The best mode of the invention
 according to figure 1 with figure 2 The simplified hydrostatic control circuit 100 has in particular a controlled and/or in particular reversible hydraulic pump 20.
 The hydraulic pump 20 has two ports 21, 22, and each of the ports 21 or 22 is connected to a hydraulic steering cylinder with a regulating member 34 and a bidirectional function through a connecting pipe 41, 42 or 51, 52. One of the two chambers 31, 32 of the actuator 30.
 The two chambers 31, 32 preferably have the same effective area.
 A check valve 25, 28 is provided between each of the two ports 21, 22 and the chambers 31, 32 corresponding to the ports 21, 22.
 According to the present invention, the two chambers 31, 32 are connected to a device 70 via pipes 61, 62. The device 70 has an especially electromagnetically controllable directional valve 60 which is locked or closed in one of its operating positions and opened in its other operating position, in particular due to the force of the spring 65.
 The reversing valve 60 is preferably designed to be sealed in its locked position or closed position.
 Also as from figure 1 with figure 2 As shown in the view, the device 70 has two non-return valves 66, 68 arranged in a mirror image of each other in the circuit technology, and each has an unlocking mechanism 67 or 69. The respective unlocking mechanism 67 or 69 can be operated mechanically, especially by a partially modified piston rod 36.
 In the normal operation of the circuit 100, especially in the application as a power steering mechanism for trucks, the solenoid coil of the reversing valve 60 is activated, so that its switching member is switched to the locked position against the force of the spring 65, by This device 70 and:
 -The position of the piston rod 36,
 -Perhaps the unlocking mechanism 67, 69 that is operated, and
 -The resulting on-off state of the two check valves 66 or 68,
 The communication between the two chambers 31, 32 of the hydraulic actuator 30 via the connecting lines 51, 61 or 52, 62 is independently blocked or shut off.
 After the hydraulic pump 20 is controlled once, a certain volume of hydraulic fluid flows through a port 21 as a pressure port:
 -The first section 41 of the first connecting pipeline,
 -The first check valve 25, and
 -The second section 51 of the first connecting pipeline,
 When the adjusting element 34 moves, it is guided into a chamber 31. At this time, a pressure control opposing pressure valve 27 for overcoming the reaction force of the steering system is generated.
 In this case, the hydraulic fluid passes through the other chamber 32:
 -The second section 52 of the second connecting line,
 -A second pressure valve 27 controlled by the inflow volume flow, and
 -Tank line 45,
 被出到液箱46。 Is discharged to the liquid tank 46.
 In the above process, the piston rod 36 of the steering cylinder 30 is moved in. When the piston rod 36 is moved out, the hydraulic pump 20 is controlled such that a certain amount of hydraulic fluid volume flow is transferred from the other port 22 of the hydraulic pump 20 as a pressure port to the circuit 100 through the first section 42 of another connecting pipe. (Due to the symmetrical design of the circuit 100, there is no need to further specify the working mode).
 In the case of a malfunction of the steering system, the solenoid coil of the transverse valve 60 is deactivated, whereby its switching member is switched to the open position by the force of the spring 65, so that the switching state of the device 70 is passed through the two in series with the reversing valve 60 The on-off states of the check valves 66 and 68 arranged in a mirror image with each other are determined.
 In the middle position of the hydraulic actuator 30 or close to the middle position of the hydraulic actuator 30, the unlocking mechanisms 67, 69 of the check valves 66, 68 remain unoperated, and the check valves 66, 68 are locked. Therefore, the piston rod 36 permanently stays at its intermediate position regardless of the force acting on the hydraulic actuator 30.
 In for example figure 2 In the shown position of the piston rod 36 moved out of the intermediate position relative to the hydraulic actuator 30, the second check valve 68 (which locks the chamber 31 expanded from the intermediate position relative to the hydraulic actuator 30 to the opposite The unlocking mechanism 69 of the chamber 32 contracted at the intermediate position of the hydraulic actuator 30 is operated by the modified section of the piston rod 36, thus allowing the flow from the expansion chamber 31 to the contraction chamber 32.
 This circulation occurs when there is an external force acting on the hydraulic actuator 30 toward its middle position until the hydraulic actuator 30 reaches its middle position. The external force acting in the direction of the expansion offset does not cause the position of the piston rod 36 to move at this time, because the first check valve 66 corresponding to the flow from the contraction chamber 32 to the expansion chamber 31 is still locked. In this state, the operating mechanism 67 of a check valve remains unoperated by the modified section and/or modified section of the piston rod 36.
 Therefore, the one-way communication between the two chambers 31, 32 is provided by the device 70, so that when the hydraulic actuator 30 is deflected out of its middle position, the piston rod 36 can only move to the middle position. In this way, it is ensured that the hydraulic actuator 30, especially the steering cylinder or the steering centering cylinder, is placed in its middle position due to external force in the event of a failure, and then stays in the middle position.
 At the intermediate position of the piston rod 36 relative to the hydraulic actuator 30 figure 2 At a position shifted in the opposite direction, the second check valve 66 (which locks the chamber 32 expanded relative to the intermediate position of the hydraulic actuator 30 to the chamber 31 contracted relative to the intermediate position of the hydraulic actuator 30 The unlocking mechanism 67 of the circulation) is operated by the modified section of the piston rod 36, so that the circulation from the expansion chamber 32 to the contraction chamber 31 can be realized (due to the symmetrical design of the circuit 100, no further special description of the working process is required).
 In addition, the steering cylinder or steering centering cylinder 30 may be equipped with a position sensor.
 According to the present invention, combining figure 1 with figure 2 The hydrostatic control circuit 100 shown is suitable as a power steering mechanism of a truck.