| 摘要 |
PROBLEM TO BE SOLVED: To provide a hydraulic drive system capable of easily adjusting a position of a work machine at a desired height.SOLUTION: A hydraulic oil passage 15 has a first passage 15a and a second passage 15b. The first passage connects a first pump port 12a to a first chamber 14c of a hydraulic cylinder 14. The second passage connects a second pump port 12b to a second chamber 14d of the hydraulic cylinder. The hydraulic oil passage configures a closed circuit between a hydraulic pump 12 and the hydraulic cylinder 14. A bleed-off passage 37 is a passage to bleed off a portion of hydraulic oil from the second passage. When an operation amount of an operation member 46a to lower a work machine is less than a predetermined operation amount, a control valve 16 connects the second passage to the bleed-off passage through a diaphragm 17 in a manner that keeps hydraulic pressure of the second passage less than relief pressure. The predetermined operation amount is not more than a maximum operation amount of the operation member to lower the work machine. |
