Control valve for hydraulic systems

The control valve for hydraulic systems is intended as a shutoff or relief valve for a pump (19) and has a pressure-limiting valve (28) and a pressure-holding valve (39) positioned downstream of the latter. If a high pressure is to be reduced for a short period, the pressure-limiting valve responds and relieves the pump. If the overload phase persists for a prolonged period, a control slide (43) is moved into a position in which the pump is relieved directly to the return at low delivery pressure. The movement of the control slide (43) is to take place in a delayed manner. This is achieved by the fact that a spool (49) is formed in the said control slide and the said spool has rotation imparted to it by a flow of pressure medium flowing via bores (60, 61). At each rotation, a restriction bore (65) is connected to the delivery side of the pump and a connection is thereby established to a chamber (47) which displaces the control slide (49) into the shutting-off position counter to the force of a compression spring (40).