US2892312A - Demand compensated hydraulic system - Google Patents

Description

June 30, 1959 J. R. ALLEN ETAL DEMAND COMPENSATED HYDRAULIC SYSTEM Filed Jan. 2'7. 1958 Mkbkk m Wu QWQQS INVENTOR. J. R. ALLEN J.H. KRESS P. R. BUNNELLE United States Patent 2,892,312 DEMAND coMi ENsATE'DHYDRAUuc SYSTEM *J'ames'R. Allen, Cedar Falls, and James H. Kress, Waterloo, Iowa, and Philip R. Bunnelle, San Jose, Calif 'assignors, by mesne assignments, to Deere 8: Com- .pany, a corporation of Delaware "Application January 27,-1958, Serial No. 711,360 10 Claims. (Cl. 60-52) This invention relates to a fluid pressure system and .more'particularly -to an improved system embodyinga demand-compensated principle in which the pump dis charge power from a variable-displacement pumpcan be made proportional tothe-instantaneous load demand of one or more fluidmotors powered by the pump.

In general, the important objects of theinvention are to "provide an arrangement-in which fluid power is pro- ,portional to the momentary demand of the system in both pressure and volume, and particularly a system hav- -ing acceptably high overall efficiency at all load conditions, thereby-significantly reducing heat problems. This includes a standby phase in which power consumption is relatively low. It is a feature of the invention to apply the system to a plurality of motors, such as hyof motors.

The system ls-designed particularlyfor use in agricultural tractors wherein a number of hydrauliefunctions must be performed, such aspower steering of the tractor front'wheelsandpower operated actuation of one or more rockshafts for adjusting implements associated with the tractor.

In the drawings, a; variable displacement pump is shown at lll as hav'ing-mechanism 12 *by means of which the output of the pump may be varied. The specific type of pump is not material, as longas it is any 'ofthe well known variable displacement'types in'which the-stroking "mechanism 12 is arranged-so that its'output or stroke is increased by a mechanical force, such asa compression spring 14 acting against a member or piston-16 to move that member in the direction of the labeled arrow. The

' piston 16' is carried .in a cylinder '18 so' that its face is 'subjectable to fluid pressureadmitted to the cylinder at the side of the piston opposite to thespring 14. The pump -10 is connected to reservoir in a suitable manner, an intake line bein'gsh'own' 'atI20' as being connected to a reservoir 22.

The-dischargeside of the pump 10 is'conne'cted via a pressure line 24am a motor control valve26 toa twoway fiuidpressuremotor 28. The motor control valve "26 may bemannau and/erservo operated by any suitable means, represented here' schematically'at30. The valve likewise is illustrated schematically, as'are many of the well known hydraulic components employed in the system. A typical flow control valve 32 'is'in'te'rposed 26, for obvious reasons.

line .38.

ice

'2 in the pressure line 24 ahead of the motor control valve The motor control valve .is shown in its neutral position and is of the closed-center type, which permits no flow .when in neutral. The valve is connected by apair otmotor lines 34 and 36 respec= tively to opposite ends of .the cylinder of the motor 28; When the motor control valve 26 is shifted downwardly from its neutral position, the high ,pressure line 24 is connected to the motor'line 34 so as to retract the motor 28 and return is via theline 36 to an exhaust or reservoir Movement ofthe'control valve 26 to an upper position, just the opposite of that just described, will interchange the motor lines 34 and 36 so that the .latter is connected to .the pressure line 24 to extend the motor 28. In'the embodiment.from which the present system .is taken, the motor 28 represents the hydraulic cylinder that is associatedwith the tractor steering mechanism,.and is therefore subjected to little if any'residual load.

A basicpart of the demand-compensated system comprises a stroking mechanism control or demand valve means indicated in its entiretyby the numeral 40 and comprising a cylinder 42 which forms a chamber carrying a shiftable valve piston 4-4 which separates the chamber into pressure and pilot sides 46 and 48 respectively. Biasing means in the form of a relatively light spring 5%) urges the piston 44 toward the pressurevside 46 forthe purpose of controlling a port 52 which is permanently connectedby a control? line 54to the cylinder 18 of the stroking mechanism, the connection of the line '54 being made to the cylinder 18 at the side of the piston inoppositionto the spring '14. The line 54'has a restricted connection or orifice56 to the reservoir 22. The ,pressure side 46 of the valve means 40 is connected by a line 24a to the pressure line 24 intermediate the discharge side of the pump 10 and the motor control valve 26. To the extent thus far described, it will be seen that the valve piston 44 is subjected to twoforces; namely, pressure via the line 24 and .the spring load via the springSi). It will be further seen that when the valve 44 is .open as respects the ports 52, pressure from the pressure line 24 is transmitted via 24a52-54 to the fluid-pressurereceivable end of the piston 16 in the stroking mechanism'12, so as to oppose the stroking mechanism-spring 14 for the purpose of decreasing the. pump output.

In addition to the forces acting on the valve 44, .as just described, the-system .adds .pilotline pressure to the pilot side 48 .ofthe demandvalve meanschamber in conjunction with .the force'imposed .by the spring,150. This pilot line pressure isobtained through a pilot line .58 which'hastwo branches 60 and-62 connected respectively to the ractor lines 34 and 36 between the motor control valve 26 andthe motor 28. When the motor control valve 26 is in its neutral position, no flow or pressure occurs in the pilot line 58, but the reverse is true when the motor control valve is opened in either direction. In-order that pilot flow from one or the other of the branches 62 may not reverse itself inthe other branch, depending upon which of the motor lines '34 and 36 is pressurized, each branch carries apilot check valve,-respective1y at 64 and 66.

From the description thus far, and ignoring the components not described, it will be seen that when themotor control valve 26 is. in neutral, the pilot line pressureis zero. Therefore, the'pump discharge pressure via 24 and -24a.is balanced only against :the spring 50 at the opposite side of the valve piston 44, and these components are calibrated to regulate IhCJSiIOkfi of'the pump ,10 via the mechanism 12so'thata relatively small-standby pressure existsfin the line 24.

pilot pressure will occur in one of the other or the lines pump must necessarily be a fixed increment greater than the pilot pressure for a balance to exist. The magnitude of this increment is obviously determined by the force of the spring 50.

The maximum pressure that the pump can produce is determined by pressure limiting means indicated in it's entirety by the numeral 68 and comprising a relief valve 70 biased to close against a pressure line branch 72 by a spring 74. The value of the spring 74 is considerably higher than that of the spring 50. The pressure limiting means dumps via a branch 76 to the control line 54 rather than directly to reservoir, and hence discharges into the mechanism 12 if an excess discharge pressure occurs, having the effect of decreasing the output of the pump to zero.

After the motor control valve 26 is again closed or neutralized, there is no fluid entering the pilot line, and the pilot pressure must reduce to zero in order to allow the system pressure to be reduced to its standby value. For this purpose, the pilot line 58 is connected to reservoir via a restriction or orifice 78. This prevents the trapping of fluid in the pilot line between the bottom of the piston valve 44 and the pilot line check valves 64 and 66.

The operation of the system to the extent described is as follows. When the motor control valve 26 is open, fluid at standby pressure is admitted to the motor 28 and simultaneously to the pilot line that is connected to whichever motor line is pressurized. This pressure in the pilot line will signal for higher pump discharge pressure, and this pressure then increases the pilot line pressure which in turn signals an increase of pump discharge pressure. During this build-up cycle, which is very nearly instantaneous, practically no flow occurs. The increase of pump discharge pressure will continue until the resistance encountered by the motor 28 is overcome and the motor begins to move. At this point, appreciable flow begins so that there is a pressure drop across the control valve 26 and in fact throughout the entire fiow path. Since the pilot pressure is taken in such a place where it is equal to the pressure acting on the motor 28, it will signal the pressure acting on this motor only, and this signal will be transmitted to the pump via the means 40. Since it is clear that no more pressure can act on the motor 28 than is required to move the resistance to which the motor is connected, the pilot line pressure can reach no greater magnitude than that necessary to activate the motor. Since the pump discharge pressure is thus determined by the pilot pressure plus the fixed increment established by the spring 50, the pump will discharge at a pre-set pressure increment above that required to move the motor. Theoretically, all this pressure increment should be dissipated across the inlet metering port of the motor control valve 26. As this valve is opened Wider, allowing more flow to the motor, the stroking mechanism control valve means 40 will compensate by putting the pump into an increased displacement, since the piston valve 44 will close the port 52 and fluid trapped in the mechanism 12 below the piston 16 will escape to reservoir via the orifice 56. This increased flow can continue until the motor control valve 26 is opened to its maximum position, subject to the limit of the flow control valve 32. At this point, it should be noted that the flow control valve may be dispensed with, but is an expedient particularly desirable in a multi-motor system.

As the motor control valve 26 is moved toward neutral,

4 the flow rate is decreased until it ultimately reaches-zero; When no further flow is admitted to the motor and thus to the pilot line 58, the pressure in the pilot line bleedsthrough the pilot line bleed orifice 78, thereby reducing the pilot pressure to zero which in turn allows a reduction: in the discharge pressure since the bottom of the piston! valve 44- is no longer subjected to any force in addition to the spring force 50. Since this spring force determines the standby' pressure, the system accordingly returns to standby operation;

As previously indicated, it is a feature of the invention to incorporate a plurality of motors in the demand-- compensated system. A second motor, here a one-way motor 80, is connected to a pressure line extension 82 via a flow control valve 84 and a second motor control valve 86 under control of suitable manual and/or servo actuating means suggested at 88. The motor 30 is here representative of a hydraulic cylinder useful on an agricultural tractor for rocking a tractor-mounted rockshaft. The motor control valve 86 for the motor is connected to that motor by a motor line 90 and a pressure line 94 incorporating a check valve 92. A pilot branch 96 conmeets the pressure branch 94 to the pilot line 58 and includes a pilot line check valve 98 for preventing reversal of pilot flow when the pilot line 58 is pressurized by the motor 28. Similarly, the pilot line check valves 64 and 66 prevent reverse pilot flow when the pilot line is pressurized by operation of the motor 80. The motor control valve 86 is connected to reservoir at 100.

The system employs a third motor here, a two-way motor 102 controlled by a motor control valve 104 which may be connected to a suitable manual and/or servo actuator as at 106. This valve is connected to an extension 108 of the pressure line 24-82 and a flow control valve 119 is interposed in the line 108 ahead of the valve 104. One side of the valve is connected to reservoir at 112 and its opposite side is connected by motor lines 114 and 116 respectively to opposite ends of the motor 102. Because the motor 102 is of the two-way type and is connected to a rockshaft on a tractor, for example, it will be subjected to residual load and for this purpose it is desired to use pilot-operated check valves as at 118 and 120, respectively, in the motor lines 114 and 116. These check valves may be of any suitable type and the rectangles at 122 and 124, respectively, indicate that these valves are hydraulically operated. A pair of pilot line branches 126 and 128, respectively incorporating check valves 130 and 132, are connected respectively to the motor lines 114 and 116, the system in this respect duplicating the arrangement previously described at 60, 62, 64 and 66.

When two or more motor control valves are opened simultaneously, the pressure will build up to the level required of the most heavily loaded motor. The motor encountering the lighter load will move first and will accelerate to the flow rate set by either its flow control valve (32, 84 or or the amount of opening of its motor control valve (26, 86, or 104). There will be, at this time, no motion in the most heavily loaded motor, and therefore, the pilot signal will indicate the requirement for still more pressure until the most heavily loaded motor is moved. Pilot pressure in the line 58 will of course build up and oppose the pressure at 24-2411, the valve piston 44 moving up to cut 011 the port 52 and relieving any counter pressure against the piston 16 in the stroking mechanism 12, whereby the pump discharge pressure will build up until it equals that required to move the most heavily loaded motor at a flow rate equal to the total of the number of motors in action. On the basis of the foregoing, any number of motors can be operated independently and simultaneously to th limit of the pump capacity.

It will thus be seen that the demand-compensated hydraulic system is a type of closed-center arrangement that provide both pressure and flow rate propor- .pres'sure line causes a decrease in displacement.

tional to the instantaneous load demand condition. 'lhe fvariab le displacement pump "is automatically "controlled control valve is opened, the pressure in the signal "or pilot line will indicate the exact pressure (excluding line losses) needed to move the "motor. The'p'ilot pressure cannot signal a higher pressure than that needed 'toovercon1e the resistance encountered by the'n'ioto'r. This resistance is'the resultant of two loadcornponents. First, there 'is the component due'to the Lphysieal'load acting on the operating 'rrioto-r and is madeup 'of'static load, inertiaand viscous resistance. Second there is a componentfdue to the return line pressure. The'second component'is applicable o'nly'infthe case of a'do'uble acting motor as at 28 and 102. The ump'wiu'preduce the nevi/rare hecessary to's'ustain the signaled p1" "sure. Or, as compared to the pressuie-compensated arrangement which will produce the flow rate necessary "to maintain one iix'edpressure, the demand-compensated arrangement will produce any new from minimum "to maximum at anypressure from standby to maximum.

As already indicated, the variable displacement pump may be of any type, and the stroking mechanism -12 is of a design such that an increase of pressure in the 'As indicated, a spring or other mechanical device may cause the increase of stroke.

The stroking mechanism control or demand valve means 40 is actually a difierential typepressure transducer. The diflerential is pre-set by the spring 50 so that the pressure in the line 24a will bleed through the port 52 and control line until it becomes a set increment greater than the pilot pressure in the line 58. If the pressure at 24a or at the pressure side 46 of the valve 44 is greater than the pilot pressure (the pressure at the pilot side 48 of the valve 44, plus the spring force of the spring 50) the valve-44 moves downwardly, opening the port 52 to the control line 54. If the values are reversed, the valve 44: moves up and closes the port 52, allowing 'a reduction ofpressure in the line 54. When equilibrium has been reached, a small steady flow is metered to the line 54 via the port'52.

The magnitude of how allowed past'the transducer valve 40 determines the control pressure inthe line 54, which cannot be greater than that existing in the line 24a, but can be any desired lesser value depending upon the size of the fixed bleedorifice'SG.

On the basis of the foregoing, it will be'relatively easy tocalculate and design for certain desirable factors,

afiecting, for example, the magnitude of the controlflow to orifice 56. The size of this orifice is influenced by the fiow'necessary to maintain pump stability 'and the permissible time interval for the pump to alter "stroke. Other factors such as efiiciency, motor control valve design and type of actuationof the motor control valve can be varied as desired while still enabling exploitation of the novel demand-compensated system. These and other factors not categorically covered, as well as variations in the' preferred sys'tem'disclosed, will' readily occur to those versed in the art, all without departurefrom the: spirit and scope 'of the invention.

What is claimed "is:

"1. A fluidlpressure'system er the class describedjcomprising: "a variable displacement pump having *output contmImec'hanism including a"pis'ton, means biasing the piston in cne direction to increase :puinp output and a cylinder carrying the piston" for receiving fluid pressure to move the-piston inthe-opposite direction to decrease pump outputya fiuidpressure motor; a high pressure line connected between-thepump and motor; a motor" control valve interposed in said line for selectively opening and "erasing s id line =to the'motor; deniand'vnve inicludin'g "-a chamber "and '"a piston valve shiftable therein se'p'a'fa rn said *charriber-into a pressure side coni'neet ed 'toithe "prs'sure li'lie ahead of the motor control a pjilot sid'e connectedtos'aid pressure line bed fridtor "central valve and the motor, a part 'inthe chamber atsaid pressure side and openable by the piston valve 'in response to-linepressure, and means biasing he'pistonvalve to 'cltises aid p'ort in opposition to line pressure and in conjiinetionwith pilot pressure when the morer'eontmlvalve is s'et to open the pressure line-tothe "motorfa restrieted-bleedline connecting the pilot 'side iSf thefdemarid *valve means to 'res'ervoir; a control line conne'eting said "to the pum courier mechanism cylinder forsfipply g fiuid'from the'pr'es sure'side of th'e'deniand Valveineans to the mechanism pistom'a re's'tricted'res'er- 'voir' lirieleading from the control line to reservoir; and

pressure-limitin mean connecting the control line to'fthe p essure line between the ump and the demand valve -rneans "a'nd incIuding'a relief valve and biasingnieans closing said relief valve, said relief valve biasing means being of greaterval'ue than-the biasing'meansin the -de'mand'valve means.

'ne'eted'to the pr'es'sure lineahead of the motor control valv'eaiid a pilot-side connected to said pressure line-be- 't'ween's'aid'motor control valve and the motor, a port in "the chamber at 's'aidpres'sur'e side and "openable by the piston valve in response to line pressure, and means biasing' the piston valveto close said port in opposition to 'line pressure andin conjunctionvvith pilot pressure when the motor control valve is set to open the pressure line *to the'mdtor; 'a restricted bleed line connecting the pilot -"side-'o'f the demand-valve means to reservoir; a control 'line connecting said port to the pump control mechanism 'for supplying fluid from the pressure side of the demand valve means to the mechanism member; a restricted reservoir line leading from the control line to reservoir; and pressure-limiting'means connecting the pressure line between the pump and the demand valve means to reservoir and including a relief valve and biasing means closing said relief valve, said relief valve biasing means being of greater value than the biasing means in the demand valve means.

3 A fiuid pressure system of the class described, comprising: avariable displacement'pump having output control -rnechanis'm including a piston, means biasing the piston in one direction to increase pump output and "a cylinder carrying the piston for receiving fluid pressure to move the piston in the opposite direction to *decrease pump output; atwo-wa'y fluid pressure motor; a high pressure line connected to thepump; a motor control valve connected to the high pressure line and having'a pair of motor lines connected respectively to opposite ends of the motor, said motor control valve being selectively settable to 'closethe pressure line or to pressurize either motor line'while connecting the other motor line to reservoir; demandwalve means including a chamber and apiston valve shiftable'therein'and separating said chamberinto a'pressure side'connected to the pressure line ahead of the motor control valve and a pilot side'con- 'nected' to both motor lines for receiving pilot flow from 7 lines including a pair of check valves for preventing pilot flow from the pressurized motor line to the other motor line, said demand valve means further including a port in the chamber at said pressure side and openable by the piston valve in response to line pressure ahead of the motor control valve and means biasing the piston valve to close said port in opposition to line pressure and in conjunction with pilot pressure when the motor control valve is set to pressurize either motor line; a restricted bleed line connecting the pilot side of the demand valve means to reservoir; a control line connecting said port to the pump control mechanism cylinder for supplying fluid from the pressure side of the demand valve means to the mechanism piston; a restricted reservoir line leading from the control line to reservoir; and pressure-limiting means connecting the control line to the pressure line between the pump and the demand valve means and including a relief valve and biasing means closing said relief valve, said relief valve biasing means being of greater value than the biasing means in the demand valve means.

4. A fluid pressure system of the class described, comprising: a variable displacement pump having output control mechanism including a member movable in one direction by fluid pressure to decrease pump output and biased to move in the opposite direction to increase pump output; a two way fluid pressure motor; a high pressure line connected to the pump; a motor control valve connected to the high pressure line and having a pair of motor lines connected respectively to opposite ends of the motor, said motor control valve being selectively settable to close the pressure line or to pressurize either motor line while connecting the other motor line to reservoir; demand valve means including a chamber and a piston valve shiftable therein and separating said chamber into a pressure side connected to the pressure line ahead of the motor control valve and a pilot side connected to both motor lines for receiving pilot flow from whichever motor line is pressurized by the motor control valve, said connection between said pilot side and motor lines including a pair of check valves for preventing pilot flow from the pressurized motor line to the other motor line, said demand valve means further including a port in the chamber at said pressure side and openable by the piston valve in response to line pressure ahead of the motor control valve and means biasing the piston valve to close said port in opposition to line pressure and in conjunction with pilot pressure when the motor control valve is set to pressurize either motor line; a restricted bleed line connecting the pilot side of the demand valve means to reservoir; a control line connecting said port to the pump control mechanism for supplying fluid from the pressure side of the demand valve means to the mechanism member; a restricted reservoir line leading from the control line to reservoir; and pressure-limiting means connecting the pressure line between the pump and the demand valve means to reservoir and including a relief valve and biasing means closing said relief valve, said relief valve biasing means being of greater value than the biasing means in the demand valve means.

5. A fluid pressure system of the class described, comprising: a variable displacement pump having output control mechanism including a piston, means biasing the piston in one direction to increase pump output and a cylinder carrying the piston for receiving fluid pressure to move the piston in the opposite direction to decrease pump output; a pair of fluid pressure motors; a 'high pressure line connected to the pump; a pair of motor control valves connected in parallel to the pressure line and respectively having independent motor lines leading respectively to the motors, each motor control valve being selectively settable to close or open the pressure line to its respective motor line; demand valve means including a chamber and a piston valve shiftable therein and separating said chamber into a pressure side con- .s V W 2,892,812

nected to the pressure line ahead of the motor control valve and a pilot side connected to both motor lines for receiving pilot flow from whichever motor line is opened tothe pressure line by its associated motor control valve, said connections to the motor lines including check valves operative respectively on the motor lines to prevent pilot flow from the opened motor line to the closed motor line, said demand valve means further including a port in the chamber at said pressure side and openable by the piston valve in response to line pressure ahead of both motor control valves, and means biasing the piston valve to close said port in opposition to line pressure and in conjunction with pilot pressure from whichever motor line is opened by its associated motor control valve; a restricted bleed line connecting the pilot side of the demand valve means to reservoir; a control line connecting said port to the pump control mechanism cylinder for supplying fluid from the pressure side of the demand valve means to the mechanism piston; a restricted reservoir line leading from the control line to reservoir; and pressurelimiting means connecting the control line to the pressure line between the pump and the demand valve means and including a relief valve and biasing means closing said relief valve, said relief valve biasing means being of greater value than the biasing means in the demand valve means.

6. A fluid pressure system of the class described, comprising: a variable displacement pump having output control mechanism including a member movable in one direction by fluid pressure to decrease pump output and biased to move in the opposite direction to increase pump output; a pair of fluid pressure motors; a high pressure line connected to the pump; a pair of motor control valves connected in parallel to the pressure line and respectively having independent motor lines leading respectively to the motors, each motor control valve being selectively settable to close or open the pressure line to its respective motor line; demand valve means including a chamber and a piston valve shiftable therein and separating said chamber into a pressure side connected to the pressure line ahead of the motor control valve and a pilot side connected to both motor lines for receiving pilot flow from whichever motor line is opened to the pressure line by its associated motor control valve, said connection to the motor line including check valves operative respectively on the motor lines to prevent pilot flow from the opened motor line to the closed motor line, said demand valve means further including a port in the chamber at said pressure side and openable by the piston valve in response to line pressure, ahead of both motor control valves and means biasing the piston valve to close said port in opposition to line pressure and in conjunction with pilot pressure from whichever motor line is opened by its associated motor control valve; a restricted bleed line connecting the pilot side of the demand valve means to reservoir; a control line connecting said port to the pump control mechanism for supplying fluid from the pressure side of the demand valve means to the mechanism member; a restricted reservoir line leading from the control line to reservoir; and pressure-limiting means connecting the pressure line between the pump and the demand valve means to reservoir and including a relief valve and biasing means closing said relief valve, said relief valve biasing means being of greater value than the biasing means in the demand valve means.

7. A fluid pressure system of the class described, comprising: a variable displacement pump having output control mechanism including a pressure chamber operative to vary pump output according to changes in pressure in said chamber; a fluid pressure motor; a high pressure line connected between the pump and motor; a motor control valve interposed in said line for selectively opening and closing said line to the motor; demand valve means including a chamber and a piston valve shiftable therein and separating said chamber into a pressure side connected to the pressure line ahead of the motor control valve and a pilot side connected to said pressure line between said motor control valve and the motor, a port in the chamber at said pressure side and operable by the piston valve in response to line pressure, and means biasing the piston valve to close said port in opposition to line pressure and in conjunction with pilot pressure when the motor control valve is set to open the pressure line to the motor; a restricted bleed line connecting the pilot side of the demand valve means to reservoir; a control line connecting said port to the pump control mechanism chamber for supplying fluid from the pressure side of the demand valve means to said chamber; and a restricted reservoir line leading from the control line to reservoir 8. A fluid pressure system of the class described, comprising: a variable displacement pump having output control mechanism including a pressure chamber operative to vary pump output according to changes in pressure in said chamber; a two-way fluid pressure motor; a high pressure line connected to the pump; a motor control valve connected to the high pressure line and having a pair of motor lines connected respectively to opposite ends of the motor, said motor control valve being selectively settable to close the pressure line or to pressurize either motor line while connecting the other motor line to reservoir; demand valve means including a chamber and a piston valve shiftable therein and separating said chamber into a pressure side connected to the pressure line ahead of the motor control valve and a pilot side connected to both motor lines for receiving pilot flow from Whichever motor line is pressurized by the motor control valve, said connection between said pilot side and motor lines including a pair of check valves for preventing pilot flow from the pressurized motor line to the other motor line, said demand valve means further including a port in the chamber at said pressure side and openable by the piston valve in response to line pressure ahead of the motor control valve and means biasing the piston valve to close said port in opposition to line pressure and in conjunction with pilot pressure when the motor control valve is set to pressurize either motor line; a restricted bleed line connecting the pilot side of the demand valve means to reservoir; a control line connecting said port to the pump control mechanism chamber for supplying fluid from the pressure side of the demand valve means to said chamber; and a restricted reservoir line leading from the control line to reservoir.

9. A fluid pressure system of the class described, comprising: a variable displacement pump having output control mechanism including a pressure chamber operative to vary pump output according to changes in pressure in said chamber; a pair of fluid pressure motors; a high pressure line connected to the pump; a pair of motor control valves connected in parallel to the pressure line and respectively having independent motor lines leading respectively to the motors, each motor control valve being selectively settable to close or open the pressure line to its respective motor line; demand valve means including a chamber and a piston valve shiftable therein and separating said chamber into a pressure side connected to the pressure line ahead of the motor control valve and a pilot side connected to both motor lines for receiving pilot flow from whichever motor line is opened to the pressure line by its associated motor control valve, said connections to the motor lines including check valves operative respectively on the motor lines to prevent pilot flow from the opened motor line to the closed motor line, said demand valve means further including a part in the chamber at said pressure side and openable by the piston valve in response to line pressure ahead of both motor control valves, and means biasing the piston valve to close said port in opposition to line pressure and in conjunction with pilot pressure from whichever motor line is opened by its associated motor control valve; a restricted bleed line connecting the pilot side of the demand valve means to reservoir; a control line connecting said port to the pump control mechanism chamber for supplying fluid from the pressure side of the demand valve means to said chamber; and a restricted reservoir line leading from the control line to reservoir.

10. A fluid pressure system of the class described, comprising: a variable displacement pump having output control mechanism including a pressure chamber operative to vary pump output according to changes in pressure in said chamber; a fluid pressure motor; a high pressure line connected between the pump and motor; a motor control valve interposed in said line for selectively opening and closing said line to the motor; a control line connected to the high pressure line ahead of the control valve and leading to the control mechanism chamber; a pilot line connected to the high pressure line between the control valve and motor; a diflerential pressure transducer for regulating the pressure and flow in the control line to vary the pressure in the control mechanism chamber, said transducer having pre-set therein a predetermined diiferential enabling the transmission of pump pressure to said chamber at a predetermined value when the control valve is closed whereby the pump output is a predetermined value, said transducer being connected to the pilot line and responsive to pressure therein when the control valve is opened for changing pressure in the control line so as to vary pump output in accordance with demand on the motor; and a restricted bleed line leading from the pilot line to enable the escape of fluid therefrom when the control valve is closed so that the transducer will again incur the aforesaid predetermined value of pressure in said chamber.

References Cited in the file of this patent UNITED STATES PATENTS 2,408,303 Ernst Sept. 24, 1946 2,432,305 Geiger Dec. 9, 1947 2,716,946 Hardy Sept. 6, 1955

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