Abstract:
An air-oil seal having a runner surface comprising at least one component selected from the group consisting of: polyetheretherketone; polyetherimide; polyphenylene sulfide; and polyetherketoneketone.

Description:
TECHNICAL FIELD 
   The invention relates generally to rotary seals and, more particularly, to a seal for sealing a rotating shaft. 
   BACKGROUND OF THE ART 
   Labyrinth seals are often used as air-oil seals to provide sealing of rotating shafts of bearing compartments and gear cases, etc. Labyrinth seals typically comprise knife edges or teeth or fins which rotate relative to a surrounding seal land or runner, the knife edges having tight tip clearances with a surface of the land or runner. The relative rotation between the knife edges and the runner of labyrinth seals, however, can tend to cause rubbing wear on the runner surface, which can eventually lead to particles of the runner being shaved off and ingested into an adjacent bearing supporting the shaft. Knife edge wear also results. These rubs can significantly reduce the sealing capability and ingested particles can also damage bearings, gears and/or other components adjacent to such seal if these components ingest the particles. Polymers, for example, are avoided as runners because they rub easily and the polymeric particles could adversely effect bearings, gear surfaces, etc. 
   An improved seal runner surface for a labyrinth seal is therefore desired, particularly for use as a shaft seal. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of this invention to provide an improved runner for a labyrinth air-oil seal. 
   In one aspect, the present invention provides a labyrinth seal comprising at least one knife edge and a runner surface, the runner surface including a material selected from the group consisting of: polyetheretherketone; polyetherimide; polyphenylene sulfide; and polyetherketoneketone. 
   In a second aspect, the present invention provides a shaft assembly comprising a rotatable shaft supporting an oiled component and at least one labyrinth seal providing air-oil sealing between the rotating shaft and an adjacent element, the labyrinth seal having at least one knife edge and a runner mounted for relative rotation, the runner having a runner surface facing the knife edge comprised of a material selected from the group consisting of: polyetheretherketone; polyetherimide; polyphenylene sulfide; and polyetherketoneketone. 
   In a third aspect, the present invention provides a method of sealing an air-oil interface, the method comprising the steps of: providing a labyrinth seal with a runner material on a runner surface thereof; and selecting the runner material from the group consisting of: polyetheretherketone; polyetherimide; polyphenylene sulfide; and polyetherketoneketone. 
   Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below. 

   
     DESCRIPTION OF THE DRAWINGS 
     Reference is now made to the accompanying figures depicting aspects of the present invention, in which: 
       FIG. 1  is a cross-sectional view of a bearing cavity labyrinth seal having a runner with an abradable surface in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a shaft bearing assembly  20  comprises generally at least one a bearing  22  supporting a rotating shaft  24  and at least one labyrinth seal  26  providing sealing between the rotating shaft  24  and an adjacent element  28 , such as a portion of the stationary outer casing  30 . In the embodiment depicted in  FIG. 1 , a second labyrinth seal  27  is provided between the rotating shaft  24  and a second rotating shaft  29  disposed concentrically within the rotating shaft  24 . The labyrinth seals  26 ,  27  generally comprise a plurality of knife edges  34  which rotate with the shaft relative to a juxtaposed runner  38 . Tips  36  of the knife edges are disposed adjacent to the runner surface  40  of the runner  38  in very tight clearance thereto such that a substructure fluid seal is provided there between. Although generally the knife edges are disposed on a shaft which rotates within a stationary surrounding runner, it is to be understood that the converse is also possible, namely that the runner surface rotates and the knife edges of the labyrinth seal disposed in close juxtaposition thereto remains stationary. Further, as depicted for the labyrinth seal  27 , both portions of the labyrinth seal may be rotating. The configuration of the seal described above is thus far generic. 
   According to the present invention, the runner  38  is preferably an insert made of reinforced composite thermal plastic material which is bonded or otherwise affixed to the base element adjacent the rotating shaft. Particularly, the reinforced composite thermal plastic used includes at least one of polyetheretherketone, polyetherimide, polyphenylene sulfide, and polyetherketoneketone. 
   Such reinforced composite thermal plastics provide several advantages over traditionally used metallic abradable seal materials and other non-metallic polymer abradable materials. Surprisingly, unlike other polymers, when repeated rubs and/or deep rubs which liberates particles from the runner surface  40 , such debris particles of the mentioned reinforced composite thermal plastics will not cause any significant damage, to bearings, gears, etc, should they be ingested by such systems. This is not the case with metallic runner materials of the prior art, or other non-metallic abradable materials of the prior art, such as Teflon™ or thermoset plastics (see U.S. Pat. No. 4,460,185). Particularly, thermoset plastics may trap and embed metallic particles (eg. from debris contained in the oil) which could also cause damage to a bearing or gear if the thermoset plastic particles were ingested. Halogenated hydrocarbons, such as Teflon™ for example, may also release halogens, as a result of decomposition of the material caused by frictional heating of the surface, which may tend to react with titanium components and cause stress corrosion cracks, making the use of such materials undesirable with titanium engine components. The present invention is therefore useful in aerospace applications, where titanium is often used, such as in gas turbine engines. 
   In contrast to these prior art materials, reinforced composite thermal plastics such as polyetheretherketone (PEEK), polyetherimide, polyphenylene sulfide, and polyetherketoneketone overcome these problems, as the inventors have discovered that these thermal plastics, unlike thermoset materials, do not cause the type of abrasive damage to bearings, gears, etc. which is usually associated with plastic seal runners. 
   The use of PEEK is known for use as an abradable layer in blade tip sealing applications—see U.S. Pat. No. 6,352,264 to Dalzell et al. Such air-only seals, however, do not have the same concerns indicated above regarding rub debris fouling components in oil lubricated system adjacent the seal. Consequently, Dalzell et al. is unhelpful in directing one to a solution to the problem addressed by the present invention. 
   The composite mix of the thermal plastics of the present invention is preferably chosen such that a predetermined thermal expansion coefficient of the final composite substantially corresponds to that of the matching metallic structural components. Thus, improved clearance control is achieved as both the runner seal insert and the surrounding seal material will shrink and expand together compatibly. 
   The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. For example, although the present invention is described preferably in reference to its use in a gas turbine engine, it is to be understood that the labyrinth seal of the present invention may be used in any other applications in which there is a need to provide a seal about a rotating shaft, such as in pump, compressors and the like. Further, the runner surface of the present invention may be composed of a single named material, or may be a composite material made up of any combination of the named materials, or may comprise a layer(s) of the novel material over a more conventional base material. Still other modifications will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the scope of appended claims.