Abstract: A hydraulic spool valve is provided in which two inlet bores and an outlet bore lead into a housing. A spool-valve piston which is under the influence of compression springs on both sides can be displaced longitudinally in the housing. When a difference in pressure of the hydraulic fluid in the inlet bores occurs, the inlet bore which conducts the lower pressure in each case is connected to the outlet bore. The two compression springs are pre-stressed and have an additional expansion path. During the displacement of the spool-valve piston from a neutral central position into an extended intermediate position, the two compression springs therefore remain in operative connection with the spool-valve piston, in which case the spring forces counterbalance each other until the intermediate position is reached.

Abstract: A system for automatically holding a predetermined course in a vehicle with a two-train hydrostatic traction drive, comprising a measurement device (5) for detecting the actual rotational movement of the vehicle around a vertical axis of the vehicle, and for providing a first signal representing the rotational movement. An electronic controller (1) operatively associated with the measurement device (5) to evaluate the first signal to determine the deviation of the first signal from the predetermined course of the vehicle to form a control signal to hydrostatic transmissions of the two-train hydrostatic drive to operate and to provide a course correction.