Abstract:
A single point load structure transfers a first load from a first bearing cone and a second load from a second bearing cone of a gas turbine engine to a plurality of struts. The single point load structure includes a stem, a branch connected to the stem and a torque box connected to the plurality of struts for absorbing the first and second loads from the stem and the branch. The stem has a concave surface that opens in a radially outward direction with respect to a rotational axis of the gas turbine engine.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This is a divisional of U.S. patent application Ser. No. 11/397,157, entitled “INTEGRATED STRUT DESIGN FOR MID-TURBINE FRAMES WITH U-BASE,” filed Apr. 4, 2006 by Keshava B. Kumar et al, the disclosure of which is incorporated by reference in its entirety. Reference is also made to application Ser. No. ______ entitled “MID-TURBINE FRAME” which is a continuation of U.S. patent application Ser. No. 11/397,157, and is filed on even date and is assigned to the same assignee as this application. 
     
    
     BACKGROUND 
       [0002]    The present invention generally relates to the field of gas turbine engines. In particular, the invention relates to a mid-turbine frame for a jet turbine engine. 
         [0003]    Turbofans are a type of gas turbine engine commonly used in aircraft, such as jets. The turbofan generally includes a high and a low pressure compressor, a high and a low pressure turbine, a high pressure rotatable shaft, a low pressure rotatable shaft, a fan, and a combuster. The high-pressure compressor (HPC) is connected to the high pressure turbine (HPT) by the high pressure rotatable shaft, together acting as a high pressure system Likewise, the low pressure compressor (LPC) is connected to the low pressure turbine (LPT) by the low pressure rotatable shaft, together acting as a low pressure system. The low pressure rotatable shaft is housed within the high pressure shaft and is connected to the fan such that the HPC, HPT, LPC, LPT, and high and low pressure shafts are coaxially aligned. 
         [0004]    Outside air is drawn into the jet turbine engine by the fan and the HPC, which increases the pressure of the air drawn into the system. The high-pressure air then enters the combuster, which burns fuel and emits the exhaust gases. The HPT directly drives the HPC using the fuel by rotating the high pressure shaft. The LPT uses the exhaust generated in the combuster to turn the low pressure shaft, which powers the fan to continually bring air into the system. The air brought in by the fan bypasses the HPT and LPT and acts to increase the engine&#39;s thrust, driving the jet forward. 
         [0005]    In order to support the high and low pressure systems, bearings are located within the jet turbine engine to help distribute the load created by the high and low pressure systems. The bearings are connected to a mid-turbine frame located between the HPT and the LPT by bearing support structures, for example, bearing cones. The mid-turbine frame acts to distribute the load on the bearing support structures by transferring the load from the bearing support structures to the engine casing. Decreasing the weight of the mid-turbine frame can significantly increase the efficiency of the jet turbine engine and the jet itself. 
       SUMMARY 
       [0006]    A single point load structure transfers a first load from a first bearing cone and a second load from a second bearing cone of a gas turbine engine to a plurality of struts. The single point load structure includes a stem, a branch connected to the stem and a torque box connected to the plurality of struts for absorbing the first and second loads from the stem and the branch. The stem has a concave surface that opens in a radially outward direction with respect to a rotational axis of the gas turbine engine. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a partial sectional view of a gas turbine engine having a mid-turbine frame. 
           [0008]      FIG. 2  is a perspective view of the mid-turbine frame. 
           [0009]      FIG. 3A  is a cross-sectional view of a first embodiment of the med-turbine frame. 
           [0010]      FIG. 3B  is a schematic diagram of the first embodiment of the mid-turbine frame. 
           [0011]      FIG. 4  is a free body diagram of the first embodiment of the mid-turbine frame. 
           [0012]      FIG. 5A  is a cross-sectional view of a second embodiment of the mid-turbine frame. 
           [0013]      FIG. 5B  is a schematic diagram of the second embodiment of the mid-turbine frame. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  shows a partial sectional view of an intermediate portion of gas turbine engine  10  about a gas turbine engine axis centerline. Gas turbine engine  10  generally includes mid-turbine frame  12 , engine casing  14 , mounts  16 , first bearing  18 , and second bearing  20 . Mid-turbine frame  12  of gas turbine engine  10  has a lightweight design that transfers the loads from first and second bearings  18  and  20  to a single point load. The design of mid-turbine frame  12  is also capable of withstanding a large amount of load without deflecting, increasing its structural efficiency. 
         [0015]    Mid-turbine frame  12  is housed within engine casing  14  of gas turbine engine  10 . Mid-turbine frame  12  is connected to engine casing  14  and first and second bearings  18  and  20 . Engine casing  14  protects mid-turbine frame  12  from its surroundings and transfers the loads from mid-turbine frame  12  to mounts  16 . Mid-turbine frame  12  is designed to combine the loads from first and second bearings  18  and  20  to one point for a single point load transfer. Due to the design of mid-turbine frame  12 , mid-turbine frame  12  has reduced weight. The weight of mid-turbine frame  12  will depend on the material used to form mid-turbine frame  12 . In one embodiment, mid-turbine frame  12  has a weight of less than approximately 200 pounds. For example, mid-turbine frame  12  formed of a Nickel-based alloy has a weight of approximately 175 pounds. Mid-turbine frame  12  is also designed as a functional plenum and does not require an independent heat transfer plenum. In addition, mid-turbine frame  12  can be integrally cast as one piece with a cooling air redistribution device as an integral component. 
         [0016]    First and second bearings  18  and  20  are located at forward and aft ends of gas turbine engine  10 , respectively, below mid-turbine frame  12 . First and second bearings  18  and  20  support thrust loads, vertical tension, side gyroscopic loads, as well as vibratory loads from high and low pressure rotors located in gas turbine engine  10 . All of the loads supported by first and second bearings  18  and  20  are transferred to engine casing  14  and mounts  16  through mid-turbine frame  12 . Second bearing  20  is typically designed to support a greater load than first bearing  18 , so mid-turbine frame  12  is designed for stiffness and structural feasibility assuming that second bearing  20  is the extreme situation. 
         [0017]      FIG. 2  shows an enlarged, perspective view of mid-turbine frame  12  within a cross-section of engine casing  14 . Mid-turbine frame  12  generally includes torque box  22  and struts  24 . First and second bearings  18  and  20  (shown in  FIG. 1 ) are connected to mid-turbine frame  12  by first bearing cone  26  and second bearing cone  28  (shown in  FIG. 1 ), respectively. First and second bearings cones  26  and  28  are continuously rotating with high and low pressure rotors and transfer the loads from first and second bearings  18  and  20  to mid-turbine frame  12 . 
         [0018]    Torque box  22  has a shell structure and is positioned between first and second bearing cones  26  and  28  and struts  24 . Torque box  22  takes the loads, or torque, from first and second bearing cones  26  and  28  and combines them prior to transferring the loads to struts  24 , which extend from along the circumference of torque box  22 . 
         [0019]    Struts  24  of mid-turbine frame  12  transfer the loads from first and second bearing cones  26  and  28  entering through torque box  22  to engine casing  14 . Each of struts  24  has a first end  30  connected to torque box  22  and a second end  32  connected to engine casing  14 . The loads travel from torque box  22  through struts  24  to engine casing  14 . In one embodiment, struts  24  have an elliptical shape and are sized to take a load and transfer it in a vertical direction toward engine casing  14 . In one embodiment, nine struts are positioned approximately forty degrees apart from one another along the circumference of torque box  22 . In another embodiment, twelve total struts are positioned approximately thirty degrees apart from one another along the circumference of torque box  22 . 
         [0020]      FIGS. 3A and 3B  show a cross-sectional view and a schematic diagram of a first embodiment of torque box  22   a , respectively, and will be discussed in conjunction with one another. Torque box  22   a  is U-shaped and generally includes U-stem  34   a  and U-branch  36   a . U-stem  34   a  of mid-turbine frame  12  has a first portion  38 , a second portion  40 , and a U-shaped center portion  42 . U-stem  34   a  is positioned below torque box  22  and connects first and second bearing cones  26  and  28  to each other as well as to torque box  22   a . First portion  38  of U-stem  34   a  extends from center portion  42  towards first bearing  18  and also functions as first bearing cone  26 . Second portion  40  of U-stem  34   a  extends from center portion  42  towards second bearing  20  and also functions as second bearing cone  28 . First and second bearing cones  26  and  28  are thus part of U-stem  34   a  and merge together at center portion  42 . The loads of first and second bearing cones  26  and  28  are introduced into torque box  22   a  at center portion  42  U-stem  34   a . Due to the shell shape of U-stem  34   a , mid-turbine frame  12  can handle large loads at a time without deflecting. U-stem  34   a  also acts as a protective heat shield and provides thermal protection to torque box  22   a.    
         [0021]    U-branch  36   a  has a first end  44  and a second end  46 . First end  44  of U-branch is connected to torque box  22   a  and second end  46  of U-branch  36   a  is connected to U-stem  34   a  at center portion  42  of U-stem  34   a . By connecting U-branch  36   a  to center portion  42  of U-stem  34   a , U-branch  36   a  can function as a bearing arm load transfer member. 
         [0022]      FIG. 4  is a free body diagram of torque box  22   a  connected to first and second bearings  18  and  20 . The loads, or reaction forces, from first and second bearings  18  and  20  come through first and second bearing cones  26  and  28 , Fbearing 1  and Fbearing 2 , respectively. Reaction forces Fbearing 1  and Fbearing 2  come in at an angle and intersect at U-stem  34   a . The reaction forces are then broken up into simple vectors with horizontal components Hbearing 1  and Hbearing 2  and vertical components Vbearing 1  and Vbearing 2 . The horizontal components Hbearing 1  and Hbearing 2  come in at opposite directions and cancel each other out a center portion  42  of U-stem  34   a . Because the horizontal components Hbearing 1  and Hbearing 2  cancel each other out, only the vertical components Vbearing 1 +bearing 2  are transferred through U-stem  34   a  and U-branch  36   a  to torque box  22   a . The total load is thus reduced due to the absorptive components being cancelled at center portion  42  of U-stem  34   a.    
         [0023]      FIGS. 5A and 5B  show a cross-sectional view and a schematic diagram of a second embodiment of torque box  22   b , respectively, and will be discussed in conjunction with one another. Torque box  22   b  is X-shaped and generally includes X-stem  34   b  and X-branch  36   b . Similar to torque box  22   a , first and second bearings  18  and  20  are connected to X-shaped mid-turbine frame  22   b  by first and second bearing cones  26  and  28 , respectively. The loads from first and second bearings  18  and  20  travel through first and second bearing cones  26  and  28  respectively, and are transferred to torque box  22   b . Torque box  22   b  then transfers the load to engine casing  14  and mounts  16 . 
         [0024]    X-stem  34   b  of torque box  22   b  has a first portion  48 , a second portion  50 , and an X-shaped center portion  52 . X-stem  34   b  is positioned below torque box  22   b  and connects first and second bearing cones  26  and  28  to each other as well as to torque box  22   b . First portion  48  of X-stem  34   b  extends from center portion  52  towards first bearing  18  and also functions as first bearing cone  26 . Second portion  50  of U-stem  34   b  extends from center portion  52  towards second bearing  20  and also functions as second bearing cone  28 . First and second bearing cones  26  and  28  are thus part of X-stem  34   b  and merge together at center portion  52 . X-stem  34   b  acts as a protective heat shield and provides thermal protection to torque box  22   b . The loads of first and second bearing cones  26  and  28  are also introduced into torque box  22   b  at X-stem  34   b.    
         [0025]    X-branch  36   b  has a first end  54  and a second end  56 . First end  54  of X-branch  36   b  is connected to torque box  22   b  and second end  56  of X-branch  36   b  is connected to X-stem  34   b  at center portion  52  of X-stem  34   b . By connecting X-branch  36   b  to center portion  52  of X-stem  34   b , X-branch  36   b  can function as a bearing arm load transfer member. 
         [0026]    In operation, X-stem  34   b  of torque box  22   b  functions similarly to U-stem  34   a  of torque box  22   a  except that due to the X-shape of center portion  52 , there is a scissor action that causes an additional load and local state of stress at center portion  52 . Thus, while torque box  22   b  also has increased structural efficiency, the amount of load that torque box  22   b  can support before deflecting will be less than the amount of load that torque box  22   a  can support. 
         [0027]    The torque box designs of the mid-turbine frame offer a lightweight structure with increased structural efficiency. The torque box has a single point transfer structure that delivers the loads from a first second bearing in the gas turbine engine. The single point transfer structure thus functions partly as a first and a second bearing cone. The loads from the first and second bearings combine at the single point transfer structure to a single load transfer point. Because the loads from the first and second bearings enter the single point transfer structure at an angle, the horizontal components of the loads cancel each other out. The only remaining force is in the vertical direction. The loads are combined and transferred to the torque box, which subsequently transfers the loads to a plurality of struts attached to the torque box. The struts are attached to an engine casing surrounding the mid-turbine frame, and delivers the load from the torque box to the engine casing. In one embodiment, the single point transfer structure has a U-shape. In another embodiment, the single point transfer structure has an X-shape. 
         [0028]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.