DE3909205C1

DE3909205C1 -

Info

Publication number: DE3909205C1
Application number: DE19893909205
Authority: DE
Inventors: Wolfgang Dipl.-Ing. 3355 Kalefeld De Reich; Franz-Werner Dr.-Ing. 4600 Dortmund De Adrian
Original assignee: HANOMAG AG 3000 HANNOVER DE
Current assignee: KOMATSU HANOMAG AG, 30449 HANNOVER, DE
Priority date: 1989-03-21
Filing date: 1989-03-21
Publication date: 1990-05-23
Anticipated expiration: 2009-03-22
Also published as: AT88773T; EP0388641B1; DE59001284D1; EP0388641A3; EP0388641A2; DK0388641T3; ES2041061T3

Description

The invention relates to a hydraulic system for Construction machines, especially wheel loaders, tractors and. Like., with a work tool that can be actuated by hydraulic cylinders, in particular loading shovel, the actuation of Hydraulic cylinder one from a pressure source over one Control valve leading main line to the hydraulic cylinders is provided, of which at least one after the control valve a connecting line leading a hydraulic accumulator branches in which a switchable shut-off valve is arranged is. In such hydraulic systems, this is done manually actuatable control valve the work tool, so especially the loading shovel raised, lowered, tilted etc., whereby goods are picked up and on with the loading shovel can be dumped elsewhere. In addition to that of the pressure source via the control valve to the Main cylinder leading hydraulic cylinders is of course also a return line for the hydraulic medium is provided, the however, is arranged and used in a known manner.

A hydraulic system of the type described at the beginning is out DE-PS 28 56 583 known. In addition to the hydraulic cylinders for The loader bucket can of course also be raised and lowered another pitch circle hydraulic cylinder for tilting the Loading shovel be provided. Via the control valve additional tools can also be switched. These Hydraulic system is provided with a damping system, which is responsible for the pitching vibrations of a wheel loader with the loading shovel filled at higher driving speeds to dampen. For this purpose branches from the main line between control valve and hydraulic cylinder one Connection line leading to one or more Hydraulic accumulator leads through an air or gas cushion to a gas-side pressure of approximately 15 to 20 bar are biased. In this connecting line is a switchable shut-off valve provided, which by hand is switchable. In addition, in the connecting line to the Hydraulic accumulator still be provided a throttle. The Manual operation of the switchable shut-off valve allows the damping circuit formed by the hydraulic accumulator consciously switch on or off to reduce the damping properties consciously change as required. If the construction machine works in the state and, for example, a sensing process Carries out loading shovel, it makes sense to get the system out To design wheel loaders and loading shovels as rigidly as possible e.g. B. the loading shovel when entering into goods to be picked up can be deliberately kept plan and not down into that Sinks well. The reverse is the case with higher ones Driving speeds (higher than about 5 km / h) make sense Switch on damping system, that is to say to that Hydraulic cylinder leading main line over the Connect the connecting line to the hydraulic accumulator so that the entire system of wheel loader and filled Loading shovel has improved damping properties and in this way that of bumps in the road resulting push forces not to increased pitching movements of the wheel loader. Such occurring anyway Due to the damping, pitching movements should quickly become Decay. At the start of a filling cycle Loading shovel of a wheel loader must u. U. Rock from the Recording site first be broken off, being in the Hydraulic cylinders have increased breakaway pressures that occur up to can rise to 220 bar. During this tearing process the hydraulic accumulators are shut off from the main line, because the switchable shut-off valve is in its Closed position. As a result, the Hydraulic accumulator not because of the high tear pressure applied and thus not be overloaded. The Hydraulic cylinders are against incorrect operation of the Manually switchable shut-off valve, however, is not protected. When properly operated, however, the shut-off valve is closed. After the tearing process, the loading shovel tilted and raised, the lifting movement by corresponding actuation of the control valve is ended. It then the carrying pressure in the hydraulic cylinders one that depends on the weight of the bucket and the weight of the recorded goods in the loading shovel is determined. This Carrying pressure can be up to 100 or 120 bar. Now the wheel loader starts to move and leads one appropriate trip, then becomes one from the driver selectable time manually or automatically the switchable Shut-off valve in the sense of establishing a connection between the two parts of the connecting line switched so that the hydraulic cylinder with the carrying pressure of about 100 bar with the hydraulic accumulator (s), in which only the gas-side preload pressure in the The order of magnitude of 15 to 20 bar prevails liquid-side pressure, however, in extreme cases also 0 bar can be connected. By making this Hydraulic fluid flows out of the connection Hydraulic cylinders in the direction of the or Hydraulic accumulator until the corresponding pressure equalization is done. This lowers the wearing pressure in the Hydraulic cylinders suddenly, causing the loading shovel to the load drops by a distance proportional to Pressure difference between the boost pressure and the preload pressure is. This drop can happen suddenly, the bucket being moved by distances up to the order of Can fall 0.5 m, which in turn creates the risk of that part of the load falls out of the loading shovel. If the damping system is not switched on, then The loader avoided falling, the wheel loader then behaves at higher driving speeds as a comparatively rigid system and the pitching movements can, especially if they are in the resonance range, assume such dimensions that not only cargo is lost can, but also for example the rear axle of the Wheel loader is raised so that the wheel loader is no longer is steerable. To avoid these situations, the Connection of the damping system provided. If you have this Damping system does not switch on and off manually, but that switchable shut-off valve for example controls depending on driving speed, then there is Danger of the driver falling from the bucket when a certain driving speed is exceeded is surprised. This drop in the bucket is uncomfortable and has the disadvantages already described.

The invention has for its object a Hydraulic system of the type described at the beginning improve that despite turning on or off a Damping system, which has hydraulic accumulator to the Hydraulic cylinders of the work tool not worth mentioning Lowering of the work tool when switching on the Damping system occurs more.

According to the invention this is achieved in that the Shut-off valve bridging filling line is provided connects the main line to the hydraulic accumulator, and that A pressure reducing valve is arranged in the filling line. The invention is based on the idea that Hydraulic accumulator, usually only with your Preload are charged in the course of a filling process of the working tool or the loading shovel to a higher one Charging pressure that is at least about the expected Carrying pressure corresponds, so when connecting the Hydraulic cylinder and the hydraulic accumulator Shut-off valve no significant amount compensation Hydraulic fluid between the hydraulic cylinder and Hydraulic accumulator takes place and so when the Damping system the carrying pressure in the hydraulic cylinders changed as little as possible. Of course, the wearing pressure of the degree of filling of the work tool, from the specific Weight of the load and the weight of the loading bucket be dependent so that this wearing pressure within certain limits with a certain construction machine with several working cycles can fluctuate. However, these fluctuations are not too great large. If the pressure reducing valve to a relatively high Pressure is set and the loading shovel at one certain filling process is only partially filled, it can even occur that the boost pressure in the hydraulic cylinders is greater than the carrying pressure in the hydraulic cylinders. At the connection of the hydraulic cylinders with the Hydraulic accumulators, however, is not disadvantageous because then the loading shovel is raised slightly if necessary can be avoided and in any case falling. Of course, the filling line and the pressure reducing valve must be such Provide cross sections so that the charging of the Hydraulic accumulator at the beginning of a filling process, so especially during the breakaway phase and maybe also still during the subsequent tilting and lifting process can take place. On the other hand, it is necessary to full tear pressure of up to 220 bar from the Keep hydraulic accumulators away because these hydraulic accumulators are not designed for such a high pressure. These The pressure reducing valve, which in can be realized in different ways. It's important, that the pressure reducing valve to the pressure in the filling line Hydraulic accumulators limited and on the other hand ensures that the boost pressure remains in the hydraulic accumulators, so in the subsequent connection between hydraulic cylinders and hydraulic accumulators the lowest possible pressure compensation he follows. For this purpose, you can also choose between Pressure reducing valve and the hydraulic accumulators Check valve is provided in this part of the filling line be. The filling line must in any case the shut-off valve bridge, d. H. it can either be before or after Control valve connected to the main line. The Different connection options depend on the Main purpose of the construction machines concerned and after desired result. The conscious pressure charging of the Hydraulic accumulator to a boost pressure that is about the same corresponds to the expected carrying pressure of the work tool it advantageously possible that the manually switchable Shut-off valve also dependent on vehicle speed or dependent can be controlled by the tilt angle of the work tool. Because the loading shovel is prevented from falling and itself the height of the loading shovel when the Hydraulic accumulator not or at least not worth mentioning changes, the driver no longer becomes uncomfortable surprised. Rather, he only notes that on the one hand for example leveling work with a fixed loading shovel can be carried out and on the other hand the Pitch vibrations very well at higher driving speeds be dampened.

The pressure reducing valve can be adjusted to the carrying pressure Hydraulic cylinder must be set. This is conveniently done Attitude to an average expected Carrying pressure or to the maximum possible wearing pressure of the Hydraulic cylinder. This boost pressure is in the On the order of 100 to 120 bar, in one Pressure range for which the hydraulic accumulators are designed, without overloading or overstressing.

The pressure reducing valve can be used as a pressure relief valve or be designed as a pressure cutter. Under a Pressure relief valve is understood to be such a valve which only has a limited pressure in the filling line Passes direction to the hydraulic accumulator and at Exceeding this adjustable pressure hydraulic medium via a return line into an oil reservoir is dissipated. This return of the hydraulic fluid can make itself felt in a noteworthy current, on the other hand, it can only absorb one leak oil rate. On the other hand, it is also possible to use the pressure reducing valve To train pressure cutters. Under one Pressure cutters is understood to be a valve which at Reaching the set pressure closes, i.e. the parts of the filling line connected on both sides of each other separates and seals on both sides. You can be one such a pressure cutter also as a shut-off valve or Grasp the two-way valve, with one in the one position Passage is provided while in the other position a completion function is achieved. In the different Embodiments, the pressure reducing valve can either control yourself or via a tax body, which can be designed in particular pressure-dependent will.

If the filling line also bridges the control valve and on the main line is connected before the control valve The special advantage achieved is that the highest in the Main occurring pressures to charge the Hydraulic accumulator can be used. Such pressures occur for example during the tear-off phase of the filling process, if the loading shovel is tilted at the same time. The Utilization of the highest pressures in the main line to Charging the hydraulic accumulator ensures that the Charging process in a comparatively short time takes place, whereby the cross sections in the filling line and correspondingly lower in the pressure reducing valve can and in any case it is ensured that at an exceptionally short tear-off process Boost pressure in the hydraulic accumulator is safely reached.

The pressure reducing valve can be connected to a return line an oil reservoir be connected so that Hydraulic medium, especially as a leak oil, in the tank or Oil reservoir is returned.

It is possible that the pressure reducing valve is time dependent is designed to flow through or with a particular adjustable throttle is provided. This time dependency is quite reasonable, so depending on this way the length of the breakaway phase different charging pressures in the hydraulic accumulators can be saved. A short breakaway phase indicates that the loading shovel only is partially filled so that the wearing pressure in this case turns out lower and also a lower boost pressure in the Hydraulic cylinders are most likely to bear this lower carrying pressure corresponds. Conversely, a longer breakaway phase indicates more difficult working conditions, including a higher one Bucket filling level is to be expected, a higher loading weight occurs and thus a correspondingly higher wearing pressure Balance by a higher boost pressure best is held.

The pressure reducing valve can be designed as a solenoid valve and via a pressure switch in the to the hydraulic accumulator leading part of the filling line can be controlled. In this Case is the pressure reducing valve normally, i. H. at Pressure increase in the filling line, opened. Will in the branch the filling line leading to the hydraulic accumulators, the the intended boost pressure is reached, then the Pressure switch around the solenoid valve or pressure reducing valve, see above that part of the filling line leading to the hydraulic accumulator is cordoned off. The desired boost pressure is then in the Hydraulic accumulators available. It can be seen that the Pressure reducing valve, for example, also controlled time-dependent can be expected if it can be expected or ensured that the intended boost pressure is approximately constant at a certain level Time span is reached.

It is also possible that the switchable shut-off valve, which the hydraulic cylinders with the hydraulic accumulators connects, as driving speed dependent or depending on Tool angle controlled solenoid valve is trained. This eliminates an arbitrary manual Actuation of the shut-off valve by the driver, with what at the same time the risk of incorrect actuation is avoided. This in turn turns on a Safety valve to protect the hydraulic accumulator against Excessive pressures can be dispensed with.

When driving speed dependent control of the Solenoid valve, the switching point can be set so that this can only be exceeded in second gear. In order to on the one hand it is ensured that the retraction of a Wheel loader in a good to be picked up, which experience has shown done in first gear, can not lead to the fact that Damping system is switched on. This leaves the Loading shovel during retraction and pulling away relatively stiff, as u. a. also for grading purposes it is asked for. Even spinning the drive wheels during the breakaway phase in first gear cannot do this lead the hydraulic accumulator with the hydraulic cylinders get connected. This can even cause the wheels to spin in the first course not a connection between Guide the hydraulic cylinder and hydraulic accumulator. These Rather, connection is only possible in second gear made with a correspondingly smaller torque spinning the wheels is no longer possible.

The pressure reducing valve, which is designed as a pressure cutter, can also be designed to be pulse-width-modulated, ie the boost pressure in the hydraulic accumulators can be set or reached via a time dependency and the width of a control pulse.

The invention is based on preferred exemplary embodiments further explained and described. It shows:

Fig. 1 is a circuit diagram of the hydraulic system in a first embodiment;

Fig. 2 shows a detail of the hydraulic system of FIG. 1 with another embodiment of the essential for the invention parts,

Fig. 3 shows a section of the hydraulic system in a third embodiment and

Fig. 4 shows a section of the hydraulic system in a fourth embodiment.

The hydraulic system shown in Fig. 1 has a pressure source, not shown, in particular a continuously driven pump, from which a main line 1 leads to a hydraulic cylinder 3 via a manually operated control valve 2 of conventional design. The hydraulic cylinders 3 can be acted upon on both sides. The main line 1 leads to the side of the pistons of the hydraulic cylinders 3 , which are acted upon during the lifting process of the loading shovel, which is confirmed via the hydraulic cylinders 3 and is not shown. The control valve 2 has a valve unit 4 for raising and lowering the loading shovel, the four positions shown being possible for the lines concerned. A valve unit 5 in the control valve 2 serves to carry out tilting movements on the loading shovel via the lines 6 and 7 . A valve unit 8 permits the actuation of additional devices, for example a tree clip, via the control valve 2 . On the other hand, a line 9 leads from the control valve 2 to the other side of the pistons of the hydraulic cylinders 3 , a lowering process being initiated via this line 9 . A return line 10 leads in a known manner from the control valve 2 to an oil reservoir 11 , which is usually also referred to as a tank.

One or more hydraulic accumulators 12 are provided, which are preloaded to a preload pressure of the order of 15 to 20 bar via an air or gas cushion, not shown. A connecting line 13 leads from the main line 1 between the control valve 2 and the hydraulic cylinders 3 via a switchable shut-off valve 14 , which, as shown, can be designed as a solenoid valve, to the hydraulic accumulators 12 . A line 15 leads from line 9 to the shut-off valve 14 . On the other hand, the shut-off valve 14 is connected to the oil reservoir 11 via a return line 16 via an orifice 17 and a filter 18 , via which the return line 10 can also be guided. It can be seen that the shut-off valve 14 must have two positions. In the position shown, the four connected lines 13 , 15 , 13 , 16 are each terminated. In the other position, the two parts of the connecting line 13 are connected to one another and, on the other hand, the line 15 is connected to the return line 16 . The latter position corresponds to the stage when the damping system is switched on, that is to say the hydraulic accumulators 12 are connected to the main line 1 and thus to the hydraulic cylinders 3 via the connecting line 13 . In the simplest form, the shut-off valve 14 can be operated manually, in which case a safety valve should be provided in the connecting line 13 to protect the hydraulic accumulators 12 against overload. However, if the shut-off valve 14 cannot be switched manually, but rather depending on the driving speed or depending on the reaching of a certain tilt angle of the working device, such a safety valve, as shown, can be dispensed with.

The control valve 2 and the shut-off valve 14 are bridged via a filling line 19 , 20 . The filling line 19 branches off from the main line 1 upstream of the switching valve 2 and leads to a pressure reducing valve 21 , which is designed as a controlled pressure limiting valve according to the symbols shown. The second part or the filling line 20 leads from the pressure reducing valve 21 to the hydraulic accumulators 12 via a check valve 22 . A return line 23 connects the pressure reducing valve 21 to the oil reservoir 11 .

The pressure reducing valve 21 is designed such that it is open at low pressures in the filling line 19 , that is to say connects the filling lines 19 and 20 to one another. The pressure reducing valve 21 is set to a limit pressure which corresponds to the desired boost pressure in the hydraulic accumulators 12 and which corresponds approximately to the expected pressure when the loading shovel is raised in the main line 1 . If this set limit pressure is reached and exceeded, the pressure reducing valve 21 closes the filling line 20 and excess oil or leak oil can flow back into the oil reservoir 11 via the return line 23 . The check valve 22 ensures that the set and desired boost pressure is stored and maintained in the hydraulic accumulators 12 .

A filling process of the loading shovel proceeds as follows: First of all, the continuously driven pressure source conveys hydraulic medium without any significant overpressure in circulation, whereby the individual components, valves and the like. Like., Are set as shown in FIG. 1. The shut-off valve 14 is closed. The control valve 2 is in its illustrated starting position and the pressure reducing valve 21 is open. Since there is no significant pressure in the filling line 19 , the hydraulic accumulators 12 are only loaded with their preload pressure. The shut-off valve 14 may be controlled depending on the driving speed. This control may be set and designed so that a certain speed limit of the order of 6 km / h can only be reached in second gear. If the wheel loader approaches a point for material to be picked up, for example rock in a quarry, this takes place in first gear, so that the shut-off valve 14 assumes its closed position shown in FIG. 1. The filling process now begins with the tear-off phase, in which rock has to be torn away from the pick-up point. This is done by corresponding actuation of the control valve 2 with the wheel loader largely inoperative. The valve units 4 and / or 5 are shifted such that the corresponding movement on the loading shovel is controlled via the main line 1 and / or the line 6 or 7 . Here occurs in the part of the main line 1 , which is brought from the pressure source to the control valve 2 , a very high pressure, which can rise to the order of magnitude of 220 bar at the top. As a result, the pressure in the filling line 19 and via the opened pressure reducing valve 21 in the filling line 20 and thus in the hydraulic accumulators 12 also increases . The hydraulic accumulators 12 are charged to a desired boost pressure, which is limited by the setting of the pressure reducing valve 21 . The set boost pressure can be in the order of 100 to 120 bar. As soon as this pressure is reached, the pressure reducing valve 21 switches over, so that the hydraulic accumulators 12 are protected against this increased pressure even as the pressure in the filling line 19 increases. When the tear-off phase has ended and the loading shovel has been raised to the height desired for driving the wheel loader, the control valve 2 is switched accordingly, as a result of which the part of the main line 1 between the control valve 2 and the hydraulic cylinders 3 is shut off. In this part of the main line 1 , a pressure will build up, which is referred to as the carrying pressure and which corresponds to the weight of the loading shovel and the goods taken up. This carrying pressure can also be of the order of magnitude of 100 to 120 bar and is dependent on the degree of filling of the loading shovel and the specific weight of the load as well as other factors. However, its fluctuations are relatively small. If the wheel loader starts to move in the sense of a driving process, then when a certain driving speed is exceeded, for example 6 km / h, the set switching point of the shut-off valve 14 is exceeded in second gear, so that it switches to its other position shown in FIG. 1 . The two parts of the connecting line 13 are now connected to one another, so that ultimately the hydraulic cylinders 3 are abruptly connected to the hydraulic accumulator 12 via the lines 1 and 13 . Since the carrying pressure corresponds approximately to the boost pressure of the hydraulic accumulator 12 , there is no significant shift in the hydraulic medium and therefore no appreciable pressure equalization, so that the disadvantageous dropping or lowering of the loading shovel after the hydraulic accumulators 12 are switched on to the hydraulic cylinders 3 via the shut-off valve 14 is omitted . As soon as the wheel loader has reached the unloading point, for example a truck, the vehicle speed falls below 6 km / h towards the end of the movement, as a result of which the shut-off valve 14 in turn switches back to the position shown in FIG. 1, so that the hydraulic accumulator 12 is removed from the hydraulic cylinders 3 are separated. Here, too, the pressure in line 1 does not change, so that there is no vertical movement of the loading shovel. The load is then tipped in a known manner.

The hydraulic accumulators 12 are released from the included boost pressure to the preload pressure while the wheel loader is returning from the unloading point to the charging point when the set driving speed of 6 km / h is exceeded. The damping system is switched on here, which is used for the conscious damping of pitching vibrations while driving. Since the loading weight is absent due to the dumped load, a comparatively low residual pressure will also occur in the hydraulic accumulators 12 . The shut-off valve 14 switches over again at the charging point or when the set driving speed is undershot and a new filling cycle can begin.

FIG. 2 shows only the section of the hydraulic system that is essential to the invention, which is otherwise unchanged from the configuration according to FIG. 1. The filling line 19 does not branch off from the main line 1 here, but takes hydraulic medium from the oil reservoir 11 . A separate pump 24 is provided in the filling line 19 . The pressure reducing valve 21 is designed here as a two-position solenoid valve. It is controlled between its two positions shown by a pressure switch 25 , which is set to a desired boost pressure in the filling line 20 . At the beginning of a filling process of the loading bucket, e.g. B. during the tearing phase, the pump 24 is actuated and the hydraulic accumulators 12 charged via the interconnected fill lines 19 and 20 . If the boost pressure set on the pressure switch 25 is reached, this switches the pressure reducing valve 21 into its other position, so that the boost pressure in the hydraulic accumulators 12 is completed and the possibly following pump 24 circulates via the return line 23 . The pump 24 can then also be stopped. Switching the shut-off valve 14 then in turn leads to the connection of the hydraulic cylinders 3 to the hydraulic accumulators 12 .

In the embodiment according to FIG. 3, the pressure reducing valve 21 is designed as a pressure cutter. It has the two positions in which the filling lines 19 and 20 are either blocked off from one another or connected to one another. The pressure reducing valve 21 is controlled via the pressure switch 25 in accordance with the pressure prevailing in the filling line. The filling line 19 is connected to the main line 1 upstream of the control valve 2 , as in the exemplary embodiment in FIG. 1. Hydraulic medium is not returned to the reservoir 11 here. The hydraulic system is protected by the pressure relief valve integrated in the control valve 2 .

FIG. 4 shows an embodiment of the pressure reducing valve 21 , which corresponds to that according to FIG. 1. The circuit is also quite similar, with the one exception that the filling line 19 is not connected here to the main line 1 upstream of the control valve 2 , but rather via part of the filling line 13 to the main line 1 downstream of the control valve 2 . So u. U. not used the highest pressures occurring in the main line 1 for storing the boost pressure in the hydraulic accumulators 12 ; however, this circuit can be useful or desirable. A throttle 26 can be assigned to the pressure reducing valve 21 or integrated therein. The throttle 26 can also be arranged in the filling line 19 or the filling line 20 . The throttle cross section can be adjustable in order to introduce a time dependency in this way, so that depending on the time different charging pressures can be stored in the hydraulic accumulators 12 , which can correspond to different filling levels of a loading scoop, so that in this way an even better adaptation of the respective boost pressure to the respective carrying pressure in the hydraulic cylinders 3 . The pressure difference between the two pressures mentioned is thus even smaller. In extreme cases, any vertical movement of the loading shovel is prevented during a connection process via the shut-off valve 14 . However, it should be expressly pointed out that pressure differences between the carrying pressure and the boost pressure of the order of 10 to 20 bar when connecting the hydraulic cylinders 3 to the hydraulic accumulators 12 are not disadvantageously noticeable, because a resulting lowering of the loading shovel within the spring movements of the wheel loader perish while driving and are regularly not noticed by the driver at all.

Reference symbol list:

1 = main line
2 = control valve
3 = hydraulic cylinder
4 = valve unit
5 = valve unit
6 = line
7 = line
8 = valve unit
9 = line
10 = return line
11 = oil reservoir
12 = hydraulic accumulator
13 = connecting line
14 = shut-off valve
15 = line
16 = return line
17 = aperture
18 = filter
19 = filling line
20 = filling line
21 = pressure reducing valve
22 = check valve
23 = return line
24 = pump
25 = pressure switch
26 = throttle

Claims

1. Hydraulic system for construction machinery, especially wheel loaders, tractors and. Like., With a hydraulic tool ( 3 ) operable work tool, in particular loading shovel, wherein to actuate the hydraulic cylinder ( 3 ) from a pressure source via a control valve ( 2 ) to the hydraulic cylinders ( 3 ) leading main line ( 1 ) is provided, from the after the control valve ( 2 ) branches off to a connecting line ( 13 ) leading to at least one hydraulic accumulator ( 12 ), in which a switchable shut-off valve ( 14 ) is arranged, characterized in that a filling line ( 19 , 20 ) bridging the shut-off valve ( 14 ) is provided which connects the main line ( 1 ) to the hydraulic accumulator ( 12 ), and that a pressure reducing valve ( 21 ) is arranged in the filling line ( 19 , 20 ).

2. Hydraulic system according to claim 1, characterized in that the pressure reducing valve ( 21 ) is set to the carrying pressure of the hydraulic cylinder ( 3 ).

3. Hydraulic system according to claim 1 or 2, characterized in that the pressure reducing valve ( 21 ) is designed as a pressure relief valve or as a pressure cutter.

4. Hydraulic system according to claim 1, characterized in that the filling line ( 19 , 20 ) also bridges the control valve ( 2 ) and is connected to the main line ( 1 ) before the control part ( 2 ).

5. Hydraulic system according to claim 3, characterized in that the pressure reducing valve ( 21 ) via a return line ( 23 ) is connected to an oil reservoir ( 21 ).

6. Hydraulic system according to claim 1, characterized in that the pressure reducing valve ( 21 ) is designed to be flow-dependent as a function of time or is provided with an in particular adjustable throttle ( 26 ).

7. Hydraulic system according to claim 1 or 3, characterized in that the pressure reducing valve ( 21 ) is designed as a solenoid valve and is controlled via a pressure switch ( 25 ) in the part leading to the hydraulic accumulator ( 12 ) of the filling line ( 20 ).

8. Hydraulic system according to claim 1, characterized in that the switchable shut-off valve ( 14 ) is designed as a speed dependent or dependent on the tilt angle of the working tool controlled solenoid valve.

9. Hydraulic system according to claim 8, characterized in that when driving speed-dependent control of Solenoid valve the switching point is set so that this can only be exceeded in second gear.

10. Hydraulic system according to claim 3, characterized in that the pressure reducing valve ( 21 ) designed as a pressure cutter is pulse-width modulated.