Abstract:
One embodiment of the face seal described herein includes a seal seat and a seal element carried by a seal housing. The seal element cooperates with the seal seat to establish a seal. The housing includes a seal element support and a shroud. The shroud includes a stem and a tip, and the tip is an insert secured to the stem.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This disclosure is a divisional of U.S. application Ser. No. 12/826,629, which was filed on 29 Jun. 2010. U.S. application Ser. No. 12/826,629 is a divisional of U.S. application Ser. No. 11/266,454, which was filed on 11 Nov. 2005 and has issued as U.S. Pat. No. 7,837,199. Both applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to face seals and particularly to a carbon face seal whose performance deteriorates in a relatively benign way in comparison to conventional seals. 
     BACKGROUND 
     Carbon face seals are used in machinery, such as turbine engines, to effect a fluid seal between regions of high and low fluid pressure. For example, carbon seals are used to prevent hot, high pressure air from entering a bearing compartment operating at a lower pressure. A typical carbon seal for a turbine engine includes an annular carbon ring secured to an annular, nonrotatable, axially translatable seal housing. The seal also includes a seal seat affixed to a rotatable shaft and positioned axially adjacent to the carbon ring. The carbon ring comprises a base (or blank) and a nose projecting axially from the base. The nose is urged into contact with the seal seat by a combination of spring forces acting on the seal housing and the net resultant of axially opposing fluid pressure forces acting on the seal housing and the carbon ring. The contact area between the carbon ring and the seal seat equals the annular area of the nose. The contact between the nose and the seal seat resists fluid leakage across the seal in the radial direction, i.e. toward or away from the axis of rotation of the seal seat. 
     During operation, the nose gradually wears away. Ordinarily, the seal is replaced or refurbished before the nose is completely worn away. Occasionally, however, accelerated seal wear can result in complete wear of the nose so that the base of the carbon ring contacts the seal seat. As a result, the contact area between the carbon ring and the seal seat equals the annular area of the base, which is larger than the contact area of the nose. This affects the resultant of the axially opposing fluid pressure forces such that the net pressure force is less favorable for maintaining reliable, positive contact between the carbon ring and the seal seat. Unfortunately, the transition between the normal condition in which the nose contacts the seal seat, and the highly deteriorated condition in which the base contacts the seal seat, although it occurs very infrequently, can occur with little warning. In addition, more abrupt failure or deterioration of the carbon ring can have a similar adverse effect on the resultant of the fluid pressure forces. As a result there may be an unanticipated period of engine operation during which fluid leaks past the seal 
     What is needed is a carbon seal that deteriorates gracefully in order to exhibit a detectable and benign operating characteristic that clearly indicates that maintenance is required. 
     SUMMARY 
     A face seal assembly according to an exemplary aspect of the present disclosure includes, among other things, a seal seat and a seal element carried by a seal housing and cooperating with the seal seat to establish a seal. The seal housing includes a seal element support and a shroud. The shroud includes a stem and a tip, and the tip is an insert secured to the stem. 
     In another exemplary embodiment of the above-described face seal assembly the shroud is axially elongated relative to the support. 
     In another exemplary embodiment of any of the above-described face seal assemblies, the carbon ring includes a blank with a nose extending axially therefrom. The blank being stepped so that a first radial region extends axially beyond a second radial region of the blank. 
     In another exemplary embodiment of any of the above-described face seal assemblies, the first and second radial regions define respective first and second steps, and the tip is axially between the first and second steps. 
     In another exemplary embodiment of any of the above-described face seal assemblies, the seal seat is a seal ring, and the seal element is a carbon ring residing radially between the support and the shroud. 
     In another exemplary embodiment of any of the above-described face seal assemblies, the shroud is radially inboard of the support. 
     In another exemplary embodiment of any of the above-described face seal assemblies, the seal housing exclusive of the tip is made of a parent material and the tip comprises a second material different than the parent material. 
     In another exemplary embodiment of any of the above-described face seal assemblies, the second material is selected from the group of materials consisting of: a) materials more lubricious than the parent material; b) materials harder than the parent material; and c) materials more abradable than the parent material. 
     In another exemplary embodiment of any of the above-described face seal assemblies, the insert is affixed to the stem via a snap. 
     In another exemplary embodiment of any of the above-described face seal assemblies, the snap is secured against a radially outward facing surface of the snap. 
     In another exemplary embodiment of any of the above-described face seal assemblies, the snap is secured against a radially innerward facing surface of the stem. 
     In another exemplary embodiment of any of the above-described face seal assemblies, the insert comprises a molded tip. 
     In another exemplary embodiment of any of the above-described face seal assemblies, the molded tip is secured to the stem through a set of circumferentially distributed countersunk holes. 
     In another exemplary aspect of the present disclosure, a seal housing for a face seal includes a base with a seal element support and a shroud both extending from the base in a common direction, The seal element support and the base defining a space for receiving a seal element. The shroud includes a stem and a tip, and the tip is an insert secured to the stem. 
     In another exemplary embodiment of the above-described seal housing, the seal element includes a blank with a nose extending axially therefrom. The blank being stepped so that a first radial region extends axially beyond a second radial region of the blank. The first and second radial regions define respective first and second steps, and the tip is axially between the first and second steps. 
     In another exemplary embodiment of any of the above-described seal housings, the insert is affixed to the stem via a snap. 
     In another exemplary embodiment of any of the above-described seal housings, the snap is comprises a radially outer snap. 
     In another exemplary embodiment of any of the above-described seal housings, the snap is a radially inner snap. 
     In another exemplary embodiment of any of the above-described seal housings, the insert comprises a molded tip. 
     In another exemplary embodiment of any of the above-described seal housings, the molded tip is secured to the stem through a set of circumferentially distributed countersunk holes. 
     The foregoing and other features of the various embodiments of the disclosed seal will become more apparent from the following detailed description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional side elevation view showing an improved carbon seal. 
         FIGS. 2 ,  3  and  4  are schematic views similar to  FIG. 1 , but circumferentially offset from  FIG. 1 , showing fluid pressure forces acting on a traditional seal in a normal or normally deteriorated condition, a highly deteriorated condition and a damaged or severely degraded condition respectively. 
         FIGS. 5-8  are schematic views similar to  FIGS. 2 through 4  showing fluid pressure forces acting on an improved seal in normal, highly deteriorated, severely deteriorated and damaged conditions respectively. 
         FIG. 9  is a view illustrating a seal housing with a shroud whose tip is made of the same material as the rest of the seal housing. 
         FIG. 10  is a view similar to  FIG. 9  showing a seal housing made of a parent material and having a shroud with a bonded or impregnated tip made of a second material. 
         FIGS. 11-13  are views similar to  FIG. 9  but with a shroud having a tip in the form of an insert or attachment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a shaft  20  for a rotary machine, such as a turbine engine, is rotatable about an axis  22 . A seal seat in the form of an annular ring  24  is secured against a shoulder on the shaft by a nut  26 . The seal seat extends radially outwardly from the shaft and circumscribes the axis. The seal seat is one component of a face seal assembly. 
     The face seal assembly also includes an annular, nonrotatable seal support  28  and a pair of annular seal housings  32 . Each seal housing includes a base  34  and a grooved secondary seal holder  36  at one end of the base. The secondary seal holder holds a secondary seal  38  in contact with a cylindrical bore of the seal support. The other end of the seal housing includes an axially extending shroud  42  and an axially extending support lip  44  that serves as a seal element support. The shroud  42  is radially offset from the lip  44  to define an annular space  46  for receiving a seal element. The shroud is also axially elongated relative to the lip. An annular flange  48  with circumferentially distributed slots  50  projects radially outwardly from the lip  44 . 
     The face seal assembly also includes a seal element  52  residing in the space  46  and secured to the lip  44  by an interference fit. The seal element includes a base or blank  54  and a nose  56  extending axially from the blank. The blank is double stepped such that a first, radially inboard region  58  of the blank extends axially beyond a second radially outboard region  60  of the blank to define a first or radially inner step  61  and a second or radially outer step  63 . Moreover, inner step  61  resides axially beyond the tip of shroud  42  whereas outer step  63  does not reside axially beyond the shroud tip. In other words, the tip of the shroud is axially between the steps  61 ,  63 . The seal element is typically made of a graphitic carbon material and is often referred to as a carbon element even though it is not made of pure carbon. In the illustrated application, the carbon element is annular and therefore can be referred to as a carbon ring. 
     A set of circumferentially distributed support pins such as representative pin  64 , each projects axially from the seal support  28  and passes through a corresponding slot  50  in the flange  48 . Springs  66  (depicted in  FIGS. 5-8 ) are circumferentially offset from the pins  64 . The springs are compressed between the flange  48  of housing  32  and the support  28  so that they exert a force on the flange  48  to urge the nose of the carbon ring into contact with the seal seat  24 . The interface between the nose and the seal seat may be unlubricated or “dry” as seen at the left side of the illustration, or it may be lubricated or “wet” as seen at the right side of the illustration. In a wet seal, lubricant flows to the interface by way of circumferentially distributed lubricant passages  68  in the seal seat. 
     During engine operation, high pressure air is present in the annular cavity  70  radially inboard of the seal and radially outboard of the shaft  20 . Lower pressure air intermixed with oil occupies a bearing compartment  72 , which is the region outboard of the seal. The seal resists leakage of the higher pressure air into the lower pressure bearing compartment. 
     Referring additionally to  FIG. 2 , the operation of the above-described shrouded seal is best understood by first considering a conventional seal.  FIG. 2  shows the conventional seal in a normal or substantially undeteriorated condition.  FIG. 2  also suffices to show the seal in a normally deteriorated condition, i.e. with the nose only partially worn away. The arrow F.sub.s represents the force exerted on the seal housing  32  by the springs  66 . Force graphs f.sub.o and f.sub.c show the axially opposing, radially distributed forces F.sub.O, F.sub.C acting on the seal housing, carbon ring and secondary seal as result of the disparate pressures in cavity  70  and compartment  72 . The force vectors in graphs f.sub.o and f.sub.c are illustrated as terminating on respective common planes to facilitate comparisons of the aggregate pneumatic forces. However those skilled in the art will recognize that the forces actually act on the axially facing surfaces of the seal housing, carbon ring and secondary seal. Graph f.sub.c shows a relatively high pressure acting on the high pressure side of the seal and a low pressure acting on the low pressure side of the seal. Graph f.sub.o shows high pressure acting on the high pressure side of the seal, low pressure acting on the low pressure side of the seal, and a radially varying pressure in a transition region across the nose  56  of the carbon ring. As is evident, the nose throttles the high pressure down to the low pressure across a narrow radial region. The combination of F.sub.S and F.sub.C exceeds F.sub.O to keep the seal closed. 
       FIG. 3  shows the conventional seal in a highly deteriorated condition in which the nose has been entirely worn away. F.sub.C is the same as in  FIG. 2 . However because the nose has been worn away, the base portion  54  of the carbon ring throttles the high pressure down to the low pressure across a radial transition region that is relatively wide in comparison to the transition region of  FIG. 2 . As a result higher pressure, and therefore higher forces, act over a larger radial region than is the case in  FIG. 2 . Accordingly, the aggregate force F.sub.O acting on the highly deteriorated seal of  FIG. 3  exceeds the aggregate force F.sub.O acting on the normal or normally deteriorated seal of  FIG. 2 . Furthermore, F.sub.S is slightly smaller than it is in  FIG. 2  due to the increased spring elongation (decompression) and consequent reduction in spring force. Due to the change in forces acting on the seal, there is a potential for F.sub.O to exceed the combination of F.sub.S and F.sub.C resulting in separation of the carbon ring  52  from the seal seat  24 . This separation will allow leakage through the resulting gap as indicated by the small fluid flow arrows. The force graphs and forces would be as shown in  FIG. 4  if the carbon ring were broken away along part or all of its circumference. This would also result in the potential for leakage as indicated in  FIG. 4 . 
     As mentioned previously, the transition between the normal condition in which the nose contacts the seal seat, and the highly deteriorated condition or severely deteriorated conditions occurs very infrequently, but can occur with little warning. As a result there may be an unanticipated period of engine operation during which fluid leaks past the seal. 
       FIG. 5  corresponds to  FIG. 2 , but shows the improved, double stepped shrouded seal in an undeteriorated or normally deteriorated condition. As is evident, the forces are substantially the same as those of  FIG. 2 , with the result that the seal is urged closed. 
       FIG. 6  shows the improved, double stepped shrouded seal in a highly deteriorated condition similar to the condition of the conventional seal in  FIG. 3 . The blank of the carbon ring of  FIG. 6  includes the first radial region  58  and its associated step  61  extending axially beyond the second radial region  60  and its associated step  63 . In addition, the seal of  FIG. 5  includes the shroud  42  on the seal housing. The axially extended first region  58  throttles the high pressure across a radial transition region that is radially narrower than the transition region of  FIG. 3 . Accordingly, the aggregate force F.sub.O of  FIG. 6  is less than the aggregate force F.sub.O of  FIG. 3 . As a result, the carbon ring  52  of  FIG. 6  is less likely to separate from the seal seat  24  than is the carbon ring of  FIG. 3 . 
       FIG. 7  shows the improved, shrouded seal in a more severely deteriorated condition. In comparison to  FIG. 6 ,  FIG. 7  shows the carbon ring  52  worn back essentially to the shroud  42  and therefore shows a throttling effect attributable to the shroud. The shroud and the axially extended first region  58  of the carbon ring throttle the high pressure across a radial transition region that is radially narrower than the transition region of  FIG. 3 . Accordingly, the force magnitude F.sub.O of  FIG. 7  is less than the force magnitude F.sub.O of  FIG. 3 . As a result, the carbon ring of  FIG. 7  is less likely than the carbon ring of  FIG. 3  to separate from the seal seat  24  and permit leakage. As further wear of the carbon ring occurs, the shroud tip will eventually contact the seal seat  24  resulting in a more pronounced throttling effect. 
       FIG. 8  shows the improved, shrouded seal in a damaged condition in which the carbon ring has been broken away over all or part of its circumference. The shroud  42  contacts the seal element and throttles the high pressure across a radially narrow transition so that the seal remains closed and resists leakage. 
     As is evident, the improved, shrouded seal deteriorates more gradually than a conventional unshrouded seal. The gradual deterioration is desirable because it manifests itself as noticeable but minor anomalies in engine performance. These minor anomalies make the engine operator aware that seal replacement or repair is required. Such replacement or repair may then be effected before the seal deteriorates enough to cause more significant problems. 
     With the construction and operation of the seal having now been described, certain variants may now be better appreciated. 
       FIG. 9  shows a seal like that of FIGS.  1  and  5 - 8  in which the housing  32  is made of a selected material. The shroud has a tip  74  at its axial extremity remote from the housing base  34 . The tip is made of the same material as the rest of the housing. 
       FIG. 10  shows a seal in which the housing  32  is made of a parent material and the shroud has a tip  74  which is a region of the shroud impregnated with a second material. Alternatively, the shroud tip may be a feature made of or impregnated with a second material and bonded to the rest of the shroud or may be a coating. The second material may be any material having characteristics that are desirable when the tip contacts the seal seat  24 . These include materials more lubricious than the parent material, materials harder than the parent material and materials more abradable than the parent material. 
       FIGS. 11-13  show a seal in which the shroud comprises a stem  76  and a tip in the form of an insert or attachment  78  affixed to the stem. In  FIG. 11  the insert is affixed with a radially outer snap  82 . In  FIG. 12  the insert is affixed with a radially inner snap  84 . In  FIG. 13  the insert is a molded tip secured to the stem  76  through a set of circumferentially distributed countersunk holes  86 . The tip insert may be made of a material having characteristics that are desirable when the tip contacts the seal ring  24 . These include materials more lubricious than the parent material, materials harder than the parent material and materials more abradable than the parent material. 
     Although the improved seal has been shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the invention as set forth in the accompanying claims.