Abstract:
An hydraulic control arrangement which is chiefly used for a mobile working machine, especially for a wheel loader, and has at least one hydraulic cylinder with the aid of which a working tool can be moved, a directional control valve for controlling the pressure-fluid channels between the hydraulic cylinder, a pressure-fluid source and a tank, a hydraulic accumulator which can be connected to the pressure-fluid source via a filling line, and a control valve with the aid of which a connection between the hydraulic accumulator and the hydraulic cylinder can be controlled to open and close. The aim of such a hydraulic control arrangement is to avoid undesired movements of the hydraulic cylinder. This is achieved wherein the fact that the hydraulic accumulator can be filled up only when the directional control valve is actuated.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The invention relates to a hydraulic control arrangement which is used for a mobile working machine, especially for a wheel loader. 
     It is known from DE 39 09 205 C1 to make use for the purpose of damping the pitching vibrations of wheel loaders which occur, especially, with a full loading shovel and at a high driving speed of a damping system which is a component of the hydraulic control arrangement of the wheel loader. For the purpose of vibration damping, the generally two hydraulic lifting cylinders for raising and lowering the loading shovels can be connected via a shutoff valve to a hydraulic accumulator which can be charged by a hydraulic pump via a fill line which branches off from the pump line upstream of the directional control valve block. The shutoff valve, which is arranged between the hydraulic accumulator and the lifting cylinders, is closed as long as the loading shovels are working, and can be opened by the driver or automatically as soon as pitching vibrations occur during driving, or as soon as the driving speed is above a specific value, for example above 6 km/h. 
     As a consequence of the fact that the filling line branches off upstream of the directional valve control block, the hydraulic accumulator is charged not only upon actuation of the directional control valve assigned to the lifting cylinders, but upon actuation of any direction control valve which leads to a build up of pressure in the pump line. For example, the actuation of a steering valve belonging to a hydraulic steering system of the working machine can also lead to an inflow of pressure fluid to the hydraulic accumulator. If the shutoff valve is then also open, an uncontrolled movement of the lifting cylinders can take place. 
     Another damping system against pitching vibrations, which is likewise part of the hydraulic control arrangement of a working machine, is known from DE 41 29 509 C2. In this case, the filling line branches off from a working line which runs between the lifting cylinders and the directional control valve assigned thereto. The shutoff valve arranged in the filling line is pressure-controlled and can be opened by the load pressure, prevailing in the working line, of the lifting cylinders against the accumulator pressure, which can be applied to a rear control chamber on the valve element of the shutoff valve, and against the force of a weak compression spring. The accumulator pressure is thus in each case only slightly lower than the highest load pressure of the lifting cylinders which occurs during a working cycle. In order to damp the pitching vibrations, the rear control member of the shutoff valve is unloaded via a pilot valve to the tank, with the result that the shutoff valve can be opened and pressure fluid can be pushed back and forth freely between the hydraulic accumulator and the lifting cylinders. 
     It has emerged that because of special kinematic relationships on a wheel loader when the shovel is being pushed into the earth, a higher pressure can be built up in the lifting cylinders than prevails in the hydraulic accumulator, and then pressure fluid is displaced into the hydraulic accumulator from the lifting cylinders even when the assigned directional control valve is not actuated. The wheel loader is raised at the front, with the result that the front wheels turn in the air and the rear wheels dig in. This is disadvantageous for the loading operation. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention further to develop a hydraulic control arrangement of the introductory-mentioned type so as largely to avoid uncontrolled movements of the hydraulic cylinder which can be connected to the hydraulic accumulator in order to damp pitching vibrations of the working machine. 
     According to the invention, this object is achieved in the case of a hydraulic control arrangement having the features from the preamble of claim  1  by virtue of the fact that the hydraulic accumulator can be filled up only when the directional control valve, which is assigned to the hydraulic cylinder, is actuated. Consequently, upon actuation of another directional control value no pressure fluid can flow into the hydraulic accumulator, with the result that the actuation of another directional control valve cannot lead to a movement of the hydraulic cylinder. It is also prevented that pressure fluid is displaced into the hydraulic accumulator from the hydraulic cylinder when the directional control valve is not actuated. The enclosed pressure fluid therefore prevents a movement of the cylinder piston and thus a change in the relative position between the boom and the body of the working machine, with the result that raising of the front axle is prevented. 
     The hydraulic cylinder is usually a double-acting one and can be controlled with the aid of a directional control valve which can be adjusted into working positions in two opposite directions starting from a middle neutral position. In accordance a feature of the invention it is provided that the hydraulic accumulator can be filled up only when the directional control valve is actuated in one direction. The control arrangement is thereby simplified. In the case of a wheel loader, for example, it suffices when pressure fluid can flow into the hydraulic accumulator only given an actuation of the directional control valve in the direction of raising the lifting cylinders. 
     In accordance with another feature of the invention, use is advantageously made for the purpose of controlling the capacity of the hydraulic accumulator to be filled of a control valve which can be switched over by actuating the directional control valve from a first control position, in which it prevents filling of the hydraulic accumulator, that is to say in which the filling line is blocked off such that no pressure fluid flows into the hydraulic accumulator even when the pump pressure or the load pressure is higher than the accumulator pressure into a second control position, in which the hydraulic accumulator can be filled. 
     The control valve which can be switched over by actuating the directional control valve is advantageously a seated valve, thus ensuring the filling line is blocked off in a largely leak-free fashion. 
     It is conceivable in principle for the control valve which can be switched over by actuating the directional control valve to be coupled mechanically to the latter. Directional control valves which are used in mobile hydraulics are currently ever more frequently driven no longer directly mechanically but with the aid of precontrol signals which can be prescribed with the aid of a precontrol unit. In accordance with still another feature of the invention, a precontrol signal for the directional control valve is also used for the purpose of producing a control signal for switching over the control valve from the first control position into the second control position. 
     Particularly preferred is a refinement according to which the filling line branches off from a working line running between the directional control valve and the hydraulic cylinder, and there is located in the filling line a pressure-controlled shutoff valve with a valve element to which the load pressure prevailing in the working line can be applied in the opening direction and the pressure in a rear control chamber can be applied in the closing direction. The rear control chamber is connected to the hydraulic accumulator in order to fill the hydraulic accumulator. An open connection between the hydraulic accumulator and the hydraulic cylinder must be present in order to damp the pitching vibrations of the working machine. The rear control chamber is unloaded with respect to the tank for this purpose. The rear control chamber can be blocked off against the outflow of pressure fluid in order to prevent filling of the hydraulic accumulator when the directional control valve is not actuated. The shutoff valve then cannot be opened by the load pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A plurality of exemplary embodiments of a hydraulic control arrangement according to the invention are represented in the drawings. The invention will now be explained in more detail with the aid of these exemplary embodiments. 
     In the drawings: 
     FIG. 1 shows the first exemplary embodiment, in which the filling line is connected to the working line, which leads to two hydraulic cylinders, and there is located in the filling line a shutoff valve which is controlled by two pilot valves which can be actuated electromagnetically, 
     FIG. 2 shows the second embodiment, which is largely of identical design to the first embodiment, and in the case of which two individual pilot valves of the first embodiment are combined to form a single pilot valve, and 
     FIG. 3 shows a third embodiment, in which the filling line branches off from the pump line upstream of the directional control valve, and is located in the filling line of a control valve with the aid of which the filling line can be blocked off when the directional control valve is not actuated. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The hydraulic control arrangements shown are provided in each case for wheel loaders, tractors, telescopic handling equipment or other machines, and comprise a control block  10  with a plurality of directional control valves, especially also with a directional control valve  11  which can assume a spring-centered mid-position, and with the aid of which it is possible to drive two hydraulic cylinders  12  which are designed as differential cylinders and with the aid of which the boom of a wheel loader can be raised and lowered. The directional control valve  11  has a first working port  13  from which a first working line  14  leads to the base-side pressure chambers  15  of the hydraulic cylinders  12 . A second working line  16  runs between a second working port  17  of the directional control valve  11  and the pressure chambers  18 , on the piston-rod side, of the hydraulic cylinders  12 . 
     In the embodiment according to FIG. 1, a filling line  20  which leads to a hydraulic accumulator  21  branches off from the working line  14 . Arranged in the filling line is a shutoff valve  22  which is designed as a 2-way cartridge valve and has a movable valve element  23 . The latter is a differential piston which can be seated on a seating cone with the end face of the piston section of smaller diameter, in the manner of a seated valve. The pressure prevailing in the working line  14 , that is to say the load pressure of the two hydraulic cylinders  12  is applied in the opening direction to the valve element  23  at said end face. The accumulator pressure acts in the opening direction at the annular surface between the two piston sections of the valve element  23 . A pressure prevailing in a rear control chamber  24 , and a weak compression spring  25  are applied to the valve element  23  in the closing direction. 
     Three further valves are constructed on the plate  26  with the 2-way cartridge valve  22 . The first valve  30  is a 3/2-way valve with a first inlet  31 , which is connected to the section of the filling line  20  located between the working line  14  and the shutoff valve  22 , and with a second inlet  32 , which is connected to the hydraulic accumulator  21 . An outlet  33  of the directional control valve  30  can be connected either to the inlet valve  31  or to the inlet  32 , depending on the load pressure in the working line  14 . To be specific, a settable compression spring acts on the valve element (not represented in more detail) of the valve  30  in order to connect the outlet  33  to the inlet  32 . The pressure in the inlet  31 , that is to say the load pressure of the hydraulic cylinder  12 , is applied to the valve element in order to connect the outlet  33  to the inlet  31 . 
     Mounted on the directional control valve  30  is a pilot valve  40  which is designed as a 2/2-way seated valve and can be regarded as a pilot-operated check valve. A control channel  41  emerging from the outlet  33  of the directional control valve  30  leads through the housing of the pilot valve  40  to a port P of a further pilot valve  50 , which is a 4/2-way valve. Under the action of a compression spring  51 , the valve element of the said 4/2-way valve assumes a neutral position in which there is a passage between the port P and a port A which is connected to a port of the pilot valve  40 . A tank port T and a further port B of the pilot valve  50  are blocked off in the neutral position of the latter. The tank port is connected to a leak port Y of the plate  26  via channels leading through the housings of the various valves. The port B of the pilot valve  50  is connected to the rear control chamber of a second 2-way cartridge valve  60 , which is located in the plate  26  and via which the pressure chambers  18 , on the side of the piston rods, of the hydraulic cylinders  12  can be connected to a tank port T of the plate  26 . The valve element of the pilot valve  50  can be brought by an electromagnet  52  into a control position in which the port P is blocked off and two ports A and B are connected to the port T. 
     The pilot valve  40  is arranged in a line  53  between the port A of the pilot valve  50  and the rear control chamber  24  of the shutoff valve  22 . A compression spring  42  and the pressure prevailing in the rear control chamber of the shutoff valve  22  are applied to the valve element of said pilot valve  40  in the direction of a neutral position in which said valve element prevents an outflow of pressure fluid from the rear control chamber  24  to the port A of the pilot valve  50 , but permits an inflow of pressure fluid from the port A of the pilot valve  50  into the rear control chamber  24 . The valve element can be brought by an electromagnet  43  into a control position in which there is a free passage between the port A of the pilot valve  50  and the rear control chamber  24  of the shutoff valve  22 . 
     The directional control valve  11  of the control block  10  can be actuated in a hydraulically proportional fashion, the precontrol pressures being produced with the aid of a hydraulic precontrol unit  55  and being given to the directional control valve  11  via control lines  56 . An electric pressure switch  57  which can be switched over at a specific precontrol pressure is connected to the precontrol line, into which a precontrol pressure can be introduced when the hydraulic cylinder  12  is to be actuated in the direction of raising the boom and for this purpose pressure fluid is applied to the base-side pressure chambers  15 . This specific precontrol pressure is preferably the precontrol pressure at which the spring-centered sliding spool of the directional control valve  11  begins to move. The switching over of the pressure switch  57  excites the electromagnet  43  of the pilot valve  40 , and said pilot valve is brought into the control position in which an open connection prevails between the port A of the pilot valve  50  and the rear control chamber  24  of the shutoff valve  22 . 
     Voltage is applied to the electromagnet  52  of the pilot valve  50  arbitrarily by the vehicle driver when pitching vibrations occur, or automatically at a specific speed of the mobile working machine, for example at a speed of 6 km/h. 
     In the neutral position of the directional control valve  11 , there is no precontrol pressure in the precontrol line  56 , to which the pressure switch  57  is connected, with the result that the pressure switch  57  is in its neutral position and the electromagnet  43  of the pilot valve  40  is de-excited. The pilot valve  40  therefore assumes the control position shown in FIG. 1, in which an outflow of pressure fluid from the rear control chamber  24  of the shutoff valve  22  is prevented. Should a pressure build up in the pressure chambers  15  of the hydraulic cylinders  12  when the loading shovel is put into the earth or other bulk material, the shutoff valve  22  cannot be opened by this pressure, since opening the shutoff valve would be possible only if pressure fluid were displaced from the control chamber  24 . The charging state of the hydraulic accumulator  21  is therefore not influenced by the build up of pressure in the pressure chambers  15 . The same holds when the directional control valve is actuated in the direction of lowering the hydraulic cylinders  12 . The pilot valve  40  then also remains in the control position shown, with the result that the shutoff valve  22  cannot be opened by the pressure in the pressure chambers  15  of the hydraulic cylinders  12 . 
     When, by contrast, the directional control valve  11  is actuated in the raising direction, the pressure switch  57  switches over, as a result of which the electromagnet  43  is actuated and the valve element of the pilot valve  40  reaches the second control position. As long as the pressure in the pressure chambers  15  remains below the pressure set at the compression spring  34  of the directional control valve  30 , the latter switches the accumulator pressure through via the pilot valve  50  and the pilot valve  40  to the rear control chamber  24  of the shutoff valve  22 . The load pressure in the pressure chambers  15  of the hydraulic cylinders  12  now opens the shutoff valve  22  whenever it is above the accumulator pressure by at least the small pressure difference equivalent to the force of the compression spring  25 . Pressure fluid can then reach into the hydraulic accumulator, with the result that, neglecting the source of the weak compression spring  25  for once, said hydraulic accumulator is always charged up to the highest load pressure occurring in the pressure chambers  15  during a working cycle. Only when the load pressure at the valve  30  is able to overcome the force of the compression spring  34  does the shutoff valve  22  remain closed. The point is that after the valve  30  is switched over the load pressure is present in the rear control chamber  24  of the shutoff valve  22 , with the result that the shutoff valve  22  is reliably closed in conjunction with the compression spring  25 . The pressure in the hydraulic accumulator  21  can therefore not exceed the value set at the compression spring  34  of the valve  30 . However, for safety reasons provision is also made of a relief valve  58  the inlet of which is connected to the hydraulic accumulator  21 . 
     It may now be assumed that the loading shovel is loaded and that the wheel loader is being driven to an unloading site. As soon as in this case a specific speed is exceeded, current flows through the two magnets  43  and  52  of the pilot valves  40  and  50 , with the result that the two pilot valves switch over from the neutral positions shown in FIG. 1 into the other control position. The rear control chamber  24  of the shutoff valve  22  is now connected via the pilot valve  40  and via the pilot valve  50  to the port Y of the plate  26 , and thus unloaded with respect to the tank  27 . The valve element  23  of the shutoff valve  22  is raised from its seat by the load pressure and accumulator pressure, with the result that there is an open connection between the hydraulic accumulator  21  and the pressure chamber  15  of the hydraulic cylinders  12 . Since the accumulator pressure  21  corresponds to the maximum pressure reached during the working cycle, upon opening of the shutoff valve  22  no sinking of the loading shovel occurs, but rather a slight raising. It may well be that load pressures occur during the working cycle which cause the valve  30  to switch, and which therefore are not followed by the loading state of the hydraulic accumulator  21 . However, these load pressures occur only in special situations, for example when an object anchored in the ground tears free, or when the loading shovel is driven against a stop, but are not caused by the weight of the loading shovel and the loading material, which acts solely when the wheel loader is being driven. The loading state of the hydraulic accumulator  21  is therefore always sufficient to keep the loading shovel at the level which the latter assumes upon opening of the shutoff valve. 
     In order to balance volumetric changes in the pressure chambers  18  which occur during the open connection of the pressure chambers  15  to the hydraulic accumulator  21 , pressure fluid can be displaced from the pressure chambers  18 , on the side of the piston rod, of the hydraulic cylinders  12  into the tank, or fed in out of the tank via the valve  60 , which is likewise opened by the switching over of the pilot valve  50 . 
     The design according to FIG. 2 differs only with regard to the pilot valves  40  and  50  from that according to FIG.  1 . Reference is therefore made to the corresponding description of FIG. 1 with regard to the control block  10 , the hydraulic cylinder  12 , the hydraulic accumulator  21 , the shutoff valves  22  and  60 , the directional control valve  30  and the precontrol unit  55 . By contrast with the design according to FIG. 1, the two pilot valves  40  and  50  are now combined to form a single pilot valve  60  which is mounted on the directional control valve  30 . The pilot valve  60  has a spring-centered mid-position in which the ports T and B are blocked off and only a throughflow from P to A is possible between the ports P and A. By driving an electromagnet  52 , the valve element of the pilot valve  60  can be brought from the mid-position into a lateral control position in which the ports A and B are connected to the port T. The valve element can be adjusted from the mid-position in the opposite direction by applying to it the pressure prevailing in the precontrol line  56 , in which a precontrol pressure is built up in order to feed pressure fluid into the base-side pressure chambers  15  of the hydraulic cylinders  12 . The port A is freely connected to the port P in this other control position of the directional control valve  60 . The port A is now connected directly to the rear control chamber  24  of the shutoff valve  22 . 
     The pilot valve  60  assumes its mid-position in the mid-position or in the lowered position of the directional control valve  11 , if the speed of the working machine does not exceed a specific value or the electromagnet  52  has not arbitrarily been switched on. In the raised position of the directional valve  11 , pressure is applied to the valve element of the directional control valve  60 , which reaches the control position in which the port P is connected to the port A. Depending on the position of the valve  30 , the accumulator or the load pressure is then present in the rear control chamber  24  of the shutoff valve  22 . The hydraulic accumulator is charged up to the maximum load pressure achieved or to the pressure set at the compression spring  34  of the valve  30 . The electromagnet  52  is excited in order to damp pitching vibrations during a rapid drive, with the result that the valve element of the directional control valve  60  reaches the control position in which the ports A and B are connected to the port T. The rear control chambers of the shutoff valves  22  and  60  are relieved of pressure, with the result that the two shutoff valves  22  and  60  open, and an open connection exists between the hydraulic accumulator  21  and the pressure chambers  15  of the hydraulic cylinders  12 , and pressure fluid can be displaced from the pressure chambers  18 , and pressure fluid can be fed into the pressure chambers  18 . Reference may be made explicitly at this juncture to the fact that the pressure at the port T of a valve block is usually a few bars above the tank pressure which prevails at the leakage oil port Y, with the result that the shutoff valve  60  can be opened against the tank pressure in the rear control chamber by the pressure at the tank terminal T. 
     It is particularly advantageous that after slowing down the drive and de-exciting the electromagnets  43  and  52  in the case of the design according to FIG. 1, and de-exciting the electromagnet  52  alone in the case of the design according to FIG. 2, because of the mode of operation of the pilot valves  40  and  60  as check valves accumulator pressure is immediately introduced into the rear control chamber  24  of the shutoff valve  22 , and the shutoff valve therefore closes. An influence exerted on the hydraulic accumulator  21 , or an influence exerted by the hydraulic accumulator  21  on the control is then excluded upon a subsequent lowering of the hydraulic cylinders  12  or an unloading of the shovel, during which time the directional control valve  11  is in its mid-position. 
     In the design according to FIG. 3, the filling line  20  branches off from a pump line  65  upstream of the valve control block  10 . Seen in the direction of flow of the pressure fluid from the pump line  65  to the hydraulic cylinders  21 , firstly a 2/2-way valve  70 , a pressure control valve  71  and a check valve  72  are arranged in the filling line  20  leading to a plurality of hydraulic accumulators  21 . In the neutral position of the directional control valve  70  brought about by a compression spring, the two ports thereof are blocked off against one another. By driving an electromagnet  73 , the directional control valve can be switched to a through position. To be specific, the electromagnet  73  is always excited when the directional control valve  11  located inside the control block  10  is actuated in the raising direction in order to drive the hydraulic cylinder  12 . The directional control valve  70  is in its initial position in the mid-position of the directional control valve  11  and when the latter is actuated in the lowering direction. 
     A maximum pressure up to which the hydraulic accumulators  21  can be charged is set at the pressure control valve  71 . As long as this pressure has not been reached, the pressure control valve switches through the outlet of the directional control valve  70  to the check valve  72 . When the pressure is reached, the inlet of the valve  71  which is connected to the outlet of the directional control valve  70  is blocked, and the port connected to the check valve is connected to the tank. 
     The hydraulic accumulators  21  can be connected via a line  74  to the working line  14  running between the working port  13  of the directional control valve and the pressure chambers  15  of the hydraulic cylinders  12 . Installed in this line  74  is 4/2-way valve  75  which assumes under the action of a compression spring  76  a neutral position in which two sections of the line  74  are blocked off from one another so that there is no connection between the hydraulic accumulators  21  and the working line  14 . In addition to the two ports required for opening and closing the line  74 , the directional control valve  75  has two further ports for two sections of a line  77  which leads to the tank  27  from the working line  15  between the directional control valve  11  and the pressure chambers  18  of the hydraulic cylinders  12 . The two sections of the line  77  are also blocked off from one another in the neutral position of the directional control valve  75 . The directional control valve  75  can be brought by an electromagnet  78  into a control position in which the two sections of the line  74  and the two sections of the line  77  are respectively connected to one another. The electromagnet  78  is excited when the working machine, which is equipped with the indicated hydraulic control arrangement, exceeds a specific driving speed. The hydraulic accumulators  21  are then connected to the pressure chambers  15  of the hydraulic cylinders  12 , with the result that pitching vibrations can be damped. Volumetric changes in the pressure chambers  18  can be balanced via the line  77 . 
     Since the electromagnet  73  of the directional control valve  70  is excited only when the directional control valve  11  is actuated in the direction of raising the hydraulic cylinders  12 , that is to say when the pressure chambers  15  are fed pressure fluid, it is only then that the hydraulic accumulators  21  can be charged. It cannot, for example, happen that in the case of a pressure present in the line  65  when the working machine is being driven fast reaches the pressure chambers  15  of the hydraulic cylinders  12  via the filling line  20  and valve  75 , which is switched during rapid driving, even if the directional control valve  11  is in the mid-position and it is not desired for the piston rods of the hydraulic cylinders  12  to be extended.