Abstract:
A turbine housing section includes a radially inner case centered on a first axis, and a radially outer case spaced radially outwardly of the inner case, and centered on a second axis. The first and second axes are offset relative to each other. A plurality of tie rods include a threaded nut received on a tie rod, with the plurality of tie rods connecting the inner and outer cases. The plurality of tie rods are spaced circumferentially about both of the first and second axes, and extend for distinct lengths between the inner and outer cases such that the inner and outer cases are held at a position wherein the first and second axes are offset

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application relates to a method and apparatus for adjusting a center line for a bearing mount relative to a turbine casing utilizing adjustable tie rods. 
         [0002]    Gas turbine engines are known, and typically include a compressor compressing air and delivering it into a combustion section where it is mixed with fuel and ignited. Products of this combustion pass downstream over turbine rotors, driving them to rotate. The turbine rotors drive the compressor rotors. 
         [0003]    In a gas turbine engine utilized on an aircraft, a fan typically delivers a portion of its air into a bypass duct, and a portion of air into the compressor. There are often two turbine rotors, and two compressor rotors. A first turbine rotor sees a higher pressure and rotates at a higher speed, and rotates a downstream or higher pressure compressor rotor. A second turbine rotor is downstream of the first, and thus sees a lower pressure and rotates at a lower speed than the first turbine rotor. 
         [0004]    Historically, the lower pressure turbine drove the lower pressure compressor rotor and the fan at one speed. More recently, a gear reduction has been included between the fan and the low pressure turbine rotor, such that the fan and the low pressure compressor can be rotated at different speeds. 
         [0005]    This development has led to freedom in the design of the turbine section. In one turbine section there is a mid-turbine casing or frame mounted between the low and higher pressure turbine rotors. 
         [0006]    The mid-turbine casing includes an inner case and an outer case which are connected by threaded tie rods. A bearing is mounted within an inner bore in the inner case, and a turbine flow area is defined between an outer periphery of the inner case and an inner periphery of the outer turbine case. 
         [0007]    For any number of reasons, it is often the case that a center line of the outer turbine case may desirably be offset from a center line of the bearing. This may occur due to a desire to optimize blade tip clearances within the engine. Further, the rotor may sag due to weight, deflection from maneuvers, and case distortion from thrust, pressure, temperature, and maneuvers. All of this may result in a nominal tip clearance being uneven around an engine circumference. 
         [0008]    Thus, it is known to offset the bearing by machining the inner case bearing flanges to be nonconcentric with the outer case flanges. This requires a long lead time to coordinate manufacturing, additional offset tool setup, and is non-adjustable once machined. 
       SUMMARY OF THE INVENTION 
       [0009]    In a featured embodiment, a turbine section has a turbine housing section including a radially inner case centered on a first axis, and a radially outer case spaced radially outwardly of the inner case, and centered on a second axis. The first and second axes are offset relative to each other. A plurality of tie rods includes a threaded nut received on a tie rod, with the plurality of tie rods connecting the inner and outer cases. The plurality of tie rods are spaced circumferentially about both of the first and second axes, and extend for distinct lengths between the inner and outer cases such that the inner and outer cases are held at a position wherein the first and second axes are offset. 
         [0010]    In another embodiment according to the previous embodiment, the tie rods include a nut positioned radially outwardly of the outer case. The tie rods include a pin head positioned radially inwardly of the inner case. The nut is tightened on the tie rod to adjust the length of the tie rod and the distance between the inner and outer cases to adjust the location of the first and second axes. 
         [0011]    In another embodiment according to any of the previous embodiments, a flange is positioned within the inner case to mount a bearing for mounting a turbine rotor, and such that a center line of the bearing will be offset from a center line of the outer case. 
         [0012]    In another featured embodiment, a gas turbine engine has a fan and a compressor. The fan delivers air into the compressor, and into a bypass duct, a combustor section, and a first and second turbine rotor downstream of the combustion section. The first turbine rotor is positioned upstream of a second turbine rotor, and drives a first compressor rotor which is downstream of a second compressor rotor. The second turbine rotor drives the second compressor rotor and the fan. A gear reduction is positioned between the fan and the second turbine rotor. There is a mid-turbine frame positioned between the first and second turbine rotors to communicate products of combustion downstream of the first turbine rotor to the second turbine rotor. The mid-turbine frame includes a radially inner case centered on a first axis, and a radially outer case spaced radially outwardly of the inner case, and centered on a second axis. The first and second axes are offset relative to each other. A plurality of tie rods includes a threaded nut received on a tie rod. The plurality of tie rods connect the inner and outer cases. The plurality of tie rods are spaced circumferentially about both of the first and second axes, and extend for distinct lengths between the inner and outer cases such that the inner and outer cases are held at position wherein the first and second axes are offset. 
         [0013]    In another embodiment according to the previous embodiment, the tie rods include a nut positioned radially outwardly of the outer case. The tie rods include a pin head positioned radially inwardly of the inner case. The nut is tightened on the tie rod to adjust the length of the tie rod and the distance between the inner and outer cases to adjust the location of the first and second axes. 
         [0014]    In another embodiment according to any of the previous embodiments, a flange is positioned within the inner case to mount a bearing mounting the first turbine rotor, and such that a center line of the bearing will be offset from a center line of the outer case. 
         [0015]    In another featured embodiment, a method of adjusting the location of a bearing in a turbine section includes the steps of connecting an inner turbine case to an outer turbine case with a plurality of tie rods with the tie rods secured with nuts between the inner and outer turbine cases. An offset is determined for a bearing to be mounted within the inner turbine case relative to a center line of the outer turbine case. The nuts are adjusted on the tie rods to move a center line of the inner turbine case relative to a center line of the outer turbine case to a desired location such that the bearing is at the desired location. 
         [0016]    These and other features may be best understood from the following specification and drawings, the following which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  schematically shows a gas turbine engine. 
           [0018]      FIG. 2  shows a feature in a mid-turbine frame section. 
           [0019]      FIG. 3  shows an adjustment of the mid-turbine frame section. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  schematically illustrates a gas turbine engine  20 . The gas turbine engine  20  is disclosed herein as a two-spool turbofan that generally incorporates a fan section  22 , a compressor section  24 , a combustor section  26  and a turbine section  28 . Alternative engines might include an augmentor section (not shown) among other systems or features. The fan section  22  drives air along a bypass flow path B in a bypass duct defined within a nacelle  15 , while the compressor section  24  drives air along a core flow path C for compression and communication into the combustor section  26  then expansion through the turbine section  28 . Although depicted as a turbofan gas turbine engine in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are not limited to use with turbofans as the teachings may be applied to other types of turbine engines including three-spool architectures. 
         [0021]    The engine  20  generally includes a low speed spool  30  and a high speed spool  32  mounted for rotation about an engine central longitudinal axis A relative to an engine static structure  36  via several bearing systems  38 . It should be understood that various bearing systems  38  at various locations may alternatively or additionally be provided. 
         [0022]    The low speed spool  30  generally includes an inner shaft  40  that interconnects a fan  42 , a low pressure compressor  44  and a low pressure turbine  46 . The inner shaft  40  is connected to the fan  42  through a geared architecture  48  to drive the fan  42  at a lower speed than the low speed spool  30 . The high speed spool  32  includes an outer shaft  50  that interconnects a high pressure compressor  52  and high pressure turbine  54 . A combustor  56  is arranged between the high pressure compressor  52  and the high pressure turbine  54 . A mid-turbine frame  57  of the engine static structure  36  is arranged generally between the high pressure turbine  54  and the low pressure turbine  46 . The mid-turbine frame  57  further supports bearing systems  38  in the turbine section  28 . The inner shaft  40  and the outer shaft  50  are concentric and rotate via bearing systems  38  about the engine central longitudinal axis A which is collinear with their longitudinal axes. 
         [0023]    The core airflow is compressed by the low pressure compressor  44  then the high pressure compressor  52 , mixed and burned with fuel in the combustor  56 , then expanded over the high pressure turbine  54  and low pressure turbine  46 . The mid-turbine frame  57  includes airfoils  59  which are in the core airflow path. The turbines  46 ,  54  rotationally drive the respective low speed spool  30  and high speed spool  32  in response to the expansion. 
         [0024]    The engine  20  in one example is a high-bypass geared aircraft engine. In a further example, the engine  20  bypass ratio is greater than about six (6), with an example embodiment being greater than ten (10), the geared architecture  48  is an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3 and the low pressure turbine  46  has a pressure ratio that is greater than about 5. In one disclosed embodiment, the engine  20  bypass ratio is greater than about ten (10:1), the fan diameter is significantly larger than that of the low pressure compressor  44 , and the low pressure turbine  46  has a pressure ratio that is greater than about 5:1. Low pressure turbine  46  pressure ratio is pressure measured prior to inlet of low pressure turbine  46  as related to the pressure at the outlet of the low pressure turbine  46  prior to an exhaust nozzle. The geared architecture  48  may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.5:1. It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine and that the present invention is applicable to other gas turbine engines including direct drive turbofans. 
         [0025]    A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The fan section  22  of the engine  20  is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet. The flight condition of  0 . 8  Mach and 35,000 ft, with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (‘TSFC’)”—is the industry standard parameter of 1 bm of fuel being burned divided by 1 bf of thrust the engine produces at that minimum point. “Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.45. “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram ° R)/(518.7° R)] 0.5 . The “Low corrected fan tip speed” as disclosed herein according to one non-limiting embodiment is less than about 1150 ft/second. 
         [0026]      FIG. 2  shows a portion of a casing  80 , which may be at a mid-turbine section of an engine, which may be like the engine  20  of  FIG. 1 . An outer casing  82  is spaced from an inner casing  84 . An inner bore  86  of the inner casing  84  receives a bearing mount or flange  400 . The bearing  403  in the mount (see  FIG. 3 ) mounts one of the turbine rotors, and in one embodiment a high pressure turbine rotor. Flange  400  is shown schematically, and provides the function of mounting the bearing. 
         [0027]    A plurality of circumferentially spaced tie rods  101  have an inner pin head  102  received within apertures in the inner casing  84 , and extend outwardly through apertures in the outer casing  82 . Nuts  100  are secured on the tie rods, and may be tightened to adjust preload and length. 
         [0028]    In the prior art, these tie rods have generally all been adjusted to an identical length such that a center line C of the outer casing  82  is centered on a center line A of the inner casing  84 . 
         [0029]    The outer casing  82  is mounted within the engine utilizing an outer casing flange  401 , shown schematically. Thus, when the center lines C and A are aligned, the outer casing flange  401  mounts the outer casing  82  such that it is centered on the same axis x as the bearing mounted to the flange  400  within the inner casing  84 . A flow area between the casings communicates the products of combustion from the high pressure turbine rotor to the low pressure turbine. 
         [0030]    As mentioned above, under certain conditions, it becomes desirable to adjust the center line of a bearing which is mounted to the bearing flange  400 . The bearing is shown in part and schematically at  403  in  FIG. 3 . 
         [0031]    Thus, for reasons that would be readily apparent to a worker of ordinary skill in the art, it may be desirable that a center line A of the bearing  403  be offset from the center line C by a distance, which is typically small, but can be determined by a worker of ordinary skill in the art. Thus, as shown in  FIG. 3 , by adjusting the lengths of the tie rods  101 , and by tightening the nuts  100  to different extents, the center y of the inner casing  84  can be moved such that its center line A is offset from the center line C of the outer casing  82  defined by a no offset center z. Thus, the tie rod  112  is tightened to be shorter than it was in the  FIG. 2  embodiment, as are the tie rods  110  and  111 . The tie rods  114  that had been at the center line in  FIG. 2 , now extend at an angle, and are longer than they would have been in the  FIG. 2  position. The tie rods  116  are also made to be longer. Now, with the adjustment, the center lines A and C are offset between the inner and outer casings  82  and  84 . 
         [0032]    Airfoils or vanes (see  59  in  FIG. 1 ) are assembled together and radially fixed to the outer casing  82 . These airfoils have their own inner and outer diameter flow path surfaces. One main purpose for adding the ability to adjust the eccentricity provided by this application is to better balance high pressure compressor and high pressure turbine blade tip clearances and rotating seal clearances about their periphery. As can be appreciated, seal clearances are generally measured in a few thousandths of an inch, so tight control of the clearance around a perimeter is valuable. 
         [0033]    A designer of the gas turbine engine turbine section would recognize how to offset the desirability of properly centering the center line of bearing  403  with the change between the flow areas F 1  and F 2  However, by utilizing the adjustable tie rods  101  to provide this adjustment a worker of ordinary skill in this art is provided with a very simple way of adjusting the center line of the bearing  403 , and is also provided with a system that allows it to be easily readjusted as the structure of the turbine section changes with wear and use. 
         [0034]    Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.