Circuit pressure control device, hydraulic control circuit using circuit pressure control unit, and hydraulic control circuit of construction machine

1. A circuit pressure control unit, comprising:

a relief valve connected to a passage communicating with an actuator at an upstream side of the relief valve, the passage including a branch passage, the branch passage being branched from a connecting passage, the connecting passage connecting the actuator and an operation valve; and

a variable throttle valve disposed in the branch passage upstream of the relief valve, the variable throttle valve being configured to change a degree of opening thereof according to a control signal from a controller.

2. The circuit pressure control unit according to claim 1 , wherein the variable throttle valve is configured to decrease the degree of opening in response to the control signal for increasing a pressure supplied to the, and is configured to increase the degree of opening in response to the control signal for decreasing the pressure supplied to the actuator.

3. The circuit pressure control unit according to claim 1 , wherein the controller is configured to increase the degree of opening of the variable throttle valve to set a load of the actuator to be small, and is configured to reduce the degree of opening of the variable throttle valve to set the load of the actuator to be large.

4. The circuit pressure control unit according to claim 1 , wherein the controller is configured to change a pressure supplied to the actuator linearly when pressure upstream of the relief valve is higher than a setting pressure of the relief valve.

5. A hydraulic control circuit for controlling a pressure-controlled actuator, the hydraulic control circuit comprising a circuit pressure control unit, including:

a relief valve connected to a connecting passage communicating with the actuator to be controlled, the actuator to be controlled being disposed upstream of the relief valve; and

a variable throttle valve responsive to a control signal from a controller to change a degree of opening of the variable throttle valve, the variable throttle valve being disposed upstream of the relief valve,

wherein an upstream side of the variable throttle valve is connected to the connecting passage, the connecting passage communicating with the actuator to be controlled, and

wherein the relief valve is connected to a downstream side of the variable throttle valve, and a downstream side of the relief valve is connected to a supply passage, the supply passage providing communication between the relief valve and an actuator other than the actuator to be controlled, the variable throttle valve and the relief valve controlling a circuit pressure of a system of the actuator to be controlled.

6. A hydraulic control circuit for a construction machine, the hydraulic control circuit comprising:

a rotating motor;

a hydraulic pump as a pressure source of the rotating motor;

an operation valve disposed between the rotating motor and the hydraulic pump, an upstream side of the operation valve being connected to the hydraulic pump or a tank, a downstream side of the operation valve being connected to the rotating motor; and

a circuit pressure control unit, including:

a relief valve connected to a connecting passage communicating with the rotating motor, the rotating motor being disposed upstream of the relief valve; and

a variable throttle valve responsive to a control signal from a controller to change a degree of opening of the variable throttle valve, the variable throttle valve being disposed upstream of the relief valve,

wherein the variable throttle valve has an upstream side that is connected to the connecting passage, the connecting passage connecting the operation valve to the rotating motor, and

wherein the relief valve has a downstream side that is connected to a supply passage, the supply passage connecting the relief valve to a hydraulic motor for rotating an electric generator.