added between a given central bolt such as  28 C and respective distal bolts such as  28 A,  28 B. The elongations of such intermediate bolt holes may be the same as the elongations of the distal bolt holes  24 A,  24 B, or less, according to intermediate relative expansion vectors. 
         [0027]      FIG. 11  shows a geometry  207  with left and right bolts  28 L,  28 R, left and right holes  24 L,  24 R, in which the left hole  24 L is elongated, and the right hole  24 R is circular. This embodiment expands toward the left. 
         [0028]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.