Abstract:
An air turbine starter with an improved seal assembly disposed between the turbine and the housing is provided. The seal assembly comprises a face seal, a non-contacting seal axially spaced therefrom to define an air filled annular chamber therebetween and a flow passage having an exit fluidly communicating with said chamber and an inlet fluidly communicating with a source of air.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     Priority is claimed to provisional application Ser. No. 60/097,467, filed Aug. 21, 1998. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to air turbine starters and in particular to seals used in such starters. 
     BACKGROUND OF THE INVENTION 
     Referring to the drawings, FIG. 2 shows a prior art turbine face seal commonly used in air turbine starters. The face seal includes a rotor  2  mounted to shaft  24  adjacent a bearing  26 . The rotor  2  has an axially facing flange  3 . The face seal further includes a stator  4  mounted to turbine exhaust housing  27 . Disposed between the rotor  2  and the stator  4  is a carbon seal ring  5  that sealingly engages the flange  3 . 
     In typical air turbine starters such the ATS 100 shown in FIG. 1, the rotation of the turbine wheel  22  can generate under certain circumstances a low pressure or “vacuum” on the downstream side of the wheel which is also the airside of the turbine seal. This differential pressure results in large oil leakage rates if the turbine seal has any flaws or coke build up in the contact zone. The coke build-up destroys the flatness of the sealing contact surfaces between the rotor and the seal carbon ring. The coke separates the two surfaces resulting in a clearance therebetween. As a result an air/oil mist is pulled from the bearings through the seal and into the turbine exhaust air. If 100 to 300 ccs of oil is displaced to the turbine exhaust air without detection by the pilot or ground crew, loss of the air turbine starter by turbine bearing failure is possible. This sensitivity of the system design to turbine seal flaws results in the turbine seal being in the top three causes of air turbine starter returns from the field for cause. 
     Accordingly, there is a need for a turbine seal for use in air turbine starters that prevents oil leakage even in the presence of a coke build up. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide for a turbine seal for use in air turbine starters that prevents oil leakage even in the presence of coke build up. 
     The present invention achieves this object by providing an air turbine starter with an improved seal assembly disposed between the turbine and the housing. The seal assembly comprises a face seal, a non-contacting seal axially spaced therefrom to define an air filled annular chamber therebetween and a flow passage having an exit fluidly communicating with said chamber and an inlet fluidly communicating with a source of air. 
     The air filled chamber acts as a buffer separating the differential pressure generated by pumping of the turbine from the face seal. Thus if the face seal develops a flaw (due to coke build up on the contact face or grain pull out from the carbon face) the seal leakage is low even if the sealing faces separate. 
     These and other objects, features and advantages of the present invention, are specifically set forth in, or will become apparent from, the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 presents a plan cross-sectional view of an AlliedSignal air turbine starter having the designation ATS 100. 
     FIG. 2 is a blow-up of the turbine section of the air turbine starter of FIG. 1 with a prior art turbine seal. 
     FIG. 3 is a blow-up of the turbine section of the air turbine starter of FIG. 1 with a turbine seal contemplated by the present invention. 
     FIG. 3A-3E show various flow passage inlet arrangements. 
     FIG. 4 is a partial cross-sectional view of the air-to-air seal of the seal assembly contemplated by the present invention. 
     FIG. 5A-5F show various air-to-air seal arrangements. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, FIG. 1 shows an air turbine starter  10  embodying the present invention. The air turbine starter  10  has a first housing assembly  12  and a second housing assembly  13 . The housing assembly  12  defines a flow path  14  extending from an inlet  16  to an outlet  18 . The housing assembly  13  includes a mounting flange  19  for mounting the air turbine starter to an aircraft engine (not shown). An air pressure duct  15  delivers pressurized air from an air supply to the inlet  16 . Typically, the air pressure at the inlet  16  is in the range of 30-40 psig. 
     Within the air turbine starter  10 , the housing assemblies  12  and  13  support a turbine section  20 , a compound planetary gear train  40 , and an overrunning clutch  60 . 
     The turbine section  20  is comprised of a turbine wheel  22  having a rotatable shaft  24  extending therefrom, journaled by bearings  26  to a turbine exhaust housing  27  which is part of housing  12 . A gear  25  is secured to the shaft  24 . A plurality of turbine blades  28  are circumferentially mounted to the turbine wheel  22  and are positioned within the flow path  14 . Upstream of the blades  28  are a plurality of nozzles  29  mounted to the housing assembly  12  which provide the proper flow angle to the air flow before it enters the turbine blades  28 . In operation, pressurized air entering through inlet  16  is properly aligned by the nozzles  29  and is then expanded across the blades  28  before exiting through outlet  18 . The blades  28  convert the pressure energy of the air into rotary motion causing the turbine wheel  22 , the shaft  24  and the gear  25  to rotate at the same speed as the blades  28 . 
     The compound planetary gear train  40  is comprised of a plurality of shafts  42  each having a gear  44  that meshes with the gear  25 . The gear  44  engages a portion of the shaft  42 , a ring gear  48  and a hub gear  62  which is the input side of the overrunning clutch  60 . In operation, the gear train  40  converts the high speed, low torque output of the turbine section  20  into low speed, high torque input for the clutch  60 . 
     The clutch  60  is a pawl and ratchet type clutch. The clutch  60  has the hub gear  62  on its input side and a clutch drive shaft  70  on its output side. The hub gear  62  has a hollow cylindrical hub portion  63  which is supported on a bearing  64  and has a ratchet  65  circumferentially positioned along its external surface. Adjacent to the hub gear  62  is a hollow drive shaft assembly comprising a clutch housing  68  integral with a clutch drive shaft  70 , and mounted on bearing  71  for rotation. A portion of the drive shaft  70  extends beyond the housing  13  and has an air turbine starter output shaft  90  mounted thereon. The output shaft  90  can be coupled, for example, to a starter pad on the gearbox of a gas turbine engine, (not shown). 
     Referring to FIG. 3, a turbine seal assembly  100  includes a face seal having a rotor  102  mounted to shaft  24  adjacent a bearing  26 . The rotor  102  has an axially facing flange  103 . The face seal further includes a stator  104  mounted to turbine exhaust housing  27 . Disposed between the rotor  102  and the stator  104  is a carbon seal ring  105  that sealingly engages the flange  103 . The assembly  100  further includes an air-to-air seal  110  disposed on the air side of the stator  104  and axially spaced therefrom to define an annular chamber  112 . The air-to-air seal  110  is typically a non-contacting clearance gap type seal and extends radially inwardly from the turbine exhaust housing  27  to a radially inner tip that seals against a radially outward facing surface of the wheel  22 . In the preferred embodiment, the seal  110  is a labyrinth seal with the inner tip being a single knife-edge. In alternative embodiments, the inner tip could be a multiple knife edges with or without abradeable contact zones  120  such as honeycomb or soft plasma sprayed high porosity coatings. In other embodiments the seal  100  can be a lip seal  110 A (FIG.  5 A), a floating ring seal  110 C (FIG.  5 C), such as a clearance floating ring seal or an arch bound floating ring seal, static long bushings  110 F (FIG.  5 F), static short bushings  110 E (FIG.  5 E), wind backs, either outside diameter or inside diameter contacting piston rings  110 B (FIG.  5 B), floating rotating free rings, brush seals  110 D (FIG.  5 D), or face seals. These seals can be oriented in either the radial or axial plane. If a lip seal is used it should be a contacting seal where the contact lip is allowed to abrade to a line-to-line fit to a clearance after rotation of the turbine wheel. The lip seal may be filled with polytetrafluoroethylene, nylon or plastics, or rubber. 
     Air is brought to chamber  112  through a flow passage  114 . In the preferred embodiment, the inlet  116  of the passage  114  receives ambient air surrounding the air turbine starter  10 . Alternatively, the inlet  116 A can be located in the exhaust duct  18  (FIG. 3A) or at the flange  19  where the starter  10  is coupled to the engine ( 116 B, FIG.  3 B). In another embodiment, high pressure from duct  15  can be directly ducted to the inlet  16  ( 116 C, FIG.  3 C), or the pressure in the air can be regulated with a reduction orifice  117  (FIG. 3D) or pressure regulated air supply. The flow passage  114  can be formed in a number of ways such as by tubing  114 A (FIG.  3 E), annular passages, or cast-in chambers and/or tubes (FIGS.  3  and  3 A- 3 D). Importantly, the flow area of the flow passage  114  needs to be larger than the maximum flow area of the air-to-air seal  110  so that the acceleration of the air at the air-to-air seal absorbs the turbine wheel disk centrifugal pressure generation leaving no differential pressure across the face seal. Preferably, the flow area of the flow passage  114  is at least three times larger than the flow area of the air-to-air seal  110 . 
     The redundant air-to-air turbine seal assembly  100  reduces the system&#39;s sensitivity to flaws in the turbine seal by eliminating the differential pressure across the seal. Because the chamber  112  provides a buffer section separating the differential pressure from the turbine wheel pumping from the turbine seal, if the seal develops a flaw (due to coke build up on the contact face or grain pull out from the carbon face) the seal leakage is low even with the two faces of the face seal separate. The elimination of the differential pressure across the seal also reduces the pressure loading of the carbon face or eliminates the requirement for a more expensive and larger envelope package pressure balanced carbon face seal. 
     Various modifications and alterations to the above-described preferred embodiment will be apparent to those skilled in the art. For example, the present invention can be used with other gas turbine engine configurations. Accordingly, these descriptions of the invention should be considered exemplary and not as limiting the scope and spirit of the invention as set forth in the following claims.