HYDRAULIC SYSTEM OF WORK MACHINE

摘要

PROBLEM TO BE SOLVED: To improve the responsiveness until a negative brake for braking a motor for driving a driven body and releasing the braking is actuated.SOLUTION: A hydraulic system of a work machine includes: a negative brake for braking a motor for driving a driven body with spring force, and releasing the braking by hydraulic pressure; a brake switching valve that can be freely switched between a braking position where hydraulic pressure is released from the negative brake and a braking release position where hydraulic pressure is supplied to the negative brake; a hydraulic pressure supply/discharge oil passage that connects the brake switching valve and the negative brake; a pressure-relief oil passage connected to the hydraulic pressure supply/discharge oil passage that releases the hydraulic pressure of the hydraulic pressure supply/discharge oil passage to a tank; and a first contraction provided to the pressure-relief oil passage. A second contraction is provided on an upstream side of a junction of the pressure-relief oil passage in the hydraulic pressure supply/discharge oil passage.