Patent Publication Number: US-10329938-B2

Title: Aspirating face seal starter tooth abradable pocket

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to aspirating face seals between rotor and stator assemblies and, more particularly, to an abradable seal land for an aspirating face seal starter tooth. 
     Aspirating face seals minimize leakage of a fluid, such as compressed air or combustion gases, by restricting flow between an area of high pressure and an area of low pressure. Aspirating face seals (AFS) control leakage by compensating for variations in the gap which may exist between a rotor and stator. Such seals have been disclosed for use in rotating machinery, including but not limited to, gas turbine engines used for power generation and for aircraft and marine propulsion. 
     Fluid leakage through gas turbine engine seal assemblies may significantly increase fuel consumption and adversely affect engine efficiency. Additionally, fluid leakage may cause damage to other components and/or increase overall engine maintenance costs. Because of the location of the seal assemblies and/or the operating environment, some known seal assemblies may deteriorate over time. 
     Some embodiments of aspirating face seals are configured as oppositely facing rotatable first and non-rotatable second seal elements. The rotatable first seal element is attached to, or is a monolithic portion of, the rotor. Likewise, such seals typically have the stator supporting the non-rotatable second seal element which is attached to, or a monolithic portion of, a slider. Retraction springs, typically coil springs, are used to separate or retract the non-rotating second seal element from the rotating first seal element during low or no power conditions. The non-rotatable second seal element is mounted on the slider supported by the stator. Examples of such aspirating face seals are disclosed in patent applications from General Electric Company in Ser. Nos. 2016/41013072 and 2016/41016504, filed in INDIA, assigned to the present Assignee, the General Electric Company, and incorporated by reference. Ser. No. 2016/41013072 is entitled “ANTI-CONING ASPIRATING FACE SEAL” and was filed in India on Apr. 14, 2016. Ser. No. 2016/41016504 is entitled “ASPIRATING FACE SEAL TOOTH CONFIGURATION” and was filed in India on May 11, 2016. 
     U.S. Pat. No. 6,676,369 to Brauer, et al., issued Jan. 13, 2004, and entitled “Aspirating Face Seal with Axially Extending Seal Teeth”, discloses a gas turbine engine aspirating face seal including a rotatable engine member and a non-rotatable engine member and a leakage path therebetween. Annular generally planar rotatable and non-rotatable gas bearing face surfaces circumscribed about a centerline are operably associated to the rotatable and non-rotatable engine members respectively. Radially inner and outer tooth rings axially extend away from a first one of the rotatable and non-rotatable gas bearing face surfaces across the leakage path and towards a second one of the gas bearing face surfaces. An auxiliary seal includes an annular restrictor tooth extending radially across the leakage path from a second one of the rotatable and non-rotatable gas bearing face surfaces towards the first one of the rotatable and non-rotatable gas bearing face surfaces. Coiled springs are utilized to separate the gas bearing face surfaces. 
     Known seal designs include a starter tooth mounted on a rotatable engine member. The starter tooth is an annular labyrinth seal tooth designed and operable to sealingly engage a corresponding abradable starter seal land. The starter seal abradable land is typically an abradable coating on an interior surface of an annular slider axially slidingly mounted on the annular non-rotatable engine member. 
     It is also important to note that aspirating face seal technology uses phrases such as “air bearing”, “air dam”, and “air flow”, wherein it is understood that the word “air” is used to describe the working fluid of the seal. The working fluid of an aspirating face seal can include, without limitation, compressed air, combustion gases, and/or steam. Note that an aspirating face seal is a non-contacting seal in that the first and second parts or rotatable and non-rotatable seal elements of the seal are not intended to touch, but may for short periods of time, during which they experience what are known as rubs. 
     One potential cause of air bearing contact is an aggressive rub between the rotor starter tooth and the slider abradable land or coating. As the tooth wears into the coating, heat generated by the rub causes the slider air bearing surface to distort. In addition, the starter tooth rub forces prevent or inhibit the slider from retracting. These two effects lead to air bearing contact. Heat generated by the contact creates a large thermal gradient across the slider air bearing face, which can cause the surface to crack. To prevent this problem, starter tooth rubs must be minimized or eliminated when the seal is closed. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A turbomachine aspirating face seal assembly includes an aspirating face seal circumscribed about a centerline axis and operable for restricting leakage of high pressure air flow from a relatively high pressure region of the turbomachine to a relatively low pressure region of the engine at a juncture between a non-rotatable member of the turbomachine and a rotatable member of the turbomachine. The rotatable and non-rotatable members include gas bearing rotatable and non-rotatable face surfaces respectively. A starter seal tooth mounted on the rotatable member is designed and operable to sealingly engage a corresponding abradable starter seal land on the non-rotatable member and an annular pocket is in an abradable coating or other abradable material of the abradable starter seal land. 
     The starter tooth may be an annular labyrinth seal tooth. The assembly further includes a primary seal tooth, and the starter and primary seal teeth are annular labyrinth seal teeth designed and operable to sealingly engage corresponding abradable starter and primary seal lands respectively on the non-rotatable member. 
     The abradable coating or the abradable material may be disposed in a radially inwardly facing groove extending radially outwardly into the non-rotatable member. The inwardly facing groove includes a radially inwardly facing cylindrical groove surface along the non-rotatable member, and the radially inwardly facing groove includes annular forward and aft groove side surfaces extending radially inwardly from the groove surface and axially bounding the abradable coating or the starter seal land. The annular pocket may extend radially outwardly from a cylindrical radially outer abradable surface of the starter seal land or the abradable coating to a pocket bottom and the pocket bottom includes a thin abradable material layer of the abradable material of the starter seal land or the abradable coating surrounding the radially inwardly facing cylindrical groove surface along the non-rotatable member. The annular pocket may extend axially aftwardly from the annular forward groove side surface into the abradable coating or the starter seal land. 
     The annular pocket may extend radially outwardly from a cylindrical radially outer abradable surface of the starter seal land or the abradable coating to a pocket bottom, and the pocket bottom may include a portion of the radially inwardly facing cylindrical groove surface. 
     The annular pocket may extend radially outwardly from a cylindrical radially outer abradable surface of the starter seal land or the abradable coating to a pocket bottom and be bounded axially by the abradable material of the abradable coating or the starter seal land. The assembly may further include a pocket width between axially spaced apart annular forward and aft sides of the pocket, a tip width of a radially outer tip of the starter tooth, and the pocket width greater than the tip width. 
     The annular pocket may be tapered and have a taper decreasing axially aftwardly away from the annular forward groove side surface and a thickness of the coating in the annular pocket increasing axially aftwardly away from the annular forward groove side surface. The tapered annular pocket may extend axially aftwardly from the annular forward groove side surface into the starter seal land or the abradable coating. 
     The assembly may further include an annular slider axially slidingly mounted on the non-rotatable member, the starter seal land and the non-rotatable face surface mounted on the slider, a retracting means for retracting the annular slider away from the rotatable member and the non-rotatable face surface away from the rotatable surface, and a primary tooth. The starter and primary teeth may be annular labyrinth seal teeth designed and operable to sealingly engage corresponding abradable starter and primary seal lands. The primary tooth may be on the rotatable member and the primary seal land on the slider or the primary tooth may be on the annular slider and the primary seal land on the rotatable member. The retracting means may include a plurality of circumferentially spaced apart springs, and each of the springs may be axially disposed between the slider and the non-rotatable member. 
     The starter tooth may be mounted on a seal teeth carrier on the rotatable member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustration of a portion of an exemplary gas turbine engine with a first exemplary embodiment of an aspirating face seal having a starter tooth land abradable coating with a pocket. 
         FIG. 2  is an enlarged cross-sectional view illustration of the aspirating gas bearing face seal illustrated in  FIG. 1  in an opened engine off position. 
         FIG. 3  is a cut-away perspective view illustration of a stator portion of the aspirating gas bearing face seal illustrated in  FIG. 2 . 
         FIG. 4  is a cross-sectional view illustration of the aspirating gas bearing face seal illustrated in  FIG. 2  with feed holes extending radially inwardly through an aft ring of the stator of the aspirating gas bearing face seal in a closed position. 
         FIG. 5  is a diagrammatical illustration of forces acting on the aspirating gas bearing face seal illustrated in  FIG. 4 . 
         FIG. 5A  is a diagrammatical illustration of air flows through the aspirating gas bearing face seal illustrated in  FIG. 4 . 
         FIG. 6  is a cross-sectional view illustration of a slider and the aspirating gas bearing face seal illustrated in  FIG. 4 . 
         FIG. 7  is a radially inwardly looking perspective view illustration of the slider illustrated in  FIG. 6 . 
         FIG. 8  is perspective view illustration of an annular flange around and fixed to the stator illustrated in  FIG. 3 . 
         FIG. 9  is perspective view illustration of the slider illustrated in  FIG. 3 . 
         FIG. 10  is perspective view illustration of a groove in the slider for receiving a tongue extending inwardly from a housing of a spring cartridge illustrated in  FIG. 3 . 
         FIG. 11  is perspective view illustration of the housing of the spring cartridge mounted to the flange illustrated in  FIG. 3 . 
         FIG. 12  is a cross-sectional view illustration of an alternative embodiment of the aspirating gas bearing face seal illustrated in  FIG. 2  with an oil dam on the stator. 
         FIG. 13  is an exemplary graphical and diagrammatical cross-sectional view illustration of flow through the aspirating gas bearing face seal illustrated in  FIG. 2 . 
         FIG. 14  is a diagrammatical illustration of a first alternative embodiment of the pocket illustrated in  FIG. 2 . 
         FIG. 15  is a diagrammatical illustration of a second alternative embodiment of the pocket illustrated in  FIG. 2 . 
         FIG. 16  is a diagrammatical illustration of a third alternative embodiment of the pocket illustrated in  FIG. 2 . 
         FIG. 17  is a cross-sectional view illustration of an alternative aspirating gas bearing face seal with a primary tooth mounted on an annular slider and starter and deflector teeth mounted on a rotatable member of the aspirating gas bearing. 
         FIG. 18  is a cross-sectional view illustration of one embodiment of the aspirating gas bearing face seal illustrated in  FIG. 2  in a closed position and the pocket sized too small. 
         FIG. 19  is a cross-sectional view illustration of another embodiment of the aspirating gas bearing face seal illustrated in  FIG. 2  in a closed position and the pocket sized too large. 
         FIG. 20  is a cross-sectional view illustration of the aspirating gas bearing face seal illustrated in  FIG. 2  in a closed position and the pocket desirably sized. 
         FIG. 21  is a cross-sectional view illustration of the aspirating gas bearing face seal illustrated in  FIG. 2  in an open position with a starter tooth directly below the starter tooth land. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Illustrated in  FIGS. 1-3  is a first exemplary embodiment of an aspirating face seal assembly  12  having an annular aspirating face seal (AFS)  16  and a secondary seal  18  which is illustrated herein as including a piston ring  20  as illustrated in  FIG. 2 . The face seal assembly  12  is designed for controlling leakage or sealing between a high pressure region  48  and a low pressure region  46  such as may be found in a turbomachine such as a gas turbine engine  10  as illustrated in  FIG. 1 . Turbomachines include, but are not limited to, steam turbines, compressors, and turbocompressors such as may be used in the gas and oil industry, or similar apparatus. 
     Referring to  FIG. 1 , the exemplary embodiment of the turbomachine or gas turbine engine  10  is circumscribed about a centerline axis  8  of the engine  10  and includes an annular stationary stator or non-rotatable member  102  coupled to an annular frame  103  and a rotating or rotatable member  104  coupled to a rotor  105 , at least in part, rotatably supported by an aft bearing  108 . The frame  103  is illustrated herein as an annular turbine center frame  37  circumscribed about the centerline axis  8  of the engine  10 . Additionally, the non-rotatable member  102  is a stationary annular member circumscribed about the centerline axis  8  of the gas turbine engine  10 . In the embodiments illustrated herein, the non-rotatable member  102  is bolted to the frame  103  and the rotatable member  104  is rotatably coupled within the engine  10  to rotate about the centerline axis  8 . The high pressure region  48  is located radially outwardly of the low pressure region  46 , and the non-rotatable member  102  is located radially between the high and low pressure regions  48 ,  46 . The frame  103  supports a middle bearing  107  in an annular sump  109  bounded by a generally conical sump member  66  located radially inwardly of the non-rotatable member  102 . 
     A drain hole  142  in the non-rotatable member  102  is located upstream or forward of the aspirating face seal  16  and the secondary seal  18 . A drain tube  144  is connected to and in fluid communication with drain hole  142 . The drain tube  144  and the drain hole  142  provides a drain assembly  146  to help prevent oil from flowing into the aspirating face seal  16 . 
     Referring to  FIGS. 1, 4, and 5 , the aspirating face seal  16  is used to restrict leakage of high pressure air flow  120  from the relatively high pressure region  48  to a relatively low pressure region  46  between the non-rotatable member  102  and the rotatable member  104 . The high pressure AFS air flow  120  passes through the aspirating face seal  16  between the rotatable and non-rotatable members  104 ,  102  and between gas bearing rotatable and non-rotatable face surfaces  125 ,  124  respectively. The rotatable and non-rotatable face surfaces  125 ,  124  are circumscribed around and generally perpendicular to the engine centerline axis  8 . An air bearing film is formed between the rotatable and non-rotatable face surfaces  125 ,  124  which function as a slider bearing face and a rotor bearing face, respectively. 
     The embodiment of the aspirating face seal  16  illustrated in  FIGS. 4 and 5  includes a rotatable seal teeth carrier  30  which may be an annular flange on the rotatable member  104 . The rotatable face surface  125  is on the carrier  30 . Primary, starter, and deflector teeth  34 ,  32 ,  36  are mounted radially outwardly of the rotatable face surface  125  on the seal teeth carrier  30 . The primary and starter teeth  34 ,  32  are annular labyrinth seal teeth designed and operable to sealingly engage corresponding annular abradable primary and starter seal lands  40 ,  38  located and mounted on an annular slider  42  axially slidingly mounted on the annular non-rotatable member  102  illustrated in  FIGS. 2 and 3 . The annular slider  42  includes a central ring  45  and annular forward and aft extensions  47 ,  51  extending forwardly and aftwardly, respectively, from the central ring  45 . 
     The primary tooth  34  extends axially forward and slightly radially outwardly from a forward carrier extension  35  of the seal teeth carrier  30 . The starter seal land  38  faces radially inwardly from and is carried on the annular aft extension  51  of the annular slider  42 . The exemplary annular starter seal land  38  disclosed herein includes an abradable coating  56  disposed in an annular inwardly facing groove  58  extending radially outwardly into the annular aft extension  51 . The annular inwardly facing groove  58  includes an axial portion  61  of a radially inwardly facing cylindrical groove surface  59  along the annular aft extension  51  of the slider  42  of the non-rotatable member  102 . The annular inwardly facing groove  58  includes annular forward and aft groove side surfaces  64 ,  65  extend radially inwardly from the groove surface  59  and axially bound the abradable coating  56  or the starter seal land  38 . 
     An annular pocket  60  in the abradable coating  56  or the starter seal land  38  reduces or eliminates contact between the starter tooth  32  and the abradable coating  56  or the starter seal land  38  when the aspirating face seal  16  is closed. Reducing or eliminating starter tooth contact prevents undesirable forces from acting on the slider  42  and minimizes thermal distortion, which reduces the probability of non-rotatable face surface  124  cracking due to an air bearing rub. 
     The pocket  60  extends radially outwardly from a cylindrical radially outer abradable surface  67  of the starter seal land  38  or the abradable coating  56  to a pocket bottom  62 . The pocket  60  includes axially spaced apart annular forward and aft sides  52 ,  54  extending radially inwardly from the pocket bottom  62 . Thus, the pocket  60  is axially bounded by the forward and aft sides  52 ,  54  and radially inwardly bounded by the pocket bottom  62 . The pocket bottom  62  may be a thin abradable material layer  63  of the starter seal land  38  or the abradable coating  56  surrounding the radially inwardly facing cylindrical groove surface  59  along the non-rotatable member  102 , as illustrated in  FIG. 2 . The embodiment of the pocket  60  illustrated in  FIGS. 2-5  extends axially aftwardly from the annular forward groove side surface  64  into the starter seal land  38  or the abradable coating  56 . The pocket  60  extends substantially along the axial portion  61  of the radially inwardly facing cylindrical groove surface  59  along the annular aft extension  51  of the slider  42  of the non-rotatable member  102 . 
     Alternatively, the pocket  60  may extend radially outwardly to the pocket bottom  62  which may be a portion  78  of the radially inwardly facing cylindrical groove surface  59 , as illustrated in  FIG. 15 . The pocket bottom  62  illustrated in  FIG. 15  is on the metallic radially inner facing surface  59  along the annular aft extension  51  of the slider  42  of non-rotatable member  102 . 
     The primary seal land  40 , in the embodiment of the aspirating face seal  16  illustrated in  FIGS. 4 and 5 , includes faces axially aftwardly from and is carried on the central ring  45  of the annular slider  42 . The starter seal land  38  is located forward of the non-rotatable face surface  124  on the central ring  45 . The non-rotatable face surface  124  is mounted on the central ring  45 . The deflector tooth  36  extends axially forward and slightly radially inwardly from the forward carrier extension  35  of the seal teeth carrier  30 . The forward carrier extension  35  extends forwardly from the seal teeth carrier  30  and supports the primary and the deflector teeth  34 ,  36 . The starter tooth  32  extends substantially radially from the seal teeth carrier  30  and substantially normal to the centerline axis  8  of the engine  10 . The primary and starter seal lands  40 ,  38  may be made of or include an abradable material. The abradable material may be a honeycomb material, thermal spray abradable material such as nickel graphite, or other abradable material. 
     The non-rotatable face surface  124  is located radially inwardly of the primary and starter seal lands  40 ,  38  on the annular slider  42  and is substantially parallel to the rotatable face surface  125  on the rotatable member  104 . The non-rotatable and rotatable face surfaces  124 ,  125  are axially spaced apart a variable distance  123 . Under a pressure differential between the high and low pressure regions  48 ,  46 , the slider  42  moves axially aft, closing the non-rotatable and rotatable face surfaces  124 ,  125 . A variable axial length annular plenum  69  extends axially between the slider  42  and the rotatable face surface  125 . A gas bearing space  100  extends axially between the non-rotatable and rotatable face surfaces  124 ,  125 . 
     Referring to  FIGS. 3-5 , air feed passages  110  extend through the central ring  45  of the annular slider  42  and from the high pressure region  48  to the gas bearing space  100  between the non-rotatable and rotatable face surfaces  124 ,  125 . The exemplary embodiment of the air feed passages  110  illustrated herein includes feed holes  112  extending generally radially inwardly from the high pressure region  48  through the central ring  45  to corresponding axially extending orifice bores  114  in the central ring  45 . The orifice bores  114  extend axially through the central ring  45  from the feed holes  112  through the non-rotatable face surface  124  to the gas bearing space  100 . 
     First and second pluralities  93 ,  95  of circumferentially spaced apart first and second vent passages  96 ,  98  through the central ring  45  of the annular slider  42  provide pressure communication between the plenum  69  and low pressure region  46  as illustrated in  FIG. 4 . The first and second vent passages  96 ,  98  vent the plenum  69  to the low pressure region  46  during engine operation when there is a substantial pressure differential between high and low pressure regions  48 ,  46 . The first vent passages  96  are inclined radially inwardly and extend from the plenum  69  forward and radially inwardly. The second vent passages  98  extend substantially radially inwardly from the plenum  69  through the central ring  45  of the annular slider  42 . 
     The starter tooth  32  is used to initiate closure of the aspirating face seal  16 . The starter tooth  32  is located on the seal teeth carrier  30  mounted on the rotatable member  104  and extends radially towards the non-rotatable abradable starter seal land  38 . This design allows the starter tooth to rub into an abradable during high radial excursions rather than have metal to metal contact. The deflector tooth  36  is used to help reduce build-up of interior pressures in the gas bearing space  100  and the annular plenum  69  between the stationary and rotating seal surfaces. 
       FIGS. 5A and 21  illustrates various air flows and tooth gaps for the aspirating face seal  16  during engine operation when the aspirating face seal  16  is partially open. Primary tooth and starter tooth gaps G 1 , G 2  between the primary and starter teeth  34 ,  32  and the primary and starter seal lands  40 ,  38  respectively allow room to draw flow between the teeth and lands. Bearing flow  901  comes from the high pressure region  48  through the air feed passages  110  into the gas bearing space  100  between the non-rotatable and rotatable face surfaces  124 ,  125 . The bearing flow  901  exits the gas bearing space  100  as radially outward bearing flow  903  and radially inward bearing flow  902 . The radially outward bearing flow  903  passes through the first and second vent passages  96 ,  98  and together with the radially inward bearing flow  902  passes through a gap between the rotatable member  104  and the non-rotatable member  102  to reach the low pressure region  46 . 
     Seal flow  121  leaks or flows between the starter seal tooth  32  and the starter seal land  38  and then between the primary seal tooth  34  and the primary seal land  40 . During engine operating conditions with the aspirating face seal  16  closed, the primary tooth  34  is the main restriction to air flow through the aspirating face seal  16 . The seal flow  121  merges with the radially outward bearing flow  903  in the annular plenum  69 , and the merged flows exit the aspirating face seal  16  as vent flow  904  passing through the first and second vent passages  96 ,  98  respectively. The merged flows then pass through the gap between the rotatable member  104  and the non-rotatable member  102  to reach the low pressure region  46 . 
     The primary seal flow  121  across the primary tooth  34  and radially outward bearing flow  903  enter the plenum  69  as jets, due to a pressure drop across the aspirating face seal  16  from the high pressure region  48  to the low pressure region  46 . The primary seal flow  121  exits the primary tooth gap G 1  between the primary tooth  34  and the primary seal land  40  traveling substantially radially inward towards the first and second vent passages  96 ,  98 . The radially outward bearing flow  903  enters the plenum  69  traveling radially outwardly and is redirected by deflector tooth  36  towards the first and second vent passages  96 ,  98 . The radially outward bearing flow  903  and the primary seal flow  121  merge into the axial and radially inward vent flows  904 ,  905  which flow out from plenum  69  through the first and second vent passages  96 ,  98  respectively to the low pressure region  46 . 
     The redirection of radially outward bearing flow  903  by the deflector tooth  36  increases flow into the vent passages  96  causing a higher discharge coefficient (Cd) and greater effective passage area. This causes the air pressure in plenum  69  to approach that of the low pressure region  46 . Similarity in pressure between plenum  69  and the low pressure region creates a more stable force balance acting on the slider  42 , which results in a more determinate operating clearance between air bearing surfaces. Cd is a standard engineering ratio used to find the effective area of a hole or passage that a fluid is passing through, i.e actual area*Cd=effective area. A perfect Cd=1, but Cd for real holes is lower. 
     During higher power operation, the primary tooth  34  restricts the AFS air flow  120  flowing from the relatively high pressure region  48  to the relatively low pressure region  46 , thereby, causing an increase in the pressure differential between high and low pressure regions  48 ,  46 . A high pressure differential between high and low pressure regions  48 ,  46  acts on areas of the slider  42  upstream of the starter tooth  32  resulting in a net axial force that pushes the slider  42  and the primary and starter seal lands  40 ,  38  located on the slider  42  toward the rotatable face surface  125  on the rotatable member  104  and the primary, starter, and deflector teeth  34 ,  32 ,  36 . The aspirating face seal  16  is illustrated in an open position in  FIG. 12  and in a closed position in  FIG. 4 . 
     Illustrated in  FIGS. 1-4  is a retracting means  82  for retracting the annular slider  42  and the non-rotatable face surface  124  away from the rotatable member  104  and the rotatable surface  125  during low or no power conditions. This causes the gas bearing space  100  and the annular plenum  69  to axially lengthen and the primary seal land  40  on the slider  42  to retract from the primary tooth  34 . 
     Referring to  FIGS. 2-4 , the exemplary embodiment of the retracting means  82  includes a plurality of circumferentially spaced apart coil springs  84  disposed within spring chambers  185  of circumferentially spaced apart cartridges  85 . Each of the cartridges  85  includes an annular housing  187  surrounding the spring chamber  185  attached to the annular non-rotatable member  102 . An aft end wall  87  of the annular housing  187  may be attached to the annular non-rotatable member  102 . A forward end  190  of the coil spring  84  rests against an axially forward static stop finger  86  which extends radially outwardly from and is attached to or part of the axially translatable annular slider  42  as further illustrated in  FIG. 9 . The stop finger  86  may be integrally formed with the axially translatable annular slider  42  as illustrated herein. A plug  192  disposed in an aperture  198  in the stop finger  86  extends into the chamber and anchors the coil spring  84  as illustrated in  FIGS. 3-4 . 
     The stop finger  86  extends radially through an axially extending slot  194  in the annular housing  187  into the spring chamber  185  as illustrated in  FIGS. 3-4 . This allows the slider  42  to translate axially and allows the coil spring  84  to compress and expand, thus, biasing the slider  42 . A tongue  199  extends radially inwardly from the housing  187  into a groove  200  in the slider  42 . This tongue and groove arrangement helps guide the axially translatable slider  42  during axial translation relative to the static housing  187  of the static cartridge  85 . The slider  42  is thus capable of axial translation and limited gimballing motion in response to an axial force and tilt moments respectively. 
     Referring to  FIGS. 2-4 and 6-11 , the cartridge  85  is connected or attached to the annular non-rotatable member  102 . The exemplary embodiment of the seal illustrated herein includes an annular flange  130  around and fixed to the annular non-rotatable member  102 . The cartridges  85  are attached to the annular flange  130 . The cartridges  85  may be attached to the annular flange  130  using pairs  133  of lugs  132  extending radially outwardly from the annular flange  130 . The cartridges  85  may be bolted to the lugs  132  with bolts  136  disposed through ear bolt holes  138  through ears  140  attached to the cartridges  85  and through lug bolt holes  134  disposed through the lugs  132 . Thus, the cartridges  85  may be removably mounted to the annular non-rotatable member  102 . The annular flange  130  is illustrated herein as being continuous but may be segmented. 
     The retracting means  82  and the coil springs  84  are upstream, with respect to the bearing airflow in the gas bearing space  100 , of the annular slider  42  and aspirating face seal  16  in the high pressure region  48 . The retracting means  82  and the springs  84  are positioned upstream from the secondary seal  18  with respect to bearing airflow through the aspirating face seal  16 . The retracting means  82 , including the coil springs  84  are positioned radially outwardly of the forward extension  47 , and the secondary seal  18  is positioned radially inwardly of the forward extension  47 . The secondary seal  18  is in sealing engagement with an annular radially inner slider surface  21  of the annular slider  42  and is located on a border between the high and low pressure regions  48 ,  46 . The retracting means  82  and the coil springs  84  are located radially outwardly of the annular slider  42  and the secondary seal  18  is located radially inwardly of the annular slider  42 . The arrangement of the retracting means  82  and the secondary seal  18  reduces deflection of the non-rotatable face surface  124  on the annular slider  42 . 
     The central ring  45  of the annular slider  42  is designed to translate between axial retracted and sealing positions RP, SP as illustrated in  FIGS. 2 and 4 , respectively, as a result of forces, illustrated in  FIG. 5 , acting on the central ring  45 . The forces are the result of pressures in the relatively low and high pressure regions  46 ,  48  acting on surfaces and spring forces of the retracting means  82 . 
     Referring to  FIG. 2 , as the engine is started, the pressure in the high pressure region  48  begins to rise because the starter tooth  32  restricts the AFS air flow  120  flowing from the relatively high pressure region  48  to the relatively low pressure region  46 . The pressure differential between the low and high pressure regions  46 ,  48  results in a closing pressure force acting on central ring  45 . The pressure force acts against a spring force from the retracting means  82  to push the central ring  45  and non-rotatable face surface  124  mounted thereupon towards the gas bearing rotatable face surface  125 .  FIG. 5  illustrates high and low pressure closing forces acting on the aspirating face seal  16  during engine start-up and how the closing forces overcomes the spring force. Referring to  FIG. 4 , during shutdown of the engine, pressure in the high pressure region  48  drops off and the springs  84  of the retracting means  82  overcome the closing force and retract the aspirating face seal  16 . Opening forces from high pressure air in the air bearing between the rotatable and non-rotatable face surfaces  125 ,  124  are also illustrated in  FIG. 5 . 
       FIG. 13  graphically illustrates modeling of total airflow through the aspirating face seal  16 , the high pressure AFS air flow  120 , for aspirating face seals with and without the annular pocket  60  in the abradable coating  56 . The solid line represents total airflow through the aspirating face seal  16  with the annular pocket  60 . The dashed line represents total airflow through the aspirating face seal  16  without the annular pocket  60 . Results of the simulation indicate that for large primary tooth clearances  70 , configuration A, the starter tooth  32  is the metering feature and the AFS flow remains within acceptable limits  72 . 
     As the primary tooth clearance  70  gets smaller, configuration B, (in the model), the metering feature transitions from the starter tooth  32  to the primary tooth  34 . In a transition region  74  between configurations B and C, the AFS flow  120  for the abradable coating  56  with the pocket  60  increases slightly compared to the seal without the pocket  60 . For primary tooth clearances  70  which are small, configuration D, the AFS flow is the same for both the abradable coating  56  with and without the pocket  60 . 
     The starter tooth abradable pocket  60  is sized to ensure the AFS flow  120  does not exceed an acceptable limit  72  as the seal metering feature transitions from the starter tooth  32  to the primary tooth  34 . As a result, there is no impact to the sealing function. In addition, the pocket  60  is sized to reduce or eliminate starter tooth rubs in a transition region and closed position. Reducing or eliminating starter tooth rubs minimizes undesirable slider forces and thermal distortion, which minimizes the air bearing deflection and reduces the risk of an air bearing rub. 
       FIGS. 14-16  illustrate alternative configurations of the annular pocket  60  and the abradable coating  56 . Illustrated in  FIG. 14  is a first alternate configuration with a U-shaped pocket  60  which may simplify manufacturing and be less expensive. The U-shaped pocket  60  is bounded axially by the abradable material  57  of the abradable coating  56  or the starter seal land  38 . The pocket bottom  62  may include a thin abradable material layer  63  of the abradable material  57  of the starter seal land  38  or the abradable coating  56  surrounding the radially inwardly facing cylindrical groove surface  59  along the non-rotatable member  102 . A pocket width PW of the pocket  60  between the axially spaced apart annular forward and aft sides  52 ,  54  is greater than a tip width TW of a radially outer tip  28  of the starter tooth  32 . 
     Illustrated in  FIG. 15  is a second alternate configuration having the coating  56  above the starter tooth  32  completely removed. Coating in this region is not necessarily required. The pocket  60  extends radially outwardly to the metallic radially inner facing surface  59  of the annular aft extension  51  of the slider  42 . 
     Illustrated in  FIG. 16  is a third alternate pocket  60  with a tapered pocket  60  in the coating  56 . A taper  76  of the pocket  60  decreases and a thickness T of the coating  56  in the pocket  60  increases aftwardly away from the non-rotatable face surface  124  on the annular slider  42 . The taper  76  of the pocket  60  decreases and the thickness T of the coating  56  in the pocket  60  increases aftwardly away from the annular forward groove side surface  64 . The taper may not completely eliminate a starter tooth rub, but it reduces the severity. 
     Referring to  FIG. 18 , if the annular pocket  60  is too small, it will not prevent the starter tooth  32  from rubbing when the aspirating face seal  16  is closed. In this case, a first axial distance X 1  from the primary seal land  40  to a pocket aft end of the pocket  60 , is significantly smaller than a second axial distance X 2  from the primary seal land  40  to the starter tooth  32 . 
       FIG. 19  illustrates a pocket  60  which is too big and allows a starter tooth gap G 2  to get large before the primary tooth  34 , which controls the primary tooth gap G 1 , takes over as the flow metering feature. In this case, the first axial distance X 1  from the primary seal land  40  to the pocket aft end is significantly larger than the second axial distance X 2  from the primary seal land  40  to the starter tooth  32 . A transition in which the starter tooth gap G 2  does not get significantly large before the primary tooth gap G 1  gets small is important for minimizing flow through the aspirating face seal  16 . 
       FIG. 20  illustrates a desirably sized embodiment of the pocket  60 . It is big enough to prevent starter tooth rubs when the aspirating face seal  16  is closed and small enough to prevent excess leakage during the starter tooth  32  to the primary tooth  34  transition phase. In this case, the first axial distance X 1  from the primary seal land  40  to the pocket aft end of the pocket  60  is slightly larger than the second axial distance X 2  from the primary seal land  40  to the starter tooth  32 . In one exemplary engine AFS design, the first and second axial distances X 1 , X 2  are 0.395 inches and 0.360 inches, respectively. The 0.035 inch difference could vary for other applications but, in general, it is a good starting point for sizing the first and second axial distances X 1 , X 2  of the pocket  60 . 
     An alternative embodiment of the aspirating face seal  16 , illustrated in  FIG. 17 , includes a rotatable seal teeth carrier  30  in the form of a flange on the rotatable member  104 . The rotatable face surface  125  is on the carrier  30 . The primary tooth  34  is mounted on an annular slider  42  instead of the rotatable seal teeth carrier  30  on the rotatable member  104  as the embodiment illustrated in  FIGS. 1-4 . The starter and deflector teeth  32 ,  36  are mounted radially outwardly of the rotatable face surface  125  on the seal teeth carrier  30 . 
     The primary and starter teeth  34 ,  32  are annular labyrinth seal teeth designed and operable to engage corresponding abradable primary and starter seal lands  40 ,  38 . The primary seal land  40  faces axially forwardly from and is mounted on the teeth carrier  30 . The primary seal land  40  located radially outwardly of the rotatable face surface  125  and the deflector tooth  36 . The primary tooth  34  extends axially aftwardly from the annular slider  42  radially between the aft extension  51  and the central ring  45  of the annular slider  42 . The deflector tooth  36  extends axially aftwardly from the seal teeth carrier  30 . The starter tooth  32  extends substantially radially from the teeth carrier  30  and substantially normal to the centerline axis  8  of the engine  10 . 
       FIG. 21  illustrates the starter tooth gap G 2  between the starter tooth  32  and the abradable starter seal land  38  when the aspirating face seal  16  is partially open. The starter tooth gap G 2  is measured as the minimum distance between the starter seal tooth  32  and the abradable starter seal land  38 . 
     While there have been described herein what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein and, it is therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention. Accordingly, what is desired to be secured by Letters Patent of the United States is the invention as defined and differentiated in the following claims.