Abstract:
A gearbox for use in an auxiliary power unit has a plurality of gears received within a housing. A lube pump is mounted within the housing, and delivers oil to the plurality of gears and bearings. The lube pump has an oil supply tube for supplying oil to the pump for delivery to the plurality of gears and bearings. An oil reservoir maintains a quantity of oil above a lowermost end of the inlet tube, and is shaped such that a cross-sectional area at a lower end of the inlet tube has a first cross-sectional area. Other cross-sectional areas spaced upwardly from the first cross-sectional area are greater. A ratio of the second cross-sectional area to the first cross-sectional area is less than 3.

Description:
STATEMENT OF GOVERNMENT INTEREST 
       [0001]    This invention was made with government support under Contract No. N00019-06-C-0081 awarded by the United States Navy. The Government has certain rights in this invention. 
     
    
     BACKGROUND 
       [0002]    This application relates to an oil reservoir for a gearbox, wherein the shape is optimized to ensure adequate oil flow even when a vehicle carrying the gearbox is put through extreme changes in orientation. 
         [0003]    Modern aircraft, and in particular helicopters, must be able to perform a variety of extreme maneuvers. They must be able to have nose up and nose down, and left and right inclination attitudes. In addition, the attitudes may be combined between nose up/down and left/right inclination. 
         [0004]    One component that is typically mounted on modern helicopters is an APU, or auxiliary power unit. An APU is a small gas turbine engine which generates electrical power. The APU includes a gear train mounted in a gear housing. The gear train is provided with a lubrication system, including a lube pump moving oil from an oil reservoir across the gear train. 
         [0005]    An inlet tube takes oil from the reservoir and delivers it to the pump. The extreme changes in attitude that can occur with the use of such a system on a helicopter raise challenges with ensuring there is an adequate oil level to supply oil into the inlet tube. On the other hand, it is also important to not have too high an oil level. Typically, a designer would not want the gear train to be immersed in the oil. 
       SUMMARY 
       [0006]    A gearbox for use in an auxiliary power unit has a plurality of gears and bearings received within a housing. A lube pump is mounted within the housing, and delivers oil to the plurality of gears and bearings. The lube pump has an oil supply inlet tube for supplying oil to the pump for delivery to the plurality of gears and bearings. An oil reservoir maintains a quantity of oil above a lowermost end of the inlet tube, and is shaped such that a cross-sectional area at a lower end of the inlet tube has a first cross-sectional area much smaller than that of the second. Other cross-sectional areas spaced upwardly from the first cross-sectional area are greater. A ratio of the second cross-sectional area to the first cross-sectional area is less than 3. 
         [0007]    These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a perspective view of an exemplary rotor wing aircraft. 
           [0009]      FIG. 2  is a partial phantom view of a power plant system for the exemplary rotor wing aircraft. 
           [0010]      FIG. 3  is a side view of the exemplary auxiliary power unit. 
           [0011]      FIG. 4A  is a back view of a gearbox such as for use on the APU. 
           [0012]      FIG. 4B  is a front view of the gearbox assembly with front cover removed illustrated in  FIG. 4A . 
           [0013]      FIG. 4C  is a detailed view of the back of the gearbox as shown in  FIG. 1A . 
           [0014]      FIG. 4D  shows a side view of the gearbox. 
           [0015]      FIG. 5  schematically shows oil levels within the oil reservoir. 
           [0016]      FIG. 6  shows examples of possible attitudes for a helicopter. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring to  FIGS. 1 and 2 , a rotary wing aircraft  10  includes a main rotor system  12 , an airframe  14  and an extended tail  16 . The extended tail  16  supports an anti-torque system  18 . The main rotor system  12  is driven about an axis A by multiple power plants ENG 1 , ENG 2  and ENG 3 . The power plants ENG 1 , ENG 2  and ENG 3  generate the power for flight operations and the APU  120  provides power utilized for operating various aircraft systems. Although a particular aircraft and power plant configuration is illustrated, other aircraft configurations and machines will also benefit from this disclosure. Operation of the example aircraft  10  includes high attitude orientations about the pitch and roll axes. Such high attitudes present challenges to the operation of systems that otherwise operate without issue at normal or less extreme orientations. 
         [0018]    Referring to  FIG. 3  with continued reference to  FIGS. 1 and 2 , the example APU  120  includes a gearbox  20  that is driven by a power plant  124 . The example power plant  124  comprises a small gas turbine engine in comparison to the main propulsion power plants ENG 1 , ENG 2  and ENG 3 . The gearbox  20  includes a plurality of gears that drive various pumps, generators and other aircraft systems. The gears operate at high speeds and require sufficient lubrication without becoming submerged within stored lubricant. Moreover, bearings supporting those gears also should not be completely submerged during operation. Submersion of rotating gears and bearings increases wear, reduces power output, and generates heat within the lubricant. Accordingly, the disclosed gearbox  20  includes features that prevent submersion of the gears and bearings during operation at extreme aircraft attitudes, but also ensures oil is available to be moved by an associated pump. 
         [0019]    A gearbox  20  is illustrated in  FIG. 4A , and includes a lube pump  22  that takes oil from an oil reservoir  26 , and delivers the oil upwardly across the gears and bearings in a gear train  24 , shown schematically. 
         [0020]    The oil reservoir  26  is configured to have an optimized shape such that oil will be available for supply to the lube pump  22  under any change of attitude of the aircraft  10  carrying the gearbox  20 . On the other hand, it is also desirable that the oil level not reach the gear train  24 , as the gears and bearings would churn and aerate the oil causing high oil temperature shutdowns. As such, the oil reservoir  26  is optimized such that it ensures oil will always be above a lowermost end of an oil supply tube, but below a lowermost end of the gear train  24 . 
         [0021]    To that end, the oil reservoir  26  includes enlarged portion  28  at a relatively upper part of the housing for oil reservoir  26 . An intermediate channel  30  is formed between the enlarged rear portions  28  to assist in the producibility of the casting. A smaller back portion  34  of the rear face of the oil reservoir  26  does not extend as far from nominal face  27  of the back as does enlarged portion  28 . As shown, the enlarged forward portion  28  ramps through a ramp portion  32  to a smaller portion  34 . 
         [0022]    As shown in  FIG. 4B , the enlarged portion  28  extends to enlarged sides  50  on the sides  62 . Enlarged sides do not extend to front face  64 , but instead end at point  200 . A ramp  52  merges downwardly to a nominal side face  62 . A front face  64  includes a fill opening  60  that allows oil to be delivered into the housing. Gear train  24  is shown including gears and bearings. 
         [0023]      FIG. 4C  shows the shape of the rear face, including the enlarged portions  28 , and the ramped portions  32  ramping down to the smaller portions  34 . In addition, the sides  54  and  55  of the back face can be seen to ramp inwardly below the enlarged sides  50  to a smaller, lower portion, and then ramp even further in at  56  to a lowermost point  58 . 
         [0024]    As can be appreciated from  FIG. 4C , the oil reservoir  26  will have a smaller cross-sectional area at lower positions than it does at positions toward the top of the enlarged portions  28 . The enlarged portions  28  further have top face  29  extending rearwardly to the nominal face  27 . 
         [0025]    As shown in  FIG. 4D , a distance extending along a length of the rotational axis of the lube pump  22 , and from the smaller portion  34  can be defined as L 1 . In one embodiment, L 1  was 5.29″ (13.44 cm). A distance L 2  can be defined from the front face  64  to the rear end of the forward enlarged portion  28 . In one embodiment, L 2  was 6.04″ (15.34 cm). 
         [0026]    Another distance L 3  can be defined from a forward end  200  of the enlarged side portion  50  to the rear end of enlarged portion  28 . In one embodiment, L 3  was 4.55″ (11.55 cm). 
         [0027]    A distance L 4  can be defined as the distance along the enlarged portion  28  to the ramped portion  32 . In one embodiment, L 4  was 1.53″ (3.89 cm). A distance L 5  can be defined as the distance to the lowermost end of the oil reservoir  36  from the bottom of the enlarged portion  28  and the beginning of the ramped portion  32 . In one embodiment, L 5  was 4.87″ (12.37 cm). 
         [0028]    In embodiment, a ratio of L 1  to L 2  was between 0.8 and 0.9. A ratio of L 4  to L 5  was between 0.3 and 0.4. A ratio of L 3  to L 2  was between 0.7 and 0.8. 
         [0029]    As shown in  FIG. 5 , a highest desired level of oil that would typically be provided within the oil reservoir  26  is shown at X. A worst case low oil level is also shown at W. The desired worst case line W is above a bottom  102  of an inlet tube  100 . Inlet tube  100  delivers oil to the lube pump  22 , shown schematically in this Figure. 
         [0030]    As can be seen in  FIG. 5 , three cross-sectional areas A 1 , A 2 , and A 3  can be defined within the interior of the oil reservoir  26 . A 1  is selected to be at the lowermost end  102 . In one embodiment, A 1  has a cross-sectional area of 23.2 in 2  (149.6 cm 2 ). A 3  is selected to be at a location  5 ″ (12.7 cm) above the bottom surface  102 . In one embodiment, A 3  was 42.4 in 2  (273.5 cm 2 ). A 2  is selected to be adjacent to the oil fill line X. In one embodiment, A 2  was 40 in 2  (258 cm 2 ). 
         [0031]    In embodiments of this invention, it is preferred that a ratio of A 1  to A 2  be less than 3, and between 1.5 and 2.5. It is preferred a ratio of A 1  to A 3  be between 0.5 and 0.6. A 3  could be the largest cross-sectional area, and is at a spot 75% between the A 1  area and the full line A 2 . 
         [0032]    With the oil reservoir  26  as shown, the smaller cross-sectional area in the lower portions, such as at A 1 , ensures that even a minimum amount of oil in the reservoir  26  will maintain oil upwardly toward the worst case line W, and certainly above the bottom  102  at normal or extreme attitudes. Even a small quantity of oil will be sufficient to fill the small area A 1 . 
         [0033]    On the other hand, the larger cross-sectional areas, A 2  and A 3 , for example, and all of the areas in the immediate vicinity, will ensure that even greater quantities of oil will not result in the fill line ramping upwardly and into the gear case, such that the gears themselves would be not submerged. That is, having dramatically larger cross-sectional areas ensures that a good deal more oil will be required to raise the level once the oil begins to reach into the larger cross-sectional area portions of the oil reservoir. 
         [0034]      FIG. 6  shows the possible aircraft attitude limits for a modern helicopter. As shown, the helicopter might experience any combination of nose down or nose up, along with left or right inclination or roll. The interior portion of this graph shows conditions that must be provided for continuous operation, and that means long-term oil supply. The areas between the innermost shape and the outermost limit require operation for a predetermined limited period of time (30 seconds for example). With the shape-optimized oil reservoir of this application, these goals are achieved. 
         [0035]    Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.