Hydraulic lift device for an attaching device

A hydraulic lift device for an attaching device of a tractor, a self-propelled working machine or an implement provided with a hydraulic pressure supply device and a hydraulic tank, said attaching device having two lower steering arms, said hydraulic lift device comprising:

• a first double acting hydraulic cylinder for actuating one of the two lower steering arms, said first double acting hydraulic cylinder having a first cylinder housing, a first piston rod and a first piston, said first piston including a first piston-end piston area and a first piston rod-end piston area, said first double acting hydraulic cylinder further having a first piston-end cylinder chamber and a first piston rod-end cylinder chamber;
• a second double acting hydraulic cylinder for actuating the other of the two lower steering arms, said second double acting hydraulic cylinder having a second cylinder housing, a second piston rod and a second piston, said second piston including a second piston-end piston area and a second piston rod-end piston area, said second double acting hydraulic cylinder further having a second piston-end cylinder chamber and a second piston rod-end cylinder chamber;
• a first switching valve adapted to take up at least three switching positions and comprising a first port representing a connection to the pressure supply device, a second port representing a connection to the hydraulic tank, a third port representing a connection to the first piston-end cylinder chamber of the first hydraulic cylinder and a fourth port representing a connection to the second piston rod-end cylinder chamber of the second hydraulic cylinder;
• a line connecting the first piston rod-end cylinder chamber of the first hydraulic cylinder to the second piston-end cylinder chamber of the second hydraulic cylinder, wherein the first piston rod-end piston area of the first hydraulic cylinder and the second piston-end piston area of the second hydraulic cylinder have the same size or a specific predetermined size relationship to each other;
• and
• a second switching valve arranged in line between the first piston rod-end cylinder chamber of the first hydraulic cylinder and the second piston-end cylinder chamber of the second hydraulic cylinder, said second switching valve adapted to take up a basic position and at least two further switching positions and comprising a first port to a pressure supply device and a second port to a hydraulic tank, wherein in said basic position the connection to the pressure supply device and to the hydraulic tank as well as to a fifth port to the second piston rod-end cylinder chamber of the second hydraulic cylinder are closed, and wherein a third and a fourth port are open to the line between the first piston rod-end cylinder chamber and the second piston-end cylinder chamber.