Abstract:
A bearing support housing for a gas turbine engine includes: an annular mounting flange; a first bearing cage including: an annular first bearing support ring; and an annular array of axially-extending first spring arms interconnecting the first bearing support ring and the mounting flange; and a second bearing cage including: an annular second bearing support ring; and an annular array of axially-extending second spring arms interconnecting the second bearing support ring and the mounting flange, the second spring arms defining spaces therebetween. The first spring arms are received between the second spring arms, and the bearing cages are sized so as to permit independent flexing motion of the first and second spring arms.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to gas turbine engine bearings and more particularly to mounting arrangements for such bearings. 
         [0002]    It is known to support bearings, such as the large rolling-element bearings used in gas turbine engines, using spring centering cages. The spring constant of such cages can be manipulated to provide a desired stiffness and consequently affect the dynamics and vibration modes of the engine. Particularly in large aircraft turbofan engines, it has been demonstrated that engine dynamics will suffer significantly if such cages are not used. 
         [0003]    Many gas turbine engines have at least one sump that includes two or more rolling element bearings positioned in close proximity to each other. These sumps have limited axial and radial space available to be used for bearings, spring cages, intermediate gearbox mounting, damper housings, air and oil seals, air pressurization channels, and oil transport between parts of the sump. The axial and radial space needed for an individual spring centering cage for each bearing, which is greater than required for a conventional stiff bearing mounting, is inconsistent with the need to keep the engine as small and light as possible. 
         [0004]    Accordingly, there is a need for a bearing support adapted to mount multiple rolling element bearings in a confined space. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    This need is addressed by the present invention, which provides an integral component incorporating two spring cages that are nested within each other, so as to operate independently, while only occupying the space normally required for a single spring bearing cage. 
         [0006]    According to one aspect of the invention, a bearing support housing for a gas turbine engine, includes: an annular mounting flange; a first bearing cage including: an annular first bearing support ring; an annular array of axially-extending first spring arms interconnecting the first bearing support ring and the mounting flange; and a second bearing cage including: an annular second bearing support ring; and an annular array of axially-extending second spring arms interconnecting the second bearing support ring and the mounting flange, the second spring arms defining spaces therebetween; wherein the first spring arms are received between the second spring arms, and the bearing cages are sized so as to permit independent flexing motion of the first and second spring arms 
         [0007]    According to another aspect of the invention, a bearing assembly for a gas turbine engine includes: an annular mounting flange secured to a stationary member of the engine; a first bearing cage including: an annular first bearing support ring; and an annular array of axially-extending first spring arms interconnecting the first bearing support ring and the mounting flange; a rolling-element first bearing mounted in the first bearing support ring; a second bearing cage including: an annular second bearing support ring; and an annular array of axially-extending second spring arms interconnecting the second bearing support ring and the mounting flange, the second spring arms defining spaces therebetween; a rolling-element second bearing mounted in the second bearing support ring; and a shaft mounted in the first and second bearings; wherein the first spring arms are received between the second spring arms, and the bearing cages are sized so as to permit independent flexing motion of the first and second spring arms. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0009]      FIG. 1  is a half-cross-sectional view of a gas turbine engine incorporating nested bearing spring cages constructed according to an aspect of the present invention; 
           [0010]      FIG. 2  is an enlarged view of a bearing compartment of the gas turbine engine of  FIG. 1 ; 
           [0011]      FIG. 3  is a perspective view of a bearing support housing shown in  FIG. 2 ; 
           [0012]      FIG. 4  is a sectional perspective view of a portion of the bearing support housing shown in  FIG. 3 ; 
           [0013]      FIG. 5  is an enlarged view of a bearing compartment, showing an alternative bearing support housing; 
           [0014]      FIG. 6  is a sectional view of a portion of the bearing support housing shown in  FIG. 5 ; and 
           [0015]      FIG. 7  is a perspective view of the bearing support housing shown in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  depicts a gas turbine engine  10 . The engine  10  has a longitudinal axis  11  and includes a fan  12 , a low pressure compressor or “booster”  14  and a low pressure turbine (“LPT”)  16  collectively referred to as a “low pressure system”. The LPT  16  drives the fan  12  and booster  14  through an inner shaft  18 , also referred to as an “LP shaft”. The engine  10  also includes a high pressure compressor (“HPC”)  20 , a combustor  22 , and a high pressure turbine (“HPT”)  24 , collectively referred to as a “gas generator” or “core”. The HPT  24  drives the HPC  20  through an outer shaft  26 , also referred to as an “HP shaft”. Together, the high and low pressure systems are operable in a known manner to generate a primary or core flow as well as a fan flow or bypass flow. While the illustrated engine  10  is a high-bypass turbofan engine, the principles described herein are equally applicable to turboprop, turbojet, and turboshaft engines, as well as turbine engines used for other vehicles or in stationary applications. 
         [0017]    The inner and outer shafts  18  and  26  are mounted for rotation in several rolling-element bearings. The bearings are located in enclosed portions of the engine  10  referred to as “sumps”. 
         [0018]      FIG. 2  shows a portion of a sump of the engine  10  in more detail. The forward end of the outer shaft  26  is carried by a ball-type first bearing  32  and a roller-type second bearing  34  which in common nomenclature are referred to as the “# 3 B bearing” and the “# 3 R bearing”, respectively. A static annular frame member referred to as a fan hub frame  36  surrounds the first and second bearings  32  and  34 . The first and second bearings  32  and  34  are connected to the fan hub frame  36  by a bearing support housing  35 . A stationary damper housing  42  with a cylindrical inner surface  44  surrounds the second bearing  34 . 
         [0019]    As best seen in  FIGS. 3 and 4 , the bearing support housing  35  is a single monolithic component incorporating first and second bearing cages  38  and  40 . The first bearing cage  38  supports the first bearing  32 , and the second bearing cage  40  supports the second bearing  34 . The bearing support housing  35  includes a single annular, radially-extending mounting flange  46  including a plurality of mounting holes  48  which receive fasteners  49  ( FIG. 2 ). The first bearing cage  38  comprises an annular, generally axially-extending first bearing support ring  50 , and a plurality of first spring arms  52  interconnecting the mounting flange  46  and the bearing support ring  50 . In this example the inner surface of the first bearing support ring  50  includes a bearing stop lip  56  and a plurality of holes  58  for receiving bolts  60  ( FIG. 2 ) used to secure the first bearing  32 . Each first spring arm  52  comprises a radially-outwardly extending portion  62  joining the aft end of the first bearing support ring  50 , and an axially-extending portion  64  joining the mounting flange  46 . The first bearing support ring  50  extends generally parallel to the axially-extending portions  62  of the first spring arms  52  and thus lies radially inside the ring of first spring arms  52 . The number, shape, and dimensions of the first spring arms  52  may be modified to suit a particular application, in particular to achieve a desired stiffness of the first bearing cage  38 . It is noted that the first spring arms  52  extend axially aft from the aft face  47  of the mounting flange  46 . Because the relatively large surface area of the aft face  47  serves as a base for the first spring arms  52 , there is significant design freedom to alter the individual cross-sectional shape and dimensions of the first spring arms  52 . 
         [0020]    The second bearing cage  40  is similar in construction to the first bearing cage  38 . It comprises an annular second bearing support ring  66  and a plurality of second spring arms  68  interconnecting the mounting flange  46  and the second bearing support ring  66 . The second bearing support ring  66  includes a generally axially-extending body with a cylindrical inner surface. The outer surface  74  of the second bearing support ring  66 , in cooperation with the damper housing  42 , forms a portion of an oil film damper  76  of a known type. In this example the inner surface of the second bearing support ring  66  defines a bearing stop lip  78 . Each of the second spring arms  68  comprises a radially-outwardly extending portion  82  joining the forward end of the bearing support ring  66 , and an axially-extending portion  86  joining the mounting flange  46 . The number, shape, and dimensions of the spring arms  80  may be modified to suit a particular application, in particular to achieve a desired stiffness of the second bearing cage  40 . 
         [0021]    The first and second bearing cages  38  and  40  are sized such that the first bearing support ring  50  fits inside of and axially overlaps or “nests” within the second bearing cage  40 . More specifically, the outside diameter over the first bearing support ring  50  is less than the inside diameter of the second spring arms  68  of the second bearing cage  40 . Furthermore, the spaces between adjacent second spring arms  68  of the second bearing cage  40  are selected so that the first spring arms  52  of the first bearing cage  38  will fit between them, resulting in an interdigitated configuration. The inner and/or outer radii of the first spring arms  52  may be equal to the inner and/or outer radii of the second spring arms  68 . 
         [0022]    The bearing cages  38  and  40  may be preferentially “clocked” or angularly offset from a symmetrical orientation relative to each other. As seen in  FIG. 3 , the bearing cages  38  and  40  are offset such that a first gap “G 1 ” between each first spring arm  52  and the adjacent second spring arm  68  on one side is less than a second gap “G 2 ” between the same first spring arm  52  and the adjacent second spring arm  68  on the other side. This clocking is useful to provide space for the passage of oil lines or other similar structures (not shown), where equal gaps might provide insufficient clearance. 
         [0023]    In operation, the spring arms of the first and second bearing cages  38  and  40  are free to move independently of one another, as required by flight loads and the dynamics of the first and second bearings  32  and  34 . This allows the harmonic response of the bearings  32  and  34  to be controlled independently. 
         [0024]      FIG. 5  shows a portion of a sump of an engine, similar to the engine  10 , including an outer shaft  126 , and incorporating an alternative bearing mounting arrangement. The forward end of the outer shaft  126  is carried by a ball-type first bearing  132  and a roller-type second bearing  134  which in common nomenclature are referred to as the “# 3 B bearing” and the “# 3 R bearing”, respectively. A static annular frame member referred to as a fan hub frame  136  surrounds the first and second bearings  132  and  134 . The first and second bearings  132  and  134  are connected to the fan hub frame  136  by a bearing support housing  135 . A stationary damper housing  142  with a cylindrical inner surface  144  surrounds the second bearing  134 . 
         [0025]    As seen in  FIGS. 6 and 7 , the bearing support housing  135  is a single monolithic component incorporating first and second bearing cages  138  and  140 . The first bearing cage  138  supports the first bearing  132 , and the second bearing cage  140  supports the second bearing  134 . The bearing support housing  135  includes a single annular, radially-extending mounting flange  146  including a plurality of mounting holes  148  which receive fasteners  149  ( FIG. 5 ). The first bearing cage  138  comprises an annular, generally axially-extending first bearing support ring  150 , and a plurality of first spring arms  152  interconnecting the mounting flange  146  and the first bearing support ring  150 . In this example the inner surface of the first bearing support ring  150  has a bearing stop lip  156  and a plurality of holes  158  for receiving bolts  159  ( FIG. 5 ) used to secure the first bearing  132 . Each of the first spring arms  152  comprises a radially-outwardly extending portion  156  joining the aft end of the bearing support ring  150 , and an axially-extending portion  160  joining the mounting flange  146 . The number, shape, and dimensions of the first spring arms  152  may be modified to suit a particular application, in particular to achieve a desired stiffness of the first bearing cage  138 . 
         [0026]    The second bearing cage  140  is similar in construction to the first bearing cage  138  and comprises an annular second bearing support ring  166 , and a plurality of second spring arms  168  interconnecting the mounting flange  146  and the second bearing support ring  166 . The second bearing support ring  166  includes a generally axially-extending body  170  with a cylindrical inner surface  172 . The outer surface  174  of the second bearing support ring  166 , in cooperation with the damper housing  142 , forms a portion of an oil film damper  176  of a known type. In this example the inner surface of the second bearing support ring  166  defines a bearing stop lip  178 . Each of the second spring arms  168  comprises a radially-outwardly extending portion  182  joining the forward end of the second bearing support ring  166 , and an axially-extending portion  186  joining the mounting flange  146 . The number, shape, and dimensions of the second spring arms  168  may be modified to suit a particular application, in particular to achieve a desired stiffness of the second bearing cage  140 . As with the bearing support housing  35  described above, there is wide flexibility to change the specific shape and dimensions of the first and second spring fingers  152  and  168 . 
         [0027]    The first and second bearing cages  138  and  140  are sized such that the first bearing support ring  150  fits inside of and axially overlaps or “nests” within the second bearing cage  140 . More specifically, the outside diameter over the first bearing support ring  150  is less than the inside diameter of the second spring arms  168  of the second bearing cage  140 . Furthermore, the spaces between adjacent second spring arms  168  of the second bearing cage  140  are selected so that the first spring arms  152  of the first bearing cage  138  will fit between them, resulting in an interdigitated configuration. The inner and/or outer radii of the first spring arms  152  may be equal to the inner and/or outer radii of the second spring arms  168 . 
         [0028]    The bearing cages  138  and  140  may be preferentially “clocked” or angularly offset from a symmetrical position relative to each other, as described above. In the example illustrated in  FIGS. 5-7 , the bearing cages  138  and  140  are clocked symmetrically to each other. One or more release slots  180  are formed at the forward end of the second bearing support ring  166  to provide for the passage of an oil line or nozzle (not shown). 
         [0029]    The operation of the bearing support housing  135  is substantially identical to the operation of the bearing support housing  35  described above. 
         [0030]    The bearing support housing configurations described above significantly reduce the axial and radial space required to fit multiple spring bearing cages into a bearing sump by nesting the cages together so they occupy the axial and radial space of one bearing cage. Engines which previously would have been unable to accommodate multiple spring bearing cages and dampers in the available sump space can now be arranged to include these features. While the nested bearing cage concept has been described with respect to a particular bearing arrangement, the concept may be used in any sump or location in the engine where it is desirable to provide multiple spring cages in a limited space. In addition to the overall product benefits of reduced part count (e.g. simplified logistics, handling, assembly), the single-piece design described herein also allows for the elimination of a joint between bearing cages, thus simplifying the flange configuration and reducing the overall stack-up. 
         [0031]    The foregoing has described a bearing support housing for a gas turbine engine. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.