HYDRAULIC SYSTEM FOR A BACKREST AND WORK EQUIPMENT

Description

[0001] The present invention relates to a working device, in particular a mobile crane or cable excavator, comprising a pivotable boom, an adjusting device for adjusting the boom, at least one fall-back support cylinder connected to the boom and following its movements, and a hydraulic system for operating a hydraulic fall-back support of a working device according to the preamble of claim 1.

[0002] For a number of work machines, such as crawler cranes or rope excavators, it is known to use so-called fallback supports to increase operational safety in adverse weather conditions. For example, document JPH05278995A discloses a work machine comprising a pivoting boom, an adjusting device for adjusting the boom, and a hydraulic system for operating a hydraulic fallback support of the work machine.

[0003] wherein the hydraulic system comprises a hydraulic fallback support cylinder and a hydraulic pump,

[0004] a switchable valve is arranged between the hydraulic pump and the fall-back support cylinder,

[0005] and wherein a hydraulic accumulator connected to the load-bearing cylinder chamber is provided between the check valve and the fallback support cylinder.

[0006] To illustrate the principle of a fallback support, the Figure 1a generic crawler crane 1 in a side view. The crane 1 has an undercarriage 2 with a chassis (e.g. crawler track, track chain chassis, etc.) and an uppercarriage 3 mounted on the undercarriage 2 for rotation about a vertical axis, to which a boom 4 is bolted for pivoting about a horizontal axis so that its angle of inclination relative to the uppercarriage 3 can be adjusted. The boom 4 is adjusted via a retracting cable 5 by means of a retracting winch 6. A load handling device 7 or crane hook is connected to a hoist winch 9 via a hoist cable 8. By operating the hoist winch 9, the free length of the hoist cable 8 can be changed in order to raise or lower loads attached to the load handling device 7.

[0007] The weight of the boom 4 together with the lifting load generates a counterclockwise load moment M around the pivot point of the boom 4 on the superstructure 3. This load moment M is counteracted by the rope force of the retraction rope 5, whereby it is tensioned and the boom 4 is held in a defined angular position to the superstructure 3.

[0008] Boom 4 offers a large surface area for wind to attack. Wind load from the front (in the Figure 1from the left) thus counteracts the load moment M and reduces the rope force in the retracting rope 5. This creates the risk that the load moment M will be exceeded; the result is slack in the retracting rope 5 and, in extreme cases, the boom 4 tipping backwards, which would lead to machine damage and personal injury. As a countermeasure, it is known to limit the maximum achievable boom angle to values ​​below 90° by means of a mechanical stop or by timely shutdown of the retracting winch 6. Furthermore, a maximum wind speed can be defined; if exceeded, machine operation must be stopped and the boom 4 must be placed on the ground.

[0009] In order to allow steeper boom angles and higher wind speeds, the use of fall-back supports 12 has been proven effective: hydraulically operated cylinders which support the effect of the load moment M with their force and are arranged between the boom 4 and the superstructure 3. The fall-back support cylinder(s) 12 maintain constant contact with the boom 4 and follow all its movements. These can be movements resulting from the operation of the retractable winch 6 (i.e. working movements with a large cylinder stroke - typically in the range of several decimetres) or smaller movements such as elastic deformation of the entire system due to load change reactions when lifting or setting down hoist loads (i.e. very small movements with a very small cylinder stroke - typically in the range of a few centimetres).

[0010] In existing devices, the power supply to the fallback supports is usually provided by specially installed hydraulic pumps, which build up pressure in the fallback support cylinders throughout the entire operating time. Typically, the piston surfaces of the fallback support cylinders are continuously pressurized by the specially provided hydraulic pumps during operation. In advantageous and proven designs, these pumps are demand-controlled (so-called "load sensing"), so that at a constant pressure, they adjust the oil flow rate to the movement of the fallback support cylinders, thus keeping power consumption within limits.

[0011] A disadvantage of such conventional energy supply systems is that each installed hydraulic pump consumes base load at 100% of the primary energy source's duty cycle (bearing friction, churning losses, energy requirements of control devices, etc.), thus reducing the efficiency of the entire machine. In the past, this fact was largely ignored in diesel-powered machines. However, efforts to avoid unnecessary emissions are now increasing. In contrast, with electric drives using accumulators, for example, there is a greater economic and technical interest in saving base load. Furthermore, the installation space and number of pump mounting locations at the primary energy source are limited.

[0012] Against this background, the object of the present invention is to reduce the energy requirement for providing the fall-back support function in such work devices and to ensure energy-efficient compensation of minimal movements of the boom.

[0013] According to the invention, this object is achieved by a work device with a hydraulic system having the features of claim 1. Accordingly, a hydraulic system for operating a hydraulic fallback support of the work device is provided, which comprises at least one hydraulic fallback support cylinder for tracking and limiting movements of a boom of the work device and a hydraulic pump by means of which the at least one fallback support cylinder and at least one further hydraulic consumer can be supplied with hydraulic fluid. The hydraulic pump is therefore not intended exclusively for supplying the fallback support cylinder. Rather, it serves as the primary energy source for supplying a wide variety of consumers such as radiator drives, winches or the like, and is also used to supply the fallback support function.

[0014] According to the invention, a shut-off valve is connected between the hydraulic pump and the fallback support cylinder, which can assume a blocking and a passing position and by means of which a load-bearing cylinder chamber of the fallback support cylinder can be shut off.

[0015] By switching the shut-off valve to the locked position, the load-bearing cylinder chamber of the fallback support cylinder can be shut off to prevent energy exchange with the rest of the hydraulic system. This means that the hydraulic pressure in the shut-off area or in the load-bearing cylinder chamber is the same as when the shut-off valve was shut off. The hydraulic pump therefore no longer has to provide energy for the fallback support function if no boom movement is to occur. This reduces the energy requirement or the base load of the primary energy source and increases overall efficiency. During active movement of the boom, i.e. movement desired by the operator of the work equipment, the shut-off valve can be switched to the open position to allow hydraulic fluid to be supplied or discharged and thus to adjust the fallback support cylinder.

[0016] Since the boom is not moved by the fall-back support cylinder, but by an actuating device, such as a retracting winch, no power needs to be provided by the hydraulic pump when the boom is raised, i.e., when the fall-back support cylinder piston rod retracts. Instead, hydraulic fluid simply needs to be diverted or fed to a tank while maintaining a certain pressure in the load-bearing cylinder chamber. Only when the fall-back support cylinder is extended, i.e., when the boom is lowered, does a supply need to be provided by the hydraulic pump. The same can apply to an assembly operation when the fall-back support cylinder is detached from the boom.

[0017] According to the invention, a hydraulic accumulator connected to the load-bearing cylinder chamber is also provided between the check valve and the fallback support cylinder. This allows for the compensation of minor movements of the boom, even when the load-bearing cylinder chamber is blocked by the check valve and thus decoupled from the hydraulic pump or a tank. The fallback support cylinder can thus follow even the smallest movements of the boom, with the required hydraulic fluid flow being drawn from or supplied to the hydraulic accumulator.

[0018] Advantageous embodiments of the invention emerge from the subclaims and the following description.

[0019] In one embodiment, at least two fallback support cylinders are provided, each with a shut-off valve and a hydraulic accumulator, which can be jointly supplied with hydraulic fluid by the hydraulic pump.

[0020] In a further embodiment, a pressure relief valve is provided between the check valve and the fallback support cylinder. This can serve in particular to protect against pressure in the event of malfunctions and is therefore preferably not involved in the function of the fallback support during normal operation. However, it is also conceivable for the pressure relief valve to be used to control or regulate the pressure and flow rate of the hydraulic fluid flow to and from the fallback support when the check valve is simultaneously open, i.e. in the through position, in order to keep the hydraulic pressure in the load-bearing cylinder chamber within an optimal range during boom adjustment. The value of the maximum pressure ensured by the pressure relief valve can be adjustable.

[0021] In a further embodiment, a check valve is provided which blocks off the area of ​​the hydraulic system that can be blocked off by the blocking valve in the direction of the hydraulic pump. The check valve can be connected in parallel to the blocking valve, i.e. can operate independently of the switching state of the blocking valve, or it can only be actuated when the blocking valve is in the blocking position, i.e. can only be used when the blocking valve is in the blocking position. In the latter case, the check valve can in particular be part of the blocking valve. The check valve makes it possible in particular to first create pressure equalization on both sides of the blocking valve before the blocking valve is opened for the purpose of adjusting the fallback support cylinder when the boom moves, and in this way to reduce or avoid load change reactions during transitions between different operating phases.

[0022] In a further embodiment, an additional valve is arranged between the check valve and the hydraulic pump, which valve can preferably be switched electrically. The valve can be a directional control valve, in particular a 4 / 2-way or 4 / 3-way valve. One possible function of the additional valve is to decouple the check valve from the hydraulic pump or the tank. Alternatively or additionally, it is conceivable that it is used to control or regulate the pressure and flow rate of the hydraulic fluid flow from and to the fallback support when the check valve is simultaneously open, in order to keep the hydraulic pressure in the load-bearing cylinder chamber within an optimal range when the boom is adjusted. The valve can be electrically controllable. If there are several fallback support cylinders, preferably only a single such valve is provided.

[0023] In a further embodiment, the load of the fallback support cylinder can be measured by means of a load measuring device, which preferably comprises a pressure sensor arranged between the shut-off valve and the fallback support cylinder. This allows, for example, a load-sensing application to be implemented. In particular, it is possible to monitor or regulate the pressure and flow rate of the hydraulic fluid flow to and from the fallback support and enable optimal tracking of the fallback support cylinder.

[0024] In a further embodiment, a pressure control device is provided, by means of which the pressure and volume flow or delivery rate to and from the fallback support cylinder can be adjusted and preferably regulated depending on a measured load of the fallback support cylinder. This allows the hydraulic pressure in the load-bearing cylinder chamber of the fallback support cylinder to be maintained within an optimal range during boom tracking. When the fallback support cylinder is retracted, particularly due to a corresponding active movement of the boom, the pressure control device generates a resistance that ensures optimal tracking while maintaining the fallback support function.

[0025] In a further embodiment, the pressure control device comprises a recuperation device on / in the fallback support cylinder, in which hydraulic fluid flows to the opposite side of the fallback support cylinder piston, at least one pressure relief valve arranged between the shut-off valve and the hydraulic pump and in particular electrically controllable, a load-sensing arrangement, and / or a means for controlling a previously described valve arranged between the hydraulic pump and the shut-off valve. The control or regulation of the latter valve can be hydraulically-mechanical or software-supported electrical. Values ​​from a load-sensing arrangement can be taken into account.

[0026] In a further embodiment, the shut-off valve is hydraulically controllable by means of a switching valve, in particular an electrically controllable one. The switching valve is preferably arranged between the hydraulic pump and a control port of the shut-off valve and can, for example, have a blocking and a passing position.

[0027] In a further embodiment, the check valve is hydraulically controllable and has a control port connected to the output of a hydraulic shuttle valve, wherein one inlet of the shuttle valve is preferably connected to a non-load-bearing cylinder chamber of the fallback support cylinder and the other inlet is connected to a switching valve, in particular an electrically controllable one. With the help of the shuttle valve, it is possible to switch the check valve in two ways: either by pressurizing the non-load-bearing cylinder chamber of the fallback support cylinder (for example, in an assembly operation) or by controlling the switching valve (for example, in normal operation, in which no pressure is applied to the non-load-bearing cylinder chamber, but rather the fallback support cylinder is retracted by "pushing" the actuated boom).

[0028] InIn a further embodiment, the hydraulic accumulator is designed to compensate for minor movements of the fallback support cylinder by removing and releasing hydraulic fluid when the load-bearing cylinder chamber is blocked by the shut-off valve. This occurs, in particular, with moderate pressure and thus force changes according to the loading characteristic of the hydraulic accumulator.

[0029] In One embodiment provides that the shut-off valve is in the locked position and seals off a load-bearing cylinder chamber of the fall-back support cylinder when the boom is not actively moved by the actuating device. This reduces energy consumption or the base load when the boom is not being moved by the operator.

[0030] In a further embodiment, a control is provided by means of which the check valve and preferably the actuating device can be controlled directly or indirectly and which is designed to switch the check valve into the passage position upon actuation of the actuating device so that hydraulic fluid can be conveyed to or diverted from the load-bearing cylinder chamber in order to keep the pressure in an optimal range for the purpose of tracking the boom.

[0031] In a further embodiment, an additional valve arranged between the shut-off valve and the hydraulic pump, as well as a check valve for each fallback support cylinder, as described above, are provided. The control system is configured to receive a signal from an input unit for actuating the actuating device for the purpose of moving the boom, then to switch the valve to a through position and to leave the shut-off valve in the blocking position, to switch the shut-off valve to the through position (and, if necessary, to close the additional valve), and then to actuate the actuating device. This allows the hydraulic pump to initially build up sufficient pressure in the non-blocked area of ​​the hydraulic system.As soon as the check valve opens (and thus pressure equalization is achieved), the shut-off valve is opened so that in the event of a subsequent movement of the boom, which leads to the retraction or extension of the fall-back support cylinder, hydraulic fluid can be pumped into or discharged from the load-bearing cylinder chamber.

[0032] Further features, details, and advantages of the invention will become apparent from the following exemplary embodiments explained with reference to the figures. They show: Figure 1: a side view of a crawler crane of the type mentioned with a fall-back support; Figure 2: a circuit diagram of a first embodiment of the hydraulic system according to the invention; and Figure 3: a circuit diagram of a second embodiment of the hydraulic system according to the invention.

[0033] The Figure 1shows a schematic side view of a generic crawler crane 1 and has already been described at the beginning, so that repeated explanations are omitted here. The working device 1 according to the invention can in principle be such a crawler crane, which, in contrast to known devices, is equipped with a hydraulic system 10 according to the invention. Figure 1In the crawler crane 1 shown, the retractable winch 6 arranged on the superstructure 3 represents the adjusting device. Furthermore, it is possible for an A-frame to be pivotably mounted on the superstructure 3 about a horizontal axis, which frame is coupled to the boom 4 via guying means, for example guy rods, and can be pivoted together with the boom by actuating the retractable winch 6. However, other boom configurations and adjusting devices for boom movement are also conceivable, without this having any influence on the functioning of the fall-back support or the hydraulic system 10 according to the invention.

[0034] In the Figure 2 is a circuit diagram of the hydraulic system 10 according to the invention according to a first embodiment. The hydraulic system 10 serves to operate the fall-back support function of a working device 1, wherein for the two embodiments discussed below, a crawler crane 1 according to Figure 1The primary energy source is a hydraulic pump 14 driven by a motor 15, which operates a series of hydraulic consumers 50 (not shown in detail). These can be, for example, radiator drives or winches such as the hoist rope and retractable winches 6, 9. According to the invention, the hydraulic pump 14 additionally supplies two fallback support cylinders 12, whose piston rods are pivotally coupled to the boom 4 of the working device 1.

[0035] Instead of the system configuration shown here with two symmetrically connected fallback support cylinders 12, other configurations with only one or more than two fallback support cylinders 12 are of course also possible. The exact design of the primary energy source 14, 15 is not relevant to the functioning of the invention. The functioning of the hydraulic system 10 is explained below using one of the two symmetrical branches.

[0036] The fallback support cylinder 12 is separated from the output of the hydraulic pump 14 by an electrically switchable shut-off valve 16, which has a shut-off position and a pass-through position. In the shut-off position, a check valve 24 closes off the load-bearing cylinder chamber 18 of the fallback support cylinder 12; the blocking direction thus points toward the hydraulic pump 14. In the pass-through position, hydraulic fluid can be supplied to or discharged from the fallback support cylinder 12. A hydraulic accumulator 20 and a pressure sensor 28 are located between the shut-off valve 16 and the fallback support cylinder 12, i.e., in the shut-off area.

[0037] An electrically switchable 4 / 3-way valve 26 connects the hydraulic pump 14 with the hydraulic lines leading to the check valve 16 and to the non-load-bearing cylinder chamber or annular chamber 19 of the fall-back support cylinder 12. In the Figure 2the directional control valve 26 is in the (middle) blocking position, so that the connection between the hydraulic pump 14 and the blocking valve 16 is disconnected. The "cross" switching position of the directional control valve 26 shown on the left in the figure is not required during normal operation, since the retraction of the fall-back support cylinder 12 is not hydraulically activated, but rather mediated by the actively operated boom 4. This switching position is required, for example, for assembly operations in which the work tool is partially disassembled for transport, i.e., the boom 4 is removed from the superstructure 3, and the fall-back support cylinder 12, which protrudes beyond the superstructure 3, is retracted to prepare it for shipping. For a work tool without a transport situation or assembly operations, the use of a simpler 4 / 2-way valve without a cross position would also be possible.

[0038] A pressure relief valve 30 is also arranged between the directional control valve 26 and the shut-off valve 16. Another such valve 30 is located in the line connecting the directional control valve 26 to the annular space 19. The maximum pressure limited by the pressure relief valve 30 is electrically adjustable. The pressure relief valve 30, in conjunction with the directional control valve 26, makes it possible to control the pressure and flow rate of the hydraulic fluid or oil flow to and from the fallback support cylinder 12.

[0039] All electrically switchable or controllable valves 16, 26, 30 are controlled by a control system of the implement 1, which, in particular, also controls various actuators of the implement 1, such as the hoist winch 9 or retractable winch 6. The control system also receives values ​​from the pressure sensor 28 regarding the pressure prevailing in the load-bearing cylinder chamber 18 of the fall-back support cylinder 12, thereby implementing a load-sensing function. For this purpose, additional sensors can be provided inside and / or outside the hydraulic system 10.

[0040] In contrast to known systems with a fallback support function, the hydraulic system 10 according to the invention does not require a permanent supply of the fallback support cylinder 12 via a specially provided hydraulic pump. Instead, the already existing hydraulic pump 14 of the work device 1 is used to provide energy for the fallback support function only in certain situations. If the boom 4 is not moved by actuating the retractable winch 6, the load-bearing cylinder chamber 18 and the area connected to the pressure sensor 28 and hydraulic accumulator 20 are blocked off by the shut-off valve 16, so that no energy is exchanged with the rest of the system. The cylinder pressure prevailing before the shut-off valve 16 was last switched to the blocked position is thus maintained in the closed volume. The hydraulic pump 14 then only supplies the remaining consumers 50.

[0041] In order to ensure the fall-back support function of the boom 4 when the load-bearing cylinder chamber 18 is closed off, even when minimal movements of the boom 4 are caused by external forces, for example, when lifting or lowering loads, gusts of wind, etc., a hydraulic accumulator 20 is provided. The required hydraulic fluid flow is drawn from this accumulator when the fall-back support cylinder 12 is extended or added when the fall-back support cylinder 12 is retracted, so that the fall-back support cylinder 12 can follow the smallest movements of the boom 4. The hydraulic accumulator 20 thus functions both as an energy source and as an energy sink for micro-movements of the boom 4.

[0042] When the operator actuates the retracting winch 6 for the purpose of actively moving the boom 4, the blocking valve 16 is opened, i.e. switched to the through position. When the boom 4 is lowered, i.e. when the fall-back support cylinder 12 is extended, the necessary pressure in the hydraulic accumulator 20 or in the load-bearing cylinder chamber 18 for tracking the boom 4 can be maintained or adjusted by supplying hydraulic fluid. For this purpose, the directional control valve 26 is also switched to a through position. When the boom 4 is raised, i.e. when the fall-back support cylinder 12 is retracted, the directional control valve 26 remains in the blocking position, so that hydraulic fluid is now discharged from the load-bearing cylinder chamber 18 to the tank 36 against the resistance generated by the pressure relief valve 30.The adjustable resistance of the pressure relief valve 30 ensures optimal tracking of the fall-back support cylinder 12 even when the boom 4 is raised.

[0043] Thus, the hydraulic pump 14 only needs to supply the fall-back support cylinder 12 when lowering the boom 4 or during any assembly operation. Optionally, it can be provided that the shut-off valve 16 is not switched to the open position when lowering the boom 4, but rather that hydraulic fluid is supplied via an opening in the check valve 24.

[0044] The check valve 24 has, among other things, the function of reducing load change reactions that occur during a change in operating phase, but is not absolutely necessary. The following describes an advantageous operating method of the hydraulic system 10 when raising the boom 4. In the initial situation, in which no active movement of the boom 4 occurs, the check valve 16 and the directional control valve 26 are each in the locked position, so that the hydraulic line between these valves 16, 26 is depressurized. The line between the check valve 16 and the fallback support cylinder 12, on the other hand, is pressurized with accumulator pressure. The operator of the work device 1 now wishes to raise the boom 4 or steepen it and makes the corresponding input via an input unit.

[0045] The control system switches the directional control valve 26 to the open position ("parallel" position), increasing the pressure in the line up to the shut-off valve 16, i.e., the pressure applied to the check valve 24. If the pressure generated by the hydraulic pump 14 exceeds the accumulator pressure in the shut-off area, the check valve 24 opens, establishing pressure equality with the hydraulic accumulator 20. Optionally, the directional control valve 26 can be closed after this point.

[0046] Only now does the control system switch the shut-off valve 16 to the open position. Optionally, however, this step can be omitted, and hydraulic fluid can be supplied via the check valve 24 if necessary (i.e., if the minimum permissible cylinder force is undershot). Subsequently, the retraction winch 6 is actuated to raise the boom 4. This retracts the fallback support cylinder 12 against the resistance of the pressure relief valve 30, so that the fallback support cylinder 12 can be adjusted when the pressure in the load-bearing cylinder chamber 18 is sufficient or when the holding force is sufficient.

[0047] The Figure 3 shows a second embodiment of the hydraulic system 10 according to the invention. In comparison to the first embodiment of the Figure 2The check valve 16, preloaded into the locking position, is hydraulically switchable here and is connected at the control input to the output of a hydraulic shuttle valve 34. The shuttle valve 34 is connected at one input to the annular space 19 of the fallback support cylinder 12, so that the check valve 16 can be switched to the open position by targeted pressurization of the annular space 19. However, this does not occur during normal operation of the work device 1, since the fallback support cylinder 12 is retracted mechanically via the boom 4.

[0048] The other input of the shuttle valve 34 is connected to an electrically switchable switching valve 32. By controlling the switching valve 32 via the control system, the shut-off valve 16 can be switched to the open position. In the closed position, the shut-off valve 16 does not have a check valve; instead, the check valve 24 is permanently connected in parallel. However, the operation of the hydraulic system 10 corresponds to the operation described in the first exemplary embodiment.

[0049] Furthermore, an additional pressure relief valve 22 is provided in the lockable area of ​​the hydraulic system 10, which serves to protect against pressure in the event of a malfunction and is not involved in the function of the hydraulic system or the fallback support during normal operation.

[0050] Overall, the essential advantages of the hydraulic system 10 according to the invention can be summarized as follows: The energy required to supply the fallback support function is only required when the fallback support cylinder 12 is extended. This only occurs when the work tool 1 is operating during "boom lowering" or during any assembly operations that may be required. Minimal movements of the boom 4, which occur due to external forces, are compensated for by the hydraulic accumulator 20, thus ensuring the fallback support function. List of reference symbols:

[0051] 1Working equipment 2Undercarriage with chassis 3Uppercarriage 4Boom 5Retraction rope 6Adjustment device (retraction winch) 7Load handling device (load hook) 8Hoist rope 9Hoist winch 10Hydraulic system 12Fall-back support cylinder 14Hydraulic pump 15Motor 16Lock valve 18Load-bearing cylinder chamber 19Non-load-bearing cylinder chamber 20Hydraulic accumulator 22Pressure relief valve 24Check valve 26Valve (directional control valve) 28Pressure sensor 30Pressure relief valve 32Switching valve 34Shuttle valve 36Tank 50Consumer MLoad torque

Claims

1. A work machine (1), in particular a mobile crane or cable excavator, comprising a pivotable boom (4), a setting device (6) for adjusting the boom (4), at least one further consumer (50), and a hydraulic system (10) for operating a hydraulic fall-back support of the work machine (1) wherein the hydraulic system comprises at least one hydraulic fall-back support cylinder (12) connected to the boom (4) and following its movements for tracking and limiting movements of the boom (4) and comprises a hydraulic pump (14) that operates the fall-back support cylinder (12) and the at least one further hydraulic consumer (50), wherein the fall-back support cylinder (12) has a load-bearing cylinder space (18) that can be acted on by pressure and a non-load bearing cylinder space (19) that can be acted on by pressure in an installation operation, wherein a switchable stop valve (16) having a blocking line and a passage line is arranged between the hydraulic pump (14) and the fall-back support cylinder (12) by means of which stop valve (16) the load-bearing cylinder space (18) of the fall-back support cylinder (12) is blockable; and wherein a hydraulic store (20) connected to the load-bearing cylinder space (18) is provided between the stop valve (16) and the fall-back support cylinder (12).

2. A work machine (1) in accordance with claim 1, characterized in that at least two fall-back support cylinders (12) are provided each having a stop valve (16) and each having a hydraulic store (20) that can together be supplied with hydraulic fluid by the hydraulic pump (14).

3. A work machine (1) in accordance with claim 1 or claim 2, characterized in that a pressure relief valve (22) is provided between the stop valve (16) and the fall-back support cylinder (12).

4. A work machine (1) in accordance with one of the preceding claims, characterized in that a check valve (24) is provided that blocks the region of the hydraulic system (10) blockable by the stop valve (16) in the direction of the hydraulic pump (14), with the check valve (24) being connected in parallel with the stop valve (16) or only being acted on in the blocking position of the stop valve (16).

5. A work machine (1) in accordance with one of the preceding claims, characterized in that a further valve (26), in particular a directional valve, that is preferably electrically switchable is arranged between the stop valve (16) and the hydraulic pump (14).

6. A work machine (1) in accordance with one of the preceding claims, characterized in that the load of the fall-back support cylinder (12) can be sensed by means of a load measurement device that preferably comprises a pressure sensor (28) arranged between the stop valve (16) and the fall-back support cylinder (12).

7. A work machine (1) in accordance with one of the preceding claims, characterized in that a pressure setting device is provided by means of which the pressure and the conveying amount to and from the fall-back support cylinder (12) are settable and can preferably be regulated in dependence on a sensed load of the fall-back support cylinder (12).

8. A work machine (1) in accordance with claim 7, characterized in that the pressure setting device comprises a recuperation device at / in the fall-back support cylinder (12), at least one pressure relief valve (30) arranged between the stop valve (16) and the hydraulic pump (14), a load sensing arrangement, and / or a means for controlling a valve (26) in accordance with claim 5.

9. A work machine (1) in accordance with one of the preceding claims, characterized in that the stop valve (16) is hydraulically controllable by means of a switching valve (32) that is in particular electrically controllable.

10. A work machine (1) in accordance with one of the preceding claims, characterized in that the stop valve (16) is hydraulically controllable and has a control connection that is connected to the output of a hydraulic shuttle valve (34), with an input of the shuttle valve (34) preferably being connected to a non-load bearing cylinder space (19) of the fall-back support cylinder (12) and with the other input being connected to a switching valve (32) that is in particular electrically controllable.

11. A work machine (1) in accordance with one of the preceding claims, characterized in that the hydraulic store (20) is configured to compensate very small movements of the fall-back support cylinder (12) with a load-bearing cylinder space (18) blocked by the stop valve (16) by removing and discharging hydraulic fluid.

12. A work machine (1, in accordance with one of the preceding claims, characterized in that the stop valve (16) is in the blocking position when the boom (4) is not actively moved by means of the setting device (6).

13. A work machine (1) in accordance with one of the preceding claims, characterized in that a control is provided by means of which the stop valve (16) and preferably the setting device (6) are indirectly or directly controllable and that is configured to switch the stop valve (16) into the passage position on actuation of the setting device (6).

14. A work machine (1) in accordance with claim 13, characterized in that a valve (26) in accordance with claim 5 and one check valve (24) in accordance with claim 4 per fall-back support cylinder (12) are provided, with the control being configured to receive a signal for actuating the setting device (6) from an input unit for the purpose of a movement of the boom (4), to thereupon switch the valve (26) into a passage position, and to leave the stop valve (16) in the blocking position, to switch the stop valve (16) into the passage position after opening the check valve (24), and to subsequently actuate the setting device (6).