Abstract:
The present invention meets these objectives providing an IDG hydraulic system that utilizes in flow series arrangement a scavenge pump, a spool valve and a boost pump. When spool valve detects an interruption in the supply pressure to the boost pump due to an adverse “g” condition, it reconfigures the system to a closed loop system. In this mode, the oil returning from the CVT is re-circulated back to the boost pump instead of back to the scavenge pump. Oil lost to leakage is replenished by an oil accumulator that use the gas pressure in the IDG&#39;s casing to expel fluid from the accumulator into the recirculating flow.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of copending U.S. patent application Ser. No. 09/482,212, filed on Jan. 12, 2000 which claims priority to provisional patent application Ser. No. 60/161,157 filed Oct. 22, 1999 which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to accumulators and in particular to an accumulator, used in an airborne, hydraulically operated integrated drive generator (IDG), for supplying hydraulic fluid during negative or zero “g” conditions. 
     BACKGROUND OF THE INVENTION 
     An integrated drive generator, (IDG), is an integral unit having a constant speed drive continuously variable transmission (CVT) and an electric generator in the same housing. The IDG converts variable speed rotary input from a shaft of an aircraft propulsion engine into a constant speed shaft drive which drives the electrical generator typically producing constant frequency three phase 400 Hz power. The transmission can be any of a plurality of continuously variable transmissions such as a hydrostatic pump/motor assembly, a traction drive, or pulley varidrive. 
     Referring to FIG. 1, a typical IDG  10  includes a pressurized casing  12  which acts as an oil sump by collecting hydraulic fluid  26  at its bottom. To maintain constant speed control of the CVT, oil from the sump is pumped to the CVT controls. In a negative “g” condition, the oil is forced from the bottom of the sump to the top and in a zero “g” condition the oil floats in the middle between the top and the bottom. Both these conditions are referred to as adverse “g” conditions and when either occurs, the oil quantity at the bottom of the sump drops and the flow to the CVT is interrupted. Such an interruption or drop in oil flow to the CVT will result in loss of speed control. This means that the CVT will be unable to hold a constant generator speed which will cause an automatic shut down of the IDG. 
     Commercial aircraft experience zero or negative “g” conditions under a variety of circumstances such as severe weather or emergency maneuvers. Clearly, when these circumstances occur it is important that the CVT continue to maintain constant generator speed, otherwise the aircraft will lose electric power. As a consequence, IDGs used on commercial aircraft are typically required to operate normally for a duration of 15 seconds of zero or negative “g” forces. 
     One method used to meet this 15 second requirement is to provide a second pump for pumping oil from the top of the sump when negative “g” is experienced. This method has had only limited success, because mounted in the sump are a plurality of rotating components which inhibit the flow of oil from the bottom to the top. Instead of the oil flowing smoothly to the top, it gets flung around the casing by these rotating components. Further, this method does not address the zero “g” conditions where oil tends to float in the middle of the sump. 
     Copending U.S. patent application Ser. No. 09/482,212, which is assigned to the assignee of this application, discloses an IDG hydraulic system that overcomes the disadvantages of the prior art systems by utilizing in flow series arrangement of a scavenge pump, a spool valve and a boost pump. When spool valve detects an interruption in the supply pressure to the boost pump due to an adverse “g” condition, it reconfigures the system to a closed loop system. In this mode, the oil returning from the CVT is re-circulated back to the boost pump instead of back to the scavenge pump. A small oil accumulator is used to make up for leakage in the system. Typically, these oil accumulators utilize a mechanical spring acting on a piston or a bladder acted upon by a charged gas volume. Both of these types of accumulators add cost and volume to the design. 
     Accordingly, there is a need for an improved piston type accumulator that does not require a spring or other mechanical actuator. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a fluid accumulator for use in an hydraulic control system for an integrated drive generator having a constant speed drive variable transmission for providing hydraulic fluid during an adverse “g” event. 
     The present invention meets these objectives providing an IDG hydraulic system that utilizes in flow series arrangement of a scavenge pump, a spool valve and a boost pump. When spool valve detects an interruption in the supply pressure to the boost pump due to an adverse “g” condition, it reconfigures the system to a closed loop system. In this mode, the oil returning from the CVT is re-circulated back to the boost pump instead of back to the scavenge pump. Oil lost to leakage is replenished by an oil accumulator that uses the gas pressure in the IDG&#39;s casing to expel fluid from the accumulator into the recirculating flow. 
     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 is a cross section of an IDG of the type that can be used on an aircraft having the accumulator contemplated by the present invention. 
     FIG. 2 is a schematic of the hydraulic system with the accumulator contemplated by the present invention. 
     FIG. 3A is an enlarged view of the spool valve of the hydraulic system of FIG. 2 during normal, positive “g” operation. 
     FIG. 3B is an enlarged view of the spool valve of the hydraulic system of FIG. 2 during adverse “g” operation. 
     FIG. 4 is a cutaway, perspective view of the IDG having the accumulator contemplated by the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an integrated drive generator (IDG) is generally denoted by reference numeral  10 . The IDG is comprised of a pressurized casing or housing  12 . Mounted in the casing  12  is a CVT  14  having a input pad  16  that receives a variable speed input and an output gear  20  that drives an idler gear  22  and generator  24  at a constant speed of typically 12,000 or 24,000 rpm. The generator  24  is a conventional brushless generator that produces 400 Hz power. The casing  12  acts as the sump for the hydraulic fluid or oil that is used to control the CVT. During the normal operation the oil collects at the bottom of the casing as represented by area  26 . 
     Referring to FIG. 2, the flow of the hydraulic fluid to the IDG is managed by a hydraulic system  30 . Oil is pumped through oil pickup  27  in the sump  12  by a scavenge pump  32  and then flowed to an air/oil separator  34  which, in a manner familiar to those skilled in the art, removes air from the fluid. From the air/oil separator, the oil flows to a supply pump  36  which pressurizes the oil. A pressure regulator  38  regulates the output of the supply pump to a preferable 65 psi (4.5 bar). A bypass  39  is provided to return bleed off flow from the regulator  38  back to the scavenge pump inlet. From the pressure regulator  38 , the oil flows through an external heat exchanger  40  where the oil is cooled. From the heat exchanger  40  approximately 85% of the oil is flowed, as represented by arrow  41 , to various parts of the IDG where it is used for cooling and/or lubrication. The remaining 15% flows to a spool valve  50 . 
     Referring to FIGS. 3A and 3B, the spool valve  50  is a conventional spool valve having a piston  52  slideably mounted within a casing  54 . The piston  52  is comprised of three block portions  56 ,  58 ,  60  connected by rod portions  57  and  59 . The rod portions have a diameter less than the block portions and the block portions are sized to seal against the inner surface of the casing thereby defining chambers  62  and  64 . The end of the block portion  56  is adjacent to a supply port  70  and the end of the block portion  60  is mounted against a spring  65  which is mounted within the casing  54 . In addition to the supply port  70 , the valve  50  has inlet ports  72 ,  74 ,  76  and outlet ports  71 ,  73 , and  75 . It also has a vent  61  through which any leakage flow around block portion  60  can escape. This vent  61  prevents leakage flow from being trapped which could inhibit the movement of the piston  52 . Inlet port  72  and the supply port  70  receive the 15% flow from the heat exchanger  40  while inlet ports  74  and  76  receive the oil flow returning from the CVT. Outlet ports  71  and  73  communicate with a boost pump  44  which delivers pressurized oil to the CVT. Outlet port  75  communicates back to an intermediate point  31  between the scavenge pump  32  and the sump  12  where the return flow is added to the oil flow from the sump. Between the spool valve  50  and the intermediate point  31 , the return flow from the generator  24  is added to the return flow from the CVT  14 . 
     Referring to FIG. 3A, during normal operation the pressure at the supply port  70  is about  65  psi which forces the piston  52  to the right against the spring. This places chamber  62  in fluid communication with inlet port  72  and outlet port  71  and places chamber  64  in fluid communication with inlet port  76  and outlet port  75 . Oil from the heat exchanger  40  flows through the inlet port  72 , outlet port  71  to boost pump  44  and then to the CVF  14 . Oil returning from the CVT  14  flows through inlet port  76 , outlet port  75  and then back to the intermediate point  31 . 
     When an adverse “g” condition is encountered, see FIG. 3B, the oil is thrown away from the bottom of the casing  12 . As there is no oil to be scavenged from the sump, the pressure at the supply port  70  drops to almost case ambient pressure so that the pressures on both sides of the piston  52  are nearly balanced. Looking at the figures, the spring  65  now pushes the piston  60  towards the left so that inlet ports  72  and  76  and outlet ports  71  and  75  are blocked by the piston  60  and inlet port  74  and outlet port  73  open and communicate through chamber  64 . As a result, the oil returning from the CVT is re-circulated through the spool valve  50  back to the boost pump  44  which continues to supply the CVT with oil at an appropriate pressure. 
     Because oil leaks from the CVT, an accumulator  100  is activated to maintain a sufficient flow of oil. Referring to FIGS. 2 and 4, the accumulator  100  comprises a cylindrical casing  102  extending from an air port  104  to an oil port  112 . The air port  104  has a hole, also referred to as a vent,  106  which places the interior of the casing  102  in fluid communication with the interior of the casing  12 . During the starting of the CVT, the air pressure within the casing  12  is about 14.7 psi (1.0 bar) at sea level and then increases due to the heating of the air within the casing  12  and is maintained throughout the operating envelope of the CVT by appropriate sealing of the casing. Disposed within the casing  102  is a piston  108  that is slidably mounted within the casing  102  on low friction seals  110 . A passage  114  places the interior of the casing  102  with a conduit  116 , shown in FIG. 2, extending from the spool valve  50  to the boost pump  44 . The accumulator  100  is mounted in the internal structure of the casing  12  by an annular retaining plate  118 . The plate  118  is mounted around the casing  102  and bolted to the internal structure of the casing  12 . In the preferred embodiment, two low friction seals are used. Alternatively, the piston can be replaced with a diaphragm or bladder. 
     During normal operation of the CVT, the oil pressure in the conduit  116  is greater than the air pressure in the casing  12 . As a result, the piston  108  is pushed toward the air port  104  expelling the air in the cylindrical casing  102  and replacing it with oil flowing through oil port  112  and passage  114  . When an adverse “g” event occurs, the oil pressure in the conduit  116  drops below the air pressure in the casing  12  and the piston is pushed away from the air port  104  to the oil port  112  forcing the stored oil out through the passage  114  and to the conduit  116 . This out flow of oil from the accumulator  100  is also assisted by a suction created by the boost pump  44 . The duration for which this system  30  can operate depends on the size of the accumulator  100 . 
     Once normal operation returns, pressure in the conduit rises and the piston  60  moves to the right until its return to its original position. At this time, the accumulator  100  is recharged. 
     Although the invention has been described in terms of an IDG used on an aircraft, it will be appreciated by those skilled in the art that the invention can be used for any hydraulically controlled mechanical system that may experience adverse “g” conditions. Accordingly, various changes and modifications may be made to the illustrative embodiment without departing from the spirit or scope of the invention. It is intended that the scope of the invention not be limited in any way to the illustrative embodiment shown and described, but that the invention be limited only by the claims appended hereto.