A valve arrangement for supplying pressure medium to a hydraulic consumer has two utility ports (A, B) for fluid connection to the consumer and has a first control valve. The first control valve (20) has a first intake (24) via which a first intake flow from the first utility port (A) to the consumer (10) is controllable, has a first return control orifice (26) via which a first return flow (28) from the consumer (10) via the second utility port (B) is controllable simultaneously with the first intake (24), has a second intake (34) via which a second intake flow from the second utility port (B) to the consumer (10) is controllable, and having a second return control orifice (36), via which a second return flow (38) from the consumer (10) via the first utility port (A) is controllable simultaneously with the second intake (34). A second control valve (40) has an intake control orifice (42) used to control the respective intake (24, 34) of the first control valve (20).

An asymmetric linear actuator is provided which integrates a hydraulic dissipater and an electric motor and power screw which generates small forces. The actuator is configured so that an electric motor drives a power screw which drives a rod through a cylinder to provide linear actuation. The cylinder is fluid-filled and incorporates a piston that separates the cylinder into a first and second fluid chamber which are filled with a first and second volume of working fluid. Movement of the piston and rod assembly results in fluid movement between the first and second volumes of working fluid and through the fluidic restriction. The fluidic restriction can be proportionally controllable via an electric motor which enables controllable power dissipation via control of the fluidic restriction motor and controllable power generation via control of the power screw motor.

A hydraulic system includes a first fixed displacement pump having a first pump output and a second fixed displacement pump having a second pump output. A first actuator requires a first required flow rate to perform a first function. A second actuator requires a second required flow rate to perform a second function. A combined flow control valve is controllable between a first state connecting fluid communication between the first fixed displacement pump and the tank so that only the second pump output is directed to the second actuator, and a second state disconnecting fluid communication between the first fixed displacement pump and the tank to combine the first pump output and the second pump output such that the combined first pump output and the second pump output is directed to the first actuator.

This hydraulic drive system includes: a hydraulic pump capable of changing a discharge flow rate of a working fluid; a meter-in control valve that controls a meter-in flow rate of the working fluid flowing from the hydraulic pump to a hydraulic actuator; a meter-out control valve that is provided separately from the meter-in control valve and controls a meter-out flow rate of the working fluid being drained from the hydraulic actuator into a tank; an operation device that outputs an operation command; a first pressure sensor that detects a drainage pressure of the hydraulic actuator; and a control device that sets a target meter-out flow rate according to the operation command from the operation device and controls an opening degree of the meter-out control valve on the basis of the drainage pressure detected by the first pressure sensor and the target meter-out flow rate.

Provided is a construction machine which can promptly adjust the rotational speed of a swing motor to a target rotational speed. A controller calculates a target rotational speed of the swing motor based on input from an operation device, calculates a degree of deviation of a rotational speed detected by a rotational speed sensor from the target rotational speed, sets a target driving pressure of the swing motor according to a moment of inertia about a swing axis of a swing structure and a work device and controls a pressure adjusting device in such a manner as to reduce a difference between a driving pressure detected by a pressure sensor and the target driving pressure, when the degree of deviation is larger than a predetermined value, and controls the pressure adjusting device in such a manner as to reduce a difference between the rotational speed detected by the rotational speed sensor and the target rotational speed, when the degree of deviation is equal to or smaller than the predetermined value.

The object of the invention of the present application resides in provision of a work machine that can be improved in operability at start of an action of a hydraulic actuator in a fine operation in which an operation lever is operated by a small amount. To this end, on the basis of a signal from a first timing sensor and a signal from a second timing sensor, a machine body controller controls, before the first timing is sensed, the pump delivery flow rate to a minimum delivery flow rate, controls, after the first timing is sensed but before the second timing is sensed, the pump delivery flow rate to a predetermined delivery flow rate that is greater than the minimum delivery flow rate, and controls, after the second timing is sensed, the pump delivery flow rate to a delivery flow rate according to an operation amount of the operation lever.

Problem to be solved: To provide a hydraulic circuit for the construction machine which enables to use the relief valve of low capacity in the work tool circuit. Solution: The hydraulic circuit 2 for a construction machine has: a hydraulic pump 4 of variable capacity, a work tool 6 operated by hydraulic oil delivered by the hydraulic pump 4, a work tool operating device 10 to output a signal for operating the work tool 6, a control valve 14 allowing the hydraulic pump 4 to supply the hydraulic oil to the work tool 6 based on the signal output from the work tool operating device 10, a tool's relief valve 44 to release the hydraulic oil flowing between the control valve 14 and the work tool 6, a pressure sensor 46 to detect a pressure of hydraulic oil flowing into the work tool 6, and a controller 48 to reduce a delivery rate from the hydraulic pump 4 when the pressure detected by the pressure sensor 46 exceeds a predetermined value.

To make it possible to control supply, discharge, and recycled flow rates independently of each other for the boom cylinder in a construction machine comprising first and second boom spool valves respectively connected to first and second hydraulic pumps. When the boom cylinder is contracted, the first boom spool valve is configured to control the recycled flow rate from head side oil chamber to rod side oil chamber, the second boom spool valve is configured to control the discharge flow rate from head side oil chamber to oil tank, and both first and second boom spool valves are configured not to supply pressure oil from first and second hydraulic pumps to the boom cylinder.

An electro-hydraulic actuator includes: a motor output rotative power; an external gear pump activated by the motor; a hydraulic actuator operated by a pressurized working fluid supplied by the external gear pump; a manifold block in which a flow channel forming a working fluid circuit of the hydraulic actuator is incorporated; a first portion to store the motor; a second portion to store the external gear pump, the hydraulic actuator, and a reservoir; and a coupling portion to couple the first portion and the second portion in a liquid-tight state. The coupling portion includes a communication hole through which the first portion and the second portion communicate, a rotational shaft of the motor and a driving shaft of the external gear pump are joined to each other, and the external gear pump is attached to the coupling portion while being stored in the manifold block.

Provided is a construction machine in which travel operability is not impaired when a combined operation is performed in which a single-rod hydraulic cylinder is driven during a travel operation. A hydraulic excavator includes a controller that controls the delivery direction and delivery flow rate of a closed-circuit pump, opens and closes a travel switching valve and an assist switching valve, and controls the delivery flow rate of the open-circuit pump, according to a travel operation lever and a work operation lever. The controller holds the assist switching valve in a closed position irrespective of whether or not the work operation lever is operated in a case where the travel operation lever is operated.