HYDRAULIC AXIAL PISTON UNIT AND METHOD FOR CONTROLLING OF A HYDRAULIC AXIAL PISTON UNIT

Abstract

Hydraulic axial piston unit includes a rotatable cylinder block and a valve segment with two pressure ports. An IDC control port and an ODC control port are located on the valve segment in circumferential direction between the circumferential ends of the pressure ports such that a cylinder bore can be fluidly connected to the IDC control port or the ODC control port when the associated working piston is at or close to its inner dead center or outer dead center. The circumferential distance from the control ports to the pressure ports is smaller than the circumferential extension of the cylinder bores. A first and a second bypass line each connecting one of the control ports are provided with an adjustable orifice in the first bypass line, capable of continuously variably opening and closing the first bypass line in order to enable an adjustable fluid flow connection between the connected pressure port and the connected pressure port.

Claims

1. A hydraulic axial piston unit with a rotating group whose displacement volume is set by means of a displacement element, the rotating group comprising a rotatable cylinder block with cylinder bores in which working pistons are mounted reciprocally moveable, and with a valve segment comprising a kidney-shaped first pressure port and a kidney-shaped second pressure port, wherein a cylinder bore can be fluidly connected to an IDC control port or an ODC control port when the associated working piston is at or close to its inner dead center (IDC) or at or close to its outer dead center (ODC), respectively, wherein the IDC control port and the ODC control port are located in circumferential direction between the respective circumferential ends of the first pressure port and the second pressure port, wherein the circumferential distance from the IDC control port to the first and second pressure ports and the circumferential distance from the ODC control port to the first and second pressure ports is smaller than the circumferential extension of the cylinder bores, and wherein a first bypass line and a second bypass line are provided, each connecting one of the control ports, with an adjustable orifice arranged in the first bypass line, capable of continuously variably opening and closing the first bypass line in order to enable an variably adjustable fluid flow connection between the connected control port and a connected pressure port, wherein, the first bypass line and the second bypass line each connects the next pressure port after the connected control port in rotational direction of the cylinder block.

2. (canceled)

3. (canceled)

4. (canceled)

5. The hydraulic axial piston unit according to claim 1, wherein the second bypass line is connected to a pressure compensation chamber.

6. (canceled)

7. The hydraulic axial piston unit according to claim 1, wherein the openings of the cylinder bores facing the valve segment show a kidney-shaped cross section, wherein the circumferential extensions of the kidney-shaped openings of the cylinder bores are smaller than the circumferential distance between the adjacent ends of the first and second kidney-shaped pressure ports.

8. (canceled)

9. The hydraulic axial piston unit according to claim 1, wherein an adjustable orifice is arranged in each of the bypass lines.

10. (canceled)

11. (canceled)

12. The hydraulic axial piston unit according to claim 1, wherein a parallel bypass line comprising an adjustable orifice or a non-adjustable orifice establishes a fluid flow connection parallel to the fluid flow connection between the pressure port and the control port connected by the first bypass line or between the pressure port and the control port connected by the second bypass line.

13. (canceled)

14. The hydraulic axial piston unit according to claim 1, wherein each control port is additionally connected to the other pressure port via a third and a fourth bypass line, wherein an adjustable orifice is arranged in each of the four bypass lines.

15. (canceled)

16. (canceled)

17. The hydraulic axial piston unit according to claim 1, wherein, in case the rotating group is at maximum displacement in its initial position, a safety pressure limiter switching valve is arranged in at least one of the bypass lines in order to close the associated bypass line when a system pressure level exceeds a threshold value.

18. (canceled)

19. The hydraulic axial piston unit according to claim 1, wherein the opening size of the orifices is controlled mechanically or by an electronic control unit (ECU) comprising a micro-controller, and being connected to at least one sensor selected from a group of sensors comprising a tilt angle sensor, a shaft position sensor, a pressure sensor, a flow sensor, a rotational speed sensor, a temperature sensor, a direction sensor, a torque sensor, an acceleration sensor or any other sensor capable of monitoring at least one operational parameter of the hydraulic unit.

20. The hydraulic axial piston unit according to claim 1, comprising at least two adjustable orifices one provided in the first bypass line and one provided in the second bypass line, wherein the opening sizes of the two adjustable orifices are adjustable separately or by means of a shared mechanical, electromechanical, hydraulic or pneumatic mechanism.

21. The hydraulic axial piston unit according to claim 1, wherein the IDC and/or the ODC control port comprise a circular shape, an elongated shape, an ellipse shape, a triangle shape, a kidney-shape or any other shape.

22. The hydraulic axial piston unit according to claim 1, wherein the IDC control port and/or the ODC control port are located on the valve segment in circumferential direction with an angular offset to the rotational position on the valve segment at which the working pistons are at its inner dead center (IDC) and/or outer dead center (ODC), respectively.

23. (canceled)

24. The hydraulic axial piston unit according to claim 22, wherein a second ODC control port is located on the valve segment such that the first and second ODC control ports being located in circumferential direction on both sides of the rotational position on the valve segment which corresponds to the outer dead center (ODC) position of the working pistons.

25. The hydraulic axial piston unit according to claim 24, wherein a second IDC control port is located on the valve segment such that the first and second IDC control ports being located in circumferential direction on both sides of the rotational position on the valve segment which corresponds to the inner dead center (IDC) position of the working pistons.

26. The hydraulic axial piston unit according to claim 24, wherein the second IDC control port and/or the second ODC control port are respectively connected to a third bypass line and/or a fourth bypass line, wherein at least one of the third and fourth bypass lines comprises an adjustable orifice capable of continuously and variably opening and closing the associated bypass line.

27. (canceled)

28. The hydraulic axial piston unit according to claim 1, operated in a closed hydraulic circuit and comprising a shuttle valve having two inlets and one outlet, which inlets are in fluid connection with the first and second pressure ports and which outlet is in fluid connection with the IDC control port or the ODC control port, such that the shuttle valve is capable of conducting the higher system pressure from one of the first and second pressure ports to the IDC control port or to the ODC control port and/or to a control valve.

29. The hydraulic axial piston unit according to claim 28, wherein a control valve is provided with a first inlet connected to the outlet of the shuttle valve, and a second inlet connected to lower system pressure or to a hydraulic reservoir, and with a first outlet connectable to the IDC control port or the ODC control port, and a second outlet connectable to the other control port, wherein the control valve is capable of selectively connecting the first inlet with the first outlet and the second inlet with the second outlet, or of connecting the first inlet with the second outlet and the second inlet with the first outlet, or of short-circuiting the first outlet with the second outlet.

30. The hydraulic axial piston unit according to claim 28, comprising a charge pump capable of providing a hydraulic fluid flow to one of the first or second pressure port to generate an initial pressure difference between the first and second pressure ports and/or to switch the shuttle valve when the hydraulic axial piston unit is in its neutral position.

31. (canceled)

32. The hydraulic axial piston unit according to claim 1 any of the preceding claims, wherein the at least one adjustable orifice is controlled by the electronic control unit (ECU) based on a pressure and/or displacement feedback of at least one adjustable orifice.

33. The hydraulic axial piston unit according to claim 1, wherein the control ports are inclined with respect to a rotational axis of the hydraulic axial piston unit.

34. (canceled)

35. A method for variably controlling the displacement volume of a hydraulic rotating group driving or being driven by a driving shaft, having a displacement element tiltable for adjusting the displacement volume of the rotating group, wherein the rotating group comprises a rotatable cylinder block in which working pistons are mounted reciprocally moveable in cylinder bores, and a valve segment with a kidney-shaped first pressure port and with a kidney-shaped second pressure port, wherein an IDC control port and an ODC control port are located on the valve segment in circumferential direction between the respective circumferential ends of the first pressure port and the second pressure port, wherein a cylinder bore can be fluidly connected to the IDC control port or the ODC control port when the associated working piston is at or close to its inner dead center (IDC), or is at or close to its outer dead center (ODC), respectively, wherein the circumferential distance from the IDC control port to the first and second pressure ports and the circumferential distance from the ODC control port to the first and second pressure ports is smaller than the circumferential extension of the cylinder bores, wherein the method comprises the following steps: draining or supplying of hydraulic fluid from or to the passing cylinder bores via the IDC control port by means of a first bypass line having a first orifice, supplying or draining of hydraulic fluid to or from the passing cylinder bores via the ODC control port by means of a second bypass line having a second orifice, adjusting an opening size of the first orifice, or an opening size of the second orifice, or adjusting both opening sizes of the first orifice and the second orifice in order to set or adjust the angle of tilt of the displacement element and to control the displacement volume of the hydraulic rotating group, wherein the hydraulic fluid from the ODC and IDC control ports is supplied or drained with the pressure level of the next pressure port in rotational direction of the cylinder block.

36. (canceled)

37. The method according to claim 35, further comprising the step of: processing a command of a control unit or an operator by means of an electronic control unit (ECU) having a microcontroller for adjusting the opening sizes of the orifices in the first bypass line and/or in the second bypass line, in order to control the pressure in the cylinder bores for controlling the displacement volume of the hydraulic axial piston unit.

38. (canceled)

39. The method according to claim 35, further comprising the step of: continuously monitoring the operational parameters of the hydraulic axial piston unit in order to smoothen pressure transition between the first and second pressure ports and vice versa, and/or for controlling the pressure in the cylinder bores, and/or for adjusting the tilt angle of the displacement element.

40. The method according to claim 35, further comprising in case the rotating group is used in a closed circuit hydraulic application having a charge pump, the steps of: supplying charge pressure to one of the first or second pressure ports via a charge pressure valve, when the rotating group is in its neutral position guiding of hydraulic fluid by means of one of the first or the second bypass line from the pressure port with the higher pressure to the associated control port; draining of hydraulic fluid by means of the other bypass line from the other control port to a hydraulic fluid reservoir.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0067] FIG. 1 a design of a first embodiment of a hydraulic axial piston unit according to the invention;

[0068] FIG. 2 schematically the first embodiment of a hydraulic axial piston unit according to the invention;

[0069] FIG. 3 schematically a valve segment of the first embodiment of a hydraulic axial piston unit according to the invention;

[0070] FIG. 4 schematically a valve segment of a second embodiment of a hydraulic axial piston unit according to the invention;

[0071] FIG. 5 schematically a valve segment of a third embodiment of a hydraulic axial piston unit according to the invention;

[0072] FIG. 6 schematically a valve segment of a fourth embodiment of a hydraulic axial piston unit according to the invention;

[0073] FIG. 7 schematically a valve segment of a fifth embodiment of a hydraulic axial piston unit according to the invention;

[0074] FIG. 8 schematically a valve segment of a sixth embodiment of a hydraulic axial piston unit according to the invention;

[0075] FIG. 9 schematically a valve segment of a seventh embodiment of a hydraulic axial piston unit according to the invention;

[0076] FIG. 10a schematically a hydraulic circuit of an eighth embodiment of a hydraulic axial piston unit according to the invention;

[0077] FIG. 10b schematically a hydraulic circuit of a ninth embodiment of a hydraulic axial piston unit according to the invention;

[0078] FIG. 11 schematically a valve segment of the eighth embodiment of a hydraulic axial piston unit according to the invention;

[0079] FIG. 12 schematically a valve segment of a tenth embodiment of a hydraulic axial piston unit according to the invention;

[0080] FIG. 13 a first embodiment of an adjustable orifice according to the invention in an open position;

[0081] FIG. 14 the first embodiment of an adjustable orifice according to the invention in a closed position;

[0082] FIG. 15 a second embodiment of an adjustable orifice according to the invention in an open position;

[0083] FIG. 16 the second embodiment of an adjustable orifice according to the invention in a closed position;

DETAILED DESCRIPTION

[0084] In the Figures same reference numerals are used for same components of different embodiments throughout the description to improve readability.

[0085] FIG. 1 shows an exemplary design of a first embodiment of a hydraulic axial piston unit according to the invention. The hydraulic pump comprises a displacement element 4 which can be tilted with respect to a tilt axis 9 in order to adjust the displacement volume of a rotating group 2 of the hydraulic unit. The rotating group 2 comprises a cylinder block 3 rotatable around a rotational axis 13, having cylinder bores 5 in which working pistons 6 are mounted reciprocally moveable between an outer dead centre (ODC) and an inner dead centre (IDC). The working pistons 6 abut against the displacement element 4 via gliding shoes. A valve segment 20 is located on the other side of the cylinder block 3 comprising a first pressure port 21 and a second pressure port 22 serving as interfaces for connecting the hydraulic unit to an open or closed hydraulic circuit. The valve segment 20 further comprises an IDC control port 23 arranged near the rotational position of the IDC of the working pistons 6 of the hydraulic unit and an ODC control port 24 arranged near the rotational position of the ODC of the working pistons 6 of the hydraulic unit.

[0086] FIG. 2 depicts a hydraulic scheme of the first embodiment of a hydraulic axial piston unit, here exemplarily a hydraulic axial piston pump. The ODC control port 24 is fluidly connected to the second pressure port 22 via a first bypass line 27. The IDC control port 23 is fluidly connected to a hydraulic reservoir 100 via a second bypass line 28.

[0087] In the embodiment according to FIG. 2, the hydraulic unit is operated in an open hydraulic circuit, for example. The first pressure port 21 is connected to a low system pressure, e.g. to the hydraulic reservoir 100. The second pressure port 22 is connected to high pressure line. Thus, at the ODC control port 24 high system pressure is present, whereas at the IDC control port 23 low system pressure is present. An orifice 29 with a variably adjustable opening size is arranged in the first bypass line 27 for adjusting the fluid flow in the bypass line 27 and therewith the pressure level at the ODC control port 24. As the first bypass line 27 is connected to the second pressure port 22, i.e. to the outlet of the hydraulic pump and thus to the high pressure side, opening of the adjustable orifice 29 increases the flow and pressure at the ODC control port 24. The second bypass line 28 comprises an orifice 31, whose opening size in this embodiment is not adjustable. Thus, in the embodiment shown with FIG. 2, the flow resistance in the second bypass line 28 is not adjustable.

[0088] The hydraulic unit further comprises a return mechanism 10 that forces the displacement element 4 of the hydraulic unit back into its initial position, when the displacement element 4 is titled out of this initial position. The initial position of the displacement element 4 can be at a tilt angle of zero degrees, e.g. However, especially preferred for hydraulic units operated as a hydraulic pump or motor in an open hydraulic circuit, the displacement element 4 can be initially tilted towards a non-zero tilt angle. For this purpose the rotational axis 12 of the displacement element 4, respectively the rotational axis 12 of the sliding surface for the guiding shoes on the displacement element 4, can comprise in direction of the tilt axis 9 of the displacement element 4 an offset with respect to the rotational axis 13 of a driving shaft or the cylinder block 3 (c.f. FIG. 1). Alternatively the displacement element 4 of the hydraulic unit can be biased to a non-zero displacement angle by means of an elastic force, e.g. provided by a spring.

[0089] FIG. 3 schematically shows the valve segment 20 of the first embodiment of a hydraulic axial piston unit according to the invention. The valve segment 20 comprises a first pressure port 21 and a second pressure port 22. Both pressure ports show a kidney-shape. As mentioned above, the first pressure port 21,-the suction port of the hydraulic unit in the example of the first embodiment according to FIGS. 2 and 3is connected to a hydraulic reservoir 100. A dash-dot-dot line represents a dead centre plane 7 in which the rotational position of the outer dead centre (ODC) and the inner dead centre (IDC) are located. The dead centre plane 7 represents a plane which is orthogonal to the tilt axis 9 of the displacement element 4 and which contains the rotational axis 13 of the cylinder block 3 in case of a hydraulic axial piston unit of the swashplate type of construction, or in case of a hydraulic axial piston unit of the bent axis type of construction, which contains the rotational axis of a driving shaft.

[0090] The working pistons 6 (c.f. FIG. 1) of the hydraulic unit abut with one side against the displacement element 4 via gliding shoes. On the other side, the working pistons 6 seal with a pressure chamber which is formed by the cylinder bores 5 in combination with the valve segment 20. When the cylinder block 3 rotates and the displacement element 4 comprises a non-zero angle of tilt, the working pistons 6 move reciprocally in the cylinder bores 5 and the volume of the pressure chambers in the cylinder bores 5 increases, when a piston 6 is moving away from the valve segment 20. The volume of a pressure chamber decreases when a piston 6 is moving towards the valve segment 20. At the outer dead centre (ODC), the volume of a pressure chamber is maximum, as the distance between the piston 6 and the valve segment 20 is maximum. At the inner dead centre (IDC) the distance between piston 6 and valve segment 20 and therewith the volume of the pressure chamber is minimum.

[0091] In the first embodiment, in case an (open circuit) hydraulic pump is considered, at the position of the ODC a working piston 6 transitions from the suction phase, in which the pressure chamber extends, and hydraulic fluid enters the pressure chamber, to a pressure phase, in which hydraulic fluid is pressed out of the pressure chamber. At the IDC the phases are inverted, i.e. a working piston 6 transitions from a pressure phase to a suction phase.

[0092] According to the invention, an ODC control port 24 is provided at or near the rotational position of the ODC. Similarly, an IDC control port 23 is provided at or near the rotational position of the IDC. In the first embodiment of the invention, both control ports 23 and 24 are arranged in positions, where an offset-angle .sub.o/.sub.i is provided between the rotational position of the working pistons 6 at ODC and IDC (dead centre plane 17) and the rotational position of the ODC control port 24 and the IDC control port 23, respectively. The position of the ODC and IDC control ports 23, 24 is essential for the functionality of the invention, especially the offset-angle .sub.o/.sub.i. Depending on the algebraic sign and the magnitude of the angles .sub.o/.sub.i, the point in time, at which overlap of the control ports 23, 24 with the passing cylinder bores 5 starts and ends, can be influenced. Modifying the position of the ODC and the IDC control ports 23, 24 influences the timing and time span, when the pressure in a cylinder bore 5 passing/overlapping one of the control ports 23, 24 can be changed/adjusted in a controlled manner. According to the invention, e.g. for a hydraulic pump, the IDC and ODC control ports 23, 24 can preferably be arrangedseen in rotational direction of the pumpbehind the respective IDC or ODC rotational positions. According to FIG. 3 the IDC control port 23 is connected via the second bypass line 28 with a non-adjustable orifice 31 to the hydraulic reservoir 100 and the first pressure port 21, here the low system pressure side (inlet or suction side) of the hydraulic pump. The ODC control port 24 is connected via the first bypass line 27 comprising an adjustable orifice 29 to the second pressure port 22, here the high system pressure side (outlet or pressure side) of the hydraulic pump.

[0093] The openings of the cylinder bores 5 facing towards the valve segment 20illustrated with dashed lines in the Figurescomprise a kidney shape, e.g., with a circumferential extension which is, in most applications smaller than the circumferential distance between the first pressure port 21 and the second pressure port 22. The circumferential distance between the first pressure port 21 and the second pressure port 22 is the sum of the circumferential distance between the first pressure port 21 and the position of the ODC/IDC .sub.o/.sub.i and the circumferential distance between the second pressure port 22 and the position of the ODC/IDC .sub.o/.sub.i. If the extension would be larger than the sum of .sub.o+.sub.o or the sum of .sub.i+.sub.i, the first pressure port 21 could be hydraulically short-circuited to the second pressure port 22 via the cylinder bore 5.

[0094] The tilt angle of the displacement element 4 can be adjusted by controlling the magnitude of the opening of the adjustable orifice 29. When the opening of the orifice 29 is increased, high pressure is conducted to the ODC control port 24. Therefore the pressure in the cylinder bore 5 passing the ODC control port 24 can be increased. Increased cylinder bore pressure leads to a higher force on the working piston 6 arranged in the passing cylinder bore 5. As this force is supported/abutted by the displacement element 4, respectively acts on the displacement element 4 via the gliding shoes, the tilt angle of the displacement element 4 can be increased by increasing the pressure in the cylinder bores 5 passing the ODC control port. If the opening size of the variable orifice 29 in the first bypass line 27 is the decreased, the pressure on the working pistons 6 decreases and the force with which the working piston 6 acts on the displacement element 4 decreases also. As a result, the return mechanism 10 (see FIG. 2) can exert a returning/neutralizing force which is higher than the on-stroking force on the displacement element 4 and tilts/de-strokes the displacement element 4 back towards its initial position until an equilibrium of the returning forces and the pressure forces acting on the displacement element 4 exerted by the working pistons 6 is established again. To summarize, adjusting the magnitude of the opening of the variable/adjustable orifice 29 influences the equilibrium of forces/moments with respect to the tilt axis on the displacement element 4, which is established between an on-stroking pressure force on the displacement element 4 and a neutralizing force of the return mechanism 10. The moment generated by the pressure force is maximum at full opening of the adjustable orifice 29.

[0095] A person skilled in the relevant art will appreciate that the inventive concept can be applied in order to set the displacement volume of fixed displacement units as well as in order to set and adjust the displacement volume of variable displacement hydraulic units. Moreover the inventive concept can be used to improve and/or smoothening the running behaviour of a hydraulic unit as pressure transition steps can be lowered making the provision of fishtails unnecessary. Thereby the inventive concept can be applied to hydraulic units equipped with a servo unit or to hydraulic units without a servo unit to set/adjust the displacement volume.

[0096] FIG. 4 schematically shows a valve segment 20 of a second embodiment of a hydraulic axial piston unit according to the invention. The arrangement according to the second embodiment of the invention is similar to the arrangement shown with FIGS. 1 and 2. The second embodiment comprises an adjustable orifice 29 in the first bypass line 27, in order to control the pressure at the IDC control port 23. In the second bypass line 28 a non-adjustable orifice 31 is provided. In consequence, the tilt angle of the displacement element 4 in this exemplary embodiment is controlled by means of adjusting the opening/flow resistance in the first bypass line 27 and therewith the pressure at the IDC control port 23. At the rotational area of the IDC control port 23 the pressure in a cylinder bore 5 passing the IDC control port 23 generates a moment with respect to the tilt axis of the displacement element 4, which is capable of decreasing the tilt angle. At the IDC the working piston 6 is at its most introduced point in the passing cylinder bore 5, therefore, increasing the pressure in a cylinder bore 5 passing the IDC will decrease the tilt angle of displacement element 4. On the contrary, decreasing the pressure in a cylinder bore 5 which passes the IDC, can lead to an increased angle of tilt of the displacement element 4, as the reaction force on the displacement element 4 is reduced.

[0097] As mentioned earlier, the first pressure port 21 and therewith the first bypass line 27 are connected to the low pressure side of the hydraulic unit, here to a hydraulic reservoir 100. Therefore, opening of the adjustable orifice 29 provides a reduced (back-) pressure at the IDC control port 23, as hydraulic fluid can be pushed out of the cylinder bores 5 with less resistance, and the pressure in the passing cylinder bore 5 is reduced. In consequence, the tilt angle of the displacement element 4 is increased. Closing the adjustable orifice 24 increases the resistance with hydraulic fluid can be discharged and a higher backpressure is build-up, therewith increasing the pressure in the passing cylinder bore 5 by restricting the pressure relief. Simultaneously, the pressure profile at the ODC pressure port 24 is not actively adjusted due to the non-adjustable orifice 31 in the second bypass line 28.

[0098] FIG. 5 schematically shows a valve segment of a third embodiment of a hydraulic axial piston unit according to the invention. The third embodiment can be seen as a combination of the first and second embodiments. Hence, an adjustable orifice 29 is provided in the second bypass line 29 and an adjustable orifice 30 is provided in the first bypass line 27. The working principle of the hydraulic axial piston unit according to the third embodiment is similar to the above explained. Increasing the pressure at the ODC control port 24 leads to an increased tilt angle of the displacement element 4 due to the increased force on the displacement element 4 acting in the direction of tilt. Increasing the pressure at the IDC control port 24 leads to a decreasing tilt angle of the displacement element 4, as the corresponding pressure force acts in a direction which decreases the angle of tilt. Therefore, by providing adjustable orifices 29, 30 in each of the two bypass lines 27, 28, the tilt angle of the displacement element 4 can be adjusted with a high degree of precision. Additionally, vibrations and noises can be reduced by adjusting the opening magnitudes of the orifices 29, 30 in relation to each other in order to smoothen the pressure profile and to reduce or even avoid pressure peaks or cavitation nearby or at the dead centre points IDC and ODC.

[0099] FIG. 6 schematically shows a valve segment 20 of a fourth embodiment of a hydraulic axial piston unit according to the invention. The fourth embodiment is a further development of the above mentioned embodiments. In the area of the ODC, the valve segment 20 of the hydraulic unit comprises a second ODC control port 26. Compared to the first ODC control port 24, the second ODC control port 26 is arranged on the opposite side of the ODC/IDC connection line, respectively of the dead centre plane 7,-seen in circumferential direction. This, e.g., means that the first ODC control port 24 is arranged in clockwise direction behind the ODC, whereas the second ODC control port 26 is arranged in clockwise direction before the ODC. The second ODC control port 26 is connected to the second pressure port 22 via an additional bypass line 33, which can comprise, e.g. an adjustable orifice 30 as well as a non-adjustable orifice. This arrangement provides an enhanced possibility to precisely adjust the pressure profile which is provided to a cylinder bore 5 via the first and second ODC control ports 24, 26, when the cylinder bore 5 is travelling on its circular path due to the rotation of the cylinder block 3. Therewith it is further possible to reducing noises and vibrations when operating the hydraulic unit and provides for a shorter reaction time to control signals due to the higher flow rate which can pass through the two ODC control ports 24, 26.

[0100] FIG. 7 schematically shows a valve segment 20 of a fifth embodiment of a hydraulic axial piston unit according to the invention. The fifth embodiment of the hydraulic unit can for example represent a hydraulic motor which can be arranged in a closed circuit. In contrast to the embodiments described above, which all comprise an offset angle between the rotational position of the ODC/IDC control ports 23, 24 and the actual rotational position of the IDC and ODC, the ODC control port 24 and the IDC control port 23 of the hydraulic unit according to the fifth embodiment are arranged at the exact respective rotational positions of the IDC and the ODC, i.e. on the dead centre plane 7. This means, that the offset angles .sub.O/.sub.I are equal to zero. In consequence, the control behaviour of the hydraulic unit when adjusting the tilt angle of the displacement element 4 is independent of the direction of rotation of the cylinder block 3.

[0101] In the fifth embodiment, an adjustable orifice 29 is provided in the first bypass line 27. The orifice 31 arranged in the second bypass line 28 comprises a non-adjustable, constant opening size. Typically, hydraulic motors used in closed circuit applications are capable of rotating in two directions. Even though the displacement element 4 of such a hydraulic motor is tiltable only in one direction, and the pressure levels which are present at the first pressure port 21 and at the second pressure port 22 can be interchanged, in order to invert the direction of rotation of a rotating group 2 of the hydraulic unit. In the embodiment shown in FIG. 7, the ODC control port 24 is connected to a hydraulic reservoir 100 via the second bypass line 28. Consequently, and regardless of the direction of rotation, lower system pressure is present at the ODC control port 24. According to the invention, high pressure can be provided to the IDC control port 23. For this purpose, a shuttle valve 35 is provided whose outlet 38 is connected to the first bypass line 27. A first inlet 36 of the shuttle valve 35 is connected to the second pressure port 22. A second inlet 37 of the shuttle valve 35 is connected to the first pressure port 21. The shuttle valve 35 is capable of always conducting the higher pressure level of the first pressure port 21 or of the second pressure port 22 to the first bypass line 27 via its outlet 38. Therefore, the adjustments in the opening size of the adjustable orifice 29 are always related to the high pressure level regardless of the direction of rotation of the hydraulic unit. The control of the tilt angle of the displacement element 4 of the hydraulic unit works similar to the control of the hydraulic unit according to the embodiments 1 to 4. For the sake of shortness of the present explanations, a detailed repetition is omitted.

[0102] FIG. 8 schematically shows a valve segment 20 of a sixth embodiment of a hydraulic axial piston unit according to the invention. The sixth embodiment is similar to the fifth embodiment. However, the adjustable orifice in the bypass line 27 connecting the IDC control port 23 with the outlet 38 of the shuttle valve 35 is replaced by a non-adjustable orifice 31. Instead, an adjustable orifice 29 is provided in the first bypass line 28 which connects the ODC control port 24 to the hydraulic reservoir 100. The working principle of the displacement control by means of an adjustable orifice 29 in the first bypass line 28 connected to the ODC control port 24 was already descripted analogously above with respect to the pump of the second embodiment in FIG. 4, where the IDC control port 23 is connected to the low system pressure side. Therefore, for the sake of shortness of the present explanations, it is refrained from a detailed repetition.

[0103] FIG. 9 schematically shows a valve segment 20 of a seventh embodiment of a hydraulic axial piston unit according to the invention. The valve segment 20 according to the seventh embodiment comprises a first ODC control port 24 and a second ODC control port 26 both connected to the second pressure port 22 via bypass lines 27 and 33, wherein each bypass line 27 and 33 comprises an adjustable orifice 29 and 30. Further, the hydraulic unit comprises a first IDC control port 23 and a second IDC control port 25 both connected to the first pressure port 21 via bypass lines 28 and 32 which both as well comprise adjustable orifices 34 and 39. The first and second ODC control ports 24 and 26 and the first and second IDC control ports 23 and 25 are arranged in rotational direction on both sides of the rotational position of the dead centre plane 7 containing the rotational position of the ODC and the IDC. If the hydraulic unit is capable to rotate bidirectional, as indicated in FIG. 9 with the two-sided arrow 80, the circumferential distance from the first IDC/ODC control ports 23 and 24 to the IDC/ODC positions on the valve segment 20 can be equal to the circumferential distance of the second IDC/ODC control ports 25 and 26 to the rotational IDC/ODC positions on the valve segment 20. Therewith, the control behaviour of the hydraulic unit is symmetrical and regardless of the direction of rotation. During operation of the hydraulic unit, the pressure levels which are present at the first pressure port 21 and the second pressure port 22 can be interchanged, e.g. due to a change of operation mode, e.g. from motor mode to pump mode or because a change of the direction of rotation of the hydraulic unit is desired. Therefore, a person with skills in the relevant art may arrange a shuttle valve 35 in the bypass lines 27 and 33 conducting high pressure to the ODC pressure ports 24, 26 if the hydraulic unit is operated as hydraulic pump, or to the IDC pressure ports 23, 25 if the hydraulic unit is operated as hydraulic motor.

[0104] FIG. 10a, FIG. 10b and FIG. 11 schematically show an eighth embodiment of a hydraulic axial piston unit according to the invention, which can for example serve as pump in a closed hydraulic circuit. Similar to the above described examples, the hydraulic unit according to the eighth embodiment comprises a first pressure port 21 and a second pressure port 22. Preferably, the hydraulic pump comprises only one direction of rotation, as indicated by the arrow on the driving shaft 8 in FIG. 10 and the arrows 80 on the valve segments 20 shown in FIGS. 10b and 11. In order to be able to supply hydraulic fluid in both directions, the displacement element 4 of the hydraulic unit is tiltable to positive and to negative angles. The displacement element 4 is forced into its neutral position, which normally is also the initial position of the displacement element 4, by a return mechanism 10.

[0105] FIG. 10b additionally shows a valve arrangement comprising charge pressure valves 51 and 52 as common check valves each combined with a proportional flow valve and drained to a hydraulic reservoir 100. With this valve arrangement a start mechanism of hydraulic pump having a neutral return mechanism 10 can be achieved. When one of the flow valves, e.g. the one next to charge pressure valve 52 is opened, charge pressure is guided to the respective other charge pressure valve, in this example to charge pressure valve 51, whose flow valve is closed. Therewith an initial pressure difference at the valve plate 20 is created when the charge pump starts working. At same time by opening both adjustable orifices 29 and 30 a system pressure difference between the two control ports 23, 24 is established, enabling an initial tilt of the displacement element 4. Due to this initial tilt of the displacement element 4 system pressure is generated by the hydraulic pump which increase the pressure delta between the two control ports 23 and 24 as long as the higher system pressure is higher than the charge pressure, and the adjustable orifice 29 is kept at least partially open. With varying the flow passages through the adjustable orifices 29 and 30 the displacement volume can be set/adjusted as described above.

[0106] Due to the potential bi-directional inclination of the displacement element 4, the rotational positions of the IDC and of the ODC are not fixed but are interchanged when the algebraic sign of the tilt angle of the displacement element 4 changes. Therefore, the allocation of the control ports 23 and 24 to the ODC and IDC is not constant throughout the operation of the hydraulic unit, but changes with over-zero displacement of the displacement element 4. In a specific operational state, the rotational position of the ODC can be located on the left side of valve segment 20, e.g. as shown with FIG. 11. Accordingly, the rotational position of the IDC can be located on the right side of FIG. 11 and the corresponding control ports are labelled ODC control port 24 and IDC control port 23. The control ports 23, 24 are connected to the pressure ports 21, 22 via first and second bypass lines 27 and 28, each comprising an adjustable orifice 29 and 30.

[0107] The inlets 36 and 37 of a shuttle valve 35 whose working principle has already been explained above, are in fluid connection with the first pressure port 21 and the second pressure port 22. The outlet 38 of the shuttle valve 35 is fluidly connected to the first inlet 41 of a control valve 40 which further comprises a second inlet 42 connected to a hydraulic reservoir 100, or another source of low system pressure. Therefore the first inlet 41 of the control valve 40 is always connected to high system pressure which is provided via the shuttle valve 35. The second inlet 42 of the control valve 40 is always connected to low system pressure. The control valve 40 further comprises a first outlet 43 connected to the first bypass line 27, and a second outlet 44 connected to the second bypass line 28. The position of the control valve 40 is selected depending on the algebraic sign of the tilt angle of the displacement element 4 and depending on the use of the hydraulic unit as a hydraulic pump or as a hydraulic motor. The control valve 40 can connect the first inlet 41 with the first outlet 43 and the second inlet 42 with the second outlet 44. In consequence, high pressure is conducted to the first bypass line 27 and low pressure is conducted to the second bypass line 28. Alternatively, the control valve 40 can connect the first inlet 41 with the second outlet 44 conducting high pressure to the second bypass line 28 and can connect the second inlet 42 to the first outlet 43 conducting low pressure to the first bypass line 27. The control valve 40 can further comprise a third position, in which the bypass lines 27 and 28 are hydraulically short-circuited and the connection between the inlets 41 and 42 and the outlets 43 and 44 are blocked. Depending on the type of use, shifting of the control valve 40 can be discrete or continuously. If the control valve 40 can be positioned continuously the control valve 40 can even serve as a variably adjustable orifice(s).

[0108] In the operating state of the control valve 40 shown with FIG. 11 the pressures at the ODC control port 24 and at the IDC control port 23 are equal, as the control valve 40 is in its third position in which the ODC control port 23 is connected to the IDC control port 24. Therefore, no tilting moment is generated by a pressure difference between the ODC control 24 and the IDC control port 23. As a result, only thenormally relatively high-forces of the neutralizing/returning mechanism 10 and thenormally relatively low-kit moments of the pistons in the cylinder bores 5 of the hydraulic unit contribute to the equilibrium of moments on the displacement element 4, and the displacement element 4 is forced is into its neutral position.

[0109] According to the invention no additional servo piston is present in the hydraulic unit and the return mechanism 10 forces the displacement element 4 to a tilt angle of zero degrees. However, to enable start-up of the hydraulic unit, an initial pressure difference has to be provided at the control ports 23 and 24, such that a hydraulic flow can be generated by the hydraulic axial piston unit according to the invention and the tilt angle of the displacement element 4 can be controlled by means of different pressure levels at the ODC/IDC control ports 23 and 24 generated at the pressure ports 21 and 22 with different pressure levels. For this purpose and in order to start-up the hydraulic axial piston unit, a charge pump 50 is provided capable of providing a pressure level to the shuttle valve 35, which is sufficient to generate a force overcoming the neutralizing forces of the return mechanism 10 at one of the control ports 23, 24. This charge pressure is necessary as long as the pressure difference generated in the working lines of the hydraulic axial piston unit in the starting phase is not high enough to create a tilt moment on the displacement element 4 via the pressure levels at the control ports 23 and 24 being sufficient to overcome the neutralizing forces of the return mechanism 10. Once a pressure difference high enough is reached, hydraulic fluid supply to the shuttle vale 35 from the charge pump 50 can be stopped. Additionally, the charge pump 50 can be capable of replacing hydraulic fluid via the low pressure side which has been discharged, e.g. by leakage or for cooling purposes from the closed circuit.

[0110] FIG. 12 shows a ninth embodiment of a hydraulic axial piston unit according to the invention. The presented embodiment can for example be used as a hydraulic pump in a closed hydraulic circuit. Similar to the embodiment shown with FIGS. 10 and 11, the hydraulic unit comprises only one direction of rotation, but the displacement element 4 of the hydraulic unit can be tilted in both directions with respect to its tilt axis 9 (c.f. FIG. 1). Therefore, the position of the IDC and the ODC and the position of the corresponding control ports 23 and 24 can be inverted when the algebraic sign of the tilt angle of the displacement element 4 is changed.

[0111] A valve arrangement 55 is arranged fluidly between the first and second pressure ports 21 and 22 and the IDC and ODC control ports 23 and 24. By means of the valve arrangement 55, appropriate pressure levels can be provided to the control ports 23 and 24, for example high pressure to the ODC control ports 24 and low pressure to the IDC control port 23, when the pump is operated. The functionality of the valve arrangement 55 is similar to the functionality of the shuttle valve 35 in combination with the control valve 40 which has been described before. The valve arrangement 55 comprises a pressure operated valve 57 which comprises two inlets and two outlets. The pressure operated valve 57 is adapted to conduct higher pressure to one outlet, e.g. the first outlet, and lower pressure to the other outlet, e.g. the second outlet, regardless of whether the higher pressure is present at the first or the second inlet.

[0112] The outlets of the pressure operated valve 57 are connected to inlets of a start-up valve 59, which in the embodiment of FIG. 12 is a 5-3-directional valve. The outlets of the start-up valve 59 are connected to the first and second bypass lines 27 and 28. In the operational position of the start-up valve 59 which is shown in FIG. 12, the high pressure, and the low pressure present at the outlets of the pressure-operated valve 57 are conducted further by the start-up valve 59 to the bypass lines 27 and 28 without changing the direction of fluid flow.

[0113] However, when there is no pressure difference between the first and the second pressure ports 21 and 22,-e.g. when the hydraulic unit is startedno pressure difference is present at the control ports 23 and 24 and in consequence, no force can be generated in order to tilt the displacement element 4 of the hydraulic unit. To solve this problem, a third inlet of the start-up valve 59 is connected to a hydraulic reservoir 100, e.g. a tank, which is at a low pressure level. A charge pump 50 is provided which is capable of providing a charge pressure to the inlets of the pressure operated valve 57. This charge pressure is also present at the first and second inlet of the start-up valve 59. When the hydraulic unit is started and the cylinder block 3 is forced to rotate and the start-up valve 59 can be shifted, in order to conduct charge pressure to one of the bypass lines 27 or 28 and to conduct low pressure from the hydraulic reservoir 100 to the other bypass line 28 or 27.

[0114] Therefore, a pressure difference between the two bypass lines 27 and 28 and in consequence between the ODC/IDC control ports 23 and 24 is established, which is capable of generating a torque on the displacement element 4 that is high enough to tilt the displacement element 4out of the initial position. After the initial tilting of the displacement element 4 a pressure difference is generated at the first and second pressure ports 21 and 22 by the fore and aft movement of the working pistons 6 in the cylinder bores 5. This pressure difference can be conducted to the control ports 23 and 24 via the valve arrangement 55 when it is operated to its operational position shown with FIG. 12, and the hydraulic unit can be operated as described in context with the preceding embodiments.

[0115] FIGS. 13 to 16 show two embodiments of adjustable orifices 29 according to the invention.

[0116] FIG. 13 shows a first embodiment of an adjustable orifice 29 according to the invention in an open position. The orifice 29 comprises a valve body 60 in which a first valve port 66 and a second valve port 68 are arranged. A rotary valve spool 62 comprises a recess in its circumferential surface, which overlaps with the first and second valve port 66 and 68 such that a fluid connection between the two valve ports is established.

[0117] FIG. 14 shows a first embodiment of an adjustable orifice 29 according to the invention in a closed position. When the rotary valve spool 62 is rotated, the first and second valve ports 66 and 68 do not overlap anymore with the recess in the rotary valve spool 62 and the fluid connection is interrupted. Needless to say that the rotary spool 62 could be replaced by a linear moving spool with a corresponding recess in the spool surface without departing from the scope of the invention.

[0118] FIG. 15 shows a second embodiment of adjustable orifices 29 and 30 according to the invention in an open position. FIG. 16 shows this second embodiment of the adjustable orifices 29 and 30 according to the invention in a closed position. In contrast to the embodiment shown with FIGS. 13 and 14, FIGS. 15 and 16 present a linear spool valve, however, the inventive concept can also be applied to a rotary spool valve. The adjustable orifices 29 and 30 comprise a common valve body 60 with a first valve port 66, a second valve port 68, a third valve port 70, and a forth valve port 72. The linear movable spool 64 is slidably accommodated in a central bore of the valve body 60 and comprises two circumferential recesses which can be brought into overlap with the valve ports in order to establish a fluid connection between the first and second valve port 66 and 68, and between the third and the forth valve port 70 and 72. The adjustable connection of the first valve port 66 with the second valve port 68 represents a first adjustable orifice 29. The adjustable connection of the third valve port 70 with the forth valve port 72 represents a second adjustable orifice 30. As the first orifice 29 and the second orifice 30 share a common spool 62, the opening of the first adjustable orifice 29 and the opening of the second adjustable orifice 30 are mechanically coupled to each other. Thus, only one actuator/actuation mechanism is required in order to adjust the opening of both orifices. Preferably, the pressure levels which are present at the valve ports 66, 68, 70, and 72 are symmetrical with respect to a plane between the second valve port 68 and the third valve port 70. For example, the second valve port 68 and the third valve port 70 can be connected to a higher pressure, and the first valve port 66 and the forth valve port 72 can be connected to a lower pressure level or vice versa. This requirement can be fulfilled e.g., when an ODC control port 24 of a hydraulic unit, e.g. a hydraulic pump, is connected to the second valve port 68 and an IDC control port 23 of the hydraulic unit, e.g. a hydraulic pump, is connected to the forth valve port 72. Then, the forces generated on the valve spool 62 by the hydraulic flow (illustrated by arrows in FIGS. 11 to 14) balance each other and only a low force is required to hold the spool 62 in place or to shift the spool 62.

[0119] From the above disclosure and accompanying Figures and claims, it will be appreciated that the hydraulic axial piston unit according to the invention offers many possibilities and advantages over the prior art. It will be appreciated further by a person skilled in the relevant art that further modifications and changes known in the art could be made to a hydraulic axial piston unit according to the invention without parting from the spirit of this invention. Therefore all these modifications and changes are within the scope of the claims and covered by them. It should be further understood that the examples and embodiments described above are for illustrative purposes only and that various modifications, changes, or combinations of embodiments in the light thereof, which will be suggested to a person skilled in the relevant art, are included in the spirit and purview of this application.

[0120] 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.