Hydraulic system for construction machine, provided with protection device

A hydraulic system for a construction machine having a protection device, comprising:

• a variable displacement hydraulic pump;
• a hydraulic actuator operated by hydraulic fluid that is supplied from the hydraulic pump;
• a control valve installed in a flow path between the hydraulic pump and the hydraulic actuator to control a flow rate of the hydraulic fluid that is supplied from the hydraulic pump to the hydraulic actuator;
• an oil cooler installed in a return flow path from the control valve to a hydraulic fluid tank to cool the hydraulic fluid;
• a hydraulic fluid temperature sensor detecting in real time a hydraulic fluid temperature of the hydraulic fluid tank;
• a hydraulic pump regulator controlling a discharge flow rate of the hydraulic pump through adjustment of an inclination angle of a swash plate of the hydraulic pump;
• a pressure detection sensor detecting pressure of the hydraulic pump;
• a flow rate detection sensor detecting the discharge flow rate of the hydraulic pump;
• and
• a controller comparing the hydraulic fluid temperature detected by the hydraulic fluid temperature sensor with a predetermined hydraulic fluid temperature, calculating a torque in accordance with the detected discharge flow rate and pressure of the hydraulic pump if the detected hydraulic fluid temperature is equal to or lower than a predetermined lower limit value, or equal to or higher than a predetermined upper limit value, and outputting a control signal to the hydraulic pump regulator so as to limit the discharge flow rate of the hydraulic pump to or below a predetermined flow rate through reduction of an input torque value of the hydraulic pump against the calculated torque value with a predetermined ratio.