HYDRAULIC HOISTING CIRCUIT WITH ELECTRICAL CONTROL FOR RELIEF VALVE ADJUSTMENT PILOT AND PILOT DISABLE VALVE

摘要

A hydraulic circuit for a construction and working machine includes hydraulic signal generating means (30,36) for generating a hydraulic signal from a hydraulic source utilizing a discharge pressure oil from a pilot pump (4) for an operating system or a pressure oil from a load-side oil chamber for an actuator for supporting a working device when a receiving section of the hydraulic signal generating means receives an external signal, a main relief valve (23,24) and a port relief valve both having boost receiving means for boosting a relief pressure of these relief valves to a value higher than a normal set pressure when the hydraulic signal from the hydraulic signal generating means is applied to the boost receiving means, operation suppressing means for holding a neutral position of a hydraulic selector valve for operating a specific one of the actuators irrespective of an operation command to the hydraulic selector valve (29) when a receiving section of the operation suppressing means receives an external signal, and signal generating means for arbitrarily generating a signal and stopping the generation of the signal by an operator, wherein the signal from the signal generating means is supplied directly to the receiving sections of the hydraulic signal generating means and the operation suppressing means, or supplied thereto through a relay circuit comprised of a pressure switch and a make contact, wherein the relay circuit functions to hold outputting of the signal received from the signal generating means as far as the operation switch is in a closed state.