Patent Application
20250223951
The present invention relates to hydraulic axial piston units and a method for controlling hydraulic axial piston units. More specifically, the invention relates to hydraulic axial piston units of the swashplate type as well as hydraulic axial piston units of the bent-axis type of construction. The invention also relates to a method for controlling both types of hydraulic axial piston units. The hydraulic axial piston units to which the invention refers to can be used in open hydraulic circuits as well as in closed hydraulic circuits and can comprise a fixed displacement volume or a variable displacement volume.
Hydraulic axial piston units of the swashplate or the bent-axis type of construction are widely known in the state of the art and are used as fixed or variable displacements units. All of them can be operated in pumping or motoring mode. The displacement volume of the hydraulic axial piston units can be set/controlled by means of setting/changing the tilt angle of a displacement element, i.e. the swashplate or the yoke. In order to transform mechanical power into hydraulic power and vice versa hydraulic axial piston units comprise a rotational group. This rotational group has a rotatable cylinder block with cylinder bores in which working pistons are arranged reciprocally movable for conveying hydraulic fluid from a kidney-shaped inlet port to a kidney-shaped outlet port located on a valve segment of the hydraulic axial piston unit. When the displacement element is inclined with respect to the drive shaft axis of the hydraulic unit, the working pistons are forced to reciprocate between their inner dead centre (IDC) and their outer dead centre (ODC), when the cylinder block is turning. Thereby, a piston is at its inner dead centre when the direction of motion of the piston changes from a movement towards the valve segment to a movement towards the displacement element. A piston is at its outer dead centre when its direction of movement changes from a movement towards the displacement element to a movement towards the valve segment. As known, one of the inlet port or the outlet port is serving as a high pressure port and the respective other port serves as a low pressure port. It depends on the operational mode of the hydraulic unit and the hydraulic flow direction, which port serves as high pressure port and which port serves as low pressure port.
For setting the tilt angle of a displacement element of a hydraulic axial piston unit manual, hydraulic or electronic control units are used. To set/adjust the displacement volume of the hydraulic axial piston unit these control units frequently control the movement of servo pistons in servo units by selectively directing hydraulic pressure into pressure chambers of the servo unit by means of shifting a control spool. These control and servo arrangements are complex due to the high level of demand in manufacturing and operation precision and are prone to errors. Thus, they are costly in manufacturing and installation work. Furthermore, control and servo units known in the art, and-due their amount of parts-are bulky and space consuming so that the overall size of hydraulic axial piston units is increased. The known controls of hydraulic axial piston units are developed for specific applications and require a specific adaptation of the control parts for each and every application, like specific valve plates and/or valve segments as well as specifically adapted servo and control spools and springs, which all require narrow tolerances. The components of the displacement control units are exposed to wear and therefore require continuous maintenance or replacement. Furthermore, these specific components are not suitable to be changed on the fly, i.e. once installed they cannot be adapted to individual load situations, and moreover they often cannot be used in different hydraulic axial piston units of different volumetric size/cubic capacity.
It is therefore an objective of the invention to provide a hydraulic axial piston unit with a control system for setting and controlling the displacement volume of hydraulic axial piston units, which compared to the solutions in prior art, comprises a lower amount of components or at least components with a simpler design, but which is capable of reliably setting and controlling the displacement volume of hydraulic axial piston unit. In consequence, the hydraulic axial piston unit according to the invention shall be less costly and shall require less construction space compared to the solutions known in the art. The control system for a hydraulic axial piston unit according to invention is intended to be adaptable to different hydraulic axial piston units, even on-the-fly, i.e. without having to disassemble the hydraulic axial piston unit.
The objective is solved by a hydraulic axial piston unit according to claim 1 and a method for controlling the displacement volume of a hydraulic rotation group according to claim 35. Preferred embodiments are presented in the subclaims dependent thereon.
A hydraulic axial piston unit according to the invention comprises a rotating group whose displacement volume is set by means of a displacement element. The rotating group comprises a rotatable cylinder block with cylinder bores in which working pistons are mounted reciprocally moveable. When the cylinder block rotates and the displacement element is inclined with respect to the rotational axis of the cylinder block, the pistons perform a fore-and-aft movement in the corresponding cylinder bores. When one full rotation of a cylinder bore and of the working piston arranged in the cylinder bore is observed, the piston changes its direction of motion two times. At the inner dead centre (IDC), the working piston changes its direction of motion from travelling towards fluid exchange opening of the cylinder bore to travelling away from the fluid exchange opening of the cylinder bore. Accordingly, within one revolution, the inner dead centre is the position in which the working piston is closest to the fluid exchange openings of the cylinder bore, that is, it is inserted furthest into the cylinder bore, and the fluid volume in the cylinder bore is minimum. At the outer dead centre (ODC), the movement of the working piston is changed from travelling away from the fluid exchange opening of the cylinder bore to a movement towards the fluid exchange openings of the cylinder bore. In consequence, when one full revolution of the cylinder block is considered, at the outer dead centre the working piston is at the position most distant from the fluid exchange opening of the corresponding cylinder bore, i.e. where the working piston is furthest extracted out of the cylinder bore, and the fluid volume in cylinder bore is the largest the set tilt angle permits.
For example, when a working piston of a hydraulic axial piston pump is at the ODC, the pressure in the corresponding cylinder bore changes from low inlet pressure to high outlet pressure, whereas for a piston at the IDC the cylinder bore pressure changes from high outlet pressure to low inlet pressure. For a hydraulic motor, the situation is inverted: At the ODC, the pressure on a working piston and in the corresponding cylinder bore pressure change from high inlet pressure to low outlet pressure, whereas at the IDC, the pressure on the piston and in the corresponding cylinder bore accommodating the piston change from low outlet pressure to high inlet pressure.
In consequence and known in the art, the adjustable longitudinal position of the inner dead centre and of the outer dead centre of the working pistons, i.e., the position seen along the rotational axis of the rotational group, depends on the inclination angle/tilt angle of the displacement element. However, the angular position of the inner and the outer dead centre is set fixedly by the rotational group design as long as the orientation and the position of the tilt axis of the displacement element is not changed, i.e. independent of the tilt angle of the displacement element.
The hydraulic axial piston unit according to the invention further comprises a valve segment with a kidney-shaped first pressure port and a kidney-shaped second pressure port. Hydraulic fluid can be conducted to and drained from the cylinder bores when a cylinder bore overlaps with the first or the second pressure port. Further, according to the invention, 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 kidney-shaped first pressure port and the kidney-shaped second pressure port. In other words, the pressure and the control ports are arranged alternately in circumferential direction, e.g. ODC control port, first pressure port, IDC control port, second pressure port.
The IDC and ODC control ports are arranged on the valve segment in such a way 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 centre (IDC) or is at or close to its outer dead centre (ODC), respectively. As mentioned above, the circumferential position of the IDC and the ODC of the working pistons is constant. In consequence, according to the invention and independently of the tilt angle, the circumferential position of the IDC control port on the valve segment is always at or near the IDC of the working pistons and analogously the ODC control port on the valve segment is always at or near the circumferential position of the ODC of the working pistons.
According to the invention the circumferential distance of the IDC control port to the first and second pressure ports and analogously the circumferential distance of the ODC control port to the first and second pressure ports is smaller than the circumferential extension of the cylinder bores or their openings towards the valve segment. When, during the rotational motion of the cylinder block, a cylinder bore leaves the circumferential area in which the cylinder bore overlaps with the first or second pressure port, hydraulic fluid which remains in the cylinder bore can be compressed further due to an ongoing motion of the piston. This effect can for example occur in a hydraulic pump when the piston is close to its inner dead centre but has not yet reached its inner dead centre. Further compressing of the hydraulic fluid in the cylinder bore leads to a pressure shock or pressure peak in the cylinder bore and consequently on the valve segment, as the hydraulic fluid in the cylinder bore cannot be drained via the first or second kidney shaped pressure port. Also in other scenarios and situations pressure peaks, shocks, or a non-uniform distribution of pressure over the valve segment might occur, e.g. a kind of cavitation near ODC. In order to weaken these effects, circumferentially oriented pressure elongation grooves (also called: “fishtails”) are often provided in the valve segment in prolongation of the pressure kidneys.
Further according to the invention, a cylinder bore is simultaneously in contact with the IDC or with the ODC control port, when it stops overlapping with the first or second pressure ports, e.g. in case of a hydraulic axial piston pump, overpressure or excess hydraulic fluid can be drained via the IDC control port or cavitation can be avoided by additional hydraulic fluid supply via the ODC control port. In case of a hydraulic axial piston motor cavitation may occur at the ODC of the working piston, therefore, in this case, hydraulic fluid supply over the ODC control port can avoid or at least can reduce the cavitation effect. As a result according to the invention, pressure peaks and a disadvantageous pressure distribution over the valve segment is avoided as well as elongation grooves (fishtails) mentioned before.
Further according to the invention, a first bypass line and a second bypass line are provided each connecting one of the control ports, i.e. the IDC control port or the ODC control port, with one of the first or the second pressure port or with a pressure compensation chamber. In the first bypass line an adjustable orifice is arranged capable of continuously and variably opening and closing the first bypass line in order to enable an adjustable fluid flow connection between the connected pressure port and the passing cylinder bore via the first bypass line and the allocated control port. A second bypass line is connected to the respective other control port. The orifice in the first bypass line can be provided as an additional part, e.g. in form of a flow valve or similar, especially preferred an adjustable flow opening. However, non-adjustable orifices can be formed also integrally with the first and/or second bypass line, in which they are arranged.
The opening of the at least one orifice and its magnitude of opening influences the sum of static pressure forces which are present at the displacement element. The pressures which are present at the IDC control port and at the ODC control port each generate a force which acts on the displacement element via the working pistons. The ODC and the IDC control ports are arranged on the valve segment on opposite sides with respect to the tilt axis of the displacement element each with a lateral offset to the tilt axis. These offsets can be the same but does not have to be. Therefore, each pressure force at the ODC and the IDC control ports generate a kit moment/torque with respect to the tilt axis on the displacement element, wherein the moment at the ODC control port comprises a different algebraic sign than the moment at the IDC control port. The resulting kit moment-including the kit moments generated by the pressure ports-sets the tilt angle of the displacement element and therefore-depending on the direction of tilt-cause a corresponding displacement of the displacement element. If the pressure level at the ODC control port is adjusted with respect to the pressure level at the IDC control port, the resulting kit moment changes. The resulting kit moment is also influenced by other parameters and forces, which are explained later.
By controlling the opening size of the at least one variable orifice or the ratio of the opening sizes of more than one orifice in the bypass lines the tilt angle of the displacement element of the hydraulic unit can be adjusted and set. The magnitude of the opening of the orifice(s) can e.g. be controlled by an electronic control unit. Thereby, it is not required that hydraulic fluid is injected into a passing cylinder bore in a short time interval, e.g. in the range of milliseconds. Quite to the contrary, static pressure is used to control and set the pressure profile, which is encountered by a cylinder bore, when passing one of the control ports. The tilt angle of the displacement element and the opening of an adjustable orifice do not regularly change with a high frequency. Especially not every time when a cylinder bore passes the IDC or the ODC control port. As static pressure is used to influence the pressure profile along the valve segment for controlling the tilt angle of the displacement element, the frequency with which the opening of orifice(s) has to be adjusted is relatively low.
The hydraulic axial piston unit according to the invention can comprise a variable displacement volume whose displacement volume is controlled by means of adjusting the opening size of the adjustable orifice, i.e. its opening magnitude. The adjustable orifice is arranged at least in the first bypass line in order to adjust the pressure at one control port in relation to the pressure at the other control port. Therewith the pressure profile, during a transition of a cylinder bore from one pressure port to the other pressure port, can be modified and therewith the kit moments acting on the displacement element can be changed/varied.
A hydraulic axial piston unit according to the invention can alternatively comprise a fixed displacement volume. In this embodiment of the invention, the displacement volume is set by means of setting the opening of the adjustable orifice in the first bypass line in order to adjust the pressure at one control port in relation to the pressure at the other control port. The fixed displacement volume is maintained throughout the operation of the hydraulic unit. Even though the displacement volume of the hydraulic unit is maintained constant, the opening of the adjustable orifice can be adjusted or controlled, during the operation of the hydraulic unit. In consequence, the pressure profile in a cylinder bore which overlaps with the control port connected to the bypass line with the adjustable orifice can be adjusted. Therewith vibrations of the displacement element as well as pressure peaks, oscillations or cavitation can be reduced or even eliminated. As a result, noise generated during operation of the hydraulic unit can be reduced and the running behaviour of the hydraulic unit according to the invention can be enhanced and therewith the lifetime of the hydrostatic unit can be extended.
According to the invention the second bypass line which is connected to the other control port can be connected to a pressure compensation chamber. In such an embodiment, the second bypass line establishes a fluid connection between the other control port and the pressure compensation chamber. The pressure compensation chamber can be adapted to dampen pressure peaks and cavitation in the passing cylinder bores, respectively on the vale segment, and thereby avoid pressure oscillations.
In one embodiment of the invention, the second bypass line is connected with the first pressure port or the second pressure port in order to establish a fluid flow connection between the connected pressure port and the passing cylinder bore via one of the control ports. Preferably, the first bypass line with the adjustable orifice connects either the IDC control port or the ODC control port with the first or second pressure port and the second bypass line connects the other respective control port with the respective other pressure port.
Preferably according to the invention, the first bypass line and the second bypass line each connects the next pressure port after the connected control port seen in rotational direction of the cylinder block. E.g., in an open circuit pump, the control port at IDC is preferably connected via the allocated bypass line to the pressure port at low system pressure and the control port at ODC is connected via the allocated bypass line to the pressure port at the higher system pressure. As a closed circuit pump contrary to an open circuit pump is often operated also in motoring mode, the invention may provide additionally for a possibility to switchable connecting the ODC control port with the respective high system pressure port, for instance by the help of a switching or shuttle valve. As known by a person with skills in the relevant art, the system pressure port at the valve segment changes sides when the hydrostatic unit changes from pumping mode to motoring mode and vice versa.
If the hydraulic unit is configured as a hydraulic pump, one bypass line can for instance connect the ODC control port with the pressure port at higher system pressure and the other bypass line can connect the IDC control port with the pressure port at lower system pressure. If the hydraulic unit is operated as hydraulic motor the situation can be inverted and one bypass line can, for example, connect the IDC control port with the high pressure port, wherein the other bypass line connects the ODC control port with the low pressure port.
The connection of the control port with the next, coming pressure port—seen in direction of the intended rotation of the rotational group—helps to avoid “fishtails” in the valve segment, as described above, as the next/coming pressure is guided back to the control port located in direction of rotation before on the valve segment. Hence pressure peaks or cavitation as described above can at least be reduced or even avoided, as the circumferential way/distance for a cylinder bore opening from the end of one pressure port to the beginning of a control pressure port with the other system pressure is shortened. For this, it is preferred by the invention that the circumferential extension of the cylinder bore opening is dimensioned such that the opening intersects on its way leaving a pressure port overlaps with both ports, the control port and the next coming pressure port at least partially. According to invention when the cylinder bore opening overlaps with the control port connected to the next system pressure, the pressure level in the cylinder bore can be adjusted/tuned/trimmed with the aim to reduce noise and vibrations caused by steps in the pressure profile. Preferably these small adjustments are done during constant tilt angle operation of the hydrostatic unit, wherein these small adjustments may not lead to a change in displacement volume, however the running behaviour of the hydrostatic unit is improved.
In one embodiment of the invention, the openings of the cylinder bores facing the valve segment show a kidney-shaped cross section. According to the prior art, cylinder bores often comprise round openings, the diameter of which being substantially equal to the radial extension of the kidney shaped pressure ports on the valve segment. If the openings of the cylinder bores comprise a kidney-shaped cross section and the longer dimension of the kidney-shape opening is oriented in circumferential direction, a bigger area is covered by the opening in circumferential direction compared to a round opening. Therefore, given the requirement that the opening of a cylinder bore shall be capable of simultaneously overlapping with a leaving pressure port and a coming control port, also the distance between a control port and its adjacent pressure ports can be increased, which improves the robustness of the valve segment.
Further preferred, the circumferential extension of the kidney-shaped openings of the cylinder bores is smaller than the circumferential distance between the adjacent ends of the first and second kidney-shaped pressure ports. Otherwise, there would be a rotational position of the cylinder bore, in which the cylinder bore could fluidly short circuit the first and the second pressure port.
According to the invention, an orifice with adjustable magnitude of opening (in the following just “adjustable orifice”) can be arranged in each of the two or more bypass lines, i.e., in the first bypass line and in the second bypass line as well as in potentially existing additional bypass lines. Alternatively, a non-adjustable orifice can be arranged in any of the bypass lines if this bypass line does not comprise an adjustable orifice. In consequence, either both the first and the second bypass line can comprise an adjustable orifice or only the first bypass line can comprise an adjustable orifice. In this case, the other, second bypass line preferably comprises a non-adjustable orifice, in order to provide a constant hydraulic resistance to the hydraulic flow in that bypass line. Depending on the selected arrangement, the tilt angle of the displacement element can be adjusted or set by influencing the ratio of the opening of the orifice in the first bypass line with respect to the opening of the orifice in the second bypass line, or by influencing the ratio between the opening of the adjustable orifice in one bypass line with respect to the opening of the non-adjustable orifice in the other bypass-line. In case of adjustable orifices, for example proportional flow valves, the pressure at the allocated control port can be adjusted variably, such that the pressure difference between the two control ports and consequently the force and kit moment situation on the valve segment is controllably influenced.
In one embodiment of the invention, one or two further, parallel bypass lines comprising an adjustable orifice, or a non-adjustable orifice can establish an additional 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. When providing two parallel connections between the same pressure port and the same control port allows splitting of the operating range of an adjustable orifice that is required for operating the hydraulic unit into two fractions is possible. The design of the orifices in the parallel bypass lines can be chosen accordingly. For example, one non-adjustable orifice providing a small, constant pressure drop could be combined with an adjustable orifice providing an adjustable pressure drop in addition to the constant pressure drop. This enables a cost efficient design of the orifices which are arranged in the bypass lines. In comparison to one single orifice with a wide operating range each operating range of parallel orifices can be reduced maintaining the total operating range of the orifice arrangement, as the control range of both orifices is summed up.
According to the invention, various arrangements of the bypass lines are covered by the present disclosure. For example, both control ports can be connected to the same pressure port via the first and second bypass lines. Additionally, each control port can be connected to the other pressure port via third and fourth bypass lines, wherein adjustable orifices can be arranged in each of the four bypass lines.
As known in the art, the displacement element can be biased into an initial position by means of an elastic force, in which the displacement volume of the rotational group is at maximum, minimum or at zero. The displacement element can also be biased into an initial position by means of an offset of the tilt axis of the displacement element with respect to the rotational axis of the cylinder block. As a result, the pistons on either side of the tilt axis comprise different lever arms with respect to the tilt axis. Even if a small pressure, e.g. charge pressure, is provided to the pressure ports/cylinder bores and the pressure force is the same on all pistons, the pistons generate a kit moment on the valve segment with respect to the tilt axis due to the different lever arms. Therefore, as soon as pressure is supplied to the cylinder bores, the displacement element starts to tilt and a pressure difference is present at the first and second pressure ports, which can be conducted to the IDC control port and to the ODC control port via the first and second bypass line.
According to the invention, the hydraulic axial piston unit can further comprise a return mechanism capable of generating a restoring force on the displacement element, when the displacement element is pivoted out of its initial position. The restoring force can generate a torque which is directed opposite to the resulting torque generated by the pressure forces at the IDC and the ODC control port. For example, the return mechanism can comprise an elastic component which provides a restoring force/torque which increases when the tilt angle of the displacement element increases. For every tilt angle of the displacement element, the equilibrium of moments between the pressure forces at the IDC control port and at the ODC control port and the restoring force of the return mechanism determines the movement of the displacement element and therewith the tilt angle of the displacement element. By arranging control ports on the valve segment circumferentially located between the pressure ports, the kit moments on the valve segment can be influenced/varied such that a restoring torque from the return mechanism can be traversed and the displacement element can be on-stroked, that is, its angle of tilt increases. Thus, for each angle of tilt a balance/equilibrium of forces/torques can be adjusted and the displacement volume of the hydrostatic unit can be controlled. So, in increasing or lowering the pressure difference present at the control ports the displacement element can be on-or de-stroked only by adjusting the opening size of the at least one variable orifice arranged in one or in both bypass lines connecting the control port with the pressure level coming next in direction of rotation.
The valve segment can be formed integrally with the housing of the hydraulic axial piston unit, or with an end cap of the hydraulic axial piston unit, or with a housing lid or with another component within the housing of hydraulic unit. Alternatively, the valve segment can be provided as a separate part.
The first pressure port at the valve segment can comprise more than one kidney-shaped pressure port opening, and/or the second pressure port of the valve segment can comprise more than one kidney-shaped pressure port opening. This can improve the mechanical stability of the valve segment in the region of the first and/or second pressure port, while negative impacts on the flow of hydraulic fluid to the cylinder bores which overlap with the first and/or second pressure port are minimised.
According to the invention, the opening of the orifice(s) can be controlled mechanically or by an electronic control unit (ECU). The electronic control unit can comprise a micro-controller and can be 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. The control unit can be capable of controlling the opening of the orifices based on measurements provided by the at least one sensor. For this purpose, the control unit can be capable of performing calculations, e.g. of calculating an error signal, and of adjusting the opening of the orifice(s), such that the error signal is reduced, e.g. by applying a PID control rule. The opening of the orifice(s) can be calculated based on the circumferential position of the first and second kidney shaped pressure ports, the circumferential position of the IDC and the ODC control ports, the diameter of the orifice(s), the restoring force of the return mechanism, the angular velocity of a shaft of the hydraulic unit, the pressure at the first and second kidney shaped pressure ports, the operating temperatures, and/or other parameters.
According to the invention, the adjustable orifice can be a rotary spool valve or a linear spool valve which is accommodated in a valve bore. The rotary or linear spool of the valve can comprise recesses or openings which overlap with channels in the valve bore, i.e. valve ports, wherein the magnitude of overlapping can be continuously adjusted by rotating the rotary spool or by longitudinal moving the linear spool. The orifice can be a linear operating valve/orifice, a rotary valve/orifice, or a flow valve. Flow valves are generally less costly than linear or rotationally operating orifices. Here, a person skilled in the relevant art will find plenty of solutions how to provide an adjustable orifice, i.e. an orifice whose magnitude of opening is adjustable.
As mentioned above, a hydraulic axial piston unit according to the invention can comprise two adjustable orifices, one provided in the first bypass line and another one provided in the second bypass line. The openings of the two adjustable orifices can be adjustable by means of a shared mechanism, which can be mechanical, electromechanical, hydraulic, or pneumatic. For example, a common, i.e. shared, spool can be provided, which serves as a shared valve spool for the adjustable orifice in the first bypass line as well as for the adjustable orifice in the second bypass line. Such a single spool can for example comprise one recess for adjusting the fluid flow in the first bypass line, and another recess for adjusting the fluid flow in the second bypass line.
According to the invention the shape of the control ports is relevant for achieving a good controllability of the tilt angle of the displacement element. A round/circular shape of the control ports requires low manufacturing effort and therefore represents a solution causing low costs. However, the shape of the control ports can also be adapted to the shape of the opening of the cylinder bores. Basically, the control ports can comprise any desired shape. For example, the control ports can show an elongated shape in circumferential direction of the valve segment with a radial extension that matches the radial extension of the cylinder bores. This design provides an increased overlap between the opening of the cylinder bore and the control port. The control port can also comprise a kidney shape, wherein the longer side of the kidney preferably extends in circumferential direction. The control port can also comprise an ellipse shape, a triangle shape, or any other shape, wherein even the manufacturing direction must not coincide with rotational axis of the valve segment.
Not only the shape of the IDC and the ODC control port is considered to be relevant for setting and adjusting the displacement of a hydraulic axial piston unit, but also the position of the IDC and the ODC control port on the valve segment. According to the invention, the IDC control port and/or the ODC control port can be located on the valve segment in circumferential direction with an angular offset to the rotational position at which the working pistons are at their inner dead centre and/or outer dead centre, respectively. This arrangement is often referred to as “indexing” and is especially preferable for hydraulic pumps. The specific location of the IDC and/or the ODC control port is selected based on the type of use of the hydraulic unit and the requirements derived therefrom. In one exemplary embodiment, the IDC control port and/or the ODC control port can be located clockwise slightly behind the actual rotational/circumferential position of the IDC or the ODC.
As a result, to continue the example of a hydraulic pump, the pressure at the ODC control port influences the motion of the working pistons when the pistons are entering the cylinder bore, that is at the start of the pressure phase, and the pressure at the IDC control port influences the motion of the piston, when the piston is withdrawn from the cylinder bore, i.e. at the start of the suction phase. In an exemplary case, the ODC port can be connected to the high pressure line, wherein the IDC port can be connected to the low pressure line. Increasing the opening of an orifice in the bypass line connected to the ODC control port generates a higher pressure in the pressure chamber which is enclosed by the working piston in the cylinder bore. As the higher pressure has to be supported by the displacement element, the tilt angle of the displacement element is increased. Increasing the opening size of an orifice arranged in that bypass line which is connected to the IDC control port reduces the hydraulic resistance which has to be overcome when hydraulic fluid is sucked into the above mentioned pressure chamber when passing the IDC control port. The resulting reaction force acting on the displacement element in the area of the inner dead centre (IDC) is reduced and the tilt angle of the displacement element is increased.
In another embodiment of the invention, the IDC control port and/or the ODC control port can be located on the valve segment exactly at that rotational position at which the working pistons are at their inner dead centre (IDC) and/or their outer dead centre (ODC), respectively. This arrangement can for example be preferable for hydraulic units, especially hydraulic motors, which are operated with changing directions of fluid flow, but whose displacement element can be tilted only in one direction. As the algebraic sign of the tilt angle does not change, the rotational position of the inner dead centre and the outer dead centre remains the same, even if the direction of fluid flow is changed. However, when the direction of the fluid flow is inverted and the direction of tilt remains constant, the direction of rotation of a hydraulic motor is inverted. If the IDC pressure port and the ODC pressure port on the valve segment does not coincide with the IDC or the ODC but shows an angular offset to the IDC and to the ODC position, the behaviour of the hydraulic unit would be different depending on the rotational direction of the cylinder block, as-considering exemplarily only one of the control ports-the control port would be in one direction of rotation before the respective dead centre position and in the other rotational direction after the respective dead centre position.
In another embodiment of the invention, a first ODC control port can be located on the valve segment with an angular offset to the rotational position at the valve segment at which the working pistons are at their outer dead centre, and a second ODC control port can located on the valve segment such that the first and second ODC control ports are located on both sides of the rotational position on the valve segment, which corresponds to the outer dead centre position of the working pistons. This arrangement increases the options for controlling the tilt angle of a displacement element of a hydraulic unit, as the pressure in a cylinder bore can be influenced before reaching as well as after leaving the outer dead centre position, i.e. when the working piston is moving outwards, as well as after leaving the inner dead centre, i.e. when the working piston is moving inwards. This increases the angular range within which the tilt angle can be influenced by the displacement control according to the invention.
Similar to the above mentioned embodiment, according to the invention, a second IDC control port can be located on the valve segment. If the first IDC control port is located on the valve segment with an angular offset to the rotational position at the valve plate at which the working pistons are at their inner dead centre the first and second IDC control ports can be located on both sides of the rotational position on the valve plate which corresponds to the inner dead centre position of the working pistons. This can be done either as an additional feature or as an alternative to providing a second ODC control port on the valve segment.
Providing a second IDC control port as well as a second ODC control port can be especially useful when the direction of fluid flow which is conveyed/supplied by another hydrostatic unit can be inverted. In this case, the rotational direction of the cylinder block and therewith the direction in which the cylinder bores move from to the ODC to the IDC interchanges. When four control ports are arranged symmetrically on the valve plate/segment, the control possibilities remain the same regardless of the direction of fluid flow.
According to the invention, the second ODC control port and/or the second IDC control port can correspondingly be connected to a fourth bypass line and/or to a third 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. Providing additional orifices with adjustable openings in separate, additional bypass lines increases the possibilities of adjusting the pressure ratio at the control ports and enhances the possibilities of influencing/controlling/adapting the pressure profile of hydraulic pressure acting on a working piston during one revolution of the cylinder block, and thus further enhances the controllability of the hydraulic unit.
A hydraulic axial piston unit according to the invention can be operated in an open hydraulic circuit or a closed hydraulic circuit. The hydraulic unit can be operated as hydraulic motor or as hydraulic pump. A person with skills in the relevant art is aware of the fact that a hydraulic pump which is arranged in an open hydraulic circuit often comprises only one rotational direction of the cylinder block and consequently shows only one conveying direction. A hydraulic pump arranged in a closed hydraulic circuit typically comprises only one rotational direction, wherein the displacement element of the hydraulic pump can be tilted bidirectional, such that hydraulic fluid can be conveyed in two/both directions through the closed hydraulic circuit. In many embodiments, the cylinder block of a hydraulic motor in a closed hydraulic circuit can be rotated in two directions depending on which of the pressure ports of the hydraulic motor is connected to higher system pressure and which is connected to lower system pressure. In most applications according to the state of the art, the tilt angle of a hydraulic motor in a closed circuit can only be stroked/adjusted in one direction.
When a hydraulic unit according to the invention is operated in a closed hydraulic circuit and the fluid flow direction is inverted, the pressure at the first and second pressure ports and thus the pressure in the first bypass line and in the second bypass line is changed also. However, depending on the type of use, it can be preferred to always have the same kind of system pressure present in the first bypass line and at he connected control port (e.g. always high system pressure at the IDC control port), as well as in the second bypass line and at the connected control port (e.g. always low system pressure at the ODC control port). This is particularly preferred for hydraulic motors whose displacement element is tiltable only in one direction and whose outer dead centre is not interchanged with its inner dead centre during operation of the hydraulic unit neither in the one nor the other rotational direction.
In this case, the hydraulic unit can comprise a component capable of always conducting one pressure level to the IDC control port and the other pressure level to the ODC control port regardless of the direction of rotation of the hydraulic unit. This function can e.g. be fulfilled by a shuttle valve having two inlets and one outlet, wherein the inlets of the shuttle valve are in fluid connection with the first and second pressure ports and the outlet is in fluid connection with the IDC control port, the ODC control port, or with a control valve (whose functionality is explained later) or a similar device. E.g. for a hydraulic motor the outlet of the shuttle valve can be connected to the IDC control port. According to this arrangement, the shuttle valve is capable of conducting the higher system pressure from the first or second pressure ports to the IDC control port, or to the ODC control port; or to the control valve. E.g., for a hydraulic motor the IDC control port is connected to a higher pressure, e.g. inlet pressure, and the ODC control port can be connected in this case to a lower pressure, e.g. outlet pressure or to a hydraulic reservoir. In consequence, the one-directional tilt angle of the displacement element can be controlled independently of the direction of rotation of the hydraulic unit, here exemplarily a hydraulic motor. According to the invention, the opening of an orifice in the second bypass line which could—for example—be connected to the ODC control port, could be adjusted in order to influence the tilt angle of the hydraulic unit. Additionally or alternatively, the opening of an orifice in the first bypass line connected to the IDC control port could be adjusted and therewith the pressure level present at the IDC control port can be controlled. This influences the profile of pressures acting on a working piston during one revolution around the cylinder block axis, at least until the working pistons reaches the other control port.
If the displacement element of a hydraulic unit can be tilted in two directions, the IDC and the ODC can be interchanged depending on the direction of tilt of the displacement element. According to the invention a control valve or a similar device can be provided capable of guiding high system pressure to the ODC control port and low pressure to the IDC control port, or vice versa. The control valve may comprise a first inlet connected to the outlet of a shuttle valve and a second inlet connected to a hydraulic reservoir. In this way, the first inlet of the control valve is connected to a high pressure level and the second inlet is connected to a lower pressure level, wherein a first outlet of the control valve can be connected to the IDC control port or the ODC control port, and a second outlet can be connected to the respective other control port. The control valve is capable of selectively connecting the first inlet with the first outlet and the second inlet with the second outlet or connecting the first inlet with the second outlet and the second inlet with the first outlet or short-circuiting the first outlet with the second outlet. Because of this, and regardless of the actual rotational position of the ODC and the IDC, the higher pressure can always be conducted to the ODC control port (for a hydraulic pump) or to the IDC control port (for a hydraulic motor), and the lower pressure can always be conducted to the respective other control port.
In order to facilitate the start-up of a hydraulic unit according to the invention, the hydraulic unit can comprise an (additional) charge pump capable of providing an initial start-up pressure which can be conducted to one of the control ports, e.g. by a valve arrangement via one of the bypass lines. For such a start-up the charge pressure provided by the charge pump is sufficient to initially tilt the displacement element towards a start position in which a small pressure difference is generated between the first and second pressure ports. Via the bypass lines this pressure difference is conducted via the bypass lines to the IDC control port and to the ODC control port and can be used to initially tilt the displacement element about a small angle. From thereon, hydraulic fluid flow from the charge pump to one of the control ports is no longer necessary, in particular when the high system pressure at one of the pressure ports exceeds the charge pressure, the pressure difference generated by the started hydraulic unit is sufficient to control the tilt angle by means of the least one variably adjustable orifice.
According to the invention, at least one of the adjustable orifices can provide a pressure and/or a displacement feedback signal. Displacement or pressure feedback means, that the hydraulic displacement unit comprises a feedback loop in order to mechanically, electrically, or hydraulically transmit the pressure level in the cylinder bores or at the control ports or the tilt angle of the displacement element to the adjustable orifice, or from the adjustable orifice to a control unit of the hydraulic system. According to the invention, a mechanical feedback or an electronic feedback signal could be provided by the orifices to an electronic control unit. Additionally or alternatively, the opening of the at least one adjustable orifice can be influenced by the magnitude of the feedback signal. For example, when the tilt angle of the displacement element is increased, and the increased tilt angle is sent either directly as feedback signal to the adjustable orifice or as feedback signal to an electronic control unit controlling the opening of the adjustable orifice, the opening of the adjustable orifice can be reduced, in order to limit or to stop the movement of the displacement element. Thus the pressure in the cylinder bores and the tilt angle of the displacement element can be controlled by means of the adjustable orifices in the bypass lines. As a result, a servo piston/servo unit according to the state of the art can be omitted, at least in some cases.
According to the invention, the control ports can pass through the valve segment perpendicular to its front surfaces. of. However, the control ports can also pass through in a direction inclined with respect to a rotational axis of the valve segment or the hydraulic axial piston unit, respectively. This means, that the orientation and position of the hydraulic bypass lines in which the adjustable or non-adjustable orifices are arranged can be chosen depending on space restrictions imposed by the design of the components of the hydrostatic unit surrounding the rotating group, e.g. For example, the control ports can be drilled, and the drilling direction can be oriented perpendicular to the front surface of the valve segment or can be oriented with an angle to the front surface different to the perpendicular to valve segment plane.
In one embodiment, the radial position of the control ports can deviate from the pitch diameter defined by the circumferential extension of the first and second pressure ports. In other words, the radial distance of the control ports to the rotational axis of the hydraulic unit must not be equal to the pitch radius of the pressure ports.
According to the invention, a method for controlling the displacement volume of a hydraulic rotating group driving or being driven by a driving shaft is provided further. The hydraulic rotational group comprises a displacement element which can be tilted in order to adjust the displacement volume of the rotating group. The rotating group comprises a rotatable cylinder block with cylinder bores in which working pistons are mounted reciprocally moveable, and a valve segment with a kidney-shaped first pressure port and with a kidney-shaped second pressure port. 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. 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 centre (IDC) or is at or close to its outer dead centre (ODC), respectively. 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.
The method for controlling the displacement volume of a hydraulic rotating group comprises the following steps:
The pressure in the cylinder bores can be increased when hydraulic fluid under high pressure is supplied to the passing cylinder bores at the ODC control port (for hydraulic pumps) or at the IDC control port (for hydraulic motors). Due to the higher pressure the force on the working piston which seals the cylinder bores increases. This increased force is transferred by the piston to the displacement element and is supported there. According to the principle “actio=reactio”, the supporting force influences the balance of forces and torques present at the displacement element. When high pressure is supplied at the ODC control port to the passing cylinder bore an increased tilting force acting on the displacement element is generated. If the tilting (kit) moments at the displacement element are higher than the resetting forces/moments which force the displacement element back to its initial or neutral position, the tilt angle of the displacement element is increased.
In the exemplary case of a hydraulic pump, increasing the opening of an adjustable orifice in the bypass line which connects the pressure ports with the higher system pressure to the ODC control port, leads to a higher pressure at the ODC control port and in the passing cylinder bore, therewith causing a bigger tilt angle. In short, opening of an adjustable orifice in this bypass line increases the tilt angle of the displacement element. The other way round, closing or reducing the size of the adjustable orifice in the bypass line decreases the tilt angle of the displacement element as the force on the working pistons passing the ODC control port decreases.
At the IDC control port, there is a similar/analogous situation at low system pressure level. To continue the case of a pump, an increasing pressure in the cylinder bore passing the IDC control port leads due to the aforementioned transmission of forces to a neutralizing/de-stroking moment which is capable of decreasing the tilt angle of the displacement element. Therefore, the higher the pressure in the cylinder bore passing the IDC control port, the more neutralizing/de-stroking moment is generated that de-strokes/tilts the displacement element back towards zero displacement. In contrast to that when the pressure in the cylinder bore passing the IDC is lowered, the tilt angle of the displacement element can be increased. The pressure at the IDC control port can be influenced by an adjustable orifice arranged in the connected bypass line. Preferably for a hydraulic pump, the bypass line connected to the IDC control port is connected to a low (system) pressure level. An orifice with an adjustable opening size can be arranged in the bypass line. Therewith the flow resistance or a backpressure in the bypass line connecting the IDC control port with the low pressure port can be adjusted. As a result, increasing the opening size of a variable orifice in the bypass line can increase the angle of tilt of the displacement element.
According to the invention supply or drain of hydraulic fluid is done with the pressure level of the subsequent/next coming pressure port-seen in direction of rotation of the cylinder block or the rotating group. For instance, in a hydraulic pump the pressure level at the IDC is changing from the high system pressure to the low system pressure such that according to the invention, the IDC control port located between the two pressure kidneys at, or nearby IDC is preferably connected via a bypass line to the low system pressure kidney. Optionally an adjustable orifice is arranged in this bypass line, in particular for hydraulic pumps operable with positive as well as negative tilt angles, as in this case the IDC changes with the ODC by tilting the displacement element over zero, and vice versa, such that the control port can be the IDC control port as well as the ODC control port.
To summarize, according to the invention, controlling the pressure in the cylinder bores passing the control port at the ODC, and in the cylinder bores passing the control port at the IDC can make an additional servo system for adjusting the tilt angle of the displacement element superfluous, as the tilt angle of the displacement element of the hydraulic unit can be adjusted by means of a controlled opening or closing of variable orifices provided in the first and/or second bypass lines connected to the ODC and IDC control ports for changing in a controlled way the pressure levels in the cylinder bores passing the control ports.
The invention is also applicable to hydraulic units equipped with a servo system for supporting the adjustment of the displacement volume and/or for tuning and/or balancing the running behaviour of the hydraulic unit as pressure steps during change of pressure in the cylinder bores from the high system pressure to the low system pressure and vice versa. These pressure transitions can be smoothened by the arrangement of control ports on the valve segment, through which a preferable variably adjustable pressure can be supplied via bypass lines connected to the two system pressure ports. For this, preferably at least one adjustable orifice is arranged in one of the two bypass lines. Thereby it is further preferred to arrange two or more bypass lines in such manner that for each rotational direction the next coming system pressure level can be guided back to the control port laying on the valve segment in rotational direction before the pressure port to with the bypass line is connected to.
The method according to the invention may further comprise the step of processing a command of a control unit or an operator by means of an electronic control unit (ECU). The electronic control unit can comprise a microcontroller for adjusting the size of the openings of the orifices in the first bypass line and/or in the second bypass line, in order to adapt/control the pressure in the cylinder bores for controlling the displacement volume of the hydraulic axial piston unit. According to the invention, any of the adjustable orifices can be controlled by the electronic control unit, regardless of whether the bypass line in which the adjustable orifice is arranged, connects one of the control ports with one of the pressure ports, or with a hydraulic reservoir, or with a pressure compensation chamber or closed cavity.
The method according to the invention may further comprise the step of sensing of at least one operational parameter of the hydraulic axial piston unit by means of a sensor. The sensor can be 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.
According to the invention, the method may further comprise the step of continuously monitoring the operational parameters of the hydraulic axial piston unit in order to smoothen pressure transitions between the kidney-shaped first and second pressure ports and vice versa, and/or for controlling the pressure level in the cylinder bores and thus the pressure profile in the cylinder bores on its way around the rotational axis of the hydraulic axial piston unit, i.e. the course of pressure in dependency of the rotational angle of the cylinder block. The method according to the invention further enables adjusting the tilt angle of the displacement element by controlling the pressure level present at the control ports by means of opening and closing the opening size of an adjustable orifice. For this purpose, measured operational parameters of the hydraulic unit can be processed by the electronic control unit.
With the help of the enclosed Figures preferred embodiments of a hydraulic axial piston unit according to the invention are explained in more detail in order to enhance the understanding of the basic idea of the invention. The present embodiments do not limit the scope of the idea of the invention, but only represent possible design alternatives, to which within the knowledge of a person with skills in the relevant art modifications can be made without leaving the scope of the invention. Therefore all those modifications and changes are covered by the claimed invention. In the Figures it is shown in:
In the Figures same reference numerals are used for same components of different embodiments throughout the description to improve readability.
In the embodiment according to
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.
The working pistons 6 (c.f.
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.
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 γo/γi 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 γo/γi. Depending on the algebraic sign and the magnitude of the angles γo/γi, 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 arranged—seen in rotational direction of the pump—behind the respective IDC or ODC rotational positions. According to
The openings of the cylinder bores 5 facing towards the valve segment 20—illustrated with dashed lines in the Figures—comprise a kidney shape, e.g., with a circumferential extension τ 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 σo/σi and the circumferential distance between the second pressure port 22 and the position of the ODC/IDC βo/βi. If the extension τ would be larger than the sum of σo+βo or the sum of σi+βi, the first pressure port 21 could be hydraulically short-circuited to the second pressure port 22 via the cylinder bore 5.
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
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.
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.
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
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
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).
In the operating state of the control valve 40 shown with
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.
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.
The outlets of the pressure operated valve 57 are connected to inlets of a start-up valve 59, which in the embodiment of
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 started—no 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.
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
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102022107860.4 | Apr 2022 | DE | national |
This application is a National Stage application of International Patent Application No. PCT/IB2023/020007, filed on Feb. 10, 2023, which claims priority to German Patent Application No. 102022107860.4, filed Apr. 1, 2022, each of which is hereby incorporated by reference in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2023/020007 | 2/10/2023 | WO |
