Abstract:
A disconnect assembly for an integrated drive generator includes an input shaft configured to receive a rotational input. A coupling member is selectively coupled to the input shaft by dog teeth. A biasing element is configured to disconnect the dog teeth and decouple the coupling member from the input shaft. A thermal coupling opposes the biasing element to maintain engagement between the dog teeth in an unmelted state. The thermal coupling is constructed from a eutectic solder material of 91.3% tin and 8.7% zinc by weight.

Description:
BACKGROUND 
       [0001]    This disclosure relates to a disconnect assembly for an integrated drive generator, for example. The integrated drive generator may be used in aerospace applications, for example. 
         [0002]    One example type of integrated drive generator (IDG) includes a generator, a hydraulic unit and a differential assembly arranged in a common housing. The differential assembly is operatively coupled to a gas turbine engine via an input shaft. The rotational speed of the input shaft varies during the operation of the gas turbine engine. The hydraulic unit cooperates with the differential assembly to provide a constant speed to the generator throughout engine operation. 
         [0003]    The differential assembly receives an input from an input shaft, which may be operatively coupled to a gas turbine engine spool. The hydraulic unit converts the variable rotational speed of the input shaft to provide a fixed output rotational speed to the generator. It may be desirable to disconnect the rotational drive to the generator during certain undesired conditions. For example, during high temperatures, the rotational drive from the input shaft may be disconnected from the generator to protect the generator or other components from damage. 
         [0004]    The input shaft, which is steel, typically includes a weakened area in the event that components within the IDG bind. The weakened area breaks under such circumstances, protecting the mechanical components upstream from the IDG. Previously, this weakened area had a nominal diameter of 0.477 inch (1.21 cm) at a nominal hardness of 40 HRC. The input shaft had a shear torque (in-lbs) to nominal diameter (in) ratio of 6960-7470, which may break earlier than desired in some applications. 
         [0005]    One type of disconnect assembly has used a thermal coupling constructed from a eutectic solder made from an alloy of tin, lead and silver, which melts at 354° F. (179° C.). The thermal coupling melts at undesirably high temperatures to uncouple the input shaft from the differential assembly and cease rotational drive to the generator, preventing the generator from overheating. The disconnect assembly includes a biasing element that urges engageable dog teeth between the input shaft and another structure out of engagement with one another once the thermal coupling material has melted. This thermal coupling material is subject to creep and may compress more than desired during a period of operation that is less than the desired maintenance period, enabling the dog teeth to at least partially disengage from one another. 
       SUMMARY 
       [0006]    In one exemplary embodiment, a disconnect assembly for an integrated drive generator includes an input shaft configured to receive a rotational input. A coupling member is selectively coupled to the input shaft by dog teeth. A biasing element is configured to disconnect the dog teeth and decouple the coupling member from the input shaft. A thermal coupling opposes the biasing element to maintain engagement between the dog teeth in an unmelted state. The thermal coupling is constructed from a eutectic solder material of 91.3% tin and 8.7% zinc by weight. 
         [0007]    In a further embodiment of any of the above, the disconnect assembly includes a generator operatively coupled to the input shaft by the coupling member with the dog teeth engaged with one another. 
         [0008]    In a further embodiment of any of the above, the disconnect assembly includes a differential assembly operatively connected between the input shaft and the generator. 
         [0009]    In a further embodiment of any of the above, the differential assembly includes a carrier. The coupling member is rotationally fixed relative to the carrier by a splined connection. The coupling member is axially slidable relative to the carrier. 
         [0010]    In a further embodiment of any of the above, the thermal coupling has a melting temperature of 390° F. (199° C.). 
         [0011]    In another exemplary embodiment, a method of disconnecting rotational drive elements from one another, includes the steps of melting a thermal coupling at a melting point of 390° F. (199° C.), and urging dog teeth out of engagement with one another. 
         [0012]    In a further embodiment of any of the above, the thermal coupling is a eutectic solder material of 91.3% tin and 8.7% zinc by weight. 
         [0013]    In another exemplary embodiment, a disconnect assembly for an integrated drive generator includes an input shaft configured to receive a rotational input. The input shaft has first and second ends. Splines are arranged at the first end and have a spline diameter provided by a spline contact line. Dog teeth are arranged at the second end and has a dog tooth contact line. A weakened area has a weakened diameter. The input shaft has a shear torque (in-lbs) to nominal weakened diameter (in) ratio of 9390 to 10,090. 
         [0014]    In a further embodiment of any of the above, the ratio of the spline diameter relative to the weakened diameter is 1.39 to 1.48. 
         [0015]    In a further embodiment of any of the above, the input shaft has a length extending from the dog tooth contact line to the first end at the spline contact line, and a ratio of the length to the weakened diameter of 5.38 to 5.70. 
         [0016]    In a further embodiment of any of the above, the length is in the range of 3.083 inch (7.83 cm) to 3.109 inch (7.90 cm). 
         [0017]    In a further embodiment of any of the above, the weakened area is an arcuate groove circumscribing an outer diameter of the input shaft. 
         [0018]    In another exemplary embodiment, an integrated drive generator includes a generator. The integrated drive generator includes a differential assembly and a hydraulic unit arranged within a common housing. An input shaft is configured to receive a rotational input. The hydraulic unit is configured to cooperate with the differential assembly to convert the variable rotational speed from the input shaft to provide a fixed rotational output speed to the generator. The input shaft has first and second ends. Splines are arranged at the first end and has a spline diameter provided by a spline contact line. Dog teeth are arranged at the second end and have a dog tooth contact line. The input shaft includes a weakened area having a weakened diameter. The input shaft has a shear torque (in-lbs) to nominal weakened diameter (in) ratio of 9390 to 10,090. A coupling member is selectively coupled to the input shaft by dog teeth. A biasing element is configured to disconnect the dog teeth and decouple the coupling member from the input shaft. A thermal coupling opposes the biasing element to maintain engagement between the dog teeth in an unmelted state. The thermal coupling is constructed from a eutectic solder material of 91.3% tin and 8.7% zinc by weight. 
         [0019]    In a further embodiment of any of the above, the integrated drive generator includes a generator operatively coupled to the input shaft by the coupling member with the dog teeth engaged with one another. 
         [0020]    In a further embodiment of any of the above, the integrated drive generator includes a differential assembly operatively connected between the input shaft and the generator. 
         [0021]    In a further embodiment of any of the above, the differential assembly includes a carrier. The coupling member is rotationally fixed relative to the carrier by a splined connection. The coupling member is axially slidable relative to the carrier. 
         [0022]    In a further embodiment of any of the above, the thermal coupling has a melting temperature of 390° F. (199° C.). 
         [0023]    In a further embodiment of any of the above, the ratio of the spline diameter relative to the weakened diameter is 1.39 to 1.48. 
         [0024]    In a further embodiment of any of the above, the input shaft has a length extending from the dog tooth contact line to the first end at the spline contact line, and a ratio of the length to the weakened diameter of 5.38 to 5.70. 
         [0025]    In a further embodiment of any of the above, the length is in the range of 3.083 inch (7.83 cm) to 3.109 inch (7.90 cm). 
         [0026]    In a further embodiment of any of the above, the weakened area is an arcuate groove circumscribing an outer diameter of the input shaft. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The disclosure can be further understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
           [0028]      FIG. 1  is a highly schematic view of a generator system. 
           [0029]      FIG. 2  is a cross-sectional schematic view of an example integrated drive generator. 
           [0030]      FIG. 3  is a schematic perspective view of a generator, a hydraulic unit and a differential assembly of the integrated drive generator shown in  FIG. 2 . 
           [0031]      FIG. 4  is a partial cross-sectional view of a differential assembly having a disconnect assembly with an input shaft. 
           [0032]      FIG. 5  is partial cross-sectional view of the input shaft decoupled from the differential assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    An example generator system  10  is schematically illustrated in  FIG. 1 . The system  10  includes a gas turbine engine  12  that provides rotational drive to an integrated drive generator (IDG)  16  through an accessory drive gearbox  14  mounted on the gas turbine engine  12 . The accessory drive gearbox  14  is coupled to a spool of the engine  12 , and the speed of the spool varies throughout engine operation. 
         [0034]    Referring to  FIGS. 2 and 3 , an example IDG  16  is illustrated. In the example, the IDG  16  includes a housing  18  having generator, center and input housing portions  20 ,  22 ,  24  secured to one another. A generator  40  is arranged in the generator housing portion  20 . Seal plates  23  are provided on either side of the center housing  22  to seal the center housing  22  relative to the generator and input housing portions  20 ,  24 . 
         [0035]    An input shaft  26  receives rotational drive from the accessory drive gearbox  14 . The rotational speed of the input shaft  26  varies depending upon the operation of the engine  12 . To this end, as a result, a hydraulic unit  32  cooperates with the differential assembly  75  to convert the variable rotational speed from the input shaft  26  to provide a fixed rotational output speed to the generator  40 . 
         [0036]    The input shaft  26  rotationally drives a differential input gear  30  that is coupled to a hydraulic input gear  34  of the hydraulic unit  32 . The differential input gear  30  is operatively coupled to the input shaft  26  by the disconnect assembly  27 . The hydraulic output gear  36  is coupled to a speed trim output gear  38 . The hydraulic unit  32  increases or decreases the rotational speed provided to the differential unit  75  from the hydraulic output gear  36  to provide a fixed rotational output speed, such as a 12,000 rpm speed. The variable rotational speed of the differential input gear  30  combines with the speed of the differential trim gear  38  to provide a fixed rotational speed to a differential output gear  28  and generator input shaft  42 . 
         [0037]    In the example, a gear train  44  cooperates with the generator input shaft  42 , which rotates at a constant speed to rotationally drive a charge pump  46 , deaerator  48 , main scavenge pump  50 , inversion pump  52  and generator scavenge pump  54 . Thus, these components may be designed efficiently to operate at a fixed speed. 
         [0038]    Referring to  FIG. 4 , the differential assembly  75  includes a carrier  60  that cooperates with the input shaft  26  to transfer rotational drive from the input shaft  26  to the differential input gear  30 . In the example, the carrier  60  includes an internal spline connection  62  that cooperates with a coupling member  66 . An external spline connection  64  cooperates with the differential input gear  30  to rotationally fix the differential input gear  30  relative to the carrier  60 . 
         [0039]    The internal spline connection  62  rotationally fixes the coupling member  66  to the carrier  60  while permitting the coupling member  66  to slide axially with respect to the carrier  60 . A coupling of dog teeth  68  between the input shaft  26  and the coupling member  66  rotationally couples the input shaft  26  to the carrier  60  with the dog teeth  68  engaged with one another. The disconnect assembly  27  urges the coupling member  66  axially away from the input shaft  26  during high heat conditions enabling the dog teeth  68  to disengage from one another and provide a gap  89 , as illustrated in  FIG. 5 . With the coupling member  66  in the position illustrated in  FIG. 5 , the input shaft  26  spins freely relative to the carrier  60 , transferring no rotational input to the differential input gear  30 . In this manner, rotational drive to the generator  40  ( FIG. 1 ) ceases to prevent the generator  40  ( FIG. 1 ) from overheating and protects the IDG  16  ( FIG. 1 ). 
         [0040]    The carrier  60  supports an array of gears  72 , shown in  FIGS. 3 and 4 . It should be appreciated that the positioning of the components as illustrated in  FIG. 3  is highly schematic in nature and may not necessarily correspond to the physical location of these components relative to one another, as appreciated by reference to  FIG. 4 . The gear  72  includes teeth  74  that cooperate with one another and intermesh with the differential speed trim gear  38  to receive rotational input from the hydraulic unit  32 . 
         [0041]    The disconnect assembly  27  includes first and second members  76 ,  78  that are concentric with one another. The second member  78  is slideably supported with respect to an outer diameter of the first member  76 . A fastener  80  secures the first member  76  to the carrier  60 . A thermal coupling  84  is arranged between the first and second members  76 ,  78 . The dog teeth include a small engagement angle that permits a compressive force to be applied to the thermal coupling  84 . The second member  78  engages a flange  82  of the coupling member  66 . The spacing of the first and second members  76 ,  78  and the thermal coupling  84  are such that full engagement of the dog teeth  68  is maintained during normal operating temperatures. A biasing element  88 , such as a helical spring, is provided between the flange  82  and a seat  86  to apply a biasing force to the coupling member  66  in a direction away from the input shaft  26 . 
         [0042]    At undesirably high temperatures, the thermal coupling material melts, which permits the biasing element  88  to urge the coupling member  66  away from the input shaft  26  such that the dog teeth  68  are separated from one another to provide the gap  89 , as shown in  FIG. 5 . In one example, the thermal coupling  84  is provided by a eutectic solder material of 91.3% tin and 8.7% zinc by weight, for example. This composition melts at 390° F. (199° C.). The thermal coupling  84  exhibits good creep characteristics, for example, about  4 . 4  times high compressive stress capability and  49  times longer life for the same operating parameters than the previous thermal coupling material that melted at 354° F. (179° C.). 
         [0043]    The input shaft  26  includes splines  91  at one end, which engage with corresponding structure of the accessory drive gearbox  14 . The dog teeth  68  are arranged at the opposite end of the shaft relative to the splines  91 . An intermediate region of the input shaft  26  includes a weakened area  90  provided by an arcuate groove circumscribing an outer diameter of the input shaft  26 . The weakened area  90  is designed to break at a particular torque to prevent damage to the accessory drive box  14  and engine  12 . The weakened area  90  includes a diameter  94 , which may be 0.559 inch (1.42 cm) nominally at a nominal 40 HRC. The input shaft  26  has a shear torque (in-lbs) to nominal diameter (in) ratio of 9390-10,090. The splines  91  include a spline pitch diameter  92  defined relative to a spline tooth contact line  98 . The ratio of the spline diameter  92  relative to the weakened diameter  94  is 1.39 to 1.48. 
         [0044]    The dog teeth  68  of the input shaft  26  include a dog tooth contact line  100 . The input shaft  26  has a length  96  extending from the dog tooth contact line  100  to an end of the input shaft  26  at the spline contact line  98 . The length  96  is in the range of 3.083 inch (7.83 cm) to 3.109 inch (7.90 cm). The ratio of the length  96  to the weakened diameter  94  is 5.38 inch (13.7 cm) to 5.70 inch (14.5 cm). In this manner, the input shaft is able to withstand greater torque without fracturing. 
         [0045]    Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the claims. For that reason, the following claims should be studied to determine their true scope and content.