In a hydraulic drive system of a work machine which drives a plurality of actuators using three or more pumps, for an operation not including traveling, a highly efficient combined operation in a front implement and excellent combined operability of a swing and the front implement are enabled, while for an operation including traveling, a highly efficient traveling operation and a highly efficient combined operation of traveling and the front implement are enabled, and a sufficient operation speed of the front implement is achieved. To this end, each of the flow rates of the first, second, and third pumps (101, 201, 301) can be controlled independently by performing the load sensing control, and in a combined operation for driving a boom (511) and an arm (512), ether one of them is driven by the first pump while the other one is driven by the second pump, and the swing is driven by the third pump. In the traveling operation, the maximum capacity of the first and second pumps is switched to the maximum capacity for the traveling operation and driven by an open center circuit. In a combined operation of traveling and the front implement, the front implement is driven by performing the load sensing control using the third pump.

In a construction machine with a hydraulic pilot type hydraulic control device, occurrence of jerking is prevented. A hydraulic excavator 1 having hydraulic pumps 51L and 51R, travel motors 22L and 22R, traveling operating levers 34L and 34R, a pilot pump 54, hydraulic pilot valves 55La, 55Lb, 55Ra, and 55Rb, and directional control valves 53L and 53R includes: changeover switches 35L and 35R which change the operating mode of the traveling operating levers 34L and 34R; pilot pressure adjusting devices 5L and 5R which adjust the pilot pressure applied to the directional control valves 53L and 53R; and pilot pressure sensors 56La, 56Lb, 56Ra, and 56Rb. The pilot pressure adjusting devices apply the pilot pressure at the time when the operating mode of the traveling operating levers 34L and 34R is changed to a control mode, to the directional control valves 53L and 53R.

A hydraulic system in of a work machine includes a hydraulic pump to supply pilot hydraulic oil in a hydraulic oil reservoir. A travel pump is to drive a travel motor according to a pilot pressure of the pilot hydraulic oil. A target pilot pressure of the pilot hydraulic oil is input through a travel operation device to control, according to the target pilot pressure, the pilot pressure supplied from the hydraulic pump to the travel pump. A pilot oil path connects the travel operation device and the travel pump to supply the pilot pressure to the travel pump. A drain oil path is divided from the pilot oil path. A pressure adjustment valve is provided in the drain oil path to adjust the pilot pressure to be less than the target pilot pressure when a condition is satisfied.

A fluid pressure control device includes a first switching valve provided on a downstream of a first travel control valve in the first main passage and having an open position and a priority position, a second switching valve provided on a downstream of a second travel control valve in the second main passage and having an open position and a priority position, and a merging passage connecting a downstream of the first switching valve and a downstream of the second switching valve. In a state where first work control valves and second work control valves are not operated and the first travel control valve is operated, the first switching valve is at a priority position, and in a state where the first work control valves and the second work control valves are not operated and the second travel control valves is operated, the second switching valve is at a priority position.

A new means of achieving a valve float position in a closed centered hydraulic control valve. The valve has a main spool position whereby pressure compensator load sense control pressure is drained to tank through the main spool. Secondly, a pin acts on the pressure compensator to prevent the main pump flow from reaching the main section spool. Finally, the new spool position adds a connection from the consumer to tank, enabling a float position.

A work machine has a controller which has an area limiting control section correcting the pilot pressures of pilot lines, a regeneration control section adjusting the flow rate of the hydraulic fluid caused to flow from a tank side line of an arm cylinder into a pump side line thereof between zero and a predetermined upper limit value, and a regeneration control switching section that issues an order to the regeneration control section to set the predetermined upper limit value to a first set value when the function of the area limiting control section is invalid and that issues an order to the regeneration control section to set the predetermined upper limit value to a second set value that is smaller than the first set value when the function of the area limiting control section is effective.

A hydraulic circuit is disclosed. The hydraulic circuit may comprise an actuation valve configured to actuate a flow of hydraulic fluid to and from the hydraulic consumer, a first control valve in fluid communication with the actuation valve through a first pilot line and configured to displace the actuation valve to a first position when actuated, and a second control valve in fluid communication with the actuation valve through a second pilot line and configured to displace the actuation valve to a second position when actuated. The first pilot line and the second line may each have a fixed minimum back pressure sufficient to maintain dissolved air in the hydraulic fluid.

A work machine including a front control section configured to calculate a limit command value for restricting an operation of a front work implement includes: for example, a bypass line that bypasses, for example, the proportional solenoid valve in, for example, the pilot line; for example, a bypass valve disposed in, for example, the bypass line; a switch configured to output a signal to turn on or off control by the front control section; an on/off determining part configured to determine whether the signal from the switch is an on signal that brings front control into an on state or an off signal that brings the front control into an off state; an open/close command part configured to generate an open/close command signal to open the bypass valve when the signal is determined to be the off/off signal.

An example valve section includes: a valve body configured to be fluidly coupled to the source and the actuator; a spool movable in the valve body intermediate the source and the actuator; a pressure compensator valve disposed upstream from the spool and configured to regulate flow received from the source, where the valve body defines (i) a first passage disposed upstream from the spool and configured to fluidly couple the pressure compensator valve to the spool, and (ii) a second passage disposed downstream from the spool and configured to fluidly couple the spool to the actuator; and a counterbalance valve disposed in the second passage downstream from the spool, where the counterbalance valve is opened to permit flow therethrough from the actuator to the spool in response to a pilot pressure derived from the first passage when the spool is shifted from a neutral position.

A fluid pressure control device includes a switching valve that has a pilot chamber to which the working oil is supplied from the supply passage through the second control valve, and a first relief valve that is provided on the downstream side of the switching valve. The second control valve allows supply of the working oil to the pilot chamber in a case where a flow of the working oil to a tilt cylinder is allowed, and blocks the supply of the working oil to the pilot chamber in a case where the flow of the working oil to the tilt cylinder is blocked. The switching valve allows a flow of the working oil to the first relief valve in a case where the working oil is supplied to the pilot chamber, and blocks the flow of the working oil to the first relief valve in a case where the supply of the working oil to the pilot chamber is blocked.