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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
     The present invention relates to an actuator for a machine, especially an excavator, with at least one supply port and one working port and with at least one hydraulic accumulator. 
     BACKGROUND OF THE INVENTION 
     Actuators for machines are readily available on the market in a plurality of embodiments. The supply port is used to connect a pump unit, for example, in the form of a hydraulic pump, to supply the hydraulic circuit connected to the working port with pressurized hydraulic fluid. These hydraulic circuits are generally divided into a high pressure part and a low pressure part. The actuating means for the machine is connected to the low pressure part through which low pressure part the operator, for example, the excavator operator, actuates the equipment of the high pressure circuit for working processes. A hydraulic accumulator ensures hydraulic supply for a certain interval in case of a supply failure within the hydraulic circuit to prevent the working equipment, for example, in the form of a drag, from falling down in the raised state or the like. This falling would constitute a safety risk. 
     In the known or conventional actuators, the hydraulic accumulator must first be charged to produce this safety function. To ensure emergency supply, the machines they cannot immediately begin working. Rather, a defined actuation function must first be carried out by the excavator operator, for example, by lifting the drag, to guarantee that the accumulator in this process is charged for a later emergency function. Generally, this working process is described in the operating instructions of the machine and must be carried out with the corresponding accuracy. 
     A generic actuator for a machine is disclosed DE-A-30 34 467, and monitors a connection between a supply port and a storage tank (tank T) on one side and a hydraulic accumulator on the other side. A relay valve has a valve element impactable by the pressure in the hydraulic accumulator, and a switching valve controllable by the relay valve. The valve element of the relay valve optionally connects a control line leading to the switching valve with the supply port or the storage tank (tank T). The known actuator promotes reliable on and off switching of the hydraulic accumulator. The valve element of the relay valve does not assume a control function, i.e., the connection between the hydraulic pump and the hydraulic accumulator is either completely open or completely closed. During reversing from pump pressure to accumulator pressure and vice versa, the valve element completes a rapid switching motion and hence acts like a flip-flop connection. This arrangement results in reliable on and off switching of the hydraulic pump and the storage tank (tank T) and/or the respective consumer. Realization of emergency supply within the meaning outlined above is, however, not possible with the disclosed actuator, which merely ensures a monitoring function. 
     SUMMARY OF THE INVENTION 
     Objects of the present invention is to provide improved actuators to ensure an emergency function before the machine begins to perform work. 
     The pressure prevailing in a secondary line and corresponding essentially to the fluid-side accumulator pressure is used for switching the switching valve. Because of a pressure reducing valve being situated between the switching valve and the secondary line, the pressure reducing valve separates within a hydraulic circuit a low pressure part from a high pressure part. Both parts are supplied via the supply port. The high pressure circuit is separated from the supply port and moreover from the supply circuit. The hydraulic accumulator is charged via the supply port when the machine is turned on, before the machine begins to perform work. Thus, immediately after the drive assembly and moreover the hydraulic pump are turned on, emergency supply pressure is available. The operator can immediately begin working without needing first to consider the emergency function. This arrangement enhances safety and facilitates operation of the machine, and therefore, guarantees immediate operating availability. Separation of the supply port from the working port can be complete in the sense of fluid blocking. Preferably, there is only partial separation, for example, by means of a choke which can still ensure partial supply of the high pressure circuit for certain working functions, at the same time emergency supply being ensured. 
     Furthermore, supply of the high pressure part as well as the low pressure part can be ensured by only one supply port. In contrast to optional electric control of the switching valve by electric switching magnet, the actuation of the switching valve is pressure controlled and hence functionally reliable. 
     In another preferred embodiment of the actuator of the present invention, between the switching valve and the secondary line, a pressure reducing valve is within a hydraulic circuit to separate the low pressure part from the high pressure part. The two parts are supplied via the a supply port. In this way, supply of the high pressure part and the low pressure part can be guaranteed via only one supply port. 
     In another preferred embodiment of the actuator of the present invention, between the secondary line and the hydraulic accumulator, a return valve opens in the direction of the hydraulic accumulator ensuring therethrough reliable separation in the case of supply failure, especially of the pump flow, from the low pressure part to the high pressure part. Preferably, the hydraulic accumulator with its fluid side is connected to the low pressure part which supplies the control unit, especially with a joystick, via another valve, especially a sliding valve. The control unit, as part of the actuating means of the machine, is thus supplied via the fluid side of the hydraulic accumulator and can influence the low pressure part with the equipment connected to it via corresponding control units. 
     In another preferred embodiment of the actuator of the present invention, the secondary line discharges into another working port. On the low pressure side, a third working port can be cut off via a third valve, especially in the form of another sliding valve. In this way, within the actuator still other working ports are available for actuating processes on the machine. Preferably, the sliding valves, the pressure limitation valves and the pressure reducing valves are connected to a tank port having essentially the ambient pressure. 
     In one especially preferred embodiment of the actuator of the present invention, the hydraulic accumulator is a membrane accumulator. The hydraulic accumulator and all the valves are made as screw-in parts, especially in the form of screw-in cartridges. Preferably, the actuator is made in a block design, the actuator block on the outer periphery side being provided with the supply port, the working ports and the tank port and with connection points for accommodating the screw-in parts. Within the actuator block, the connecting lines preferably extend between the ports and the connection points. In this way, a modular structure of the actuator concept is accomplished, with a compact structure requiring little installation space within the machine. Furthermore, when one component fails, it can be easily and economically replaced by a replacement part. The reversing valve and the return valve are preferably made as kits. 
     Preferably, the low pressure part has a working pressure of at least 20 bar, and the high pressure part has a working pressure of at least 200 bar. 
    
    
     Other objects, advantages and salient features of the present invention will become apparent form the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the drawings which form a part of this disclosure: 
     FIG. 1 is a schematic hydraulic diagram of an actuator according to an embodiment of the present invention; and 
     FIG. 2 is an exploded perspective view of the actuator of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The actuator, as shown in the drawings, is designed for a machine, especially an excavator, and has a supply port P and a working port P′. Furthermore, a hydraulic accumulator  10  is a membrane accumulator. As FIG. 1 further shows, between the supply port P and the working port P′, a switching valve  12  is used as a reversing valve. In the first operating position shown in FIG. 1, switching valve  12  separates the supply port P from the working port P′, or chokes it and guarantees fluid-side charging of the hydraulic accumulator  10  via the supply port P. The gas side of the hydraulic accumulator  10  is provided with a working gas, especially in the form of nitrogen, with a pressure which can be stipulated. In the other or second operating position of the switching valve  12 , the supply port P would be connected to the working port P′, routing fluid. 
     In its initial position shown in FIG. 1, the switching valve  12  at least partially separates the supply port P from the working port P′. The switching pressure prevails in a secondary line  14 , corresponds essentially to the fluid-side pressure of the hydraulic accumulator  10 , and is used to switch the switching valve  12  into its supply position which links the supply port P to the working port P′. 
     Between the switching valve  12  and the secondary line  14  a pressure reducing valve DM separates the low pressure part  16  from the high pressure part  18  within the hydraulic circuit. The two parts are supplied via the supply port P. Between the secondary line  14  and the hydraulic accumulator  10 , a return valve RV opens in the direction of the hydraulic accumulator  10 . Furthermore, the hydraulic accumulator  10  has its fluid side connected to the low pressure part  16  which via another valve  20 , especially in the form of a sliding valve, supplies a control unit (not shown) for actuating the equipment, especially with a joystick (not shown) for operation by an operator. The pertinent port is labeled Azg in FIG.  1 . 
     Another or second working port Ast discharges into the secondary line  14 . On the low pressure side  16 , a third working port Ap can be blocked via a third valve  22 , especially in the form of another sliding valve. The two sliding valves  20  and  22  are spring-loaded or biased toward their base positions shown in FIG. 1, and can be electrically actuated via switching magnets WK 1 , WK 2 . Due to the other two working ports Ap and Ast, on the low pressure side other units of the machine can be hydraulically connected to the actuator. Furthermore, the high pressure part  18  is safeguarded via a pressure limitation valve DB 3 . The low pressure part  16  is protected via the pressure limitation valve DB 4 . The two sliding valves  20 ,  22 , the two pressure limitation valves DB 3  and DB 4  and the pressure reducing valve DM are connected to carry fluid to a tank port T having essentially the ambient pressure. 
     As shown especially by FIG. 2, the hydraulic accumulator  10  is a membrane accumulator. The hydraulic accumulator  10  and all valves  12 ,  20 ,  22 , DM, RV, DB 3  and DB 4  are made as screw-in parts, especially in the form of screw-in cartridges. As FIG. 2 furthermore shows, the actuator is made in a block design, the actuator block  24  being in the form of a cube or cuboid. On the outer peripheral side, actuator block  24  has the supply port P, the working ports P′, Ast and Ap and the tank port T. Furthermore, it has a port Azg and the corresponding connection points  26  for accommodating the screw-in parts. Within the actuator block  24  (not detailed), the corresponding connecting lines establish the connections between the indicated ports and the connection points  26 . 
     This actuator is now described with respect to its function. When the machine, especially in the form of an excavator, is turned on, the drive unit, especially in the form of a diesel engine, runs and drives a hydraulic pump (not shown). The hydraulic pump delivers to the supply port P a working fluid pressure, for example at a level of 230 bar. When the machine starts, the switching valve  12  is in its interlocked or choked operating position shown in FIG.  1 . Also, the fluid pressure of the high pressure part  18  is brought via the pressure reducing valve DM to the working pressure of the low pressure part  16 , for example by setting the pressure reducing valve to a set pressure of 45 bar. The return valve RV is opened against its spring force, and the supply port P supplies the hydraulic accumulator  10  on its fluid side with reduced pressure. 
     The accumulator is now filled or loaded on the fluid side against the gas working pressure until the pressure in the secondary line  14  corresponds roughly to the desired working pressure in the hydraulic accumulator  10 , for example, 35 bar at a delivery volume of 6.6 l/min. If pressure equilibrium is established in the low pressure part  16  between the fluid side of the hydraulic accumulator  10  and the prevailing pressure in the secondary line  14 , including the closing force of the reset springs of the return valve RV, the switching valve  12  is moved into its fluid-carrying operating position. In the operating position, the supply port P is connected to the working port P′ so that the working pressure of the high pressure part  18  of the actuator can then prevail on or be conveyed to the equipment of the machine, for example, in the form of hydraulic cylinders. If the sliding valves  20 ,  22  are switched via electromagnets WK 2  and WK 1 , the pressure in the low pressure part  16  prevails on the ports Azg and Ap which are otherwise connected to the tank side  16  to carry fluid. Via the ports Azg and Ap, the control unit for the machine can be supplied with low pressure, as can another hydraulic consumer of the entire hydraulic circuit (not shown). 
     If malfunctions occur, emergency function is ensured via the hydraulic accumulator  10  which then guarantees emergency supply via the respective port Azg and/or Ap. The return valve is blocked relative to the high pressure part  18 . In this way, for example, a raised drag upon failure of the hydraulic supply can be kept in its position or lowered in a defined manner. There is another working port for another hydraulic consumer on the low pressure side via the working port Ast. With the actuator of the present invention, it is therefore ensured that, before the actual start of operation with the machine, the emergency accumulator in the form of a hydraulic accumulator  10  is filled with a pressurized fluid reserve which can then later guarantee emergency supply in any case. 
     For partial separation of the fluid supply for the high pressure circuit, a choke DR can be provided so that the switching valve  12 , as shown in FIG. 1, can be replaced by a switching valve with a choke, as shown. In this way, emergency supply can be ensured via a hydraulic accumulator  10  and at the same time at least one working function within the high pressure circuit can be achieved. 
     While one embodiment has been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.

Summary:
A switching device for a machine, especially a digger, has at least one supply connection and one useful connection in addition to at least one hydraulic accumulator. An on-off valve is arranged between the supply connection and the useful connection, and separates the supply connection at least partially from the useful connection in one of its switching positions, in addition to ensuring that the hydraulic accumulator is charged on the fluid side via the supply connection. In another switching position, the supply connection is connected to the useful connection. A substantial improvement relative to conventional switching devices is attained by being able to guarantee an emergency function before the machine is used to perform work.