阅读说明：本技术 无伺服马达、液压活塞单元及其控制方法 (Servo-free motor, hydraulic piston unit and control method thereof ) 是由 史蒂文·李格 斯蒂芬·弗兰茨 杰弗里·C·汉塞尔 于 2019-07-31 设计创作，主要内容包括：本申请提供了一种无伺服马达、液压活塞单元及其控制方法。液压活塞单元包括用于驱动驱动轴或由驱动轴驱动的旋转组,并且具有可倾斜的位移元件,用于在最小或最大位移之间调节旋转组的位移容积,其中,在肾脏入口和肾形出口之间的阀段上在可往复运动的工作活塞的相应死端位置处,第一和第二控制端口与缸体中的缸孔流体连接,用于控制位移元件的位置。液压活塞单元还包括控制阀,该控制阀具有可转换的控制阀阀芯,该阀芯通过高压端口流体连接到液压活塞单元的高压侧。控制阀阀芯被配置成将液压流体从高压侧传导到第一或第二控制端口中的一个。(The application provides a servo-free motor, a hydraulic piston unit and a control method thereof. The hydraulic piston unit comprises a rotary group for driving or driven by a drive shaft and has a tiltable displacement element for adjusting a displacement volume of the rotary group between a minimum or maximum displacement, wherein on a valve section between a kidney inlet and a kidney outlet at respective dead end positions of a reciprocatable working piston, a first and a second control port are in fluid connection with a cylinder bore in the cylinder for controlling the position of the displacement element. The hydraulic piston unit also includes a control valve having a switchable control valve spool fluidly connected to the high pressure side of the hydraulic piston unit through a high pressure port. The control valve spool is configured to conduct hydraulic fluid from the high pressure side to one of the first or second control ports.)

1. A hydraulic piston unit comprising:

a rotating group for driving or driven by the drive shaft, the rotating group comprising a piston and a cylinder having a cylinder bore, the piston being mounted for reciprocal movement in the cylinder bore for conveying hydraulic fluid from the kidney-shaped inlet port to the kidney-shaped outlet port of the valve section,

a displacement element tiltable between a first end position and a second end position relative to the rotational axis of the drive shaft for adjusting a displacement volume of the rotating group between a minimum or maximum displacement,

wherein, on the valve section between the inlet port and the outlet port at respective dead end positions of the reciprocatable working piston, a first control port and a second control port are positioned in fluid connection with the cylinder bore for controlling the position of the displacement element, and

wherein the hydraulic piston unit further comprises a control valve having a switchable control valve spool fluidly connected to a high pressure side of the hydraulic piston unit by a high pressure port capable of conducting hydraulic fluid from the high pressure side to one of the first and second control ports.

2. The hydraulic piston unit of claim 1, wherein the control valve spool is held in an initial position by a control valve spring in which hydraulic fluid at high pressure can be directed to the first control port to adjust the displacement element into the first end position; and wherein the control valve spool is shiftable to a shift position by means of an actuator, wherein hydraulic fluid may be conducted from the high pressure port to the second control port in order to adjust the position of the displacement element.

3. The hydraulic piston unit of claim 2, wherein the control valve is a two-position valve or a proportional valve.

4. The hydraulic piston unit of claim 2, wherein the hydraulic piston unit is a two-position configuration type hydraulic piston unit.

5. The hydraulic piston unit of claim 2, wherein the actuator for shifting the control valve spool is electromechanical, hydraulic, pneumatic, or a combination thereof.

6. The hydraulic piston unit of claim 5, wherein the actuator is controllable by a control unit.

7. The hydraulic piston unit of claim 5, wherein a hydraulic or pneumatic actuator is configured to shift the control valve spool when a predetermined system parameter is exceeded or fallen below.

8. The hydraulic piston unit of claim 7, wherein the displacement element is held in the initial position by a spring force generated by a spring.

9. The hydraulic piston unit of claim 2, wherein the actuator is controllable by a control unit.

10. The hydraulic piston unit of claim 2, further comprising a high pressure selector valve fluidly connected to both pressure sides of the hydraulic piston unit for supplying hydraulic fluid from the high pressure side to the high pressure port of the control valve.

11. The hydraulic piston unit of claim 10, further comprising a low pressure selector valve fluidly connected to both pressure sides of the hydraulic piston unit for supplying hydraulic fluid from a low pressure side to a low pressure port of the control valve, wherein the control valve spool is configured to direct hydraulic fluid at a low pressure to the respective other of the first and second control ports that is not charged with high pressure.

12. The hydraulic piston unit of claim 1, wherein the control valve further comprises a low pressure port fluidly connected to a charge pressure source of the hydraulic piston unit or any other system pressure line, wherein the control valve spool is configured to direct hydraulic fluid at charge pressure or system pressure to the respective other of the first and second control ports that is not charged by high pressure.

13. The hydraulic piston unit of claim 1, wherein the control valve is a two-position valve or a proportional valve.

14. The hydraulic piston unit of claim 1, wherein the hydraulic piston unit is of the two-position configuration type.

15. The hydraulic piston unit of claim 1, further comprising a high pressure selector valve fluidly connected to both pressure sides of the hydraulic piston unit for supplying hydraulic fluid from the high pressure side to the high pressure port of the control valve.

16. The hydraulic piston unit of claim 1, further comprising a low pressure selector valve fluidly connected to both pressure sides of the hydraulic piston unit for supplying hydraulic fluid from a low pressure side to a low pressure port of the control valve, wherein the control valve spool is configured to direct hydraulic fluid at a low pressure to the respective other of the first and second control ports that is not charged with high pressure.

17. A method for controlling displacement volume of a rotating group of a hydraulic piston unit, the hydraulic piston unit comprising: a control valve having a switchable control valve spool fluidly connected to a high pressure side of the hydraulic piston unit through a high pressure port; and a valve section having a first control port and a second control port located at respective dead-end positions of working pistons on the valve section that are reciprocable in cylinder bores of the rotating group of cylinders, the method comprising:

maintaining the control valve spool in its initial position to direct hydraulic fluid from the high pressure side to one of the first or second control ports through the control valve if an initial displacement volume of the rotating group is to be maintained; and

if the displacement volume of the rotating group is to be changed, the control valve spool is shifted by means of an actuator to direct hydraulic fluid at high pressure through the high pressure port to the respective other of the first or second control ports.

18. The method of claim 17, further comprising:

the system parameters are controlled by means of a control unit and the control valve spool is switched by commanding the actuator by the control unit in case one system parameter is exceeded or undershot.

19. The method of claim 18, wherein the control valve further includes a low pressure port and the control valve spool conducts hydraulic fluid from a low pressure side to the respective other of the first or second control ports that is not pressurized with high pressure.

20. The method of claim 17, wherein the control valve further includes a low pressure port and the control valve spool conducts hydraulic fluid from a low pressure side to the respective other of the first or second control ports that is not pressurized with high pressure.

Technical Field

The invention relates to a hydraulic piston unit having a displacement element for adjusting a displacement volume of the hydraulic unit. The invention relates in particular to a hydraulic axial piston unit and a bent axis construction of the type of swash plate. However, the invention is also applicable to hydraulic radial piston units or vane pumps and motors. In the sense of the present invention, the term hydraulic piston unit covers both hydraulic piston pumps as well as hydraulic piston motors. In a particular embodiment, the invention relates to a two-position hydraulic axial piston unit, wherein a displacement element is positionable between two end positions for adjusting a displacement volume of the hydraulic axial piston unit to a maximum or a minimum value. Although the invention is described with an example of an axial piston unit having in particular an axial piston unit of the swash plate structure type, the given example can be switched by a person having the skills associated with all of the above mentioned types of hydraulic piston units.

Background

In the prior art, the displacement volume of a hydraulic piston unit is usually adjusted by means of a servo unit having a servo cylinder and a movable servo piston, the position of which in the servo cylinder ultimately determines the displacement volume of a rotating group within the hydraulic piston unit. Such servo units form a separate group of components, in particular in the case of two-position hydraulic piston units, which leads to increased costs and consumes installation space. Furthermore, such servo units are subject to wear or other possible malfunctions, which may lead to inoperability of the entire hydraulic piston unit. Furthermore, each cylinder-piston unit shows a certain leakage, as does the servo unit.

Disclosure of Invention

It is therefore an object of the present invention to simplify the adjustment and control device of the displacement volume of a hydraulic piston unit to overcome the drawbacks of the prior art. The hydraulic piston unit according to the invention should therefore comprise fewer components for performing the control of the displacement volume. The hydraulic piston unit should be easier and more economical to manufacture, in particular simpler to assemble.

The object of the invention is solved by a hydraulic piston unit having a rotating group for driving or driven by a drive shaft. The hydraulic piston unit further comprises a displacement element which is tiltable with respect to the axis of rotation of the drive shaft between a first end position and a second end position. The displacement volume of the rotating group is adjusted between a maximum or minimum displacement volume depending on the angle of inclination of the displacement element. Within the cylinder block, a working piston is reciprocally movably mounted in the cylinder bore for delivering hydraulic fluid from a kidney-shaped inlet port to a kidney-shaped outlet port of the valve section. In the sense of the invention, the rotary assembly comprises at least a cylinder with a working piston reciprocable therein and a drive or driven shaft and, at the lower end of the cylinder, a valve section with the aforementioned kidney-shaped inlet and outlet ports. In the case of hydraulic piston units of the swash plate type, the swash plate also forms part of the rotating group, since the piston heads are usually fixed to the swash plate by means of piston shoes. However, these piston shoes are designed to slide on the swash plate surface facing the cylinder block. Furthermore, in the case of these hydraulic piston units, the swash plate and the valve section are located on opposite sides of the cylinder block. The same applies to a hydraulic piston unit of the bent-axis construction type; in this case, however, the piston head is fixed directly on the drive shaft in a ball joint to allow the cylinder block to flex relative to the axis of rotation of the drive shaft.

The hydraulic piston unit of the present invention comprises a first control port and a second control port positioned on the valve section between the inlet port and the outlet port, in fluid connection with the cylinder bore at respective dead end positions of the reciprocatable working piston. Thus, the hydraulic fluid pressure difference between the two control ports and the hydraulic fluid pressure in the cylinder bore determine the inclination angle of the displacement element and ultimately the displacement volume of the hydraulic piston unit.

The hydraulic piston unit of the present invention also includes a control valve having a switchable control valve spool. The control valve is connected to the high pressure side of the hydraulic piston unit by a high pressure inlet and is capable of directing hydraulic fluid from the high pressure side to one of the first and second control ports at the valve section. Thereby, one control port is supplied with high-pressure hydraulic fluid at one dead-end position of the working piston, and a low pressure, e.g. tank or housing pressure, is present at the other control port at the other dead-end position of the working piston movement. According to the present invention, high pressure acts on the working piston passing through the control port to push the working piston outward along the cylinder bore. This forces the displacement elements at the opposite ends of the cylinder to deflect or be oriented perpendicular relative to the axis of rotation of the cylinder, thereby defining a displacement volume. Thus, according to the invention, the working piston fulfils the function of moving the displacement element, which is accomplished in the prior art hydraulic piston units by means of the above-mentioned additional servo unit.

In the field of hydraulic piston units, the displacement element is preferably held at least in its initial first end position by a spring force, preferably caused by a spring or the like, against a stop, as is common knowledge in the art. In other angular positions, a force balance between the deflection forces, here performed by the working piston when passing through the control ports at the respective upper and lower dead ends, and a spring force acting in the direction of the initial first position can be achieved. In particular for a two-position hydraulic piston unit, the second end position of the displacement element is provided by a second stop against which the displacement element can abut at its maximum or minimum deflection/inclination angle. It should be readily understood that the displacement element may also be positioned at an intermediate position between the maximum and minimum tilt angles, for example by means of an elastic or hydraulic force that varies with the tilt angle.

For example, in an embodiment in which the initially set displacement volume of the hydraulic piston unit is at its minimum, i.e. the displacement element is in its neutral or zero position and the inclination angle of the displacement element is equal to zero, the displacement element may be held in this position as long as the control spool of the control valve has not been shifted out of its initial position in which hydraulic fluid at high pressure is directed to the first control port. In this example case, as the pressure conducted to the first control port on the valve section decreases, the displacement volume of the hydraulic piston unit increases with the displacement of the control valve spool, and the pressure directed to the second control port increases with the displacement of the controller spool. This will continue until hydraulic fluid at high pressure is directed only to the second control port. At that moment, for this embodiment, the displacement element is at its maximum displacement, i.e. at its maximum inclination angle. However, it should be readily understood that this example could be performed in reverse without difficulty.

The hydraulic piston unit of the present invention may be applied alternatively in substantially all hydraulic piston unit applications known in the art that use a servo unit to adjust displacement volume. Accordingly, the present disclosure also encompasses all of these possibilities. However, unlike prior art solutions, the present invention provides a hydraulic piston unit with fewer components, since no servo unit is required anymore, and for the same reason a smaller hydraulic piston unit is provided which consumes less construction space. Thus, control of displacement volume can be accomplished by merely controlling the position of the control valve spool in the control valve, which in a simple embodiment may be a three/two position valve.

Since the invention only affects the way the displacement element is positioned and controlled in its positioning, the hydraulic piston unit according to the invention can be operated in a motor mode as well as in a pump mode, including the associated propulsion and towing modes in case the hydraulic piston unit of the invention is applied in a closed hydraulic circuit.

In a basic embodiment, the hydraulic piston unit of the invention is of the two-position construction type, i.e. only a minimum displacement and a maximum displacement can be selected. In this case, the control valve is also only an on-off valve, which conducts hydraulic fluid to the first control port or the second control port. In this case, by switching the control valve from the first position to the second position and vice versa, the hydraulic piston unit changes its displacement from a maximum value to a minimum value and vice versa. It should be readily understood that the control valve spool as well as the hydraulic piston unit may also be adjusted in a neutral position and may also be adjusted across a "zero" displacement of the displacement member. So that the absolute value of the maximum displacement can be as large as the minimum displacement, only the sign of the preamble changes, i.e. the transport direction, or the propulsion mode changes to the towing mode.

Thus, according to the invention, the control valves may be two-position valves as well as proportional valves. In case the control valve is a proportional valve, the displacement of the rotating group may also be proportionally adjusted. The control of the control valves can here preferably be done by actuator means commanded by the control unit. For example, the control unit itself monitors system parameters or receives input from an operator of the hydraulic propulsion application and sends corresponding signals to the actuator device in case the tilt angle of the displacement element has to be changed or maintained (e.g. when a value of the system parameters is exceeded or fallen below). The actuator device thus switches the control valve spool in an electromechanical, hydraulic, pneumatic manner or a combination of these possibilities. Examples of such electromechanical, hydraulic or pneumatic devices are known in the art. For example, springs, solenoids, cylinder-piston-units, levers, electric or thermal relays, etc., are exemplary such actuation devices to actuate a control valve spool to maintain or change its position. The present invention thus covers all these general technical possibilities on the actuation control valve spool and the inventive directing of the pressurized hydraulic fluid to the first and second control ports in the valve section.

Furthermore, all these means for switching the control valve spool may be controlled by means of a control unit which is preferably able to command the aforementioned actuator means to act on the control valve spool. The control means may thus be a CPU, microprocessor or the like, which sends corresponding signals to the actuator means for changing or maintaining the position of the switchable control valve spool. Of course, the control unit receives input information from other components and/or from, for example, an operator of the hydraulically driven vehicle and sends corresponding signals to the actuator means. In another embodiment of the invention, the control unit monitors system parameters and commands the actuator device to hold or shift the control valve spool in the event that the system parameters are exceeded or undershot.

As mentioned above, the hydraulic piston unit of the invention is suitable for open as well as closed hydraulic circuits, with a conveying direction and/or the ability to operate in a propulsion mode or a towing mode. In the case of implementing the hydraulic piston unit of the invention in such a closed hydraulic circuit, the hydraulic piston unit of the invention preferably comprises a high pressure selection valve which is fluidly connected to both pressure sides of the hydraulic piston unit and which is capable of supplying hydraulic fluid from the high pressure side to the high pressure inlet port of the control valve. In a basic embodiment, this high pressure selection valve is in the form of a double check valve with two inlets and one outlet, wherein the inlet on the low pressure side is closed by a check valve ball under high pressure. In this way, it is ensured that hydraulic fluid at high pressure is directed to the high pressure inlet port of the control valve under any operating conditions.

In another preferred embodiment of the invention, the control valve is further capable of directing hydraulic fluid at low or lower pressure to a respective other control port on the valve section that is not charged with hydraulic fluid at high pressure. Thus, the control valve is connected via a low pressure inlet port to the low pressure side of the hydraulic piston unit or to a source of charging pressure (e.g. in the form of a charging pump), or another system pressure below the high pressure level. In the case of a hydraulic piston unit embodied as a closed hydraulic circuit, the control valve is fluidly connected to both pressure lines of the hydraulic circuit by means of a low-pressure selector valve in order to appropriately direct hydraulic fluid at low pressure to the respective other, non-high-pressure-charged control port in both conveying directions. The low pressure selection valve may be implemented, for example, by a switching valve, wherein the switching valve spool is referenced on one side to a high pressure level and on the opposite side to a lower pressure level, which is not necessarily the pressure level forwarded by the low pressure selection valve.

In summary, with the hydraulic piston unit of the invention, a control method for adjusting the displacement volume of at least a two-position hydraulic unit can be carried out in a simplified manner, wherein if an initial displacement volume of the hydraulic piston unit of the invention is to be maintained, a control valve spool is held in its initial position, in which the control valve conducts hydraulic fluid from the high pressure side to one of a first or a second control port located on a valve section of the hydraulic piston unit at a respective dead end position of a reciprocating working piston, and if the initial displacement volume of the two-position hydraulic unit has to be changed, the control valve spool is switched by means of an actuating device into a switching position for supplying high-pressure hydraulic fluid to the respective other control port. As mentioned above, the control method of the invention is performed in the basic embodiment on a two-position hydraulic piston unit, but it is also applicable to a proportionally adjusted hydraulic piston unit in order to proportionally change the displacement volume of the rotating group.

It is further preferred that the control of the displacement volume of the rotating group of the hydraulic piston unit of the invention is controlled by a control unit able to command actuator means acting on a control valve spool of a control valve. The control valve may be a two-position type valve or a proportional valve.

The inventive method for controlling the displacement volume of a hydraulic piston unit may further comprise controlling the directing of hydraulic fluid at low pressure to a respective further control port on the valve section that is not charged with hydraulic fluid at high pressure. The control method of the invention is applicable to hydraulic piston units in open circuit as well as in closed hydraulic circuits, wherein the hydraulic piston units of the invention are operable in both directions of rotation as well as in a propulsion mode or a towing mode.

Drawings

Exemplary embodiments of the inventive hydraulic piston unit according to the present disclosure are shown in more detail in the accompanying drawings, which do not limit the scope of the disclosure. All features of the disclosed and described embodiments can be combined with each other in any desired combination within the scope of the invention. For this purpose, it is shown that:

fig. 1 schematically shows a first embodiment of the hydraulic piston unit of the invention in a first initial position;

fig. 2 schematically shows a second embodiment of the hydraulic piston unit of the invention, with the control valve spool in a switching position;

fig. 3 schematically shows a third embodiment of the hydraulic piston unit of the invention in a first initial position;

FIG. 4 schematically illustrates the embodiment of FIG. 3 with the control valve spool in a shift position; and

fig. 5 schematically shows a valve plate according to the invention.

Detailed Description

In fig. 1 to 5, the invention is schematically disclosed by means of a simplified hydraulic piston unit, illustratively a hydraulic axial piston unit of swash plate construction. The embodiments shown are only for reasons of simplicity and merely illustrate the invention in a simple manner and do not limit the scope of the inventive concept. On the contrary, as mentioned above, the invention is also applicable to hydraulic axial piston units of the bent-axis or rolling-plate structure type, as well as radial piston units and vane units. In addition, in the drawings, like reference numerals denote like features of the hydraulic piston unit of the present application. Thus, although some descriptions may refer only to certain figures and reference numbers, it should be understood that the descriptions may apply equally to the same reference numbers in other figures.

The hydraulic axial piston unit 1 shown in fig. 1 comprises a cylinder block 3, wherein a cylinder bore 5 is positioned generally parallel to the axis of rotation 9 of the cylinder block 3. The working pistons 6 are reciprocable in these cylinder bores 5, guided by the displacement element 4 according to the invention, which displacement element 4 is shown in fig. 1 as an exemplary non-rotating swash plate. As known to the person skilled in the art, in a bent-axis hydraulic axial piston unit, the displacement element 4 may be a yoke which bends the axis of rotation of the cylinder block relative to the axis of rotation of the drive shaft of the hydraulic axial piston unit.

In the exemplary embodiment of fig. 1, the displacement element 4 in the form of a swash plate can be tilted about a pivot axis 29 in order to adjust the displacement volume displaced by the working piston 6 during one revolution of the cylinder block 3. The cylinder bore 5 is fluidly connected to the first and second control ports 23 and 24 on the valve section 20 on the lower side of the cylinder block 3 (i.e., on the bottom side of the piston) through cylinder ports 31 and 32. The valve section 20 is rotationally fixed relative to the housing of the hydraulic piston unit and does not rotate with the cylinder 3. The drive shaft 8 is attached to the cylinder block 3 in a rotationally fixed manner, so that the drive shaft 8 can be driven in a rotatable manner or by the cylinder block 3. Depending on which kidney-shaped port of the valve section 20 forms the inlet port 21 or the outlet port 22 and in which position, i.e. the angle of inclination at which the swash plate 4 is oriented, the rotating group 2 of the hydraulic piston unit 1, which usually comprises at least the swash plate 4, the working pistons 6, the cylinders 3, the drive shaft 8 and the valve section 20, acts as a pump or a motor and operates in a propulsion mode or a drag mode. Since these details are well known to those of ordinary skill in the art, further explanation of this aspect is omitted.

For example, the hydrostatic piston unit 1 in fig. 1 is shown as a hydraulic piston motor in a propulsion mode, wherein the inlet port 21 is arranged at the high pressure side 14 of the hydraulic piston unit 1. Accordingly, the outlet port 22 is arranged at the low pressure side 15. Thus, hydraulic fluid under high pressure enters the rotating group 2 at the inlet port 21 and presses the working pistons 6 against the swash plate 4, which results in a rotational movement of the cylinder blocks 3. Because the lower working piston 6 is in its lower dead-end position, hydraulic fluid at low pressure at the outlet port 22 exits the rotating group 2, which is compressed by the swashplate 4, through the working piston 6 and a cylinder port 32 (not shown in fig. 1), which cylinder port 32 is aligned with the control port 24 in the simplified embodiment of fig. 1. The arrangement of the two kidney ports 21 and 22 and the arrangement of the control ports 23 and 24 of the present invention of the present disclosure are shown schematically in fig. 5. From fig. 1 in combination with fig. 5 it can be easily understood that the cylinder block 3 of the hydraulic piston unit 1 in fig. 1 shows an even number of cylinder bores 5, here four cylinder bores 5, which is a special configuration (since usually an odd number of cylinder bores 5 are used), e.g. to enhance a smoother operation of the hydraulic piston unit.

In the embodiment of fig. 1, the swash plate 4 is urged and held in its inclined or deflected position by hydraulic fluid at high pressure, which is directed from the control valve 10 to the cylinder ports 31 via the first control port 23. This provides a sufficiently high hydraulic pressure in the cylinder bore 5 to push the working piston 6 to its upper dead end position and hold it there, so that the swash plate 4 is in the maximum deflection/tilt position, as shown in fig. 1. Hydraulic fluid at high pressure enters the control valve 10 through a high pressure port 11, and the high pressure port 11 is fluidly connected to the high pressure side 14 through a high pressure selector valve 30. As an example, the high-pressure selector valve 30 can be designed as a double check valve which is connected to both pressure sides of the hydraulic piston unit 1.

In the operating state of the hydraulic piston motor 1 according to fig. 1, the control valve 10 with the control valve spool 16 is in a first (initial) position, in which the control valve 16 conducts hydraulic fluid at high pressure to the first control port 23. The control valve spool 16 is held in this first initial position by means of a control valve spring 17. The actuator 13 is arranged on the opposite side of the control valve spool 16, the actuator 13 being controlled by a suitable control unit 100 and configured to generate a reaction force against the spring force of the control valve spring 17 in order to move the control valve spool 16 to the second switching position. The actuator 13 may be, for example, an electromechanical actuator 41, a hydraulic actuator 42, a pneumatic actuator 43, or a combination thereof. In this second switching position, since the hydraulic fluid under high pressure enters the cylinder bores 5 through the cylinder ports 32 and presses the working pistons 6 toward the swash plate 4, the control valve spool 16 can conduct the hydraulic fluid under high pressure to the second control port 24, thereby changing the inclination angle of the swash plate 4. By this change in the inclination angle of the swash plate 4, the strokes of all the working pistons 6 are changed, and the displacement volume of the hydraulic piston unit 1 is also changed. Even if the displacement volume of the hydraulic piston unit 1 changes, the high pressure selector valve 30 is held at the same position as long as the high pressure side 14 and the low pressure side 15 are not exchanged so that the side surface 15 becomes the high pressure side and the side surface 14 becomes the low pressure side.

The switched position of the control valve 10 is shown in fig. 1 with a dashed line, in which the swash plate 4 is in a zero position against the stop 33, i.e. the angle of inclination is equal to zero. In this operating condition, the hydraulic piston unit 1 does not show displacement volume, because the swash plate is oriented perpendicular to the axis of rotation 9, which means that the working pistons do not perform any stroke, because the distance between the swash plate and the cylinder block, seen in the direction of the axis of rotation, does not change in the circumferential direction. In this state, therefore, the hydraulic piston unit is idling without performing any work.

However, it should be readily understood that the swash plate 4 may not abut against the stop 33 but be inclined to a negative angle relative to the position of fig. 1, i.e. the deflection of the swash plate 4 is opposite to that shown in fig. 1. This would be the setting when the hydraulic motor of fig. 1 is operated in a drag mode, for example, now working as a pump and performing an interrupting effect on the hydraulic propulsion application, for example.

Fig. 2 schematically shows a second embodiment of the hydraulic piston unit of the invention, in which the control valve spool 16 is in a switching position. Unlike the embodiment of fig. 1, the high pressure side 14 varies with the low pressure side 15 in fig. 2. Assuming that the direction of delivery is the same as in the embodiment of fig. 1, i.e. counterclockwise, the hydraulic piston unit 1 acts as a pump, wherein the pressure present at the control port 24 determines the inclination angle of the swash plate 4 and the displacement volume of the hydraulic piston unit 1 (here a hydraulic axial piston pump).

Fig. 3 shows another embodiment of the hydraulic piston unit 1 of the invention, which differs from the embodiment of fig. 1 and 2 in that the control valve 10 is a 4-way/2-position valve, which directs hydraulic fluid at high pressure to a control port 23 and directs hydraulic fluid at low pressure to a respective further control port 24. This configuration provides a better lubrication effect in the lower dead end region of the working piston 6 on the valve section 20. In order to supply low-pressure hydraulic fluid to the control valve 10 from the low-pressure side, a low-pressure selector valve 35 is arranged upstream of the control valve 10. The low pressure selection valve 35 is connected to the high pressure side 14 as well as the low pressure side 15. The spool of the low pressure selection valve 35, e.g. a three/two position valve, is thus referenced to the high pressure side on one front side and to low pressure on the other side to ensure that the low pressure port 12 connected to the control valve 10, which is always the low pressure side, is always the low pressure side. Similar to the embodiment of fig. 1 and 2, the high pressure selection valve 30 ensures that the high pressure port 11 of the control valve 10 is connected to the high pressure side 14 of the hydraulic piston unit 1.

As can be seen from fig. 3, the inclination angle of the swash plate 4 can be changed to the other side, i.e. the leading sign of the angle can be changed to a respective maximum value in each direction. By doing so, it is possible to change the conveying direction so that the high pressure side is exchanged with the low pressure side, but the direction of rotation of the hydraulic piston unit 1 remains unchanged. In addition, the switching of the control valve spool 16 from one position to another switches the hydraulic piston unit 1 from pump mode to motor mode and vice versa when maintaining the delivery direction, since it is preferred for propulsion applications when changing from propulsion mode to towing mode. Such a switching configuration is shown, for example, in fig. 4.

In addition, fig. 4 shows another embodiment of the hydraulic piston unit 1 of the invention, in which hydraulic fluid at a pressure below high pressure is provided, for example, by a charging pressure source 40. This enables the mode of operation to be preselected in the stationary state of the hydraulic piston unit 1. For example, the delivery direction can be selected in the case of a hydraulic pump simply by shifting the control valve spool 16 to the appropriate position. It should be readily understood that the charge pressure source 40 may be interchanged with any system pressure source 44 that provides hydraulic fluid at a lower pressure than the high side pressure of the hydraulic piston unit 1, 44. In addition, pressurized hydraulic fluid directed to one of the control ports 23 or 24 improves lubrication of the low pressure region of the valve section 20. As shown in fig. 5, the control ports 23 and 24 are arranged between the kidney-shaped inlet and outlet ports 21 and 22 at respective dead end positions 61 and 62 of the working piston 6. The circumferential distance between the boundaries of the kidney ports 21 and 22 and the boundaries of the control ports 23 and 24 is preferably less than twice the circumferential distance between the boundaries of the cylinder ports 31. By doing so, it can be ensured, in particular for an odd number of cylinder bores, that always one cylinder port 31, 32 overlaps at least one control port 23, 24 to ensure better and faster reactive control and smoother operation of the hydraulic piston unit 1.

In summary, with the inventive hydraulic piston unit of the present invention, a smaller hydraulic piston unit with reliable displacement volume control is provided, which eliminates the need for an (external) servo unit. This not only saves costs, but also the hydraulic piston unit is less complex and more reliable since it comprises fewer parts. Leakage is also significantly reduced as fewer components are subjected to hydraulic pressure.