Abstract:
A fluid transfer tube comprises a base, a plurality of external threads, and an annular shoulder. The base is disposed at a first axial end of the tube. The plurality of external threads are integrally formed into at least a portion of the base for securing the base into a boss. The annular shoulder is fixed to an outer diameter of the tube at a first distance from the first axial end, causing the shoulder to contact an outer surface of the boss with the base of the tube at a maximum depth in the receiving portion of the boss to prevent further downward travel of the base.

Description:
BACKGROUND 
     Gas turbine engine components such as bearings require continuous lubrication provide cooling and lubrication due to high operating temperatures, pressures, and friction. Oil is distributed to these components throughout the engine in a series of tubes. For example, a bearing housing may receive oil from a transfer tube connected to a plenum with multiple feed lines and nozzles. Oil transfer tubes and seals are subject to high stresses and torque in addition to the high temperatures and pressures, which can lead to premature leakage and failure. When a leak occurs in the vicinity of hot engine parts, the oil can coke up and catch fire, necessitating unscheduled replacement and repair of the entire engine. 
     It is relatively easy for technicians to overtorque oil transfer tubes when trying to retighten a tube to fix or prevent oil leaks. When the tube is overtorqued, it becomes difficult or impossible to remove due in part to extrusion of the copper seal material between the threads at the seal interface. Thus it would be helpful to provide a transfer assembly that simultaneously prevents or discourages overtorquing while still minimizing leaks. 
     SUMMARY 
     A fluid transfer tube comprises a base, a plurality of external threads, and an annular shoulder. The base is disposed at a first axial end of the tube. The plurality of external threads are integrally formed into at least a portion of the base for securing the base into a boss. The annular shoulder is fixed to an outer diameter of the tube at a first distance from the first axial end, causing the shoulder to contact an outer surface of the boss with the base of the tube at a maximum depth in a receiving portion of the boss to prevent further downward travel of the base. 
     A fluid transfer assembly comprises a tube and a compression seal. The tube includes a base at a first axial end for installation into a boss, a tube lip with a tube contact surface proximate the first axial end, and an annular shoulder fixed to an outer diameter of the tube at a first distance from the first axial end. The compression seal includes a seal body, a seal lip above the seal body with a seal contact surface configured to complement the corresponding tube lip. The first distance causes the shoulder to contact an outer surface of the boss with the base of the tube reaches a depth into a receiving portion of the boss to define maximum axial compression of the seal. 
     A method is disclosed for retrofitting a fluid transfer assembly comprising an existing fluid transfer tube and an existing compression seal secured into a receiving portion of a boss. The existing fluid transfer tube and existing compression seal are removed from the receiving portion of the boss. A maximum travel depth is determined for the fluid transfer tube into the boss. The tube is modified to affix an annular shoulder to an outer diameter of the fluid transfer tube to such that a bottom surface of the shoulder is a first distance from the first axial end of the tube, with the first distance substantially equivalent to the selected maximum travel depth. A new or refurbished compression seal is placed into the receiving portion of the boss. The modified fluid transfer tube is threaded into the receiving portion of boss to axially compress and radially expand the seal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of an oil transfer tube assembly secured to a boss on a portion of a rear bearing housing of a gas turbine engine. 
         FIG. 2A  is a perspective cross-section of an oil transfer tube assembly with a seal and a tube having a shoulder. 
         FIG. 2B  is a detailed view of the contact area of the tube and seal from  FIG. 2A . 
         FIG. 2C  shows a cross-section of the oil transfer tube and shoulder. 
         FIG. 2D  shows the seal and a projection of the contact area from  FIG. 2A . 
         FIG. 2E  is a perspective view of the tube shoulder. 
         FIG. 3A  shows the first stage of installing the oil transfer tube assembly. 
         FIG. 3B  shows the second stage of installing the oil transfer tube assembly. 
         FIG. 3C  shows the third stage of installing the oil transfer tube assembly. 
         FIG. 4A  depicts the transfer tube assembly with a washer disposed between the shoulder and the boss. 
         FIG. 4B  is a detailed view of the contact area of the shoulder, washer, and boss from  FIG. 4A   
         FIG. 4C  shows the washer and contact area from  FIG. 4A . 
         FIG. 5A  is a perspective cross-sectional view of a first alternative embodiment of the tube assembly having a conical convex seal and a complementary concave tube base. 
         FIG. 5B  is a detailed view of the contact area of the assembly shown in  FIG. 5A . 
         FIG. 5C  shows the convex seal from the assembly shown in  FIG. 5A . 
         FIG. 6A  is a perspective cross-sectional view of a second alternative embodiment of the tube assembly having a concave seal and a complementary conical convex tube base. 
         FIG. 6B  is a detailed view of the contact area of the assembly shown in  FIG. 6A . 
         FIG. 6C  shows the concave seal from the assembly shown in  FIG. 6A . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an area of a gas turbine engine that includes a portion of rear bearing housing  10 , boss  12 , oil transfer tube assembly  14 , tube  16 , seal  18 , tube base  20 , and shoulder  22 . Housing  10  houses bearings of a gas turbine engine (not shown). This engine may be an industrial gas turbine or one installed on an aircraft, either to provide motive power or to provide backup electrical power as an auxiliary power unit (APU). The general operation of gas turbine engines are well-known and will not be discussed in detail here except as necessary background for describing the various features of the invention. 
     Rear bearing housing  10  includes boss  12  for receiving oil transfer tube assembly  14  comprising tube  16  and seal  18 . Tube  16  and seal  18  are secured to into boss  12  by placing seal  18  into a well of boss  12  and threading tube base  20  into the well to compress seal  18  until shoulder  22  contacts boss  12 . This shown in more detail in subsequent figures. 
       FIG. 2A  is a perspective cross-section of oil transfer tube assembly  14  with tube  16 , seal  18 , tube base  20 , shoulder  22 , and tube threads  24 . Shoulder  22  extends annularly around the outer diameter of tube  16  above tube base  20  and tube threads  24 . Seal  18  is compressed in boss  12  (shown in  FIG. 1 ) in order to prevent oil in tube  16  from reaching the space around tube threads  24 . 
     In prior tube assemblies without a shoulder above the base or the threads, a high degree of torque is often required to secure the tube in place and retain the seal in compression to prevent leakage. Through normal use and repeated tightening by technicians seeking to stop leaks, the seal breaks down and leaks eventually become more frequent and severe. In many cases, high torque values exerted on the seal and threads can cause additional problems, such as extruding of the seal material into the mating threads, complicating tube and seal removal for engine maintenance or repair. 
     In contrast, shoulder  22  provides a mechanical “stop” for tube assembly  14  to limit downward travel of tube base  20  into boss  12  (shown in  FIG. 1A ). Shoulder  22  is provided at a height such that it contacts the top of boss  12  as seal  18  is compressed to an optimal degree. This reduces the total contact stresses around seal  18  and minimizes damage to seal  18  while also reducing and preventing leaks. 
     The axial position of shoulder  22  restricts the downward travel distance of tube base  20  when the bottom of shoulder  22  contacts boss  12  (as seen in  FIGS. 3A-3C ). This in turn prevents the technician from applying excessive torque to tube  14 , while also providing additional surface area over which to spread the applied torque. Shoulder  22  is positioned so as to restrict downward movement and thus torque to an appropriate degree that allows for sufficient compression of seal  18  for minimizing leaks while also extending the time before seal breakdown. Limiting downward travel and torque also minimizes extrusion of seal material into threads  22 , simplifying removal of tube  16  and seal  18  by maintaining the original thread interface. 
       FIG. 2B  shows a magnified portion of tube assembly  14  from  FIG. 2A , with seal  18 , tube base  20 , tube threads  24 , seal body  26 , seal lip  28 , tube lip  30 , and seal contact surfaces  32 A,  32 B. Seal  18  includes seal body  26  and seal lip  28 , while tube base  20  includes tube lip  30  with a number of external tube threads  24 . Tube base  20  fits over seal  18  and on seal body  26  by arranging tube lip  30  over an outer edge of seal lip  28  and engaging contact surfaces  32 A,  32 B. As shown in  FIGS. 3A-3C , seal body  26  is axially compressed by tube lip  30  and radially expands to prevent oil from flowing out of tube  16  and into the area around tube threads  24 . Details of tube  16  and seal  18  are shown individually in  FIGS. 2C and 2D . 
       FIG. 2C  is a cross-section of oil transfer tube  16  with tube base  20 , shoulder  22 , tube threads  24 , tube lip  30 , tube contact surface  32 A and tube bore  34 . As will be seen in  FIGS. 3A-3C , tube base  20  is threaded via tube threads  24 . Contact surface  32 A compresses seal  18  (shown in  FIG. 2D ) in a receiving well to minimize oil escaping from tube bore  34 . 
     Tube  16  can be manufactured from any material providing suitable mechanical and temperature resistance for the particular job, including many grades of carbon steel or stainless steel. Shoulder  22  can be integrally formed as part of the exterior of tube  16  such as by forging, casting, and/or machining. Shoulder  22  can alternatively be formed separately as a ring, toriod, or similar geometry and welded or otherwise bonded at the desired height onto a standard tube. This can be done either during initial manufacture of tube  16 , or as a repair or retrofit. 
     The effective diameter and depth of shoulder  22  should be sufficient to withstand the applied torque and provide an appropriate surface area for contacting an outer surface of a boss. However, shoulder  22  should also not be dimensioned to substantially interfere with adjacent components during installation or operation. 
       FIG. 2D  is a perspective view of seal  18  with seal body  26 , seal lip  28 , and seal contact surface  32 B. As will be shown in  FIGS. 3B and 3C , seal  18  sits loosely in a well of boss  12 . Seal body  26  is compressed as tube  16  is threaded into boss  12 .  FIG. 2D  also includes seal contact surface  32 B, which is the upper portion of seal body  26  extending outside seal lip  28 . Seal contact surface  32 B is approximately equivalent to tube contact surface  32 A to evenly distribute the compressive stresses from torque applied to tube  16 . 
     Seal  18  can be made from any material suitable for the particular application. In the example of an oil transfer tube assembly for a bearing housing, softer metals such as copper or copper alloys will be effective. These example materials provide sufficient compression and deformation to form an effective seal while also resisting relatively high temperatures and chemical interactions. In other less chemically and thermally reactive conditions, seal  18  can comprise, for example, a cured silicone, perfluorocarbon, or fluorocarbon resin. 
       FIG. 2E  shows shoulder  22  with inner diameter  35 . As noted above, tube assembly  14  can be manufactured integrally with shoulder  22 , or alternatively can be retrofitted onto an existing tube assembly. An existing tube assembly without a shoulder can be removed from an existing engine. Shoulder  22  can then be welded onto the existing tube proximate inner diameter  35 . The distance of shoulder  22  from the threaded axial end of the tube is determined to accommodate the installation or reinstallation steps depicted in  FIGS. 3A-3C . 
       FIGS. 3A-3C  progressively depict the steps of assembly  14  being received into boss  12 .  FIG. 3A  includes boss  12 , oil transfer tube assembly  14 , tube  16 , seal  18 , tube base  20 , tube shoulder  22 , tube threads  24 , seal body  26 , seal lip  28 , tube lip  30 , tube and seal contact surfaces  32 A,  32 B, tube bore  34 , seal base  36 , boss threads  40 , boss well  42 , well base  44 , first axial gap  46 , and second axial gap  48 . 
       FIG. 3A  shows the first stage of installing oil transfer tube assembly  14  with seal  18  disposed in well  42 . Seal  18  is sized to rest loosely in well  42  to provide sufficient room for radial expansion that will eventually result in filling the volume at the base of well  42 . Tube base  20  is then received by well  42  and secured via tube threads  24  and boss threads  40 . 
     At this stage, prior to contact between seal  18  and tube base  20 , there is first axial gap  46  between shoulder  24  and boss  12 , as well as second axial gap  48  between tube base  20  and seal  18 . First gap  46  is slightly larger than second axial gap  48 . This is because tube base  20  is to contact seal  18  at surfaces  32 A,  32 B prior to shoulder  22  contacting boss  12 . Seal base  36  also is not yet fully sealed against well base  44  due to relative curvature of the two surfaces and the lack of axial compression or radial expansion. To further simplify installation, tube threads  24  and/or boss threads  40  can be plated with a silver-based compound. Plated or unplated threads can also be coated with oil just before tube base  20  is first threaded into well  42  to further reduce the coefficient of friction therebetween. 
       FIG. 3B  includes boss  12 , oil transfer tube assembly  14 , tube  16 , seal  18 , tube base  20 , tube shoulder  22 , tube threads  24 , seal body  26 , seal lip  28 , tube lip  30 , tube/seal interface  32 , tube bore  34 , seal base  36 , boss threads  40 , boss well  42 , well base  44 , and first axial gap  46 . 
     Here,  FIG. 3B  shows the second stage of tube assembly  14  being received by boss  12 . As tube base  20  is threaded further into well  42 , first axial gap  46  is smaller but still visible between boss  12  and shoulder  22 . Second gap  48  between tube base  20  and seal  18  is now gone and replaced by tube/seal interface  32  (interface between surfaces  32 A and  32 B from  FIG. 3A ). Here, shoulder  22  is positioned high enough from the end of tube  16  such that it does not contact boss  12 . This ensures that seal  18  can be compressed as the remainder of tube base  20  is threaded and secured into well  42  (shown in  FIG. 3C ). Seal base  36  has not yet been fully compressed against well base  44 , leaving a small space between these surfaces as well. Tube base  20 , which is the portion of tube  16  below shoulder  22 , is roughly equal to the depth of boss well  40  less the height of seal body  26 . The difference between these values represents the degree of compression required to compress seal  18  against boss well  42  and well base  44 . 
       FIG. 3C  includes boss  12 , tube assembly  14 , tube  16 , seal  18 , tube base  20 , shoulder  22 , tube threads  24 , seal body  26 , seal lip  28 , tube lip  30 , tube/seal interface  32 , tube bore  34 , seal base  36 , boss threads  40 , boss well  42 , and boss base  44 . 
       FIG. 3C  shows tube assembly  14  finally installed and received in well  42 . At this final stage, shoulder  22  is now contacting the top of boss  12 . This contact prevents further torquing of tube  16  by interfering with downward travel of tube base  20  into well  42 . This provides a signal to the repair or installation technician without the need for an actual torque measurement or specialized tools. This also has the benefit of preventing excess compression of seal  18 , which can result in extrusion of soft seal material into areas around tube threads  24  and boss threads  40 . As noted above, extrusion of seal material into the threads complicates removal of tube  16  and seal  18  due to the relative bonding strength of the copper or other seal material as compared to the lower friction interface between respective threads  22 ,  40 . Preventing extrusion from overtorquing thus facilitates and expedites maintenance and repairs. 
     Tests indicate tube assembly  14  with shoulder  22  results in an approximate 30% reduction in applied torque as compared to a tube and seal assembly without a shoulder. Contact stresses are also spread more evenly over the greater surface area, with about ⅔ of the contact stresses being seen at tube/seal interface  32  and the remainder between shoulder  22  and boss  12 . 
       FIG. 4A  shows oil transfer tube assembly  14  with tube  16 , seal  18 , shoulder  22 , tube threads  24 , seal body  26 , seal lip  28 , tube lip  30 , tube bore  34 , bore threads  40 , bore well  42 , bore well base  44 , first axial gap  46 , and washer  50 . 
       FIG. 4A  corresponds roughly to  FIG. 3B  where tube lip  30  has been threaded over seal lip  28  but has not yet fully compressed seal body  26  against the edges of bore well  42 . Washer  50 , also shown in more detail in  FIG. 4C , is disposed in axial gap  46  between shoulder  22  and boss  12 . As can be seen here, a small oil escape path can remain between threads  24 ,  40 . Washer  50  minimizes oil leaks from these and other sources by providing another flexible and compressible sealing surface between this oil escape path and the outside of assembly  14 . When compressed by shoulder  22 , washer  50  blocks oil that has escaped tube bore  34  past compressed seal  18  and the spaces between threads  24 ,  40 . Details of washer  50  can be seen in  FIGS. 4B-4C . 
       FIG. 4B  is a detailed view of boss  12 , tube assembly  14 , tube  16 , tube base  20 , shoulder  22 , washer  50 , washer bead  52 , and washer thread  54 .  FIG. 4B  shows installation of washer  50  under shoulder  22 . Washer  50  includes washer bead  52  and washer thread  54 , which is shown in more detail in  FIG. 4C . As discussed above, washer bead  52  is compressed between shoulder  22  and boss  12 . Before tube base  20  is received into boss  12 , washer  50  can be first threaded over tube threads  24  to its location under shoulder  22 . Washer thread  54  thus simplifies placement and seating of washer  50  between shoulder  22  and boss  12 . 
     If oil leaks are minimized with one or more features described herein, technicians are less likely to try to “fix” the leak by tightening and possibly overtorquing the tube. It should be noted that the axial position of shoulder  22  on tube  16  can optionally be modified to account for the presence of washer  50 . However, washer  50  is compressible and generally has a comparatively minimal height relative to the other components. Thus any adjustment to the position of shoulder  22 , if necessary, will be small to inconsequential. 
       FIG. 4C  shows washer  50  with bead  52 , thread  54 , and projected washer contact surface  56 . Washer  50  in this example is formed from an unreinforced silicone rubber to maximize resistance to adjacent hot surfaces and limit chemical breakdown from contact with any potential leaking oil. However, the material can vary depending on the surrounding thermal, mechanical, and chemical conditions. Washer  50  has a unique design that simplifies installation and maximizes leak prevention. As was shown in  FIG. 4B , washer  50  is threaded onto tube base  20  using washer thread  54  before tube base  20  is received by boss  12 . For clarity,  FIG. 4C  includes a projection of contact surface  56 . Contact surface  56  provides the sealing interface between washer  50  and boss  12  when bead  52  is compressed. 
     Alternative embodiments of the oil transfer tube assembly are shown below and illustrate additional features for further reducing oil leaks. Washer  50  can also be adapted as necessary to be incorporated into some or all of these alternative embodiments. 
       FIG. 5A  includes tube assembly  114  with tube  116 , seal  118 , tube base  120 , shoulder  122 , tube threads  124 , tube/seal interface  132 , and tube bore  134 .  FIG. 5A  shows an alternative example embodiment of tube assembly  114 , which can be received by a boss such as was shown in  FIGS. 3A-3C . As compared to tube assembly  14 , tube assembly  114  has greater interface area  132  between tube  116  and seal  118 . This further helps to spread the torque and the resulting stresses around, as well as providing more area to block oil from leaking out of tube bore  134 . This is accomplished in part by providing a complementary concave/convex interface between tube  116  and seal  118 . Shoulder  122  has a similar effect of restricting downward movement of tube  116  and thus preventing overtorquing and extrusion of material from seal  118  into tube threads  124 . 
       FIG. 5B  is a detailed view of tube assembly  114  around tube/seal interface  132 .  FIG. 5B  also includes tube  116 , seal  118 , tube threads  124 , seal body  126 , seal lip  128 , tube lip  130 , and seal base  136 . As seen here, seal  118  includes seal body  126  with conical convex seal lip  128  which interfaces with complementary tube lip  130  to increase the area of interface  132 . Interface  132  is arranged at an oblique angle to a central axis of tube  116  to increase the contact area. Depending on the actual contact angle between tube  116  and seal  118 , the contact area of interface  132  can be more than double that of interface  32 , which is perpendicular to the central axis as shown above. Additional contact area makes it easier to compress seal  118  with a given torque value in addition to providing additional surface area to prevent and minimize oil leaks from tube bore  134  (shown in  FIG. 5A ). 
       FIG. 5C  shows conical convex seal  118  with seal body  126 , seal lip  128 , and convex contact surface  132 B. As was seen also in  FIG. 5B , convex contact surface  132 B has about double the surface area as compared to corresponding contact surface  32 B. 
       FIG. 6A  includes tube assembly  214  with tube  216 , seal  218 , tube base  220 , shoulder  222 , tube threads  224 , tube/seal interface  232 , and tube bore  234 .  FIG. 6A  is a second alternative example embodiment of tube assembly  114  that can be installed into a boss such as was shown in  FIGS. 3A-3C . Like tube assembly  114 , tube assembly  214  has greater contact area at interface  232  than tube assembly  14  to further spread the torque and the resulting stresses around, as well as provide more area to restrict oil leakage out of tube bore  234 . This is accomplished here by providing a complementary convex/concave arrangement between tube  216  and seal  218 , which is shown in more detail in  FIG. 6B . Shoulder  224  has a similar effect of restricting downward movement of tube  216  and thus preventing overtorquing and extrusion of material from seal  218  into tube threads  224 . 
       FIG. 6B  is a detailed view of tube assembly  214  around contact surfaces  232 .  FIG. 6B  also includes tube  216 , seal  218 , tube threads  224 , seal body  226 , seal lip  228 , tube lip  230 , and seal base  236 . Similar to seal  118  in  FIG. 5B , seal  218  includes seal body  226  with seal lip  228  which interfaces with complementary tube lip  230  to increase the contact area of interface  232 . Here, however, seal lip  228  is concave while tube lip  230  is convex, the opposite arrangement of tube assembly  114  shown in  FIGS. 5A-5C . 
       FIG. 6C  shows seal  218  with seal body  226 , seal lip  228 , and convex contact surface  232 B. As was seen also in  FIG. 6B , concave contact surface  232 B will have about double the contact surface area than corresponding contact surface  32 B in  FIGS. 3A-3C . 
     The invention has thus far been discussed in the context of an oil transfer tube assembly for a rear bearing housing of a gas turbine engine. However, application of the principles described herein are not so limited. The example embodiments can be readily modified by one skilled in the art to adapt these concepts to other lubrication or fluid transfer systems. Adaptations can be made for fluid transfer assemblies not just in gas turbine engines but other machinery as well. In addition, existing assemblies can be easily retrofitted as part of a repair or upgrade process so long as surrounding components do not interfere. 
     For example, the seal need not be a compressible copper seal as described here. Any known structure for restricting seepage or leakage of a fluid through connecting threads can be substituted for a seal. Examples include tape, washers, gaskets, flanges, etc. In addition, the tube need not be received by a boss on a bearing housing, but rather any structure suitable for receiving a fluid transfer tube, including internally threaded tubes. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.