Hydraulic circuit with variable relief valves

Claim 1

A hydraulic circuit for a hydraulic shovel, comprising: a variable displacement hydraulic pump; selector valve for selectively transmitting hydraulic fluid from said hydraulic pump; actuators for actuating said shovel and communicated with said selector valves such that hydraulic fluid from said hydraulic pump may be delivered to said actuators via said selector valves; at least one variable relief valve provided in a discharge oil passage of said variable displacement hydraulic pump at a fluidic position between said hydraulic pump and said selector valves and also provided in input oil passages of at least one of said actuators; a pilot hydraulic pump communicated with said variable relief valves through pilot oil passages; a solenoid valve provided in one of said pilot oil passages for changing set pressures of said variable relief valves; a regulator for controlling a discharge quantity of said variable displacement hydraulic pump; and a pilot oil passage for communicating said solenoid valve with said regulator so as to control a discharge quantity of said hydraulic pump by operation of said solenoid valve, according to the pressure of said pilot pump and independent of a discharge pressure of said variable displacement hydraulic pump.