US3809500A

US3809500A - Method and apparatus for regulating pumps

Info

Publication number: US3809500A
Application number: US00286951A
Authority: US
Inventors: G Staudenrausch
Original assignee: HANDTMANN A METALGUSSWERK ARMA
Current assignee: HANDTMANN A METALGUSSWERK ARMA
Priority date: 1972-02-25
Filing date: 1972-09-07
Publication date: 1974-05-07
Anticipated expiration: 1991-05-07
Also published as: DE2208877B2; GB1400682A; DE2208877A1

Abstract

Apparatus for regulating the discharge pressure of adjustable pumps by modifying the discharge capacity by means of a control device which is dependent on consumed load. The control device comprises an element which is positionable in a first direction through the pump discharge pressure and in a second direction opposite thereto in response to a force formed by the consumed working load pressure and a given spring bias. The control means is further urged in its second direction by a bias which is removed on reaching a given minimum of discharge pressure so that the first forces are responsive only above such pressure.

Description

i451 May 7, 1974 2,368,791 2/1945 Waldre........ 2,502,546 4/1950 Adams...... 2,975,717

[ METHOD AND APPARATUS FOR REGULATING PUMPS [75] Inventor:

3/1961 Rynders et a1 12/1962 Lambeck Georg Staudenrausch, Rissegg, Germany 6/1965 Puryear......... 3,384,019 5/1968 u a e E m m m hC m m u TV. m m m Lu H mM m .m M F 2 r 7 0 8 H .mm 1 me m 1 E m WW n 5 Mr 3 0 HH 3 PA k r e W m g "m m .r e w M .f my m .mw 8 0mm m r tuae mmm ace 1 fr... mmm mam: b m lr i AAFB e n g S s A 57 ABSTRACT v Apparatus for regulating the discharge pressure of adjustable pumps by modifying the discharge capacity by 2 7 9 1 1 75 L 0.6 08 S2 0 N .ml m w FA H 22 Foreign Application Priority Data means of a control device which is dependent on con- Feb. 25, 1972 Germany............................ sumed load. The control device comprises an element which is positionable in a first direction through the [52] US. Cl. pump discharge pressure and in a second direction opposite thereto in response to a force formed by the consumed working load pressure and a given spring [51 Int.

[58] Field of Search...... 60/450; 417/218, 220, 213,

bias. The control means is further urged in its second direction by a bias which is removed on reaching a References Cited I UNITED STATES PATENTS given minimum of discharge pressure so that the first forces are responsive only above such pressure.

2,255,783 9/1941 Keadrick,.........1..................1. 60/450 14 Claims, 3 Drawing Figures SHEET 2 BF 2 Fig. 3

METHOD AND APPARATUS FOR REGULATING PUMPS BACKGROUND OF INVENTION The present invention relates to regulating device for fluid pumps and in particular to apparatus for controlling the discharge feed pressure of an adjustable pump.

It is known to control pumps through regulating means employing a predetermined biasing force responsive to the load wherein, the discharge pressure (pl) of the pump stands at a predetermined value greater than the load or the work pressure (p2). The difference berween both pressures is thereby at least so large as the maximum apparent pressure drop between the pump and the consuming load. Generally, on start up, the pump is fixed first at its largest discharge capacity until the predetermined pressure is reached on the control device biasing force. At the start up the consumer has for its use available only the effective pressure p2=0. From then on only that output pressure equal to the pressure difference (pl p2) is maintained. The pump does not adjust at a higher pressure.

In order to obtain a spontaneously high degree of regulation of the pressure, it is necessary, however, that a biasing force substantially higher than the pressure differential be exerted or placed on the control device. That is, it is necessary to maintain a minimum pressure of about 30 atms. on the control device in order to maintain the necessary efficient function. Even though this minimumpressure is exceeded it is necessary that the differential pl p2 should, nevertheless, be maintained constant notwithstanding the height of the working pressure p2 of the consumer. This is generally not possible in the known prior art devices, since variable factors such as load changes, working pressure, etch., may change.

It is the object of the present invention to provide an improved method and apparatus for regulating the output of adjustable fluid pumps.

It is another object of the present invention to provide apparatus for regulating the operation of hydraulic pumps or the like in dependence upon variations in the difference between output pressure and consumer work pressure.

It is another object of the present invention to provide a regulating apparatus satisfying the specific needs indicated above, wherein on start up, the pump at first creates quickly a given discharge pressure which is also maintained even though in the interim the working pressure may increase or decrease by a significant magnitude.

These objects, others together with numerous advantages will be seen from the following description of the present invention.

SUMMARY OF INVENTION According to the present invention there is provided a method and apparatus for controlling the feed pressure of an adjustable pump through changes in the discharge capacity of the pump bymeans ofa load responsive adjustable control device which is movable into a first position responsive to the force of the output pressure of the pump and into a second position opposite thereto through a force responsive to the pressure necceleration and increase in power with extraordinary sure fed to the inlet of a consuming device and from a similarly directed spring biasing force.

Preferably, the control element is maintained in the direction of the second position by an additional force which is of a degree at least equal to the minimum pressure of the working force of the actuator. This second force is arranged so as to be withdrawn or inactivated on reaching this minimum pressure.

In the above manner, the control device permits the pump to reach the working pressure of the actuator and is only thereafter responsive to changes in the working pressure. Preferably, the biasing forces other than the field pressures are resilient spring means capable of accurate adjustment. In practice the pump can start up instantaneously and brought to a standstill while producing a working pressure for the actuator which can continuously be sensed through the second biasing force in response thereto desirable adjustment can be made. It is possible by employing variable pump and hydraulic motors instantaneously to obtain a large acexactness and without having the pump produce at the maximum pressure. The pressure difference pl p2 between the discharge pressure of the pump and the working pressure of the consumer can be maintained even though the pressure drop of the associated hydraulic circuit is at a minimum level. so that during minimum pressure operation efficient pump regulation can be obtained more responsively to this pressure difference, thereby loss of efficiency in the hydraulic circuit and the hydraulic fuel temperature can be maintained at a minimum level.

Full details of the present invention are given in the following description and will be seen from the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a view of a rotary pump and regulating means therefor taken in section along a plane perpendicular to the axis of rotation and a schematic representation of the connection pum with a fluid actuator representing a consuming load,

FIG. 2 is an enlargement of the regulating means shown in FIG. 1, and

FIG. 3 is a graphical representation of the pressure curves illustrating the-working of the present invention.

DESCRIPTION OF THE INVENTION To illustrate the application of the present invention it is disclosed in connection with a vane type pump, as seen in FIG. 1, bearing the general numeral 1, comprising a cylindrical housing 2 in which there is coaxially mounted a pump rotor 3 having a plurality of radially extending slidable vanes 4. Spaced from and surrounding the rotor 3 is a guide ring 5. Interposed between the guide ring 5 and the rotor 3, and adapted to move in a horizontal direction as indicated by the double arrow 6, is a pressure ring 7 of substantially smaller diameter than that of the guide ring 5. The vanes 4 are adapted to run about the surface of the pressure ring on rotation of the rotor 3 which is driven about its central axis by a conventional drive. Because of the ability to offset the pendicularly from the space 8 through the housing 1 is an outlet conduit by which the pressurized media is discharged at a pressure indicated by numeral Pl from the sections 8a. Opposite the discharge conduit 10 is a suction inlet conduit 11 which connects with the sections 8b. The conduit extends to a filter 12 which is contained in a reservoir 13 adapted to hold a supply of suitable compressible fluid media such as oil or the like.

Adjustment of the pressure ring 7 along the diametric line indicated by arrow 6 will increase ordecrease the output capacity of the pump. The variation in output pressure by such adjustment follows in dependence thereto, and conforms'to the X or indicated by the directions of the arrow 6.

To eccentrically adjust the pressure ring 7 and thereby vary the area of each of the sections a and b, a stressing piston 14 and a control piston 15 are arranged at opposite sides of the housing 1 along the horizontal diameter indicated by the double arrow 6. The

pistons 14 and 15 are adapted to reciprocate within the respective cylinders'housing them and each is provided with a rod at its end which extends througha hole in the guide ring 5 into approximate contact with the edge of the pressure ring 7. The stressing piston 14 is located in a bore 140 which is enlarged at its outer end and is provided with a set screw or suitaable adjusting means which bears against the outer end of the piston 14 so as to limit its outward movement in the bore 14a. The piston 14 is provided with suitable seal rings about its circumference which seal against the flow of fluid between it and the cylinder. The enlarged rear portion of the bore 14a defines asuitable cylinder head by which the fluid media may bear against the outward end of the piston 14. a

The piston 15 is, on the other band, formed of a hollow cylinder and is set within a bore 16 of substantially larger size than the bore 14a. The bore 16 is closed by a control and regulating mechanism generally referred to by the numeral 17 by which pressurized fluid media is delivered to the piston 15. A spring 18 is located within the interior of the hollow piston 15 and bears between its front face and the face of the regulating device l7. The spring 18 is sufficient under normal conditions to maintain the piston 15 directed toward the right as shown in FIG. 1 and to bear against the pressure ring 7, pushing the ring in the indicated plus direction of arrow 6.

As seen in more detail in F IG. 2, the pressure regulating device 17 comprising a control element embodied in a cylindrical body 19 the lower end of which is fixedly mounted to cover the cylinder 16. This lower portion 19 has a narrow axial bore in which is mounted for reciprocal movement a slide.20. The slide 20 comprises with the housing 19 a type of fluidic spool valve and has a collar at each end which is provided'with suitable seal rings, sealing the interior of the bore in which it slides. The slide 20 has a central collar which forms two

annular chambers

21 and 23 on' opposite sides thereof/The upper chamber 21 communicates with a conduit 22 while the lower chamber 23 communicates with a conduit 24, both extending into the pump housing 1. A third conduit 26 extends from substantially the middle of the bore of housing 19. The central collar of slide 20 is normally positioned over the conduit 26 closing the cylinder 16.

The slide 20 is provided further with a thin axial bore 25 which extends from its lower end upward to about the upper end of the slide, from whence it branches off in a lateral direction. A screw cap is secured at the lower end of the housing enclosing a small chamber 28 forming a cylinder head relative to the lower end of the slide. The small axial bore 25 can thus deliver fluid from the annular chamber 21 to the chamber 28. The upper conduit 22 communicates with a conduit 27 extending from one side of the pump housing 1 to the other side through the discharge conduit 10 and into the enlarged portion of the bore 14a, thus providing communication between the stressing piston 14 and the control piston 15 via the regulating means 17. The lower conduit 24 communicates with the infeed conduit 11 via the interior of the pump, namely the space between the guide ring 5 and the pressure ring 7.

The control piston 15 has a larger diameter than the stressing piston 14. The cylinder 16 in which the piston 15 is located stands under the pressure indicated by the numeral P3-which is regulated by the regulating device 17 between a base equivalent to zero (0) and an amount equivalent to the discharge pressure Pl through the conduit 10. The combination of the pressure P3 and the bias of the spring 18 forces the piston 15 to the right wherein the eccentricity of the pressure ring 7 with regard to the rotor 3 and the pressure quantity created by the pump may be enlarged as indicated by the plus of the double arrow 6. It is only when, during the operation of the rotor 3 that a pressure P1 may be built in the

conduit system

10, 27 and 14a greater than that exerted by the combined pressure P3 and the spring 18, that a sufficient working force is created on the stressing piston 14 to move the pressure ring 7 to the left and thus overcome the normal biasing effect of the piston 15.

The feeding and the discharge of control fluid from the cylinder 16 is controlled through the longitudinal adjustment of the slide member 20. The normally closed central conduit 25 maintains the cylinder 16 at a given fixed pressure P3. When the slide 20 is moved upwardly so that the cylinder 16 is in communication with the discharge outlet 11 through the conduit 24 then only the spring 18 biases the control piston 15 to the right. Accordingly, the stressing piston 14 will produce a greater force than that of the piston 15 so that the pressure ring becomes urged to the left and thus produces a decrease in the pressure quantity of sections 8a resulting in a decrease in the output of the .pump. When, to the contrary, the slide 20 is moved downwardly, the cylinder 16 is connected via the conduits 26, the annular space 21, the conduit 22 to the conduit 27 and simultaneously to the discharge pressure P1 in the conduit 10. In this case the control pressure P3 increases and approaches the discharge pressure P1 and combined with biasing force of the spring 18 pushes the pressure ring 7 to the right, resulting in an increase in the discharge pressure of the pump.

The internal bore 25 of the slide 20 opening into a space 28 in the lower end of the housing 19, permits the passage of the discharge pressure P1 into the space 28 so as to normally act on the lower end of the slide 20 maintaining the slide normally in its upward position. This maintains the conduit passage 26 into the cylinder 16 in normally closed position, tending to fix the control pre'ssure P3 therein. The control pressure P3 can be varied only by the movement of slide 20 as provided by the control and regulating means as the operating control means to be described hereinafter.

The central bore of the control housing 19 opens at its upper end above the slide 20 into an enlarged bore forming a pressure chamber 29 at the exterior end of which is sealingly inserted a hollow screw plug 30. The plug 30 forms a central hollow bore 32 on the free end of which a second screw plug 33 is secured. Both of the screw plugs are adjustably fixed in their position by an annular nut. A pressure pin 34 is arranged in chamber 29 to be axially moveable. The lower end of the pin 34 is provided with a collar plate 340 which contacts the upper end of the control slide 20, which extends slightly into the pressure chamber 29. A spring F1 is mountedsurrounding the pin 34 and bears against the collar plate 34a and the frontal face of the hollow screw plug 30. The compression ratio of the spring F1 is adjusted by changing the position of the screw plug 30 by adjustment of its retaining nut.

A stem 35 is located in central axial bore formed in the first screw plug 30. The stem 35 is axially moveable and is also provided with a collar 35a at its lower end which extends tandemly through the spring F1 from the bottom of the first screw plug 30 into engagement with the upper end of the pin 34. The upper end of the stem 35 is provided with a plate 36 which carries a second spring F2 compressed by engagement with the frontal face of the second screw plug 33. Adjustment of the second screw plug 33 will also vary the pressure on the spring F2.

The main pressure discharge conduit is, as seen in FIG. 1, connected via a regulating valve mechanism 37 with a conduit 38 connected to a fluid actuator or consumer of the pressure produced by the pump, such as a hydraulic motor 39 or if desired, any other connection fluid pump, motor, piston etc. A return line 40 extends from the hydraulic motor 39 to the reservoir 13. The regulating valve is a conventional spring loaded piston-cylinder which is adjustable as indicated by the screw adjusted piston so as to provide a predetermined pressure P2 in the line 38 which is necessary to power the consuming device, which is generally smaller than the discharge pressure Pl from the pump. In its place an accumulator, reducer or similar fluid pressure storage device may be used. In order to prevent undesirable pressure variations from being passed from the pump to the pressure consumer 39, it is desirable to maintain constant pressure difference-Pl P2 in the system. Accordingly, the valve 37 is adjusted to provide such a pressure drop from conduit 10 to conduit 38 notwithstanding the actual pressure in conduit 10. Connected to the conduit 38 is a monometer 41 which measures and shows the pressure value P2 in the conduit. The output of the monometer passes via another conduit 42 into the pressure chamber 29 of the regulating means 17. Thus, a portion of the discharged fluid passing through the conduit 38 is by-passed into the regulating means to thereby furnish the necessary sensing and impulse means by which the regulating means may be automatically caused to function to operatethe As an illustrative example of the function of the present invention, the invention will be further described in the operation of a conventional rotary hydraulic motor 39 in the medium size range and 'wherein the output minimum pressure of pump 1 lies in the order of 30 atmospheres (30kp/cm the pressure differential across valve 3'7 (P1 P2) is about Skp/cm and the pressure input to the motor 39 is at least about 22kp/cm Spring F1 is so chosen that its spring ratio or force, relative to the cross-section of the slide 20 approximates the pressure differential Pl P2 to 8kp/cm Thus, a cross-section of one centimeter for the slide 20 requires a spring force of 8kp on spring f1. The second spring F2 must then have a spring ratio exerting a pressure force of 22kp/cm so that the total of the spring forces acting on the slide 20 amounts to P PF1+ PF 8 22 30kp/cm As will be seen from FIG. 3 the pump output pressure P1 is in the initial stages of operation, at a constant (i.e. p10) which in the given example is 30 atmospheres. On opening the valve 37, the pressure P2 to the consumer, i.e., in the conduit 38, instantly jumps to a pressure 22kp/cm (P20) which, of course is less than P1 by an amount equal to PF] 8kp/cm The power delivered to the consumer 39 then is at a minimum of 22kp/cm or PF2. Should thereafter, the pressure P2 exceed the value of P20, then the flow through conduit 42 into the chamber 29 of the regulating device 17 causes'the stem 35 to be pushed back against the force of the spring F2, unseating itself from contact with the pin 34. The reference magnitude for the force of spring F2 is namely in the cross-section of the stem 35. From that moment on the force of spring F2 is overcome and it no longer exerts a downward bias on the pin 34 or on the slide 20. Meanwhile the force of the spring Fl: continues to be directed against the slide 20 through its action on the pin 34. Notwithstanding the increase in working pressure either from the pump 1 or in the motor 39, the slide 20 therefore remains in its middle position when the difference between both of the pressures pl p2 remains at the predetermined value 8kp/cm The differential between the pump and motor is maintained constant throughout the rise in working pressure. During this time the pressure P3 in cylinder 16 is constant, and it together with the force of spring 18 maintains the pressure ring 7 in its normal working position.

Should this balance be disturbed as for example through the changing of the number of revolutions (rpm) of the hydraulic motor 39 or through a change in the working moment of the motor, and the pressure in line 38 P2) varies, then the slide 20 is moved out of its middle position and changes analogously to the change in pressure P2. Thus the pressure P3 in cylinder 16 and on the control piston 15 is caused to change. If Pl P2 drops, i.e., the differential decreases below 8kp/cm and the relative value of P2 to P1 increases the pressure in chamber 29 acting on plate 34a forces the pin 34 and the slide 20 downwardly, opening conduit 25 to conduit 27 via the annular chamber 21. Pressure P3 in cylinder 16 rises to approximate the pressure P1 in line 10. Consequently the pressure ring 7 moves further to the right as shown in FIG. 1 and results in an in crease in its eccentricity and in the resultant pressure quantity of the pump. As a result an increase of the output pressure P1 is obtained until the predetermined pressure difference 8kp'lcm is again reached. Thereafter the slide 20 through the balancing of the force of the spring FI and pressure P2 once again assumes its middle position. On the other hand, should the pressure difference Pl P2 increase, thenthe pressure P2 through conduit 42 decreases. As a result the relatively I crease in the eccentricity and in the delivery characteristics of the pump, resulting in the lowering of the pres sure Pluntil the balance between P1 and P2 again approximates the desired8 kp/cmidifferential.

The present inventionthus-provides a simple, effective and instantly responsive method and apparatus for regulating the output'or discharge pressure'of an adjustable pump by modifyingthe discharge or output capacity by a control device dependent on load or consumer conditions. The control 'device'essentially consu'reof thepump and in a second opposite direction re-. sponsive to the pressure of the working media forthe actuator. In accord with'the invention resilient biasing means are provided to further additionally bias the element in the second direction, the latter bias being equal to the minimum pressure required to operate the actua-.

tor and being removable on reaching this minimum.

Thus'the element is responsive only to variations between the output pressure and the working pressure when the system as a whole is functioning above the minimum level. Thus more responsive and accurate control is obtained. v 1' h The first spring acting in conjunction'with the working media to biasfthe element functions only after.

reaching the minimum level and thus operates only to maintain'the'differential pressure between output and working media It will thus be seenrthat the control device 17 is responsive only to changes in the differential between the pump output andthe motor consumption and is not re-. sponsive to the rise or change'in the absolute value of the two'pressure forces. Thus, as seen in FIG. 3, the

pressure P1 and P2 can rise to their maximum while a constantdifferential is obtained. Further, the output of the pump is varied responsive to the control mechanism, while no operation itself is made on the motor input, orthe interposed valve 37. in this manner the motor 39 receives a steady constant input and is shieldnd fromundue and undesirable variations in pres-' sures.

Up to now the present description has described the invention by way of example in a hydraulic'pump. It

s will be understood that the same control means can be For example, a relay, solenoid, fluidic devices or other similar switching means may be used. While the biasing forces in the control device 17 are shown as mechanical spring means, they can also be provided via pneumatic, electrical, electromechanic, or fluidic arrangements as well. I 7

Various modifications and embodiments will 'of course be evident to thoseskilled in this art. Accordingly, the foregoing disclosure is to be taken as illustrative only, and not limiting of the scope of the present invention.

What is claimed:

1. In combination with afluid pump and an actuator operated by fluid under pressure received from said pump, said pump having adjustable means for varying theoutput-capacity thereof and an output connected to said fluid actuator for delivering fluid under pressure thereto, apparatus for regulating the output capacity of said pump, comprising a movable control element adapted to adjust the means for varying the capacity of the pump, said control element being movable in a first direction responsive to the output pressure of said pump and in a second direction opposite to said first direction responsive to a first biasing. force comprising the pressure of the fluid operating said actuator, a second biasing force comprising means biasing said control element further into said second position and mounted to be movable out of engagement with said control element in response to a predetermined mini mum pressure of the fluid operating said actuator,

.whereby after reaching said minimum pressure said pump output capacity is regulated by the relative difference between said, output pressure and the pressure operating said actuator. f Y 2. The apparatus according to claim 1 wherein said first biasing force includes means acting in combination with the pressure of the fluid operating said actuator and said second biasing means to normally balance said spool between the first and second position.

3. The apparatus according to claim 1 wherein said pump comprises a housing, a cylindrical pressure ring and a rotor mounted in said housing, said rotor having a plurality of vanes extending radially therefrom in contact with the inner surface of said pressure ring, said pressure ring being reciprocal along a diametric line, and includes means for transmitting the movement of said control element thereto to adjust the position thereof in response to the movement of said control element, and inlet and outlet conduit means for receiving fluid and discharging fluid under pressure.

4. The apparatus according to claim 1, wherein said first or second biasing means are first or second springs.

5. The apparatus according to claim 4 wherein said control element comprises an axial movable slide member located in a hollow housing, the output pressure of said pump being exerted against one end of said slide member and the pressure operating said actuator being exerted against the opposite end of slide member.

6. The apparatus according to claim 5 including a chamber located in said housing axially of said slide for receiving fluid under pressure from said actuator, said first spring means being located within said chamber said fluid under pressure, said first spring and said second spring combining to urge said slide in second direction.

7. The apparatus according to claim 6 including a stem mounted to movably extend into said chamber and having an end acting onto the axially end of said slide, said second spring means bearing against said stem, the end of said stem being urged by the pressure within said chamber against the bias of said second spring.

8. The apparatus according to claim 7 including means for adjusting the spring rate of said second spring means to the level of the predetermined minimum pressure for operating said actuator.

9. The apparatus according to claim 7 including means for varying the spring rate of said first spring.

10. The apparatus according to claim 3 including a guide ring surrounding said pressure ring, first and second pistons each located in a cylinder and arranged to extend through said guide ring in contact with said.

pressure ring on opposite side of said diametric line, conduit means connecting the cylinder of said first piston with the outlet of said pump, and conduit means connecting said second cylinder with said control element thereby the position of said pressure ring is determined by the balance of said control element and the output pressure of said pump.

11. The apparatus according to claim 10 wherein said control element comprises a fluid spool valve adapted to control the flow of fluid under pressure thereto and includes a central conduit means communicating with said second cylinder, said valve having a pair of inlet conduits connected to the outlet of said pump and to the inlet thereof respectively, said spool being movable in said two opposed directions to permit c 'ommunicaa fluid pump feeding a fluid actuator by adjusting its output capacity said pump having regulating means for adjusting the output capacity, comprising the steps of providing a movable control element adapted to cause adjustment of said regulating means, maintaining said control element under the urging of two opposed forces, the first of said forces being responsive to the output pressure of said pump, the second of said forces being responsive to the pressure of the fluid required to operate said actuator, further maintaining an additional biasing force on said element in the direction of said second force equal to the minimum pressure necessary to operate said actuator, and removing said additional force on reaching said minimum pressure to permit said two opposed forces to balance each other in response to changes in the output pressure of said pump and the pressure of said fluid operating said actuator, whereby said balance is transmitted to said regulating means.

13. The method according to claim 12 including the step of resiliently biasing said second force to normally maintain said second force substantially equal with said first source.

14. The method according to claim 13 wherein said resilient biasing means is maintained at a level equal to the difference between the output pressure of said pump and the pressure required to operate said actuatOl'.

Claims

1. In combination with a fluid pump and an actuator operated by fluid under pressure received from said pump, said pump having adjustable means for varying the output capacity thereof and an output connected to said fluid actuator for delivering fluid under pressure thereto, apparatus for regulating the output capacity of said pump, comprising a movable control element adapted to adjust the means for varying the capacity of the pump, said control element being movable in a first direction responsive to the output pressure of said pump and in a second direction opposite to said first direction responsive to a first biasing force comprising the pressure of the fluid operating said actuator, a second biasing force comprising means biasing said control element further into said second position and mounted to be movable out of engagement with said control element in response to a predetermined minimum pressure of the fluid operating said actuator, whereby after reaching said minimum pressure said pump output capacity is regulated by the relative difference between said output pressure and the pressure operating said actuator.

2. The apparatus according to claim 1 wherein said first biasing force includes means acting in combination with the pressure of the fluid operating said actuator and said second biasing means to normally balance said spool between the first and second position.

10. The apparatus according to claim 3 including a guide ring surrounding said pressure ring, first and second pistons each located in a cylinder and arranged to extend through said guide ring in contact with said pressure ring on opposite side of said diametric line, conduit means connecting the cylinder of said first piston with the outlet of said pump, and conduit means connecting said second cylinder with said control element thereby the position of said pressure ring is determined by the balance of said control element and the output pressure of said pump.

11. The apparatus according to claim 10 wherein said control element comprises a fluid spool valve adapted to control the flow of fluid under pressure thereto and includes a central conduit means communicating with said second cylinder, said valve having a pair of inlet conduits connected to the outlet of said pump and to the inlet thereof respectively, said spool being movable in said two opposed directions to permit communication of said central conduit means with one of said inlet and outlet conduits.

12. The method of regulating the output pressure of a fluid pump feeding a fluid actuator by adjusting its output capacity said pump having regulating means for adjusting the output capacity, comprising the steps of providing a movable control element adapted to cause adjustment of said regulating means, maintaining said control element under the urging of two opposed forces, the first of said forces being responsive to the output pressure of said pump, the second of said forces being responsive to the pressure of the fluid required to operate said actuator, further maintaining an additional biasing force on said element in the direction of said second force equal to the minimum pressure necessary to operate said actuator, and removing said additional force on reaching said minimum pressure to permit said two opposed forces to balance each other in response to changes in the output pressure of said pump and the pressure of said fluid operating said actuator, whereby said balance is transmitted to said regulating means.

14. The method according to claim 13 wherein said resilient biasing means is maintained at a level equal to the difference between the output pressure of said pump and the pressure required to operate said actuator.