Abstract:
A ball bearing assembly for a gas turbine engine comprises an inner race that couples to a rotor shaft; an outer race that couples to a primary static structural support; a plurality of ball elements between the inner race and the outer race; and a ball cage that comprises a carbon-carbon composite material to maintain the relative radial spacing of the ball elements between each other within the inner race and the outer race after subjection to high loads, heat generation and long-term storage.

Description:
[0001]    The invention occurred with government support under Contract No.: F08635-03-C0002 awarded by the United States Air Force. The government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to bearings for gas turbine engines and more particularly to ball bearings for supporting a turbine shaft in a gas turbine engine. 
       BACKGROUND OF THE INVENTION 
       [0003]    Miniature gas turbine or turbojet engines, typically of 150 lb-f thrust and smaller, are often useful for single-use airborne applications such as reconnaissance drones and other unmanned air and ground launched aeronautical vehicles. The use of such an engine greatly extends the range of such vehicles in comparison to the more conventional solid fuel rocket engine. 
         [0004]    A miniature gas turbine engine must have a relatively inexpensive manufacturing cost coupled with a high degree of starting and operational reliability when launched from air or ground systems in order to be an economically feasible extended range expendable propulsion source for such applications. The high-speed ball bearings that support the rotating turbine machine are one type of component that greatly affects mechanical performance and reliability of a miniature gas turbine engine. Reliability and efficiency of the ball bearing cage for such bearings are prime concerns for a successful expendable turbine engine. Long-term storage, excessive operational heat, or cage wear from induced loads may compromise such reliability and efficiency of the bearing cage. 
         [0005]    Current gas turbine bearing systems employ common cage materials such as silver-plated stainless steel, brass or even poly-ether-ether-keytone (PEEK). These common materials typically are not able to handle the high loads, heat generation and long-term storage typical of single use systems, and if used in practice may increase the potential of an operational failure. Accordingly, it is desirable to utilise ball bearings with an uncomplicated and inexpensive cage material that can withstand the high loads, heat generation and long-term storage typical of single use systems for supporting the rotating turbine machine to achieve a successful miniature gas turbine engine for such expendable aeronautical applications. 
       SUMMARY OF THE INVENTION 
       [0006]    In one embodiment, the invention comprises an improved ball bearing assembly that utilises a carbon-carbon ball cage for its ball elements to maintain the relative radial spacing of the ball elements between each other after the ball bearing assembly is subjected to high loads, heat generation and long-term storage. 
         [0007]    Generally, the invention comprises a ball bearing assembly for a gas turbine engine, comprising: an inner race that couples to a rotor shaft; an outer race that couples to a primary static structural support; a plurality of ball elements between the inner race and the outer race; and a ball cage that comprises a carbon-carbon composite material to maintain the relative radial spacing of the ball elements between each other within the inner race and the outer race after the ball bearing assembly is subjected to high loads, heat generation and long-term storage. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a side view of an expendable aeronautical vehicle that is suitable for incorporating at least one embodiment of the invention. 
           [0009]      FIG. 2  is a cut-away side view of a miniature turbine engine for the expendable aeronautical vehicle shown in  FIG. 1  that is suitable for incorporating at least one embodiment of the invention. 
           [0010]      FIG. 3  is a cut-away side view of a bearing assembly according to a possible embodiment of the invention. 
           [0011]      FIG. 4  is a top view of a bearing assembly according to the possible embodiment of the invention shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]      FIG. 1  is a side view of an expendable aeronautical vehicle  2  that is suitable for incorporating at least one embodiment of the invention. The vehicle  2  comprises an airframe  4  with one or more aerodynamic surfaces  6 . The vehicle  2  also comprises a propulsion engine  8 , typically of the gas turbine or turbojet type. The engine  8  mounts within or to the vehicle  2 . In  FIG. 1 , for purposes of illustration the engine  8  mounts within the vehicle  2 , as shown in dashed line. An intake  10 , shown in dashed line, supplies ambient air to the engine  8 . An exhaust pipe  12 , shown in dashed line, exhausts the thrust of the engine  8  to propel the vehicle  2 . 
         [0013]      FIG. 2  is a cut-away side view of a miniature turbine engine  8  for the expendable aeronautical vehicle shown in  FIG. 1  that is suitable for incorporating the invention. The miniature gas turbine engine  8  generally comprises a housing  14 , a rotor shaft  16  supported by a forward bearing  18  and an aft bearing  20 , a generally annular combustion chamber  22  and an exhaust pipe  24 . The forward bearing  18  and the aft bearing  20  allow the rotor shaft  16  to rotate about a longitudinal axis X. The forward bearing  19  and the aft bearing  20  are both of the ball bearing type. 
         [0014]    A multi-bladed compressor wheel  26  mounted on the rotor shaft  16  faces forward toward an intake  28  and a multi-bladed turbine wheel  30  mounted on the rotor shaft  16  faces rearward toward the exhaust pipe  24 . The forward bearing  18  and the aft bearing  20  support the rotor shaft  16  to extend it at least partially into a forward cover  32 . The forward cover  32  is preferably the forward-most portion of the engine  8  and defines an aerodynamically contoured shape. The intake  28  generally surrounds the forward cover  32  to facilitate airflow. 
         [0015]    A permanent magnet generator (PMG)  34  preferably mounts on the rotor shaft  16  between the forward bearing  18  and the aft bearing  20  to generate electrical power for the engine  8  and other accessories. The PMG  34  comprises a stator  36  that mounts to the housing  14  by way of a housing inner support  38  and a rotor  40  mounted on the rotor shaft  16 . An electrical power line  42  transfers electrical power from the PMG  34  to an electrical power system  44 . 
         [0016]    A fuel pump  46  to pump fuel from a fuel source  48  by way of a fuel source line  50  pumps fuel to the annular combustion chamber  22  by way of a pump supply line  52  through a fuel manifold  54 . The electrical power system  44  preferably drives the fuel pump  46 , although alternatively the turbine wheel  30  could drive the fuel pump  46  by way of a suitable transmission (not shown) coupled to the rotor shaft  16 . The fuel burns at high temperatures within the combustor chamber  22  to generate expanding exhaust gases that flow through a turbine nozzle  56 , the turbine wheel  30  and the exhaust pipe  24  thereby driving the turbine wheel  30  and generating a high velocity thrust out of the exhaust pipe  24 . 
         [0017]    A fastener  58 , such as a threaded rotor nut or bolt, may conveniently couple to a mating end portion  60  of the rotor shaft  16 , such as a threaded stud or aperture, to retain the rotor shaft  16  within the forward bearing  18  and the aft bearing  20 . The housing inner support  38  conveniently mounts the forward bearing  18  and the aft bearing  20  to the housing  14 . 
         [0018]    The housing  14  provides the primary static structural support for rotation of the rotor shaft  16  and the hereinbefore-described rotational components mounted on it. The fastener  58  extends at least partially within the forward cover  32 . The forward cover  32  mounts to the housing  14 . Removal of the forward cover  32  facilitates assembly and disassembly by providing access to the fastener  58 . 
         [0019]    The housing  14  includes a lubrication line  62  supplied by a lubrication source that supplies suitable bearing lubricant, such as fuel, oil or a mixture thereof to the bearings  18  and  20 . The lubrication source may conveniently be the fuel source  48 , in which case the lubrication line  62  may couple to the fuel source line  50 . The lubrication line  62  may conveniently supply a plurality radial lubricant passages (not shown) arranged about each of the bearings  18  and  20 . 
         [0020]    In any case, the lubricant delivery preferably sprays lubricant onto the bearings  18  and  20 . Such lubrication delivery still further improves reliable operation. Furthermore, the cooling airflow that passes through the forward cover  32  propagates lubricant that collects aft of the forward bearing  18  toward the aft bearing  20  and into the combustion chamber  22 . Using a fuel or a fuel oil mixture as the lubricant maintains engine efficiency, since the lubricant ultimately propagates into the combustion system  22  for combustion and thrust generation. 
         [0021]      FIG. 3  is a cut-away side view of the bearings  18  and  20  according to a possible embodiment of the invention.  FIG. 4  is a top view of the bearings  18  and  20  according to the possible embodiment of the invention shown in  FIG. 3 . Each bearing  18  and  20  has a plurality of ball elements  64  arranged radially between an inner race  66  and an outer race  68 . A ball cage  70  comprises a plurality of apertures  72  arranged around its periphery that each retain a respective ball element  64  and serve to maintain the relative radial spacing of the ball elements  64  between each other within the inner race  66  and the outer race  68 . According to one possible embodiment of the invention, the ball cage  70  may comprise a carbon-carbon or reinforced carbon-carbon (RCC) composite material. 
         [0022]    Carbon-carbon or RCC material is a composite of carbon fibre, usually made from pitch, rayon, or poly-acrylo-nitrile (PAN), in a carbon-dominated matrix. Fabrication of such materials generally uses a high-content carbon resin as the initial matrix subjected to high heat to drive off the non-carbon elements. The carbon-carbon composite material may be carbonised or graphitised, depending on the temperature of the heating process. It is lightweight, highly heat-resistant, thermal-shock-resistant, and malleable for shaping as necessary. Allcomp Inc., of City of Industry, California manufactures a suitable carbon-carbon or RCC material for this purpose. 
         [0023]    Relative positional terms as hereinbefore described, such as “forward”, “aft”, “upper”, “lower”, “above”, “below”, and the like are with reference to the normal operational attitude and should not be considered otherwise limiting. 
         [0024]    The described embodiment of the invention is only an illustrative implementation of the invention wherein changes and substitutions of the various parts and arrangement thereof are within the scope of the invention as set forth in the attached claims.