Abstract:
A seal comprises the combination of a primary seal and a secondary seal each of which acts on at least one shoe that is installed with clearance relative to one of a rotor and a stator in a position to create a non-contact seal therewith. The at least one shoe is provided with a surface geometry and labyrinth-type teeth that influence the inertia of fluid flowing across the seal, and, hence, the velocity of the fluid and the pressure distribution across the seal, ultimately affecting the balance of forces applied to the seal.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part application of U.S. patent application Ser. No. 11/953,009 filed Dec. 10, 2007, which is a continuation-in-part of U.S. patent application Ser. No. 11/669,454 filed Jan. 31, 2007, which is a continuation-in-part application of U.S. patent application Ser. No. 11/226,836 filed Sep. 14, 2005 and now U.S. Pat. No. 7,182,345, which is a continuation of U.S. patent application Ser. No. 10/832,053 filed Apr. 26, 2004, now abandoned, which claims the benefit of U.S. Provisional Application Ser. No. 60/466,979 filed May 1, 2003 under 35 U.S.C. §119(e) for all commonly disclosed subject matter. U.S. Provisional Application Ser. No. 60/466,979 is expressly incorporated herein by reference in its entirety to form part of the present disclosure. 
    
    
     FIELD OF THE INVENTION 
     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 non-contact seal especially intended for gas turbine engine applications having at least one shoe supported by a number of spring elements so that a first surface of the at least one shoe extends along one of the machine components within design tolerances. The first surface of the at least one shoe may have a number of different geometries, and one or more cavities formed by radially inwardly extending tooth members, which collectively influences the velocity and pressure distribution of the fluid flowing across the seal thus allowing the seal clearance to be controlled in both directions, e.g. a larger or smaller radial clearance with respect to a machine component. 
     BACKGROUND OF THE INVENTION 
     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 but 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. 
     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, but 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. 
     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. 
     The design for common brush seals, as seen in  FIGS. 2A and 2B , is usually an assembly of densely packed flexible wires sandwiched between a front plate and a back plate. 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. 
     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. 
     One additional limitation of brush seals is that they are essentially unidirectional 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 . 
     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 but 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. 
     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 
     This invention is directed to a non-contact seal for sealing the circumferential gap between a first machine component such as a stator and a second machine component such as a rotor which is rotatable relative to the stator. 
     In one presently preferred embodiment, the seal comprises the combination of a primary seal and a secondary seal each of which acts on at least one shoe extending along one of the rotor and stator in a position to create a non-contact seal therewith. At least one spring element is connected between one of the rotor and stator and the at least one shoe. The spring element may take the form of two or more radially spaced beams or bands, or a number of generally parallel pins axially extending between a ring and the at least one shoe. The spring elements are flexible in the radial direction, but axially stiff so that they can function to assist in preventing roll over of the shoes with respect to the rotor or stator, thus maintaining an effective seal under pressure load. The spring elements deflect and move with the at least one shoe in the radial direction in response to the application of aerodynamic forces applied to the at least one shoe to create a primary seal, within design tolerances, along the gap between the machine components. 
     The shoe(s) includes a first, sealing surface and a second surface opposite the first surface. The second surface is formed with a slot within which one end of a secondary seal may be disposed. It is contemplated that the slot may be positioned at the front (high pressure) or aft (low pressure) side of the shoe(s). The opposite end of the secondary seal is connected to one of the first and second machine components. The secondary seal deflects and moves with the shoe(s) in response to the application of aerodynamic forces to the shoe(s), and applies a radial force acting in the direction of one of the first and second machine components to assist with the creation of a secondary seal along the gap between the machine components. 
     In the presently preferred embodiment, the first, sealing surface of the shoe(s) may be formed with different geometric features, and one or more cavities located between axially spaced labyrinth-type tooth elements, to affect the clearance between the sealing surface of the shoe(s) and the first or second machine component. As discussed below, this construction influences fluid velocity and pressure resulting from the application of aerodynamic forces to the seal, allowing for improved control of the clearance between the seal and the first or second machine component. 
     The 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. 
     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 
                 High 
                 Low 
               
               
                   
                   
                 Low 
               
               
                 Hybrid Seal 
                 Low 
                 Low 
                 Low 
                 Low 
               
               
                   
               
             
          
         
       
     
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       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: 
         FIGS. 1A-1E  are schematic views of a number of prior art labyrinth seals; 
         FIGS. 2A and 2B  depict views of a prior art brush seal; 
         FIG. 3  is an isometric view of the hybrid seal of this invention; 
         FIG. 4  is a partial, perspective view of the seal depicted in  FIG. 3 , illustrating a single shoe with the secondary seal removed; 
         FIG. 5  is a cross sectional view taken generally along line  5 - 5  of  FIG. 4 ; 
         FIG. 6  is a cross sectional view taken generally along line  6 - 6  of  FIG. 3 , with a brush seal depicted as a secondary seal; 
         FIG. 7  is a view similar to  FIG. 6  except with a secondary seal comprising side-by-side plates; 
         FIG. 8  is an enlarged, side view of a portion of one of the plates shown in  FIG. 7 ; 
         FIG. 9  is a force balance diagram of a shoe depicting the aerodynamic forces, spring forces and secondary seal forces acting on the shoe; 
         FIGS. 10A-10G  depict alternative embodiments of shoe(s) having different geometric features; 
         FIG. 11  is a view similar to  FIG. 7  except with the formation of axially spaced labyrinth-type tooth elements along the first surface of the at least one shoe; 
         FIG. 12  is a perspective view of an alternative embodiment of the seal of this invention employing axially spaced spring elements; 
         FIG. 13  is a perspective view of a portion of  FIG. 12 ; and 
         FIG. 14  is a partial perspective view of a still further embodiment of the seal of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to  FIGS. 3-6 , the hybrid seal  10  of this invention is intended to create a seal of the circumferential gap  11  between two relatively rotating components, namely, a fixed stator  12  and a rotating rotor  14 . The seal  10  includes at least one, but preferably a number of circumferentially spaced shoes  16  which are located in a non-contact position along the exterior surface of the rotor  14 . Each shoe  16  is formed with a sealing surface  20  and a slot  22  extending radially inwardly toward the sealing surface  20 . For purposes of the present discussion, the term “axial” or “axially spaced” refers to a direction along the longitudinal axis of the stator  12  and rotor  14 , e.g. axis  18  shown in FIGS.  3  and  10 A- 10 G, whereas “radial” refers to a direction perpendicular to the longitudinal axis  18 . 
     Under some operating conditions, particularly at higher pressures, it is desirable to limit the extent of radial movement of the shoes  16  with respect to the rotor  14  to maintain tolerances, e.g. the spacing between the shoes  16  and the facing surface of the rotor  14 . The seal  10  preferably includes a number of circumferentially spaced spring elements  24 , the details of one of which are best seen in  FIGS. 3 and 4 . Each spring element  24  is formed with an inner band  26  and an outer band  28  radially outwardly spaced from the inner band  26 . One end of each of the bands  26  and  28  is mounted to or integrally formed with the stator  12  and the opposite end thereof is connected to a first stop  30 . The first stop  30  includes a strip  32  which is connected to a shoe  16  (one of which is shown in  FIGS. 4 and 5 ), and has an arm  34  opposite the shoe  16  which may be received within a recess  36  formed in the stator  12 . The recess  36  has a shoulder  38  positioned in alignment with the arm  34  of the first stop  30 . 
     A second stop  40  is connected to or integrally formed with the strip  32 , and, hence connects to the shoe  16 . The second stop  40  is circumferentially spaced from the first stop  30  in a position near the point at which the inner and outer bands  26  and  28  connect to the stator  12 . The second stop  40  is formed with an arm  42  which may be received within a recess  44  in the stator  12 . The recess  44  has a shoulder  46  positioned in alignment with the arm  42  of second stop  40 . 
     Particularly when the seal  10  of this invention is used in applications such as gas turbine engines, aerodynamic forces are developed which apply a fluid pressure to the shoe  16  causing it to move radially with respect to the rotor  14 . The fluid velocity increases as the gap  11  between the shoe  16  and rotor  14  increases, thus reducing pressure in the gap  11  and drawing the shoe  16  radially inwardly toward the rotor  14 . As the seal gap  11  closes, the velocity decreases and the pressure increases within the seal gap  11  thus forcing the shoe  16  radially outwardly from the rotor  14 . The spring elements  24  deflect and move with the shoe  16  to create a primary seal of the circumferential gap  11  between the rotor  14  and stator  12  within predetermined design tolerances. The purpose of first and second stops  30  and  40  is to limit the extent of radially inward and outward movement of the shoe  16  with respect to the rotor  14  for safety and operational limitation. A gap is provided between the arm  34  of first stop  30  and the shoulder  38 , and between the arm  42  of second stop  40  and shoulder  46 , such that the shoe  16  can move radially inwardly relative to the rotor  14 . Such inward motion is limited by engagement of the arms  34 ,  42  with shoulders  38  and  46 , respectively, to prevent the shoe  16  from contacting the rotor  14  or exceeding design tolerances for the gap between the two. The arms  34  and  42  also contact the stator  12  in the event the shoe  16  moves radially outwardly relative to the rotor  14 , to limit movement of the shoe  16  in that direction. 
     In the presently preferred embodiment, the seal  10  is also provided with a secondary seal which may take the form of a brush seal  45 , as shown in  FIG. 6 , or a stack of at least two sealing elements oriented side-by-side and formed of thin sheets of metal or other suitable material as shown in  FIGS. 7 and 8 . The brush seal  45  is positioned so that one end of its bristles  47  extends into the slot  22  formed in the shoe  16 . The bristles  47  deflect with the radial inward and outward movement of the shoe  16 , in response to the application of fluid pressure as noted above, in such a way as to create a secondary seal of the gap  11  between the rotor  14  and stator  12 . 
     Referring now to  FIGS. 7 and 8 , the secondary seal of this embodiment may comprise a stack of at least two sealing elements  48  and  50 . Each of the sealing elements  48  and  50  comprises an outer ring  52  formed with a number of circumferentially spaced openings  54 , a spring member  56  mounted within each opening  54  and a number of inner ring segments  58  each connected to at least one of the spring members  56 . The spring member  56  is depicted in  FIG. 8  as a series of connected loops, but it should be understood that spring member  56  could take essentially any other form, including parallel bands as in the spring elements  24 . The sealing elements  48  and  50  are oriented side-by-side and positioned so that the inner ring segments  58  extend into the slot  22  formed in the shoe  16 . The spring members  56  deflect with the radial inward and outward movement of the shoe  16 , in response to the application of fluid pressure as noted above, in such a way as to create a secondary seal of the gap  11  between the rotor  14  and stator  12 . As such, the sealing elements  58  and  50  assist the spring elements  24  in maintaining the shoe  16  within design clearances relative to the rotor  14 . 
     In the presently preferred embodiment, the spring elements  48  and  50  are formed of sheet metal or other suitable flexible, heat-resistant material. The sealing elements  48  and  50  may be affixed to one another, such as by welding, a mechanical connection or the like, or they may merely placed side-by-side within the slot  22  with no connection between them. In order to prevent fluid from passing through the openings  54  in the outer ring  52  of each sealing element  48  and  50 , adjacent sealing elements are arranged so that the outer ring  52  of one sealing element  48  covers the openings  54  in the adjacent sealing element  50 . Although not required, a front plate  60  may be positioned between the spring element  24  and the sealing element  48 , and a back plate  62  may be located adjacent to the sealing element  50  for the purpose of assisting in supporting the sealing elements  48 ,  50  in position within the shoe  16 . 
     In applications such as gas turbine engines, the seal  10  of this invention is subjected to aerodynamic forces as a result of the passage of air along the surface of the shoes  16  and the rotor  14 . The operation of seal  10  is dependent, in part, on the affect of these aerodynamic forces tending to lift the shoes  16  radially outwardly relative to the surface of rotor  14 , and the counteracting forces imposed by the spring elements  24  and the secondary seals e.g. brush seal  45  or the stacked seal formed by plates  48 ,  50  which tend to urge the shoes  16  in a direction toward the rotor  14 . These forces acting on the shoe  16  are schematically depicted with arrows in  FIG. 9 . There must be a balance of forces acting on the seal  10  to ensure that nominal clearance is maintained. 
     Local pressures acting on the seal  10 , induced by the pressure differential across the seal  10 , have considerable impact on the force balance of seal  10 . As noted above, when the seal gap  11  increases the fluid velocity increases and the pressure decreases along such gap  11  thus drawing the shoe  16  toward the rotor  14 . As the seal gap  11  closes, creating a choked flow condition, the velocity of the fluid flowing through such gap  11  decreases thus increasing the pressure and forcing the shoes  16  away from the rotor  14 . It has been found that at least two design features formed on the surface of the shoes  16  facing the rotor  14  influence the velocity of the fluid and pressure distribution across the seal. One design feature comprises the geometric surface configuration of each shoe  16  immediately upstream and downstream from a sealing area of such shoe  16 , as discussed below in connection with a description of  FIGS. 10A to 10G . The second design feature comprises the provision of two or more labyrinth-type tooth elements that form cavities along the surface of the shoes  16  that faces the rotor  14 , as described in connection with a discussion of  FIGS. 11 to 14 . These two design features collectively enhance control of the radial clearance between the shoes  16  and rotor  14 , thus improving the performance of the seal  10  herein. 
     With reference initially to  FIGS. 10A-10G , a number of preferred geometries of the shoes  16  are illustrated. For ease of illustration, only a portion of one shoe  16  is depicted in  FIGS. 10A-10G , and it should be understood that the gap or radial clearance between the shoe  16  and rotor  14  is exaggerated for purposes of illustration. Generally, each of the shoes  16  shown in  FIG. 10A-10G  include a radially inwardly extending flow contraction area  70 , and then variations of converging surfaces, diverging surfaces and other surfaces, as described individually below. For purposes of discussion of  FIGS. 10A-10D , the terms “longitudinal direction” and “axial direction” refer to a direction along the longitudinal axis  18  of the rotor  14 . 
     Referring to  FIG. 10D , the shoe  16  has a first area  72  of substantially constant radial dimension upstream from the flow contraction area  70 , and a second area  74  of substantially constant radial dimension downstream or aft of the step  70 . The radial spacing of the second area  74 , relative to the rotor  14 , is less than that of the first area  72 . A converging area  76  extends aft from the second area  74 , and connects to a diverging area  78 . A sealing area or edge  80  is formed at the juncture of the converging and diverging areas  76 ,  78 . In the embodiment of  FIG. 10A , the length of the converging area  76 , measured in a longitudinal direction along axis  18 , is less than the length of the diverging area  78 . 
     The shoe  16  illustrated in the embodiment of  FIG. 10B  has the same flow contracting area  70 , and first and second areas  72 ,  74 , as  FIG. 10A . A converging area  82  extends from the second area  74  and joins to a diverging area  86  along an edge  84  forming a sealing area of the shoe  16  in this embodiment. As seen in  FIG. 10B , the length of converging area  82 , measured along the longitudinal axis  18  of rotor  14 , is greater than the length of the diverging area  86 . 
     Referring to  FIG. 10C , a shoe  16  is illustrated having the same construction as  FIG. 10B , except that instead of a diverging area connected to the converging area  86 , a third area  88  of substantially constant radial spacing extends from the converging area  86 . The radial spacing between the third area  88  and rotor  14  is less than that of the second area  74 , which, in turn, is less than that of the first area  72 . The third area  88  forms the sealing area of this version of the shoe  16 . 
     The converging and diverging areas along the surface of the shoe  16  are eliminated in the embodiment of this invention depicted in  FIG. 10D . The same first and second areas  72  and  74  connected to step  70  are employed, as described above, but then a second flow contraction area  90  connects the second area  74  to an elongated area  91  having a substantially constant radial spacing from the rotor  14 . The radial spacing between the elongated area  91  and rotor  14  is less than that of the second area  74 , which, in turn, is less than that of the first area  72 . In the embodiment shown in  FIG. 10D , the elongated area  91  forms the sealing area of shoe  16 . 
     The shoe  16  of  FIG. 10E  is similar to that shown in  FIG. 10A , except a converging area  92  extending from the second area  74 , and a diverging area  94  connected at an edge  96  to the converging area  92 , have substantially the same length as measured along the longitudinal axis  18 . The edge  96  forms the sealing area of the shoe  16  illustrated in  FIG. 10E . 
     In the embodiment of the shoe  16  illustrated in  FIG. 10F , essentially the same construction as that depicted in  FIG. 10C  is provided except the third area  88  is eliminated and a converging area  98  extends from the second area  74  to the end of the shoe  16 . The sealing area of shoe  16  depicted in  FIG. 10F  is located at the end edge  99  where the converging area  98  terminates. The same reference numbers used in  FIG. 10C  are employed in  FIG. 10F  to indicate common structure. 
     The shoe  16  of  FIG. 10G  is similar to that of  FIG. 10D , except the elongated area  91  in  FIG. 10D  is eliminated and replaced with a diverging area  100 . The diverging area  100  extends from the second flow contraction area  90  to the end edge of the shoe  16 . A sealing area of the shoe  16  is formed at the juncture  101  of the flow contraction area  90  and diverging area  100 . All other structure of the shoe  16  shown in  FIG. 10G  that is common to that of  FIG. 10D  is given the same reference numbers. 
     Referring now to  FIGS. 11-14 , alternative embodiments of the seal of this invention are shown. The seals depicted in  FIGS. 11-14  share the common feature of the addition of labyrinth-type tooth elements to the surface of shoes  16  that faces the rotor  14 , but the spring arrangement for supporting shoes  16  is different in the embodiments of  FIGS. 12-14  than that described above and is intended for higher pressure applications.  FIG. 11  is discussed first, followed by a description of the embodiments of  FIGS. 12-14 . 
     The embodiment of the seal  10  depicted in  FIG. 11  is similar to that described above in connection with a discussion of  FIGS. 3-8 , and particularly  FIGS. 7 and 8 , except for the addition of two labyrinth-type tooth elements including a forward tooth element  110  and an aft tooth element  112  that is axially spaced (along the longitudinal axis  18 ) from the forward tooth element  110 . The same reference numbers shown in  FIG. 7  are used to identify like structures in  FIG. 11 . Each of the tooth elements  110  and  112  extends from the surface of the shoe  16  that faces the rotor  14  and has a tip  114  and  116 , respectively, located within a predetermined design tolerance from the rotor  14 . The tooth elements  110  and  112  decrease in thickness from their point of connection at the shoe  16  to the tips  114 ,  116 , and are angled in a forward direction, i.e. in a direction opposing the aerodynamic forces applied to the shoe  16 . Preferably, the forward tooth element  110  is somewhat shorter than the aft tooth element  112  to resist clogging of the gap between the tip  114  of the tooth element  110  and the rotor  14  in the event debris should become entrained in the flow of fluid toward the shoe  16 . 
     A first cavity  118  is formed between the aft tooth element  112  and the flow contraction area  120  of shoe  16 , and a second cavity  122  is formed between the forward and aft tooth elements  110 ,  112 . While not wishing to be bound by any particular theory of operation of the seal  10  of  FIG. 11 , it is believed that the flow of fluid passing between the shoe  16  and rotor  14  swirls within the cavities  118  and  122  causing a reduction in the fluid pressure in that area. In response to such pressure reduction, the shoe  16  moves toward the rotor  14  creating an improved seal. The nominal clearance between the sealing area  124  of the shoe  16  depicted in  FIG. 11  and the rotor  14  may be 0.001 inches, for example, but the addition of the cavities  118  and  122  causes the actual clearance during operation of the seal  10  to be less than 0.001 inches. 
     Despite the formation of the forward tooth element  110  somewhat shorter than the aft tooth element  112 , as discussed above, it is nevertheless possible that the area between the tips  114 ,  116  thereof and the rotor  14  could become clogged with debris. This would result in a pressure drop in the region upstream from the sealing area  124  of the shoe  16  and could cause the shoe  16  to contact the rotor  14 . To prevent this from occurring, an orifice or bleed hole  126  may be formed in the shoe  16  extending from the surface opposite the rotor  14  into the first cavity  118 , and/or the second cavity  122  may be formed with a bleed hole  127 . Alternatively, or in addition to the bleed hole  126  and  127 , a notch may be formed in the forward tooth element  110  and/or the aft tooth element  112 , such as shown in the embodiment of  FIG. 14  discussed below. The bleed hole(s)  126 ,  127 , and/or notch(es), act to prevent a sudden drop in pressure within the cavities  118  and  122  thus assisting in avoiding contact between the shoe  16  and the rotor  14 . 
     Referring now to  FIGS. 12-14 , alternative embodiments of a seal  128  and a seal  130  are illustrated which are particularly intended for higher pressure applications than the seal of  FIGS. 3-11 . The seal  128  depicted in  FIGS. 12 and 13  comprises at least one shoe  132  having a first surface  133  and a second surface  135  radially spaced from the first surface  133 . A number of shoes  132  are depicted in  FIG. 12  for purposes of illustration. The first surface  133  of each shoe  132  may have one of the surface geometries shown in  FIGS. 10A to 10G , and it may further include labyrinth-type tooth elements  110  and  112  such as depicted in  FIG. 11 . The structure and operation of such surface geometries, and the tooth elements  110 ,  112 , is the same as that described above in connection with a discussion of  FIGS. 10A to 11 , and the same reference numbers are therefore used in  FIGS. 12 and 13  to denote like structure. Additionally, the second surface  135  of the shoes  132  may be formed with a slot  137  to receive a brush seal  45  or stacked plates  48 ,  50  forming a secondary seal as described above in connection with a discussion of  FIGS. 6-8 . 
     The seal  128  of  FIGS. 12 and 13  differs from the seal  10  of this invention primarily with respect to the spring elements that support the shoes  132  of seal  128  relative to the rotor  14 . The shoes  132  of seal  128  is provided with a radially outwardly extending, circumferential flange  134  formed with a number of sockets  139 . The sockets  139  are circumferentially spaced along the flange  134 , and are preferably alternately radially spaced from one another. A ring  138  is axially spaced from the shoes  132  and connected to the stator  12 . The ring  138  is formed with a number of sockets (not shown) that align with the sockets  139  in the flange  134  of shoes  132 . A number of axially extending rods or pins  142  connect the ring  138  and shoes  132 . Each pin  142  has a first end mounted within a socket  139  of the flange  134  of a shoe  132 , and a second end mounted within an aligning socket on the ring  138 . As seen in  FIGS. 12 and 13 , the pins  142  are oriented generally parallel to one another when positioned within the sockets  136  in the shoes  132  and ring  138 . The pins  142  act as spring elements and deflect in a radial direction in response to the application of aerodynamic forces to the shoes  132 , allowing the shoes  132  to “float” at a predetermined clearance or gap  11  relative to the rotor  14 . 
     Referring to  FIG. 14 , the seal  130  of this embodiment is similar to that of  FIGS. 12 and 13  except that a number of spring elements or rods  144  are welded, brazed or otherwise permanently affixed to each of at least one shoe  146  and a ring  148 . For purposes of illustration, two shoes  146  are shown in  FIG. 14 , the shoe  146  may be provided with a number of openings  150  within which one end of a rod  144  is received and may be welded or brazed in place. The openings  150  are circumferentially spaced along the shoes  146 , and alternately radially spaced from one another. The ring  148  is axially spaced from the shoes  146  and fixed to the stator  12 . Openings (not shown) are formed in the ring  148  that align with the openings  150  in the shoes  146  to receive and mount the opposite end of each rod  144  so that they are generally parallel to one another. 
     The rods  144  of seal  130 , like the pins  142  of seal  128 , act as spring elements and deflect in a radial direction in response to the application of aerodynamic forces to the shoes  146 , allowing the shoes  146  to “float” at a predetermined clearance or gap  11  relative to the rotor  14 . The surface of shoes  146  that faces the rotor  14  may be formed with one of the surface geometries shown in  FIGS. 10A to 10G , and it may further include labyrinth-type tooth elements  110  and  112  depicted in  FIG. 11 . The structure and operation of such surface geometries and tooth elements  110 ,  112  is the same as that described above in connection with a discussion of  FIGS. 10A to 11 , and the same reference numbers are therefore used in  FIG. 14  to denote like structure. Additionally, the shoes  146  may be formed with one or more orifices or bleed holes  152  extending into the cavity  118  or  122 , one of which is shown within cavity  122  in  FIG. 14 , for the same purposes as bleed holes  126  and  127  described above in connection with a discussion of  FIG. 11 . Further, a notch  154  may be formed in one or both of the tooth elements  110  and  112 . The bleed hole  152  and/or notch  154  act to prevent a sudden drop in pressure within the cavities  118  and  122  thus assisting in avoiding contact between the shoe  16  and the rotor  14 . 
     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. 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.