Abstract:
A hybrid brush seal is provided having two bundles of axially spaced seal bristles each of which are mounted in a ring shape on a first machine component with bristle ends directed at a sealing surface of the second, rotating machine component. The bristle ends are kept from direct contact with the rotating machine component via one or more shoes which are designed such that as the shaft rotates a hydrodynamic film separates the shoe(s) from the shaft. The shoe(s) is attached to the bristle ends at discreet locations. Alternatively, one or more bundles of seal bristles are mounted at one end either to the fixed or rotating machine component, with the opposite bristle ends directed toward one or more shoes, and wherein one or more spring elements are connected between the machine component and shoes.

Description:
[0001]    This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application Serial No. 60/466,979 filed May 1, 2003 for all commonly disclosed subject matter. U.S. Provisional Application Serial No. 60/466,979 is expressly incorporated herein by reference in its entirety to form a part of the present disclosure. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to seals for sealing a circumferential gap between two machine components that are relatively rotatable with respect to each other, and, more particularly, to a hybrid brush seal having two sets of axially spaced seal bristles each of which are mounted in a ring shape on a first machine component with bristle ends directed at sealing surface of the second, rotating machine component. The bristle ends are kept from direct contact with the rotating machine component via one or more shoes which are designed such that as the shaft rotates a hydrodynamic film separates the shoes from the shaft. The shoe(s) is attached to the bristle ends at discreet locations. Alternatively, one or more sets of seal bristles are mounted at one end either to the fixed or rotating machine component, with the opposite bristle ends directed toward one or more shoes, and wherein one or more spring elements are connected between the machine component and shoes.  
         BACKGROUND OF THE INVENTION  
         [0003]    Turbomachinery, such as gas turbine engines employed in aircraft, currently is dependent on either labyrinth (see FIGS. 1A-1E), brush (see FIGS. 2A and 2B) or carbon seals for critical applications. Labyrinth seals provide adequate sealing, however, they are extremely dependent on maintaining radial tolerances at all points of engine operation. The radial clearance must take into account factors such as thermal expansion, shaft motion, tolerance stack-ups, rub tolerance, etc. Minimization of seal clearance is necessary to achieve maximum labyrinth seal effectiveness. In addition to increased leakage if clearances are not maintained, such as during a high-G maneuver, there is the potential for increases in engine vibration. Straight-thru labyrinth seals (FIG. 1A) are the most sensitive to clearance changes, with large clearances resulting in a carryover effect. Stepped labyrinth seals (FIGS. 1B and 1C) are very dependent on axial clearances, as well as radial clearances, which limits the number of teeth possible on each land. Pregrooved labyrinth seals (FIG. 1D) are dependent on both axial and radial clearances and must have an axial clearance less than twice the radial clearance to provide better leakage performance than stepped seals.  
           [0004]    Other problems associated with labyrinth seals arise from heat generation due to knife edge to seal land rub, debris from hardcoated knife edges or seal lands being carried through engine passages, and excessive engine vibration. When seal teeth rub against seal lands, it is possible to generate large amounts of heat. This heat may result in reduced material strength and may even cause destruction of the seal if heat conducted to the rotor causes further interference. It is possible to reduce heat generation using abradable seal lands, however, they must not be used in situations where rub debris will be carried by leakage air directly into critical areas such as bearing compartments or carbon seal rubbing contacts. This also holds true for hardcoats applied to knife edges to increase rub capability. Other difficulties with hardcoated knife edges include low cycle fatigue life debits, rub induced tooth-edge cracking, and the possibility of handling damage. Engine vibration is another factor to be considered when implementing labyrinth seals. As mentioned previously, this vibration can be caused by improper maintenance of radial clearances. However, it can also be affected by the spacing of labyrinth seal teeth, which can produce harmonics and result in high vibratory stresses.  
           [0005]    In comparison to labyrinth seals, brush seals can offer very low leakage rates. For example, flow past a single stage brush seal is approximately equal to a four knife edge labyrinth seal at the same clearance. Brush seals are also not as dependent on radial clearances as labyrinth seals. Leakage equivalent to approximately a 2 to 3 mil gap is relatively constant over a large range of wire-rotor interferences. However, with current technology, all brush seals will eventually wear to line on line contact at the point of greatest initial interference. Great care must be taken to insure that the brush seal backing plate does not contact the rotor under any circumstances. It is possible for severing of the rotor to occur from this type of contact. In addition, undue wire wear may result in flow increases up to 800% and factors such as changes in extreme interference, temperature and pressure loads, and rubbing speeds must be taken into account when determining seal life.  
           [0006]    The design for common brush seals, as seen in FIGS. 2A and 2B, is usually an assembly of densely packed flexible wires sandwiched between two plates. The free ends of the wires protrude beyond the plates and contact a land or runner, with a small radial interference to form the seal. The wires are angled so that the free ends point in the same direction as the movement of the runner. Brush seals are sized to maintain a tight diametral fit throughout their useful life and to accommodate the greatest combination of axial movement of the brush relative to the rotor.  
           [0007]    Brush seals may be used in a wide variety of applications. Although brush seal leakage generally decreases with exposure to repeated pressure loading, incorporating brush seals where extreme pressure loading occurs may cause a “blow over” condition resulting in permanent deformation of the seal wires. Brush seals have been used in sealing bearing compartments, however coke on the wires may result in accelerated wear and their leakage rate is higher than that of carbon seals.  
           [0008]    One additional limitation of brush seals is that they are essentially uni-directional in operation, i.e., due to the angulation of the individual wires, such seals must be oriented in the direction of rotation of the moving element. Rotation of the moving element or rotor in the opposite direction, against the angulation of the wires, can result in permanent damage and/or failure of the seal. In the particular application of the seals required in the engine of a V-22 Osprey aircraft, for example, it is noted that during the blade fold wing stow operation, the engine rotates in reverse at very low rpm&#39;s. This is required to align rotor blades when stowing wings. This procedure is performed for creating a smaller aircraft footprint onboard an aircraft carrier. Reverse rotation of the engine would damage or create failure of brush seals such as those depicted in FIGS. 2A and 2B.  
           [0009]    One attempt to limit wear of brush seals is disclosed in U.S. Pat. No. 5,026,252 to Hoffelner in which a sliding ring is interposed between the bristle pack of the seal and the moving element or rotor to avoid direct contact therebetween. The bristle ends are received within a circumferential groove in the sliding ring and are allowed to freely float or move within such groove. Although bristle wear may be reduced in this design, it is believed that the seal created at the interface of the sliding ring and rotor is unsatisfactory.  
           [0010]    An improvement of prior brush seals, including that disclosed in the &#39;252 patent to Hoffelner noted above, is found in my U.S. Pat. No. 6,428,009. In that design, one end of each of a plurality of seal bristles is fixed in an annular shape and mounted to the fixed machine component or stator while their opposite ends are attached to a number of individual shoes located proximate the rotating machine component or rotor. Prior to shaft rotation, the shoes are in contact with the rotor surface with preferably the leading edge of each shoe set to have less contact than the trailing edge of the shoe. When the rotor begins to rotate, a hydrodynamic wedge is created which lifts the shoe slightly off the surface of the shaft allowing the shoe to effectively float over the shaft at a design gap. It has been found that one limitation of the design disclosed in the &#39;009 patent is a potential problem with “roll over” under pressure load, i.e. the shoes can tip or pivot in the axial direction thus creating a leakage path.  
           [0011]    Carbon seals are generally used to provide sealing of oil compartments and to protect oil systems from hot air and contamination. Their low leakage rates in comparison to labyrinth or brush seals are well-suited to this application, however they are very sensitive to pressure balances and tolerance stack-ups. Pressure gradients at all operating conditions and especially at low power and idle conditions must be taken into account when considering the use of carbon seals. Carbon seals must be designed to have a sufficiently thick seal plate and the axial stack load path must pass through the plate as straight as possible to prevent coning of the seal. Another consideration with carbon seals is the potential for seepage, weepage or trapped oil. Provisions must be made to eliminate these conditions which may result in oil fire, rotor vibration, and severe corrosion.  
           [0012]    According to the Advanced Subsonic Technology Initiative as presented at the NASA Lewis Research Center Seals Workshop, development of advanced sealing techniques to replace the current seal technologies described above will provide high returns on technology investments. These returns include reducing direct operating costs by up to 5%, reducing engine fuel burn up to 10%, reducing engine oxides of emission by over 50%, and reducing noise by 7 dB. For example, spending only a fraction of the costs needed to redesign and re-qualify complete compressor or turbine components on advanced seal development can achieve comparable performance improvements. In fact, engine studies have shown that by applying advanced seals techniques to just a few locations can result in reduction of 2.5% in SFC.  
         SUMMARY OF THE INVENTION  
         [0013]    A hybrid brush seal is provided which is generally similar to the one disclosed in my prior U.S. Pat. No. 6,428,009, but which overcomes the tendency of the shoes to roll over under the application of a pressure load.  
           [0014]    In one presently preferred embodiment, two sets or bundles of seal bristles are axially spaced from one another, i.e. in the direction of the longitudinal axis of two relatively rotating machine components such as the rotor and stator of a gas turbine engine. One end of the seal bristles in each bundle is fixed in an annular shape to either the stator or the rotor, while the opposite end of the seal bristles in each bundle extends to one or more shoes circumferentially disposed about the other machine component. The shoes are located with respect to the rotor or stator to created a seal between the two while avoiding contact of the seal bristles with the relatively rotating component. Each of the shoes is connected at discrete points to the end of the seal bristles such that the leading edge of the shoe is oriented to have less contact with the rotor or the stator than the trailing edge of the shoe. In one embodiment, each shoe is connected at two spaced locations to the abutting seal bristles by electron beam welding or similar mounting techniques, thus creating two hinge points for the shoe to translate about.  
           [0015]    In an alternative embodiment, one or more bundles or seal bristles are mounted at one end to either the rotor or the stator, and their opposite end extends toward one or more shoes located proximate the other of the rotor or stator. A spring element is connected between the shoes and the rotor or stator which is flexible in the radial direction, but axially stiff. The spring element functions to assist in preventing roll over of the shoes with respect to the rotor or stator where it is located, thus maintaining an effective seal under pressure load. It is contemplated that the ends of the seal bristles proximate the shoes can be either connected to the shoes such as by welding or other means of attachment, or spaced from the shoes. In either case, the seal bristles act as a secondary seal between the rotor and stator in combination with the shoes.  
           [0016]    In operation, the shoes of either embodiment of this invention function very similarly to that of a tilting pad bearing shoe. Prior to rotation of the rotor, the shoe is in contact with the rotor or stator surface. Because the leading edge of the shoe has less contact with the rotor or stator than its trailing edge, when the rotor begins to rotate a hydrodynamic wedge is created that lifts the shoe slightly off of the surface of the rotor or stator. Consequently, the shoe “floats” over the rotor or stator at a design gap, such as 0.0005 to 0.0010 inches.  
           [0017]    The advantages of the hybrid brush seal of this invention are many. It has the same sealing characteristics of existing brush seals, but will never change in performance due to bristle wear. The brush seal backing plate can be moved further outboard of the I.D. because the shoe prevents the bristles from bending over in high pressure applications. Each shoe may have a certain amount of interference with the rotor or stator prior to rotation. Thus, the seal can be slightly off center during assembly but once rotation begins, each pad will lift-off. Hence, tight tolerances can be relaxed.  
           [0018]    The hybrid seal of this invention can be utilized in all seal applications, including labyrinth, brush and carbon. The robust design eliminates the careful handling now required of carbon seals utilized in lube system compartments. This seal may allow the engine designer to utilize less parts in the assembly as this seal will permit “blind” assemblies to occur.  
           [0019]    The following table provides a comparison of the seal of the subject invention with currently available technology.  
                                                           Dependence   Contamination       Seal Type   Wear Rate   Leakage   on Clearances   Potential                   Labyrinth   High   Low   High   High       Seals       Brush Seals   Medium   Low   Medium   Medium       Carbon Seals   Medium   Very Low   High   Low       Hybrid Seal   Low   Low   Low   Low                  
 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0020]    The structure, operation and advantages of this invention will become further apparent upon consideration of the following description, taken in conjunction with the accompanying drawings, wherein:  
         [0021]    [0021]FIGS. 1A-1E are schematic views of a number of prior art labyrinth seals;  
         [0022]    [0022]FIGS. 2A and 2B depict views of a prior art brush seal;  
         [0023]    [0023]FIG. 3 is a cross sectional view of one embodiment of the hybrid brush seal of this invention;  
         [0024]    [0024]FIG. 4 is a schematic, elevational view of the seal shown in FIG. 3;  
         [0025]    [0025]FIG. 5 is a view similar to FIG. 4, except of an alternative embodiment herein;  
         [0026]    [0026]FIG. 6 is a schematic, elevational view of an alternative embodiment of the seal herein employing a single bundle of seal bristles and axially spaced spring elements;  
         [0027]    [0027]FIG. 7 is a view similar to FIG. 6, except employing two sets of axially spaced seal bristles;  
         [0028]    [0028]FIG. 8 is a cross sectional view of a still further embodiment of the brush seal of this invention; and  
         [0029]    [0029]FIG. 9 is a cross sectional view taken generally along line  9 - 9  of FIG. 8. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0030]    Referring initially to FIGS. 3-5, the hybrid bush seal  10  of this invention is intended to create a seal between two relatively rotating components, namely, a fixed stator  12  and a rotating rotor  14 . In this embodiment, the seal  10  comprises a first group or bundle  16  of seal bristles  18  and a second bundle  20  of seal bristles  18  which are axially spaced from the first bundle  16 . As used herein, the term “axial” or “axially spaced” refers to a direction along the longitudinal axis of the stator  12  and rotor  14 , e.g. axis  22  in FIG. 3, whereas “radial” refers to a direction perpendicular to the longitudinal axis  22 .  
         [0031]    The seal bristles  18  in each bundle  16  and  20  have an inner end  24  and an outer end  26 . In the embodiment illustrated in FIGS. 3 and 4, the outer end  26  of the seal bristles  18  in each bundle  16 ,  20  is affixed to the stator  12 . For purposes of the present discussion, the construction and operation of the seal  10  herein is described with the seal bristles  18  in that orientation. It should be understood, however, that the inner end  24  of the seal bristles  18  could be affixed to the rotor  14 . Preferably, the seal bristles  18  are mounted to the stator  12  or rotor  14  by clamping, welding, brazing or other means of affixation. The seal bristles  18  in each bundle  16  and  20  are arranged in an annular shape corresponding to the circumferential gap between the stator  12  and rotor  14 . As best seen in FIGS. 4 and 5, a spacer plate  28  is located in the axial space between the seal bristle bundles  16  and  20 . The seal bristles  18  in bundle  16  are captured between a high pressure backing plate  30  associated with the stator  12  and the spacer plate  28 , whereas the seal bristles  18  in bundle  20  extend between a second spacer plate  31  and a low pressure backing plate  32 .  
         [0032]    In one presently preferred embodiment, the seal bristles  18  are formed of a wire material, but it is contemplated that different materials may be utilized depending upon environmental conditions of the particular sealing application. In the past, brush seal materials, including the seal bristles, were chosen primarily for their high temperature and wear capability properties. The bristle seals  18  of this invention do not contact the rotor  14 , as discussed below, and therefore different wear characteristics and other considerations are involved in the selection of appropriate materials for the bristle seals  18  as compared to conventional brush seals. The bristle seal  18  geometry may be angled in the direction of rotation of the rotor  14 , or, alternatively, the bristle seals  18  may be straight and have varied angles. The bristle seals  18  may be round, square, rectangular. or other shapes, and, if round, the diameter of each bristle seal  18  can be varied depending on the nature of the sealing environment. The outer end  26  of the bristle seals  18  in each bundle  16  and  20  may be fused together or free to move independently. Further, the number of seal bristles  18  within each bundle  16  and  20  can be varied with the understanding that more seal bristles  18  generally leads to improved sealing.  
         [0033]    The inner end  24  of the seal bristles  18  in each bundle abut one or more shoes  34  located in sealing relationship to the rotor  14 . In the embodiment of FIG. 4, the shoes  34  are formed with axially spaced ridges  36  and  38 . One side of the bundle  16  of seal bristles  18  abuts the ridge  36 , and one side of the bundle  20  of seal bristles  18  abuts the ridge  38 . FIG. 5 depicts a slightly different construction of shoes  34  in which the ridge  36  is the same as that in FIG. 4, but a ridge  40  is formed on the shoes  34  in position to contact the opposite side of the bundle  20  of seal bristles  18  compared to the FIG. 4 embodiment. In both cases, each shoe  34  is attached at discrete locations to the abutting seal bristles  18  such as by welding, brazing, clamping or other means. The arc length, width, height, geometry and surface characteristics of the shoes  34  can be varied to enhance hydrodynamic pressure between the rotor  14  and stator  12 , to balance the static pressures within the system to vary the pressure sealing capabilities of the seal  10  and for other purposes. Preferably, the shoes  34  are made from sheet metal stampings or similar materials, to reduce manufacturing costs.  
         [0034]    Referring now to FIGS. 6-9, alternative embodiments of a brush seal of this invention are shown. In FIG. 6, a brush seal  40  is shown in which a single bundle  42  of seal bristles  18  is located between a high pressure backing plate  44  and a low pressure backing plate  46 . For purposes of the present discussion, and consistent with the description of the previous embodiments, an outer end  48  of each seal bristle  18  in bundle  42  is mounted to the stator  12  while the inner end  50  extends toward the rotor  14 . It should be understood that the seal bristles  18  in bundle  42  could be affixed to the rotor  14  instead of the stator  12 .  
         [0035]    In the embodiments of FIGS. 3-5, axial rigidity and radial compliance of the seal  10  is provided by the seal bristles  18  in the bundles . 16  and  20  through their connection between the stator  12 . and shoes  34 . In the embodiment of FIG. 6, the seal bristles  18  in the bundle  42  need not be connected to a shoe  34 . Instead, a spring element  52  is connected between the high pressure backing plate  44  and the shoe  34 , and another spring element  54  is connected between the low pressure backing plate  46  and the shoe  34 . These spring elements  52  and  54  are axially spaced from one another and provide essentially the same resistance to roll over of the seal  40  as the bundles  16  and  20  of seal bristles  18  in the seal  10  of FIGS. 3-5. Preferably, the spring elements  52  and  54  are formed of spring steel or other material which is flexible in the radial direction but stiff in the axial direction.  
         [0036]    The embodiment of FIG. 7 depicts a seal  55  which is similar to the seal  40  of FIG. 6, except that two axially spaced bundles  56  and  58  of seal bristles  18  are employed instead of one. The bundle  56  of seal bristles  18  is retained between a low pressure backing plate  60  and a spacer plate  62 , whereas the bundle  58  is retained between a second spacer plate  64  and a high pressure backing plate  66 . As in the embodiment of FIG. 6; the bristles  18  of each bundle  56 ,  58  need not be connected to a shoe  34 . Axial rigidity and radial compliance are provided primarily by a spring element  68  connected between the low pressure backing plate  60  and shoe  34 , and a second spring element  70  connected between the high pressure backing plate  66  and the shoe  34 .  
         [0037]    Referring now to FIGS. 8 and 9, a still further embodiment of a seal  72  according to this invention is shown. The seal  72  is similar to that of seals  40  and  55  except for the spring elements  74 . Each spring element  74  is essentially a rectangular-shaped beam with an outer band  76  radially spaced from an inner band  78 . One end of each of the bands  76  and  78  is connected to a seat  80  formed in the stator  12 , and the opposite end of bands  76 ,  78  mounts to a ridge  82  formed in a shoe  34 . The spring element  74  functions to maintain the shoe  34  is sealing relationship with the rotor  14  in the same manner as the spring elements  52 ,  54  and  68 ,  70 . A bundle  72  of seal bristles  18  is fixed at its outer end to the stator  12 , and the inner end of each seal bristle  18  extends toward the shoe  34  where it may or may not be affixed thereto.  
         [0038]    In each of the embodiments of FIGS. 6-9, the seal bristles  18  form essentially a secondary seal. The shoes  34  are maintained in position with respect to the stator  12  and rotor  14  by the spring elements  52  and  54 ,  68  and  70 , and  74 , which cooperate with the bristle bundles to resist roll over.  
         [0039]    While the invention has been described with reference to a preferred embodiment, it should 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.  
         [0040]    For example, it has been found advantageous to provide a flow path in the shoes  34  of this invention to assist in balancing static pressure in the system. This flow path can take the form of a step  84  formed in the shoe  34 , as depicted in FIG. 6.  
         [0041]    Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the invention, but that the invention will include all embodiments falling within the scope of the appended claims.