Abstract:
The present invention solves the aforementioned problems by providing a vacuum seal assembly for a high power generator stub shaft which includes a seal stator encircling the stub shaft and a plurality of oil seals disposed on an inner diameter of the seal stator and encircling the stub shaft. Adjacent oil seals form a plurality of seal cavities between the stub shaft and the seal stator, and define gaps between each oil seal and the stub shaft for receiving a lubricant film. At least one seal feed conduit extending through the seal stator into the plurality of seal cavities to provide lubricant to the seal assembly.

Description:
FEDERAL RESEARCH STATEMENT 
       [0001]    This invention was made with Government support under contract FA8650-04-G-2466-0001 awarded by U.S. Air Force Research Laboratory. The Government has certain rights in the invention. 
     
    
     BACKGROUND 
       [0002]    This invention relates generally to electrical machines, and more particularly to a vacuum seal for sealing a rotating shaft of a high speed generator. 
         [0003]    Power density in generators can be improved by increasing the mechanical speed of the rotor. Increased rotor speed, however, results in increased friction and windage losses. The losses can be reduced by evacuating the chamber in which the rotor is located and operating the rotor in a vacuum environment. Because the rotor shaft extends from outside the vacuum chamber into the vacuum chamber, a vacuum seal around the rotor shaft is necessary to maintain the integrity of the vacuum chamber. Traditional seal types, such as brush seals, carbon seals, or other clearance seals can be employed, but the leakage rates from these types of seals are too high for high power applications. Another option is to utilize a ferrofluidic seal. A ferrofluidic seal establishes a seal by applying a magnetic field to a magnetic fluid (a ferrofluid). The ferrofluid assumes the shape of a liquid o-ring and seals the desired area. Ferrofluidic seals can reduce the seal leakage rates to acceptable levels, but are not effective when the shaft is rotating at high surface speeds (DN of about 1,200,000). Generic “O-ring” seals or “piston-ping” seals have been used in some applications, for example, high speed racing engines, but these typically have a more relaxed vacuum requirement (&gt;1 Torr). 
         [0004]    What is needed is a vacuum seal with leakage rates comparable to a ferrofluidic seal, but which is effective at the high DN values that can be achieved by operating the rotor in a vacuum chamber. 
       BRIEF DESCRIPTION 
       [0005]    Embodiments of the present invention solve the aforementioned challenges through a vacuum seal assembly for a high power generator stub shaft which comprises a seal stator disposed about the stub shaft, and a plurality of oil seals disposed on an inner diameter of the seal stator and encircling the stub shaft. The oils seals define gaps between the oil seals and the stub shaft for receiving a lubricant film, and adjacent oil seals form a plurality of seal cavities between the stub shaft and the seal stator. At least one seal feed conduit extending through the seal stator into the plurality of seal cavities to provide lubricant to the seal assembly. A lubricant film is located at an inner diameter of each oil seal bridging a gap between the stub shaft and the oil seals. The lubricant film forms the vacuum seal around a circumference of the stub shaft. 
         [0006]    These and other advantages and features will be more readily understood from the following detailed description of preferred embodiments of the invention that is provided in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a partial cross-sectional view of a high speed generator rotor and shaft assembly including the bearing assembly and an embodiment of an improved vacuum seal assembly. 
           [0008]      FIG. 2  is an enlarged view of the circled portion of  FIG. 1 , illustrating a carbon seal ring and a stub shaft. 
           [0009]      FIG. 3  is a partial cross-sectional view of a high speed generator rotor and shaft assembly including the bearing assembly and another embodiment of an improved vacuum seal assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    The present invention finds application in a variety of high speed generator applications, non-limiting examples of which include high temperature superconducting generator applications. Shown in  FIG. 1  is an embodiment of high speed generator including a rotor  10  connected to a stub shaft  12 . The stub shaft  12  is substantially cylindrical in shape, and the rotor  10  is connected to the stub shaft  12  at one end. The stub shaft  12  extends through a bearing assembly  14  and a vacuum seal assembly  16  to a drive mechanism (not shown) that drives the rotation of the stub shaft  12  and rotor  10 . 
         [0011]    The bearing assembly  14  in this embodiment includes a carbon seal ring  18  disposed at a rotor end  20  of a bearing stator  22 . The bearing assembly  14  also includes one or more main bearings  24  disposed at a bearing stator inner diameter  26 . The one or more main bearings  24  are positioned axially between the rotor end  20  and an end plate  28 . The one or more main bearings  24  are lubricated by a lubricant introduced through one or more bearing feed conduits  30 . Non-limiting examples of lubricants include fluorinated vacuum oils such as Krytox®. The one or more bearing feed conduits  30  extend through the bearing stator  22  to the bearing stator inner diameter  26 . Lubricant is urged through the one or more bearing feed conduits  30  and applied to the one or more main bearings  24 . When the lubricant is urged onto the main bearings  24 , the lubricant forms a mist suspended in a bearing chamber  32 . Excess lubricant in the bearing chamber  32  is collected in a plurality of bearing output conduits  34  and urged away from the bearing chamber  32 . 
         [0012]    The vacuum seal assembly  16  includes a seal stator  36  which is cylindrical in shape and surrounds the stub shaft  12  adjacent to the end plate  28 . The vacuum seal assembly  16  includes at least three oil seals  38 ,  40  and  42 , nonlimiting examples of which are carbon rings, Teflon®-based lip seals, and metal and carbon face seals. The oil seals  38 ,  40  and  42  define a vacuum side cavity  44  and an atmospheric side cavity  46  between an inner surface  48  of the seal stator  36  and the stub shaft  12 . Lubricant is urged into the vacuum side cavity  44  through one or more seal feed conduits  50 . The lubricant forms a mist in the vacuum side cavity  44 , and as shown in  FIG. 2  forms a lubricant film  52  on the inner diameter  54  of each of a vacuum side oil seal  38  and a center oil seal  40 . Referring again to  FIG. 1 , a small amount of lubricant mist may leak into the bearing chamber  32 , where it can be removed via the bearing output conduits  34 . An amount of the lubricant mist may leak into the atmospheric side cavity  46 . The lubricant mist will, as shown in  FIG. 2 , form a lubricant film  52  on an inner diameter  54  of an atmospheric side oil seal  42 . Returning again to  FIG. 1 , one or more seal output conduits  56  are ported into the atmospheric side cavity  46  and extend through the seal stator  36 , removing excess lubricant that leaks from the vacuum side cavity  44  to the atmospheric side cavity  46 . Because the oil seals  38 ,  40  and  42  are non contact seals, the lubricant film  52  formed on each oil seal  38 ,  40  and  42 , and the oil mist contained in the vacuum side cavity  44  and atmospheric side cavity  46  provide the necessary vacuum sealing around the circumference of the stub shaft  12  to maintain the integrity of the vacuum chamber. 
         [0013]    A bellows seal  58  spans an area between an aft face  60  of the bearing stator  22  and a forward face  62  of the seal stator  36 . The bellows seal  58  is connected to each of the aft face  60  of the bearing housing  22  and the forward face  62  of the seal stator  36 . The bellows seal  58  completes a boundary between a vacuum chamber  66  (about 3.87e-4 psia) in which the rotor  10 , bearing assembly  14 , and vacuum seal assembly  16  are included, and an atmospheric chamber  68  (about 14.7 psia) into which the stub shaft  12  extends. The boundary is defined by the aft face  60  of the bearing stator  22 , the bellows seal  58 , the seal stator  36 , and the atmospheric side carbon seal ring  42 . In addition, the bellows seal  58  provides the vacuum seal assembly  16  with a lateral degree of freedom in the event of radial movement by the stub shaft  12  without affecting performance of the vacuum seal assembly  16 . 
         [0014]    Another embodiment is shown in  FIG. 3 . The embodiment shown in  FIG. 3  has the advantage of the main bearings  24  being disposed in an atmospheric environment, rather than a vacuum environment, so that the lubricant used for the main bearings  24  does not need to be effective in a vacuum environment and can be standard aerospace grade oil. The seal assembly  16  includes the seal stator  36  which is substantially cylindrical in shape and surrounds the stub shaft  12 . The vacuum side oil seal  38  and the atmospheric side oil seal  42  are disposed between the inner surface  48  of the seal stator  36  and the stub shaft  12 , creating a seal cavity  70  between the stub shaft  12  and the seal stator  36 . Lubricant is urged into an oil cavity  72  through one or more seal feed conduits  50 . The lubricant then proceeds through the oil cavity  72  into the seal cavity  70  as a mist. As shown in  FIG. 2 , the lubricant forms a lubricant film  52  on the inner diameter  54  of each of the vacuum side oil seal  38  and the atmospheric side oil seal  42 . Returning to  FIG. 2 , a small amount of mist that leaks past the vacuum side oil seal  38  is prevented from entering the vacuum chamber  66  by a non-contact seal  74  disposed at a rotor-side face  76  of the seal stator  36 . One or more seal output conduits  56  extend from the seal cavity  70  through the seal stator  36 , removing excess lubricant from the seal cavity  36 . Because the oil seals  38  and  42  do not contact the stub shaft  12 , the lubricant film  52  formed on each oil seal  38  and  42 , and the oil mist contained in the seal cavity  70  provide the necessary vacuum sealing around the circumference of the stub shaft  12  to maintain the integrity of the vacuum chamber. 
         [0015]    The bearing assembly  14  is disposed at the non-rotor face  78  of the seal stator  36 . The bearing assembly  14  comprises the bearing stator  22  and one or more main bearings  24  disposed at the bearing stator inner diameter  26 . The one or more main bearings  24  are positioned axially between the seal stator  36  and the bearing stator  22 . The one or more main bearings  24  are lubricated by a lubricant, in this case aerospace grade oil, introduced through one or more bearing feed conduits  30 . The one or more bearing feed conduits  30  extend through the bearing stator  22  to the bearing stator inner diameter  26 . Lubricant is urged through the one or more bearing feed conduits  30  and applied to the one or more main bearings  24 . When the lubricant is urged onto the main bearings  24 , the lubricant forms a mist suspended in the bearing chamber  32 . Excess lubricant in the bearing chamber  32  is collected in a plurality of bearing output conduits  34  and urged away from the bearing chamber  32 . 
         [0016]    The bellows seal  58  spans an area between a bearing stator forward face  80  and the rotor-side face  76  of the seal stator  36 . The bellows seal  58  is connected to each of the bearing stator forward face  80  and the rotor-side face  76 . The bellows seal  58  completes a boundary between a vacuum chamber  66  in which the rotor  10  is included, and an atmospheric chamber  68  into which the stub shaft  12  extends. The boundary is defined by the non-contact seal  74 , the rotor-side face  76  of the seal stator  36 , the bellows seal  58 , the bearing stator forward face  80  and an end frame  82 . 
         [0017]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.