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BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a hydraulic system having at least two piston-in-cylinder units, an equalizing line for the pressure equalization between the piston-in-cylinder units and the control pistons, with one respective control piston being associated with a connection of a piston-in-cylinder unit. 
         [0002]    Such hydraulic systems are used, for example, in mobile construction machinery, in particular in hydraulic excavators. Hydraulic excavators and other mobile construction machinery are also inter alia used for load lifting work. In this case, burst pipe safety valves are prescribed for safety reasons which allow the controlled lowering of the load on the bursting of a hose or of a pipeline. 
         [0003]    Two parallel piston-in-cylinder units are typically used for the boom in an excavator, also for the stick with larger machines. An equalization line is installed to ensure a uniform load bearing capacity of the two piston-in-cylinder units. 
         [0004]    In accordance with ISO 8643, the oil loss on a bursting of this equalization line may not amount to more than 10 l/min per cylinder. To meet this demand, in known systems (cf. also  FIG. 1 ), nozzles with the reference numerals  40 ,  42  are used which limit the throughflow quantity to 10 l/min at a maximum pressure (at the drive side). 
         [0005]    This system allows a pressure equalization statically. To ensure a symmetrical load holding in dynamic operation, it is necessary that the control pistons of the system, provided with the reference numerals  30 ,  32  in  FIG. 1 , have the same opening cross-section at all times, which is not simple to ensure in practice due to tolerances and disturbance variables. 
         [0006]    The hydrodynamic jet force ƒ(√{square root over (Δp)};Q) inter alia counts as a disturbance variable which has the result that the control piston releases less cross-section with the same desired value and thus bears more load. This means that as soon as the pressures are not equalized, the load is only borne by one cylinder. 
         [0007]    A sufficient equalization amounts is already required between the cylinders at a low pressure difference for safe dynamic operation. Due to the throughflow behavior of nozzles ƒ(√{square root over (Δp)};A), no sufficient equalization can take place to compensate the influence of the tolerances at a low pressure difference, above all with larger machinery. The standard would no longer be satisfied by the installation of a nozzle with a larger restrictor cross-section. 
       SUMMARY OF THE INVENTION 
       [0008]    It is therefore the object of the present invention to further develop a hydraulic system of the initially named kind in an advantageous manner, in particular such that a sufficient equalization quantity of hydraulic liquid can always be provided for the compensation of pressure differences, such that the system has a simple and safe structure and the relevant standards are still satisfied. 
         [0009]    This object herein is achieved in accordance with the invention by a hydraulic system having the features herein. Provision is accordingly made that a hydraulic system has at least two piston-in-cylinder units, at least one equalization line for the pressure equalization between the piston-in-cylinder units and the control pistons, with one respective control piston being associated with a connection of a piston-in-cylinder unit and with at least one 2-way flow regulation valve with a check valve function being in communication with the at least one equalization line. 
         [0010]    The advantage thereby results that a constant equalization quantity can already be achieved with a small pressure difference, said equalization quantity being larger by a factor of 5-10 at the decisive moment, that is, on an occurrence of pressure differences, than with a solution which only has one nozzle like the previously known systems. It is at the same time ensured that the maximum oil loss on the bursting of the equalization line no longer exceeds more than 10 l/min per cylinder and the corresponding standard is thus satisfied. It is, for example, conceivable that a 2-way flow regulation valve with a check function in both directions is formed in an integrated construction and is arranged in the equalization line. It is also possible that such a 2-way flow regulation valve in an integrated construction also includes the equalization line. 
         [0011]    Provision can furthermore be made that the 2-way flow regulation valve connects the equalization line, a control piston and a connection of the piston-in-cylinder unit. This arrangement advantageously allows a safe pressure equalization in dynamic operation with a simultaneously simple construction. 
         [0012]    It is furthermore possible that a 2-way flow regulation valve is associated with each piston-in-cylinder unit. The advantage thereby results that previously used hydraulic systems can be retrofitted since the 2-way flow regulation valves can advantageously be used instead of the previously used restrictors. It is further thereby possible to create a symmetrical design of the hydraulic system. A symmetrical design advantageously facilitates a symmetrical load distribution in dynamic operation. 
         [0013]    It is furthermore conceivable that the 2-way flow regulation valves are each arranged at the end side of the equalization line. 
         [0014]    Provision can furthermore be made that the 2-way flow control valve has two parallel fluid guides, with a first restrictor element with an adjustable diameter being provided in a first fluid guide and/or with a check element and a second restrictor element being arranged in series in a second fluid guide. The check element can be a check valve, for example. The check element is in this respect preferably a check element made such that it only triggers at a specific limit pressure or an abrupt pressure drop. 
         [0015]    It is preferred if the second restrictor element is a nozzle. Provision can, however, likewise equally be made that the second restrictor element is a restrictor valve. 
         [0016]    It is furthermore possible that the 2-way flow regulation valve is arranged integrated in a housing and/or in an intermediate plate. It is thereby possible to create a compact construction of the hydraulic system. Such an integrated construction furthermore allows protection against environmental conditions such are not uncommon with mobile construction machinery, for example. The housing can also be a previously used housing of a conventional burst pipe safety valve. 
         [0017]    Provision can moreover be made that the 2-way flow regulation valve is arranged in a line branching off from the line connecting the control pistons and the connection of the piston-in-cylinder unit and connects this branching line to an end of the equalization line. 
         [0018]    It is preferred if the piston-in-cylinder units are made in the same construction. 
         [0019]    Provision can further advantageously be made that the piston-in-cylinder unit has a cylindrical piston space and a ring space, with the connection of the piston space being connected to a connection of the 2-way flow regulation valve. 
         [0020]    It can furthermore be of advantage if the 2-way flow regulation valves are made in the same construction. 
         [0021]    The invention furthermore relates to a mobile construction machine having the features herein. Provision is accordingly made that a mobile construction machine has at least one hydraulic system in accordance with these features. 
         [0022]    It is in particular of advantage if the mobile construction machine is an excavator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    Further details and advantages will now be explained with reference to an embodiment shown in more detail in the drawing. 
           [0024]    There are shown: 
           [0025]      FIG. 1 : a schematic representation of a known hydraulic system; and 
           [0026]      FIG. 2 : a schematic representation of an embodiment in accordance with the invention of a hydraulic system. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]      FIG. 1  shows, in a schematic representation, a known hydraulic system  10  for a mobile construction machine such as a hydraulic excavator, which is not shown in any more detail. The hydraulic system  10  is in this respect of a substantially symmetrical structure and has two piston-in-cylinder units  20 ,  22  which are connected in parallel and are made in the same construction. 
         [0028]    The piston-in-cylinder units  20 ,  22  have cylindrical piston spaces  21 ,  23  which are in fluid communication with the further components of the hydraulic system  10  via connections  24 ,  25 . The piston  34 ,  36  of the piston-in-cylinder unit  20 ,  22  furthermore has a piston rod  35 ,  37  which can be moved out by increasing the volume or the pressure in the piston space  21 ,  23 . 
         [0029]    To move in the piston  34 ,  36  or the piston rod  35 ,  37  of the piston-in-cylinder units  20 ,  22 , the volume in the ring space  26 ,  27  is increased, for which purpose fluid is supplied via the connections  28 ,  29  of the ring spaces  26 ,  27  while the volume in the piston space  21 ,  23  is reduced accordingly. 
         [0030]    To move the piston rods  35 ,  37  of the piston-in-cylinder units  20 ,  22  out dynamically in a uniform manner, a line  120  is provided which branches off from the line  110 ,  110 ′ connected to the connections  24 ,  25  and which has a restrictor element  40 . The components of restrictor element  40 , line  110  and line  120  or restrictor element  42  and lines  110 ′ and  120 ′ can advantageously each be combined to form a burst pipe safety valve  45 ,  45 ′. The restrictor elements  40 ,  42  bound the maximum throughflow to 10 l/min so that a maximum of 10 l/min of hydraulic fluid can be discharged per piston-in-cylinder unit  20 ,  22  in the event of a rupture of the line  50 . The equalization line  50  is in this case respectively connected to the burst pipe safety valves  45 ,  45 ′ or to the restrictors  40 ,  42 . 
         [0031]    Hydraulic fluid is pumped from the reservoir  60 ,  62  via the hydraulic lines  100  or  100 ′ to the piston spaces  21 ,  23  to move out the piston rods  35 ,  37 . Before the hydraulic fluid enters via the connections  24 ,  25  of the piston spaces  21 ,  23 , the hydraulic fluid first flows through the control pistons  30 ,  32  which set their throughflow cross-section with reference to the preset desired value, e.g. for the pressure to be applied against the pistons  34 ,  36 . 
         [0032]    After the control piston  30 ,  32 , the hydraulic fluid flows through the burst pipe safety valves  45 ,  45 ′ which are made in the same construction. 
         [0033]    As already stated above, such an embodiment of the known hydraulic system does not allow any sufficient compensation of pressure differences in dynamic operation with a simultaneous satisfaction of the standards for oil loss on the bursting of the equalization line  50 , according to which no more than 10 l/min oil may be discharged per piston-in-cylinder unit  20 ,  22 . 
         [0034]    An embodiment in accordance with the invention of a hydraulic system  10  for a mobile construction machine such as a hydraulic excavator is shown in  FIG. 2 ; this embodiment on the one hand allows a highly dynamic equalization of hydraulic fluid via the equalization lines  60 , for example also with a slightly different cross-section of the control pistons  30 ,  32 , with the standard at the same time still being satisfied, according to which a maximum of 10 l/min of hydraulic fluid may be discharged per piston-on-cylinder unit on a bursting of the equalization line so that the load raised indirectly by the moved out piston-in-cylinder units  20 ,  22  is slowly lowered. 
         [0035]    In this respect, the embodiment shown in  FIG. 2  is substantially of comparable structure as the hydraulic system  10  shown in  FIG. 1 . Comparable components are accordingly provided with the same reference numerals. Only the previously known burst pipe safety valve  45 ,  45 ′ was replaced by a 2-way flow regulation valve  70 ,  72 . The 2-way flow regulation valve  70 ,  72  is in this respect in each case arranged at the end side in the equalization line and is connected via the line  120  or  120 ′ to the line  110 ,  110 ′ leading to the connection  24 . 
         [0036]    In the event that, for example, the control piston  30  should have a larger cross-section than the control piston  32  arranged at the other side, the hereby higher pressure, which would normally act on the piston  34 , is compensated via the equalization line  50  or, in the case of bursting, a restricted discharge of hydraulic fluid such as hydraulic oil is made possible. 
         [0037]    A part of the fluid coming from the control piston  30  branches off via the line  120  into the 2-way flow regulation valve  70  which on the one hand has two fluid guide paths connected in parallel. 
         [0038]    In this respect, a restrictor element  74  with an adjustable opening cross-section is provided in a first fluid guide path of the 2-way flow regulation valve  70 . A restrictor element  76  and, downstream thereof, a check valve  75  are arranged in the other fluid guide. 
         [0039]    The 2-way flow regulation valve  72  is in this respect in the same construction of the 2-way flow regulation valve  70 . 
         [0040]    On a bursting of the line  50 , the check valves  75  prevent more than 10 l/min of hydraulic fluid from being discharged per piston-in-cylinder unit  20 ,  22  so that the pistons  34 ,  36  move in evenly and slowly. In this case, the second fluid guides with the restrictor element  76  are each blocked via the restrictor element  75  so that a fluid discharge can only take place via the first restrictor element  74  having the variable diameter. In such a case, for example, it is ensured via a controller or regulator, not shown in any more detail, that the restrictor element  74  only has an opening cross-section which allows a maximum throughflow of 10 l/min. 
         [0041]    Provision is advantageously made that a cross-section increase in the first restrictor element  74  having a variable diameter is only made possible by a direct control in a case in which an equalization requirement is recognized, that is, for example, only on the moving out of the piston-in-cylinder units  20 ,  22 , with different opening cross-sections of the control pistons  30 ,  32  playing a role, so that a highly dynamic control via the equalization line  50  can take place. 
         [0042]    After the moving out has taken place, a reset back into the zero position takes place, that is, the diameter of the restrictor element  74  again allows a maximum throughflow of 10 l/min. It is hereby ensured that an increased throughflow is only possible on an equalization, whereas in another respect throughflow quantities are only possible through the restricted opening cross-section of the restrictor element  74  which correspond to the standard so that the oil loss no longer amounts to more than 10 l/min per piston-in-cylinder unit  20 ,  22  on a bursting of the compensation line  50 . 
         [0043]    On a recognized bursting of the equalization line  50 , for instance by a recognized abrupt pressure drop in the region of the equalization line  50 , the restrictor elements  74  are reset to the zero position, if not already done, preferably automatically. 
         [0044]    The restrictor elements  74  can in this respect be self-resetting, i.e. a separate activation has to take place for an enlarged diameter. Such an activation can then be reversed at the latest in a recognized bursting case so that the restrictor elements  74  automatically reset themselves to a diameter which allows a maximum throughflow of 10 l/min.

Summary:
The present invention relates to a hydraulic system having at least two piston-in-cylinder units, at least one equalization line for the pressure equalization between the piston-in-cylinder units and the control pistons, with one respective control piston being associated with a connection of a piston-in-cylinder unit and with at least one 2-way flow regulation valve with a check valve function being in communication with the at least one equalization line.