Control unit for hydraulic variable displacement pumps and variable displacement pump with a control unit

Abstract

Control device for hydraulic variable displacement pumps operated in an open hydraulic circuit and adjustable in their displacement volume by means of a servo control device. The control device comprises a control piston with two control edges to which pressure can be applied by means of pressurized pressure fluid from a variable displacement pump, the control piston being mounted in a housing so that it shifts longitudinally. The housing of the control piston comprises an inlet for the connection of a high pressure line of a variable displacement pump, an outlet which can be connected to a tank and a servo connection which can be linked to a servo cylinder, whereby a link between the inlet and the servo connection can be made via a first control edge. It is possible to create a link between the servo connection and the outlet via a second control edge.

Claims

1. A control device (20) for a hydraulic variable displacement pump (1) which is operated in an open hydraulic circuit and which is adjustable by a servo piston (4) configured to shift inside a servo cylinder (5), to which servo piston (4) pressure is applied by pressurized pressure fluid via the control device (20), the control device (20) comprising: a housing (11) of a control piston (6), an inlet (26) for the connection of a high pressure line of the variable displacement pump (1), an outlet (18) which is configured to connect to a tank (19) and a servo connection (27) configured to link to the servo cylinder (5), the control piston (6) having a first control edge (38) and a second control edge (39) and being mounted in the housing (11) so that the control piston (6) shifts longitudinally from a first end to a second end, whereby the control piston (6) is configured to create a connection between the inlet (26) and the servo connection (27) via the first control edge (38) and the control piston is configured to create a connection between the servo connection (27) and the outlet (18) via the second control edge (3), the control piston (6) is preloaded by a first spring (7) at the first end (28) which is adjacent to the outlet (26), at the second end (29) of the control piston (6) adjacent to the outlet (18) a second spring (8) engages the control piston (6), which counteracts a hydraulic force and the preload of the first spring (7), a power-adjustable actuator (9) at one of the two ends (28 or 29) of the control piston (6) that engages the control piston (6) by a tractive or a compressive force transmitted to another of the two ends (28 or 29) in parallel with the respective spring (7 or 8) of the control piston (6) such that the force of the actuator (9) is applied to the control piston (6) toward the another of the two ends (28 or 29), whereby pressure can only be applied to the control piston (6) with the pressure fluid under pressure from the variable displacement pump (1) to generate the hydraulic force toward the direction of the another of the two ends (28 or 29) of the control piston (6), and a spring guide (13, 14) configured to provide fluid communication between a longitudinal bore (36) of the control piston (6) and a first spring chamber (21) in one position of the control piston (6).

2. The control device (20) for the hydraulic variable adjustment pump (1) according to claim 1, characterized in that the control piston (6) is stepped and the pressurized pressure fluid of the variable displacement pump (1) acts on two diameters of differing sizes, whereby the diameter acting in the direction of the outlet (18) is larger.

3. The control device (20) for the hydraulic variable displacement pump (1) according to claim 1, characterized in that the actuator (9) is an electric solenoid to which electric current is applied at adjustable levels by an electronic control unit (31).

4. The control device (20) for the hydraulic variable displacement pump (1) according to claim 1, characterized in that the spring force of the first spring (7) or second spring (8) is set by a setting device (12).

5. The control device (20) for the hydraulic variable displacement pump (1) according to claim 1, characterized in that the actuator (9) and the spring (7 or 8) positioned at the one of the two ends of the control piston (6) are arranged in series.

6. The control device (20) for the hydraulic variable displacement pump (1) according to claim 1, the spring guide (13, 14) is configured to transmit the spring force to the one end of the two ends (28 or 29) of the control piston (6).

7. The control device (20) for the hydraulic variable displacement pump (1) according to claim 1, characterized in the first spring (7) is mounted in the first spring chamber (21) and the second spring (8) is mounted in a second spring chamber (21), each of the first spring chamber (21) and the second spring chamber (21) are respectively configured so as to be adjacent to a respective one of the two ends of the control piston (6) and are connected to each other via the longitudinal bore (36) in the control piston (6).

8. The control device (20) for the hydraulic variable displacement pump (1) according to claim 7, characterized in that one of the two spring chambers (21) is connected to the outlet (18) by means of a channel (25) in the housing (11).

9. The control device (20) for the hydraulic variable displacement pump (1) according to claim 1, wherein the pressurized pressure fluid from the variable displacement pump (1) is applied to the servo piston (4) via the control device (20)) in order to set a supply pressure.

10. The control device (20) for the hydraulic variable displacement pump (1) according to claim 9, characterized in that the variable displacement pump (1) is configured as an axial piston machine of the swash plate or bent axis design.

11. The control device (20) for the hydraulic variable displacement pump (1) according to claim 1, further comprising the control piston (6) having two circumferential grooves (24) whose longitudinal limits form the first control edge (38) and the second control edge (39) respectively.

12. The control device (20) for the hydraulic variable displacement pump (1) according to claim 1, wherein the longitudinal bore (36) passes through an axial length of the control piston.

13. The control device (20) for the hydraulic variable displacement pump (1) according to claim 12, wherein the longitudinal bore (36) is configured to balance pressure between the first end (28) and the second end (29) of the piston (6).

14. The control device (20) for the hydraulic variable displacement pump (1) according to claim 1, characterized in that the actuator (9) is positioned on the opposite side of the pressure applied by the pressurized pressure fluid.

15. The control device (20) for the hydraulic variable displacement pump (1) according to claim 1, characterized in the actuator (9) having a tappet (30) that passes through the adjacent one of the two springs (7 or 8).

16. The control device (20) for the hydraulic variable displacement pump (1) according to claim 15, characterized in that the tappet (30) is configured to exert the force of the actuator at the spring guide (14).

17. The control device (20) for the hydraulic variable displacement pump (1) according to claim 1 wherein the force of the adjacent one of the two springs (7 or 8) is applied independently to the control piston (6) of the force of the actuator (9) throughout actuation of the actuator (9).

18. A control device (20) for a hydraulic variable displacement pump (1) which is operated in an open hydraulic circuit and which is adjustable by means of a servo piston (4) configured to shift inside a servo cylinder (5), to which servo piston (4) pressure is applied by means of pressurized pressure fluid via the control device (20), the control device (20) comprising: a housing (11) of a control piston (6), an inlet (26) for the connection of a high pressure line of the variable displacement pump (1), an outlet (18) which is configured to connect to a tank (19) and a servo connection (27) configured to link to the servo cylinder (5), the control piston (6) having a first control edge (38) and a second control edge (39), whereby the control piston (6) is configured to create a connection between the inlet (26) and the servo connection (27) via the first control edge (38) and the control piston is configured to create a connection between the servo connection (27) and the outlet (18) via the second control edge (3), the control piston (6) is preloaded by a first spring (7) at a proximal end (28) which is adjacent to the outlet (26), at a distal end (29) of the control piston (6) adjacent to the outlet (18) a second spring (8) engages which counteracts a hydraulic force and the preload of the first spring (7), a power-adjustable actuator (9) at the proximal end (28) of the control piston (6) engages with a tractive or a compressive force is transmitted to the distal end (29) in parallel with the first spring (7) of the control piston (6) such that the force of the actuator (9) is applied to the control piston (6) toward the distal end (29) independently of the force of the first spring (7) towards the respective the distal end (29), whereby pressure can only be applied to the control piston (6) with the pressure fluid under pressure from the variable displacement pump (1) to generate the hydraulic force in the direction of the distal end (29) of the control piston (6), a spring guide (13, 14) configured to provide fluid communication between a longitudinal bore (36) of the control piston (6) and a spring chamber (21) in one position of the control piston (6).

19. A control device (20) for a hydraulic variable displacement pump (1) which is operated in an open hydraulic circuit and which is adjustable by a servo piston (4) configured to shift inside a servo cylinder (5), to which servo piston (4) pressure is applied by pressurized pressure fluid via the control device (20), the control device (20) comprising: a housing (11) of a control piston (6), an inlet (26) for the connection of a high pressure line of the variable displacement pump (1), an outlet (18) which is configured to connect to a tank (19) and a servo connection (27) configured to link to the servo cylinder (5), the control piston (6) having a first control edge (38) and a second control edge (39), whereby the control piston (6) is configured to create a connection between the inlet (26) and the servo connection (27) via the first control edge (38) and the control piston is configured to create a connection between the servo connection (27) and the outlet (18) via the second control edge (3), the control piston (6) is preloaded by a second spring (8) at a distal end (29) which is adjacent to the inlet (28), at a proximal end (28) of the control piston (6) adjacent to the outlet (26) a first spring (7) engages which counteracts a hydraulic force and the preload of the second spring (8), a power-adjustable actuator (9) at the distal end (29) of the control piston (6) engages with a tractive or a compressive force is transmitted to the proximal end (28) in parallel with the second spring (8) of the control piston (6) such that the force of the actuator (9) is applied to the control piston (6) toward the proximal end (28) independently of the force of the second spring (8) towards the respective proximal end (28), whereby pressure can only be applied to the control piston (6) with the pressure fluid under pressure from the variable displacement pump (1) to generate the hydraulic force in the direction of the proximal end (28) of the control piston (6), a spring guide (13, 14) configured to provide fluid communication between a longitudinal bore (36) of the control piston (6) and a spring chamber (21) in one position of the control piston (6).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be explained by way of an example based on preferred embodiments shown in the figures, whereby the preferred embodiments shown in the figures do not limit the inventive concept. The following are shown:

(2) FIG. 1 A variable displacement pump with a control device according to the invention in diagrammatic form;

(3) FIG. 2 A variable displacement pump with another type of control device according to the invention in diagrammatic form;

(4) FIG. 3 A partial longitudinal cross-section of a control device according to the invention of the type shown in FIG. 1;

(5) FIG. 4 A partial longitudinal cross-section of a control device according to the invention of the type shown in FIG. 2;

(6) FIG. 5 A detailed view of the central section of a control unit according to FIG. 4;

(7) FIG. 6 An exemplary current/pressure diagram for the controlled increase of supply pressure using the control device according to the invention; and

(8) FIG. 7 An exemplary current/pressure diagram for the controlled increase of supply pressure using the control device according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(9) FIG. 1 shows a variable displacement pump 1 with a control device 20 according to the invention in diagrammatic form. The type of the variable displacement pump 1 is optional, providing that the adjustment of the displacement volume can be controlled by means of an adjustment element that can be activated by a servo piston 4.

(10) Preferred examples here are axial piston pumps with an adjustable swash plate whose angular position can be specified by means of a servo piston 4. The variable displacement pump 1 is powered by a drive shaft 35 with a drive motor not shown here operating at a constant rotational speed, for example, and it displaces pressure fluid in an open circuit. The variable displacement pump comprises and inlet 2 and an outlet 3 for the pressure fluid and is connected to a consumer not shown here via pressure lines, as well as being connected to the control device 20 via a pressure line 16 and to a tank 19 for the pressure fluid via a drain line 10.

(11) The control device 20 comprises a control piston 6 which is mounted in a housing 11 so that it shifts longitudinally. A first end 28 of the control piston 6 is exposed to high pressure at the outlet 3 of the variable displacement pump 1. The high pressure exerts a hydraulic force in the direction of the second end 29 of the control piston 6. The first end 28 of the control piston 6 is also in contact with an adjustable first spring 7. Pressure is applied to the opposite, second end 29 of the control piston 6 by a second spring 8 with which an actuator 9 is arranged in parallel in this exemplary preferred embodiment. Parallel here means that the force of the actuator 9 is applied to the control piston 6 independently of the second spring 8. In the case of the arrangement of the spring 8 and actuator 9 in series, as shown for example in FIGS. 3 and 4, the force of the actuator 9 is transmitted via the second spring 8 onto the second end 29 of the control piston 6. In this embodiment of the invention it is immaterial whether the second spring 8 is placed upstream or downstream of the moving part of the actuator 9.

(12) The housing 11 of the control device 20 comprises an inlet 26 which is connected to the outlet 3 of the variable displacement pump 1 via a pressure line 16. At this inlet 26, pressure is therefore applied by pressure fluid from the variable displacement pump 1. A servo connection 27 of the housing 11 is linked to the servo cylinder 5 via the pressure line 17. An inlet 18 of the housing 11 is linked to the tank 19 via the tank line 10.

(13) The servo cylinder 5 is connected to the control device 20 via the pressure line 17 and is supplied with pressure fluid by it. The pressure fluid acts on the servo piston 4 and shifts this against the force of a return spring 32. Shifting of the servo piston 4 adjusts the variable displacement pump 1 as required via the activation link 33. This adjustment might consist of a change in the deflection angle of a swash plate, for example. Alternatively, the control device 20 can reduce the pressure in the servo cylinder 5 in that shifting the control piston 6 creates a link between the servo connection 27, which now acts as an inlet, and the outlet 18 to the tank 19. This reduces the pressure in the servo cylinder 5, resulting in the servo piston 4 being shifted under the impact of the return spring 32 in such a way that the servo force acting on the adjustment element of the variable displacement pump 1 is reduced and the latter is deflected further, leading to an increase in supply pressure.

(14) The control piston 6 is guided in a stepped longitudinal or through-hole 21 of the housing 11 (see FIGS. 3, 4 and 5) and comprises at least two circumferential grooves 24 which form control edges 38, 39 (see FIG. 5). The respective positions of the control edges 38 in relation to the various inlets and outlets (18, 26, 27) determine the inflow and outflow of pressure fluid in relation to the servo cylinder 6. Via the actuator 9, which by way of an example here takes the form of a solenoid 15 with a movable armature designed as a tappet 30, a force which can be externally specified is exerted on the control piston 6 such that the latter changes its position. In the case of it being a solenoid 15, the actuator 9 draws its power supply from a control unit not shown here, the level of which can be set by means of an entry device which is not shown here either. This entry device can be operated manually in analog or digital form, for example, or respond to signals supplied by sensors. What is more, the actuator can also be operated mechanically, hydraulically or pneumatically without deviating from the inventive concept. These details are familiar to the person skilled in the art so they will not be expanded on here.

(15) When the actuator 9 is at zero power, a state of balance is created at the control piston 6, thereby setting a predefined position of the control edges 38 and 39 by means of which the interaction between the other forces is fixed. These forces are determined by the prevailing output pressure of the variable displacement pump 1 at the inlet 26 and the interplay of the springs 7 and 8, which act against each other. By setting the adjustable spring 7, it is possible to specify the starting position at which the actuator 9 is powerless, for example. This means that the control edges 38 and 39 of the control piston 6 determine a defined pressure in the servo cylinder 5 which results in the corresponding output pressure at the outlet 3 of the variable displacement pump 1. If the control device according to the invention is to be used for the controlled increase of the displacement volume of the variable displacement pump 1, this output pressure is relatively low (see FIG. 6) and defines the idle pressure of the variable displacement pump 1. If the control device according to the invention is to be used for the controlled reduction of the displacement volume of the variable displacement pump 1, this output pressure is relatively high (see FIG. 7) and defines the maximum pressure of the variable displacement pump 1.

(16) If the control device is of the construction type as shown in FIGS. 1 and 3, the actuator 9 is configured as pressure-generating, for example, and engages at the second end 28 of the control piston 6, which is positioned adjacent to the outlet 18 and the servo connection 27. If an electric current is applied to the actuator 9, here configured as a solenoid 15, for example, an additional force acts on the second end 29 of the control piston 6 via a tappet 30 which shifts the control piston out of its starting position. The starting position of the control piston 6 for the preferred embodiments shown in FIGS. 1 and 3 with an actuator 9 that generates a compressive force is therefore the position of the control piston 6 when shifted to maximum extent towards the outlet side, where the actuator 9 is also positioned. In this starting position, the control edge 28 therefore opens the hydraulic link between the outlet 26 and the servo connection 27 (cf. FIG. 5) so that the high pressure of the variable displacement pump 1 is passed onto the servo cylinder 5, causing pressure fluid to apply pressure to the servo piston 4. If the hydraulic force of the servo piston 6 is greater than the force of the servo piston return spring 32, the adjustment element of the variable displacement pump 1 is shifted in the direction of decreased deflection and the displacement volume of the variable displacement pump 1 is reduced until a balance of forces sets in at the servo piston 4. In the starting position shown in FIG. 2 and with an actuator 9 generating a compressive force, the variable displacement pump 1 is at a minimum flow rate corresponding to its drive speed.

(17) If a compressive force is now exerted on the control piston 4 via the actuator 9, the control piston 4 is shifted towards the inlet side, causing the control edge 38 to close the hydraulic link between the servo connection 27 and the inlet 26 as the compressive force exerted by the actuator 9 is increased, and, as the compressive force is increased, causing the control edge 39 facing the outlet side to open the hydraulic link to the outlet 18, by means of which the pressure in the servo cylinder 5 to the tank 19 can be relieved. The servo piston return spring 32 now shifts the servo piston 4 in the direction of the zero pressure position, thereby increasing the adjustment of the variable displacement pump 1 and increasing the displacement volume, until the pressure level in the servo cylinder is the same as in the tank 19. The variable displacement pump 1 then reaches its maximum flow rate in accordance with its drive speed.

(18) In this way it is possible, according to the invention, to continuously adjust and regulate the output pressure of the variable displacement pump 1 from a low level, which can be set via the setting screw 12 at idle, to a higher level by specifying the force applied by the actuator 9.

(19) FIG. 2 shows a variable displacement pump 1 with another type of the control device 20 according to the invention in diagrammatic form. In this and in the subsequent figures, analog components bear the same reference numerals as those in FIG. 1.

(20) The construction type shown in FIGS. 2 and 4 only differs from that of FIGS. 1 and 3 in that the actuator 9 is positioned on the side of the first end 28 of the control piston 6, i.e. on the inlet side of the control piston, while the now adjustable spring 8 is positioned at the second end 29, i.e. on the outlet side of the control piston. Consequently, the setting screw 12 is positioned on the outlet side. The other elements remain unchanged. As a result of this construction type, in which the actuator 9 is positioned adjacent to the inlet 26 of the housing 11, which in turns generates a compressive force, the function of the control device is altered. When the actuator 9 is inactive, the balance of the forces acting on the control piston 6 is set in such a way that a minimum pressure is applied to the servo piston 4. This is achieved in that the spring force of the first spring 7 is greater than the counterforce of the second spring 8 that is adjustable by the setting screw 12, the second spring 8 engaging on the opposite side of the control piston 6 like the actuator 9. When the actuator 9 is inactive and powerless, the control piston 6 is shifted to the maximum extent to the inlet side and the control edge 39 opens the hydraulic link of the servo connection 27 with the tank outlet 18, causing the pressure level of the servo cylinder to be the same as that of the tank 19, i.e. virtually pressure-free or equal to the pressure level of the housing. In this way, the variable displacement pump 1 is set to the structurally defined maximum displacement volume, since a variable displacement pump is assumed that is set to maximum deflection when the servo piston does not exert any force on the adjustment element of the variable displacement pump 1. The variable displacement pump 1 therefore operates in starting position at a high output pressure which, as explained above, acts on the control piston 6 and codetermines the relative position of the control piston 6 in the housing 11. In the preferred embodiment shown in FIGS. 2 and 4 with an actuator 9 capable of generating a compressive force, the hydraulic force exerted by the supply pressure of the variable displacement pump 1 on the control piston plus the spring force of the first spring 7, is not sufficiently great, when the actuator 9 is inactive, to shift the control piston 6 from its maximum position on the inlet side without the help of actuator 9.

(21) If the solenoid 15 of the actuator 9 is now supplied with electric current, this changes the balance of forces at the control piston 6 and the control piston 6 is shifted from the previously occupied maximum position on the inlet side. In FIG. 2, this is therefore towards the right, since the actuator 9 exerts additional pressure on the first end 28 of the control piston. As a result, the position of the control edges 38, 39 (see FIG. 5) of the control cylinder 6 changes in relation to the through-channels 18, 26, 27 of the housing 11 with the results as described based on the preferred embodiment shown in FIGS. 1 and 3. The pressure in the servo cylinder 5 increases in line with the specification provided by the control unit 20, which leads to a reduction in pressure at the outlet 3 of the variable displacement pump 1 since there is an increase in the force of the servo piston 4 acting on the adjustment element of the variable displacement pump 1. This construction type of the adjustment device according to the invention therefore allows the output pressure at outlet 3 of the variable displacement pump 1 to be set in such a way that, assuming a high level of pressure at which the actuator 9 is inactive, the actuator 9 can be activated to set a lower pressure level in a controlled manner. As already mentioned, this happens without requiring a change in the rotational speed of the drive shaft 35.

(22) FIG. 3 shows a partial longitudinal cross-section of a control device 20 according to the invention of the general construction type according to FIG. 1 in which the pressure at the outlet 3 of the variable displacement pump 1 can be set from a low level to a higher level. In this preferred embodiment, contrary to the depiction in FIG. 1, the spring 8 and the actuator 9, here configured as a solenoid that generates a compressive force, are arranged in series. This means that the actuator 9 engages via the spring 8 at the control piston 6, whereby the tappet of the actuator 9 is in contact with one end of the spring 8. In a parallel arrangement of the spring 8 and the actuator 9 not shown here, the tappet 30 of the actuator 9 immediately adjoins the adjacent end 28, 29 of the control piston 6, for example, without touching the spring 8. For this purpose, the pin-shaped tappet 30 passes through the inside of the spring 8 configured to exert pressure, for example, or it engages at the guide 14 of the spring 8.

(23) The control device 20 comprises a housing 11 through which a bore 21 passes from a first end face 22 to a second end face 23. The bore 21 is stepped, comprising a central section with a smaller diameter flanked on both sides by sections with a larger diameter. The control piston 6 slides in the central section. This central section is itself stepped in such a way that a first end 28 of the control piston 6 runs through a section with a smaller diameter while the adjacent section has a somewhat larger diameter. The boundary between the two sections is located, for example, in the area of the opening of the outlet 26 in the central sections of the bore 21 and forms a step 37 or control edge 38 (see FIG. 5). The control piston 6 is adapted to the shape of the central section of the bore 21 in such a way that its first end section 28 has a smaller diameter than the adjacent section up to the second end of the control piston 6. It should be emphasized that the thinner end of the control piston 6 with its stepped configuration is always located near the opening of the outlet 26 for pressure fluid in the central section of the bore 21. The control piston 6 comprises two circumferential grooves 24 whose lateral limits form control edges 38, 39. It also has a continuous through bore or longitudinal bore 36 passing through it which serves to balance the pressure level between its two end sections 28, 29. At the first end 28 of the control piston, on the left in FIG. 2, the spring 7 configured as a compression spring engages via the guide 13. The other end of the spring 7 rests on the setting screw 12 which can be adjusted in its longitudinal direction via a thread, thereby allowing the force exerted by the spring 7 on the control cylinder 6 to be adjusted. At the opposite, second end 28 of the control piston 6, a spring 8 is also positioned whose force is transmitted via the guide 14 onto the control piston 6. The end of the control piston 6 pointing away from the spring 8 rests on the tappet 30 of the actuator 9, configured here as a solenoid 15. In this way, the spring 8 and the actuator 9 are arranged in series.

(24) Several channels 18, 25, 26, 27 pass through the housing 11 of the control device 20, which are, for example, directed towards the central bore 21 starting from a base area 34 of the housing 11. The channels 18, 25, 26, 27 cross the bore 21, thereby forming the inlet 26 for pressure fluid from the variable displacement pump 1, the servo connection 27 to the servo cylinder 5 and the outlet 18 to the tank 19. When the control edges 38, 39 of the control piston 6 are appropriately positioned, the two channels 18 and 25 serve to drain pressure fluid from the servo connection 27 via the groove 24 in the control piston 6 to the outlet 18 and therefore to the tank 19.

(25) The channels 18, 25, 26, 27 are hydraulically connected to the lines 10, 16, 17 for the pressure fluid, as shown in FIG. 1. When the control piston 6 is shifted, its control edges 38, 39 defined by the grooves 24 pass over the channel 27 that discharges into the bore 21, thereby opening the connection between the channels 18 and 27 as well as 26 and 27 in a defined manner or else blocking them completely. In this way it is possible to control the pressure acting on the servo control unit, i.e. the servo control piston 6.

(26) If one considers the preferred embodiment in FIG. 4 once again, the control piston 6 is moved to the right, away from the actuator 9, under the balance of forces of the springs 7, 8 and the output pressure of the variable displacement pump 1; here the control piston resumes its starting position in which the variable displacement pump is set to minimum output. This starting position, which can be set by means of the setting screw 12 at the spring 7, causes a defined pressure in the servo cylinder 5 since the operating pressure of the variable displacement pump 1 is directed onto the servo piston 6, thereby moving the variable displacement pump 1 into a minimally deflected position, which results in a defined low pressure at the outlet 3 of the variable displacement pump 1 that is nonetheless sufficient to apply a maximum servo force to the servo piston.

(27) If the solenoid 15 of the actuator 9 in FIG. 4 is supplied with electric current, its tappet 30 moves/pulls the control piston 6 to the left, if the actuator 9 is configured so as to exert a tractive force. Due to this additional tractive force applied to the control piston, which supports the spring force of spring 8, the position of the control edges 38, 39 of the control piston 6 changes in relation to the opening of the channel 27, which blocks the hydraulic connection between the outlet 26 and the servo cylinder 5 as the tractive force of the actuator 9 increases. As a result, the pressure on the servo piston 6 is reduced, changing the position of the servo piston 4 because the servo force decreases, increasing the flow rate of the variable displacement pump 1.

(28) The increased pressure at the outlet of the variable displacement pump 1 is transferred to the inlet 26 of the control unit 20 via the line 16 and acts on the control piston 6 via the stepped diameters of the end sections 28, 29 of the control piston 6. This produces a new balance of forces at the control piston 6 which results in the automatic setting of an increased but constant pressure level at the outlet 3 of the variable displacement pump 1. This pressure level can therefore be set via the electric current at the solenoid 15 or generally by controlling the actuator 9, whether mechanically, pneumatically, hydraulically or similar, and the pressure level is regulated automatically by the control device according to the invention. FIG. 6 shows an exemplary current/pressure diagram for the construction type of the control device 20 according to FIGS. 1 and 3, where the actuator 9 is configured as a control element which generates a compressive force.

(29) FIG. 4 shows a partial longitudinal cross-section of a control device 20 according to the invention, of the general construction type according to FIG. 2, in which the pressure at the outlet 3 of the variable displacement pump 1 can be set from a high level to a lower level. In this preferred embodiment, however, contrary to the depiction in FIG. 2, the spring 7 and the actuator 9, here configured as a solenoid 15, are arranged in series. This means that the actuator 9 engages the control piston 6 via the spring 7. As previously in FIG. 3, FIG. 4 shows a state in which the control piston 6 assumes a position where the control edges 38, 39 (see FIG. 5) block the hydraulic link to the servo connection 27 as well as to the inlet 26 and the outlet 18. In this construction type, the starting position of the control piston 6 is shifted further to the right than in FIGS. 3 and 4 when the actuator 9 is inactive.

(30) The arrangement according to FIG. 4 differs from that of FIG. 3 in that the actuator 9 and the first spring 7 are allocated to the first end 28 of the control piston 6 while the second spring 8, now adjustable via the setting screw 12, acts on the second end 29 of the control piston 6. Otherwise the components in FIG. 4 are exactly the same as those in FIG. 3. The mode of action of this arrangement differs from that according to FIG. 3 in that when the actuator 9 is not activated, the control piston 6 is shifted into its starting position (towards the left in FIG. 4) in such a way that the full, maximum output pressure of variable displacement pump 1 is applied at the inlet 26 of the control piston 6. When the actuator 9 is activated and the control piston 6 is shifted to the right, the pressure on the servo piston 6 is increased such that the variable displacement pump is reduced in its deflection and therefore in its flow rate, thereby diminishing the pressure at the outlet 3. In the preferred embodiment shown in FIG. 4, therefore the output pressure of the variable displacement pump 1 can be regulated from a high initial level to lower levels by means of the actuator 9. FIG. 7 shows an exemplary current/pressure diagram for the construction type of the control device 20 according to FIGS. 2 and 4.

(31) FIG. 5 shows a detailed view of the central section of the housing 11 of the control device 20 according to FIG. 4. Here the reference numerals apply in the same way as in FIGS. 1 to 4. The stepped central section of the bore 21 with the step 37 is to be emphasized, as are the varying diameters of the two ends 28, 29 of the control piston 6. The step 37 accommodates the different diameters of the control piston 6. This difference causes the pressure acting via the inlet 26 on the control piston 6 at the outlet 3 of the variable displacement pump 1 to exert a force on the control piston 6. In the event of an arrangement according to FIG. 5 and in all other preferred embodiments, this force is always directed towards the thicker, second end 29 of the control piston 6, i.e. towards the right-hand side in the examples shown in the figures. The pressure at the outlet 3 of the variable displacement pump 1 has a direct impact on the balance of forces acting on the control piston 6.

(32) FIG. 6 shows an exemplary current/pressure diagram of the control device according to FIGS. 1 to 5 in which a controlled reduction of supply pressure is effected by the control device according to the invention. Here the actuator 9 is configured in general form as a solenoid which causes increasing deflection of the tappet 30 positioned at the armature. A low initial pressure level can be seen with the actuator 9 inactive, i.e. the starting position of the control piston 6. The starting position of the control piston 6 in the diagram according to FIG. 6 is in its position of maximum deflection towards the outlet side in which the control edge 39 entirely closes the opening of the outlet 18, causing the pressure in the servo cylinder 5 to be equal to that of the variable displacement pump 1, thereby exerting a maximum servo force on the adjustment element, for example a swash plate, the variable displacement pump 1 being in a minimally deflected state. As the electric current to the actuator 9 is increased, the control piston 6 is shifted towards the inlet side, whereby the closing of the inlet 26 and simultaneous opening of the outlet 18 lowers the pressure in the servo cylinder 5 as well as the force of the servo cylinder 4 acting on the adjustment element, such that the servo adjustment allows the adjustment element to be deflected, increasing the supply pressure of variable displacement pump 1. Across a wide range, the increase in supply pressure is preferably linear and proportional to the actuator force.

(33) FIG. 7 shows an exemplary current/pressure diagram for the construction type of the control device according to FIGS. 1 to 5 in which a controlled reduction of supply pressure is effected by the control device according to the invention. Here the supply pressure of the variable displacement pump 1 is at a maximum level in its starting position when the actuator 9 is inactive, and this pressure is continuously reduced as the electric current applied to the actuator 9 is increased. In the starting position here, the control piston is shifted to the inlet side to maximum extent with the servo control virtually powerless and the variable displacement pump 1 deflected to maximum flow rate as is inherent to its design. Controlled pressure reduction by means of controlled application of electric current to the actuator 9 can extend to the value zero, at which the variable displacement pump 1 does not pump any pressure fluid. However, pumping is recommenced as soon as the electric current falls below the relevant boundary value.