Patent Publication Number: US-2012043154-A1

Title: Hydraulic system

Description:
FIELD OF THE INVENTION 
     The present disclosure relates to a hydraulic system for a mobile working machine. 
     BACKGROUND OF THE INVENTION 
     Hydraulically controlled steering devices are used for mobile working machines, for example, farm tractors, harvesting machines, or construction machines. Usually, such working machines are equipped with other hydraulic devices, in addition to the hydraulic steering device, such as brake systems, suspensions, power lifts, hydraulic motors, etc., which are supplied hydraulically via a hydraulic main supply pump. The control of several hydraulic devices with only one hydraulic main supply pump takes place via so-called priority control valves, which are controlled as a function of load pressure (Load-Sensing (LS)-control) and guarantee that with high total hydraulic loads, a reduced hydraulic supply of safety-relevant devices, such as a brake system or a steering device, cannot occur. An interconnection of such priority control valves, which, as a rule, is relatively complex and comprehensive, and a necessarily large-volume design of main supply pumps can, in certain operating states, be at the expense of reaction times of the hydraulic components. Thus, large-volume hydraulic pumps require larger priority control valves, which, in turn, have slower response times. The higher the number of various hydraulic loads or devices, the more comprehensive is the priority control system or the load pressure-dependent control pressure lines. On one hand, longer control pressure lines and a higher number of these lines are connected in this way. Both have a negative effect on the response time and the reaction capacity of the entire hydraulic system. If a priority control valve and/or a pressure-producing hydraulic pump do not react fast enough, this can—with reference to a hydraulic steering device—be noticed as an interruption in the steering and perceived as disturbing by the operator of a vehicle. It is also disadvantageous that on some mobile working machines with high volume power requirements, in particular, construction machines, the pump reaction times are slowed down because of reasons of provisioning stability, which likewise can lead to a slowing down of the steering reactions, if high pressure and volume flow are quickly needed by the steering. 
     The response times or reaction times to the general pressure buildup of an aforementioned hydraulic system are to be seen as critical for a steering device both with systems with gear pumps (fixed displacement pumps) and also with variable displacement pumps (variable), since a hydraulic steering device will always be limited, in its reaction time, by the pressure-producing component, so that for this reason, quick reaction times are desired, which, in turn, can contribute to instabilities of the hydraulic system or the individual hydraulic devices. 
     Manufacturers of priority control valves make an effort to optimize the reaction times with the aid of optimized control edges and the use of hydraulic apertures on their valves. Other solutions provide for the use of a second or several supply pumps, .wherein a separate supply pump is proposed for the exclusive supply or also for the additional supply for a steering device with reduced hydraulic service, which, however, is connected with additional costs, or parts and construction expense. 
     Furthermore, a way is known for counteracting a reduced hydraulic supply of the steering device in that a pre-filled hydraulic storage unit is provided, which, in case of need, is correspondingly discharged and cancels a reduced hydraulic supply. Thus, EP 1293669 A2 discloses a hydraulic system for a steering device with a hydraulic storage unit, which is connected to a steering pump and to a steering actuator, wherein the specific conveying volume of the steering pump is dimensioned in such a way that with low operating speeds of the combustion engine, the operation of the steering actuator requires the supply of pressure from the steering pump and the hydraulic storage unit. Only upon exceeding a certain operating speed can the hydraulic need of the steering device be covered from only the steering pump. 
     Such an arrangement of the hydraulic storage unit counteracts a reduced hydraulic supply, but is disadvantageously connected with a delay in the total pressure buildup and, on the other hand, in that, as a whole, an “imprecise,” fluctuating or elastic pressure state is established for the entire steering device. 
     SUMMARY 
     According to an aspect of the present disclosure, a hydraulic system for a mobile working machine includes a hydraulic pump, a steering valve, a steering actuator controlled by the steering valve, a first hydraulic line communicating the hydraulic pump to the steering valve, and a hydraulic storage unit connected to the first hydraulic line by a second hydraulic line. A throttle is connected in parallel with a check valve in the second hydraulic line. The check valve permits one-way fluid flow in a direction away from the hydraulic storage unit. On one hand, such an arrangement makes it possible to have a quick discharge of the hydraulic storage unit when a reduced hydraulic supply of the steering device occurs and thus the reaction capacity of the hydraulic steering device is improved and, on the other hand, this guarantees that the hydraulic storage unit loads up only slowly during a steering movement, so that there is no substantial delay of the pressure buildup and, as a whole, the stability of the pressure state is improved and, all total, a relatively stable and relatively uniform pressure state is established. In particular, this prevents the hydraulic storage unit from being able to load up too rapidly and thus from generating a pressure decline. The aperture or throttle provides for a slow loading of the hydraulic storage unit, could also be replaced thereby by another similar volume flow-reducing means. The check valve makes possible a quick discharging of the hydraulic storage unit, in the direction of the steering device, in case of a reduced hydraulic supply, wherein, the other way around, for the loading of the hydraulic storage unit, the volume flow is exclusively conducted through the aperture or the throttle. Here too, other similar hydraulic means could perhaps be used. 
     A further check valve is located between the hydraulic pump and the second hydraulic line in the first hydraulic line. The further check valve permits one-way fluid flow away from the pump. The further check valve ensures that a discharge of the hydraulic storage unit results in a hydraulic supply taking place only in the direction of the steering device, so that the effect attainable by a discharge of the hydraulic storage unit is not reduced by a pressure relief in the direction of other components of the hydraulic system. Here too, other similar hydraulic means could perhaps be used. 
     A control valve is placed between the hydraulic pump and the second hydraulic line in the first hydraulic line, and provides a separate control of the steering device—that is, a hydraulic supply control, separate from other hydraulic devices or components. 
     The control valve could also be designed thereby as a priority control valve, by means of which the first hydraulic line could be connected with other hydraulic devices, in addition to the steering valve. Thus, the priority control valve could be a hydraulic priority control system, wherein another or several other priority control valves or other hydraulic devices or loads could follow the priority control valve. For example, a hydraulic brake system with a higher priority than the steering device or other devices, such as a hydraulic suspension or power lift with a lower priority, could be correlated with a priority valve. 
     The hydraulic pump could be designed as a variable displacement pump. Alternatively, a fixed displacement pump, for example, a gear pump could also be provided, which is placed in connection with additional volume flow-changing adjusting means. 
     In addition, load pressure-dependent control pressure lines could be provided, by means of which the hydraulic pump could be controlled as a function of the load pressure. Thus, with alternating load pressures or a changing supply need for hydraulic fluid, the hydraulic pump could be correspondingly controlled and, as a whole, the volume flow for the entire hydraulic system, in a load pressure-optimized manner. 
     Furthermore, load pressure-dependent control pressure lines could also be provided, by means of which the at least one control valve could be controlled. In this regard, it is also possible to connect correspondingly arranged priority valves of a priority control system with corresponding load pressure-dependent control pressure lines, so that the volume flow could be controlled for the entire hydraulic system in a load pressure-optimized manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The sole FIGURE is a schematic circuit diagram of a hydraulic system, according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The hydraulic system  10  can be used for mobile working machines, such as, for example, farm vehicles, such as tractors, harvesting machines, self-propelled spraying machines, loading machines, in particular, telescopic loaders, but also construction machines, such as wheel loaders or excavators, etc., and forestry machines. In particular, such machines have several hydraulic devices or loads, which are supplied via only one hydraulic supply pump. 
     The hydraulic system  10  includes a hydraulic pump  12  (preferably a variable displacement pump), which supplies hydraulic fluid from a hydraulic tank  14 , via a supply line  13 , and a steering valve  16 , for the control of a steering actuator  18 . The steering valve  16  is connected with the hydraulic pump  12  via a first hydraulic line  20 . A second hydraulic line  22  connects the first hydraulic line  20  with a hydraulic storage unit  24 . A check valve  26  and a throttle or aperture  28  are connected in parallel in the second hydraulic line  22 . The check valve  26  permits one-way fluid flow in a direction away from the hydraulic storage unit  24 . 
     A second check valve  32  is placed between a connecting site  30 , on which the second hydraulic line  22  is connected with the first hydraulic line  20 . The check valve  32  permits one-way fluid flow in a direction away from the pump  12 . Another hydraulic line  34  connects the steering valve with the hydraulic tank  14 . Furthermore, a control valve  36 , preferably a priority control valve, is provided between the other check valve  32  and the hydraulic pump  12  in the first hydraulic line  20 . Valve  36  is controlled, in a known manner, via the load pressure control lines  38 ,  40 , connected with the steering valve  16  and the first hydraulic line  20 . Another load pressure control line  42  branches off from the aforementioned load pressure control lines  38 ,  40 , and leads to a corresponding adjusting device  44  for the load pressure-dependent adjustment of a volume flow, delivered by the hydraulic pump  12 . Another hydraulic line  46  connects the priority control valve  36  in the usual known manner with other hydraulic device or components (not shown), such as a hydraulic brake system, a hydraulic suspension, a hydraulic power lift system, or another hydraulic device of the working machine. 
     Accordingly, the hydraulic system conveys hydraulic fluid from the tank  14 , via the hydraulic pump  12  and via the priority control valve  36 , to the hydraulic steering device  16 . At the same time, hydraulic fluid is conducted into the hydraulic storage unit  24  via the second hydraulic line  22 , equipped with the throttle or aperture  28  so that the storage unit is slowly loaded with hydraulic fluid parallel to the supply of the hydraulic steering device  16 . The steering actuator  18  can be controlled by actuation of the hydraulic steering device  16 . The steering actuator  18  is coupled to a corresponding steering rod (not shown). Depending on the load of the hydraulic system  10 , a corresponding volume flow is established via the load pressure lines  38 ,  40 ,  42 . If as a result of operating state changes, there should be a reduced supply of the hydraulic system to such an extent that the volume flow or the hydraulic pressure declines, then the hydraulic pressure decline or the reduced hydraulic supply is automatically caught in that the hydraulic storage unit  24  discharges in the direction of the hydraulic steering device  16  via the check valve  26 . At the same time, the second check valve  32  prevents the hydraulic fluid stored in the hydraulic storage unit  24  from flowing off also only in the direction of the hydraulic steering device  16 . 
     While the disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description is to be considered as exemplary and not restrictive in character, it being understood that illustrative embodiments have been shown and described and that all changes and modifications that come within the spirit of the disclosure are desired to be protected. It will be noted that alternative embodiments of the present disclosure may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations that incorporate one or more of the features of the present disclosure and fall within the spirit and scope of the present invention as defined by the appended claims.