Abstract:
Bolt hole stress in rotor disks having bolted joints is reduced by passing relatively hot secondary flow path air (such as compressor discharge air) through each bolt hole to heat the disk from inside the bolt hole. In doing so, the temperature distribution in the area of the bolt hole is made more uniform and the stress is dramatically reduced. The bolted joint includes a bolt hole formed in a first rotor disk and a bolt disposed in the bolt hole such that a channel is defined between the bolt and the bolt hole. A first nut or abutment is attached to a first end of the bolt, and a second nut or abutment is attached to a second end of the bolt. A first passage associated with the first abutment provides fluid communication with the channel, and a second passage associated with the second abutment provides fluid communication with the channel, thereby allowing the relatively hot fluid to pass through the channel during engine operation.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to gas turbine engines and more particularly to bolted joints for joining adjacent rotor disks in such engines.  
           [0002]    A gas turbine engine includes a compressor that provides pressurized air to a combustor wherein the air is mixed with fuel and the mixture is ignited for generating hot combustion gases. These gases flow downstream to one or more turbines that extract energy therefrom to drive the compressor and provide useful work such as powering an aircraft in flight. The compressor and turbine sections each include a plurality of rotor disks that are joined together for rotation about the engine&#39;s centerline axis. Each rotor disk comprises a central bore region, a disk rim from which a plurality of radially extending blades are supported, and a web joining the bore and rim. The bore and web are typically much more massive than the, disk rim to accommodate the stresses to which the disk is subjected.  
           [0003]    Rotating disks, particularly those in the high pressure turbine section of an engine, develop high radial thermal gradients during transient operation because of exposure of the disk rim to hot gases. In this case, the rim of the disk has a quick thermal response (i.e., temperature increase) while the web and bore react more slowly due to their high relative mass and their lower temperature environment. The thermal gradient creates large tangential and radial stresses in the web and bore of the disk that are magnified by any stress concentrations such as holes, fillets and the like.  
           [0004]    A significant challenge in disk design is to connect multiple disks together without developing high stresses. One method of connection is through the use of bolted joints connecting adjacent disks. Often, at least one of the disks must be bolted through the disk web because of space limitations. In such instances, the bolt holes are located in regions of high thermal gradient and produce high concentrated stresses. This limits the allowable time of operation of the rotor hardware.  
           [0005]    One approach to reducing bolt hole stress is to balance the radial and tangential stresses by modifying the hole pattern design, i.e., the number of holes, hole spacing, hole diameter and hole length. Generally, a bolted joint having more holes will produce lower mechanical stresses in the tangential direction but will result in higher radial stress. For every hole pattern design, there exists a certain quantity of holes that will balance the tangential stress at the top or bottom of the hole with the radial stress at the sides of the hole. However, modifying the hole pattern design to balance the radial and tangential stresses typically results in increased disk weight and even slower transient thermal response of the disk web and bore. Accordingly, there is a need for an improved method of reducing bolt hole stresses.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    The above-mentioned need is met by the present invention, which provides a bolted joint for connecting first and second rotor disks in a gas turbine engine. The bolted joint includes a bolt hole formed in the first rotor disk and a bolt disposed in the bolt hole such that a channel is defined between the bolt and the bolt hole. A first abutment is attached to a first end of the bolt, and a second abutment is attached to a second end of the bolt. A first passage associated with the first abutment provides fluid communication with the channel, and a second passage associated with the second abutment provides fluid communication with the channel. Hot fluid passing through the channel reduces thermal gradients in the first rotor disk.  
           [0007]    The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the concluding part of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:  
         [0009]    [0009]FIG. 1 is a partial cross-sectional view of a gas turbine engine having the bolted joint of the present invention.  
         [0010]    [0010]FIG. 2 is an enlarged cross-sectional view of the bolted joint of FIG. 1.  
         [0011]    [0011]FIG. 3 is a perspective view of the bolt from the bolted joint of FIG. 1.  
         [0012]    [0012]FIG. 4 is an enlarged cross-sectional view of a second embodiment of a bolted joint.  
         [0013]    [0013]FIG. 5 is an enlarged cross-sectional view of a third embodiment of a bolted joint. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1 shows a portion of a gas turbine engine  10  having, among other structures, a combustor  12  and a turbine section  14  located downstream of the combustor  12 . The turbine section  14  includes a first stage nozzle assembly  16 , a first stage turbine rotor  18 , a second stage nozzle assembly  20  and a second stage turbine rotor  22  arrange sequentially along the engine centerline axis. The combustor  12  includes a generally annular hollow body having an outer liner  24  and an inner liner  26  defining a combustion chamber  28  therein. A compressor (not shown) provides compressed air that passes primarily into the combustor  12  to support combustion and partially around the combustor  12  where it is used to cool both the combustor liners  24 ,  26  and turbomachinery further downstream. Fuel is introduced into the forward end of the combustor  12  and is mixed with the air in a conventional fashion. The resulting fuel-air mixture flows into the combustion chamber  28  where it is ignited for generating hot combustion gases. The hot combustion gases are discharged to the turbine section  14  where they are expanded so that energy is extracted.  
         [0015]    The first stage nozzle assembly  16  includes an inner nozzle support  30  to which a plurality of circumferentially adjoining nozzle segments  32  is mounted. The nozzle segments  32  collectively form a complete 360° assembly. Each segment  32  has two or more circumferentially spaced vanes  34  (one shown in FIG. 1) over which the combustion gases flow. The vanes  34  are configured so as to optimally direct the combustion gases to the first stage turbine rotor  18 . The inner nozzle support  30  is a stationary member suitably supported in the engine  10 .  
         [0016]    The first stage turbine rotor  18  is located aft of the first stage nozzle assembly  16  and is spaced axially therefrom so as to define a first wheel cavity  36 . The first stage turbine rotor  18  includes a plurality of turbine blades  38  (one shown in FIG. 1) suitably mounted to a first rotor disk  40  and radially extending into the turbine flow path. The second stage nozzle assembly  20  is located aft of the first stage turbine rotor  18 , and the second stage turbine rotor  22  is located aft of the second stage nozzle assembly  20  so as to define second and third wheel cavities  42  and  44 , respectively. The second stage turbine rotor  22  includes a plurality of turbine blades  46  (one shown in FIG. 1) suitably mounted to a second rotor disk  48  and radially extending into the turbine flow path. The second rotor disk  48  has a forward extending flange  50  that is joined to the aft side of the first rotor disk  40  at a bolted joint  52 . Thus, the first and second rotor disks  40 ,  48  are arranged to rotate together about the engine centerline axis.  
         [0017]    An annular rotating seal member  54  is fixed to the forward side of the first rotor disk  40  for rotation therewith by the bolted joint  52 . The rotating seal member  54  contacts the inner nozzle support  30  to form one or more forward seals  56  for sealing the compressor discharge air that is bled off for cooling purposes from the hot gases in the turbine flow path. In one preferred embodiment, the forward seals  56  are rotating labyrinth seals, each including a plurality of thin, tooth-like projections extending radially outward from the stationary seal member  56 . The outer circumference of each projection rotates within a small tolerance of the inner circumference of a corresponding annular stationary seal member  58  mounted on the inner nozzle support  30 , thereby effecting sealing between the cooling air and the hot gases in the turbine flow path.  
         [0018]    The nozzle assembly  16  also includes an accelerator  60  disposed radially between the two forward seals  56 . The accelerator  60  is an annular member that defines an internal air plenum. High pressure compressor discharge air is fed to the accelerator  60  via air holes  62  formed in the inner nozzle support  30 . The high pressure air passes axially through the accelerator  60  and is discharged therefrom through a plurality of aft nozzles into a chamber or cavity  63  located forward of the first rotor disk  40 . A portion of this air passes through passages  64  formed in the first rotor disk  40  for cooling turbomachinery further downstream. As will be described in more detail below, some of this high pressure air is directed through the bolted joint  52  for reducing the thermal gradient in the first rotor disk  40  and thereby reducing disk transient stresses.  
         [0019]    Referring now to FIGS. 2 and 3, the bolted joint  52  is described in more detail. The bolted joint  52  comprises a bolt  66  extending axially through a first opening  68  in the rotating seal member  54 , a bolt hole  70  in the first rotor disk  40 , and a second opening  72  in the second rotor disk flange  50 . Both ends of the bolt  66  are threaded so that a first nut  74  is threadingly received on the forward end of the bolt  66  and a second nut  76  is threadingly received on the aft end of the bolt  66 . The first nut  74  is a fixed abutment against the rotating seal member  54 , and the second nut  76  is a fixed abutment against the second rotor disk flange  50 . Thus, when the nuts  74 ,  76  are suitably tightened, the first rotor disk  40 , the second rotor disk  48  and the rotating seal member  54  are joined together for rotation about the engine centerline axis.  
         [0020]    The bolt  66  includes first and second raised shoulders  78  and  80 , respectively, that are located intermediate the threaded ends thereof. The raised shoulders  78 ,  80  are sized to fit within the bolt hole  70  and the second opening  72  with a tight tolerance such that the bolted joint  52  provides a body-bound function. That is, the bolted joint  52  will radially locate and maintain the second rotor disk  48  with respect to the first rotor disk  40 . The second, or aft, raised shoulder  80  has an axial retention lip  82  formed on the outer circumference thereof. The axial retention lip  82  abuts a recess formed in the forward face of the second rotor disk flange  50 , thereby axially locating the bolt  66  with respect to the first and second rotor disks  40 ,  48 . This facilitates assembly of the bolted joint  52 , which is normally a blind assembly.  
         [0021]    The bolt  66  is sized so as to have an annular, axially extending channel  84  formed thereabout. Specifically, except for the raised shoulders  78 ,  80 , the bolt  66  has a lesser diameter than its surrounding structure; i.e., the bore of the first nut  74 , the first opening  68 , the bolt hole  70 , the second opening  72  and the bore of the second nut  76 . Accordingly, the channel  84  is defined by the gap between the bolt  66  and its surrounding structure.  
         [0022]    One or more radial inlet passages  86  are formed in the first nut  74  for providing fluid communication between the forward cavity  63  and the channel  84 . Similarly, one or more radial outlet passages  88  are formed in the second nut  76  for providing fluid communication between the second and third wheel cavities  42 ,  44  and the channel  84 . As best seen in FIG. 3, each of the raised shoulders  78 ,  80  has a plurality of axially extending flats  90  formed thereon. The flats  90  allow air to flow down the entire length of the channel  84 , while the rest of the raised shoulders  78 ,  80  engage the inner surfaces of the bolt hole  70  and the second opening  72  to perform the body-bound function.  
         [0023]    In operation, compressor discharge air delivered to the forward cavity  63  from the accelerator  60  flows through the inlet passages  86  in the first nut  74  into the forward end of the channel  84 . This air passes through the bolt hole portion of the channel  84  due to the pressure differential between the forward cavity  63  and the second and third wheel cavities  42 ,  44 . The air is then discharged through the outlet passages  88  to the second and third wheel cavities  42 ,  44  where it rejoins the compressor discharge air that has passed through the passages  64  and contributes to cooling turbomachinery further downstream. As the compressor discharge air (which is generally hotter than the web and core of the first rotor disk  40 ) flows through the bolt hole portion of the channel  84 , it heats the first rotor disk  40  in the area around the bolt hole  70 . By heating the first rotor disk  40 , the compressor discharge air increases the thermal response of the disk&#39;s web and bore, thereby decreasing the thermal gradient between the web and bore and the disk&#39;s rim. This reduction in thermal gradient will cause a reduction in unconcentrated thermal operating stresses and result in increased hardware life. The amount of air delivered to the bolt hole  70  is determined by the size of the inlet and outlet passages  86 ,  88  and/or the size of the shoulder flats  90 . Thus, the amount of air needed to produce the desired degree of disk heating for a given system can be achieved by tightly controlling the sizes of the inlet and outlet passages  86 ,  88  and the shoulder flats  90 .  
         [0024]    Turning to FIG. 4, a second embodiment of a bolted joint  152  is shown. The bolted joint  152  of the second embodiment comprises a bolt  166  extending axially through a first opening  68  in the rotating seal member  54 , a bolt hole  70  in the first rotor disk  40 , and a second opening  72  in the second rotor disk flange  50 . The forward end of the bolt  166  has a head  174  integrally formed thereon, and the aft end of the bolt  166  is threaded so that a nut  176  is threadingly received thereon. A first washer or spacer  92  is disposed on the bolt  166  between the head  174  and the rotating seal member  54 , and a second washer or spacer  94  is disposed on the bolt  166  between the nut  176  and the second rotor disk flange  50 . The head  174  and first spacer  92  act as a fixed abutment against the rotating seal member  54 , and the nut  176  and second spacer  94  act as a fixed abutment against the second rotor disk flange  50 . Thus, when the nut  176  is suitably tightened, the first rotor disk  40 , the second rotor disk  48  and the rotating seal member  54  are joined together for rotation about the engine centerline axis. Alternatively, two threaded nuts could be used (like in the first embodiment) instead of the integral head and single nut.  
         [0025]    The bolt  166  includes first and second raised shoulders  178  and  180 , respectively. As in the first embodiment, the raised shoulders  178 ,  180  are sized to fit within the bolt hole  70  and the second opening  72  with a tight tolerance such that the bolted joint  152  provides a body-bound function and have axially extending flats formed thereon. Also like the first embodiment, the bolt  166  is sized so as to have an annular, axially extending channel  184  formed thereabout. Specifically, except for the raised shoulders  178 ,  180 , the bolt  166  has a lesser diameter than its surrounding structure; i.e., the first spacer  92 , the first opening  68 , the bolt hole  70 , the second opening  72  and the second spacer  94 . Accordingly, the channel  184  is defined by the gap between the bolt  166  and its surrounding structure.  
         [0026]    One or more radial inlet passages  186  are formed in the first spacer  92  for providing fluid communication between the forward cavity  63  and the channel  184 . Similarly, one or more radial outlet passages  188  are formed in the second spacer  94  for providing fluid communication between the second and third wheel cavities  42 ,  44  and the channel  184 . Thus, compressor discharge air will flow into the channel  184  through the inlet passages  186  and out of the channel  184  through the outlet passages  188 . The compressor discharge air will heat the first rotor disk  40  in the area around the bolt hole  70  as it flows through the bolt hole portion of the channel  184 .  
         [0027]    Turning to FIG. 5, a third embodiment of a bolted joint  252  is shown. The bolted joint  252  of the third embodiment comprises a bolt  266  extending axially through a first opening  68  in the rotating seal member  54 , a bolt hole  70  in the first rotor disk  40 , and a second opening  72  in the second rotor disk flange  50 .  
         [0028]    Both ends of the bolt  266  are threaded so that a first nut  274  is threadingly received on the forward end of the bolt  266  and a second nut  276  is threadingly received on the aft end of the bolt  266  for joining the first rotor disk  40 , the second rotor disk  48  and the rotating seal member  54 . As with the prior embodiments, the bolt  266  is sized so as to have an annular, axially extending channel  284  formed thereabout. However, in this embodiment, inlet and outlet passages for the channel  284  are not formed in nuts or spacers. Instead, one or more grooves or slots  286  are formed in the forward surface of the rotating seal member  54 , adjacent to the first nut  274 . Thus, the first nut  274  and the slots  286  define inlet passages that provide fluid communication between the forward cavity  63  and the channel  284 . Similarly, one or more grooves or slots  288  are formed in the aft surface of the second rotor disk flange  50 , adjacent to the second nut  276 . Thus, the second nut  276  and the slots  288  define outlet passages that provide fluid communication between the second and third wheel cavities  42 ,  44  and the channel  284 . Compressor discharge air will thus flow into the channel  284  through the inlet slots  286  and out of the channel  284  through the outlet slots  288 . The compressor discharge air will heat the first rotor disk  40  in the area around the bolt hole  70  as it flows through the bolt hole portion of the channel  284 . This embodiment can be implemented with or without spacers and with a bolt having an integral head and a single nut as an alternative to the two threaded nuts  274 ,  276 , as shown. While this third embodiment will simplify the manufacture of the fasteners and possibly reduce overall part count, it could also result in increased stress concentrations in the structural rotor components.  
         [0029]    The foregoing has described a bolted joint that increases the thermal response of the disk web and bore through use of a parallel air delivery system. The increased thermal response reduces the thermal gradient in the rotor disk, which in turn reduces disk transient stresses. 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 as defined in the appended claims.