Hydraulic system for supplying pressure to a hydraulic actuator

Abstract

Hydraulic system (10) for supplying pressure to a hydraulic actuator (1, 2), comprising a double-acting hydraulic cylinder (20) with a first and a second pressure chamber (21, 22) which can be pressurized via a hydraulic pump (12), and a rapid traverse device which is configured to hydraulically connect the two pressure chambers (21, 22) to each other in a rapid traverse mode, so that hydraulic fluid displaced from one pressure chamber (22) can flow into the other pressure chamber (21), and to hydraulically separate the two pressure chambers (21, 22) from each other in a normal traverse mode, characterized in that the rapid traverse device is integrated into a valve unit (50) of the hydraulic system (10) which is connected to the two pressure chambers (21, 22) via a first and a second port (61, 62) and has a third port (63) which can be pressurized via the hydraulic pump (12),wherein the valve unit (50) comprises a slidably mounted switching piston (52) which, in a normal operating position, hydraulically separates the first and second ports (61, 62) from each other and, in a rapid traverse position, hydraulically connects the first and second ports (61, 62) to each other and separates them from the third port (63), wherein a pre-tensioning device with a switchable pre-tensioning element (54) is further integrated into the valve unit (50), which is configured, in a blocking position, to separate the third port (63) from the second port (62) and thereby shut off the pressure chamber (22) connected to the second port (62) to the outside.

Description

The present invention relates to a hydraulic system according to the preamble of claim 1, as well as a valve unit and a working device, in particular a mobile crane, with such a system. Piston-cylinder units comprise a cylinder housing and a piston with a piston rod, which is slidably mounted within it. In the case of a double-acting piston-cylinder unit, there are pressure chambers on both sides of the piston, so that the piston rod is extended or retracted depending on the pressure applied to one or the other pressure chamber. A pressure chamber through which a piston rod passes is also called an annular chamber or annular space due to the ring-shaped piston surface; a pressure chamber without a piston rod passing through it can be called a piston chamber or piston space. Double-acting piston-cylinder units whose piston has a piston rod on only one side are called differential cylinders. One application for such piston-cylinder units is the telescoping cylinders of telescopic booms on well-known mobile cranes. These telescopic booms comprise an outer telescopic section and one or more inner telescopic sections slidably mounted within it. Particularly in larger telescopic booms, a single telescoping cylinder, often in the form of a hydraulic differential cylinder, is frequently used to extend and retract the telescopic sections, extending and retracting the inner sections sequentially. For this purpose, one part of the telescoping cylinder, typically the piston rod, is connected to the base of the outer telescopic section, while the other part, typically the cylinder housing, extends and retracts relative to the outer telescopic section by pressurizing the respective pressure chamber. The corresponding hydraulic lines are usually routed through the piston rod to the pressure chambers. To move the individual telescopic sections, the telescoping cylinder must be temporarily connected to them. For this purpose, a locking device (so-called locking head) is usually provided on the telescoping cylinder, particularly at the piston rod end of the housing or at the collar. This locking head engages several spring-loaded drive pins from the inside of the innermost telescopic section to be extended, so that the telescopic section extends together with the telescoping cylinder. The individual telescopic sections can also be locked to each other in defined extension positions by means of spring-loaded locking pins on the telescoping sections. To unlock or remove the locking pins, they can typically be gripped by the locking device and moved into an unlocked position.For this purpose, the locking device usually has a spring-returned yoke which can be engaged by means of pull mushrooms from rods of the locking bolts that project inwards into the telescopic sections. Both the drive pins and the yoke are pre-tensioned into their locking positions by spring elements and can be retracted into their unlocked positions by hydraulic energy against the spring force of the return springs. The necessary hydraulic supply can be provided via a pipe feedthrough integrated into the piston rod of the telescoping cylinder. This feedthrough can comprise two feedthrough tubes that are slidably mounted inside one another, sealed against each other, and extend telescoping together with the cylinder. When the telescoping cylinder is extended, for example, an inner feedthrough tube extends along with it and follows the telescoping cylinder's movement. An outer feedthrough tube with a larger diameter can be fixed to the piston rod. At the cylinder end of the telescoping cylinder, the supply line is routed to the outside and back to the locking device. A known problem with this arrangement is that, when the bolt is open (i.e., in the unlocked positions), the pressure in the pipe penetration can, in certain situations, cause the telescoping cylinder to extend unintentionally, for example, in shallow boom positions, with low friction, and / or under light load. To prevent this unwanted movement, one of the pressure chambers (usually the ring side) can be pre-tensioned with a pressure that counteracts the pressure in the pipe penetration and thus prevents extension. This pre-tension should be switchable, as otherwise unnecessarily high power losses would occur at the pre-tension during regular extension and retraction. The control block required for this adds cost and weight, requires installation space, and necessitates corresponding wiring. Furthermore, the required telescoping time, especially during the idle stroke, is a disruptive factor with such telescoping cylinders. The goal is always to minimize the time required for extending and retracting. The idle stroke of the telescoping cylinder, which can occur during both extension and retraction, is particularly noticeable because no movement is visible to the crane operator (for example, the telescoping cylinder retracts within the boom to prepare for the next telescoping section). Due to the difference in piston and ring area, a difference arises: extending the telescoping cylinder is significantly slower than retracting it, as it takes longer to fill the larger piston chamber with the same oil flow. To reduce extension times, the speed of the telescoping cylinder piston can be increased. Higher speeds require a greater oil flow to the piston side. This can be achieved with a larger pump or, if possible, by increasing the drive speed. Both options have the disadvantages of increased costs and / or weight, as well as increased noise and greater flow losses. Another possibility is to reduce the piston area, but this leads to a loss of payload capacity if the pressure cannot be increased further. Another solution involves connecting the ring and piston sides via a rapid traverse mechanism during cylinder extension. This directs the oil flowing from the ring side back to the piston side for extension, thereby increasing the oil flow to the piston side and consequently the extension speed. The available cylinder force during extension is reduced proportionally to the piston area relative to the rod cross-sectional area, but this typically does not restrict the telescoping process, or only does so in the high load-bearing capacity range of telescoping applications. From AT 249930 a generic hydraulic system for actuating a crane boom is known. The present invention aims to avoid the aforementioned disadvantages of the prior art and to develop it further in an advantageous manner. This is to be achieved in particular with a compact and weight-saving device. This problem is solved by a hydraulic system having the features of claim 1 and by a valve unit according to claim 13. Advantageous embodiments of the invention are described in the dependent claims and the following description. Accordingly, a hydraulic system for supplying pressure to a hydraulic actuator is proposed. The hydraulic actuator can be an actuator of a locking device described in the introduction (e.g., at least one actuator for actuating a pull yoke and / or at least one actuator for unlocking drive pins). However, the invention is not limited to this application. The actuator can be part of the hydraulic system. The hydraulic system comprises a double-acting hydraulic cylinder with a first and a second pressure chamber, which can be pressurized by a hydraulic pump to move a piston of the hydraulic cylinder. The hydraulic cylinder can be a telescopic cylinder. The hydraulic system also includes a rapid traverse device, which is configured to hydraulically connect the two pressure chambers in rapid traverse mode, allowing hydraulic fluid displaced from one chamber to flow into the other. This accelerates the filling of a pressure chamber that expands during the extension or retraction of the hydraulic cylinder. The rapid traverse device is also configured to hydraulically isolate the two pressure chambers in normal traverse mode, preventing hydraulic fluid displaced from one chamber from flowing into the other (and instead diverting it, for example, into a hydraulic tank). According to the invention, the hydraulic system comprises a valve unit in which the rapid traverse device is integrated. The valve unit has a first port and a second port, each connected to one of the aforementioned pressure chambers of the hydraulic cylinder. It should be noted that when the term "ports" is used here, it refers to hydraulic ports. The valve unit further comprises a third port, which can be pressurized via the hydraulic pump. Preferably, the third port can be selectively connected to either the hydraulic pump or a hydraulic tank via a control valve. The valve unit comprises a slidably mounted switching piston which, in a normal traverse position, hydraulically isolates the first and second ports from each other, while in a rapid traverse position, the switching piston hydraulically connects the first and second ports and simultaneously isolates them from the third port. In the rapid traverse position, hydraulic fluid displaced from one pressure chamber can flow through the valve unit into the other pressure chamber, as described above. According to the invention, a pre-tensioning device is additionally integrated into the valve unit. The pre-tensioning device comprises a switchable pre-tensioning element which, in a locked position, is configured to disconnect the third port from the second port, thereby sealing off the pressure chamber connected to the second port (for example, the annular chamber of a telescopic cylinder). This sealing prevents pressure building up in the hydraulic cylinder from causing it to extend or retract. This may be necessary, for example, in the case of a telescopic cylinder with a pipe passage, to prevent unintentional extension of the piston rod due to pressure buildup in the pipe passage (see above). Preferably, in the locked position, the pre-tensioning element disconnects the third port from both the first and second ports. In the normal operating position, the switching piston preferably allows a fluidic connection between the second and third ports. However, these can be separated from each other by the preload element in the locked position. Integrating the rapid traverse and pre-tensioning functions into a single valve unit results in a compact, cost-effective, and weight-saving design. Furthermore, the valve unit can be designed as a valve cartridge and integrated directly into the hydraulic cylinder. In one possible embodiment, the valve unit includes an actuating unit by means of which the switching piston can be moved between the normal travel and rapid traverse positions. The actuating unit can be mechanically, hydraulically, or electrically actuated, the latter being preferred. The actuating unit can be a solenoid valve. A valve piston of the solenoid valve can be arranged coaxially with the switching piston. Preferably, the switching piston is biased into the normal travel position by means of a first biasing device, which can include or be a spring, and can be moved into the rapid traverse position by means of the actuating unit. Thus, when the actuating unit is deactivated (e.g., no current is supplied to the solenoid valve), the valve unit is in normal travel mode. In another possible embodiment, the preload element is designed as a sleeve that surrounds the switching piston and is slidably mounted relative to it. This results in a particularly compact design of the valve unit. Preferably, the sleeve is arranged in the area of ​​the third port. The valve unit may have mechanical stops that limit axial movement of the preload element. Alternatively or additionally, a mechanical stop for the preload element may be arranged / designed on the switching piston. In another possible embodiment, the switching piston has a channel extending axially along its direction of movement. This channel can run within the switching piston and preferably coaxially. In the region of the sleeve (preload element), the channel extends radially outwards (this can be perpendicular or at an acute angle to the switching piston axis) and opens into an annular chamber formed between the switching piston and the sleeve. The sleeve can have a control surface (e.g., an end-face annular control surface) that delimits the annular chamber and can be pressurized via the channel to move the sleeve. The channel can include one or more restrictors. Preferably, an opening of the channel in the switching piston is arranged in the region of the first connection, so that the annular chamber is hydraulically connected to the first connection, particularly regardless of the position of the switching piston. The channel can open into a chamber hydraulically connected to the actuating unit on the opposite side of the switching piston. In another possible embodiment, the preload element is preloaded into the locked position by a second preloading device, which may comprise or be a second spring supported by the preload element. By pressurizing the first or third port in normal operation mode, the preload element can be moved into an open position in which the second and third ports are hydraulically connected. This preferably "deactivates" the preload when one of the pressure chambers is selectively pressurized to extend or retract the hydraulic cylinder, so that the extension or retraction does not have to occur against the preload force. In the absence of pressure at the first or third port, the pressure chamber connected to the second port is closed. In another possible embodiment, the valve unit includes a check valve arranged between the first and second ports. This check valve is designed to allow hydraulic fluid to flow from the second to the first port and to block hydraulic fluid flow from the first to the second port when the switching piston is in rapid traverse mode. If the valve unit is designed as a cartridge, the check valve can be integrated into the cartridge or arranged between the cartridge and a cartridge housing that receives the cartridge. The check valve ensures that, in rapid traverse mode, hydraulic fluid can only flow in one direction from one pressure chamber to the other (e.g., when extending the hydraulic cylinder, which fills a larger piston chamber compared to an annular chamber). The check valve can comprise a valve body that surrounds the switching piston in a ring shape and is mounted to slide relative to it, resulting in a particularly compact design. One end face of the valve body can have at least one chamfered control surface, the application of pressure to which from the second port causes the check valve to open. In another possible embodiment, the hydraulic system includes a control valve for extending and retracting the hydraulic cylinder. Depending on the switching position of the control valve, either the first or the second pressure chamber of the hydraulic cylinder is filled with hydraulic fluid. The control valve comprises a first inlet connected to the hydraulic pump, preferably a second inlet connected to a hydraulic tank, as well as a first outlet connected to a first pressure chamber of the hydraulic cylinder and a second outlet connected to a second pressure chamber of the hydraulic cylinder. Preferably, depending on the switching position, the respective outlets, and thus the pressure chambers, are connected to the hydraulic pump or the hydraulic tank. In particular, one of the control valve's outputs is connected to the third port of the valve unit, allowing, for example, the pressure chamber connected to the second port to be filled with hydraulic fluid in normal operation mode (or, preferably, conversely, allowing hydraulic fluid to flow from the pressure chamber into a hydraulic tank via the third port). Alternatively or additionally, one of the control valve's outputs can be connected to the first port of the valve unit. Preferably, the valve unit has a fourth port, which may be permanently hydraulically connected to the first port (independent of the switching position of the switching piston), with one of the control valve's outputs being connected to the third port and the other control valve output being connected to the fourth port. Hydraulic fluid can flow through the fourth port into the pressure chamber connected to the first port and vice versa (this could, for example, be a piston chamber of the hydraulic cylinder). The control valve can be actuated via one, preferably two, pilot valves. The control valve can be designed as a main spool valve. It is conceivable that the hydraulic system includes a pressure balance that ensures a constant flow of hydraulic fluid through the control valve by keeping the difference between the pressures at one of the two outputs of the control valve and the hydraulic pump constant. In another possible embodiment, the hydraulic system includes a control unit by which an actuating unit moving the switching piston can be electrically controlled to switch between rapid traverse and normal traverse modes. The actuating unit can be configured as described above. The control unit is preferably configured to determine the load on the hydraulic cylinder based on at least one pressure measurement in the hydraulic system and to compare this load with at least one stored characteristic value. For pressure measurement, the hydraulic system can include at least one pressure sensor. Preferably, the pressures prevailing in the pressure chambers are detected by two pressure sensors, and a current load is determined from this. The at least one characteristic value can be a limit value stored in a characteristic curve field and / or a load capacity table.Alternatively, it is conceivable that the characteristic value is calculated by the control unit. The maximum load that can be moved by the hydraulic cylinder may be lower in rapid traverse mode than in normal traverse mode. To prevent exceeding the maximum load when switching to rapid traverse mode, a preferred embodiment of the control unit is configured to determine the future load resulting from the switch before switching from rapid traverse to normal traverse mode or vice versa. This load is then compared with at least one stored characteristic value, and a decision is made based on this comparison as to whether or not a switch can be performed. In the case of a mobile crane with a telescopic cylinder, the crane operator would have to make this decision using load charts, but reviewing these charts would significantly distract them from operating the crane. Therefore, the switch between rapid and normal travel is preferably automated by the control unit. This unit calculates the operating pressures in the piston and ring sides before or after switching between rapid and normal travel modes, determining whether a switch is actually possible and switching accordingly. This relieves the crane operator, allowing them to concentrate on handling the load while still achieving the fastest possible telescoping time in any given situation. In another possible embodiment, the hydraulic system includes a lowering brake valve located between the valve unit and one of the pressure chambers. In a telescopic cylinder, the lowering brake valve is specifically located between the piston chamber and the valve unit (particularly the first port of the valve unit). In a first switching position, the lowering brake valve blocks backflow of hydraulic fluid from the pressure chamber (e.g., preventing the hydraulic cylinder from retracting), but preferably also allows a flow of hydraulic fluid into the pressure chamber (e.g., enabling the hydraulic cylinder to extend), particularly by means of an integrated check valve. The lowering brake valve has a second switching position in which backflow of hydraulic fluid from the pressure chamber is permitted (e.g., controlled retraction of the hydraulic cylinder under external load).For this purpose, the lowering brake valve can include a throttle that reduces the flow rate in the second switching position. In another possible embodiment, the hydraulic cylinder comprises a piston and a piston rod with a pipe passage. The latter can be constructed as described in the introduction with reference to the prior art. The pressure supply to the at least one hydraulic actuator (e.g., the locking device of a telescopic cylinder) is provided via the pipe passage. The piston rod is preferably guided from one end of a cylinder housing of the hydraulic cylinder (differential cylinder), wherein the hydraulic cylinder comprises an annular chamber, which is preferably connected to the second port of the valve unit, and a piston chamber, which is preferably connected to the first port of the valve unit. In the locked position, the pre-tensioning device integrated into the valve unit prevents, in particular, the unintended extension of the piston rod (= reduction of the annular chamber) caused by pressure build-up in the pipe passage when an actuator supplied via the pipe passage is actuated. This is achieved by blocking backflow from the annular chamber through the valve unit. If one of the pressure chambers is intentionally pressurized via the hydraulic pump for extension or retraction, the pre-tensioning device preferably opens automatically. In another possible embodiment, the hydraulic cylinder is a telescopic cylinder and the hydraulic system includes a locking device connected to the telescopic cylinder for reversibly locking the telescopic cylinder with a telescopic section and / or for reversibly locking two telescopic sections of a telescopic boom, wherein at least one hydraulic actuator of the locking device can be supplied with pressure via the hydraulic system. In another possible embodiment, the components of the rapid traverse device and the pre-tensioning device are arranged in a common housing of the valve unit. This results in a compact design. Alternatively or additionally, the valve unit can be designed as a valve cartridge and arranged within a cylinder housing of the hydraulic cylinder. In another possible embodiment, the control piston is provided with at least one control notch to prevent or at least mitigate a pressure release surge when switching between the normal and rapid traverse positions of the switching piston (delayed pressure release). The at least one control notch can be designed as an axially milled groove and / or as a radially applied chamfer (a combination of several such grooves is conceivable). The at least one control notch can be formed on a section of the control piston that is formed with an edge of a valve housing or a sleeve receiving the control piston and is in sealing contact with the control piston in the rapid traverse position. The at least one control notch mitigates an abrupt increase in the flow cross-section during the transition to the normal traverse position and thus prevents a jerky pressure release. In another possible embodiment, the hydraulic pump is designed as a variable displacement pump. The variable displacement pump can be equipped with an electroproportional controller to implement a load-sensing system. The invention further relates to a valve unit with an integrated rapid traverse device and an integrated pre-tensioning device for the hydraulic system according to the invention. The valve unit according to the invention thus comprises all the features of the device described with respect to the hydraulic system and possesses the same properties and / or offers the same advantages. A repetitive description is therefore omitted. In particular, the valve unit according to the invention can be configured according to any of the embodiments described above. The invention further relates to a work device, in particular a mobile crane, with a hydraulic system according to the invention. This preferably controls an actuator of the work device to perform a work function. In one possible embodiment, the work device is designed as a mobile crane with a telescopic boom, the telescopic boom comprising an outer telescopic section, at least one inner telescopic section slidably mounted therein, a hydraulic telescoping cylinder for extending and retracting the at least one inner telescopic section, and a locking device connected to the telescoping cylinder for reversibly locking the telescoping cylinder to an inner telescopic section and / or to two telescopic sections together. At least one actuator of the locking device can be supplied with pressure or controlled via the hydraulic system according to the invention. The actuators can be at least one actuator for actuating a drawbar and / or at least one actuator for actuating a drive pin. Further features, details and advantages of the invention will become apparent from the exemplary embodiments explained below with reference to the figures. Figure 1 shows a schematic representation of the hydraulic system according to the invention in one exemplary embodiment; and Figures 2-5 show longitudinal sections of an exemplary embodiment of the valve unit according to the invention in different switching positions. Figure 1 shows an embodiment of the hydraulic system according to the invention. The hydraulic system 10 comprises a double-acting hydraulic cylinder 20, which in the present embodiment is designed as a telescopic cylinder for a telescopic boom with a locking device attached to the outside of the cylinder housing 27. However, the following descriptions and the operation of the hydraulic system according to the invention are not limited to this application. The locking device serves the purpose described in the introduction and comprises spring-returned drive pins that can be actuated (retracted) via first hydraulic actuators 1, as well as a spring-returned pull yoke that can be actuated via a second hydraulic actuator 2. The hydraulic supply and control of the actuators 1 and 2 is provided by a hydraulic pump 12 of the hydraulic system 10 and by valves 3 and 4. Valve 3 is connected to valve 4 via supply line 5 and, depending on its switching position, connects the latter to either the hydraulic pump 12 or a hydraulic tank 11. The hydraulic cylinder 20 comprises a piston 24 and a piston rod 23 extending from one end of the cylinder housing 27, and is thus a differential cylinder. The piston rod 23 has a hydraulic pipe feedthrough 25, 26 connected to the supply line 5, through which the hydraulic supply to the actuators 1, 2 is provided. The pipe feedthrough comprises two feedthrough tubes 25, 26, which are slidably mounted within one another, sealed against each other, and extend telescopically together with the hydraulic cylinder 20. An inner feedthrough tube 26 can be connected to the cylinder housing 27 and extend with it, while an outer feedthrough tube 25 can be fixedly connected to the piston rod 23. The hydraulic cylinder 20 has a piston-side pressure chamber 21 (first pressure chamber) and a piston-rod-side pressure chamber 22 (second pressure chamber). To retract the hydraulic cylinder 20 (telescoping), the annular chamber 22 is pressurized via the hydraulic pump 12. To extend the hydraulic cylinder 20 (telescoping), the piston chamber 21 is pressurized via the hydraulic pump 12. The respective pressure chambers 21 and 22 are pressurized via a control valve 14 – in this case, a main valve actuated by two pilot valves 15. The inlets of the control valve 14 are connected to the hydraulic pump 12 and the hydraulic tank 11, while the outlets of the control valve 14 are connected via a supply line 6 to the piston chamber 21 and via another supply line 7 to the annular chamber 22. The piston rod 23 may have connections that connect the supply lines 6, 7 to the respective pressure chambers 21, 22 via internal cavities or channels (see Fig. 1). The hydraulic pump 12 can have an electroproportional controller to implement a load-sensing system, which includes the control valve 14. The hydraulic system 10 can include a pressure compensator that ensures a constant oil flow through the control valve 14 by maintaining a constant pressure differential between the inlet and outlet of the control valve 14. The pressure compensator, shown in Fig. 1, includes the pressure relief valve 42. Pressure relief valves 45, 46 may be present to limit the load-sensing pressure (see Fig. 1). Optionally, two pressure relief valves 43, 44 connected to the supply lines 6, 7 limit the pressure in the hydraulic cylinder 20. A pressure relief valve 41 may be connected in parallel to valve 3. The hydraulic system 10 can include one or more pressure sensors or pressure transmitters. For example, a pressure sensor 32 provided on the pressure balance and a pressure sensor 31 connected to the pump output can enable control of the hydraulic pump 12 based on the difference between the measured pressure values ​​of the pressure sensors 31 and 32, which serves as the control variable. Preferably, a pressure sensor 33, 34 is connected to one of the pressure chambers 21, 22 or to a supply line 6, 7 leading to the respective pressure chamber 21, 22, in order to be able to determine, for example, the current load on the hydraulic cylinder 20 based on the detected pressures (see below). Preferably, the hydraulic system 10 has a lowering brake valve 16, which allows the hydraulic cylinder 20 to be retracted or stopped in a controlled manner, even under load. This is particularly important for telescopic cylinders. In the switching position shown in Fig. 1, a check valve of the lowering brake valve 16 prevents backflow of hydraulic oil from the piston chamber 21, while allowing the piston chamber 21 to be filled for extension. In a second switching position, an integrated throttle enables controlled and slowed retraction of the hydraulic cylinder 20. A pressure relief valve 47 can be connected in parallel to the lowering brake valve 16 to prevent overpressure resulting from heating of the hydraulic oil when the lowering brake valve 16 is closed, e.g., due to sunlight. The components described above (load-sensing system, pressure relief valves, pressure balance, lowering brake valve, pressure sensors, etc.) are optional and can be provided in any combination in the hydraulic system 10. According to the invention, the hydraulic system comprises a valve unit 50, which integrates a rapid traverse function and a preload function into a common unit and is explained below with reference to a preferred embodiment shown in Figures 2-5. The essential components of the valve unit 50 are also shown schematically as valve components in Figure 1. The valve unit 50 comprises, on the one hand, a rapid traverse device (represented in Fig. 1 by components 52, 66, and 70) which, when actuated, connects the two pressure chambers 21 and 22. This ensures that when the hydraulic cylinder 20 extends, the hydraulic oil flowing from the annular chamber 22 is directly fed back into the piston chamber 21, significantly increasing the oil flow to the piston side and thus the extension speed. In the embodiment shown in Fig. 1, the valve unit 50 is arranged downstream of the lowering brake valve 16, starting from the hydraulic cylinder 20, so that the latter alone performs the safety function (shutting off the piston chamber 21). The preloading device (represented by component 54 in Fig. 1) is designed to prevent unintentional extension of the telescoping cylinder 20 when actuators 1, 2 are actuated (i.e., when the locking bolts are unlocked). In this case, the operating pressure of the locking bolts in the pipe passage 25, 26 causes the telescoping cylinder 20 to extend unintentionally in certain situations because the inner passage tube 26 is pushed out by the oil pressure acting in both passage tubes 25, 26, and thus the telescoping cylinder 20 also extends. This displaces hydraulic oil from the annular chamber 22. This is prevented by a preloading element 54 of the preloading device. The valve unit 50 has a first port 61, which is connected to the piston chamber 21. In Fig. 1, the first port 61 is connected to the supply line 6 before the lowering brake valve 16. The valve unit 50 has a second port 62, which is connected to the annular chamber 22, and a third port 63, which is connected to one of the outputs of the control valve 14. In the embodiment shown in Fig. 1, the first port 61 communicates with the other port of the control valve 14 and, via the lowering brake valve 16, thus with the piston chamber 21. In the embodiment shown in Figs. 2-5, which show a longitudinal section through the valve unit 50, an additional fourth port 64 is provided, which is connected to the control valve 14, while the first port 61 is connected to the piston chamber 21. Since the first and fourth ports 61, 64 are hydraulically connected to each other in every switching position, the functional situation is the same as in Fig. 1. Fig. 2 shows the valve unit 50 in the unactuated state, i.e. the hydraulic cylinder 20 is not actuated via the hydraulic pump 12 and none of the pressure chambers 21, 22 are pressurized. The valve unit 50 has a valve housing 68, which has the aforementioned ports 61-64. A control piston 52 is axially displaceable within the valve housing 68. The control piston 52 can be arranged within a sleeve 69, which is inserted into a recess in the valve housing 68 and has corresponding openings that correspond to the ports 61-64. For the sake of simplicity, only the valve housing 68 will be referred to below, even though the sleeve 69 may also be included. The control piston 52 is biased into the left position (see Fig. 2) by a first spring 53 (= first preloading device). Here, a control edge 51 of the control piston 52 (see Fig. 3), in conjunction with a corresponding step in the valve housing 68 / 69, closes the connection between the first port 61 and the second and third ports 62, 63. The valve unit 50 further comprises a preload element 54, which in the illustrated embodiment is designed as a sleeve 54 that is slidably mounted relative to the control piston 52 and surrounds it in an annular manner. The preload element 54 is preloaded into the right-hand position by a second spring 55 (= second preloading device) (see Fig. 2), in which a valve surface 75 of the sleeve 54 interacts with a valve seat 76 formed in the valve housing 68 / 69 (see Fig. 3) and closes the connection between the second and third ports 62, 63. The preload element 54 is arranged, in particular, in a chamber formed in the region of the third port 63. Fig. 2 thus shows the control piston 52 and the preload element 54 in their basic positions (control piston 52: normal operating position, preload element 54: closed position). Furthermore, the valve unit 50 can include a check valve 66 with a valve body annularly surrounding the control piston 52 and a spring biasing the valve body into a closed position (see Fig. 2). In the closed position, the valve body closes a connection between the first and second ports 61, 62. The valve body can have a control surface chamfered towards the control piston 52, which is designed such that the check valve 66 opens when pressure is applied from the side of the second port 62 (when the pressure is greater than the pressure prevailing at the first port 61) (see Fig. 4) and thereby connects the first and second ports 61, 62. In the embodiment shown in Figs. 2-5, the sleeve 69, which is mounted in the valve housing 68, can have an end section in the region of the first port 61. Within this end section, the control piston 52 is supported by the first spring 53. The check valve 66 is arranged in a chamber 84 formed between the end section of the sleeve 69 and the valve housing 68. This chamber 84 can be connected to the fourth port 64 (see Fig. 2) and surrounds the end section. The end section can terminate at a distance in front of the fourth port 64, which is formed on the end face of the valve housing 68, so that a hydraulic connection between the first and fourth ports 61 and 64 is always maintained. However, other configurations are also conceivable. The control piston 52 can have a bore 56 or a channel 56 which extends axially, in particular coaxially to its longitudinal axis, within the control piston 52 from an end face facing the first spring 53 to at least the area of ​​the preload element 54. There, the channel 56 is connected via a radial bore 57 to an annular chamber 58 (see Fig. 3) which is formed between the control piston 52 and the sleeve-shaped preload element 54. Towards the left side (i.e., the side facing away from the first spring 53), the annular chamber 58 is bounded by an annular control surface of the preload element 54, so that by pressurizing the annular chamber 58 via the channel 56, 57, the preload element 54 is moved to the left into an open position against the preload force of the second spring 55 (see Fig. 3). This opens the connection between the second and third terminals 62, 63 (see Fig. 3). The preloading element 54 can have an externally inclined control surface 74 in the area of ​​the third port 63, which is designed such that the preloading element 54 is moved to the left into the open position by pressurization via the third port 63. Thus, the preloading element 54 can be moved into the open position both when the third port 63 and when the first port 61 (via the channel 56) are pressurized. Consequently, the preloading device of the valve unit 50 always opens when one of the pressure chambers 21, 22 is pressurized via the hydraulic pump 12 for the active extension or retraction of the hydraulic cylinder 20. However, the preloading element 54 is designed such that it remains in the locked position (Fig. 2 and Fig. 4) when pressure is applied only via the second port 62. This prevents unintentional extension of the hydraulic cylinder 20 in the event of a pressure increase in the annular chamber 22, while the preloading device is "inactive" during regular extension and retraction of the hydraulic cylinder 20. The valve unit 50 further comprises an actuating unit 70, which, when actuated, pushes the control piston 52 from the normal operating position (see Fig. 2) into a rapid traverse position (see Fig. 4), thereby connecting the first and second ports 61, 62 and the two pressure chambers 21, 22, respectively. The actuating unit 70 can be a solenoid valve which, when energized, actuates a valve piston 72 mounted axially to the control piston 52 (see Fig. 2) and presses it against a valve seat (see Fig. 4). This pressure build-up via the channel 56 in a chamber now closed by the valve piston 72 and fluidically connected to the third port 63 causes the control piston 52 to move into the rapid traverse position. Preferably, the channel 56 extends to the other, solenoid-valve-side end of the control piston 52 and preferably opens there via a restrictor into a chamber which is connected to the aforementioned chamber 73, which can be closed by the valve piston 72. The connection between the chamber 73 and the third port 63 can also include a restrictor. Due to the restrictors, pressure can build up or decrease in the space between the chamber 73 and the control piston 52. Furthermore, the restrictors limit the switching speed. The valve unit 50 compactly integrates a rapid traverse function and a pre-tensioning function for the hydraulic cylinder and can operate as follows: In the unactuated state (see Fig. 2-3), the control piston 52 is in the normal operating position. In this normal operating mode, the hydraulic cylinder 20 can be regularly extended and retracted, during which the hydraulic oil displaced from one of the pressure chambers 21, 22 flows via the control valve 14 into the hydraulic tank 14. Depending on the pressure applied, the pre-tensioning element 54 can be in the locked position (see Fig. 2) or in the open position (see Fig. 3). When the actuating unit 70 is actuated, the control piston 52 is pressed into the rapid traverse position (see Fig. 4), in which a control edge of the control piston 52 separates the connection between the second and third ports 62, 63, and when pressure is applied to the first port 61, the first and second ports 61, 62 are connected to each other for the rapid traverse mode. When switching to rapid traverse mode, the operating pressure increases with a constant load, corresponding to the ratio of the piston area to the rod area of ​​the hydraulic cylinder 20. This pressure must be reduced again when switching back to normal traverse mode. To prevent this from causing a pressure release surge in a telescopic cylinder, which would stress the supporting structure of the mobile crane, at least one control notch 80 can be formed on the control piston 52 (see Fig. 3). The control notch(es) 80 can be designed as an axially milled groove or a radially applied chamfer and is / are located, in particular, in the area of ​​the control edge, which, in the rapid traverse position, closes the connection between the second and third ports 62, 63. The number of control notches 80 can be determined by the required total opening cross-section.The at least one control notch 80 allows for a delayed pressure release while the control piston 52 moves back to the normal operating position, and prevents a release stroke that would be disruptive and stressful. Fig. 5 shows the control piston 52 during the return to the normal operating position. An opening 82 has formed in the area of ​​the control notch 80, which connects the second and third terminals 62, 63. With a telescoping cylinder, the telescoping load capacity is lower in rapid traverse than in normal traverse. Depending on the current load, telescopic boom angle, and telescopic length, it may be possible and advantageous to switch to rapid traverse mode, or not. The crane operator would have to make this decision using load tables, which would be a significant distraction from crane operation. Therefore, the switch between rapid and normal traverse modes is preferably automated by a control unit of the hydraulic system 10 (not shown). In a preferred embodiment, the current load is determined by pressure measurement via the two pressure sensors 33 and 34. A maximum possible load is known from a load capacity table stored in a memory unit. By calculating the operating pressures in the piston chamber 21 and in the annular chamber 22 before or after switching from rapid traverse to normal traverse or vice versa, the control unit can decide whether a switchover is actually possible and accordingly switchover or not. The crane operator is not burdened with this decision and distracted by it, but can concentrate on handling the load and still always achieve the fastest telescoping time in the respective situation. The valve unit 50 can preferably be designed as a valve cartridge. This eliminates the need for external piping and allows for direct oil flow without line losses. Furthermore, the valve cartridge can be placed directly inside the hydraulic cylinder 20, saving space. Reference symbol list: 1 First actuator 2 Second actuator 3, 4 Valves 5-7 Supply lines 10 Hydraulic system 11 Hydraulic tank 12 Hydraulic pump 14 Control valve 15 Pilot valve 16 Lowering brake valve 20 Hydraulic cylinder 21 First pressure chamber 22 Second pressure chamber 23 Piston rod 24 Piston 25 Outer feedthrough tube 26 Inner feedthrough tube 27 Cylinder housing 31-34 Pressure sensors 41 Pressure relief valve 42 Pressure balance 43-47 Pressure relief valves 50 Valve unit 51 Control edge 52 Switching piston 53 First preload device 54 Preload element 55 Second preload device 56 Channel 57 Radial bore 58 Annular chamber 61 First port 62 Second port 63 Third port 64 Fourth port 66 Check valve 68 Valve body 69 Sleeve 70 Actuating unit 72 Valve piston 73 Chamber 74 Control surface 75 Valve surface 76 Valve seat 80 Control notch 82 Opening 84 Chamber

Claims

Hydraulic system (10) for supplying pressure to a hydraulic actuator (1, 2), comprising a double-acting hydraulic cylinder (20) with a first and a second pressure chamber (21, 22) which can be pressurized via a hydraulic pump (12), and a rapid traverse device which is configured to hydraulically connect the two pressure chambers (21, 22) to each other in a rapid traverse mode, so that hydraulic fluid displaced from one pressure chamber (22) can flow into the other pressure chamber (21), and to hydraulically separate the two pressure chambers (21, 22) from each other in a normal traverse mode, characterized in that the rapid traverse device is integrated into a valve unit (50) of the hydraulic system (10) which is connected to the two pressure chambers (21, 22) via a first and a second port (61, 62) and has a third port (63) which can be pressurized via the hydraulic pump (12).wherein the valve unit (50) comprises a slidably mounted switching piston (52) which, in a normal operating position, hydraulically separates the first and second ports (61, 62) from each other and, in a rapid traverse position, hydraulically connects the first and second ports (61, 62) to each other and separates them from the third port (63), wherein a pre-tensioning device with a switchable pre-tensioning element (54) is further integrated into the valve unit (50), which is configured, in a blocking position, to separate the third port (63) from the second port (62) and thereby shut off the pressure chamber (22) connected to the second port (62) to the outside. Hydraulic system (10) according to claim 1, wherein the valve unit (50) comprises a preferably electrically controllable actuating unit (70), in particular a solenoid valve, by means of which the switching piston (52) can be moved between the normal travel position and the rapid traverse position, wherein the switching piston (52) is preferably pre-tensioned into the normal travel position via a first pre-tensioning device (53) and can be moved into the rapid traverse position by means of the actuating unit (70). Hydraulic system (10) according to claim 1 or 2, wherein the preload element (54) is designed as a sleeve which surrounds the switching piston (52) and is slidably mounted relative to it, wherein the sleeve (54) is preferably arranged in the area of ​​the third connection (63). Hydraulic system (10) according to the preceding claim, wherein the switching piston (52) has a channel (56) extending along its direction of displacement, which is guided radially outwards in the region of the sleeve (54) and opens into an annular chamber (58) formed between the switching piston (52) and the sleeve (54), wherein preferably an opening of the channel (56) in the switching piston (52) is arranged in the region of the first connection (61), so that the annular chamber (58) is hydraulically connected to the first connection (61), in particular regardless of the position of the switching piston (52). Hydraulic system (10) according to one of the preceding claims, wherein the preloading element (54) is preloaded into the locking position by a second preloading device (55) and can be moved into an opening position by pressurizing the first or the third port (61, 63) in normal operating mode, in which the second and third ports (62, 63) are hydraulically connected. Hydraulic system (10) according to one of the preceding claims, wherein the valve unit (50) comprises a check valve (66) which is arranged between the first and second ports and is configured to allow the flow of hydraulic fluid from the second to the first port (62, 61) and to block the flow of hydraulic fluid from the first to the second port (61, 62) in the rapid traverse position of the switching piston (52), wherein the check valve (66) preferably comprises a valve body which surrounds the switching piston (52) in an annular manner and is slidably mounted relative to it. Hydraulic system (10) according to one of the preceding claims, comprising a control valve (14) for extending and retracting the hydraulic cylinder (20), which has a first inlet connected to the hydraulic pump (12), preferably a second inlet connected to a hydraulic tank (11), and two outlets connected to the pressure chambers (21, 22) of the hydraulic cylinder (20), wherein in particular one of the outlets is connected to the first port (61) of the valve unit (50) and / or one of the outlets is connected to the third port (63) of the valve unit (50). Hydraulic system (10) according to one of the preceding claims, comprising a control unit by means of which an actuating unit (70) moving the switching piston (52) can be electrically controlled for switching between rapid traverse and normal traverse modes, wherein the control unit is in particular configured to determine a load on the hydraulic cylinder (20) as a function of at least one pressure measurement in the hydraulic system (10) and to compare this with at least one stored characteristic value, wherein the control unit is preferably further configured to determine a future load resulting from the switching before switching from rapid traverse to normal traverse mode or vice versa, to compare this with at least one stored characteristic value and to decide on the basis of the comparison whether a switching can take place or not. Hydraulic system (10) according to one of the preceding claims, comprising a lowering brake valve (16) which is arranged between the valve unit (50) and one of the pressure chambers (21), wherein the lowering brake valve (16) in a first switching position blocks a backflow of hydraulic fluid from the pressure chamber (21) and preferably allows a flow of hydraulic fluid into the pressure chamber (21), in particular by means of an integrated check valve, and wherein the lowering brake valve (16) in a second switching position allows a backflow of hydraulic fluid from the pressure chamber (21). Hydraulic system (10) according to one of the preceding claims, wherein the hydraulic cylinder (20) comprises a piston (24) and a piston rod (23) with a pipe passage (25, 26), wherein a pressure supply to the hydraulic actuator (1, 2) is provided via the pipe passage (25, 26), wherein the piston rod (23) is preferably guided on one side from a cylinder housing (27) of the hydraulic cylinder (20) and wherein an annular chamber (22) formed on the side of the piston rod (23) is connected to the second port (62) and a piston chamber (21) formed on the opposite side of the piston (24) is connected to the first port (61) of the valve unit (50). Hydraulic system (10) according to one of the preceding claims, wherein the hydraulic cylinder (20) is a telescopic cylinder and the hydraulic system (10) comprises a locking device connected to the telescopic cylinder for reversibly locking the telescopic cylinder with a telescopic section and / or for reversibly locking two telescopic sections of a telescopic boom, wherein at least one hydraulic actuator (1, 2) of the locking device can be supplied with pressure via the hydraulic system (10). Hydraulic system (10) according to one of the preceding claims, wherein the components of the rapid traverse device and the pre-tensioning device are arranged in a common housing (68) of the valve unit (50) and / or wherein the valve unit (50) is designed as a valve cartridge and is arranged within a cylinder housing (27) of the hydraulic cylinder (20). Valve unit (50) with integrated rapid traverse device and integrated pre-tensioning device of a hydraulic system (10) according to one of the preceding claims. Working equipment, in particular mobile crane, with a hydraulic system (10) according to one of the preceding claims. Working device according to the preceding claim, which is designed as a mobile crane with a telescopic boom, wherein the telescopic boom comprises an outer telescopic section, at least one inner telescopic section slidably mounted therein, a hydraulic telescoping cylinder for extending and retracting the at least one inner telescopic section, and a locking device connected to the telescoping cylinder for reversibly locking the telescoping cylinder to an inner telescopic section and / or two telescopic sections together, wherein at least one actuator (1, 2) of the locking device can be supplied with pressure via the hydraulic system (10).

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