| 摘要 |
A lifting device for lower steering arms of a tractor has a position recording device (25). The position recording device (25) generates a position signal representing the pivot position of the lower steering arms (2, 3). A piston-cylinder unit (11, 12) acts on the tractor and on the lower steering arm (2, 3). A switching circuit is provided per piston-cylinder unit (11, 12). The switching circuit includes a directional valve (33, 33') with four ports and four switching positions. In a first position of the directional valve (33, 33') all ports are blocked. In a second position, the cylinder chamber at the piston end is connected to the pressure agent supply source and the cylinder chamber at the piston rod end is connected to the return line. In a third position, the cylinder chamber at the piston rod end is connected to the pressure agent supply source and the cylinder chamber at the piston end is connected to the return line. In a fourth position, both cylinder chambers are connected to the return line. A switching unit (28, 28') is provided with a guiding value (F). The guiding value (F) corresponds to the nominal position of the lower steering arm (2, 3). A position signal originates from the position recording device (25, 25') and represents the respective position of the lower steering arm. A comparator compares the guiding value (F) with the position signal. The switching unit (28, 28'), as a function of the result of the comparison, causes the directional valve (33, 33') to occupy one of the four positions.
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