PISTON ASSEMBLY

Abstract

The present disclosure relates to a piston assembly, the piston assembly comprising a cylinder extending along an axis and having an open axial end, a cap assembly closing the cylinder at its open axial end, wherein the cap assembly comprises an insertion portion received in the cylinder at its open axial end, and a piston axially movably disposed within the cylinder and partially received in the insertion portion of the cap assembly. The present disclosure further relates to a variable displacement hydraulic unit including said piston assembly.

Claims

1. A piston assembly, comprising: a cylinder extending along an axis and having an open axial end, a cap assembly closing the cylinder at its open axial end, the cap assembly comprising an insertion portion received in the cylinder at its open axial end, and a piston axially movably disposed within the cylinder and partially received in the insertion portion of the cap assembly.

2. The piston assembly of claim 1, wherein a cylinder wall enclosing the cylinder has a cylindrical shape and includes no machined portion or portions in or on the cylinder wall.

3. The piston assembly of claim 1, wherein the cap assembly comprises an outer sealing member disposed radially in between the insertion portion of the cap assembly and a cylinder wall enclosing the cylinder, wherein the outer sealing member is mounted on the insertion portion of the cap assembly.

4. The piston assembly of claim 3, wherein a radially outer surface of the insertion portion of the cap assembly comprises an outer annular indentation, wherein the outer sealing member is received in the outer annular indentation.

5. The piston assembly of claim 1, wherein the cap assembly comprises an inner sealing member disposed radially in between the insertion portion of the cap assembly and the piston, wherein the inner sealing member is mounted on the insertion portion of the cap assembly.

6. The piston assembly of claim 5, wherein a radially inner surface of the insertion portion of the cap assembly comprises an inner annular indentation, wherein the inner sealing member is received in the inner annular indentation.

7. The piston assembly of claim 1, further comprising a hydraulic chamber formed axially in between the cap assembly and the piston, wherein a hydraulic pressure in the hydraulic chamber is configured to bias the piston in a first direction along the axis.

8. The piston assembly of claim 1, further comprising a housing, wherein the cylinder is formed in the housing and the cap assembly is fixed to the housing.

9. The piston assembly of claim 1, further comprising: a first biasing member for biasing the piston in a first direction along the axis, and an axially displaceable zeroing member limiting axial movement of the first biasing member.

10. The piston assembly of claim 9, wherein the first biasing member is axially supported on the cap assembly.

11. The piston assembly of claim 9, further comprising a second biasing member for biasing the piston in a second direction along the axis, opposite the first direction, wherein the zeroing member limits axial movement of the first biasing member in the first direction along the axis and wherein the zeroing member limits axial movement of the second biasing member in a second direction along the axis, opposite to the first direction.

12. The piston assembly of claim 11, wherein the piston comprises a body portion and a protrusion extending from the body portion in a lateral direction perpendicular to the axis, wherein the first biasing member is configured to be axially supported on the protrusion and to bias the piston in the first axial direction when at least a portion of the protrusion extends beyond the zeroing member in the second axial direction, and wherein the second biasing member is configured to be axially supported on the protrusion and to bias the piston in the second axial direction when at least a portion of the protrusion extends beyond the zeroing member in the first axial direction.

13. The piston assembly of claim 12, wherein an axial extension of the protrusion is equal to an axial extension of the zeroing member.

14. The piston assembly of claim 12, further comprising: a first thrust ring axially disposed in between the zeroing member and the first biasing member and in between the protrusion and the first biasing member so that the first biasing member is configured to be supported on the protrusion via the first thrust ring and to move the piston in the first direction until the first thrust ring hits the zeroing member, and a second thrust ring axially disposed in between the zeroing member and the second biasing member and in between the protrusion and the second biasing member so that the second biasing member is configured to be supported on the protrusion via the second thrust ring and to move the piston in the second direction until the second thrust ring hits the zeroing member.

15. The piston assembly of claim 11, wherein the first biasing member and/or the second biasing member at least partially enclose/encloses the piston.

16. The piston assembly of claim 9, further comprising a rotatable gudgeon including an eccentric pin engaged with the zeroing member so that the zeroing member is displaceable along the axis by rotating the rotatable gudgeon.

17. The piston assembly of claim 9, further comprising a worm screw or leadscrew engaged with the zeroing member so that the zeroing member is displaceable along the axis by rotating the worm screw.

18. A variable displacement hydraulic unit, comprising: a swashplate, and the piston assembly of claim 1, wherein the piston is coupled to the swashplate for rotating the swashplate.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0025] FIG. 1 schematically illustrates a plan view of a first embodiment of a variable displacement hydraulic unit including a piston assembly of the presently proposed type.

[0026] FIG. 2 schematically illustrates a first sectional view of the variable displacement hydraulic unit of FIG. 1.

[0027] FIG. 3 schematically illustrates a second sectional view of the variable displacement hydraulic unit of FIG. 1

[0028] FIG. 4 schematically illustrates a plan view of elements the variable displacement hydraulic unit of FIG. 1, wherein a swashplate is in a tilted position.

[0029] FIG. 5 illustrates a sectional view of the variable displacement hydraulic unit of FIGS. 1 to 4, wherein a piston of the piston assembly is deflected from a first zero position.

[0030] FIG. 6 illustrates the sectional view of FIG. 5, wherein the piston of the piston assembly has been moved back to the first zero position.

[0031] FIG. 7 illustrates a sectional view of the variable displacement hydraulic unit of FIGS. 1 to 4, wherein the piston of the piston assembly is deflected from a second zero position.

[0032] FIG. 8 illustrates the sectional view of FIG. 7, wherein the piston of the piston assembly has been moved back to the second zero position.

[0033] FIG. 9 schematically illustrates a plan view of a second embodiment of a variable displacement hydraulic unit including a piston assembly of the presently proposed type.

[0034] FIG. 10 schematically illustrates a first sectional view of the piston assembly of FIG. 9.

[0035] FIG. 11 schematically illustrates a second sectional view of the piston assembly of FIG. 9.

[0036] FIG. 12 schematically illustrates a perspective view of the piston assembly of FIG. 9.

DETAILED DESCRIPTION

[0037] FIG. 1 shows a plan view of a variable displacement hydraulic unit 100 of the presently proposed type according to a first embodiment. Here, the variable displacement hydraulic unit 100 is a variable displacement axial piston unit and may be used as a hydraulic pump or as a hydraulic motor. FIGS. 2 to 8 show various views of the hydraulic unit 100 of FIG. 1, wherein here and in all of the following, features recurring in different figures are designated with the same or similar reference signs. In FIG. 1, a dashed line A-A indicates the sectional plane of FIG. 2. And in FIG. 2, a dashed line B-B indicates the sectional plane of FIG. 3. The viewing direction of FIG. 4 is perpendicular to the viewing direction of FIG. 2 and perpendicular to the viewing direction of FIG. 3.

[0038] The hydraulic unit 100 includes a casing 1, a rotatable shaft 2 such as pump shaft or motor shaft at least partially disposed in the casing 1, a cylinder block 3 (see FIG. 3) rotationally coupled to the shaft 2, pistons 4 configured to reciprocate within cylinders 3a formed in the cylinder block 3, and a tiltable swashplate 5 (see FIGS. 3 and 4) configured to control a stroke of the pistons 4, as is generally known in the art of hydraulic devices. A rotation axis 5a of the swashplate 5 is arranged perpendicular to a rotation axis 2a of the rotatable shaft 2. The cylinders 3a are fluidically connected to a fluid inlet and to a fluid outlet, for example via a valve plate 6. Within the scope of this document the term fluid may include a liquid such as oil.

[0039] The hydraulic unit 100 further includes a piston assembly 101. More detailed views of the piston assembly 101 are depicted in FIGS. 5 to 8, for example. The piston assembly 101 includes a piston 7, for example a servo piston, movable along an axis 8 and mecanically coupled to the swashplate 5, biasing members 9a, 9b for biasing the piston 7 along the axis 8, and an axially displaceable zeroing member 110 configured to limit axial movement of the biasing members 9a, 9b. The piston 7 may move or may be moved in a first direction 8a along the axis 8 and in a second direction 8b along the axis 8, wherein the first axial direction 8a and the second axial direction 8b point in opposing directions along the axis 8. The piston 7 is configured to control a swivel angle of the swashplate 5 for controlling the stroke of the pistons 4. Here, the biasing member 9a, 9b are configured as helical compression springs.

[0040] In the embodiment depicted here, movement of the piston 7 along the axis 8 is controllable via hydraulic forces. When no net hydraulic force acts on the piston 7 along the axis 8, the biasing members 9a, 9b move the piston 7 back to a zero position. Ideally, in the zero position of the piston 7 the swashplate 5 is arranged perpendicular to the rotation axis 2a of the shaft 2 so that a stroke of the pistons 4 vanishes. Or in other words, when the piston 7 is in the zero position, the hydraulic unit 100 should be in a neutral configuration in which it does not displace fluid upon rotation of the shaft 2. However, due to manufacturing tolerances and/or mechanical wear the zero position of the piston 7 may not always exactly correspond to the neutral configuration of the hydraulic unit 100. Therefore, in order to make sure that the hydraulic unit 100 is in the neutral configuration when the piston 7 is in the zero position, the zero position of the piston 7 can be set or adjusted by displacing the zeroing member 110 along the axis 8, as will be explained in further detail below.

[0041] FIG. 5 shows a detail of FIG. 2. Specifically, FIG. 5 shows a sectional view of the piston assembly 101 of the hydraulic unit 100. The piston assembly 101 includes a housing 11. Here, the housing 11 of the piston assembly 101 is formed in one piece with the casing 1 enclosing the shaft 2 and the cylinder block 3. In alternative embodiments not explicitly depicted here, the housing 11 of the piston assembly 101 and the casing 1 may possibly be separate elements which may be connected to one another. The housing 11 forms a cylindrical cavity or cylinder 12 extending along the axis 8 from a first axial end 12a to a second axial end 12b. Here, the cylinder 12 is formed as a cylindrical boring in the housing 11. A cylinder wall 18 formed by the housing 11 does not feature any additional machined portions on or in its inner surface, making the hydraulic unit 100 and/or the piston assembly 101 easy to manufacture.

[0042] Along the axis 8, the first axial direction 8a points from the first axial end 12a of the cylinder 12 to the second axial end 12b of the cylinder 12, and the second axial direction 8b points from the second axial end 12b of the cylinder 12 to the first axial end 12a of the cylinder 12. The piston 7 is disposed within the cylinder 12. A cylinder axis of the cylinder 12 coincides with the axis 8. The piston 7 is movable relative to the housing 11. Along the axis 8 the piston 7 extends from a first axial end 7a to a second axial end 7b. The piston 7 includes a cylindrical or at least partially cylindrical body portion 7c extending along the axis 8 and a protrusion 7d extending from the body portion 7c in a lateral direction perpendicular to the axis 8. Here, at least a section of the protrusion 7d has an annular shape. The protrusion 7d runs around the body portion 7c. In the embodiment depicted here, the body portion 7c and the protrusion 7d are formed in one piece. The piston 7 may be forced or moved in the first axial direction 8a by a hydraulic pressure in a first hydraulic chamber 25a, and the piston 7 may be forced or moved in the second axial direction 8b by a hydraulic pressure in a second hydraulic chamber 25b. The hydraulic chambers 25a, 25b are formed within the cylinder 12.

[0043] The piston assembly 101 further includes a first cap assembly 13a which closes or seals off the cylinder 12 at its first axial end 12a. The first cap assembly 13a includes an insertion portion 14a extending along the axis 8, and a cap portion 15a extending perpendicular to the axis 8. Here, the insertion portion 14a and the cap portion 15a are formed in one piece. The first cap assembly 13a is fixed to the housing 11, for example by a plurality of screws 16a (see FIG. 1). Here, the screws 16a extend parallel to the axis 8 and fix the cap portion 15a to the housing 11. The insertion portion 14a of the first cap assembly 13a is received in the cylinder 12. The first cap assembly 13a, more specifically the insertion portion 14a of the first cap assembly 13a, delimits the first hydraulic chamber 25a in the second axial direction 8b.

[0044] Here, the insertion portion 14a has the shape of a hollow cylinder. An outer radius of the insertion portion 14a is equal to or just slightly smaller than a radius of the cylinder 12. The piston assembly 101 further includes at least one first outer sealing member 17a such as a rubber or metal sealing ring. The first outer sealing member 17a is disposed radially in between the insertion portion 14a and the cylinder wall 18 enclosing the cylinder 12. The first outer sealing member 17a is mounted on the insertion portion 14a. More specifically, the first outer sealing member 17a is received in an outer indentation 19a formed in a radially outer surface of the insertion portion 14a of the first cap assembly 13a. The first outer sealing member 17a prevents fluid such as oil from leaking out of the cylinder 12.

[0045] The piston 7 or, more specifically, the body portion 7c of the piston 7 is partially received in the hollow cylindrical insertion portion 14a of the first cap assembly 13a. An inner radius of the hollow cylindrical insertion portion 14a is equal to or just slightly larger than a radius of the body portion 7c of the piston 7. The piston assembly 101 further includes at least one first inner sealing member 20a such as a rubber or metal sealing ring. The first inner sealing member 20a is disposed radially in between the hollow cylindrical insertion portion 14a and the body portion 7c of the piston 7. The first inner sealing member 20a is mounted on the insertion portion 14a. More specifically, the first inner sealing member 20a is received in an inner indentation 21a formed in a radially inner surface of the hollow cylindrical insertion portion 14a of the first cap assembly 13a. The first inner sealing member 20a prevents fluid such as oil from leaking into a hollow 22a formed axially in between the piston 7 and the cap portion 15a of the first cap assembly 13a.

[0046] The piston assembly 101 further includes a second cap assembly 13b which closes or seals off the cylinder 12 at its second axial end 12b. The second cap assembly 13b includes an insertion portion 14b extending along the axis 8, and a cap portion 15b extending perpendicular to the axis 8. Here, the insertion portion 14b and the cap portion 15b are formed in one piece. The second cap assembly 13b is fixed to the housing 11, for example by a plurality of screws which may extend parallel to the axis 8 and fix the cap portion 15b to the housing 11 (not shown). The insertion portion 14b of the second cap assembly 13b is received in the cylinder 12. The second cap assembly 13b, more specifically the insertion portion 14b of the second cap assembly 13b, delimits the second hydraulic chamber 25b in the first axial direction 8a.

[0047] Here, the insertion portion 14b has the shape of a hollow cylinder. An outer radius of the insertion portion 14b is equal to or just slightly smaller than a radius of the cylinder 12. The piston assembly 101 further includes at least one second outer sealing member 17b such as a rubber or metal sealing ring. The second outer sealing member 17b is disposed radially in between the insertion portion 14b and a cylinder wall 18 enclosing the cylinder 12. The second outer sealing member 17b is mounted on the insertion portion 14b. More specifically, the second outer sealing member 17b is received in an outer indentation 19b formed in a radially outer surface of the insertion portion 14b of the second cap assembly 13b. The second outer sealing member 17b prevents fluid such as oil from leaking out of the cylinder 12.

[0048] The piston 7 or, more specifically, the body portion 7c of the piston 7 is partially received in the hollow cylindrical insertion portion 14b of the second cap assembly 13b. An inner radius of the hollow cylindrical insertion portion 14b is equal to or just slightly larger than the radius of the body portion 7c of the piston 7. The piston assembly 101 further includes at least one second inner sealing member 20b such as a rubber or metal sealing ring. The second inner sealing member 20b is disposed radially in between a radially inner wall of the hollow cylindrical insertion portion 14b and the body portion 7c of the piston 7. The second inner sealing member 20b is mounted on the insertion portion 14b. More specifically, the second inner sealing member 20b is received in an inner indentation 21b formed in a radially inner surface of the hollow cylindrical insertion portion 14b of the second cap assembly 13b. The second inner sealing member 20b prevents fluid such as oil from leaking into a hollow 22b formed axially in between the piston 7 and the cap portion 15b of the second cap assembly 13b.

[0049] The zeroing member 110 is partially received in a recess 23 formed in the housing 11. Portions 11a, 11b of the housing 11 enclosing the recess 23 limit movement of the zeroing member 110 along the axis 8. More specifically, the portion 11a limits movement of the zeroing member 110 in the second axial direction 8b, and the portion 11b limits movement of the zeroing member 110 in the first axial direction 8a. The zeroing member 110 protrudes partially into the cylinder 12. Here, a length or extension of the recess 23 along the axis 8 and a length or extension of the zeroing member 110 along the axis 8 are such that a maximum stroke of the zeroing member 110 along the axis 8 within the recess 23 is at most 20 percent or at most ten percent of the axial length or of the axial extension of the recess 23. As can be seen in FIG. 3, a radially inner surface 110c of the zeroing member 110 has an arcuate shape adapted to the arcuate outer surface of the protrusion 7d of the piston 7.

[0050] In the embodiment depicted in FIGS. 1 to 8, the piston assembly 101 further includes a rotatable gudgeon 111 for adjusting a position of the zeroing member 110 along the axis 8. A rotation axis 111a of the gudgeon 111 is arranged perpendicular to the axis 8. The gudgeon 111 is received or at least partially received in the housing 11. The gudgeon is accessible from outside the housing 11. For example, the gudgeon 111 may include a feature 111b for receiving a tool such as an Allen key, a screw driver, or the like. Here, the rotatable gudgeon 111 includes a male threaded portion 111c which is received in a corresponding female threaded portion 11c formed in the housing 11. At an axial end of the gudgeon 111 facing the zeroing member 110, the gudgeon 111 includes an eccentric pin 112. The eccentric pin 112 extends parallel to and is spaced from the rotation axis 111a of the gudgeon 111 along the axis 8. The eccentric pin 112 is engaged with the zeroing member 110. More specifically, the eccentric pin 112 of the gudgeon 111 is received in a corresponding recess 110a formed in the zeroing member 110. For example, the eccentric pin 112 may be received in the recess 110a in a form-fit. The axial position of the zeroing member 110 may be adjusted by rotating or turning the gudgeon 111 with respect to its rotation axis 111a.

[0051] The first biasing member 9a biases or is configured to bias the piston 7 in the first axial direction 8a along the axis 8. In the embodiment depicted here, the first biasing member 9a is received on and encloses or at least partially encloses the piston 7. More specifically, the first biasing member 9a is received on and encloses or at least partially encloses the body portion 7c of the piston 7. Along the axis 8, the first biasing member 9a is supported on the first cap assembly 13a. Further, along the axis 8 the first biasing member 9a is supported on either the zeroing member 110 or the protrusion 7d of the piston 7 at all times. Or in other words, the first biasing member 9a is preloaded or at least partially compressed in between the first cap assembly 13a and either the zeroing member 110 or the protrusion 7d of the piston 7 at all times. More specifically, along axis 8 the first biasing member 9a is supported on the insertion portion 14a of the first cap assembly 13a. Or in other words, along the axis 8 the first biasing member 9a is supported on the housing 11 via the first cap assembly 13a.

[0052] Along the axis 8, the first biasing member 9a is disposed in between the first cap assembly 13a and the protrusion 7d of the piston 7. More specifically, along the axis 8 the first biasing member 9a is disposed in between the insertion portion 14a of the first cap assembly 13a and the protrusion 7d of the piston 7. Along the axis 8 the first biasing member 9a is disposed in between the first cap assembly 13a and the zeroing member 110. More specifically, along the axis 8 the first biasing member 9a is disposed in between the insertion portion 14a of the first cap assembly 13a and the zeroing member 110.

[0053] The piston assembly 101 further includes a first thrust ring 24a. Along the axis 8 the first thrust ring 24a is disposed in between the first biasing member 9a and the zeroing member 110 and in between the first biasing member 9a and the protrusion 7d of the piston 7. The first thrust ring 24a is received on the piston 7, more specifically on the body portion 7c of the piston 7. In the radial direction perpendicular to the axis 8, the first thrust ring 24a at least partially overlaps with the zeroing member 110. Or in other words, with respect to the axis 8 a maximum radius of the first thrust ring 24a determined perpendicular to the axis 8 is larger than a minimum radius of the zeroing member 110 determined perpendicular to the axis 8. Further, in the radial direction the first thrust ring 24a at least partially overlaps with the protrusion 7d of the piston 7. Or in other words, with respect to the axis 8 a minimum radius of the first thrust ring 24a determined perpendicular to the axis 8 is smaller than a maximum radius of the protrusion 7d determined perpendicular to the axis 8.

[0054] Thus, along the axis 8 the first biasing member 9a is configured to be supported on the protrusion 7d via the first thrust ring 24a and to move the piston 7d in the first axial direction 8a when or as long as the protrusion 7d extends or at least partially extends beyond the zeroing member 110 in the second axial direction 8d until the first thrust ring 24a hits or strikes against the zeroing member 110. A biasing force the first biasing member 9a exerts on the zeroing member 110 and a frictional force between the rotatable gudgeon 111 and the housing 11 are such that the first biasing member 9a may not move the zeroing member 110 along the axis 8 against the frictional force between the rotatable gudgeon 111 and the housing 11. In this way, the zeroing member 110 limits movement of the first biasing member 9a in the first axial direction 8a.

[0055] The second biasing member 9b biases or is configured to bias the piston 7 in the second axial direction 8b along the axis 8. In the embodiment depicted here, the second biasing member 9b is received on and encloses or at least partially encloses the piston 7. More specifically, the second biasing member 9b is received on and encloses or at least partially encloses the body portion 7c of the piston 7. Along the axis 8, the second biasing member 9b is supported on the second cap assembly 13b. Further, along the axis 8 the second biasing member 9b is supported on either the zeroing member 110 or the protrusion 7d of the piston 7 at all times. Or in other words, the second biasing member 9b is preloaded or at least partially compressed in between the second cap assembly 13b and either the zeroing member 110 or the protrusion 7d of the piston 7 at all times. More specifically, along axis 8 the second biasing member 9b is supported on the insertion portion 14b of the second cap assembly 13b. Or in other words, along the axis 8 the second biasing member 9b is supported on the housing 11 via the second cap assembly 13b.

[0056] Along the axis 8 the second biasing member 9b is disposed in between the second cap assembly 13b and the protrusion 7d of the piston 7. More specifically, along the axis 8 the second biasing member 9b is disposed in between the insertion portion 14b of the second cap assembly 13b and the protrusion 7d of the piston 7. Along the axis 8, the second biasing member 9b is disposed in between the second cap assembly 13b and the zeroing member 110. More specifically, along the axis 8 the second biasing member 9b is disposed in between the insertion portion 14b of the second cap assembly 13b and the zeroing member 110.

[0057] The piston assembly 101 further includes a second thrust ring 24b. Along the axis 8, the second thrust ring 24b is disposed in between the second biasing member 9b and the zeroing member 110 and in between the second biasing member 9b and the protrusion 7d of the piston 7. The second thrust ring 24b is received on the piston 7, more specifically on the body portion 7c of the piston 7. In the radial direction perpendicular to the axis 8, the second thrust ring 24b at least partially overlaps with the zeroing member 110. Or in other words, with respect to the axis 8 a maximum radius of the second thrust ring 24b determined perpendicular to the axis 8 is larger than a minimum radius of the zeroing member 110 determined perpendicular to the axis 8. Further, in the radial direction the second thrust ring 24b at least partially overlaps with the protrusion 7d of the piston 7. Or in other words, with respect to the axis 8 a minimum radius of the second thrust ring 24b determined perpendicular to the axis 8 is smaller than a maximum radius of the protrusion 7d determined perpendicular to the axis 8.

[0058] Thus, along the axis 8 the second biasing member 9b is configured to be supported on the protrusion 7d via the second thrust ring 24b and to move the piston 7d in the second axial direction 8b when or as long as the protrusion 7d extends or at least partially extends beyond the zeroing member 110 in the first axial direction 8a until the second thrust ring 24b hits or strikes against the zeroing member 110. A biasing force the second biasing member 9b exerts on the zeroing member 110 and a frictional force between the rotatable gudgeon 111 and the housing 11 are such that the second biasing member 9b may not move the zeroing member 110 along the axis 8 against the frictional force between the rotatable gudgeon 111 and the housing 11. In this way, the zeroing member 110 limits movement of the second biasing member 9b in the second axial direction 8b.

[0059] In FIGS. 5 and 6, the rotatable gudgeon 111 fixes the zeroing member 110 in a first position along the axis 8. For example, in the first position of the zeroing member 110 depicted in FIGS. 5 and 6, a center line 110b of the zeroing member 110 may coincide with the rotation axis 111a of the rotatable gudgeon 111. In FIG. 5, a hydraulic pressure in the first hydraulic chamber 25a is higher than a hydraulic pressure in the second hydraulic chamber 25b and deflects the piston 7 in the first axial direction 8a and away from a first zero position of the piston 7 set by the first position of the zeroing member 110, against a biasing force exerted on the piston 7 by the second biasing member 9b acting in the second axial direction 8b. In the deflected position of FIG. 5, the piston 7 at least partially compresses the second biasing member 9b in between the second thrust ring 24b and the second cap assembly 13b. At the same time, the first biasing member 9a presses the first thrust ring 24a against the zeroing member 110, the zeroing member 110 thereby limiting movement or further movement of the first biasing member 9a in the first axial direction 8a. In the deflected position of the piston 7 illustrated in FIG. 5, the piston 7 may swivel the swashplate 5 to a position such as the one shown in FIG. 4 where the swashplate 5 sets a stroke of the pistons 4 reciprocating within the cylinder block 3 to a non-zero value so that the hydraulic 100 is configured to displace fluid upon rotation of the shaft 2.

[0060] As, starting from the situation illustrated in FIG. 5, the hydraulic pressure in the first hydraulic chamber 25a is reduced to the hydraulic pressure in the second hydraulic chamber 25b so that no net hydraulic force acts on the piston 7 along the axis 8. The second biasing member 9b pushes the piston 7 in the second axial direction 8b until the second thrust ring 24b strikes against the zeroing member 110 so that the zeroing member 110 limits further movement of the second biasing member 9b in the second axial direction 8b.

[0061] The resulting situation is depicted in FIG. 6 where the biasing members 9a, 9b press the thrust rings 24a, 24b against the zeroing member 110 on opposing sides of the zeroing member 110 along the axis 8. Or in other words, in FIG. 6 the thrust rings 24a, 24b abut the zeroing member 110 on opposing sides of the zeroing member 110 along the axis 8. In this situation, the position of the protrusion 7d along the axis 8 is restricted to a space in between the thrust rings 24a, 24b set by the extension of the zeroing member 110 along the axis 8. Or in other words, in the first zero position of the piston 7 depicted in FIG. 6, the protrusion 7d overlaps with the zeroing member 110 along the axis 8. In the embodiment depicted here, a length or an extension of the protrusion 7d of the piston 7 along the axis 8 is equal to the extension of the zeroing member 110 along the axis 8. Consequently, the axial position of the zeroing member 110 set by the rotatable gudgeon 111 precisely sets the zero position of the piston 7 along the axis 8.

[0062] In FIGS. 7 and 8, the rotatable gudgeon 111 fixes the zeroing member 110 in a second axial position which is different from the first axial position of the zeroing member 110 depicted in FIGS. 5 and 6. As explained above, adjusting the axial position of the zeroing member 110 by turning the gudgeon 111 may be required to make sure that the zero position of the piston 7 along the axis 8 set or determined by the axial position of the zeroing member 110 corresponds to the neutral configuration of the hydraulic unit 100 with good accuracy. As a reminder, in the neutral configuration of the hydraulic unit 100 a stroke of the pistons 4 reciprocating in the cylinder block 3 vanishes. For example, in the second axial position of the zeroing member 110 depicted in FIGS. 7 and 8, the center line 110b of the zeroing member 110 is spaced from the rotation axis 111a of the rotatable gudgeon 111 along the axis 8.

[0063] Similar to the situations shown in FIGS. 5 and 6, FIG. 7 shows the piston 7 deflected from a second zero position of the piston 7 along the axis 8 set by the second axial position of the zeroing member 110. By contrast, in FIG. 8 no net hydraulic force acts on the piston 7 along the axis 8 and the biasing members 9a, 9b fix the piston 7 in the second zero position set by the second axial position of the zeroing member 110. Again, in FIG. 8 the position of the protrusion 7d along the axis 8 is restricted to a space in between the thrust rings 24a, 24b set by the extension of the zeroing member 110 along the axis 8. Or in other words, in the second zero position of the piston 7 depicted in FIG. 8, the protrusion 7d overlaps with the zeroing member 110 along the axis 8.

[0064] FIGS. 9 to 12 show a variable displacement hydraulic unit 200 of the presently proposed type according to a second embodiment. The hydraulic unit 200 of FIGS. 9 to 12 is a variation of the hydraulic unit 100 of FIGS. 1 to 8. The hydraulic unit 200 is a variable displacement axial piston unit and may be used as a hydraulic pump or as a hydraulic motor. FIGS. 10 and 11 show different sectional views of the hydraulic unit 200 of FIG. 9. In FIG. 9, a straight dashed line A-A indicates the sectional plane of FIG. 10, and a step-like dashed line F-F indicates the section of FIG. 11. FIG. 12 shows a perspective view of elements of a piston assembly 201 of the hydraulic unit 200. As before, features recurring in different figures are designated with the same or similar reference signs.

[0065] Like the hydraulic unit 100 of FIGS. 1 to 8, the hydraulic unit 200 includes a casing 1, a rotatable shaft 2 such as pump shaft or motor shaft at least partially disposed in the casing 1, and a piston assembly 201. Further, the hydraulic unit 200 of FIGS. 9 to 12 typically includes a cylinder block rotationally coupled to the shaft, pistons configured to reciprocate within cylinders formed in the cylinder block, and a tiltable swashplate configured to control a stroke of the pistons (not shown), as is generally known in the art of hydraulic devices. Again, a rotation axis of the swashplate may be arranged perpendicular to the rotation axis of the rotatable shaft 2.

[0066] For the sake of brevity and simplicity, in the following only those features of the hydraulic unit 200 of FIGS. 9 to 12 which distinguish it from the hydraulic unit 100 of FIGS. 1 to 8 will be described in some detail. Unless explicitly stated otherwise, the hydraulic unit 200 of FIGS. 9 to 12 may include the same features as the hydraulic unit 100 of FIGS. 1 to 8. The differences between the hydraulic units 100 and 200 primarily concern differences between the piston assembly 101 of the hydraulic unit 100 and the piston assembly 201 of the hydraulic unit 200.

[0067] In contrast to the piston assembly 101 of the hydraulic unit 100 of FIGS. 1 to 8, the piston assembly 201 of the hydraulic unit 200 of FIGS. 9 to 12 includes a worm screw or lead screw 211 which is engaged with the zeroing member 210 of the piston assembly 201 so that the zeroing member 210 is displaceable along the axis 8 by rotating the worm screw or leadscrew 211. The worm screw or lead screw 211 includes a male threaded portion 211a which is engaged with a corresponding toothed portion 210a formed in the zeroing member 210. The worm screw or leadscrew 211 is arranged or extends parallel to the axis 8. The worm screw or leadscrew 211 is received in a cylindrical recess 11d formed in the housing 11. Along the axis 8 the worm screw or leadscrew 211 is fixed relative to the housing 11 in such a way that it does not move along the axis 8 when rotating. The worm screw or leadscrew 211 is accessible from outside the housing 11. For example, the worm screw or leadscrew 211 may include a feature 211b for receiving a tool such as an Allen key, a screw driver, or the like. Here, the worm screw or leadscrew 211 extends through the zeroing member 210.

[0068] FIGS. 1-12 are drawn to scale, although other relative dimensions may be used, if desired. FIGS. 1-12ap show example configurations with relative positioning of the various components. Unless otherwise noted, if shown directly contacting each other, or directly coupled, then such elements may be referred to as directly contacting or directly coupled, respectively, at least in one example. Similarly, elements shown contiguous or adjacent to one another may be contiguous or adjacent to each other, respectively, at least in one example. As an example, components laying in face-sharing contact with each other may be referred to as in face-sharing contact. As another example, elements positioned apart from each other with only a space there-between and no other components may be referred to as such, in at least one example. As yet another example, elements shown above/below one another, at opposite sides to one another, or to the left/right of one another may be referred to as such, relative to one another. Further, as shown in the figures, a topmost element or point of element may be referred to as a top of the component and a bottommost element or point of the element may be referred to as a bottom of the component, in at least one example. As used herein, top/bottom, upper/lower, above/below, may be relative to a vertical axis of the figures and used to describe positioning of elements of the figures relative to one another. As such, elements shown above other elements are positioned vertically above the other elements, in one example. As yet another example, shapes of the elements depicted within the figures may be referred to as having those shapes (e.g., such as being circular, straight, planar, curved, rounded, chamfered, angled, or the like). Further, elements shown intersecting one another may be referred to as intersecting elements or intersecting one another, in at least one example. Further still, an element shown within another element or shown outside of another element may be referred as such, in one example.

[0069] The following claims particularly point out certain combinations and sub-combinations regarded as novel and non-obvious. These claims may refer to an element or a first element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and sub-combinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure