Servo system bolted on design

Abstract

A hydrostatic servo assembly unit (1) for being arranged inside, outside or distant from a variable displacement hydrostatic unit (100) and for controlling the displacement of the variable displacement hydrostatic unit (100). The servo assembly unit (1) includes a servo housing (10) in which at least one servo piston (40) is arranged. The piston head (42) of the servo piston (40) can be pressurized such that the servo piston (40) can move linear relative to a servo cylinder (12) formed in the servo housing (10). The servo assembly unit (100) further includes a movable output element (49) protruding outside of the servo housing (10), which can be mechanically coupled to a displacement element (102) of a variable displacement hydrostatic unit (100).

Claims

1. A hydrostatic servo assembly unit for being arranged inside or outside a casing of a variable displacement hydrostatic unit and for controlling a displacement of the variable displacement hydrostatic unit, the servo assembly unit comprising a servo housing in which at least one servo piston is arranged to move linearly relative to a servo cylinder formed in the servo housing, wherein the servo assembly unit comprises a movable output element protruding outside of the servo housing, which can be mechanically coupled to a displacement element of the variable displacement hydrostatic unit, wherein the at least one servo piston comprises two piston heads each sealing a cylindrically shaped pressure chamber arranged coaxially along a longitudinal bore axis and formed at either side of the servo housing, wherein each pressure chamber is formed by an end cap, such that outwardly facing front faces of the two piston heads are configured to be pressurized by servo pressure in the respective pressure chamber in order to move the at least one servo piston, wherein each pressure chamber is hydraulically connected to a pressure source such that the servo pressure can be alternately provided to the pressure chambers, wherein at least one servo spring is arranged between the two piston heads by means of two spring seats, the two spring seats including radial outer portions and radial inner portions with reference to the longitudinal bore axis, where the radial outer portions abut against the end caps and the radial inner portions are movable by the two piston heads in order to compress the at least one servo spring when one of the pressure chambers is pressurized with the servo pressure, wherein a servo piston rod of the at least one servo piston is connected operatively to a first end of an eccentric mechanism such that a second end of the eccentric mechanism rotates when the at least one servo piston is moving, and wherein a portion of the eccentric mechanism is located inside of the servo housing and the second end of the eccentric mechanism acts as the movable output element and protrudes outside the servo housing.

2. The hydrostatic servo assembly unit according to claim 1, wherein the output element is moveable linearly or rotatably.

3. The hydrostatic servo assembly unit according to claim 1, wherein the two piston heads comprise different diameters or the outwardly facing front faces of the two piston heads are equal with regard to the surface size on which servo pressure can act.

4. The hydrostatic servo assembly unit according to claim 1, wherein additional springs and/or dampers are arranged in at least one of the pressure chambers.

5. The hydrostatic servo assembly unit according to claim 1, configured to receive an adjustable servo pressure provided by the pressure source, the pressure source being selected from the group consisting of a hydraulic drive pedal, an electronic displacement control circuit, a hydraulic steering circuit and a charge pump.

6. The hydrostatic servo assembly unit according to claim 5, wherein the pressure source is connected to the pressure chambers via pilot valves, wherein each of the pilot valves is controlled by an actuator and/or are equipped with a pressure compensator.

7. A working machine comprising the hydrostatic servo assembly unit according to claim 6, wherein the pilot valves of the servo assembly unit are configured to be controlled according to command signals given to a control unit by an operator of the working machine.

8. The hydrostatic servo assembly unit according to claim 1, wherein a restoring force which is exerted by the servo spring and/or a center position of the at least one servo piston is/are adjustable by way of the endcaps being positionable relative to the servo housing.

9. The hydrostatic servo assembly unit according to claim 1, wherein at least one of the endcaps is formed integrally with the servo housing and/or a servo housing lid.

10. A variable displacement hydrostatic unit having the casing to which the hydrostatic servo assembly unit according to claim 1 is attached, wherein the second end of the eccentric mechanism is configured to directly or indirectly move the displacement element of the variable displacement hydrostatic unit in order to set the displacement volume of the variable displacement hydrostatic unit.

11. The variable displacement hydrostatic unit according to claim 10, wherein the servo assembly unit is arranged inside or outside of the casing of the variable displacement hydrostatic unit and the displacement element is arranged inside of the casing of the variable displacement hydrostatic unit.

12. The variable displacement hydrostatic unit according to claim 10, wherein the variable displacement hydrostatic unit is a hydrostatic pump or a hydrostatic motor of an axial piston or radial piston design.

13. A variable hydrostatic transmission with at least one variable displacement hydrostatic pump and one hydrostatic motor comprising the casing, which is a common transmission casing, to which the hydrostatic servo assembly unit according to claim 1 is attached, wherein the second end of the eccentric mechanism is configured to directly or indirectly move the displacement element of the at least one variable displacement hydrostatic pump in order to set a displacement volume of the at least one variable displacement hydrostatic pump.

14. A method of using a hydrostatic servo assembly unit to control a displacement of a hydrostatic unit, an open or closed circuit hydrostatic transmission, a steering device, a flap mechanism, or any other bi-directional moveable device/mechanism, the hydrostatic servo assembly unit comprising a servo housing in which at least one servo piston is arranged to move linearly relative to a servo cylinder formed in the servo housing, wherein the servo assembly unit comprises a movable output element protruding outside of the servo housing which can be mechanically coupled to a displacement element of a variable displacement hydrostatic unit, wherein the at least one servo piston comprises two piston heads each sealing a cylindrically shaped pressure chamber arranged coaxially along a longitudinal bore axis and formed at either side of the servo housing, wherein each pressure chamber is formed by an end cap, such that outwardly facing front faces of the two piston heads are configured to be pressurized by servo pressure in the respective pressure chamber in order to move the at least one servo piston, wherein each pressure chamber is hydraulically connected to a pressure source such that the servo pressure can be alternately provided to the pressure chambers, wherein at least one servo spring is arranged between the two piston heads by means of two spring seats, the two spring seats including radial outer portions and radial inner portions with reference to the longitudinal bore axis, where the radial outer portions abut against the end caps and the radial inner portions are movable by the two piston heads in order to compress the at least one servo spring when one of the pressure chambers is pressurized with the servo pressure, wherein a servo piston rod of the at least one servo piston is connected operatively to a first end of an eccentric mechanism such that a second end of the eccentric mechanism rotates when the at least one servo piston is moving, and wherein a portion of the eccentric mechanism is located inside of the servo housing and the second end of the eccentric mechanism acts as the movable output element and protrudes outside the servo housing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following an inventive hydrostatic servo unit is shown with the help of the attached Figures. However, the invention is not limited to the embodiments shown below even if not mentioned in the description. Different embodiments can be combined or modified by a person with ordinary skills in the art without leaving the scope of the inventive idea.

(2) The Figures show:

(3) FIG. 1 a scheme of a hydrostatic servo assembly unit according to the invention in combination with a general actuation system;

(4) FIG. 2 a scheme of a hydrostatic servo assembly unit according to the invention with a pilot valve actuation system;

(5) FIG. 3 a section of a hydrostatic servo assembly unit according to the invention;

(6) FIG. 4 an isometric view of a hydrostatic servo assembly unit according to the invention;

(7) FIG. 5 an isometric view of a variable displacement hydrostatic unit with a remotely attached servo assembly unit according to the invention;

(8) FIG. 6 a side view of a second embodiment of a hydrostatic servo assembly unit according to the invention;

(9) FIG. 7 a sectional view of the second embodiment of a hydrostatic servo assembly unit according to the invention; and

(10) FIG. 8 a sectional view of a second embodiment of variable displacement hydrostatic unit with a laterally attached hydrostatic assembly unit according to the invention.

DETAILED DESCRIPTION

(11) FIG. 1 discloses a scheme of an embodiment of an inventive hydrostatic servo unit 1 which comprises a housing 10 (illustrated as dotted-dashed-line) within which two pressure chambers 25, 35 are arranged coaxially along a longitudinal bore axis 14 at opposing sides of the housing 10. Each of the pressure chambers 25, 35 is formed as a cup-shaped cavity in end caps 20, 30 which are arranged in the housing 10, wherein the cavities are open towards the inside of the housing 10. The openings of the end caps 20, 30 towards the inner of housing 10 are sealed on each side by a piston head 42, 46 of a two head servo piston 40. In consequence the pressure chambers 25, 35 are formed by interior surfaces of the end caps 20, 30 and the outwardly facing front faces 43, 47 of the piston heads 42, 46. The piston heads 42, 46 share a piston rod 41 that protrudes on one side through the pressure chamber 25 and through the corresponding end cap 20 towards the outside of the housing 10. Both pressure chambers 25, 35 are hydraulically connected to a pressure source 60 which can guide pressurized fluid as a servo pressure into the pressure chambers 25, 35, such that servo pressure can act on the outwardly facing front faces 43, 47 of the servo piston 40. Both pressure chambers 25, 35 can alternatively be connected to a hydraulic reservoir 70 which provides the possibility to drain fluid from the pressure chambers 25, 35. As a consequence, a pressure difference can be established over the two outwardly facing front faces 43, 47, which leads to a movement of the two head servo piston 40 including first and second piston heads 42, 46 and the piston rod 41 which is connected to at least one of the piston heads 42, 46.

(12) In this embodiment, two servo springs 50 are arranged on opposing sides of the cylinder bore axis 14 between the first and second inwardly facing front faces 44 and 48 of piston heads 42, 46 of the servo piston 40. Each of the servo springs 50 abuts against two spring seats 52, 54, wherein one is located on either side of the servo springs 50. The spring seats 52, 54 comprise a radially outer portion 56 which is in a centered/initial position pushed against an end surfaces 22 and 32 of the endcaps 20, 30, such that the servo springs 50 are centered between the end caps 20, 30. The radially inner portions 58 of the spring seats 52, 54 can be contacted by the first and second inwardly facing front faces 44 and 48 of piston heads 42, 46.

(13) In the centered position of the servo piston according to the invention both springs 50 are on both sides in contact with the first and second spring seats 52, 54 which are with their radially outer regions 56 in contact with the end surfaces 22 and 32 of end caps 20 and 30 as well as with their radially inner regions 58 in contact with the piston heads 42 and 46. It is obvious to a person skilled in the art that this centering position can be adjusted when the position of the inner end surfaces 22, 32 of the end caps 20, 30 is changed, wherein, e.g., the distance between the two endcaps 20, 30 can be kept constant. However, depending on the application it can be also desired to change the distance between the inner end surfaces 22, 32 of the two end caps 20, 30, in order to adjust the pretension of the servo springs 50. This will lead to a more or less strong restoring behavior as shown in the next paragraph.

(14) Each piston head 42, 46 when being pressurized at its outwardly facing front face 43, 47 can compress the springs 50 by means of a movement in a direction towards the spring 50, respectively towards the other (non-pressurized) pressure chamber 25 or 35. This movement of the two head servo piston 40 is transduced via the spring seats 52, 54 onto the adjacent end of the springs 50. However, the springs 50 abut against the end surfaces 22 and 32 of end caps 20 and 30 via the radially outer portions 56 of the spring seats 52, 54 on the opposite side of the servo springs 50. This leadswith ongoing of the movement of the servo piston 40to an increasing compression of the servo springs 50 which in consequence exert a force counteracting the movement of the servo piston 40. The servo piston 40 will move as long as the counteracting spring forces are lower than the result of the hydraulic force balance on the outwardly facing front faces 43, 47. If the spring force and the resulting hydraulic force are equal, the servo piston 40 remains in its current position. However, if the servo piston 40 is deflected from the centered position and the resulting hydraulic force is smaller than the restoring spring force, e.g., if hydraulic fluid is drained from a pressurized chamber, the servo piston 40 will be pushed back towards its centered position by the force of the servo springs 50. Thereby, the spring forces decreases with ongoing movement towards the center position until the servo piston 40 reaches the center/initial position, in which the correspondent spring seat 52 or 54 contacts the inner end surface 22 or 32 of the end caps 20 or 30, having served before as pressure chamber 25 or 35.

(15) In the embodiment shown in FIG. 1 the piston rod 41 protrudes through the first pressure chamber 25 to the outside of the housing 10. If the two piston heads 42, 46 comprised the same diameter, different servo pressures would be required in the sealed pressure chambers 25 and 35 to obtain the same moving distance of the servo piston 40. In the embodiment of FIG. 1 the part of surface of the two outwardly facing front faces 43, 47 on which pressure can act is of equal size, as no pressure can act on the surface area which is covered by the connection of the piston rod 41 to the piston head 42. To enhance the modular usability of the inventive servo unit, it is desirable to establish a pressure surface size ratio equal to 1 between the two outwardly facing front faces 43, 47. This means, that a particular pressure difference will lead to a particular movement of the servo piston independently which pressure chamber 25, 35 comprises the higher pressure. Therefore, in the embodiment according to FIG. 1 the first piston head 42 comprises a greater diameter than the second piston head 46. This compensates the surface loss due to the piston rod connection and leads to a well-balanced movement independently from the movement direction.

(16) According to the invention, the pressure chambers 25, 35 of the servo unit 1 comprise hydraulic connection ports, i.e. at least one drain port 67 and at least one supply port 66, which can be directly connected to a hydraulic sink 70 and/or a hydraulic reservoir 70. Even though FIG. 1 does not show any additional hydraulic components that are arranged between the hydraulic source 60/reservoir 70 and the pressure chambers 25 & 35 the inventive concept covers the idea of arranging other hydraulic circuits between the connection ports of the servo unit 1, e.g. a control circuit of a hydraulic steering assembly.

(17) FIG. 2 shows another embodiment of an inventive hydraulic servo unit 1. In this embodiment, the pressure chambers 25, 35 of the servo unit 1 are connected to the pressure source 60 and the hydraulic reservoir 70 via pilot valves 63, 64, wherein the outlet 68 of a first pilot valve 63 is hydraulically connected to the first pressure chamber 25 and the outlet 68 of a second pilot valve 64 is hydraulically connected to the second pressure chamber 35. Each pilot valve 63, 64 comprises a supply port 66 which is connected to a pressure source 60, e.g. a charge pressure line, and a drain port 67 is connected to a hydraulic reservoir 70, e.g. a hydraulic tank.

(18) In a pressure less state, the pilot valves 63, 64 according to the embodiment of FIG. 2 are in a first, closed position in which the corresponding pressure chambers 25, 35 are connected hydraulically via the pilot valve 63, 64 to the hydraulic reservoir 70. In a second, open position of one of the pilot valves 63 or 64 a corresponding pressure chamber 25, 35 is connected with the pressure source 60 and a servo pressure is guided by one of the pilot valves 63, 64 from its supply port 66 via the outlet 68 to the pressure chamber 25 or 35. The respective other pressure chamber 35 or 25 remains on the pressure level of hydraulic reservoir 70 as the second pilot valve 64 or 63 remains in its first, closed position in which the supply port 66 is closed and draining of hydraulic fluid from the correspondent pressure chamber 35 or 25 to the hydraulic reservoir 70 is enabled.

(19) Each pilot valve 63, 64 comprises two front faces on which actuation forces can be applied. On a first front face a pilot valve spring 65 is arranged, which abuts against the front face and holds the pilot valves 63, 64 in its first, closed position, as long as an actuation force exerted on the opposing second front face is lower than the spring force on the opposite front face. Additionally a hydraulic feedback line 69 can guide hydraulic pressure from the outlet 68 of the pilot valves 63, 64 to the first front face. This hydraulic pressure exerts an additional closing force on the pilot valve spool proportional to the servo pressure guided to one of the pressure chambers 25 or 35.

(20) Actuators 62 are arranged on the spring 65 opposing second front face of the pilot valves 63, 64 and can act on the front faces with an opening actuation force, such that the pilot valves 63, 64 are shifted to a second, open position, when the actuators 62 are energized, e.g. In the embodiment according to FIG. 2 the actuators 62 are solenoids receiving an electrical control signal from a control unit (not shown) and exerting a corresponding force on the second front face. Although solenoids are shown in FIG. 2 as actuators 62, other types of actuators are covered by the inventive concept as well, e.g. mechanical actuators or hydraulic actuators.

(21) FIG. 3 discloses a sectional view of an inventive hydrostatic servo unit 1. The housing 10 in this embodiment comprises a housing base 11 and a housing lid 15 which is connected to the housing base 11 in a fluid tight manner. Attention is drawn to the first and second end caps 20, 30 which are accommodated by threads arranged in longitudinal bores 12 of the housing 10. These threads provide the possibility of adjusting independently the position of the end caps 20, 30 relative to each other. The connection surfaces between end caps 20, 30 and housing 10 may be sealed against leaking fluid as well as the connection surfaces between the piston heads 42 and 46 with the end caps 20, 30. Although not shown in FIG. 3, it is covered by the invention to form at least one of the end caps 20, 30 integrally with the housing 10, in particular with the housing base 11 and/or the housing lid 15.

(22) The spring seats 52, 54 according to the embodiment shown in FIG. 3 comprise a T-shaped cross section, providing a large-scale contact-area for the servo springs 50 on the associated inwardly facing front face 44 or 48. The radially outer portion 56 of the spring seats 52 & 54 contact the end caps 20, 30 at their inner ends 22 or 32, respectively. Additionally, hollow-cylinder shaped radially inner area 58 are provided that can simultaneously serve as a guidance for the movement of the servo spring 50 in the direction of the longitudinal bore axis 14.

(23) The servo piston 40 shown in FIG. 3 comprises a basically bone-like shape with a central longitudinal bore 45, in which the piston rod 41 is arranged. The piston rod 41 is attached to the second piston head 46 in a detachable manner using a screw that is accommodated by a thread arranged in the piston rod 41. The piston rod 41 projects outwardly of the housing 10 on the opposite side of the servo assembly unit 1. A multi-part design of the piston 40 can facilitate the assembling process of the servo unit 1, especially if only one servo spring 50 or one-part circumferential spring seats 52, 54 are used that surround the bone like servo piston 40. Then, splitting the two piston heads 42, 46 in different assembly units provides the possibility to arrange the surrounding parts on the bone shaped piston. However, an integral design of the two head servo piston 40 is also covered by the invention. This is especially advantageous, if more than one servo spring 50 is used and the spring seats 52, 54 are circumferentially split in more than one part.

(24) FIG. 4 shows an isometric view of an exemplary embodiment of a hydrostatic servo unit 1 according to the invention. The inventive servo unit 1 comprises a two part housing 10 with a housing base 11 and a housing lid 15. The two housing parts are connected in a releasable manner by the use of screws. At one side of the housing 10 the outwardly protruding end of the piston rod 41 is visible. The piston rod 41 is accommodated in the housing base 11 and/or the housing lid 15, such that it is movable along the cylinder bore axis 14. However, it is important that no fluid can leak through the bearing of the piston rod 41.

(25) A supply port 66 and a drain port 67 are arranged at the side of the housing 10. These two ports enable a fluid connection of the pressure chambers 25, 35 of the inventive servo unit 1 to a hydraulic pressure source 60 and to a hydraulic reservoir 70, such that the two head piston 40 of the servo assembly unit 1 can be moved forced by servo pressure supplied to one of the pressure chambers 25, 35 and draining hydraulic fluid from the other pressure chamber 25, 35. However, in the embodiment shown in FIG. 4, the pilot valves 63, 64 (not visible in FIG. 4) are used to control the fluid flow to the pressure chambers 25, 35. The pilot valves 63, 64 are actuated, e.g., by solenoids 62, which convert an electric input signal to a mechanical force which can act on a front face of the one of pilot valve spools to move it (see also FIG. 2).

(26) In FIG. 5 a hydrostatic transmission 110 with two variable displacement hydrostatic units 100 is shown. The general working principle of a hydrostatic transmission is familiar to a person skilled in the art. In the embodiment of FIG. 5, the hydrostatic transmission 110 comprises a hydraulic pump 100 with a variable displacement volume which pumps hydraulic fluid utilizing mechanical energy to a hydrostatic motor with a constant displacement, for instance. The motor converts the hydraulic energy to rotational mechanical energy. The transmission ratio of the hydrostatic transmission 110 is changed by adjusting the displacement volume of the pump 100 which, in consequence, conveys more or less hydraulic fluid flow to the motor. In the described embodiment, the displacement volume is adjusted by a tilting motion of the displacement element 102, in this case a swashplate with a control rod that is attached to the swashplate and protruding outwardly of pump 100. However, other types of hydrostatic transmission units 110 which may use a different number, type or arrangement of pumps and motors are also covered by the inventive concept.

(27) The displacement element 102 of the hydrostatic unit 100 according to the invention is rotated by means of an inventive servo unit 1. The hydrostatic servo unit 1 is arranged at the outside of the casing 106 of the controlled hydrostatic unit 100. Therefore, the control rod of the displacement element 102 protrudes to the outside of the casing 106 of the hydrostatic transmission 110. The displacement element 102 is coupled with its outwardly facing end eccentrically coupled to the piston rod 41 of an inventive hydrostatic servo unit 1 which is also arranged at the outside of the casing 106. The eccentric mechanism 104 converts a linear motion of the piston rod 41 to a rotational motion of the displacement element 102. However, other mechanical methods to convert linear motion of the piston rod 41 to a rotational motion of the control rod are known by a person skilled in the art, e.g. a leverage or cam system. Even electronic or hydraulic concepts, for instance a sensor-actor system, that couple the linear moving piston rod 41 to the rotationally deflectable control means 102 can be applied.

(28) In FIG. 5 two different concepts to pressurize the pressure chambers 25, 35 of the servo piston 1 are shown. The actuators 62 of pilot valves 63, 64 are arranged at the side surfaces of the housing 10 as already known from FIG. 4. Additionally, the servo assembly unit 1 can be actuated using drive pedals 61. These pedals 61 are connected to the pressure chambers 25, 35 of the servo unit 1 via a hydraulic sub-circuit, e.g. a control valve, which is not depicted.

(29) FIG. 6 depicts a second embodiment of a hydrostatic servo assembly unit 1 according to the invention. FIG. 7 shows a sectional view along line A-A in FIG. 6.

(30) The servo unit 1 comprises a movable output element 49 which is rotatable protrudes from the housing 10 of the servo unit 1. The movable output element 49 is the second end 82 of an eccentric mechanism 104 which transmits linear motion of a servo piston 40 (not shown) to rotational motion of the movable output element 49. The housing 10 of the servo unit 1 according to the invention is not formed integrally with the casing 106 of a variable displacement hydrostatic unit 100, but is provided separately. Therefore, the servo unit 1 according to the invention can be assembled independently from a variable displacement hydrostatic unit 100. As a consequence of this differential construction method, components of the hydrostatic servo unit 1 can be facilely attuned to each other during and after the assembling process of the servo unit 1. In particular, the center position of the servo spool can be adjusted/attuned easily to the neutral position of a hydrostatic unit 100 to be controlled with the hydrostatic servo assembly unit 1 according to the invention.

(31) The housing 10 comprises a two part construction and is therefore formed by means of a housing base 11 and a housing lid 15 (see also FIG. 7). A two head servo piston 40 is slidably arranged in a longitudinal bore 12 in the housing 10 of the servo unit 1. The two head servo piston 40 comprises a first piston head 42 and a second piston head 46. Actuators 62 are provided on the side of the housing 10 which is located opposite to the movable output element 49. The actuators 62 control the servo pressure in the pressure chambers 25, 35 which are formed between first and second outwardly facing front faces 43, 47 and the first and second endcaps 20, 30, respectively.

(32) Servo springs 50 are provided which apply a centering force on the servo piston 40, if the servo piston 40 slides to one side from its centered starting position. The servo springs 50 abut on either side via spring seats 52, 54 on inner end surfaces 22, 32 of end caps 20, 30. Similar to the embodiment shown with FIG. 3, the springs 50 are simultaneously in contact with the radially inner portions 58 and the radially outer portions 56 of the first spring seat 52 and the second spring seat 54, when the two head servo piston 40 is in its starting position. The springs 50 can be inserted between the spring seats 52, 54 in a pre-tensioned, compressed manner, such that a force is simultaneously exerted on the radially inner portion 58 of the spring seats 52, 54 and on the radially outer portion 56 of the spring seats 52, 54 and the two head servo piston 40 is centered between shoulders of the housing 10. Thereby, the outer portion 56 is supported by the housing and the radially inner portion 58 is supported by the first or second inwardly facing front faces 44, 48 of the two head servo piston 40.

(33) If one of the pressure chambers 25, 35 is charged with servo pressure and the other pressure chamber 35, 25 is simultaneously connected with a hydraulic reservoir 70, the servo piston will move towards the pressure chamber 35, 25 from which hydraulic fluid is discharged into the reservoir 70. The motion of the servo piston 40 is counteracted by restoring forces of the servo springs 50 which are compressed, when the servo piston 40 leaves its centered position. The motion of the servo piston 40 is transmitted via the inwardly facing front faces 44 or 48 to the radially inner portion 58 of the first spring seat 52 or second spring seat 54 to the springs 50. The spring seats 52, 54 are supported via the radially outer portions 56 at the housing on the opposite side of the springs 50 (the side towards which the servo piston 40 slides).

(34) The two head servo piston 40 according to the embodiment of FIGS. 6 and 7 comprises a piston rod 41 which extends in the direction of the longitudinal bore axis 14. A first end 80 of an eccentric mechanism 104 is attached to the central part of the piston rod 41. The first end 80 is therefore in operative connection with the piston rod 41 and the longitudinal movement of the two head servo piston 40 is directly transmitted to the first end 80 of the eccentric mechanism 104 in order to rotate the second end 82. A person with relevant skills in the art will chose an appropriate joint connection between the piston rod 41 and the first end 80 of the eccentric mechanism 104 that provides appropriate freedom of movement. Thereby it can be for example preferable, to minimize the play of the joint connection in the direction of the longitudinal bore axis 14. For example, a ball connection which only transmits longitudinal/lateral movements and does not impede the rotational relative movement of the first end 80 of the eccentric mechanism 104. However, also a stiff joint, a sliding joint or an alternative joint connection may be selected by the person skilled in the art.

(35) The linear/longitudinal motion of the first end 80 of the eccentric mechanism 104 is converted to rotational motion of the second end 82 which is in the embodiment according to FIGS. 6 and 7 the movable output element 49. The movable output element 49 protrudes from the housing 10 and provides a rotationally moving interface to the tilting displacement volume control means 102 for controlling the displacement of a variable displacement hydrostatic unit 100 (see FIG. 8, e.g.). The form and motion of the provided interface, i.e. the movable output element 49, can preferably be standardized, such that one model of hydrostatic servo unit 1 can control the tilt angle of various types and models of displacement control means 102. This decreases the complexity of an assembly consisting of a variable displacement hydrostatic unit 100 and a separate servo unit 1 according to the invention. Due to the higher number of common and generic parts, the servo unit 1 according to the invention is therefore capable of facilitating the assembling process and of enhancing the manufacturing quality and costs of a variable displacement hydrostatic unit 100.

(36) The separate hydrostatic servo assembly unit 1 can be fixed to a variable displacement hydrostatic unit 100 by means of bolts 85 or similar fixation means, for example, clamping means. A person skilled in the art may as well choose non-resolvable fixation techniques, such as gluing or welding. Also, other methods of fixing the hydrostatic servo unit 1 to the casing 106 of a variable displacement hydrostatic unit 100 are covered by the scope of the inventive concept.

(37) FIG. 8 shows a hydrostatic transmission 110 with one variable displacement hydrostatic unit 100 being, for example a hydrostatic pump, and another hydrostatic unit with constant displacement volume, for example a hydrostatic motor. It is clear to a person with relevant skills in the art that a hydrostatic transmission 110 can comprise different hydrostatic units, e.g., two variable displacement hydrostatic units 100. The hydrostatic transmission 110 is located inside of a transmission casing 106 which is sealed from the outside. A hydrostatic servo assembly unit 1 is provided as a separate assembly and is attached to the casing 106 of the hydrostatic transmission 110 by means of bolts 85. The housing 10 of the servo unit 1 is also closed to the outside. However, an opening is provided on one side of the housing 10. A movable output element 49 is protruding through the opening to the outside of the housing 10 of the servo unit 1 and to the inside the casing 106 of the hydrostatic transmission 110. The servo pressure acting on a servo piston 40 of the servo unit 1 can be controlled by means of actuators 62, e.g. electro-mechanical solenoids, or by moving pedals 61 which are mechanically or hydraulically coupled to the hydrostatic servo unit 1. In consequence, the pressure in one of the pressure chambers 25, 35 rises and the servo piston 1 is pushed sideways from its centered position.

(38) As an eccentric mechanism 104 is provided inside of the housing 10 of the servo unit 1, the linear motion of the piston rod 41 is transmitted to the first end 80 of the eccentric mechanism 104. The second end 82 of the eccentric mechanism 104, in this case the movable output element 49, rotates in order to tilt a displacement element 102 (not shown), to which the output element 49 is operatively connected.

(39) In summary with the hydrostatic servo assembly unit 1 according to the invention a robust and cost effective servo unit is provided, which ensures a good ability to adjust the center position of the servo unit to the neutral position of a hydraulic unit to be controlled by the inventive servo unit. The invention further provides for a more flexible design of hydraulic unit as the servo assembly unit 1 according to the invention is usable for a variety of apparatuses as a standardized servo unit. Hence, hydraulic units, in particular, can be designed smaller, as the servo assembly unit can be mounted remotely from the hydraulic unit.

(40) While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.