Patent ID: 9695842
Date: 2017-07-04
CPC Classifications: E02F,F15B

Claim:
1. A hydraulic drive system, comprising: a hydraulic pump having a first pump port and a second pump port, the hydraulic pump being switchable between a state of inducting hydraulic fluid from the second pump port and discharging hydraulic fluid from the first pump port, and a state of inducting hydraulic fluid from the first pump port and discharging hydraulic fluid from the second pump port; a driving source configured to drive the hydraulic pump; a work implement; a hydraulic cylinder driven by hydraulic fluid discharged from the hydraulic pump, the hydraulic cylinder having a first chamber and a second chamber, the hydraulic cylinder being configured to lower the work implement by exhausting hydraulic fluid from the first chamber and supplying hydraulic fluid to the second chamber and raise the work implement by supplying hydraulic fluid to the first chamber and exhausting hydraulic fluid from the second chamber; a hydraulic fluid flowpath having a first flowpath connecting the first pump port and the first chamber and a second flowpath connecting the second pump port and the second chamber, the hydraulic fluid flowpath configuring a closed circuit between the hydraulic pump and the hydraulic cylinder; a bleed-off flowpath branching off from the first flowpath, the bleed-off flowpath having flowing therein a portion of hydraulic fluid exhausted from the first chamber when the work implement is lowered; a control valve configured to control a flow amount of hydraulic fluid flowing into the bleed-off flowpath from the first flowpath; and a rotation speed sensor configured to detect a rotation speed of the hydraulic pump or the driving source; and an operating member for operating the hydraulic cylinder, wherein, a full amount of hydraulic fluid exhausted from the first chamber is returned to the first pump port through the first flowpath when an operation parameter that varies in accordance with an operation amount of the operating member is less than a prescribed value when lowering the work implement; a portion of hydraulic fluid exhausted from the first chamber when the operation parameter is equal to or greater than the prescribed value when the work implement is lowered, flows to the bleed-off flowpath, and a flow amount of hydraulic fluid to be returned to the first pump port is less than the full amount of hydraulic fluid exhausted from the first chamber; and when the rotation speed of the hydraulic pump or the driving source exceeds a prescribed value less than a prescribed allowable rotation speed, an opening of the control valve connected to the bleed-off flowpath begins to open and an opening surface area of the opening increases in accordance with an increase in the rotation speed.