Patent ID: 9051712
Filing Date: 2015-06-09
Classification: E02F,F15B

Abstract:
1. A working machine comprising: a front working implement for material lifting; and a hydraulic system connected to the front working implement, wherein the hydraulic system includes: a variable-capacity hydraulic pump; a center bypass line connected to the hydraulic pump on an upstream side thereof and connected to a tank on a downstream side thereof; a plurality of hydraulic actuators driven by a hydraulic fluid delivered from the hydraulic pump, where the hydraulic actuators include a specific hydraulic actuator that has a bottom-side cylinder chamber and a rod-side cylinder chamber and either one of the bottom-side cylinder chamber and the rod-side cylinder chamber becomes a load retaining side during heavy load fine speed operation work; a plurality of actuator lines connected to the hydraulic actuators respectively; a plurality of center bypass type, flow/directional control valves each including a center bypass passage and a meter-in passage, the center bypass passage being positioned on the center bypass line and the meter-in passage being positioned on a hydraulic line that communicates a hydraulic fluid supply line receiving the hydraulic fluid delivered from the hydraulic pump to the actuator lines, and each of the flow/directional control valves being configured such that with an increase in stroke thereof, an opening area of the center bypass passage is reduced while an opening area of the meter-in passage is increased to thereby control flows of the hydraulic fluid supplied from the hydraulic pump to the plurality of hydraulic actuators; a plurality of operating devices provided for the plurality of hydraulic actuators to operate the respective flow/directional control valves; and a pump regulator to control a capacity of the hydraulic pump such that a delivery rate of the hydraulic fluid therefrom increases depending on operations of the operating devices; a center bypass cutoff valve disposed at a position downstream of the flow/directional control valve corresponding to the specific hydraulic actuator on the center bypass line passing through the plurality of flow/directional control valves; and control means for controlling the center bypass cutoff valve such that when the one of the operating devices corresponding to the specific hydraulic actuator is operated to supply the hydraulic fluid to the load-retaining side of the specific hydraulic actuator, the center bypass cutoff valve is actuated and a fluid delivery pressure of the hydraulic pump becomes higher than a load pressure of the specific hydraulic actuator, wherein the control means includes: operation detection means for detecting whether the one of the operating devices corresponding to the specific hydraulic actuator has been operated to supply the hydraulic fluid to the load-retaining side of the specific hydraulic actuator to conduct the heavy load fine speed operation work, and bypass control means for actuating the center bypass cutoff valve when the operation detection means detects that the one of the operating devices corresponding to the specific hydraulic actuator has been operated to supply the hydraulic fluid to the to the load-retaining side of the specific hydraulic actuator to conduct the heavy load fine speed operation work.