Hydraulic circuit for a boom cylinder in a hydraulic shovel

A hydraulic circuit for a boom cylinder in a hydraulic shovel in which a pressurized discharge fluid from a hydraulic pump which is driven by an engine of the shovel is supplied via a directional control valve for a boom into a retraction pressure chamber and an extension pressure chamber of a boom cylinder for swinging the boom upwards and downwards, said hydraulic circuit comprising:

• a relief valve disposed in a circuit for connecting said retraction pressure chamber of the boom cylinder to said directional control valve, said relief valve having a relief set pressure;
• a switching means for switching said relief set pressure to a low pressure and a high pressure when said directional control valve is set at a position other than a neutral position thereof, when the engine is driven, said switching means being adapted to switch said relief set pressure to a high pressure when the engine is halted, wherein said relief valve is provided with a set pressure changing section having a pilot chamber and adapted to have the relief set pressure reduced to the low pressure when the pilot chamber is supplied with a pressure fluid and adapted to have the relief set pressure increased to the high pressure when the pilot chamber is not supplied with the pressure fluid;
• an auxiliary hydraulic pump having a discharge path and driven by the engine, said discharge path being connected to the pilot chamber through a switching valve;
• and
• a controller for setting the switching valve at a drain position thereof when said directional control valve is set at said neutral position and setting the switching valve at said pressure fluid supply position when the directional control valve is set at position other than the neutral position thereof.