HYDRAULIC WORKING MACHINE

摘要

A hydraulic circuit of a hydraulic working machine is constituted of the following components: a main pump (21), a boom cylinder (11) telescoped by pressurized oil from the main pump (21), a directional control valve (22) for regulating the flow of the pressurized oil fed from the main pump (21) to a bottom chamber (11a) of and a rod chamber (11b) of the boom cylinder (11), an operating device (23) for switching the directional control valve (22), a pilot pump (24), a jack-up switch valve (25) for regulating the flow of pressurized oil discharged from the pilot pump (24), a flow-rate control valve (26) connected, on the upstream side of the directional control valve (22), to a meter-in port of the directional control valve (22) and switched by the jack-up switch valve (25), and a center bypass switch valve (27) connected, on the downstream side of the directional control valve (22), to a center bypass port of the directional control valve (22) and switched by the jack-up switch valve (25). The jack-up switch valve (25) is switched in accordance with a bottom pressure in the boom cylinder (11).