Abstract:
Gas turbine engines and related systems involving offset hub struts are provided. In this regard, a representative bearing assembly for a gas turbine engine includes: a bearing operative to support a rotatable shaft; an annular hub positioned about the bearing; and an annular array of struts extending radially outwardly from the hub, at least two of the struts being positioned in different planes, the planes being oriented transversely with respect to the rotatable shaft. The method of assembling the struts by mounting first and second pluralities of struts, each plurality in a common plane, between the hub and the turbine exhaust case, with the second plurality of struts longitudinally offset from the first plurality of struts.

Description:
BACKGROUND 
     1. Technical Field 
     The disclosure generally relates to gas turbine engines. 
     2. Description of the Related Art 
     It is generally acknowledged that gas turbine engines for aircraft and other uses have a tendency to vibrate and generate noise at certain load ratings. To improve engine performance, it is desirable to reduce vibrations by strengthening the components of the engine and increasing the durability of the engine without increasing the weight of the engine. Also, it is desirable to shield the components of the engine from high temperatures where practical. 
     In the turbine case of a gas turbine engine, there are radially extending struts that are disposed in the path of the hot gases being exhausted from the turbine. The struts extend radially with respect to a longitudinal axis of the engine. The struts extend radially inwardly from the annular turbine exhaust case, through the path of the hot exhaust gases toward the longitudinal axis of the engine to a single axial location that is centrally located on the hub or “torque box.” As such, the struts are disposed in a single plane, positioned at an axial location as measured along the longitudinal axis of the engine. The struts support the hub, which, in turn, supports the bearings of the turbine. The struts, hub, bearings and other components of the engine are constructed in an attempt to withstand the engine vibrations and other load-bearing forces, such as gyroscopic forces and gravitational or G forces. 
     Because of the extreme heat of the exhaust gases flowing about the struts, it has become common practice to shield the struts from the high temperature and velocity of the exhaust gas by applying fairings about the struts. Typically, the fairings are aerodynamically shaped and tend to divert the hot gases around the struts. 
     It is desirable to make the struts relatively thick to increase the strength of the struts. However, by enlarging the breadth of the struts, the enlarged struts require more lateral space. The enlarged struts cause the fairings that are adjacent the struts to be larger and the larger fairings tend to apply more resistance to the flow of the hot gases through the turbine section. 
     It is also desirable to make the struts and the adjacent fairings relatively thin to reduce the drag associated with the fairings. However, by reducing the breadth of the fairings and the associated struts, the strength of the struts is also reduced. Unfortunately, the reduced strength of the struts tends to allow the hub to be more susceptible to the above described forces that may result primarily due to the offset bearing loads carried by the hub. 
     SUMMARY 
     Gas turbine engines and related systems involving a hub supported by offset struts are provided. In this regard, an exemplary embodiment of a bearing assembly for a gas turbine engine comprises: a bearing operative to support a rotatable shaft; an annular hub positioned concentrically about the bearing; and an annular array of struts extending radially outwardly from the hub, at least two of the struts being positioned in different planes, the planes being oriented transversely with respect to the rotatable shaft. 
     An exemplary embodiment of a gas turbine engine comprises: at least one set of rotatable blades operative to engage a stream of oncoming gas moving in a longitudinal, annular path through the engine; an annular turbine exhaust case positioned downstream of the rotatable blades and being operative to exhaust the gas; an annular hub positioned concentrically within the turbine exhaust case; and an annular array of struts positioned across the gas path and extending radially between the hub and the turbine exhaust case, at least one of the struts being longitudinally offset with respect to at least another of the struts. 
     Another exemplary embodiment of a gas turbine engine comprises: an annular turbine exhaust case; a hub positioned concentrically within the turbine exhaust case; a first strut connected to and extending substantially radially from the hub to the turbine exhaust case; and a second strut connected to and extending substantially radially from the hub to the turbine exhaust case, the second strut being longitudinally offset, at the hub, with respect to the first strut. 
     Other systems, methods, features and/or advantages of this disclosure will be or may become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features and/or advantages be included within this description and be within the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a side cross-sectional view of an exemplary embodiment of a gas turbine engine. 
         FIG. 2  is a cross-sectional view of the upper half of a turbine exhaust case of the embodiment of  FIG. 1 , showing a strut positioned in a forward offset position when connected to a hub. 
         FIG. 3  is a cross-sectional view of the upper half of the turbine exhaust case of the embodiment of  FIG. 1 , but showing another strut that is positioned in an aft offset position when connected to the hub. 
         FIG. 4  is an end view of the hub and the adjacent portions of the struts connected to the hub, with the fairings removed from the struts. 
         FIG. 5  is a top view of the hub and the adjacent portions of the struts with the fairings removed from the struts, taken along lines  5 - 5  of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     Gas turbine engines and related systems involving a hub with offset struts are provided, and exemplary embodiments will be described in detail. In some embodiments, some of the struts are connected to the hub at different distances along the length of the hub from others of the struts. The length of the hub is measured along the longitudinal axis of the engine. Because the connections of the struts to the hub are longitudinally offset with respect to a single axial location that is centrally located on the hub, these offset connections tend to facilitate stabilizing the position of the hub within the turbine exhaust case. In some of these embodiments, each strut may be associated with fairings adjacent the struts to facilitate shielding the struts from excessive heat of the exhaust gases of the engine. 
     Referring now to the drawings,  FIG. 1  is a diagram depicting a representative embodiment of a gas turbine engine  100 . Although engine  100  is configured as a dual-spool turbofan, there is no intention to limit the invention to use with turbofans (or dual-spool configurations) as use with other types (and configurations) of gas turbine engines is contemplated. 
     As shown in  FIG. 1 , gas turbine engine  100  incorporates an engine casing  101  that houses a fan  102 , a compressor section  104 , a combustion section  106  and a turbine section  108 . Compressor section  104  includes a low-pressure compressor  112  and a high-pressure compressor  114 , and turbine section  108  includes a high-pressure turbine  116  and a low-pressure turbine  118 . A low shaft  120  interconnects low-pressure turbine  118  with low-pressure compressor  112 , and a high shaft  122  interconnects high-pressure turbine  116  with high-pressure compressor  114 . Notably, shafts  120  and  122  are supported by a bearing assembly  130 , which generally includes bearings for the shafts and an associated hub and struts that support the bearings (described in detail later). The length of the hub and the longitudinal location of struts are herein described as measured along a longitudinal axis  124  of the gas turbine engine  100 . 
     During the operation of the gas turbine engine  100 , the combustion section  106  supplies fuel to air that is being drawn through the engine by the compressor section  104 . As a result, the combustion of the fuel creates a hot gas stream that flows through turbine section  108 . The hot gas stream impinges upon the blades of the turbines  116 ,  118  to facilitate driving the corresponding compressors through shafts  120 ,  122 . 
     Generally, the components of the gas turbine engine  100  tend to vibrate and G forces and gyroscopic forces tend to be applied through the bearings to the hub  142  (shown in  FIGS. 2 and 3 ). In particular, forces are transmitted between the hub  142 , struts  144 ,  146  and engine casing  101 . Notably, the exemplary bearing assembly  130  includes offset struts  144 ,  146 . 
     As shown in  FIGS. 2 and 3 , bearing assembly  130  includes a low bearing  168  (which supports shaft  120 ) and a high bearing  170  (which supports shaft  122 ). The bearings  168 ,  170  interconnect with the hub  142 . 
     As shown in  FIGS. 4 and 5 , multiple struts are attached to the hub  142  and extend radially outwardly from the hub  142  to the engine casing  101 . Both the forward struts  144  and the aft struts  146  extend radially from the longitudinal axis  124  of the engine. The struts  144 ,  146  are connected to a turbine exhaust case portion  152  of the engine casing  101 . Notably, the embodiment of  FIGS. 2 and 3  includes two sets of substantially cylindrical struts, with the forward struts  144  shown in  FIG. 2  and the aft struts  146  shown in  FIG. 3 . While twelve struts are shown in  FIG. 4 , in other embodiments other numbers of sets and configurations of struts may be used. 
     As shown in  FIGS. 4 and 5 , forward struts  144  are aligned in a forward plane  145  that is oriented transversely with respect to the longitudinal axis  124  of the gas turbine engine  100  and shafts  120 ,  122 . As shown in  FIG. 4 , the forward struts  144  are formed in a radial array about the hub  142  with the inner end portions  148  mounted to the hub  142  and the outer portions  150  ( FIG. 2 ) mounted to the engine casing  101 . In the exemplary embodiment, the inner end portions  148  of the forward struts  144  are threadedly mounted into threaded openings (not shown) formed in the hub  142 . 
     Likewise, the aft struts  146  are aligned in an aft plane  147  that is oriented transversely with respect to the longitudinal axis  124  of the gas turbine engine  100  and shafts  120 ,  122 . The aft struts  146  of the rearward array of struts also are threadedly connected at their inner end portions  154  to the hub  142 , and their outer end portions  156  are connected to the engine casing  101 . In other embodiments, the struts  144 ,  146  may be connected at their inner and outer ends by various means to the hub  142  and engine casing  101 , such as by threads, welding, pins, or other means. 
     Fairings, such as fairings  160  of  FIG. 2  of the forward struts  144 , are adjacent the intermediate portion of each of the forward struts  144  for the purpose of directing the hot gases about the forward struts  144 . Likewise, fairings  162  of  FIG. 3  are adjacent the intermediate portion of each of the aft struts  146  for the same purpose as the forward struts  144 . The fairings  160 ,  162  may be attached at their inner and outer ends to the hub  142  and to the engine casing  101 , respectively, thereby also tending to shield the hub and the engine casing from the hot gases. While  FIGS. 4 and 5  do not show the fairings so as to better illustrate the struts, it will be understood that a fairing may be positioned about some or all of the struts. 
     Since the array of forward struts  144  are longitudinally offset from the array of aft struts  146 , i.e., struts  144  and  146  reside in different planes that are oriented transversely with respect to the longitudinal axis  124  of the engine, as shown in  FIGS. 4 and 5 . Thus, the hub  142  is more rigidly supported than the support typically provided by similarly sized struts arranged in an annular, single plane arrangement. Therefore, the vibrations and other forces applied to the hub  142  by the load from the bearings  168  and  170  tend to cause smaller deflections. In this arrangement, the struts  144 ,  146  provide stronger support than when arranged in a single annular plane. 
     By using longitudinally offset forward and aft struts  144  and  146 , a stronger support can be provided to the hub  142  and the dimensions of the struts  144 ,  146  can be reduced without compromising the ability of the struts  144 ,  146  to stabilize the hub  142 . Thus, additional stability is applied to the hub  142  with a reduction in the breadth of the struts  144 ,  146 , thereby allowing the adjacent fairings (e.g., fairings  160 ,  162 ) to be thinner. This facilitates reducing the drag associated with the fairings. 
     In some embodiments, the hub  142  is formed in a substantially cylindrical shape and openings are formed through the hub  142  at the positions where the inner end portions  148 ,  154  of the struts  144 ,  146  are fastened. However, another exemplary hub may have a substantially non-circular shape if desired, such as an octangular shape or other shape that presents flat surfaces for receiving the inner end portions of the struts. For example, if a hub is to be supported by twelve struts, the hub may be formed in a substantially circular shape having twelve circumferentially spaced flats for receiving the inner end portions of the struts. 
     Forward struts  144  may be both longitudinally and circumferentially offset from aft struts  146 . In this regard, struts from one array may be spaced further apart from each other than the struts of another array. Additionally or alternatively, the spacing between the arrays may be different than the spacing between adjacent struts of a given array. 
     While the longitudinal spacing of the struts  144 ,  146  in  FIGS. 2 and 3  may appear to demonstrate a substantial separation of the arrays from each other, it will be understood that arrays of the struts  144 ,  146  may be positioned closer together as shown in  FIG. 5  so that they overlap one another in a common plane. However, increased longitudinal spacing between the arrays of struts tends to facilitate increasing stability applied to the hub. In some embodiments, the struts are longitudinally spaced from one another a distance equal to about one diameter of a strut, but other longitudinal spacings may be used. 
     In some embodiments, both a forward strut and an aft strut may be adjacent a single fairing, such as when the forward and aft strut are positioned in longitudinal alignment with each other. This may be used for the purpose of providing the desired strength of the hub while leaving another position empty of a strut but having a substantially hollow fairing present in the empty position. This arrangement provides more space for routing of fluids or cooling air, for example, through the empty fairing to other portions of the engine. 
     It should be emphasized that the above-described embodiments are merely possible examples of implementations set forth for a clear understanding of the principles of this disclosure. Many variations and modifications may be made to the above-described embodiments without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the accompanying claims.