Abstract:
An assembly including inner and outer overlapping annular elements with interengaging threads. An axial abutment between the two elements is spaced from the threaded section to permit a structural and fluid seal at the joint. Pilot joints are provided to stabilize the joint and provide additional sealing.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present application relates generally to disconnectable threaded joints for interconnecting two components. More particularly, one form of the present application relates to a threaded joint for coupling together gas turbine engine casings. Although the present application was developed for application in gas turbine engines, certain applications may exist in other fields.  
         [0002]     Gas turbine engines usually include a number of cylindrical components joined together to define a housing. Within the housing, there is generally a flow of working fluid. Gas turbine engine designers have strived to secure the components of the housing in a way that maintains structural and pressure integrity while at the same time facilitating assembly and disassembly for inspection and/or repair of components.  
         [0003]     A conventional system for connecting cylindrical gas turbine engine housing components has been to incorporate circumferential and abutting flanges which are secured to one another by clamps or fasteners extending through aligned openings in the abutting flanges. One limitation of this approach has been that the prior system complexity adds to the cost and potential unreliability of the joint. Further, the flanged, bolted joint and/or clamped joint may lead to an increase in the overall envelope for the engine.  
         [0004]     Accordingly, there is a continuing need for an effective disconnectable joint for gas turbine engine components.  
       SUMMARY OF THE INVENTION  
       [0005]     One form of the present invention contemplates an apparatus comprising: a first gas turbine engine component having a first annular portion with an internal thread and a first annular abutment surface spaced from the internal thread; and a second gas turbine engine component having a second annular portion with an external thread and a second annular abutment surface spaced from the external thread and abutting the first annular abutment surface, the first and second components at least partially overlapping one another and the threads interengage to couple the components together and place the abutting abutment surfaces in a first sealing relationship, wherein one of the components is in tension and the other of said components is in compression between the abutting abutment surfaces and the interengaging threads.  
         [0006]     Another form of the present invention contemplates a method of assembling a threaded joint between two gas turbine components. The method comprising: orienting a cylindrical portion of the two components in an overlapping relationship, one of the components in the overlapping relationship defining an inner overlapping section having an externally threaded portion and the other component defining an outer overlapping section having an internally threaded portion; creating a differential thermal loading between the inner overlapping section and the outer overlapping section, the outer overlapping section having a greater thermal loading; threading the components together to bring an abutment surface on each of the components into an abutting relationship and establish a seal therebetween that is spaced from the threaded portions; and allowing the components to achieve equal thermal loading thereby increasing the axial preload.  
         [0007]     Yet another form of the present invention contemplates an apparatus comprising: a first component having a first cylindrical portion with an internal thread and a first annular abutment surface spaced from the internal thread, the first component including a pair of first pilot surfaces spaced apart from the internal thread; and, a second component having a second annular portion with an external thread and a second annular abutment surface spaced from the external thread and abutting the first annular abutment surface, the first and second components at least partially overlapping one another and the threads interengage to couple the components together and place the abutting abutment surfaces in a first sealing relationship, wherein one of the components is in tension and the other of the components is in compression between the abutting abutment surfaces and the interengaging threads, and further wherein the second component including a pair of second pilot surfaces spaced apart from the external thread and in registry with the pair of first pilot surfaces to form a second sealing relationship.  
         [0008]     One object of the present invention is to provide a unique threaded joint for gas turbine components.  
         [0009]     Related objects and advantages of the present invention will be apparent from the following description.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  shows an external side view of a gas turbine engine employing a prior art technique for joining annular components.  
         [0011]      FIG. 2  is a fragmentary view of a gas turbine engine, comprising one embodiment of a threaded joint of the present invention.  
         [0012]      FIG. 3  is a fragmentary cross-section view of the threaded joint of  FIG. 2 .  
         [0013]      FIG. 4  is a fragmentary, cross section view of an alternative embodiment of the present invention showing the component joint at the beginning of assembly.  
         [0014]      FIG. 5  is a fragmentary cross-section view of the joint of  FIG. 4  shown in its assembled position.  
         [0015]      FIG. 6  is a fragmentary cross-section view showing a locking mechanism for the threaded joint of  FIG. 3 .  
         [0016]      FIG. 7  is an illustrative end view taken at arrow A in  FIG. 6 .  
         [0017]      FIG. 8  is a cross-section view taken on line B-B of  FIG. 6 .  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.  
         [0019]     With reference to  FIG. 1 , there is illustrated a gas turbine engine  10  utilizing one embodiment of a threaded joint  36  of the present invention. The gas turbine engine  10  illustrated is purely illustrative and no limitation is intended herein to any specific type of gas turbine engine. The illustrative gas turbine engine comprises an accessory drive/inlet housing  12 , compressor housing  14 , diffuser/combustor housing  16 , turbine housing  18  and exhaust housing  20 . The general operation of gas turbine engines is well known to those of ordinary skill in the art and thus it is unnecessary to describe the operation of compression, combustion and expansion to extract work out of the gas turbine engine by a power turbine providing a rotary output or pure reaction to produce thrust. Gas turbine engines operate with internal pressures at the hundreds of PSI level. In at least one form, discharge pressures can approach 600 PSI on high pressure ratio applications having overall pressure ratios of about 40:1. The capability to withstand high pressures require joints between the various housings to be structurally sound, capable of a pressure seal, and removable for maintenance. The present application is applicable to a wide variety of pressures and is not limited, unless expressed to the contrary, to any particular pressure ranges.  
         [0020]     With reference to  FIG. 2 , there is illustrated an enlarged fragmentary view of a portion of the gas turbine engine showing the joint  36  coupling the diffuser/combustor housing  16  to the turbine housing  18 . While the joint of the present invention is illustrated between the diffuser/combustor housing  16  and the turbine housing  18 , it is applicable to all joints in the gas turbine engine. Further, the present invention is contemplated for other fields, including but not limited to, rocket motors, steam turbines, liquid carrying tubes/pipes, and gaseous fluid carrying tubes/pipes.  
         [0021]     With reference to  FIG. 3 , there is illustrated an enlarged fragmentary cross-sectional view of the joint  36 . Housing  18  has an annular portion  38  with an externally threaded portion  40  formed thereon. Housing  16  has an annular portion  42  with an internally threaded portion  44  formed thereon. The portions  42  and  38  overlap, and the threads  40  and  44  interengage when in an assembled state. An end face  48  on annular portion  38  abuts a corresponding shoulder  46  on annular portion  42 . In one embodiment, the interface between shoulder  46  and end face  48  is axially spaced from the interengaging threads  40  and  44  so that when the threads are tightened there is sufficient axial force to drive the end face  48  and shoulder  46  into an abutting relationship and form a seal. The term “seal” or “sealing” as utilized herein describes the reduction of fluid flow between the components coupled together at the joint. The reduction in fluid flow at the joint may be a complete prevention of fluid leakage between the components or a partial prevention of fluid leakage that minimizes fluid leakage between the components at the joint. The interengaging of the threaded joint places at least a portion of the outer annular portion  42  in tension and the inner annular portion  38  in compression.  
         [0022]     Radial pilots  100  and  101  are formed on either side of the interengaging threaded joint. The radial pilot  100  comprises an inwardly facing pilot surface  50  formed on annular portion  42  and a corresponding outer facing pilot surface  52  formed on annular portion  38 . Radial pilot  101  comprises an outer facing pilot surface  54  formed on annular portion  38  and an inner facing pilot surface  56  formed on annular portion  42 . In one form the respective surfaces of the pilot surfaces are substantially parallel.  
         [0023]     The parameters of the pilot joints, length to the axial abutting surfaces, and thread size are all selected to facilitate assembly/disassembly during a condition where the housings  16  and  18  are subjected to differential thermal conditions. In one non-limiting example the present invention contemplates a ten inch diameter threaded joint where the length from the thread element to the mutually abutting axial surfaces  48  and  46  is about 1½ inches. The thickness of annular portion  42  which is in tension and of annular portion  38  which is in compression is about 0.1 inches. In obtaining about a 0.002 inch deformation in each annular portion, a preload of about 125,000 pounds is generated. A 40,000 PSI bearing stress is generated at the abutting surfaces  48  and  46 . A buttress thread is utilized at the threaded joint. This 125,000 pound load is sufficient to provide a fluid tight coupling with adequate bending stiffness. However, other joint sizes, threads, amount of deformation and preloads are contemplated herein.  
         [0024]     In the assembly/disassembly phase, the housing  16  is subjected to a higher localized thermal loading than housing  18 . This can be done by heating the exterior of housing  16  in the proximity of the annular portion  42  or by cooling the annular portion  38  of housing  18 . Preferably, the heating of the housing occurs between the radial pilots  100  and  101 . In this condition, the length from the threaded joint to the mutually abutting axial surfaces  48  and  46  is greater for annular portion  42  than it is for annular portion  38 ; the pilot surfaces  50  and  56  are greater in diameter than the interconnecting surfaces  52  and  54 , and; threads  44  have clearance relative to threads  40 . The heating provides clearance between the components forming the radial pilot  100  and  101 .  
         [0025]     The two components are threaded together in the state of differential thermal loading until the surfaces  48  and  46  abut one another. The threads are tightened to create a predetermined loading on these axial end faces. In one form of the invention, the preload is 125000 pounds. As the annular portions  38  and  42  reach equal thermal loading, annular portion  42  reduces in length and diameter relative to annular portion  38 . The result of the cooling of the assembly is to create an axial preload between shoulder  46  and end face  48  and a radial preload between the surfaces  54  and  56  of radial pilot  101  and surfaces  50  and  52  of pilot  100 . The practical effect is to tighten the joint and enhance the seals at the joint between the following pairs of surfaces:  50  and  52 ,  46  and  48 ,  54  and  56 . To disassemble the joint, the differential thermal loading described above is employed and the annular portions  38  and  42  are unthreaded.  
         [0026]     During operation of the gas turbine engine, the working fluid flow path is at least partially adjacent the annular portion  38  so that it is subjected to a higher thermal loading than annular portion  42 . As a consequence, there is thermal growth in annular portion  38  relative to annular portion  42 , thus increasing both the axial and pilot seals. It should be noted that the axial spacing of the pilot joints  50 ,  52  and  54 - 56  from the threads  40 ,  44  provide increased bending stiffness through the joint. In another embodiment, a secondary seal including, but not limited to, an E-seal, W-seal or C-seal can be employed in the void formed between the aft face  58  of housing  16  and the shoulder on housing  18 .  
         [0027]     The joint  46  shown in  FIGS. 4 and 5  is substantially similar to the joint  36 , but includes an enhanced self-piloting element. The utilization of like feature numbers is done to represent like features. Annular portion  60  extends from gas turbine housing  18  and has an axial end face  62 . Annular portion  64  is integral with housing  16  and overlaps annular portion  60 . An externally threaded section  66  on annular portion  60  interengages with an internally threaded section  68  on annular portion  64  as the components are threaded together. Lead-in pilot surface  72  on annular portion  60  cooperates with a corresponding pilot  74  on annular portion  64  leading from shoulder  113 . This lead-in pilot surface  72  aids in threading the joint into place in the position shown in  FIG. 5 . As the components are assembled, the radial pilot  110  is formed by the engagement of surface  111  of portion  60  with surface  112  of portion  64 . The lead-in pilot surface  72  on portion  60  does not normally contact the surfaces  74  and/or  112 . However, the lead-in pilot surface  74  is located in close proximity to the component  64  in order to provide an alignment guide for the structure. Surface  72  can be set to engage surface  112  and/or  74  prior to threads  66  and  68  engaging to prevent cross-threading at assembly. Surface  62  disposed at the end of portion  60  is brought into an abutting relationship with surface  113  when the components are assembled. The seal is formed between surfaces  62  and  113 , surfaces  111  and  112 , and between surfaces  70  and  114 . It should be noted that the same techniques for differentially thermally loading the joints can be employed for the joint illustrated in  FIGS. 4 and 5 .  
         [0028]      FIGS. 6, 7  and  8  illustrate an anti-rotation locking mechanism generally indicated at  200  which is illustrated here applied to the joint of  FIGS. 2 and 3 . It should be noted, however, that the anti-rotation mechanism may be employed with equal benefit to the other joints including, but not limited to, those set forth in  FIGS. 4 and 5 . The anti-rotation device comprises a ring  76  extending over both of annular end portions  42  and  38 . As shown in  FIG. 8 , end portion  38  has a plurality of slots  78  (only one of which is shown) spaced around the circumference of annular section  38 . A corresponding number of projections  80  extend from ring  76  into slots formed in portion  38 . Ring  76  includes an annular section  82  which overlaps annular portion  42 , including at least a portion of the slots  84  spaced around the circumference of the portion  42 . When the elements are in their secure position and the projections  80  are lined up in grooves  78 , the thin annular section  82  is deformed at  86  to extend into grooves  84 . Thus, the elements are locked against rotation. Disassembly may take place after sections  86  are bent to clear grooves  84  and permit unthreading of the elements. The present application contemplates other anti-rotation locking mechanisms, such as, but not limited to, local welding for expendable applications, locking pins and lockwire.  
         [0029]     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. It should be understood that while the use of the word preferable, preferably or preferred in the description above indicates that the feature so described may be more desirable, it nonetheless may not be necessary and embodiments lacking the same may be contemplated as within the scope of the invention, that scope being defined by the claims that follow. In reading the claims it is intended that when words such as “a,” “an,” “at least one,” “at” least a portion item unless specifically stated to the contrary in the claim. Further, when the language “at least a portion” and/or “a portion” is used the item may include a portion and/or the entire item unless specifically stated to the contrary.