Patent ID: 6253859
Filing Date: 2001-07-03
Classification: A01B

Abstract:
A lifting device for lower steering arms of an attaching device of a tractor, said lower steering arms, at one end, are pivotably attached to the tractor, and said lifting device comprises:a position recording device per lower steering arm, said position recording device generating a position signal representing the respective pivot position of the lower steering arm;a hydraulically loadable, double-acting piston-cylinder unit per lower steering arm, said piston-cylinder unit, at one end, capable of being attached to the tractor and at its other end capable of acting either directly or indirectly, via an intermediate lever drive, on said lower steering arm, said piston-cylinder unit having a cylinder chamber at a piston end and a cylinder chamber at a piston rod end;a switching circuit per piston-cylinder unit, said switching circuit having a directional valve with four ports, said directional valve transferring into at least three switching positions, two ports in said directional valve serving to connect to the piston-cylinder unit, a first port connected to the cylinder chamber at the piston end, with a second port being connected to the cylinder chamber at the piston rod end, a third port serving to connect to a pressure agent supply source and a fourth port serving to connect to a return line, and in a first position of the directional valve, all ports of said directional valve are blocked, in a second position, the cylinder chamber at the piston end being connected to the pressure agent supply source and the cylinder chamber at the piston rod end being connected to the return line, in a third position, the cylinder chamber at the piston rod end being connected to the pressure agent supply source and the cylinder chamber at the piston end being connected to the return line and said switching circuit including an electric switching unit provided with an input for introducing a guiding value signal corresponding to the nominal position of the lower steering arm and an input for introducing a position signal generated from said position recording device and representing the respective position of the lower steering arm and a comparator for comparing the guiding value signal with the position signal, with the switching unit, as a function of the result of the comparison, causing the directional valve to assume one of said at least three positions.