US3078674A

US3078674A - Hydraulic control system for variable displacement pump

Info

Publication number: US3078674A
Application number: US9461A
Authority: US
Inventors: Arthur F Anderson
Original assignee: Gar Wood Industries Inc
Current assignee: Gar Wood Industries Inc
Priority date: 1960-02-18
Filing date: 1960-02-18
Publication date: 1963-02-26
Anticipated expiration: 1980-02-26
Also published as: GB909906A

Description

3,078,674 HYDRAULIC CONTROL SYSTEM FOR VARIABLE DISPLACEMENT PUMP Filed Feb. 18, 1960 Feb. 26, 1963 A. F. ANDERSON 9 Sheets-Sheet 1 INVENTOR. flr/ z/r fxf zisr fi BY @4414 y M I 19' air/V5961 Feb. 26, 1963 A. F. ANDERSON 3,078,674

HYDRAULIC CONTROL SYSTEM FOR VARIABLE DISPLACEMENT PUMP Filed Feb. 18, 1960 9 Sheets-Sheet 2 INVENTOR.

Eb /4 ggz Feb. 26, 1963 A. F. ANDERSON 3,078,674

HYDRAULIC CONTROL SYSTEM FOR VARIABLE DISPLACEMENT PUMP 9 Sheets-Sheet a Filed Feb. 18, 1960 Feb. 26, 1963 A. F. ANDERSON 3,078,674

HYDRAULIC CONTROL SYSTEM FOR VARIABLE DISPLACEMENT PUMP Filed Feb. 18, 1960 9 Sheets-Sheet 4 Feb. 26, 1963 A. F. ANDERSON 3,073,674

HYDRAULIC CONTROL SYSTEM FOR VARIABLE DISPLACEMENT PUMP Filed Feb. 18, 1960 9 Sheets-Sheet 5 INVENTOR. 59/2217 f/f Tarn/5:

Feb. 26, 1963 A. F. ANDERSON HYDRAULIC CONTROL SYSTEM FOR VARIABLE DISPLACEMENT PUMP Filed Feb. 18, 1960 9 Sheets-Sheet 6 Feb. 26, 1963 A. F. ANDERSON HYDRAULIC CONTROL SYSTEM FOR VARIABLE DISPLACEMENT PUMP Filed Feb. 18, 1960 9 Sheets-Sheet 7 INVENTOR.

flr/Zz/r 297274 6719072 Feb. 26, 1963 A. F. ANDERSON 3,078,674

HYDRAULIC CONTROL SYSTEM FOR VARIABLE DISPLACEMENT PUMP Filed Feb. 18, 1960 9 Sheets-Sheet 8 Feb. 26; 1963 A. F. ANDERSON HYDRAULIC CONTROL SYSTEM FOR VARIABLE DISPLACEMENT PUMP Filed Feb. 18, 1960 9 Sheets-Sheet 9 IIII I United States Patent This invention relates generally to pump control systems and more particularly to a hydraulic control system for a' variable displacement pump.

In a system in which a variable displacement pump is used to operate a hydraulic device, a provision for actuatingthe pump to substantially zero displacement in the limit positions of the hydraulic device is desirable, because this eliminates unnecessary heating of the system due to circulation of the hydraulic fluid, and'makes it possible to use the power required for operating the pump for other useful purposes. By limit position is meant a position in which the hydraulic device cannot be moved in-a direction in which'fluid from the-pump tends to move it, either becauseof the structure of the hydraulic device or some external force applied to it. Theinvention is best understood by referring to asystem in which a variable displacement "pump of reciprocating piston type operates a hydraulic cylinderassembly that raises and lowers a dozer blade mounted on 'the frontend'of'a tractor. 'In a pump of'this type, which' is' descri-bed in detail in my prior application Serial No. 835,002," filed August '20, 1959, and 'now abandoned, a'movable tilt box'is angularly adjustable relative to the'axis of'rotation of the cylinder barrel to controlthe-displacement of the "pistons. When the tilt box is perp'endicularto the-axis of "cylinder barrel rotationf'thepistons -have=no stroke= and no fluidispumped. This is called'the zero displacement or neutral position of the tilt box which'is" hereinafter sometimes generally referred to as the pumpdisplacernent control member. When the tilt box 'is movedin either direction from the perpendicular position fluidis'pumped by'the pump'in one or the other directions depending on the direction of movement'of'the tilt box. The greater 'the'extent of movement of the 'tilt'box from 'theperpen- *dicular position, the'greater the stroke of the-pistonsand,

consequently, the greater the output ofthe pump. 'When the pump is driven by the tractor engine,'any diversion of engine power for unnecessary pump operation is to be avoided if possible. Consequently, in limit positions of the hydraulic cylinder assembly it is desirable to have the 'tilt box moved from a full to substantially a zero displacementposition so that substantially the full power of the engine is available for purposes other than'operating-the pump at full pressure and unnecessarily heating the control system. It is 'also' 'desirable in systems of this type to provide for a flow of replenishing fluid to'the main pump circuit to replace the piston rod when it is moving out of the cylinderassembly and to compensate for leakage at the pump, which is a necessary characteristic of pumps of this type since'a lubricating'layer of 'fiuid must be provided between the rotating cylinder barrel and the adjacent stationary valve 'fa ce through which fluid is supplied tothe pump and discharged from'the pump. When the cylinder ismaintainedsubstantially full/offluid 'at all times, an, immediate response of thecylinder assembly to operation of the control valve therefor is insured.

It is an object ofthis invention, therefore, to provide .an. improvedhydraulic control system for a variable displacement pump which provides for an automatic actua- "tion of the pump displacement control member tosubstantially a zero displacement position when theipump output pressure exceeds a predetermined maximumpressure.

.2 "This is accomplished in the :system of this invention. by communicating the high pressure fluid on the output 'side of the pump with the control apparatus whichactuates the pump displacement control member so as to overpower the control apparatus which is maintaining the displacement control mem-ber'in a-positive displacementposition.

Another object-of this invention is to provide a control system for a variable displacement pump which includes a separate hydraulic control circuit. operable to actuate the pump so that it will pump fluid in eitherof twodirections to the hydraulic device which is being controlled.

A- further object of this invention is to provide ahydraulic control system for a variable displacement 'pump and a hydraulic-device operated thereby which;,.includes a super-charge circuit that replenishes the operating circuit to maintain the operatingcircuit substantially full at all times.

Another object of this invention is to provide in'f'a hydraulic control' system 1 for *a ''variable displacement pump, means for'automatically relieving the -pressurein the operating "circuit 'during the time the pump' displa'cernent control member is being movedtowa'rd a ".zero "displacement or neutral-'position so that thepre'ssure in the system does not exceed=apredetermined-maximum pressure, thereby requiringcoriditiohing of Elie-system for withstanding only this predetermined maximum pressure.

Still another object ot this yinventi'on'is toprovide a control system *for aivariable displacementpump"which provides for movement of the pump displacement con- 'trol member to'su-bstantially a-neutral "positionfrom a positive displacementposition in either direction fromthe neutral position without any hunting of the member for the neutral position. I H

jFurther objects, featuresand'advantages of this invention will become apparent from a consideration :of the following description, theiappended claims and 'thea'e companying drawing in which: I

FIGURE ,1 is a' fragmentary side elevationalyiew' of a tractor having a dozer blade whichis, raised fandlow mu by a hydraulic cylinder'assembly .whichis actuated by the control system ofthis' invention;

' FIGUREZ is an enlarged fragmentary elevational view of a portion of the tractor and purnp'mounting'shown in FIG. 1, with some parts. broken'away forjthe purposeof clarity;

.FIGURE 3 is a diagrammatic viewofthe controlsystem of thisinvention, shown associated-with a variable'displacement pump of reciprocating piston type and-the hydraulic cylinder assembly shown'in' Fig. 1,- and illusstrating the systemis a hold condition, -namely,":a condition for holding the piston in thecylinderasser'nbly, and consequently the do zer'blade, in a fixed positioniwith respect to the front end of the tractor.

FIGURE 4 is a diagrammatic view of ;the1"system of vthis inventiomlillustrated similarly to FIG."3, showing'ithe system in'a raise condition, in'which thejdozer' blade will be moved upwardly;

FIGURE 5 is a diagrammatic view, illustrated simil'arl-y to FIG. "4, showing'the system in-a"raise overload condition, namely, a condition in which the dozer blade has been raised to a limit position and further effort by the pump to raise the blade has raised the pressure on the output side'oi' the pump to-a predetermined safety limit at which the system has operated to'move the pumpdisplaoe 'ment control memberto a substantially neutral position;

FIGURE 6 is a diagrammatic View illustrated similarly to--FIG.' 3" showing the system 'ina lower condition, namely, a condition providing,for'downward-movement of the dozer blade;

I ment pump FIGURE 7 is a diagrammatic view illustrated similarly to FIG. 6, showing the system in a lower-overload condition; v 7 FIGURE 8 is a. diagrammatic view illustrated similarly to FIG. 3, showing the system in a float condition;

FIGURES 9 and 10 are'enlarged sectional views of one of the valves in the control system of this invention, showing two positions of the valve;

FIGURE 11 is a fragmentary sectional view of a modified form of the control valve shown in FIGS. 9 and 10, showing the valve in assembly relation with the housing for the variable displacement pump and illustrating the valve in one position;

FIGURE 12 is a fragmentary sectional view of the modified valve shown in FIG. 11 illustrating the valve in another position; and

FIGURE 13 is an enlarged view of the encircled portion of FIG. 12 indicated by. the numeral 13.

, With reference to the drawing, the hydraulic control system of this invention is illustrated in connection with a variable displacement pump 12, of reciprocating piston type and described in detail in my prior application Serial No. 3 35,002, filed August 20, 1959, mounted on a tractor 14 for operating a hydraulic cylinder assembly 16 to raise and lower a dozerzblade 18. However, it is to be understood that whilethe control system of this invendor} has particular utility in the illustrated installation, the system has general utility in systems which include variable displacement pumps. As used herein, variable displacement pump means a positive displacewhich can be adjusted at a given speed 0 operation to vary the volumetric displacement.

The cylinder assembly 16 includes a cylinder member 20 which is pivotally mounted on the front end of the tractor, a piston 22 (FIG. 3), and a piston rod 24 which extends through the lower end of the cylinder and is connected at its lower end to the dozer blade 18. A dozer bladesupporting frame 26 which is pivotally supported at its rear end on shafts 28 carried by the tractor 14 has the dozer blade 18 mounted on the front end thereof so that the blade is forwardly of the front end of the tractor 14. The pump 12 is hydraulically connected to the cylinder assembly 16 for moving the dozer blade 18 between its upper position shown in broken lines and its lower position shown in full lines. A handle 30 mounted on the tractor 14 at a position for convenient operation by the operator is connected to a control valve 34 which controls the operation of the pump 12.

The control system, whichis indicated generally by the-numeral and is illustrated diagrammatically in FIGS. 3-8, inclusive, is made up of three related circuits, namely, a main or cylinder operating circuit which includes the pump 12 and the cylinder assembly 16, a supercharge or makeup circuit which replenishes fluid in the main circuit to compensate for pump leakage and movement of the piston rod 24 into and out of the cylinder 20, and a control circuit which includes the valve 34 and is provided for hydraulically actuating the pump 12 to achieve the desired movement of the dozer blade 18.

' The main circ uit includes, in addition to the pump 12 and the cylinder assembly 16, -a pair of substantially identical cylinder holding

check valves

36 and 38 connected to the upper and lower ends, respectively, of the cylinder 20 and a cylinder float control valve 40 disposed between the

valves

36 and 38 and closed except in the float condition of the system 10. Each of the

valves

36, 38 and 40 is constructed for pilot operation, as will more clearly appear hereinafter. The pressure in the main circuit, in the illustrated embodiment of the invention in which the cylinder assembly 16 raises and lowers a dozer blade 18, may be as high as 1500 psi. on the output side of the pump 12, which is capable of pumping 150 gallons or more of fluid per minute. d

The supercharge circuit'includes a gear pump 42 connected to a tank or reservoir 44 for the system 10, a check valve 46 over which the pump 42 pumps to tank, a check valve 48 which communicates with one side of the pump 12 and a pilot-operated check valve 50 which communicates with the opposite side of the pump 12. In the illustrated embodiment of the invention the check valve 46 is set to provide a pressure of about 50 psi. in this circuit and the pump 42 is capable of pumping about 28 gallons per minute.

The control circuit includes a gear pump 52 which is connected through a line 47 to one side of a check valve 54 that is connected on the opposite side by a line 49 to the supercharge circuit. In addition, the control circuit includes the control valve 34 and a pair of substantially identical control valves 56 and 58 which control a pair of tilt box actuating pistons 60 and 62, respectively, which are axially aligned and disposed on opposite sides of the pump 12 for moving the tilt box. In other words, control fluid can be directed through either one of the pilot valves 56 and 58 to extend either one of the pistons 60 and 62 to move the pump tilt box in a desired direction to obtain the desired flow of fluid from the pump 12. In the illustrated embodiment of the invention, the pressure in the control circuit is about 400 p.s.i. and the pump 52 is capable of pumping about 8 gallons per minute.

The pump 12, which is described in detail in my prior application referred to above and is only shown diagrammatically, includes a casing 61 having a tilt box 63 mounted therein for rotatable movement in opposite directions about an axis perpendicular to the plane of the paper in FIGS. 4 and 6 between the limit positions illustrated therein in which the actuating pistons 62 and 60 bottom in their respective cylinder supports. The tilt box 63 is provided with an actuating arm 65 disposed between the control pistons 60 and 62. A pair of spring assemblies 173 are associated with the pistons 60 and 62 and are initially accurately adjusted so that they constitute yieldable abutments which exert no force on the pistons 60 and 62 in the zero displacement position of the tilt box 63. Each spring assembly 173 will compress and yield so as to permit retraction of its piston 60 or 62 when a retracting force is applied to the piston. The details of assembly of the spring assemblies 173 with the pump 12 are included in my copending application referred to above.

, As shown in FIG. 2, the drive shaft 67 for the pump 12 is driven by a tractor engine-driven shaft 69 disposed below the tank 44 which is mounted on the front end of the tractor 14 and provided with a fill neck 45. The gear pumps 42 and 52 are driven from a common shaft and are mounted in a casing 55 located within the tank 44 and communicating with fluid in the tank 44 through a filter 53. The pump 12 is mounted on a valve block 71 at the front end of the tractor and a gear train 73 is provided on the block 71 for transmitting driving power from the shaft 67 to the pumps 42 and 52. The check valve 54 is.

mounted in a casing 57 disposed within the filter 53, the check valve 46 is mounted on one side of the valve block 71, and a housing 59 which projects forwardly from the valve block 71 at a position above the pump 12 encloses the

valves

36, 38, 40, 48 and 50 which communicate through passages in the valve block 71 and the housing 59 with the pump 12 and the tank 44 in a manner herein-- after described. The valves 56 and 58 are mounted on.

the pump casing 61.

While the pump 12 is normally controlled by the control circuit, under overload conditions the pump 12 is of the tilt box 63 in either direction so as to vary the stroke of the pistons in the pump.

The operation of the system 10 can be better understood if Ithevalves in thesystem are first described in :detail. The control valve 34 includes a'casing 64 in which a spool 66 is mounted for reciprocal movementand which is connected through ports 68, 70 and 72 to the tank 44. The spool 66 has a stern 66a'which is connected to the handle .30 so that the handle 30 is operable to reciprocate the spool 66. The casing 64 has three

outlets

74, 76 and 78 for fluid under, pressure 'and a pair of inlets 80 and 82. Thespool '66 is shiftable longitudinally of the casing 44 between holdj? raise, lower and float positions, shown in FIGS. 3, 4, 6 and 8, respectively, which are hereinafter described in detail in connection with a detailed description of the operation of the-system.

'The pilot-operated check valve 50 includes a valvestem which is guidably mounted in a casing 92 and is provided adjacent one end with a valve member 94. The opposite'end'of the stem engages a ball 96 which is urged by a spring'98 into a position closing'a valve passage. 100 in the casing '92. The casing 92 has one end 91provided with an opening that communicates with a line 104 and the 'opposite end 93 of the casing 92 is closed.

As used herein, the term line refers to a passage, conduit, tube or the like, through which fluid canflow from one point to another. When the'passage 100 is closed, comrnunication of a fluid line 102 on one side of passage 100 with the line 104 on the opposite side is blocked. The line 102 connects to a line 106 extending between pump 42 and relief valve 46 and line 104 connects to a line 108 which extends between pump 12rand check valve 36. The portion of the casing 92 between-the closed end 93 and the valve member 94 communicates with'one :end of a line 110 connected at its opposite end to the valve "36. When the pressure inthe line 110 is above the pressure in line 104, the fluid under pressure from line 110 "acting on valve member 94 will move the valve stem 90 so as to move the ball 96 out of a position closing. the valve passage 100, against the-pressure of spring '98, which-is small.

Since the cylinder holding

check valves

36 and 38 are substantially identical, only the valve .36 will be hereinafter describedin detail, with like numerals on the two valves indicating like parts. The valve 36'consists of 'a casing 118. in' which a hollow elongated plunger member 120, provided at one end with "a stem 122, is mounted for reciprocal'movement. The stem 122 is formed intermediate its ends with a shoulder 124, and a disk 1% is *slidably supported on thevalve stem 122 for .movement in one direction to a position engaged with one end1128 of "the casing 118' and in the opposite direction to a position engaged with the shoulder 124. A spring positioned .withinrthe hollow plunger 120and engaged with the opposite end 132 of the casing 118 urges the plunger 120 in a direction to seat an inclined'surface 134 on the plunger member 120 against a correspondingly inclined surface .136 formed in the casing 118. An opening 138 in'the plunger 120 provides for a constant communication-,ofthe spring urged side of the plunger 120 with a port 140 in the casing 118. The port 140 communicates with'a line 142 extending between the valve 40 and the closed end of the 'cylinder 20. The corresponding'port 140 in the valve38 communicates with a line 144 which-extends between the --valve 40 andthe piston rod end of the cylinder 20.

The cylinder float control valve -40'thus communicates .011 one side through the line 142 with the cylinder 20 ,on the top side of the piston 22 and 'on the opposite side through the line 144 with the cylinder 20 on thebottom side of the piston 22. The control valve40 consists of .a casing 146 having a valve member 148 disposed therein 'and provided at one end 154 with a port 150 which communicates through a line 152 with theline 106 which is connected to the output side of the supercharge pumpv42. ,A port 156 at the opposite end of' the casing 146 communicates through a line 158 with the outlet port 74 in "the control valve 34. A spring 160 :extends between the easing end 154 and the valve member 148' forurging='the port for a peaking relief valve 202.

valve member toward the oppositeend of the casing" .against-the. pressure offiuid at'the .port 156. .Whenever the pressure at the port 156 is sufficient to move the valve memberv 148 in the opposite direction, the valve member 1481's movedto thepositionshown in FIG. 8 in which the lines 142 and 144 are fluid connected through the valve 40.

Since the tilt box control valves 56 and 58 are,-sub- .stantially identical only the valve 58, enlarged views of which ares'hown inFIGS. 9 and lO, will be described-in detailwith alike numerals indicating like parts on the valves. The valve 58 includes a casing thathas a port 172 which communicates witha cylinder support 174 for :the control piston'62. A port 176 in the casing'l170communicates through a line 178 with the outlet port 78in .thescontrol valve 34. A port 180 in the casing1-70 communicates with tank 44. A pair ofspaced annular walls 182 and 184 in the casing 170 separate the ports.172, 176 and 180. Apiston186 is slidably supported onthe walls 182 and 184 andis urged by a spring 188 toward aposition engaged with an annular seat 191 which surrounds. aport at one end of the casing. Aplurality of keywayshaped grooves 192 in the piston .186 are located so that theyextend across'the wall 184 (FIG. 9) whenthe piston is engaged with the seat:191.

The port 172 for the valve 56 communicates with a supporting cylinder 175 for the control piston-60 and the -port 176.communicates .throughaline194 with the outlet port 76 in the control valve 34. I

:The ports 190, which undercircuit overload conditions hereinafter described constitute inlet ports for the valves 56 and 58, are connected to the ends oflines 196 and 200, respectively. The oppositeend of the line 196 isconnected to .the inlet port fora peakingrelief valvev 198 and .the opposite end of. the .line200 is connected to the inlet The peaking relief valves 198 and 202 are identical, each including a casing 204 having an inlet port 206 and anoutlet. port 208' and a valve member 210 urged by a spring 211 toward a seated position closing theinlet port206. In the valve 198 the port- 206 communicates with the .line 196 and the outlet .port'208communicates with a line 212 which connects to the line 108. In the-valve .202 the inlet port 206 communicates with the line 200. and the port 208 communicates with axline 214-which is connectedto -a line 216 'that-extendslbetween thepump 12 and the valve '38. A line 218'extends between the-line 216 and the line 110 which is in turn connected to the 1ine196 bya line 220 ..for a pnrposeto appear presently. A line 222 connects the line 200 and the line 108 and a line 224 connects the line 222 andv the valve 38 also for a purpose to-appear presently.

Operation of thevalve 34, the control pump 52 is supplying fluid --to the valveinlet ports 80 and 82 both of which comrnunicate through thevalvecasing 64 with the tank port 70 so that no pressure is being developed by the control pump 52 and the relief valve 54 is closed. Consequently, since nopressureis' being applied to the control pistons 60 and 62 the tilt box 63 is in a zero displacement position and no oil is being pumped by the pump 12. The

supercharge pump 42 is pumping over the relief valve 46 to tank 44 so the pressurerequired to open the valve 46 determines the pressure developed by the pump '42.

"Theva'lves 48 and 50 are set to open at a pressure b'elow this pressure and'the volume outputof pump 42 is such 'that'the full-volume of fluid from pump 42 is greater than 7 will flow through

valves

48 and 50 at a pressure below the pressure at which relief vale 46 opens. At this pressure sufiicient fluid will flow through relief valve 46 under all conditions to prevent a pressure rise in the supercharge circuit above the pressure at which valve 46 opens, which "is hereinafter referred to as supercharge pressure.

Fluid under a supercharge pressure is supplied to both the cylinder holding

check valves

36 and 38, and in the illustrated embodiment of the invention this pressure on the valve members 120 is not suificient to overcome the force of the springs 130. This is because the springs 130 are strong to insure quick and positive closing of the

check valves

36 and 38. In the illustrated embodiment of the invention, a spring 130 exerts a force of about one hundred pounds on its valve member 120. However, even if the springs 130 were weak enough to permit opening of a

valve

36 or 38 under supercharge pressure, the systern 10 would work satisfactorily so long as the weight of the dozer blade 18 is sufficient to maintain the pressure on the spring side of valve member 120 in valve 38 closed against supercharge pressure on the opposite side. If the valve 36 opens under supercharge pressure, there will only be enough flow therethrough to fill the cylinder 20 on the top side of piston 22, following which the valve member 120 will seat because the pressures on opposite sides thereof will be equal.

Normally, the cylinder 20 is full and the cylinder holding

check valves

36 and 38 are closed and prevent movement of the piston 22 in either direction in the cylinder, and the shut-off valve 40 blocks communication of opposite ends of the cylinder 20.

Since the piston 22 is unbalanced in the system of this invention, meaning that the areas on opposite sides thereof are unequal, it must be weighted sufliciently to raise the pressure on the small area side enough to maintain the valve 38 closed against supercharge pressure, in the event the spring 130 will not accomplish this purpose, to prevent the piston 22 from moving downwardly when valve 34 is in a hold position. In the use of the system of this invention with a piston 22 which is not weighted, and with a spring 130 which will not hold the valve 38 closed against supercharge pressure, the piston 22 must be balanced, as by extending the piston rod 24 completely through the cylinder 20, if the piston 22 is to be maintained in a fixed position in cylinder 20 in the hold position of the control valve 34. The

valves

48 and 50 are shown open but of course as soon as the

lines

108 and 216 are filled with fluid from the pump 42, the

valves

48 and 50 will close since the fluid pressures on both sides of each valve will be the same, and the full output of fluid 12111: pump 42 will flow through the relief valve 46 to ta It can thus be seen that in the hold position of the valve 34, the dozer blade 18 is held in a fixed position relative to the front end of the tractor 14.

When it is desired to raise the dozer blade 18, the control valve 34 is moved to its raise position shown in FIG. 4 in which fluid from the pump 52 supplied to the inlet port is blocked by the spool 66 against travel out of the casing 64 and fluid supplied to the inlet port 82 flows through the casing 64 to the outlet port 78. Line 194 is now in communication with tank port 68 since in this position of the control valve fluid may pass around the upper end of the spool 66, as well as through the grooves thereat, as can be seen in FIGURE 4. Fluid from the outlet port 78 flows through the line 178 to the inlet port 176 in the valve 58. Since the piston 186 is seated, fluid from the port 176 flows through the piston grooves 192 to the valve outlet 172 and into the support cylinder 174 for the pump control piston 62. The control piston 62 is extended by this fluid to thereby tilt the tilt box 63 in a clockwise direction to the limit position shown in FIG. 4. Excess fluid from pump 52, over and above that required to extend piston 62, flows through the check valve 54 and the line 49 into the line 106 in the supercharge circuit.

The pump 12 has two ports or openings, shown diagrammatically at 230 and 232, either one of which can function as the pump inlet or outlet depending on the direction of tilt of the tilt box 63, as described in my copending application hereinbefore referred to, and in the position of the tilt box 63 shown in FIG. 4 the valve port 230 communicates with the cylinder bores (not shown) in the pump 12 when the pistons in these bores are on their suction stroke. Consequently, the port 230 functions as the inlet port for the pump 12 and the other pump port 232, which communicates with the cylinder bores during the time the cylinders in these bores are on their discharge stroke, functions as the pump outlet. Fluid under pressure from the pump port 232 flows through the line 216 to the valve 38 and moves the plunger off its seat 136 so that fluid flows through the valve 38 and line 144 into the lower end of the cylinder 20 so as to raise the piston 22. Fluid from line 216 flows through a line 234 to the relief valve 48 and keeps the valve closed.

Fluid in the cylinder 20 hausted through the line 142 12 through the open valve valve 36 is maintained open by fluid under pressure from the pump 12 supplied through the line 218 to a pilot port 219 in valve 36 so as to move the disk 126 toward the right as viewed in FIG. 4 to unseat the valve plunger 120.

The volume of fluid exhausted from the closed end of the cylinder 20, during upward travel of the piston 22, exceeds the volume of fluid being supplied to the piston rod end of the cylinder 20 because of the space in the cylinder 20 occupied by the piston rod 24. Consequently, the volume of fluid flowing toward the inlet port 230 exceeds the volume of fluid being pumped out of the pump 12 through the outlet port 232. The excess fluid flowing to the pump inlet 230 flows back to the tank 44 through the pilot operated check valve 50 which is maintained in an open position, illustrated in FIG. 4, by fluid under main pump outlet pressure supplied to the valve 50 through the line 110 so as to move the valve member 94 in a direction to unseat the ball 96. A restriction 111 is provided in the line 110 to prevent a surge of fluid to the valve 50 tending to move the valve parts so rapidly as to decrease the wear life thereof.

The spring 188 in the pump control valve 56 is adjusted to prevent unseating of the piston 186 when the pressure of fluid on the outlet side of pump 12 is at or below the predetermined maximum pump operating pressure, which in the illustrated embodiment is about 1500 p.s.i. This pressure is determined by the capabilities of the pump 12 and the other components of the system 10 and is adequate for moving the blade 18 to accomplish the usual work operations for which the tractor and dozer assembly is intended.

However, in the event the blade 18 encounters an obstruction which prevents raising movement of the blade 18 or, as more often happens, in the event the piston 22 engages the closed end of the cylinder 20, continued operation of the pump 12 causes an immediate rise of the pump pressure to a pressure above the predetermined maximum pressure which is high enough to unseat the piston 186 in the valve 56 as shown in FIG. 5. Fluid then flows into valve 56 through port 190 and from valve 56 into the cylinder support for piston 60 through port 172 so as to extend the depressed pump operating piston 60 and move the tilt box 63 toward its zero displacement position. Since some time is involved in moving the tilt box 63 to substantially its zero displacement position, and since the pressure in the system 10 on the output side of the pump 12 is continually rising during this time, the peaking relief valve 198 is provided to limit the pressure. In the illustrated embodiment of this invention, the spring 211 in valve 198 is adjusted so that the valve member 210 above the piston 22 is exand is returned to the pump 36 and the line 108. The

. moves off its seat at a peak pressure of about 1700 psi.

Consequently, the valve 198 will open to dump enough fluid from the pump outlet line 216 to the inlet line 108 to 9 lower the pump output pressure to below 1700 p.s.i. 'To accomplish this it is not necessary-thatthe relief valve 198 be of a large size suflicient to carry the entire pump I output. It is only necessary that the'valve-198 be capable of carrying about 30 to 40 gallons per minute of fluid.

'Since no relief has beenprovided for the fluid on'the outputside of the pump 12, otherithan to prevent the pressure from rising above 1700 p.s.i., a pump output pressure I of about 1500 psi is maintained. Since'there is no'flow through'valve 38, it closes tightly to trap fluid in cylinder 20 which is on the underside of the piston 22 and which d-sat-about the 1500 psi. pressureso that the blade 18 does not fall.

'When the piston 186 is unseated (FIG. 10) some of the fluid suppliedthrough poi-1:190 to valve 56 flows through the grooves 192 to the port 176-and'into line 194which communicates with the tank port-68 in control valve 34.

-The grooves'192 are located relative to thewall 184-and are of a-size such that theycon-stitute: a restriction to flow I through the valve'56 from port 190 to tank when the, pump 5.

12*is pumping at full volume capacity. Consequently,

sorne fluid entering valve .56 through portl190flows out through port 172 into cylinder'175 and causes arise in the pressurein cylinder 175 until the combined force of the fiuid. and spring assembly 173 tending to extend pistoni60 exceeds the oppositely directed force on piston 62. so that the piston 60 is extended. During extension of piston 60,

the tilt .box 63 is rotated toward its neutral position.

This causes a corresponding reduction in the volume of fluid being pumped by the pump 12 through the port 190 f *for valve 56, until at someposition of piston 60 short of -.a centered position of tilt'box 63, the volume of fluid being pumped into valve 56 through port190-equals the volume of'fluid flowing through the-grooves 192. .In: this equi- 'Jibrium position of the piston 186 in valve 56,'the-force exerted on the piston by the compressed spring188 is "b'al- -.ance'd by an equal and oppositely directed force-exerted on the piston 186 by the fluid entering through port 190.

.The' flow throughport 190 in this equiblibrium position of piston 186 at a pressure drop-of from about 1500 p;.-s.i.:to the lower pressure existingat port 172 and in cylinder175 equals the 'flOW: through the grooves, 192 at,lthe pressure drop existing between ports 172 and 176. By forming the valve .56 so that "a -flowjthrough grooves .192;is;.insured when the valve member 186 is unseated,-a-movement'gof the pumptilt box 63 ton substantially-zerojdisplacement positionwithout any hunting is insured.

All-of the above described action of valve 56 during rnovernent of tilt box 63 toward a substantially zero dis- 1 placement position takes place very quickly. In the event the piston 186 hould momentarilymove against the spring 188 to a position in which the-grooves 192 are closed ;-by

the wall 184, as in'the case of a sudden pressure above 1700 p.s.i., the quick response of the tilt -box"63 to provide for a reduced pump output and the action of the peaking relief valve 198 provide for a reduced pressure at port190 so that the spring 188 again moves the piston 186 to its equilibrium position. The spring 188 is of a strength such that at pressures at port 190 between the equilibrium pressure and the peaking relief pressure fluid can flow through grooves -192-to tank.

In the equilibrium position'of piston 186 in valve 56, the

pump 12 will pump suflicient fluid to take careof flow 18 is prevented and the control valve 34is in the raise position, the tiltbox 63 is automatically moved to a sub stantially zero displacement position, by theaction of valve 56, so that very little power is required from the tractor engine to operate the pump 12,.and the blade 18 6 1s 1n,tens1on,'the pressure in the cylinder 20 below the piston 22 willv riseqand the pressureabove the piston 22 plunger the top side of the is "maintained in thegpositio-n "at which raisingixmovem'ent i-WaS -StOPPEd.

In the event "downward movement-'ofithe blade 18. is'de- :Isired,- the. control'valve34 is moved. to'its .-down' position shown in :FIG. .6, in .which .the valve spool "66' is intra 'jpOSltiOH'lIl which thei'nlet port 82 is' blocked'and'the inlet :..port communicates with the. outletport 76. The outlet -port '74communicateswith the tank-port 68 and theoutlet :port '78communicates with the tank port 72. Fluid-under -.';pressure :from the-port. 7.6. flows through. line .194 ito the sinlet-tport 176-in valvec56rjfor flow. outtheiport 172:.into i the cylinder 175 so .as to extend the'control piston 60 and ;rotatethetilt box- 63 .in-aa counterclockwise jdirection,ias viewed ,-in;.-FI'G.; 6. Some: of the. fluid in .the cylinder.174 j-is-exhausted,-during1 retraction of piston 62,.through the valve 58, the.line 178 and the control valve tank port 72.

Assume that the blade 18 is encountering an obstacle,"such .-?as the ground surface which resists wits downward -move- ..ment so that the piston r0d'24 isiunder compression.

The pump 12 now'operates-so that the port-230 functions.as theoutlet port and the port 232 functions asthe inlet port. Fluid is pumped out of the port -230..and .travels through the. line 108 to thecheck valve 36,'where .it, unseats the valveplunger and travels out the .port

and through the line,142 to the closed end. of the cylin- ..der .20 so as to exert a. pressure on the'top-side ofpiston T22 which moves-the piston downwardly, to infturn move the dozer-blade 18 downwardly. .Since the float: control .shut-ofivalve 40 is closed no fluid under pressure is. supplied tothepiston rod end of the cylinder 20.

.As the-piston22-moves downwardly, fluid is exhausted from the cylinder 20 through the line 144, theiopen valve 38..and .the line 216. to the ;pump inlet 232. Thevalve 38 is .maintainedopen by fluid under pump. outletgpressnre supplied to.,the pilotinlet 219 for the valve 38. through the ...1ines.222 and.224. Thisffluid moves the (1181(126'011'1116 stem .122 into engagement with the shoulder :124; so ':as

,to. move the plungerlZG-to an unseated position.

Theiarea of the disk126 subjected to pressuresbyfluid entering port 219 is-suflicientlygreater.than, the :area on .,.the spring-u rged side plunger 120. toprov-ide for opening of valve 38 at :a pressure considerably below Ether limit ,pressureuo'f 1500. psi-"when the pressureof fluid. trapped .in cylinder.ZOonthe-underside of piston 22,is 1'S00;p;sli. The ratio of this area of =disk-.126. to the areafo'fthet spring- .urgedside of,plunger -120 is greater than the 'ratioxof the .areas. on the. topand .bottornsides of piston 22 to prevent a,bu.ildup.of thevforce on the plunger 120'resisting' opening fast er than, the buildup ofthe force on the-disk 126 'in'the .conditionof thev system 10 shown in FIG. 6 followingaa condrtionsuchas shown in FIG. 5 in which fluid is trapped in cylinder 20 on the underside of the piston .22. In the illustrated embodiment of'the invention, the ratio :of .the

areaof disk.126 to the area of the spring-urged sideofthe 120 is about 2 to 1 and the ratiooffthe :area of piston 22 to the area ofithesbottom 'sideof piston .22 is less than this.

In a free fallcondition of the dozer blade 18, namely, a condition .in which there is no external force resisting downward travel of; the blade 18 so that the piston rod124 will fall. Consequently, ,the.plunger 120'in the valve.38 w ll-be moved by the .pressureon the underside of the .piston 2 2 toward a seated position against'the pressure applied to.the disk 126, which latter .pressureis the same means of lines 222 and 224. As soon as the plunger 120 :seats against the seat 136, the pressure onvthe top side of thepiston 22 will rise, because of 'the resistance to -downwar'd movement of the piston 22 created-by the fluid .trappe'din cylinder 20 belowthe piston 22. This will :zcausethe disk 126 mm move'd in-adirection to open the valve 38. The result-of this type of operation is thatthe plunger 120.

1 blade 18'is a start and stop type of movement.

In order to overcome this tendency of the system to operate so that the dozer blade 18 moves downwardly in a,

this manner when the piston rod 24 is under tension, the line 194 which is connected to the port 176 in the control valve 56 is also connected to the inlet port 254 in a oneway check valve 250 that includes a casing 252 having an outlet port 256 which is connected by a line 258 to the line 224 communicating with the pilot port 219 in the valve 38. The check valve 250 has a ball member 260 therein which is urged by a spring 262 toward a seated position closing the inlet port 254. There is a restriction 264 in the line 224 at a position between the line 222 and the connection of the line 258 to the line 224 for a purpose to appear presently.

When fluid under control pressure is supplied through the line 194 to the check valve 250, in response to movement of valve 34 to a down position (FIGS. 6 and 7.),

the ball member 260 is unseated so that fluid under control pressure is supplied through the line 258 to the pilot port 219 in the valve 38 so as to move the disk member 126 against the valve stem shoulder 124 and unseat the The restriction 264 prevents free flow of this valve actuating fluid into the main pump operating circuit. When the pressure in the line 108 exceeds the pressure in line 258, the check valve 250 is closed. The valve opens again only if the pressure in line 108 drops below the pressure in the control circuit. At such time valve 250 opens to prevent hunting of the piston 22 during downward movement.

The area of the disk 126 in valve 38 is proportioned with respect to the area on the underside of the piston 22 and the weight of the blade 18 so that the total force exerted on the disk 126 by the fluid under control pressure is sufficient to maintain the plunger 120 in an unseated position against the combined forces in the opposite direction exerted by the fluid communicating with the underside of the plunger 120 and the spring 130.

It is seen, therefore, that the fluid from the control circuit is used to insure an open position of the valve 38 when the pressure in line 108 is not high enough to keep the valve 38 open during a powered movement of the dozer blade 18 downwardly.

When the pump 12 is operating to pump fluid into the upper end of the cylinder 20, to move the dozer blade 18 downwardly, fluid under operating pressure from the pump 12 is supplied from the line 108 through the line 104 to the pilot operated check valve 50 so as to maintain the ball member 96 in a seated position. Fluid under operating pressure is supplied from the line 222 to the line 200 which communicates with the control valve 58 and the peaking relief valve 202, and so long as the pressure of this fluid does not exceed the maximum normal operating pressure, about 1500 p.s.i. in the illustrated embodiment of the invention, the valve members 58 and 202 remain closed as shown in FIG. 6. The volume of fluid being supplied to the pump inlet 232 through the line 216 is less than the volume of fluid which must be supplied to the cylinder 20 on the top side of the piston 22 to keep the cylinder 20 full because the piston rod 24 is moving out of the cylinder 20. The necessary additional volume of fluid is supplied to the inlet port 232 from the supercharge pump 42 which pumps through the open check valve 48 to the line 216. So long as replenishing fluid is required at the pump inlet 232, the pressure in the line 216 will be below supercharge pressure so that the valve 48 will be open. The communication of the relief valve 54 in the control circuit with the line 106 provides for a dumping of control circuit fluid into the supercharge circuit as a precaution against the creation of voids in the cylinder 20 above the piston 22. The valve 54 can be connected to tank, instead of to the supercharge circuit,'but it is advantageous to utilize the control circuit fluid to increase the volume of fluid available in the supereharge circuit.

In the event the downward movement of the piston 22 is stopped, because it engages the lower end of the cylinder 20 (FIG. 7), the pump operating pressure immediately rises above the predetermined maximum pressure for which the valve 58 is set, 1500 p.s.i. in the example, resulting in unseating of the piston member 186, so that fluid flowing to port 190 in valve 58 is supplied to the cylinder 174 so as to extend the control piston 60 and stroke the tilt box 63 toward its neutral position. The valve 58 operates like the valve 56 previously described to insure stroking of the pump to a substantially zero displacement position without any hunting.

The peaking relief valve 202 bleeds of! enough fluid in the line 200 to prevent the pressure from exceeding a predetermined maximum pressure. In the illustrated embodiment of the invention this pressure is about 1700 p.s.i., and as previously explained, the peaking relief valve 202 functions to prevent undue pressure rise in the system 10 during the time involved in moving the tilt box 63 to a substantially zero displacement position. Since there is no flow through the valve 36 the piston 120 is moved by the spring 130 to a position engaged with the seat 136 to there-by hold the pressure on the top side of piston 22. In this position of the tilt box,

the pump 12 pumps enough fluid to take care of leakage at the pump, flow through the valve 58, and any other leakage in the system on the output side of the pump. In the event the pressure falls below the pressure necessary to maintain the piston member 186 in valve 58 in an unseated position, for any reason, the piston member 186 moves to the seated position shown in FIG. 9 in which the cylinder 174 communicates with the tank port 176 in the valve 58, and the control pressure in the cylinder 175 again extends the piston 60 to provide for tilting of the tilt box 63 to a position in which the pumping of an increased volume of fluid out the pump outlet 230 is resumed to again raise the pressure at the port 190 to the predetermined maximum.

In FIGS. 11-13, a valve 58a is illustrated which may be substituted for the valves 56 and 58, and is shown associated with the cylinder 174 only for purposes of illustration, it being understood that an identical valve may be mounted on the opposite side of the tilt box 63 and associated with the cylinder 175 if desired. The valve 58a includes an elongated casing 300 which is provided with a longitudinally extending bore 302 and is integrally formed with the cylinder 174 which is at substantially right angles to the bore 302. The valve 58a is assembled with the pump 12, only fragmentary portions of which are shown in FIG. 11, by inserting the cylinder 174 in an opening 304 in the pump casing 61. The cylinder is similarly inserted on the diametrically opposite side of the casing 61 so that it is in substantial alignment with the cylinder 174, as shown in FIGS. 3-8, and as described in detail in my copending application Serial No. 835,002. The spring assembly 173 is positioned in the cylinder 174 and engaged with the piston 62 so that the spring assembly constitutes a yieldable abutment which exerts no force on the piston 62 in the neutral position of the tilt box 63 in which the actuating arm 65 is centered between the pistons 60 and 62. The spring assembly 173 will compress and yield so as to permit retraction of its piston 62 when a retracting force is applied to the piston and of course the more the spring assembly 173 is compressed the greater the force it exerts on the piston 62.

The valve casing 300 is secured, such as by bolts (not shown) to the pump casing 61 so as to maintain the cylinder 174 in the position shown in FIGS. 11 and 12.

A spool sleeve 306 is threaded into the bore 302 in the casing 300 and is provided with an axial bore 308 which extends inwardly from the inner end 309 of the vland 328 to the openings 3-16 whichlco nrnnnieatet the tank. tFluid suppliedthronghithe. pvort 17, 2 to. the cylinder 174 extends the depressed pump pperating piston 62 and moves the tiltboxiqfa toward a position centered sleeve 306 and has its inner end''3101ocatedintermediate the ends of the sleeve 306. 'lhree-sets'of radially extending

openings

312, 314 and 316, are formed'at 'longi- ,tudinally spaced positions in magnet/@ 396 and intersect the bore308. A spool 318 is 'slidably supportedinfithe bore 308 so'that the innerend 20thereof isi spaced from the bore end 310. An axialpassagethe spool 318 at the end 320 thereof is in constant :fluid communication with the sleeve opening 312 vthrough; the space between the spool end 320..an'd the bo re en df3l0."'The= spool 318 is formed with three longitudinally spaced lands 326 328 and 330 and the passage. 32 2 has, a transversely extending portion 332 at its inner end intersects'the outer surface of the spool;318' be t ween the lands 326 and 328. V

.The opposite end of the spool' i318;hasva reduced diameter extension, 334 which :supports a. guide 336 for the spring 188 that extends'be'tween the inner end of the bore-302 and the guide 336. The springll88 urges .the spool 318 toward the rightas viewed'in FIGSgll and 1'2 tothe position shown in FIG.-1,11 in which anendflange 337 on the guide336 engages the inner'endofjthesleeve In use,vthe valve 58a is connectedtin the control system so't hat the sleeve openings 312 communicatew'ith the end of line 200 opposite the.f 'aea king v relief.=valv'e 202, the s1eeveopenings314 communicate with the connecting port 172 between the cylinder 1 74and the. bore.30 8,iand

system to are in the position showniniFlG. emin ne dozer blade 18 is being.moved-downwardlL the valve 58a is still in the position shown in;'FlG.. 1"1,fhowever the line 178 which communicatestwithi the sleegeopenings 316 connects through the control valve 3 4 withjthe. tank 144,

the sleeve openings-312i communicate with the outputside v t the openings 316vcommunicate throngh'theport l76 in the weasing300 with the line .178 iwhieh is connectedtito the of the pump "1'2, andthef tilt.box operating piston fllis" depressed by the action of pis'tonf60l When. theipiston 22vengagjes the. bottom end of the cylinder 20 (FIG. 7) .so that continued operation of 'the pump 12 causes an immediateriseto f th e pump pressure ing pressure, which in theillustratedleinbodirnentlof the invention is about 1500 p.s .i., the ffo rce of the fluid at the openings 312 is suflicient to move .;the .t spool .0318

v against'the force of the spring; 1 88 to e -position in yvhich the land 32 8; is positionedrelative to thetsleeve penings 314 such-thatfluid supplied throng passage 22510 the. space between the lands Q26 v and 328 12311 fl ow out the openings 314 to; the cylinder porttl7z andcaifound the between the pistons 60and62 fIhelpeakingajreliefsvalve 202 .will open totpreventthe press ure ong th e outputlside of the pump 12 fromtrising above-the 1700 p.s.i.. pressure during the time involvedin moving thetilt boififi toward a neutral position.

This varrangement provides for "a continual flow 0f fluid through the valve 58atottian k, during ithe i t irne -t he valve 58a is in aposition providing-for;a flqw.ofdiuid to extend the piston 62. Duringentension ,of theepist on 62, the volume of fluid being pnrnped by the -pump v 12 is :eontinually being reduced andtat some :position 0f piston 62 short of a fully extended position in which thetilt box a to a pressure above thepredeternnned. max mum operat- 163 is centered, thevolume Of fluid' being pumped into the valve 584 ,through 'theopenings312equals the volume of'fiuidflowing out the valvef58athrough tankport 176.

In; an equilibrium ,position of the spool 318, shown in IQ-and 13, the amount of fluid flowing past land [2:28 to the opeiiingsfiilyt' for cylinder "174i equals'the 'amount offluid flowing 'from'the openings 314' past land $528; to tank'port-176. fAs shown-in FIG. l3,'-the'land 328 is slightly to "the right" of :a centered position relative to the openings3'1 4, in the equilibrium ipo'sition of spool I 318, hecausefthe pressure dropgin the illustrated embodi- 11161 of theinventiony is greater between "the fluid on'the right hand sideof land '328 and :the'fluidin openings 3 -14 andpylinderglflthan the fdrop-between' the fluid in openings f 141and 'thefluid on' the left-hand side of land 328, v which eommnnicates with tank port 176. In this'position v ofthei tilt box-63,?the volum'e' of fluid being pumped by theypnnipf12issi1ificient to take care of the flow" through "the; vaIve' S Sa an'd' leakage at the pump 12, as-well= as any 20 ;eating with' the output-side ofith'e-pump 1 2. At a pressure of the fluid at'theopenings 3l2labove theequilibrium pressure filldjblOW the peaking'pressure of 1700 p;s.i., the "spool 318 is moved slightly to the left-ofthe position shown ,in "FIGS. Hand 13' to ;a posit-ion -in-' which fluid other leakage in the portion of the system;10-'communileaks past" the "land-328"totank" and around the land 328 tothe port7,172 and into the cylinder 174 so as to extend the; piston 62 torotate the tilt box 63 to-aposition in which the amount'of fluid pumped is'insuflicient to maintain the pressure above the equilibriumpressure. Con- ,sequently the pressure at; the openings'312-quickly' falls ]to" the equilibrium pressure and the spring 188 moves the ,spool i318 to the "equilibrium position.

fIn the 'equilibriuin' position of the spoolfil'sfshown in Bl GS. 12 "and i 13, "the force exerted on the'spool by the compressed spring- 18 8 is" balanced by an equal and oppositely directed 'forceexerted on -'the spool 318 bythe ffl uidsuppliedthrough the openings 312. --'lfhe-eonstruc- "tion of the valve =58a to provide for {a continual flow around the f land 328, insures stroking of the" pump {12 I to a substantially zero displacement; position without any iihugtiugfi The above described action'of the valve 58a to provide for movement of the tilt "box --63 to 'a substantially centeredposition takes place veryquicklyeridthe pump 12 responds'very ,quickly tOTdUQf the volume *offluid' being purnpe'd sinee'thedistance the actuating 'arm '65 nust be 'lmoved'totcenter' the tilt'b X 63 issrnall. In the event fthe puinp output 'pressure 'at the openings-3'12 should momentarilyijbe high enough tomove the spool- 3 18 to a, position in which the land 1328 v blocks [flow of fluidto the tank openings 316, "the quick response ofthetilt box f 6 3; t o extension of the piston '6Zr to- 'pr'ovidefor a reduced volume of fluidf beingpurnped by the pump 12 'and the operation of the peaking relief valve, provide "for an immediate reduotion gof thepress ure'of the fluid at the open .ings 312.

The valve "58a thus operates in "conjunction 'with the I peaking relief j valve 202 "to provide for movement of the .tilt" box 1 63 to 'a substantially neutral'position, in' response 'to a riseiin the pressure' on'the ontpntside of thepump gtrated'e-m bodiment of the.invention, this pressure is about 1500;p;s i., and is determinedbyfthe strength of "the spring188. lfthe sprin g l1 88"is of a slightly "reduced strength, this pressure will 'be slightly lower .and 'if the spring is ofaslightly greater; strength, this :pressure will be slightly higher. The size of the openings 3 14 relative to. the.land ti3 28 determinesthe volumetoffluid which will fi e t a th an t ank andth s al e th qlu n iis-fth'ef less work thatfisrequiredItromthe pump 12 in-the substantially zero i displacement position of -,the tilt ,box

6 3. 1n,the illustrated, embodiment. of theinvention about 19 1 ne a llq infiltiiiinn l a e s he n 1628- move freely up and down with respect to the front end of the tractor 12, the valve member 34 is moved to the float position illustrated in FIG. 8 in which the valve spool 66 is in a position blocking the inlet port 82 and providing for communication of the inlet port 80 with the

outlet ports

74 and 76 which are blocked by the spool 66 against communication with the tank port 68. The outlet port 78 communicates with the tank port 72.

Fluid from the outlet port 74 flows through the line 158 to the inlet port 156 in the 40 and operates to move the valve member 148 aga nst the pressure of spring 160 to a position in which the lines 142 and 144 on opposite sides of valve 40 are communicated, by a groove 149 in the valve member 148 as shown in FIG. 8, to provide for a fluid communication of opposite ends of the cylinder 20. The piston 22 is thus free to float up and down in the cylinder 20 as the dozer blade 18 moves over the ground surface, since the fluid is free to flow from one side to the other of the piston 22, complicated only by the fact that during movement of the piston rod 24 into the cylinder 20 some of the fluid being forced out of the upper end of the cylinder 20 must be bled off because it is more than enough to fill the opposite end and during movement of the piston rod out of one end of the cylinder, additional fluid must be supplied to the opposite end of the cylinder if it is to be kept full of fluid.

Fluid from the control valve outlet 76 is 'supphed through line 194 to the valve 56, and through the valve 56 into the cylinder 175 so as to extend the control piston 60 and rotate the tilt box 63 in a counterclockwise direction as shown in FIG. 8. The pump 12 thus pumps its full volume of fluid out the port 230 'and through line 108 to holding valve 36. However, since the piston 22 1S floating in the cylinder 20, the only pressure developed in line 108 is that required to overcome line loss and

open valves

36 and 38, as shown in FIG. 8, and provide a complete circuit for the fluid from the pump outlet port 230 through the line 108,

valves

36, 40 and 38 and the line 216 to the inlet port 232. Fluid under pressure from the control circuit supplied through the open check valve 250 to the valve 38 maintains the valve 38 open at all times in the float position of control valve 34 so that the piston 22 can move downwardly without huntingfi During downward floating movement of the piston 22 in the cylinder 20, the pressure in line 216 falls because there is insuflicient fluid flowing out of the lower end 'of cylinder 20 to supply the pump 12 with enough fluid to fill the upper end thereof since the piston rod 24 is moving out of the cylinder 20. Consequently, the fluid under supercharge pressure in line 106 opens the check valve 48 and enough of this fluid flows into the pump inlet 232 to insure filling of the upper end of the cylinder 20. During upward movement of the piston 22 a greater volume of fluid is forced out of the top end of cylinder 20 than is required to fill the cylinder 20 on the underside of piston 22 because the piston rod 24 is moving into cylinder 20. This additional fluid in the main circuit causes a rise in the pressure in line 216 suflicient to close valve 48 and move the valve member 94 to open the pilot valve 50 so that a volume of fluid equal to the volume of the fluid displaced in the cylinder .20 by the piston rod 24 can flow through the valve 50. to tank; For this reason the area of the surface of valve member 94 exposed to pressure fluid from line 110 is proportioned tothe area of ball 96 to insure unseating of ball 96 when the fluidpressuresapplied to the ball from line 104 and to the valve member 94from the line 110 are substantially equal.

It can thus be seen that the-checkvalves 48 and '50 function to continuously maintain the cylinder 20 full of fluid. As a result, if downward movement of the blade 18 is desired, when the control valve 34 is in the float position, it is only necessaryto move the control valve 34 to the down position shown in FI GS'. 6'and 7 and the piston 22 will be immediately moved downwardly without any time delay. As soon as-the valve 34 is moved float control shut-off valve from the float" position (FIG. 8) to any other position,

, the port 74, which is connected by the line 158 to the which constitutes a work member, can be raised, lowered,

held, or floated, without requiring unnecessary operation of the pump 12 while still providing for quick response of the piston 22 to manipulation of the control valve 34. As a result, the power required for operation of the pump 12 is then made available for other useful purposes required in the particular installation of the pump. The supercharge pump 42 provides the fluid necessary to maintain the main circuit full at all times with the

check valves

48 and 50 separating the supercharge and main circuits when no fluid is to be added to or taken from the main circuit. The use of control circuit fluid to open the valve 38 prevents hunting of the piston 22 during the time the piston 22 is being powered down, and if desired control circuit fluid may be similarly utilized to open the valve 36 if there is any tendency for the piston 22 to hunt in the opposite direction. In the illustrated system this is not a problem and in a system where hunting of the piston 22 is not objectionable, use of control fluid to open either or both of the

valves

36 and 38 is not necessary.

It will be understood that the specific construction of the improved control system for variable displacement pumps and similar devices which is herein disclosed and described is presented for purposes of explanation and illustration and is not intended to indicate limits of the invention," the scope of which is defined by the following claims.

displacement, and a hydraulically actuable device operable by fluid from said pump: actuating means movable in response to a supply of fluid under pressure thereto for moving said'actuating member to and from a first position in which the displacement of said pump is zero and a second position in which said displacement is maximum; fluid supply and return lines connecting said pump and said device; means independent of said pump for supplying fluid at a control pressure; valve means for communicating said fluid at control pressure to said actuating means to move said member to increase the displacement and hence output of said pump, and means responsive to the pressure of the output fluid from said pump in said supply line to communicate said output fluid to said actuating means to move said member in the opposite direction to decrease the displacement of said pump to a substantially zero-flow equilibrium position when said output pressure reaches a predetermined high pressure.

2. In a hydraulic control system for a variable displacement pump having an actuating member for varying displacement, and ahydraulically actuable device oper- 1 able by fluid from said pump: actuating means movable in response to a supply of fluid under pressure thereto for moving said actuating member to and from a first at-rest position in which the displacement of said pump is zero and a second position in which said displacement is maximum; spring means for resisting movement of said member by said actuating means away from said first position, said spring means being restrained and prestressed so as to allow movement of said member away from said 'first'position -only when a substantial force is applied thereto ,by sa1d actuating means; fluid supply and return lines connectingsaid pump and said device; means independent of said pump for supplying fluid at a control pressure; valve means for communicating said fluid at control pressure to said actuating means to move said member to increase the displacement and hence output of said pump, and means responsive to the pressure of the --output-fluid from-said pump in said supply line to communicate said output fluid to said actuating means to move said member in the opposite direction to decrease thedisplacement of said pump to a substantially zero-flow equi librium position when said output pressure reaches a predetermined high pressure.

3. In a hydraulic control system for a variable displacement pump having an actuating member for varying displacement, and a hydraulically actuable device operable by fluid from said pump: actuating means including a double-acting piston unit movable in response to a supply of fluid under pressure thereto for moving said actuating member to and from a position in which the displacement of said pump is zero and a position in which said displacement is maximum; fluid supply and return lines connecting said pump and said device; a fluid reservoir; means independent of said pump for supplying fluid from said reservoir at a control pressure; and valve means operable in a first position to communicate said fluid at control pressure to one side of said double-acting piston unit to move said member to increase the displacement and hence output of said pump, said valve means being responsive to the pressure of the output fluid from said pump in said supply line and operable in a second position to communicate said output fluid to the opposite side of said double-acting piston unit to move said member in the opposite direction to decrease the displacement of said pump to a substantially zero-flow equilibrium position when said output pressure reaches a predetermined high pressure.

4. A system as claimed in claim 3, wherein said valve means is operable in said first position to communicate said opposite side of said double-acting piston unit with said reservoir, and in said second position to substantially block communication between said opposite side and said reservoir.

5. A system as claimed in claim 3, wherein said valve means comprises a housing having a bore therein, a passageway connecting said bore to the side of said doubleacting piston unit not subject to control pressure, a spool having a land slidable in said bore relative to said passageway, said bore adapted to communicate on one side of said land with said supply line and on the other side of said land with said reservoir, said spool land normally blocking communication between said passageway and said supply line and being movable from said normal position to progressively restrict communication between said passageway and said reservoir and progressively admit fluid from said supply line to said passageway in response to a predetermined high pressure in said supply line, whereby said spool assumes an equilibrium position in which the amount of fluid flowing into said passageway from said supply line substantially equals the amount of fluid flowing from said passageway to said reservoir and the total forces exerted on said unit are substantially balanced to maintain said pump in an equilibrium positive displacement position in which said predetermined high pressure is maintained and a limited flow is provided through said valve for said unit.

6. In a hydraulic control system for a reversible variable displacement pump having an actuating member for varying displacement, and a hydraulically actuable device operable by fluid from said pump; actuating means including a double-acting piston unit movable in response to a supply of fluid under pressure thereto for moving said actuating member through an at-rest zero displacement position to and from a position in which the displacement of said pump is maximum for one direction of flow and a position in which said displacement is maximum for the opposite direction of flow; fluid supply and return lines connecting said, pump and said device; a fluid reservoir; means independent of said pump for supplying fluid from said reservoir at a control pressure; valve means for selectively communicating said fluid at control pressure to one side or the other side of said piston unit to move said member to increase the displacement and hence output of saidpumpin either said one-or said opposite directions resp ti ly, and m an respons o h p e r of the output fluid from said pump in said supply line to cornmunicate said output fluid to the side of said piston unit not subject to control pressure to move said member to decrease the displacement of said pump to a substantially zero-flow equilibrium position when said output pressure reaches a predetermined high pressure.

7. in a hydraulic control system for a reversible variable displacement pump having an actuating member for varying displacement, and a hydraulically actuable device operable by fluid from said pump: actuating means including a double-acting. piston unit movable in response to a supply of fluid under pressure thereto for moving said actuating member through an at-rest zero displacement position to and from a position in which the dis-. placement of said pump is maximum for one direction of flow and a position in which said displacement is maximum for the opposite direction of flow; spring means for resisting movement of said member by said double-acting piston unit in either direction away from said at-rest zero-displacement position, said spring means being restrained and prestressed so as to allow movement of said member away from said zero displacement position in either direction only when a substantial force is applied theretoby said actuating means, fluid supply and return lines connecting said pump and said device; a fluid reservoir; means independent of said pump for supplying fluid from said reservoir at a control pressure; and valve means for selectively communicating said fluid at control pressure to one side-or the other side of said piston unit to move said member to increase the displacement and hence output of said pump ineither said one or said opposite directions respectively, said valve means being responsive to the pressure of the output fluid from said pump in said supply line to communicate said output fluid to the side of said piston unit not subject to control pressure to move said member to decrease the displacement of said pump to a substantially zero-flow equilibrium position when said output pressure reaches a predetermined high pressure.

8. In a hydraulic control system for a reversible variable displacement pump having an actuating member for varying displacement, and a hydraulically'actuable device operable by fluid from said pump: actuating means including a double-acting piston unit movable in response to a supply of fluid under-pressure thereto for moving said actuating member through an :at-rest zero displace, ment'positionto and from a position in which the dis? placement of said pump is maximum for one direction of flow and aposition in which said displacement is maxi mum for the opposite direction of flow; fluid supply and return lines connecting-said pump and said device; a fluid reservoir; means independent of said pump for supplying fluid from said reservoir at-a control pressure; and valve means for selectively communicating said fluid at control pressure to one side or the other side of said piston unit and said reservoir with the opposite side thereof to move said member to increase the displacement and hence output of said pump in either said one or said opposite directions respectively, said valve means being responsiveto the pressure of the output fluid from said pump in said supply line to substantially block said communication be tween said reservoir'and the side ofsaid'piston unit not subject to control pressure and to communicate said out put fluid thereto to move said member to decrease the displacement of said pump to a substantially zero-flow equilibrium position when said output pressure reaches a predetermined high pressure;

9. A system as claimed in claim 7, wherein said valve means comprises a housing having a bore therein, a pas, sageway connecting said bore to the side of said doubles acting piston unit not subject to control pressure, a spool having a land slidable in said bore relative to said passage way, said bore adapted to communicate on one side of said land with said supply line and on the other side of said land with said reservoir, said spool land normally blocking communication between said passageway and said supply line and being movable from said normal position to progressively restrict communication between said passageway and said reservoir and progressively admit fluid from said supply line to said passageway in response to a predetermined high pressure in said supply line, whereby said spool assumes an equilibrium position in which the amount of fluid flowing into said passageway from said supply line substantially equals the amount of fluid flowing from said passageway to said reservoir and the total forces exerted on said unit are substantially balanced to maintain said pump in an equilibrium positive displacement position in which said predetermined high pressure is maintained and a limited flow is provided through said valve for said unit.

10. In a hydraulic control system for a reversible variable displacement pump and a reversible hydraulically actuatable device operable by fluid from said pump: a member that is movable in opposite directions to control the direction and magnitude of the pump displacement; hydraulically operable means for moving said member in opposite directions between a zero pump displacement position and a maximum pump displacement position for each direction of flow; spring means arranged to resist movement of said member by said hydraulically operable means in either direction away from said zero pump displacement position, said spring means being restrained and prestressed so as to allow movement of said member away from said zero displacement position in either direction only when a substantial force is applied thereto by said hydraulically operable means; and means including a control valve operable in one position to supply fluid to said hydraulically operable means to move said member in one direction against said spring means to increase the displacement of said pump in one direction and in a second position to supply fluid to said hydraulically operable means to move said member in an opposite direction against said spring means to increase the displacement of said pump in the opposite direction.

11. In a hydraulic control system for a variable displacement pump having inlet and outlet ports and a hydraulically actuatable cylinder assembly consisting of a cylinder, a piston, and a piston rod projecting through one end of the cylinder, first passage means communicating the inlet and outlet ports on said pump with opposite ends of said cylinder, a supercharge circuit including a second pump, second passage means connecting said second pump and said first passage means, check valve means in said second passage means movable to an open positron when the pressure in said second passage means is above the pressure in said first passage means to provide for flow of fluid from said second pump into said first passage means when said piston rod is moving out of said cylinder, tank means communicating with said second passage means, second check valve means in said second passage means, and means responsive to fluid pressure in the rod end of said cylinder for moving said second check valve means to an open position to provide for flow of fluidtherethrough to said tank when said pistonrod is moving into said cylinder.

12. In a hydraulic control system for a variable displacement pump having a pair of ports operable to pump fluid in either of 'two directions so that when the pump is pumping in one direction one port functions as an inlet port and the other port functions as an outlet port and when the pump is pumping in the opposite direction said other port functions asan inlet port and said one port functions as an outlet port, and wherein said pump is connected to a hydraulic cylinder assembly including a cylinder having a working piston slidably mounted therein, said system comprising first passage means extending between one of said ports and one end of said cylinder, second passage means extending between said other one of said ports and the opposite end of said cylinder, a first pilotoperated check valve in said first passage means operable in response to the pressure of fluid from said pump in said first passage means to admit said fluid to said cylinder to move said piston in one direction, a pilot line connecting said first check valve and said second passage means for opening said first check valve in response to a predetermined high pressure in said second passage means, a second pilot-operated check valve in said second passage means operable in response to the pressure of fluid from said pump in said second passage means to admit fluid to said cylinder to move said piston in the opposite direction, means for opening said second check valve when a predetermined high pressure exists in said first passage means, a pilot-operated float-control valve connected to opposite ends of said cylinder, a pilot line communicating with said float-control valve, and means for selectively supplying fluid under pressure to said float-control pilot line to open said float-control valve, whereby when all of said valves are closed said piston is fluid-locked against movement in either direction in said cylinder, when said float-control valve is open said piston is freely movable in said cylinder, and when said float-control valve is closed and said pump is pumping fluid under pressure through said first passage means said piston is moved in said one direction and when said pump is pumping fluid under pressure through said second passage means said piston is moved in said opposite direction.

13. In a hydraulic control system for a variable displacement pump having a pair of ports operable to pump fluid in either of two directions so that when the pump is pumping in one direction one port functions as an inlet port and the other port functions as an outlet port and when the pump is pumping in the opposite direction said other port functions as an inlet port and said one port functions as an outlet port, and wherein said pump is connected to a hydraulic cylinder assembly including a cylinder having a working piston slidably mounted therein: first passage means extending between one of said ports and one end of said cylinder, second passage means extending between said other one of said ports and the op posite end of said cylinder, a first pilot-operated check valve in said first passage means operable in response to the pressure of fluid from said pump in said first passage means to admit said fluid to said cylinder to move said piston in one direction, a first pilot line connecting said first check valve to said second passage means for opening said first check valve in response to a predetermined high pressure in said second passage means, a second pilot-operated check valve in said second passage means operable in response to the pressure of fluid from said pump in said second passage means to admit fluid to said cylinder to move said piston in the opposite direction, a second pilot line communicating with said second check valve, a hydraulic control circuit operatively associated with said pump for controlling the direction in which said pump pumps, means connecting said control circuit with said second pilot line to communicate fluid thereto to open said second check valve when said pump is pumping into said first passage means to move said piston in said one direction, a pilot-operated float-control valve connected to opposite ends of said cylinder, a pilot line communicating with said float-control valve, and means for selectively supplying fluid under pressure to said float-control pilot line to open said float-control valve, whereby when all of said valves are closed said piston is fluid-locked against movement in either direction in said cylinder, when said float-control valve is open said piston is freely movable in said cylinder, and when said floatcontrol valve is closed and said pump is pumping fluid under pressure through said first passage means said piston is moved in said one direction and when said pump is pumping fluid under pressure through said second passage means said piston is moved in said opposite direction.

14. In a hydraulic control system for a variable displacement pump having a pair of ports and operable to pump fluid in either of two directions so that when the pump is pumping in one direction one port functions as an inlet port and the other port functions as an outlet port and when the pump is pumping in an opposite direction said other port functions as an inlet port and said one port functions as an outlet port, and wherein said pump is connected to a hydraulic cylinder assembly which includes a cylinder having a piston mounted therein and a load carrying piston rod attached to the piston and extending through one end of the cylinder, said system comprising first passage means extending between said one of said ports and said one end of said cylinder, second passage means extending between said other one of said ports and the opposite end of said cylinder, a first pilotoperated check valve in said first passage means operable in response to the pressure of fluid from said pump to admit said fluid to said cylinder, a first pilot line extending between said first check valve and said second passage means providing for opening of said check valve in response to a predetermined high pressure in said second passage means, a second pilot-operated check valve in said second passage means operable in response to the pressure of fluid from said pump to admit fluid to said cylinder, 21 second pilot line extending between said sec ond check valve and said first passage means providing for opening of said second check valve in response to a predetermined high pressure in said first passage means, a pilot-operated float control valve connected to opposite ends of said cylinder, a third pilot line connected to said float control valve for supplying fluid under pressure to the float control valve so as to open the valve, and means for selectively supplying fluid under pressure to said third pilot line, whereby when all of said valves are open said piston is freely movable in said cylinder, when all of said valves are closed the piston is fluid-locked against movement in either direction in said cylinder, when said float control valve is closed and said pump is pumping fluid under pressure out of said first port and through said first passage means to said one end of the cylinder the piston is moved toward the opposite end of the cylinder, and when said pump is pumping fluid under pressure out said second port and through said second passage means to said other end of the cylinder the piston is moved toward said one end of the cylinder.

15. In a hydraulic control system for a variable displacement pump wherein said pump is connected to a hydraulic cylinder assembly which includes a cylinder hav'ng a piston mounted therein and a load-carrying piston rod attached to the piston and extending through one end of the cylinder, said system comprising first passage means extending between said pump and said one end of said cylinder, second passage means extending between said pump and the opposite end of said cylinder, said pump being operable to pump fluid in a first direction such that it travels out said first passage means for return through said second passage means and in a second direction such that it travels out said second passage means for return through said first passage means, a first pilotoperated check valve in said first passage means operable in response to the pressure of fluid from said pump to admit said flu'd to said cylinder, a first pilot line extending between said first check valve and said second passage means providing for opening of said check valve in response to a predetermined high pressure in said second passage means, a second pilot-operated check valve in said second passage means operable in response to the pressure of fluid from said pump to admit flud to said cylinder, a second pilot line extending between said second check valve and said first passage means providing for opening of said second check valve in response to a predetermined high pressure in said first passage means, a pilot-operated float control valve connected to opposite ends of said cylinder, a pilot line connected to said float control valve for supplying fluid under pressure to the float control valve so as to open the valve, and means for selectively supplying fluid under pressure to said pilot line, whereby when all of said valves are open said piston is freely movable in said cylinder, when all of said valves are closed the piston is fluid-locked against movement in either direction in said cylinder, when said float control valve is closed and said pump is pumping fluid under pressure through said first passage means to said one end of the cylinder the piston is moved toward the opposite end of the cylinder, and when said pump is pumping fluid under pressure through said second passage means to said other end of the cylinder the piston is moved toward said one end of the cylinder, a hydraulic control circuit operatively associated with said pump for actuating the pump to determine the direction of flow in said first and second passage means and means connecting said control circuit and said first pilot line to provide for a flow of fluid into said first pilot line so as to open said first check valve when said variable displacement pump is pumping fluid in said second direction.

16. In a hydraulic control system for a variable displacement pump connected by a main circuit to opposite ends of a hydraulc cylinder having a piston therein and a piston rod connected thereto and extending through one end of the cylinder, and wherein said main circuit includes a first line extending between one pump port and the closed end of said cylinder and a second line extending between another pump port and the rod end of'said cylinder, a fluid reservoir, a supercharge circuit connected to said reservoir and said second line in said main circuit for supplying fluid from the reservoir to the main circuit when said p'ston rod is moving out of said cylinder, means providing a path from said first line to said reservoir for fluid displaced by the piston rod during movement of the rod into the cylinder, said supercharge circuit includ ing a pump connected to said reservoir, a first check valve commun cating on one side with said supercharge pump and on the opposite side with said second main circuit line and movable to an open position by a high pressure on said one side, a normally closed pilot-operated check valve in said path and communicating on one side with sa'd reservoir and on the opposite side with said first line, a pilot line communicating said pilot-operated check valve with said cylinder on the piston rod side of said piston so that when fluid under pressure is supplied to said cylinder so as to move the piston in a drection moving the piston rod into the cylinder the pressure in the pilot line maintains said pilot-operated check valve in an open position so that excess fluid in the main circuit can flow through the pilot-operated check valve to the reservoir.

17. In a hydraulic control system for a variable dis placement pump having a pair of inlet and outlet ports and a hydraulically acuatable device operable by fluid from said pump, a member that is movable from a zero displacement position to control the pump displacement, means engaged with one side of said movable member for urging the member in one direction to a position cor,- responding to a first positive displacement condition of said pump in which fluid is pumped out one of said ports, a piston and cylinder unit engaged with the opposite side of said movable member, means associated with said unit for resisting retraction thereof on movement of said movable member in said one direction, said unit being extendible by fluid under pressure supplied thereto to theremove said movable pump member in an opposite direction to a position corresponding to a second positive displacement condition of said pump in which fluid is pumped out another one of said ports, a reservoir for fluid, a valve for said unit controlling communication between said unit and the outlet port for said pump and communication between said unit and said reservoir when said pump is in said first positive displacement condition, said valve including a member formed with a bore and an opening communicating with said bore and said unit, a spool member having a land slidable in said bore rela-

Claims

1. IN A HYDRAULIC CONTROL SYSTEM FOR A VARIABLE DISPLACEMENT PUMP HAVING AN ACTUATING MEMBER FOR VARYING DISPLACEMENT, AND A HYDRAULICALLY ACTUABLE DEVICE OPERABLE BY FLUID FROM SAID PUMP: ACTUATING MEANS MOVABLE IN RESPONSE TO A SUPPLY OF FLUID UNDER PRESSURE THERETO FOR MOVING SAID ACTUATING MEMBER TO AND FROM A FIRST POSITION IN WHICH THE DISPLACEMENT OF SAID PUMP IS ZERO AND A SECOND POSITION IN WHICH SAID DISPLACEMENT IS MAXIMUM; FLUID SUPPLY AND RETURN LINES CONNECTING SAID PUMP AND SAID DEVICE; MEANS INDEPENDENT OF SAID PUMP FOR SUPPLY ING FLUID AT A CONTROL PRESSURE; VALVE MEANS FOR COMMUNICATING SAID FLUID AT CONTROL PRESSURE TO SAID ACTUATING MEANS TO MOVE SAID MEMBER TO INCREASE THE DISPLACEMENT AND HENCE OUTPUT OF SAID PUMP, AND MEANS RESPONSIVE TO THE PRESSURE OF THE OUTPUT FLUID FROM SAID PUMP IN SAID SUPPLY LINE TO COMMUNICATE SAID OUTPUT FLUID TO SAID ACTUATING MEANS TO MOVE SAID MEMBER IN THE OPPOSITE DIRECTION TO DECREASE THE DISPLACEMENT OF SAID PUMP TO A SUBSTANTIALLY ZERO-FLOW EQUILIBRIUM POSITION WHEN SAID OUTPUT PRESSURE REACHES A PREDETERMINED HIGH PRESSURE.