Patent Application
20200040867
The present invention relates to hydraulic piston units having a displacement element for adjusting the displacement volume of the hydraulic unit. The invention relates in particular to hydraulic axial piston units of the swashplate and the bent axis construction type. However, the invention is applicable also on hydraulic radial piston units or vane pumps and motors. In the sense of the invention the term hydraulic piston unit covers hydraulic piston pumps as well as hydraulic piston motors. In a particular embodiment the invention relates to two position hydraulic axial piston units in which the displacement element can be positioned between two end positions for adjusting the displacement volume of the hydraulic axial piston unit to a maximum or minimum. Even though the invention is described with example of axial piston units in concrete with axial piston units of the swash plate construction type, the given examples are transferable by a person with skills in the relevant to all above mentioned types of hydraulic piston units.
In the state of the art, the displacement volume of hydraulic piston units is usually adjusted by the help 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 the rotational group inside the hydraulic piston unit. Such a servo unit constitutes a separate assembly group, especially in two position hydraulic piston units, which causes increased costs and consumes construction space. Furthermore, such a servo unit is subjected to wear or other possible failures that may lead to inoperability of the whole hydraulic piston unit. Furthermore, every cylinder piston unit shows a certain leakage and, therefore, so does such a servo unit as well.
Therefore it is object of the present disclosure to simplify adjusting and control means of the displacement volume of hydraulic piston units in order to overcome the deficiencies of state of the art. Thereby the inventive hydraulic piston units should comprise fewer parts for carrying out control of the displacement volume. The hydraulic piston unit should be easier and more economic to manufacture, in particular more simple in assembling the same.
The object of the invention is solved by a hydraulic piston unit having a rotational group for driving or being driven by a driving shaft. The hydraulic piston unit further comprise a displacement element being tiltable with respect to a rotational axis of the driving shaft between a first end position and a second end position. According to the angle of tilt of the displacement unit the displacement volume of the rotational group is adjusted between a maximum or a minimum displacement volume. Within a cylinder block working pistons are mounted reciprocally movable in cylinder bores for conveying hydraulic fluid from a kidney-shaped inlet port to a kidney-shaped outlet port of a valve segment. In the sense of the invention the rotational group comprises at least the cylinder block with the reciprocally movable working pistons therein as well as the driving or driven shaft and, at the lower end of the cylinder block, a valve segment with the before mentioned kidney-shaped inlet and outlet port. In case of a hydraulic piston unit of the swashplate construction type the swashplate also forms part of the rotational group as the piston heads are commonly fixed on the swashplate by the help of piston shoes. However, these piston shoes are designed to slide on the swashplate surface facing the cylinder block. Further, in case of these hydraulic piston units the swashplate and the valve segment are located on opposing sides of the cylinder block. The analogous is valid for hydraulic piston units of the bent axis construction type; however, in this case, the piston heads are fixed to the driving shaft in ball joints directly in order to allow the cylinder block to be bent with respect to the rotational axis of the driving shaft.
The inventive hydraulic piston unit includes, on the valve segment between the inlet port and the outlet port at the respective dead end positions of the reciprocally moveable working pistons, a first control port and a second control port are located with fluid connections with the cylinder bores. Such that the hydraulic fluid pressure difference between these two control ports and the hydraulic fluid pressure in the cylinder bores determines the tilt angle of the displacement element and finally the displacement volume of the hydraulic piston unit.
The inventive hydraulic piston unit further comprises a control valve with a shiftable control valve spool. The control valve is connected via a high pressure inlet port to the high pressure side of the hydraulic piston unit and is capable to conduct hydraulic fluid from the high pressure side to one of the first and second control port at the valve segment. Therewith one control port is fed at one dead end position of the working pistons with hydraulic fluid under high pressure, on the other control port at the other dead end position of the movement of the working pistons low pressure is present, for example tank or casing pressure. According to the invention high pressure acts on the working piston passing over the control port intending to push the working piston in direction outwardly of the cylinder bore. This forces the displacement element located at the opposite end of the cylinder block into either a deflected or perpendicular orientation with respect to the rotational axis of the cylinder block, thereby defining the displacement volume. Herewith, according to the invention, the working pistons are fulfilling the function of moving the displacement element, which in hydraulic piston units of the state of the art is done by via the above mentioned additional servo unit.
As it is common sense in the art for hydraulic piston units the displacement element is hold at least in its initial first end position preferably by an elastic force, preferably caused by a spring or the like, against a stop. In other angle positions can be obtained by a force balance between the deflecting forces—here performed by the working pistons as they pass the control ports at the respective upper and lower dead ends—and the elastic force acting in direction towards the initial first position. In particular with two position hydraulic piston units a second end position of the displacement element is provided by a second stop, against which the displacement element can abut in its maximum or minimum deflection/tilt angle. It should be readily understood that the displacement element can be positioned also in intermediate positions between the maximum and minimum angle of tilt, for instance by means of elastic or hydraulic forces varying with the angle of tilt.
For example, in one embodiment in which the initially set displacement volume of the hydraulic piston unit is at its minimum, i.e. the displacement element in its neutral or zero position and the tilt angle of the displacement element is equal to zero, the displacement element can be held in this position as long as the control spool of the control valve is not shifted out of its initial position, in which hydraulic fluid and high-pressure is guided to the first control port. In this exemplary case the displacement volume of the hydraulic piston unit increases with the displacement of the control valve spool as the pressure conducted to the first control port on the valve segment is decreases and the pressure guided to the second control port increases with displacing the control valve spool. This will continue until hydraulic fluid under high-pressure is guided only to the second control port. In that moment, for this embodiment, the displacement element is at its maximum displacement, i.e. at its maximum angle of tilt. However, it should be readily understood that this example can be carried out without difficulties also the other way around.
In essentially all hydraulic piston unit applications know in the art, in which a servo unit is used for adjusting the displacement volume, the inventive hydraulic piston unit can be applied instead. Hence all these possibilities are covered by the present disclosure too. However, in difference to the state of the art solutions, the invention provides for hydraulic piston units with fewer parts, as a servo unit is no longer necessary, and provides for the same reason for smaller hydraulic piston units which consume less construction space. Thereby the control of the displacement volume can be done merely by controlling the position of the control valve spool in the control valve which can be in a simple embodiment a 3-way/2-position valve.
As the invention only effects the manner in which the displacement element is positioned and controlled in its positioning the hydraulic piston units according to the invention can be operated in motor mode as well as in pump mode, including the associated propel and drag modes in case the inventive hydraulic piston unit is applied to a closed hydraulic circuit.
In a basic embodiment the inventive hydraulic piston unit is of the two position construction type, i.e. only minimum displacement and maximum displacement is selectable. In this case the control valve is also only a two position valve which either conducts hydraulic fluid to the first control port or to the second control port. In this case by switching the control valve from a first position to a second position and vice versa the hydraulic piston unit changes its displacement volume from maximum to minimum and vice versa. It should be readily understood that the control valve spool as well as the hydraulic piston unit can be adjusted in intermediate position too and over “zero” displacements of the displacement element are possible as well. Such that a maximum displacement can be as big in absolute value as a minimum displacement only the leading sign changes, i.e. the conveying direction or the propel mode changes to drag mode.
Therefore, according to the invention, the control valve can be a two position valve as well as a proportional valve. In case the control valve is a proportional valve the displacement of the rotational group is adjustable proportionally as well. Here the control of the control valve can be done preferably by actuator means commanded by a control unit. The control unit itself, for instance, monitors system parameters or receives inputs from an operator of a hydraulic propel application and send a corresponding signal to the actuator means in case the angle of tilt of the displacement element has to be changed or maintained, e.g. when a value for a system parameter is exceeded or underrun. Thereby the actuator means shift the control valve spool in an electro-mechanical, hydraulic, pneumatic way or in a combination of these possibilities. Examples for such electro-mechanical, hydraulic, or pneumatic means are known in the art. For example springs, solenoids, cylinder-piston—units, levers, electric or thermic relays and the like are exemplarily of such actuation means to actuate the control valve spool in order to maintain or change its position. Therefore all these common art possibilities of actuation on the control valve spool in combination with the inventive guidance of pressurized hydraulic fluid to the first and the second control port in the valve segment are covered by the present disclosure.
Furthermore, all these means for shifting the control valve spool can be controlled by means of a control unit which preferably is capable to command the before mentioned actuator means to act on the control valve spool. Thereby the control means can be a CPU, a microprocessor or the like which sends a corresponding signal to the actuator means for changing or holding the position of the shiftable control valve spool. Naturally the control unit receives input information from other components and/or, for instance, from the operator of a hydraulically driven vehicle, and transmits a corresponding signal to the actuator means. In another embodiment of the invention the control unit supervises system parameters and commands the actuator means to hold or to shift the control valve spool in case a system parameter is exceeded or underrun.
As mention already above the inventive hydraulic piston unit is applicable to open hydraulic circuits as well as to close hydraulic circuits with to conveying directions and/or the capability to operate in propel mode or drag mode. In case of implementing the inventive hydraulic piston unit in such a closed hydraulic circuit the inventive hydraulic piston unit preferably comprises a high pressure selecting valve which is fluidly connected to both pressure sides of the hydraulic piston unit and which is capable to supply hydraulic fluid from the high pressure side to a high pressure inlet port of the control valve. Such a high pressure selecting valve in a basic embodiment is in the form of a double check valve having two inlets and one outlet, wherein the inlet of the lower pressure side is closed by the check valve ball under the action of the high pressure. Such that in any operational condition it is ensured that hydraulic fluid under high-pressure is guided to the high-pressure inlet port of the control valve.
In a further preferred embodiment of the invention the control valve is also capable to conduct hydraulic fluid under low or lower pressure to the respective other control port on the valve segment which is not charged with hydraulic fluid under high pressure. Thereby the control valve is connected via a low pressure inlet port to the low pressure side of the hydraulic piston unit or to a charge pressure source, e.g. in form of a charge pump, or to another system pressure being lower than the high pressure level. In case the hydraulic piston unit is in implemented into a closed hydraulic circuit the control valve is fluidly connected to both pressure lines of the hydraulic circuit by means of a low pressure selecting valve, in order to conduct properly in both conveying directions hydraulic fluid under a lower pressure to the respective other control port not charged with high-pressure. This low pressure selecting valve can be performed, for instance, by a shifting valve with a shifting valve spool referenced on one side to the high pressure level and on the opposite side to a lower pressure level, which do not have to be the pressure level which is forwarded by the low pressure selecting valve.
In summary, with the inventive hydraulic piston unit a control method for adjusting the displacement volume at least of a two position hydraulic unit can be performed in a simplified manner, wherein 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 second control port located on the valve segment of the hydraulic piston unit at the respective dead end positions of the reciprocating working pistons, if the initial displacement volume of the inventive hydraulic piston unit is to be maintained, and the control valve spool is shifted by means of actuator means into a shifted position for supplying hydraulic fluid under high pressure to the respective other control port, if the initial displacement volume of the two position hydraulic unit has to be changed. As mentioned above the inventive control method is performed in a basic embodiment on two position hydraulic piston units, however it is applicable also on proportional adjustable hydraulic piston units in order to change the displacement volume of the rotational group proportionally.
Further preferably the control of the displacement volume of the rotational group of the inventive hydraulic piston unit is controlled via a control unit capable to command actuator means, which act on the control valve spool of the control valve. This control valve can be of the two position type or a proportional valve.
The inventive method for controlling the displacement volume of a hydraulic piston unit can also include the control of guidance of hydraulic fluid under low pressure which is conducted to the respective other control port on the valve segment not being charged by hydraulic fluid under high pressure. The inventive control method is applicable on hydraulic piston units in an open circuit as well as on hydraulic piston unit in a closed hydraulic circuit, where the inventive hydraulic piston unit can be operated in both rotational directions as well as in propel mode or in drag mode.
Exemplary embodiments of the inventive hydraulic piston unit according to the present disclosure are shown in more detail in the enclosed drawings which do not limit the scope of the present disclosure. All features of the disclosed and illustrated embodiments may be combined in any desired combination with one another within the scope of the present invention. For this purpose it is shown in:
In the
The hydraulic axial piston unit 1 shown in
In the exemplary embodiment of
For example, the hydrostatic piston unit 1 in
In the embodiment of
In the operational state of the hydraulic piston motor 1 according to
A switched position of control valve 10 is shown in
However, it should be readily understood that the swashplate 4, rather than abutting against stop 33, may instead be tilted to negative angles with respect to the position of
As can be seen from
Additionally
In summary, with the inventive hydraulic piston unit of the present disclosure, a smaller hydraulic piston unit with a reliable displacement volume control is provided, which eliminates the need of a (external) servo unit. This not only saves costs but also renders the hydraulic piston unit less complex and more reliable as it comprises fewer parts. Also leakage is reduced significantly as less parts are subjected to hydraulic pressure.
