US3001368A - Negative gravity hydraulic system - Google Patents

Description

Sept. 26, 1961 s. R. BARR ETAL NEGATIVE GRAVITY HYDRAULIC SYSTEM 2 Sheets-Sheet 1 Filed Aug. 13, 1959 Sept. 26, 1961 s. R. BARR ETAL 3,001,368

NEGATIVE GRAVITY HYDRAULIC SYSTEM Filed Aug. 15, 1959 2 sheets-sheet 2 United States Patent O ice assgnors to General Electric Company, a corporation of New York Filed Aug. 13, 1959, ser. No. 533,586 claims. (ci. ca -s3) Our invention relates to negative gravity hydraulic systems, and in particular to a negative gravity system for hydraulic constant speed drives.

As is well known in the art, a hydraulic constant speed drive is a device which -accepts a variable speed input and, through a variable ratio `transmission formed in part by a hydraulic transmission element, provides a constant speed output. Devices of this kind are commonly used in aircraft to drive alternating current generators at `a constant speed. In such an installation the drive is usually mounted on an accessory gear box of a jet engine and is connected to be driven mechanically by the engine.

The hydraulic transmission portion of this type of drive requires a supply of hydraulic lluid, usually oil, to make up for leakage flow from the hydraulic element and to provide cooling. The main oil supply reservoir of the engine is normally used as the supply source for the drive with the engineoil cooling system forming the heat sink for the system. In order for the drive to perform its function as a variable ratio transmission, it is necessary to supply hydraulic uid to the hydraulic element of thedrive on a substantially continuous basis n order to make up for leakage in the drive. One of the problems that is commonly encountered in this regard arises out of the interruption in the engine hydraulic supply to `the drive Which may occur during negative gravity conditions induced either by Hight maneuvers or by turbulence.

It is the usual practice to make at least some provision in the engine oil supply system to the drive for negative G operation. Such arrangements may typically take the form of a baffled supply tank capable of trapping a portion of the oil in the tank and retaining it in communication with the supply pump inlet during negative G operation. However, such systems usually can supply hydraulic iluid for only a very limited time and are intended only to provide continued ilow through negative G transient conditions of relatively short time duration. It has been considered desirable therefore, and `in some applications a requirement, to provide in the drive itself an emergency system capable of sustained operation for a relatively long period of time under negative G conditions where the flow of hydraulic iluid from the main supply is interrupted.

Accordingly, it is a primary object of our invention `to provide in a hydraulic drive an improved emergency hydraulic system capable ofoperation for'relatively long 3,0@1368 Patented Sept. 26, 1961 air bleed system does permit, however, the restoration of the system to its normal configuration upon the generation of hydraulic pressure of a preselected magnitude at the pump discharge. *In one arrangement of our system, we provide additional hydraulic fluid storage in the drive which is supplemented by a further quantity of hydraulic Huid which our system is capable of introducing into the drive during the transient condition under which the emergency system is actuated.

Our invention will be further described, and various other advantages and objects thereof will be pointed out in the following description taken in connection with the accompanying drawings in which:

FIG. 1 is a schematic presentation of a hydraulic drive having a hydraulic supply and negative gravity system embodying our invention and,

FIG. 2 is a cross-sectional View of the drive of FIG. l showing the internal storage and oW passage arrangement in greater detail.

Referring now to FIG. l there is shown in schematic form a hydraulic constant speed drive having an outer casing 10 provided with an internal reservoir 11. The drive includes a supply pump 12 and a scavenge pump 13, both of which are mechanically coupled to and driven by the constant speed output of the drive. Hydraulic uid, in the usual case oil, is supplied to the drive from a main supply reservoir or tank 14 which is mounted remotely from the drive and which normally forms the main supply source for the engine on which the drive is mounted. Oil is supplied to the drive through a boost pump 15 and conduit means 16 connected to the inlet of the supply pump 12 of the drive. The drive casing 10 is vented to the main oil tank 14 through conduit 17 so that the internal pressure in the casing remains substantially at the atmospheric level.

High pressure oil discharged from the supply pump 12 passes through a conduit 18, a filter 19', which has in series with it a pair of check valves 20 and 21, the purpose of which will be described later, and then through a conduit 22 to the pintle 23 of the hydraulic unit, which is the point in the drive where makeup oil is introduced to compensate for leakage in the system. The manner in which the oil is introduced into the pintle and the internal oil ow arrangement therein will be set forth later in connection with the description of FIG. 2, although it will be understood that this is merely supplementary material which is introduced to permit a better understanding of our invention and that it forms no part of it.

The ow rate of oil discharged from the supply pump 12 is substantially greater than that required to supply the normal leakage flow to the pintle, although tlow rates considerably greater than the leakage iiow are required during system transients created by load and input speed changes on the drive. Therefore, under steady state operating conditions, it is necessary to bypass a substantial portion of the supply pump discharge ow around the periods `of time undernegative gravity conditions with the llow from the main hydraulic supply system interrupted.

Briefly stated, we accomplish this, in one form of our invention, by the provision of a iluid pressure sensitive valving system which detects the reduction in lluid supply pressure accompanying an interruption or decay in flow fromthe main supply and which resets the drive system to anv emergency conguration in which hydraulic iluid is recirculated within the drive. We :also provide an air bleed system which is automatically actuated to bleed ofi the air discharged from the drive pump during emergency operation, thereby preventing the air pressure which would otherwise be developed by the pump from restoring the system to'fits normal configuration. The

pintle. This is accomplished by a bypass conduit 24, which has in series with it a regulating valve 25. The regulating valve Z5 includes a plunger type valve member 26 loaded by a spring 27 and set to allow bypass flow at a pressure which in the embodiment shown is in the vicinity of 18() p.s.i. Oil discharged from the bypass conduit 24 is directed into a storage space 28 which is formed within the drive casing by a wall or bulkhead 29, which in this case also forms the support member for the pintle 23. The lower portion of the wall 29 also forms an inlet passage 30 for the scavenge pump 13, so that oil spilling over the top of the wall 29 and into the chamber 31 formed in the casing on-the right hand side of the wall 29 falls to the bottom ofthe casing 3 and is introduced to the scavenge pump 13 through inlet 30.

The bypass system is provided with an overpressure relief valve 32, which is connected to the conduit 22 through a conduit 3%` and which comprises a plunger 34 loaded by a spring 35 and set to provide bypass ilow at an excess pressure level, in the case shown at about 30() p.s.i. By way of example, the pressure relief valve 32 could be expected to operate under startingy conditions at very low temperatures where the bypass 'valve 25 would be incapable of providing sufficient bypass capacity at the high oil viscositiesy encountered under these conditions. Oil discharged from the relief valve 32 is also directed into the cha'mber 28 on 'the left side of the vwall 29.

The leakage flow from Ythe hydraulic unit, which is relatively small in comparison to the bypass ow, is allowed to fall directly int'o the chamber 31 on the right side ofthe Wall 29. Under 'normal operating conditions, the bypass :flow therefore keeps the chamber 28 filled with oil, the excess '-ow spilling over the top ofthe wall 29 and returning to the inlet 30 of Vthe scavenge pump. Oil 'is Vscav'enged from the chamber 31, however, Which therefore runs'sub'stantially dry.

The s'cavenge: pump 13 discharges into the reservoir 11 'through a conduit 3'6 which passes through the wall ofthe reservoir. The reservoir 11 is provided with a pressurizing valve 38 which comprises a plungerva'lve 39 resiliently loaded by a spring 40 set to permit the discharge of oil from the reservoir through a discharge opening 441 when theuid pressure in the reservoir 11 exceeds a preselected level, which in 'the embodiment presented is in the vicinity of 130 p.s.i.g. A conduit connects 'the internal case pressure to the opposite side of the plunger 39 so that the valve responds to the `difference lbetween reservoir pressure and drive case pressure. The reservoir 11 is also provided with an `overpressure relief valve 42 which allows fluid to be'discharged from kthe reservoir through a conduit 43y into the chamber 31 when the pressure exceeds some preselected level in excess of that which the regulating valve 3S is set to maintain. In the embodiment shown, the relief valve 42 is set to permit flow from the reservoir '11 to the chamber 31 at pressures in excess of about 200 p.s.i.g. The relief valve 42 comprises a piston 44 loaded by a spring 4S and `includes a conduit 46 connectingcthe internal drive casing pressure to one side of the piston so that the piston is caused to respond to the differences between reservoir pressure and case pressure.

Oil Apassing through the pressurizing valve 39 is directed out of the reservoir through a conduit 47 which has connected in series with it a pressure responsive negative gravity valve 48. After passing through the pressure responsive valve 48, the oil is returned to the main supply tank 14 of the system).

'Ihe pressure responsive valve 4S comprises an outer casing 49, having inlet and discharge ports 50 and 51 respectively connected in series with the conduit 47, a spool V52 slidable in the casing and having lands 53 and 54 thereon, and a "spring 55 bearing against one end of the spool as shown. The chamber 6 formed by the casing and one end of 'the'spool 52 is vented to atmospheric pressure'through an opening 557 and the chamber 5S formedby the casing and the other end of the spool is connected 'to the discharge conduit 18 of the Ysupply pump 12 through'conduit 59. The longitudinal position of the spool 52 in the casing is thus a function of the difference between the supply pressure of the pump'and atmospheric pressure. The valve 43 is designed vsuch that for the normal range of discharge pressures'of the supply pump 12, the spool 52 is in approximately the position illustrated in FIG. 1, with the lands '53 and154 being cleary ofthe inlet and discharge -ports k50' fand 51, thereby allowing unobstructed flow through the valve 48. The spool 52 is provided with an air bleed conduit '60 which extends through the spool from the `chamber 58 and radially out through the land 54. The function of the air bleed conduit A60 will be explained later.

The -system is provided with an emergency line 61 having in series therewith a check valve 62 which allows ow to pass from the reservoir 11 Vto the conduit 22 but blocks ow in the opposite direction. Since under nor mal operating conditions, the pressure in the conduit 22 established `by the regulating valve 25 is higher 'than that established in the reservoir 11 by the pressurizing valve 38, the check valve 62 will normally remain closed. However, in the event the pressure in the line 22 falls below that of the reservoir, the checkvalve Vv62 opens to permit flow from the reservoir throughvthe line 61 into the line 22. 'Ihe pressure in the line 22 may fall below reservoir pressure either by reason of a failure in the hydraulic supply system or because, under certain severe transient conditions creating a very high ow demand, the capacity of the supply pump 12 is inadequate to supply the full flow requirement. Thus, Vthe capacity of vthe scavenge pump 1'3, which maintains hydraulic pressure in the reservoir 11, is made available for transient and emergency ow operation.

Upon the occurrence of a negative gravity condition, the hydraulic fluid in the main tank 14 moves to what is normally the top of the tank. The tank 14 is provided with a bafe 63, however, which traps a certain .quantity of the hydraulic fluid as it moves in the direction of the top of the `tank to maintain a limited supply of the fluid in contact with the inlet conduit 16a. Thus, vthe Vsystem is provided with a limited supply o'f 'hydraulic -iluid which is available under negative gravity conditions.

The arrangement of the various elements within the drive and the internal oil flow configuration is shown in greater detail in FIG. 2. "Referring to FIG. 2, the hydraulic/element is shown at 64 and comprises an input shaft -65 connected to drive an elliptical race 66 and a cylinder block V67 integrally formed with the race -66. The cylinder block =67 is .provided with la longitudinal extension 68, on which there is rotatably mounted a cylinder block 69. The cylinder block x69' is connected to an internally splined output shaft 70 through interconnected elements 71, 72, 73, vand 'a unidirectional clutch 74. The supply .pump 12 and the scavengepump '13 as well as various other accessories, such as `speed governor, are driven Vby the output .shafting of the .drive .through suitable gearing, not shown. l Y

The cylinder block w67 is provided with a series of radial cylinders .75 having ballpistons slidably 4mounted therein. yThe cylinder block .692is of similar construction, having :ajseries of hall pistons 76 slidably mounted in cylinders 77 Vfor radial reciprocating movement therein. Oil is supplied to the .pintle `23 through an linternal -passage 78 from the discharge line 22 of the supply Vpump 12 and is ducted through internal porting to thecylinder blocks 67 and `69 as shown. Further description-of the operation of the hydraulic unit -64 is not deemed necessary since the construction and-operation of suchdevices are well lknown lin the art and such additional material `is not essential to an understanding of our invention. The operation-of the system describedabove -underfneg tive gravity or emergency conditions will now be set forth.

As has been stated above, upon the voccurrence of Ya negative gravity condition, the bale63 trapsra -limited quantity of fluid in the vicinity xof thevpump inlet 16a. It will be observed vby reference to lFIGS. '1 and v2 that by reason of the location of Vthe scavenge pump inlet passage 30 at the lower portion of the-drivefcasing, `the occurrence of a negative 'gravity condition, in causing the hydraulic fluid in fthe'drive to :move to ftheftop ofthe casing .and-away Vfrom the pump inlet, tprevents the `pump A13 from returningto -the reservoir fl-1 Yanyfofthe fluidin the casing.` lUnder these conditions, however, fthelsupply pump 12 continues to pump fluid into the drive.

is exceeded, by the reservoir relief valve 42.

georges Therefore, as long as the fluid delivered to the drive remains out of contact withthe'scavenge pump inlet 39, the supply pump 12 will continue to pump additional hydraulic iluid into the drive casing from the emergency Supply contained in the baille 63. Thus, during this initial period, the drive takes on or gulps an additional quantity of hydraulic fluid over and above that normally stored in the drive in anticipation of sustained operation under negative gravity conditions. `As this phase of the cycle proceeds, the oil level in the drive reaches the point Where it comes into contact with the exposed rotating elements of the hydraulic unit, i.e. the race 66 and the cylinder block 67. When this occurs, the oil in the casing is set into motion around the inside of the casing by reason of the contact with the rotating parts. 'Ihe resulting vortex or swirl motion of the oil in the casing brings it into contact Wtih the inlet 30 of the scavenge pump 13 and the scavenge pump again begins to pump oil from the casing into the reservoir 11, the pressurizing valve 318 again allowing oil to return to the main reservoir 14 through the pressure responsive valve 48. It will be observed that during this initial period of negative gravity operation, `the drive automatically takes on a sulicient quantity of additional oil to allow the Setting up of the oil vortex within the drive to thereby reestablish fluid communication with the inlet 30 to the scavenge pump under negative gravity conditions. In other words, a sufiicient quantity of additional oil is taken into the drive to supplement that already contained in the chamber 28 to maintain the circulation of oil through the scavenge pump during operation under negative gravity conditions.

Upon depletion of the reserve oil supply stored in the bae chamber 63, thepressure at the discharge line 18 of the supply pump decreases rapidly, causing a corresponding decrease in pressure in chamber 58 of the pressure responsive valve 48. This allows the spring 55 to force the spool 52 against the upper limit of the casing 49, bringing the land 54 into a position `blocking flow between the inlet and discharge ports 50 `and 51, thus shutt ting oir" flow from the reservoir 11 to the main tank 14. It will be observed that when this occurs, the pressurizing valve 38 is no longer capable of regulating the pressure in the reservoir 11 and that the pressure in the reservoir will therefore be regulated at the higher pressure level established by the relief valve 42.

At the same time that the decrease in the discharge pressure of the pump 12 causes the valve 48 to close off flow to the main tank 14, the decrease in pressure in line 22 below that existing in the reservoir 11 allows the check valve 62 to open, thereby permitting flow from the reservoir 11 through the emergency line 61 and the check valve 62 to the line 22. The check valves 20 and 21 act to prevent the application of reservoir pressure back through lines 18 and 59, a condition which would otherwise cause the valve 48 to re-open and allow flow from the reservoir 11 back to the tank 14 thereby gradually depleting the oil supply stored in the drive. Only one of the two check valves 20 and 21 is required to perform this function, there being two provided in the particular embodiment illustrated in order to block flow from the lines 18 and 22 when the `filter 19 is removed for maintenance purposes.

With the drive system reset as just described for internal recirculation, hydraulic fluid is supplied to the pintle line 22 through the emergency line 61 from the pressurized reservoir 11. Under these conditions, the pressure in the reservoir 11 is regulated by the regulating valve 25 and, to the eXteut the capacity of the valve 25 In order to maintain'the reservoir pressure at the desired level, it is necessary that the scavenge pump 13 continue to supply oil to the reservoir through its discharge line 36 at a rate equal to or greater than that at which oil -is removed through the emergency line 61; Under these conditions the pressure in the reservoir 11 will normally be established by the reservoir relief valve 42. 'Ihe vortex of VAoil set up within the drive casing during the initial period of negative gravity operation by the taking on of additional oil into the drive maintains the supply of oil to the scavenge pump inlet 30. The oil vortex is continuously supplemented by oil discharged back into the casing from the regulating valve 42, the bypass valve 25 and by leakage from the hydraulic system so that an internal recirculation system is formed which is capable of continuous operation under the conditions just described. In other words, the amount of oil in the casing remains suti'icient to maintain the oil level in contact with the rotating elements of the hydraulic unit so that the vortex which feeds the scavenge pump 13'is maintained so long as the negative gravity condition exists.

Referring to FIG. 1, it will be observed that the movement of the spool `52 of the valve 48 against the upper extremity of the casing aligns the radial portion of the air bleed passage 60 with the discharge port 51. This allows air to be bled from the discharge line 18 of the supply pump 12 through the bleed passage 60 back to the main tank 14, `thereby preventing the gradual buildup of air pressure which would otherwise be developed by the Acontinued running of the supply pump and which would cause the valve 48 to reopen and allow oil to escape from the reservoir 11, and which would also result in pressurization of the drive casing 10.

Thesystem will continue to operate in the manner just Vdescribed -as long as the negative gravity condition remains. When the -system is returned to normal gravity conditions, the oil supply in the main tank 14 is again brought into contact with the pumpinlet 16a and becomes available to the supply pump 12. The crosssectional area of the air bleed passage 60 is suiciently small such that while it can accommodate the necessary flow of air to prevent the pressure in the discharge line 1S from building up beyond an undesired level when the supply pump 15 is pumping air, the amount of liquid ow it can accommodate is small in comparison to the capacity of the supply pump. Therefore, when hydraulic fluid is made available to the supply pump, the pressure in the discharge line 1S builds up to the level established by the regulating valve 25, the spool 52 being forced back` to the position shown in FIG. 1 in the process, thus once again permitting the reservoir pressurizing valve 3'8 to return iiuid from the reservoir 11 to the main supply tank 14. The return of the reservoir pressure to a level lower than that established in the line 22 by the regulating valve 25 causes the check valve 62 to close and the system is returned to its normal operating configuration. As the system continues to operate under normal conditions, the excess fluid is scavenged from the chamber 31 and the chamber 28 is again filled with hydraulic fluid in the manner already described.

While we have shown and described a particular embodiment of our invention for the purpose of making a full disclosure thereof, it is to be appreciated that our invention is not-limited to the embodiment set forth and that various changes, substitutions and modifications may be made in the embodiment presented without departing from the true scope and spirit of our invention as de- `lined in the appended claims.

. What We claim and desire to secure by Letters Patent .of the United States is:

.sure'responsive valve connected to respond to the duid vpressure in said fluid system on the discharge side of said 'supply pump, said pressure responsive valve being connected in series with said pressurizing valve to shut oi iiow from said reservoir -to said main supply source when the pressure on the discharge side of said supply pump falls below a preselected level.

2. ln a system for a hydraulic drive rhaving a .main hydraulic iiuid source, asupply pump for supplying -fluid under pressure -from said main source to the drive, a reservoir associated with the drive, anda scavenge .pump connected to scavenge uid from the drive andfdischarge it into said reservoir; an emergency hydraulic system `comprising a reservoir pressurizing 'valve connected to Ireturn iluid to said main supply source from said reservoir when the fluid `pressure in said reservoir exceeds a preselected level, a pressure relief valve connected to discharge fluid -from said reservoir linto the drive when -the pressure in said reservoir exceeds -a second preselected level higher than that normally maintained by said pressurizing valve, a .pressure responsive valve connected 'to respond -to the fluid pressure in .said lhydraulic drive system at apoint on the discharge side of said supply pump, said pressure responsive valve being connected to shut oi the ilow of uid 'from said reservoir to -the main supply source fin response to a decrease in the discharge pressure of said supply pump below a preselected level, conduit means connecting said reservoir to the hydraulic supply system of the driveat a point downstream of said supply pump, `and valve means permitting lio'w 'from 'said 'reservoir into the drive supply system through said conduit means when the fluid pressure generated in the supply system by said supply pump falls 'below a preselected level.

3. Apparatus as set forth in vclaim 2 including check valve means connected between said conduit 'means and said 'pressure 'responsive valve to vprevent the application of reservoir pressure to said pressure responsive valve through said conduit means.

4., A fluid system for a hydraulic vdrive 4comprising la supply pump for supplying tluid under pressure from a main supply source to said drive, a reservoir associated with said drive, a scavenge pump connected to scavenge fluid from said drive land discharge it into the said reservoir, a .pressurizing valve connected to return 4lluid 'to said main supply source when the pressure lin said reservoir'exceeds a preselected level, a pressure responsive valve connected to respond to the 'fluid pressure in said 'fluid system onthe discharge side kof said supply'pump, said pressure reponsive valve being connected to shut ot ll'ow from said reservoir to said main supply source when the pressure on the vdischarge side of said supply pump falls below a preselected level, and air bleed means kactuated upon movement of said pressure responsive valve to a position shutting oi ilow from said reservoir to said main supply source to bleed olf the air discharged by said supply pump Aunder such conditions.

5. 4'In `a system for a hydraulic ydrive having a Vmain hydraulic duid source, a supplypump for supplying Huid vrunder pressure from said main source to ithe drive, a

reservoir associated with the drive, and a scavenge pump connected to scavenge fluid from the drive and discharge it Yinto said reservoir; an emergency -hydraulic system comprising a reservoir pressurizing valve connected to'return fluid to said main supply lsource 'from said reservoir 'when the fluid ypressure in said reservoir exceeds apreselected level, a pressure relief Valve connected to ldisvcharge iluid from said `reservoir'into'the drive -when Ethe pressure in said'reservoir exceeds Ea 'second preselected level 'high'erth'an that ynormallymaintained by said-pressuri'zing valve, va pressure responsive valve connected to respond to the `fluid pressurein-said hydraulic-*drive'systern vatapoint on the discharge side of said supply pump, said 'pressure `responsive valve being 'connected `to -'shut oilthe flow-of Aliluid froms'aid'r'eservoir tothe' mainsupp'ly source in responseto 'a decrease'inthe discharge presdrive supply system through said conduit means when 'the iiuid pressure generated in the Ihydraulic drive sys'temby said supply pump falls below 'a preselected level, and air bleed meanskactuated upon movement of 'said 'pressure responsive valve blocking iiow 'from-said 'reservoir to 'said main supply source'to'bleed olf 'the air discharged .by said supply pump under such conditions.

6. I'Apparatus 'as 'set forth rin claim 5 'including check valve means connected between said conduit means 'and said pressure 'responsive valve 'to 'prevent the application o'f reservoir pressure `to said pressure 'responsive valve through said .conduit means. l

7. `A system 'for 'a hydraulic drive comprising avarible speed ratio transmission including a rotatable cylinder 'block 'and race assembly, ran outer casing extending around said transmission, a supply pump for supplying duid under pressure to said transmission from a ylluid source, a scavenge pump connected to scavenge fluid 1from said casing, a partition in `.said casing vextending partially across transmission, means for 'discharging 'the bypassed Vflow into said Llirs't `chamber to 'maintainsaid lirst chamber 'substantially full of uid under normal operatingconditions,

.said transmission being located in said second chamber,

and inlet 'means 'for 'said scavenge pump located in said second chamber, whereby said transmission normally operates in a lsubstantially dry chamber, 'the iiuid stored 4in "said iirst chamberv 'being thrown to vthe top fof "said casing under negative gravity'conditions into communica- 'tion 'with said passage to become available for iilooding said second chamber, the duid entering said second chamber d coming into `contact with said rotatable cylinder -block vand race assembly and being thereby induced into a swirl motion whereby said fluid is :brought into contact with the lscavenge vrpump inlet means during negative gravity conditions.

8. A hydraulic drive "fluid system as set forth inclaim 7 including means associated with said fluid source :for :making available to said drive under negative gravity conditions a limited quantity of hydraulic fluid to supplefmentthe `fluid stored in said drive.

9. A hydraulic'drive 'fluid system as set forth in claim 7 including a reservoir associated with said drive, said fscavenge pump being connected `to discharge in'to said reservoir, a pressurizing valve connected to return uid 'from the 'reservoir t'o the main supply source when the pressure ins'aid reservoir exceeds a preselected level, a `pressure relief kvalve 'connected to 'discharge -luid from "said reservoir '-into `the casing when vthe pressure in 4said reservoir exceeds a second Ypreselected level higher than 'that normally maintained by said pressurizing valve, a

.pressure responsive va'lve connected to respond 'to 'the 1'duit means when the lfluid pressure generated in said drive uidfsystem by the supply VV.pump falls below the pressu're in said reservoir.

110.:A :uid system Vfor a hydraulic drive comprising a A'supply `pump "foi-:supplying duid 'under pressure ifi-om Sa main supply source to said drive, a scavenge system including a scavenge pump for scavenging uid from said drive and returning it to said main supply source, a pressure responsive valve connected in series ow re1ati0nship With said scavenge system and operable to one position blocking return ilow `from said drive to said supply and to a second position permitting said return flow, said pressure responsive valve being connected to respond to the fluid pressure in said fluid system on the discharge side of said supply pump, whereby return ow from said drive to said supply source through said scavenge system is shut off by said pressure responsive valve when the pressure on the discharge side of said supply pump falls below a preselected level.

No references cited.

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