Hydraulic spool valve for use in a hydraulic vehicle drive

A hydraulic vehicle drive having a closed hydraulic circuit which comprises a variable displacement pump, a hydraulic motor driving the drive wheels of the vehicle and two lines which connect the variable displacement pump and the hydraulic motor and are intended for the hydraulic fluid, it being possible for each of the lines to be the high-pressure line or low-pressure line of the closed hydraulic circuit, depending in each case on the direction of travel of the vehicle, having a flushing spool valve which is situated parallel to the hydraulic motor on the lines of the hydraulic circuit and, with the difference in pressure of the hydraulic fluid that prevails in the normal driving mode, connects the particular low-pressure line to a flushing-pressure limiting valve, which conducts a flushing flow of the hydraulic fluid through the hydraulic motor, but interrupts the connection to the flushing-pressure limiting valve if the difference in pressure fails to appear, characterized by such a design of the flushing spool valve that, with a difference in pressure between the high-pressure and low-pressure line that is smaller in comparison to the normal driving mode, although the connection between the low-pressure line and the flushing-pressure limiting valve is likewise produced, the volume of the flushing flow is, however, significantly reduced in comparison to the normal driving mode;

• wherein the flushing spool valve has the features of the hydraulic spool valve piston, which is arranged in a housing bore in a manner allowing it to be displaced longitudinally and which is assigned, on each end side, a respective compression spring which counteracts a displacement of the spool-valve piston towards this compression spring, while the two compression springs together, owing to their pre-stress, effect a neutral central position of the spool-valve piston, which position encompasses one region and in which the through flow of hydraulic fluid through the spool-valve is blocked, having two inlet bores and an outlet bore arranged between them, for hydraulic fluid, the inlet bores and the outlet bore all leading into the housing bore and a difference in pressure of the hydraulic fluid situated in the two inlet bores causing the spool-valve piston to be displaced, with the consequence that, in the extended end position of the spool-valve piston, the outlet bore is connected to that inlet bore in which the lower pressure of the hydraulic fluid prevails, characterized in that for each compression spring an expansion path, starting from the pre-stressed state and limited by a stop, is provided in such a manner that the spool-valve piston, when displaced on both sides out of its neutral central position into an intermediate position defined by the stop, is acted upon by the two compression springs, but, after the stop has come into effect and the intermediate position has been exceeded in the direction of the extended end position, is only acted upon by the one compression spring, which in each case counteracts the displacement of the said spool-valve piston, and in that even as the spool-valve piston approaches the intermediate position from the neutral central position a connection between the outlet bore and the inlet bore of lower pressure is produced, the flow cross section of this connection being smaller than when the spool-valve piston has exceeded the intermediate position.