Patent Publication Number: US-6705349-B2

Title: Weep plug

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to gas turbine engines and more particularly to a weep plug for recovering oil used to lubricate the bearings of a gas turbine engine. 
     Gas turbine engines typically include a core having a compressor for compressing air entering the core, a combustor where fuel is mixed with the compressed air and then burned to create a high energy gas stream, and a pressure turbine which extracts energy from the gas stream to drive the compressor. In aircraft turbofan engines, a low pressure turbine located downstream from the core extracts more energy from the gas stream for driving a fan. The fan provides the main propulsive thrust generated by the engine. 
     Bearings are used in the engine to accurately locate and rotatably mount rotors with respect to stators in the compressor and high and low pressure turbines of the engine. The bearings are enclosed in oil-wetted portions of the engine called sumps. 
     In order to prevent overheating of the bearings, lubricating oil and seals must be provided to prevent the hot air in the engine flowpath from reaching the bearing sumps, and lubricating oil flows must be sufficient to carry away heat generated internally by the bearings because of their high relative speed of rotation. 
     Oil consumption arises from the method used to seal the engine sumps. The sealing method makes it necessary for an air flow circuit to exist that flows into and out of the sumps. This flow ultimately contains oil that is unrecoverable unless adequately separated and delivered back to the sumps. In one particular configuration the forward engine sump is vented through the forward fan shaft and out the engine through a center vent tube. Once the air/oil mixture exits the sump, it swirls, depositing oil on the inside of the fan shaft. Oil that is contained in the air/oil mixture is lost when it is unable to centrifuge back into the sump through the vent hole due to rapidly escaping vent air. 
     Some designs allow for oil recovery by using weep holes, which are passages whose function is to provide a dedicated path for oil to re-enter the sump, integrated into the forward fan shaft design. Weep holes are typically smaller in diameter and longer in length than holes designed to route vent flow. However, in other designs, the fan shaft has no dedicated weep holes, only vent holes. Forming weep holes in fan shafts of the latter design after their manufacture and installation in an engine would be prohibitively expensive. 
     Accordingly, there is a need for a method to recover oil in existing sump structures without modifying the existing hardware. 
     BRIEF SUMMARY OF THE INVENTION 
     The above-mentioned need is met by the present invention, which provides a weep plug having a central vent passage and one or more weep passages parallel to the central passage. 
     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 
     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: 
     FIG. 1 is a longitudinal axial sectional view of a gas turbine engine incorporating a weep plug of the present invention. 
     FIG. 2 is an enlarged fragmentary view of a region of the engine enclosed by dashed box  2 — 2  of FIG.  1 . 
     FIG. 3 is a perspective view of a weep plug of the present invention. 
     FIG. 4 is an end view of a weep plug of the present invention. 
     FIG. 5 is a side elevational view of the weep plug of FIG.  4 . 
     FIG. 6 is a cross-sectional view taken along lines  6 — 6  of FIG.  5 . 
     FIG. 7 is a perspective view of a weep plug constructed in accordance with an alternate embodiment of the present invention. 
     FIG. 8 is a cross-sectional view of a portion of a gas turbine engine fan forward shaft having a weep plug of the present invention installed therein. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1 illustrates a gas turbine engine, generally designated  10 , in which is incorporated weep plug  90  of the present invention, as shown in detail in FIGS. 3-8. The engine  10  has a longitudinal center line or axis A and an outer stationary annular casing  14  disposed concentrically about and coaxially along the axis A. The engine  10  includes a gas generator core  16  which is composed of a multistage compressor  18 , a combustor  20 , and a high pressure turbine  22 , either single or multiple stage, all arranged coaxially about the longitudinal axis or center line A of the engine  10  in a serial, axial flow relationship. An annular outer drive shaft  24  fixedly interconnects the compressor  18  and high pressure turbine  22 . 
     The core  16  is effective for generating combustion gases. Pressurized air from the compressor  18  is mixed with fuel in the combustor  20  and ignited, thereby generating combustion gases. Some work is extracted from these gases by the high pressure turbine  22  which drives the compressor  18 . The remainder of the combustion gases are discharged from the core  16  into a low pressure turbine  26 . 
     An inner drive shaft  38  is mounted for rotation relative to the outer drive shaft  24  via rear bearings  32 , differential bearings  40 , and via suitable forward bearings  42  interconnected to the outer stationary casing  14 . The inner drive shaft  38 , in turn, rotatably drives a forward fan shaft  62 , which in turn drives a forward fan disk/booster rotor  44 . Fan blades  48  and booster blades  54  are mounted to the fan disk/booster rotor  44  for rotation therewith. 
     Referring now to FIG. 2, there is illustrated the region of the gas turbine engine  10  where a conventional bearing sump  58  is defined about the forward bearings  42 . The bearing sump  58  is generally defined by an outer annular structure  60  which is interconnected to the outer casing  14  and the forward fan shaft  62  which rigidly interconnects the forward end of the inner drive shaft  38  to the forward fan disk/booster rotor  44 . The forward fan shaft  62 , being connected with an inner annular race  42 A of the forward bearings,  42  rotates with the inner drive shaft  38  relative to the stationary outer annular structure  60  of the bearing sump  58  which is connected to an outer annular race  42 B of the forward bearings  42 . 
     Conventional labyrinth air and oil seals  64 ,  66  are provided adjacent to the forward bearings  42  and between the forward ends of the relatively rotating outer annular structure  60  and the forward fan shaft  62  to seal the forward end of the bearing sump  58 . Oil is pumped to the forward bearings  42  and therefore into the sump  58  through an oil supply conduit  68 . Pressurized air is injected to the labyrinth air seal  64  through an air supply conduit  70  in order to prevent oil from leaking through the labyrinth oil seal  66 . 
     A portion of the injected pressurized air which enters the bearing sump  58  must be vented from the sump  58  in a controlled manner in order to maintain sump pressure at a proper balance. However, the pressurized air becomes mixed with particles of the oil in the sump  58 . Therefore, the forward fan shaft  62  has one or more vent holes  84  extending through its thickness in a generally radial direction. Typically, the fan shaft  62  has a plurality of these holes  84  arranged in a band around its circumference. The vent holes  84  provide a passage for air flow from the sump  58  into a vent plenum  78  and subsequently into the center vent tube  80 . A cover  74  is attached to the forward fan shaft  62  with fasteners  76 . 
     Referring now to FIGS. 3-6, a weep plug  90  has a unitary body  92  having a first end  96  and a second end  98 , defining an axis  94  extending therebetween. A generally cylindrical central passage  100  passes axially through the body  92  from the first end  96  to the second end  98 . A generally circular head  116  having a flat end surface  118  is disposed at the first end  96 . Adjacent the head  116  is a generally annular flange  104  which has a pair of opposed flats  108  formed on laterally opposite sides thereof. An annular groove  117  separates the circular head  116  and the flange  104  and provides a surface for a tool to pry against when removing the plug  90 . A generally cylindrical elongated portion  102  extends between a proximate end  110  adjacent the flange  104  and a distal end  112  at the second end  98  of the body  92 . An annular groove  114  disposed at the junction of the elongated portion  102  and the flange  104 . A rim  106  is disposed on the flange  104  and extends axially towards the second end  98  of the body  92 . The rim  106  is divided into two annular sections by the presence of the opposed flats  108  of the flange  104 . The flats  108  provide a clearance space between the weep plug  90  and other nearby structures when the weep plug  90  is installed. 
     A pair of slots  122  are formed in opposite sides of the elongated portion  102 . The slots  122  begin at the distal end  112  of the elongated portion  102  and extend partially down the length of the elongated section  94 . The slots  122  divide the elongated portion  102  into two prongs  124 . Each of the prongs  124  has a pair of chamfered surfaces  120  formed at its distal end  112 , on opposite sides of the prong  124 . An annular protruding lip  126  extends from the distal end  112  of each of the prongs  108 . Although the illustrated example shows two slots  122 , it should be noted that three or more slots  122  could be formed in the elongated portion  102 , dividing it into three or more prongs  108 . At least one weep passage  130  is formed in the outer surface  128  of the elongated portion  102 . As best seen in FIGS. 4 and 5, in the illustrated example the weep passages  130  are in the form of grooves having a generally semicircular cross-section, although other shapes may be used. The weep passages have an outlet  132  disposed at the distal end of the elongated portion  102 . The weep passages then extend axially towards the flange  104 . At the point where the weep passages  130  intersect the annular groove  114 , they turn at a corner  134  and then extend radially outward, terminating at an inlet  136  disposed in the flange  104 , in alignment with the flat  108 . 
     The weep plug  90  is manufactured from a material which is capable of withstanding the temperatures prevailing in the sump  58 , which is approximately 149° C. (300° F.), and resisting attack from the engine lubricating oil. Also, because the fan shaft  62  is a life-limited part whose characteristics must not be compromised, the plug  90  must be made of a material which will itself wear rather than cause wear of the fan shaft  62 . Furthermore, the weight of the plug  90  is preferably minimized both to avoid extra weight in the engine  10  generally, and to preclude imbalance problems in the fan shaft  62 , especially if the plugs  90  should be improperly installed. One suitable material is VESPEL polyimide, available from E.l. DuPont de Nemours and Company, Wilmington, Del. 19898 USA. Another suitable material is PEEK polyetheretherketone, which is available from Victrex USA Inc., 3 Caledon Court, Suite A, Greenville, S.C. 29615 USA. In general, any material that satisfies the requirements described above may be used, for example aluminum or other relatively soft metals may also be suitable materials. The weep plug  90  may be formed by any known method, for example injection molding, compression molding a near-net shape followed by machining, or by machining from a blank of material. 
     Another embodiment of the present invention is illustrated in FIG. 7. A weep plug  290  is similar to weep plug  90 , having a body  292  comprising an elongated portion  102 , a flange  104  having a rim  106 , and a circular head  116 . An annular groove  114  encircles the body  292  at the junction of the flange  104  and the elongated portion  102 . A pair of weep passages  294  are disposed in the outer surface of elongated portion  102  on opposite sides thereof. In this embodiment, the inlets  296  of the weep passages  294  do not extend into the flange  104 . The flange  104  extends completely around the circumference of the body  292 . A pair of opposed channels  298  are formed in the rim  106 . The channels  298  are recessed from the surface of the rim  106  and provide additional area for oil to flow to the groove  114  and the weep passages  294 . 
     FIG. 8 illustrates a more detailed view of the weep plug  90  installed in the forward fan shaft  62 . The weep plug  90  is installed in the vent hole  84  from the radially inner direction. The chamfered surfaces  120  assist in aligning the body  92  of the weep plug  90  with the vent hole  84 . In the uninstalled condition the width W (FIG. 4) across the outer edges of the lips  126  is slightly greater than the diameter D (FIG. 2) of the vent hole  84 . The presence of the slots  122  allows the prongs  124  to flex inward slightly as the weep plug  90  is installed. When the weep plug  90  is fully inserted and the lips  126  clear the radially outward edge of the vent hole  84 , the prongs return to their original position and bear against the radially outer edge  302  of the vent hole  84 , retaining the weep plug  90  in the vent hole  84 . In operation, the weep plug  90  rotates with the forward fan shaft  62  and tends to move radially outward. This causes the rim  106  of the flange  104  to bear against the radially inner edge  300  of the vent hole  84  and retain the weep plug  90  in the vent passage. 
     In operation, an air/oil mixture exits the sump  58  through the central passage  100  of the weep plug  90 , as shown by the arrows marked B in FIG.  8 . In the illustrated example the length L of the weep plug  90  is about 30.5 mm (1.2 in.) and the diameter c of the central passage is about 8.8 mm (0.35 in.), resulting in a length-to-diameter ratio of about 3.5. The mixture then swirls, depositing oil on the inside of the forward fan shaft  62 . Oil that is contained in the air/oil mixture flows into the inlets  136  of the weep passages  130 , along the length of the weep passages  130 , and then through the outlets  132  into the sump  58  where it can be recovered, as shown by arrows C. In the illustrated example the weep passages  130  have a length I of about 21 mm (0.83 in.) and a width W of about 0.76 mm (0.03 in.), resulting in a length-to-diameter ratio of about 28. This greater LID ratio allows the oil to flow up the weep passages  130  without being affected by the flow of air sump  58 , as it would be without the weep plug  90 . Additionally, the diameter d of the elongated portion  102  of the weep plug  90  may be selected relative to the diameter D of the vent hole  84  to leave a clearance between the vent hole  84  and the plug body  92 , creating an additional annulus for oil to centrifuge through without being sheared by escaping vent air. The diametrical clearance of the illustrated example is about 0.177 mm (0.007 in.). 
     The weep plug  90  as disclosed herein provides a tangible oil consumption benefit and is yet simple to implement. The weep plugs  90  can be assembled on field engines while they are still mounted on the aircraft with very little disassembly of the engine. In one particular example, the assembly of four plugs into the forward fan shaft in a total of 22 holes resulted in about an 8% reduction in overall engine oil consumption. The optimum number of plugs  90  will vary for each particular application. If there are too few plugs, the benefit of reduced oil consumption will not be realized. If too many plugs are used they may excessively restrict the flow through vent holes  84 , upsetting the sump pressurization balance. Analysis of another possible configuration has shown that 12 plugs used in a possible of 20 holes would result in the optimum reduction in oil consumption while minimally affecting engine system pressurization characteristics. It might also be desirable to use a reduced number of plugs, for example 10, or the equivalent of one plug every other hole, to provide simplified installation. 
     The foregoing has described a weep plug having a central vent passage and one or more weep passages. 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.