Abstract:
A lubrication system includes a bearing compartment. A main reservoir is fluidly connected to the bearing compartment by a main supply passage. A main pump is arranged in the main supply passage configured to provide fluid from the main reservoir to the bearing compartment during a positive gravity condition. A secondary supply passage fluidly connects the main reservoir to at least one segment of the main supply passage, thereby providing fluid from the main reservoir to the bearing compartment during a negative gravity condition. A method of supplying a bearing compartment with fluid includes pumping a fluid from a main reservoir to a bearing compartment through a main supply passage during a positive gravity condition, and providing fluid from the main reservoir to the bearing compartment through a secondary supply passage, fluidly connected to at least one segment of the main supply passage, in response to a negative gravity condition.

Description:
BACKGROUND 
     This disclosure relates to a lubrication system, and more particularly, to a lubrication system for a fan drive gear system in gas turbine engines. 
     In many gas turbine engines, a low pressure spool includes a low pressure turbine connected to and driving a low pressure compressor, and a high pressure spool includes a high pressure turbine connected to and driving a high pressure compressor. A main pump is typically driven by the high pressure spool, connected through gearing, and is used to pump lubricating and cooling fluid to all engine components that require lubrication and cooling. 
     The main pump typically pumps fluid from a passage connected to a main reservoir that holds both liquid and air. During normal operating conditions, the fluid settles at the bottom of the main reservoir and displaces air to the top. However, in a gas turbine engine mounted on an aircraft, the main reservoir may experience “negative gravity” conditions such as the aircraft turning upside down, the aircraft accelerating toward the Earth at a rate equal to or greater than the rate of gravity, or the aircraft decelerating at the end of a vertical ascent. Under negative gravity conditions, the fluid in the main reservoir can rise to the top, which can expose an opening of the passage to air and interrupt the supply of fluid to the main pump and, consequently, interrupt supply to the engine components. Certain engine components, such as gears and bearings, can be damaged by a relatively short period of non-lubricated operation during negative gravity conditions. 
     In some gas turbine engines, a fan at the front of the engine is connected to the low pressure spool through a fan drive gear system. When the high pressure spool stops rotating or rotates at a reduced rpm (revolutions per minute), the fan drive gear system can continue rotating even though the main pump will ordinarily provide little or no fluid during this time. For example, wind may rotate the fan and corresponding gears and bearings while the aircraft is parked on the ground or during an in-flight engine shutdown. Certain gears and bearings can also be damaged by a relatively short period of non-lubricated operation during windmilling as well. 
     SUMMARY 
     In one exemplary embodiment, a lubrication system for a gas turbine engine includes a bearing compartment. A main reservoir is fluidly connected to the bearing compartment by a main supply passage, which includes one or more passage segments therein. A main pump is arranged in the main supply passage configured to provide fluid from the main reservoir to the bearing compartment during a positive gravity condition. A secondary supply passage fluidly connects the main reservoir to at least one segment of the main supply passage, thereby providing fluid from the main reservoir to the bearing compartment during a negative gravity condition. 
     In a further embodiment of any of the above, the main reservoir includes upper and lower portions. The main supply passage is in fluid communication with the lower portion, and the secondary supply passage is in fluid communication with the upper portion. 
     In a further embodiment of any of the above, the system includes a first check valve arranged in the secondary supply passage fluidly between the main reservoir and the main supply passage. 
     In a further embodiment of any of the above, the system includes a bleed passage that is fluidly connected to the secondary passage and is configured to bypass the first check valve. 
     In a further embodiment of any of the above, the main reservoir includes a vent having a pressure valve that is configured to maintain a desired pressure in the main reservoir and open the first check valve in the negative gravity condition. 
     In a further embodiment of any of the above, the desired pressure is at least 20 psia (137.90 kPa). 
     In a further embodiment of any of the above, the system includes a second check valve in the main supply passage and is arranged upstream from an intersection of the main supply passage with the secondary supply passage. 
     In a further embodiment of any of the above, the bearing compartment includes a sump, and a scavenge pump that fluidly connects the sump to the main reservoir. 
     In a further embodiment of any of the above, the system includes an auxiliary reservoir and an auxiliary pump that is fluidly connected between the auxiliary reservoir and the bearing compartment via an auxiliary passage. 
     In a further embodiment of any of the above, the system includes a shuttle valve that interconnects the auxiliary passage and the main supply passage and is arranged upstream from the auxiliary and main pumps. 
     In a further embodiment of any of the above, the system includes a check valve that is provided in the main reservoir selectively movable in response to the positive and negative gravity conditions. 
     In a further embodiment of any of the above, the main supply passage includes an ejector that is fluidly connected to the secondary supply passage at an ejector throat. 
     In another exemplary embodiment, a method of supplying a bearing compartment with fluid includes the steps of pumping a fluid from a main reservoir to a bearing compartment through a main supply passage, which includes one or more passage segments therein, during a positive gravity condition, and providing fluid from the main reservoir to the bearing compartment through a secondary supply passage, fluidly connected to at least one segment of the main supply passage, in response to a negative gravity condition. 
     In a further embodiment of any of the above, the providing step includes flowing fluid past a first check valve arranged in the secondary supply passage. 
     In a further embodiment of any of the above, the pumping step includes flowing fluid past a second check valve in the main supply passage. 
     In a further embodiment of any of the above, the providing step includes pressurizing the main reservoir to open the first check valve. 
     In a further embodiment of any of the above, the pumping step includes bleeding a fluid past the first check valve to fill the secondary supply passage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure can be further understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
         FIG. 1  is a schematic cross-sectional side view of a gas turbine engine with a fan drive gear system. 
         FIG. 2  is a schematic view of a pump system for the example gas turbine engine. 
         FIG. 3A  is a schematic view of a second shuttle valve in a first position. 
         FIG. 3B  is a schematic view of the second shuttle valve in a second position. 
         FIG. 4A  is a schematic view of a first shuttle valve in a first position. 
         FIG. 4B  is a schematic view of the first shuttle valve in a second position. 
         FIG. 5A  is a schematic view of another embodiment of the first shuttle valve in a first position. 
         FIG. 5B  is a schematic view of another embodiment of the first shuttle valve in a second position. 
         FIG. 6  is a schematic view of a portion of the pump system having a check valve that provides lubrication fluid to the bearing compartment during a negative gravity condition. 
         FIG. 7A  schematically depicts another check valve during a positive gravity condition. 
         FIG. 7B  schematically depicts the check valve of  FIG. 7A  in a negative gravity condition. 
         FIG. 8  is a schematic view of another example fluid supply system for a negative gravity condition. 
     
    
    
     DETAILED DESCRIPTION 
     In general, the disclosure relates to a pump system for lubricating bearings in a fan drive gear system. The pump system includes a main pump for supplying lubricating fluid, such as oil, during ordinary engine operating conditions, an auxiliary pump for supplying the fluid when the main pump loses pressure, and a valve for selecting between the two. The pump system also includes a sump for supplying the fluid to the auxiliary pump during windmill conditions, an auxiliary reservoir for supplying the fluid to the auxiliary pump during zero and negative gravity conditions, and another valve for selecting between those two. 
       FIG. 1  is a schematic cross-sectional side view of gas turbine engine  10 . Gas turbine engine  10  includes low pressure spool  12  (which includes low pressure compressor  14  and low pressure turbine  16  connected by low pressure shaft  18 ), high pressure spool  20  (which includes high pressure compressor  22  and high pressure turbine  24  connected by high pressure shaft  26 ), combustor  28 , fuel pump  29 , nacelle  30 , fan  32 , fan shaft  34 , and fan drive gear system  36  (which includes star gear  38 , ring gear  40 , and sun gear  42 ). The general construction and operation of gas turbine engines is well-known in the art, and therefore detailed discussion here is unnecessary. However, a more detailed understanding of fan drive gear system  36  can be helpful. 
     As shown in  FIG. 1 , low pressure spool  12  is coupled to fan shaft  34  via fan drive gear system  36 . In the illustrated embodiment, fan drive gear system  36  is a “star gear system”. Sun gear  42  is attached to and rotates with low pressure shaft  18 . Ring gear  40  is rigidly connected to fan shaft  34  which turns at the same speed as fan  32 . Star gear  38  is coupled between sun gear  42  and ring gear  40  such that star gear  38  revolves about its axis, when sun gear  42  rotates. When low pressure spool  12  rotates, fan drive gear system  36  causes fan shaft  34  to rotate at a slower rotational velocity than that of low pressure spool  12 . This allows fan  32  and low pressure spool  12  to rotate at different speeds for improved operation of both of fan  32  and low pressure spool  12 . In an alternative embodiment, fan drive gear system  36  can be a “planetary gear system”. In a planetary gear system, ring gear  40  is fixed and fan shaft  34  is attached to a carrier (not shown) that carries star gear  38  (also called a planet gear). Star gear  38  orbits about sun gear  42  as it spins between sun gear  42  and ring gear  40 . Other fan drive gear systems may also be employed. 
     Pump  44  is coupled to and is driven by fan shaft  34  via pump gear  46  such that pump  44  can operate whenever fan shaft  34  is rotating. Pump  44  supplies fluid to lubricate gears and bearings of fan drive gear system  36 . Fan drive gear system  36  benefits from a relatively continuous supply of lubricating fluid whenever fan shaft  34  is rotating. At least some of the fluid supplied to fan drive gear system  36  drains to sump  48  and is eventually pumped back through pump  44 . In an alternative embodiment, pump  44  can be an electrically driven oil pump. 
       FIG. 2  is a schematic view of pump system  50 . Pump system  50  includes bearing compartment  52  having a compartment cavity that contains fan drive gear system  36  (including bearings  54 ), auxiliary pump  44 , gutter  56 , auxiliary reservoir  58 , and first shuttle valve  60 . Pump system  50  also includes second shuttle valve  62 , main reservoir  64 , main pump  66 , and scavenge pump  67  positioned outside of bearing compartment  52 . Passages  68 ,  70 ,  72 ,  74 ,  76 ,  78 ,  80 ,  82 ,  84 ,  86 , and  88 , referred to interchangeably as passages and passage segments, connect the various components as illustrated and as further described, below. 
     As fan drive gear system  36  spins, lubricating fluid drips or flies off fan drive gear system  36  into bearing compartment  52  in different directions. A portion of that fluid is caught and collected by gutter  56  and funneled to auxiliary reservoir  58 . During normal operating conditions, auxiliary reservoir  58  is kept substantially full of fluid for later use. In one embodiment, auxiliary reservoir  58  contains enough fluid to provide adequate lubrication for fan drive gear system  36  for at least 10 seconds. Gutter  56  does not collect all fluid leaving fan drive gear system  36 . The remaining fluid that is not collected by gutter  56  falls to sump  48 , which is an open-top reservoir at a bottom of bearing compartment  52 . Bearing compartment  52  can be sealed to reduce fluid flow out of bearing compartment  52 , except through designated passages as herein described. 
     First shuttle valve  60  is fluidly connected to auxiliary reservoir  58  via passage  68 , to sump  48  via passage  70 , to auxiliary pump  44  via passage  72 , and again to sump  48  via passage  74 . First shuttle valve  60  selectively directs fluid flow from auxiliary reservoir  58  or sump  48  to auxiliary pump  44  in a manner further described below with respect to  FIGS. 3A and 3B . 
     Second shuttle valve  62  is fluidly connected to auxiliary pump  44  via passage  76 , to main pump  66  via passage  78 , to bearings  54  via passage  80 , and to main reservoir  64  via passages  82  and  88 . In the illustrated embodiment, passage  76  is an auxiliary supply passage and passage  78  is a main supply passage. Second shuttle valve  62  selectively directs fluid flow from auxiliary pump  44  or main pump  66  to bearings  54  in a manner further described below with respect to  FIGS. 4A and 4B . Main reservoir  64  is further connected to main pump  66  through passage  84 . Scavenge pump  67  is connected to sump  48  via passage  86  and to main reservoir  64  via passage  88 . Scavenge pump  67  pumps a portion of the fluid in sump  48  to main reservoir  64  for use by main pump  66 . It should be appreciated that passages  84 ,  78  and  80 , can be referred to as passage segments that, collectively, form a main supply passage between the main reservoir and the bearing compartment during normal conditions. 
     As part of pump system  50 , first shuttle valve  60  and second shuttle valve  62  work together as a valve system. This valve system directs lubricating fluid to bearings  54  from one of sump  48 , auxiliary reservoir  58 , or main reservoir  64 . The pump system selects among these potential sources of lubricating fluid based upon sensed engine operating conditions. 
       FIG. 3A  is a schematic view of second shuttle valve  62  in a first position. Second shuttle valve  62  includes valve passages  100 ,  102 ,  104 , and  106 , sensor  108 , sensor passage  110 , and spring  112 . In the first position, valve passage  100  is aligned with passages  78  and  80  to direct flow from main pump  66  to bearings  54 . Valve passage  102  is aligned with passages  76  and  82  to direct flow from auxiliary pump  44  to passage  88  which is connected to main reservoir  64 . In an alternative embodiment, passage  82  can be connected directly to main reservoir  64 , bypassing passage  88 . In yet another embodiment, passage  82  can be connected to sump  48  instead of main reservoir  64 . In the illustrated embodiment, valve passages  104  and  106  are not aligned with any exterior passages. Valve passages  104  and  106  are, effectively, unused in the first position. 
     Sensor  108  can be one of a variety of sensors for sensing an engine operating condition such as pressure. Sensor  108  is connected to second shuttle valve  62  for actuating second shuttle valve  62  between first and second positions based upon the sensed engine operating condition. Sensor  108  is also fluidly connected to passage  78 , through sensor passage  110 , to sense pressure in passage  78 . When sensor  108  senses pressure in passage  78  exceeding a threshold, it provides a force greater than that of spring  112 , actuating second shuttle valve  62  into the first position and compressing spring  112 . Second shuttle valve  62  can be actuated mechanically, electronically, or a combination of mechanically and electronically. 
       FIG. 3B  is a schematic view of second shuttle valve  62  in a second position. When sensor  108  senses the pressure in passage  78  below the threshold, it provides a force less than that of spring  112 . Thus, spring  112  can extend and actuate second shuttle valve  62  into the second position. In the second position, valve passages  100  and  102  are no longer aligned with any exterior passages and are, effectively, unused. Valve passage  104  is aligned with passages  78  and  82  to direct flow from main pump  66  to main reservoir  64 . Valve passage  106  is aligned with passages  76  and  80  to direct flow from auxiliary pump  44  to bearings  54 . In an alternative embodiment, passage  78  need not be connected to passage  82 . Instead, valve passage  104  could be replaced with a valve passage ending (a dead-head) to reduce flow. 
     Under normal operating conditions, main pump  66  can supply adequate fluid for lubricating most components in gas turbine engine  10 , including fan drive gear system  36  and bearing  54 . When main pump  66  is supplying adequate fluid, pressure in passage  78  is above a threshold. Sensor  108  senses that pressure and actuates second shuttle valve  62  to the first position so long as the pressure is above the threshold. In the first position, main pump  66  supplies fluid to bearing  54 . Fluid from auxiliary pump  44  is not needed at that time. Consequently, fluid from auxiliary pump  44  is directed to main reservoir  64  by second shuttle valve  62  in the first position. 
     If, however, sensor  108  senses that pressure in passage  78  is below the threshold, that indicates that main pump  66  may not be supplying adequate fluid to bearings  54 . Then sensor  108  exerts a reduced force on second shuttle valve  62 , allowing spring  112  to expand and actuate second shuttle valve  62  to the second position. In the second position, auxiliary pump  44  supplies fluid to bearings  54 . Main pump  66  is then connected to main reservoir  64  through passage  82  to direct any fluid that is still being pumped through passage  78  back to main reservoir  64 . Second shuttle valve  62  remains in the second position unless and until pressure in passage  78  exceeds the threshold again. 
     In one embodiment, bearings  54  are journal bearings. Journal bearings can benefit from having a supply of substantially continuous lubricating fluid. Consequently, bearings  54  can benefit from having fluid supplied from auxiliary pump  44  when engine operating conditions prevent main pump  66  from supplying adequate fluid. This benefit depends on auxiliary pump  44  having an adequate supply of fluid during those engine operating conditions. 
       FIG. 4A  is a schematic view of first shuttle valve  60  in a first position. First shuttle valve  60  includes valve passage ending  120 , valve passages  122 ,  124 , and  126 , sensor  128 , and spring  130 . In the first position, valve passage ending  120  is aligned with passage  70  to reduce or prevent flow from sump  48 . In an alternative embodiment, valve passage ending  120  could be replaced with a valve passage aligned with and connecting passages  70  and  74 . Valve passage  122  is aligned with passages  68  and  72  to direct flow from auxiliary reservoir  58  to auxiliary pump  44 . Valve passages  124  and  126  are not aligned with any exterior passages. Valve passages  124  and  126  are, effectively, unused in the first position. The ultimate effect of the first position is to supply fluid from auxiliary reservoir  58  to auxiliary pump  44 . 
     Sensor  128  can be one of a variety of sensors for sensing an engine operating condition. Sensor  128  is connected to first shuttle valve  60  for actuating first shuttle valve  60  between first and second positions based upon the sensed engine operating condition. When sensor  128  senses the sensed engine operating condition having a value in a first range, it provides a force less than that of spring  130 , allowing spring  130  to extend and actuate first shuttle valve  60  into the first position. First shuttle valve  60  can be actuated mechanically, electronically, or a combination of mechanically and electronically. 
       FIG. 4B  is a schematic view of first shuttle valve  60  in a second position. When sensor  128  senses the sensed operating condition having a value in a second range, it provides a force greater than that of spring  130 , actuating first shuttle valve  60  into the second position and compressing spring  130 . In the second position, valve passage ending  120  and valve passage  122  are no longer aligned with any exterior passages and are, effectively, unused. Valve passage  124  is aligned with passages  70  and  72  to direct flow from sump  48  to auxiliary pump  44 . Valve passage  126  is aligned with passages  68  and  74  to direct flow from auxiliary reservoir  68  to sump  74 . The ultimate effect of the second position is to dump excess fluid from auxiliary reservoir  58  to sump  48  and to supply fluid from sump  48  to auxiliary pump  44 . 
     In one embodiment, sensor  128  can be a gravity sensor, such as a simple weight connected to first shuttle valve  60 , and the sensed engine condition can be gravity. When gravitational forces acting on sensor  128  are below a threshold, such as zero and negative gravity conditions, the weight of sensor  128  is reduced, and spring  130  can extend such that first shuttle valve  60  is in the first position as illustrated in  FIG. 4A . When gravitational forces acting on sensor  128  exceed a threshold, such as normal gravity conditions, the weight of sensor  128  can push down on first shuttle valve  60 , compressing spring  130 , such that first shuttle valve  60  is in the second position as illustrated in  FIG. 4B . In an alternative embodiment, spring  130  can be omitted, and first shuttle valve  60  can be actuated solely by sensor  128 . 
     Normal gravity conditions can occur when gravity is positive, such as when gas turbine engine  10  is parked on the ground, flying level, ascending, or gradually descending. Negative and zero gravity conditions can occur when gravity is sensed to be approximately zero or negative, such as when gas turbine engine  10  is upside down, accelerating toward the Earth at a rate equal to or greater than the rate of gravity, or decelerating at the end of a vertical ascent. 
     Under zero and negative gravity conditions, fluid in sump  48  and main reservoir  64  can rise to their respective tops, interrupting supply to passages  70  and  84 , respectively. On the other hand, auxiliary reservoir  58  is kept substantially full of lubricating fluid and is adapted to supply that fluid during negative gravity conditions. In one embodiment, however, auxiliary reservoir  58  only holds enough fluid to supply for a limited amount of time, such as about 10 seconds. Auxiliary reservoir  58  does not collect fluid fast enough to supply the fluid for long durations. Thus, first shuttle valve  60  supplies fluid from sump  48  to auxiliary pump  44 , under ordinary gravity conditions, which is most of the time. First shuttle valve  60  then switches and supplies from auxiliary reservoir  58  only for brief periods of zero or negative gravity. 
     Using first shuttle valve  60  in combination with second shuttle valve  62  can provide substantially continuous fluid to bearings  54 . As described above with respect to  FIGS. 3A and 3B , second shuttle valve  62  directs fluid from auxiliary pump  44  to bearings  54  when main pump  66  does not supply adequate fluid. If main pump  66  is not supplying adequate fluid due to zero or negative gravity conditions, then first shuttle valve  60  directs fluid from auxiliary reservoir  58  to bearings  54 . If, however, main pump  66  is not supplying adequate fluid due to some reason other than negative gravity conditions, then first shuttle valve  60  directs fluid from sump  48  to bearings  54 . 
       FIG. 5A  is a schematic view of a different embodiment of first shuttle valve  60 ′ in a first position. First shuttle valve  60 ′ includes valve passage ending  120 , valve passages  122 ,  124 , and  126 , sensor  128 ′, spring  130 , and sensor passage  132 . In the first position, valve passage ending  120  is aligned with passage  70  to reduce or prevent flow from sump  48 . Valve passage  122  is aligned with passages  68  and  72  to direct flow from auxiliary reservoir  58  to auxiliary pump  44 . Valve passages  124  and  126  are not aligned with any exterior passages. Valve passages  124  and  126  are, effectively, unused in the first position. 
     Sensor  128 ′ can be one of a variety of sensors for sensing an engine operating condition. Sensor  128 ′ is connected to first shuttle valve  60 ′ for actuating first shuttle valve  60 ′ between first and second positions based upon the sensed engine operating condition. When sensor  128 ′ senses the sensed engine operating condition exceeding a threshold, it provides a force greater than that of spring  130 , actuating first shuttle valve  60 ′ into the first position and compressing spring  130 . 
       FIG. 5B  is a schematic view of first shuttle valve  60 ′ in a second position. When sensor  128 ′ senses the sensed operating condition below the threshold, it provides a force less than that of spring  130 . Thus, spring  130  can expand and actuate first shuttle valve  60 ′ into the second position. In the second position, valve passage ending  120  and valve passage  122  are no longer aligned with any exterior passages and are, effectively, unused. Valve passage  124  is aligned with passages  70  and  72  to direct flow from sump  48  to auxiliary pump  44 . Valve passage  126  is aligned with passages  68  and  74  to direct flow from auxiliary reservoir  68  to sump  74 . 
     First shuttle valve  60 ′ as illustrated in  FIGS. 5A and 5B  is substantially similar to first shuttle valve  60  as illustrated in  FIGS. 4A and 4B  except for two differences. The first difference is that sensor  128 ′ swaps positions with spring  130 . Thus, spring  130  is compressed when first shuttle valve  60 ′ is in the first position, whereas spring  130  is extended when first shuttle valve  60  is in the first position. The second difference is a difference between sensor  128 ′ and sensor  128 . 
     Sensor  128 ′ as illustrated in  FIGS. 5A and 5B  is a buffer pressure reference sensor and the sensed operating condition is pressure. Sensor  128 ′ is fluidly connected to a component in gas turbine engine  10 , through sensor passage  132 , for sensing pressure at that component. When sensor  128 ′ senses pressure exceeding a threshold, it provides a force greater than that of spring  130 , actuating first shuttle valve  60 ′ into the first position and compressing spring  130 . When sensor  128 ′ senses the pressure below the threshold, it provides a force less than that of spring  130 , allowing spring  130  to extend and actuate first shuttle valve  60 ′ into the second position. 
     Pressure sensed by sensor  128 ′ can be one of a variety of pressures related to gas turbine engine  10 . In one embodiment, the pressure is air pressure downstream of high pressure compressor  22 . In another embodiment, the pressure is fuel pressure downstream of fuel pump  29 . In yet another embodiment, the pressure is lubricating fluid pressure downstream of auxiliary pump  44 . In each of these embodiments, pressure is measurably higher when high pressure spool  20  is rotating at operating speed than when high pressure spool  20  is rotating below operating speed. In still other embodiments, the pressure sensed by sensor  128 ′ can be virtually any pressure that is measurably higher when high pressure spool  20  is rotating at operating speed than when high pressure spool  20  is rotating below operating speed. 
     Rotational speed of high pressure spool  20  is important because main pump  66  is driven by high pressure spool  20 . If high pressure spool  20  rotates slower than operating speed or even stops, then main pump  66  will pump a reduced amount of fluid. In some situations, fan  32  can continue rotating at relatively high speeds when high pressure spool  20  rotates slowly or even stops. This can occur when gas turbine engine  10  is shut down but air still flows across fan  32 , such as during an in-flight engine shut-down or when gas turbine engine  10  is on the ground and fan  32  is “windmilling”. Speed of high pressure spool  20  is also an indicator of whether gas turbine engine  10  is operating, overall. 
     Using first shuttle valve  60 ′ in combination with second shuttle valve  62  can also provide substantially continuous fluid to bearings  54 . As described above with respect to  FIGS. 3A and 3B , second shuttle valve  62  directs fluid from auxiliary pump  44  to bearings  54  when main pump  66  does not supply adequate fluid. If main pump  66  is not supplying adequate fluid due to slow rotation of high pressure spool  20 , then sensor  128 ′ can sense pressure below a threshold and first shuttle valve  60 ′ can direct fluid from sump  48  to bearings  54 . This allows fluid to be supplied to bearings  54  for an extended period of time of windmilling. If, however, high pressure spool  20  is not rotating slowly, then sensor  128 ′ can sense pressure above the threshold and first shuttle valve  60 ′ can direct fluid from auxiliary pump  68  to bearings  54 . This allows fluid to be supplied to bearings  54  for a brief period of zero or negative gravity conditions. This can be beneficial because an interruption in supply from main pump  66  when high pressure spool  20  is rotating at operating speed is most likely due to zero or negative gravity conditions. 
     Thus, pump system  50 , including the valve system combining either first shuttle valve  60  or first shuttle valve  60 ′ with second shuttle valve  62 , can provide substantially continuous fluid to bearing  54  under a variety of engine operating conditions. This is done by selecting an appropriate source of fluid depending on the conditions. 
     Additional passages may be provided to better ensure that fluid is provided to the bearing compartment  52  under negative gravity conditions in which fluid may be at the top of the main reservoir  64 . Returning to  FIG. 2 , the pump system  50  includes a supply passage  140  that fluidly connects the main reservoir to the passage  80 . The main reservoir  64  includes a lower portion  61  that holds the lubrication fluid during positive gravity conditions. The fluid in the lower portion  61 , connected to the passage  80  via passage  84 , supplies a steady amount of fluid to the main pump  66 , which supplies the bearing compartment  52  with fluid through the passage  80 . A vent  65  permits air to escape the main reservoir  64  during positive gravity conditions. During a negative gravity condition, the fluid within the main reservoir  64  floats to an upper portion  63 , connected to the supply passage  140 , such that the main pump  66  is starved of lubrication fluid. In this negative gravity condition, the fluid in the upper portion  63  flows through the supply passage  140  to supply fluid to the passage  80  and the bearing compartment  52 . 
     It is desirable to maintain normal flow through the passage  80  during positive gravity conditions. In the example illustrated in  FIG. 6 , a check valve  142  blocks fluid flow from the main reservoir  64  during positive gravity conditions. A housing  144  includes an inlet  146  that is in fluid communication with the main reservoir  64 . The housing  144  also includes an outlet  148  connected to the supply passage  140 . In one example, the housing  144  is part of the vent  65 . 
     A ball  150  is arranged within the housing  144  against a seat  152  to block the inlet  146  in positive gravity conditions. In one example, the ball  150  is positioned against the seat  152  from the weight of the ball  150  without an additional biasing element. Under negative gravity conditions, the ball  150  floats upward in the housing  144  against a stop  153 . In this position, fluid is permitted to flow from the main reservoir  64  past the ball  150  and through slots  154  within the housing  144  to an outlet  148 . The outlet  148  is in fluid communication with the passage  80 . 
     In systems in which the pressure within the main reservoir  64  is not sufficient to cause the fluid to flow from the main reservoir  64  to the passage  80 , an ejector  156  may be provided in the passage  80 . The ejector  156  includes a converging portion  158  connected to a diverging portion  162  by a throat  160 . An inlet  164  provides fluid communication between the supply passage  140  and the throat  160 . Fluid flow through the ejector  156 , whether air or liquid, draws the oil from the supply passage  140  into the passage  80 . However, the ejector  156  does not create vacuum sufficient to pull the ball  150  from the seat  152  without the assistance of a negative gravity condition. 
     Referring to  FIGS. 7A and 7B , another example check valve  242  is illustrated.  FIG. 7A  illustrates the check valve  242  in the positive gravity condition in which the ball  250  is seated against a lower seat  252  within the housing  244 . During a negative gravity condition, as shown in  FIG. 7B , the ball  250  moves upward against an upper seat  253  to permit the fluid from the main reservoir  64  to flow past the ball  250  and through slots  254  to supply the fluid to the passage  80  through the supply passage  140 . 
     Another example check valve arrangement  342  is illustrated in  FIG. 8 . The supply passage  340  includes a first check valve  170  that permits fluid to flow from the upper portion  63  of the main reservoir  64  in a negative gravity condition in a first flow direction  178 . The first check valve  170  may include an element that is spring biased to a closed position during the positive gravity condition to prevent flow in the first flow direction  178 . 
     A second check valve  172  is arranged in the passage  80  upstream from an intersection  182  between the supply passage  340  and the passage  80 . The fluid flows past the second check valve  172  in a second direction  180  during the positive gravity condition. The second check valve  172  is open during a positive gravity condition to permit the main pump  66  to pump fluid from the lower portion  61  of the main reservoir  64  through the shuttle valve  62  to the bearing compartment  52 . During the positive gravity condition, a bleed passage  174  permits a small amount of fluid to bypass the first check valve  170  to fill the supply passage  340 . As a result, in the negative gravity condition, oil will begin to immediately flow from the main reservoir  64  to the bearing compartment  52  through the supply passage  340 . 
     The main reservoir  64  includes vent  65 , which includes a pressure valve  176  configured to maintain a desired pressure within the main reservoir  64  in the example, unlike the arrangements illustrated in  FIGS. 6-7B . In one example, the pressure valve  176  is configured to maintain at least between 20-30 psia (137.90-206.84 kPa) in the main reservoir  64 . This main reservoir pressure is sufficient to open the spring-biased first check valve  170 , as the pressure in the passage  80  drops due to a lack of fluid, without the need of an ejector in the supply passage  80 . 
     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.