Abstract:
A fan rotor apparatus includes: a rotatable fan disk defining a central aperture and carrying an array of airfoil-shaped fan blades around its periphery, the disk having a forward end and an aft end; and an annular, generally axially-extending forward fan shaft extending through the aperture and coupled to the fan disk for rotation therewith, where the forward fan shaft joins the fan disk at or near the forward end.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to gas turbine engines and more specifically to bearing assemblies and load reduction devices for gas turbine engines. 
         [0002]    A gas turbine engine includes a turbomachinery core having a high pressure compressor, combustor, and high pressure turbine in serial flow relationship. The core is operable in a known manner to generate a primary flow of propulsive gas. A typical turbofan engine adds a low pressure turbine driven by the core exhaust gases which in turn drives a fan rotor through a shaft to generate a bypass flow of propulsive gas. In the case of a high bypass engine this provides the majority of the total engine thrust. 
         [0003]    The fan rotor includes a fan that includes an array of fan blades extending radially outward from fan disk. The fan shaft transfers power and rotary motion from the low pressure turbine to the fan disk and is supported in several rolling-element bearing assemblies spaced along its length. The bearings are commonly referred to as no. 1, no. 2, and no. 5 bearings, identifying their sequential position in the engine. 
         [0004]    During operation of the engine, a fragment of a fan blade may become separated from the remainder of the blade as a result of impact with a foreign object. Accordingly, a substantial rotary unbalance load may be created within the damaged fan and carried by the bearings, bearing supports, and the fan support frames. 
         [0005]    To minimize the effects of potentially damaging abnormal imbalance loads, known engines include support components for the fan rotor support system that are sized to provide additional strength for the fan support system. However, increasing the strength of the support components undesirably increases an overall weight of the engine and decreases an overall efficiency of the engine when the engine is operated without substantial rotor imbalances. 
         [0006]    Other known engines include a bearing support that includes a mechanically weakened section, or primary fuse, that decouples the fan rotor from the fan support system. During such events, the fan shaft seeks a new center of rotation that approximates that of its unbalanced center of gravity. This fuse section, in combination with a rotor clearance allowance, is referred to as a load reduction device, or “LRD”. The LRD reduces the rotating dynamic loads to the fan support system. 
         [0007]    For the LRD to operate successfully, it is often desirable to have a specific ratio of the axial distance from the fan disk to the #5 bearing, divided by the axial distance from the #2 bearing to the #5 bearing. However, newer engine designs with long cores and short forward overhangs do not provide sufficient axial length for this configuration. 
         [0008]    Accordingly, there is a need for a fan rotor load reduction device which is effective in a limited axial space. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0009]    These and other shortcomings of the prior art are addressed by the present invention, which provides a forward fan shaft with increased flexibility in a given space. 
         [0010]    According to one aspect, the invention provides a fan rotor apparatus including: a rotatable fan disk defining a central aperture and carrying an array of airfoil-shaped fan blades around its periphery, the disk having a forward end and an aft end; an annular, generally axially-extending forward fan shaft extending through the aperture and coupled to the fan disk for rotation therewith, where the forward fan shaft joins the fan disk at or near the forward end. 
         [0011]    According to another aspect of the invention, a turbofan engine includes: a turbomachinery core operable to produce a flow of pressurized combustion gases; a turbine disposed aft of the core; a rotatable fan disk mounted forward of the core, the fan disk defining a central aperture and carrying an array of airfoil-shaped fan blades around its periphery, the disk having a forward end and an aft end; and an annular, generally axially-extending forward fan shaft mechanically coupled to the turbine, the forward fan shaft extending through the aperture and coupled to the fan disk for rotation therewith, where the forward fan shaft joins the fan disk at or near the forward end. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0013]      FIG. 1  is a schematic cross-sectional view of a prior art gas turbine engine; 
           [0014]      FIG. 2  is an enlarged view of a portion of a gas turbine engine incorporating a load reduction device constructed according to an aspect of the present invention of ; and 
           [0015]      FIG. 3  is a cross-sectional view of a portion of a gas turbine engine showing an alternative load reduction device. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  schematically depicts a prior art 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”. 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 other types of turbine engines. 
         [0017]    The inner shaft  18  comprises a forward fan shaft  28  and a rear fan shaft  30  coupled together and mounted for rotation in several rolling-element bearings. The forward fan shaft  28  is carried by a first bearing  32  (commonly referred to as a “no. 1 bearing”) and a second bearing  34  (commonly referred to as a “no. 2 bearing”). The rear fan shaft  30  is carried by a bearing  36  (commonly referred to as a “#5 bearing”). 
         [0018]    The fan  12  of the engine  10  shown in  FIG. 1  is coupled to the forward fan shaft  28  in accordance with prior art principles. In contrast,  FIG. 2  illustrates a fan  112  and surrounding structure which are constructed according to an aspect of the present invention, and which may be incorporated in the engine  10 . The fan  112  comprises a fan disk  138  with a central aperture  139 . The fan disk  138  has an annular array of airfoil-shaped fan blades  140  mounted around its periphery. The fan disk  138  has a forward end  142  and an aft end  144 . An annular disk arm  150  extends at an angle axially forward and radially inward from the forward end  142  of the disk  138 . A forward fan shaft  128  extends between the fan disk  138  and a rear fan shaft  130 , and is coupled to the rear fan shaft  130  for rotation therewith, for example by a bolted joint or a splined connection. The forward fan shaft  128  comprises part of a load reduction device. 
         [0019]    A no. 1 bearing  132  is mounted to a surrounding structural support frame  146  by an annular, generally axially-extending fuse  148 . In accordance with known principles, the size, material, and mechanical design of the fuse  148  is selected to fail at a predetermined radial load, such as a load that might occur after separation of a fan blade  140 . Failure of the fuse  148  allows the fan disk  138  to rotate about a new axis of rotation without imposing excessive radial loads on the surrounding structure. Other types of fuse structures are known, such as bolted joints or fuse pins designed to fail in tension or in shear, or collapsible member(s) in a frame designed to crush at designated loads. The specific type of fuse structure is not critical to the present invention. 
         [0020]    In contrast with prior art designs, the forward fan shaft  128  extends axially forward past the aft end  144  of the fan disk  138 , traversing the longitudinal extent of the fan disk  138 , and is coupled to the fan disk  138  at a point at or near the forward end  142  of the fan disk  138 . As used herein, the term “coupled to the fan disk at or near the forward end” means that torque is transferred from the forward fan shaft  128  to the fan disk  138  through a load path passing at or through the disk&#39;s forward end. It does not necessarily imply any specific type of mechanical connection between the forward fan shaft  128  and the fan disk  138 , or require any specific location of a mechanical joint between the two components. In the example shown in  FIG. 2 , the forward fan shaft  128  includes a tapered aft portion  152 , a generally cylindrical axial portion  154 , and a flange  156  which extends radially outward from the forward end of the axial portion  154 . The flange  156  is coupled to the disk arm  150  for rotation therewith, for example using a bolted or splined connection. As a result, the forward fan shaft  128  is substantially less stiff in bending than the prior art design shown in  FIG. 1 ). 
         [0021]    The disk arm  150  shown in  FIG. 2  could extend axially forward or aft of the forward end  142  of the fan disk  138 . The angle and cross-sectional shape of the disk arm  150  may be varied to provide a bending stiffness suitable for each particular application. Also, while the forward fan shaft  128  is shown as being a single integral component, it could be built up from two or more sections joined together, for example using bolted joints. 
         [0022]      FIG. 3  illustrates an alternative fan  212  and surrounding structure, including a frame  246 , fuse  248 , and bearing  232 . A forward fan shaft  228  has a tapered aft portion  252  coupled to a rear fan shaft  230  and a generally cylindrical axial portion  254 . An annular fan disk  238  carries fan blades  240  and has a forward end  242  and aft end  244 . An annular disk arm  250  extends generally axially forward and radially inward from the forward end  242  of the disk  238 . The disk arm  250  has a forward portion  256  which extends forward, then curves backward in a “C”-shape, and an aft portion  258  which extends generally axially aft. The aft portion  258  is coupled to the forward fan shaft  228  for rotation therewith, for example with a bolted or splined joint. The additional arc length of the curved portion of the disk arm  250  provides an opportunity to further increase and tune the flexibility of the forward fan shaft  228 . Alternatively, the additional curve and arc length could be incorporated into the forward fan shaft  228  instead of the disk arm  250 . Furthermore, any of the fan shafts described herein could me made all or partially integral with the fan disk. 
         [0023]    In operation, the forward fan shaft design described herein permits the fan rotor to safely windmill after a blade release event while limiting the bending loads applied to the core. This can be achieved without the need for any specific engine length or bearing position requirements. 
         [0024]    The foregoing has described load reduction device 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, the invention being defined by the claims.