Hydraulic control system in working machine

A hydraulic control system in a working machine equipped with an option control circuit that is shared for a plurality of option hydraulic actuators that is selectively mounted in the working machine, the option control circuit further comprising:

• an option control valve for performing oil supply and discharge control to and from an option hydraulic actuator in accordance with an operation of an option operation lever;
• a pair of actuator oil passages that connect the option control valve and the option hydraulic actuator;
• a pressure sensor for detecting a pressure of the actuator oil passages;
• a pressure compensating valve which is disposed on an upstream-side of the option control valve and is actuated in order to maintain a differential pressure, by an inlet-side pressure and an outlet-side pressure of the option control valve being introduced, between the introduced inlet-side pressure and outlet-side pressure at a predetermined pressure;
• and
• a control device for controlling an actuation of the option control valve;
• wherein the option control circuit is configured such that, by connecting a variable relief valve capable of varying a relief setting pressure in response to control signals from the control device, to a load pressure introducing oil passage that introduces the outlet-side pressure of the option control valve to the pressure compensating valve, and lowering a pressure of the load pressure introducing oil passage down to the relief setting pressure by the variable relief valve and introducing the pressure of the load pressure introducing oil passage to the pressure compensating valve, a pressure of the inlet-side of the option control valve can be variably controlled in accordance with change of the relief setting pressure of the variable relief valve;
• wherein the option control circuit is configured such that, by providing a load pressure relief position at which oil supply and discharge to and from the option hydraulic actuator is not performed, but the pressure of the actuator oil passages is allowed to flow into the load pressure introducing oil passage, in a switching position of the option control valve, and by switching the option control valve to the load pressure relief position, when the pressure in the actuator oil passages exceeds an upper limit pressure that is preset depending on individual optional hydraulic actuator during non-operation of the option operation lever;
• as well as by controlling the relief setting pressure of the variable relief valve so as to be less than or equal to the upper limit pressure, the pressure in the actuator oil passages exceeding the upper limit pressure is released to an oil tank via the option control valve, the load pressure introducing oil passage, and the variable relief valve.