Abstract:
Apparatus for simplifying maintenance of gas turbine engines. Ordinarily, to remove a high-pressure compressor in a twin-spool engine, many components of must be removed, such as parts of a booster, in order to gain access to a mounting nut which is threaded onto a shaft supporting the high-pressure compressor. The removal is necessary because the nut must be extracted from the engine after disengagement from the shaft. Under the invention, this removal-of-components is largely eliminated, by providing a stowage location for the nut within the engine. The nut is removed, and then threaded onto a set of threads specifically provided for storage of the nut, and for nothing else.

Description:
TECHNICAL FIELD  
         [0001]    The invention concerns a simplified approach to removal of the core from a gas turbine engine.  
         BACKGROUND OF THE INVENTION  
         [0002]    [0002]FIG. 1 is a schematic of a twin spool gas turbine engine found in the prior art. Incoming air  1  is compressed by a booster  2 , and then delivered to a high-pressure compressor  3 , which further compresses the air and delivers the compressed air to a combustor  6 . Fuel is added to the combustor  6  and ignited, producing a high-velocity gas stream  9 .  
           [0003]    The gas stream  9  is delivered to a high-pressure turbine  12 , which extracts energy to drive the high-pressure compressor  3 , through high-speed shaft  15 . The gas stream  9  is then delivered to a low-pressure turbine  18 , which extracts energy to drive the booster, through low-speed shaft  21 , and to drive a fan  24 . The fan produces thrust. indicated by arrow  27 . Some additional thrust  28  is produced by the gas stream exiting the low-pressure turbine  18 .  
           [0004]    Bearings  30  support the high-speed shaft  15  on the low-speed shaft  21 . Bearings  33  support the latter low-speed shaft  21  on stationary structures, not shown.  
           [0005]    Maintenance and repairs are required on the engine, some of which require removal of the core of the engine. The core, indicated by dashed box  29 , includes the high-pressure compressor  3 , the high-pressure turbine  18 , the shaft  15 , and a casing  31  which contains these three elements. Typically, in the prior art, major disassembly of the engine is required for removal of the core  29 .  
           [0006]    The inventor has developed a strategy for removal of the core  29  which avoids the need for major disassembly of the engine.  
         SUMMARY OF THE INVENTION  
         [0007]    In one form of the invention, a nut which locks a shaft in place in a gas turbine engine is equipped with a second thread. The second thread is used to engage another thread provided nearby, when the nut is removed. Thus, when the nut is removed from the shaft, it can be threaded onto the second thread, and need not be removed from the engine. The latter type of removal ordinarily requires removal of extensive amounts of additional components. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a schematic of a generic gas turbine engine.  
         [0009]    [0009]FIG. 2 illustrates components located generally within region  36  in FIG. 1.  
         [0010]    [0010]FIG. 3 illustrates some of the components of FIG. 2, in exploded form.  
         [0011]    [0011]FIG. 4 illustrates obstacles  65  which block access to nut  45 .  
         [0012]    [0012]FIGS. 5 and 6 illustrate one form of the invention.  
         [0013]    [0013]FIGS. 7, 8, and  9  illustrate a sequence of events undertaken in one form of the invention.  
         [0014]    [0014]FIGS. 10, 11,  12 , and  13  illustrate additional forms of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    [0015]FIG. 2 illustrates components found in the region generally designated  36  in FIG. 1, plus additional components. A seal  39  blocks fluid passage, and mates with the high-speed shaft  15  at rotating interface  42 . A threaded nut  45 , sometimes called a coupling nut, locks the high-speed shaft  15  into position, as perhaps best explained by reference to FIGS. 2 and 3 together.  
         [0016]    [0016]FIG. 3 illustrates several components in exploded view. FIG. 3 shows high-speed shaft  15 , and a bevel ring gear  50 . A gear tooth  52  is identified. The bevel ring gear  50  is used to extract power from the engine, to drive components such as electrical generators and hydraulic pumps, neither of which is shown. FIG. 3 also shows a seal  55  and the threaded nut  45 .  
         [0017]    When the components of FIG. 3 are assembled, as in FIG. 2, threaded nut  45  applies axial clamping force to drive the seal  55  and bevel ring gear  50  against bearing race  60 , thereby locking shaft  15  in position.  
         [0018]    When one wishes to remove the core  29  of the engine, one must remove threaded nut  45 . However, a problem arises, because access to the threaded nut  45  is blocked by obstacles  65  in FIG. 4. These include parts of the booster  2  in FIG. 1, and other components. Removal of these components is labor intensive, and thus removal of the core is an expensive operation.  
         [0019]    It may be thought that access to the threaded nut  45  may be obtained through the path indicated by dashed arrow  70  in FIG. 4. However, that is not so. The threaded nut  45  cannot be removed along path  70 , because the diameter of bearing  33  is too small. It may be thought that bearing  33  could be re-designed with a larger diameter. That is possible in theory. However, in practice, bearing  33  is made in the smallest feasible diameter to minimize relative speeds of the bearing races, not shown.  
         [0020]    The Inventor has developed a stratagem for eliminating the need to remove most, if not all, of the obstacles  65  which presently must be removed in order to extract the core  29  from the engine.  
         [0021]    [0021]FIG. 5 illustrates some of the components shown in FIG. 2. In addition, FIG. 5 shows threaded nut  45  in modified form, and labeled  80 . Threaded nut  80  is fabricated with two sets of threads. A set of internal threads  85 , and a set of external threads  90 . The internal threads  85  engage threads in region  95 , which are formed in the high-speed shaft  15 , and are used in normal operation.  
         [0022]    If the core  29  is to be removed, the nut  80  is removed from high-speed shaft  15 , and threaded onto an additional set of internal threads  100 , as shown in FIG. 6. The additional threads  100  are supported, in this example, by seal  39 .  
         [0023]    FIGS.  7 - 9  illustrate a sequence of events undertaken in removal of the core  29 . FIG. 7 shows the engine in assembled form. FIG. 8 shows the dual-threaded nut  80  removed from shaft  15 , and stowed on threads  100 . FIG. 9 shows shaft  15  being extracted from the bevel ring gear  50 .  
         [0024]    Several significant features and characterizations of the invention are the following. Thread  100 , as well as external thread  90 , in FIGS.  5  and  6 , need not be designed to carry any significant load. These threads only support dual-threaded nut  80  during maintenance of the engine. The dual-threaded nut  80 , in general, will weigh less than about ten pounds, and supporting it in a stationary state only requires overcoming the force of gravity.  
         [0025]    From another point of view, in one form of the invention, either thread  90 ,  100 , or both, are not designed to withstand any design load for which the thread  95 , or thread  85 , are designed. The latter threads  85  and  95  hold the high-speed shaft  15  in place during engine operation. The former threads  90  and  100  hold a nut in place during maintenance.  
         [0026]    Thread  100  is of the same pitch, pitch diameter, and type as thread  90 .  
         [0027]    Thread  100  is shown as supported by seal  39  in FIG. 5. However, seal  39  is chosen because it is a structure conveniently located to thread  95 . Other components can be used to support thread  100 . In one form of the invention, a thread such as thread  100  can be located at any convenient station which is axially displaced from thread  95 .  
         [0028]    Threaded nut  80  is shown as bearing internal threads  85  which engage external threads  95  in FIG. 5. However, as stated above, nut  80  serves to apply axial force to a bearing race analogous to race  60  in FIG. 2. Consequently, other threading arrangements are possible. FIG. 10 illustrates one possibility.  
         [0029]    Nut  150  contains two external threads  155  and  160 . External thread  155  engages an internal thread  165  on shaft  15 . Shoulder  170  applies axial force to seal  55  and the bevel ring gear  50  (not shown). Thread  160  engages thread  100  during maintenance.  
         [0030]    It is observed that two functions are involved: locking high-speed shaft  15  into its operational position by applying axial force from nut  45  to bearing race  60  in FIG. 2, and securely storing nut  80  in FIG. 5 during maintenance. The former function can be accomplished by either an external or internal thread on shaft  15 , with nut  80  being designed with a complementary thread.  
         [0031]    The latter function can be accomplished by another thread, and the type of thread, external or internal, is not necessarily dictated by the type of thread used for the former function. That is, any of the following combinations of thread pairs can be used, wherein the first element of the pair represents the thread used to lock shaft  15  and the second element represents the thread used to stow the nut  80  during maintenance: (internal, internal) (internal, external), (external, internal) and (external, external).  
         [0032]    In one form of the invention, threads  100  may not be necessary. Threads  100  may be replaced by a smooth surface, or annulus  250  in FIG. 11. The annulus  250  acts as a parking place for nut  255 . During maintenance, annulus  250  surrounds the nut  255 . Alternately, a smaller annulus  260  may be provided, as in FIG. 12. Nut  255  would slide onto annulus  260 , like a ring on a rod. A stop  265  may be provided, and a similar stop may be provided in FIG. 11, to restrict axial travel of nut  255 .  
         [0033]    After nut  255  is positioned on the storage annulus  250  or  260  during maintenance, a retainer is added. For example, a snap-type retaining ring of C-configuration can be placed in a groove partially indicated by path  280 . As another example, a pair of cotter keys, not shown, can be inserted through holes  290  in FIG. 12.  
         [0034]    The forms of the invention shown in FIGS. 11 and 12 suffer the disadvantage that additional parts are required, namely, the retaining ring or cotter keys. However, cotter keys can be jury-rigged using simple electrical wire, which is expected to be found in nearly all maintenance shops.  
         [0035]    In this connection, the Inventor points out that the specific embodiment shown in FIGS. 5 and 6 offers two specific advantages. One is that, since nut  80  is stowed within the engine, it is unlikely that the nut  80  will be misplaced. A second is that, since the internal thread  85  is fully exposed, it can be inspected, cleaned of dirt and debris, and lubricated, with the possible addition of corrosion inhibitor.  
         [0036]    Thus, in one form of the invention, nut  80  is removed from the core as described above, stowed as shown in FIG. 6, and then inspected, cleaned, and lubricated.  
         [0037]    One form of the invention comprises a particular process. A core of a gas turbine engine is transported to a disassembled gas turbine engine. The disassembled engine contains a nut in a stowed position. The core is inserted into the engine, the nut is withdrawn from its stowed position, and engaged with a thread on the core.  
         [0038]    Sometimes, in the prior art, aircraft technicians have used nuts of the type  45  in FIG. 2 to apply axial force to the shaft  15 , during insertion or withdrawal. In one form of the invention, this is preferably avoided. Instead, if axial force is required, a hydraulic jack, represented by block  300  in FIG. 9, is used.  
         [0039]    [0039]FIG. 13 illustrates one form of the invention. Block B represents the apparatus of one or more of FIGS. 5, 6, or  10 , for example. FIG. 13 shows the engine  305 , core  29 , casing  31 , high-pressure compressor  3 , high-pressure turbine  12 , and shaft  15 .  
         [0040]    Numerous substitutions and modifications can be undertaken without departing from the true spirit and scope of the invention. What is desired to be secured by Letters Patent is the invention as defined in the following claims.