Abstract: 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.

Abstract: 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.

Abstract: An object of the present invention is to solve such problems associated with installing of a spool-type regenerative valve, in that an overall installation space of a hydraulic valve increases and processing that requires accuracy becomes complex. A regenerative circuit includes: a regenerative oil path provided in a valve body of a direction switching valve so as to connect a supply oil path connected to one side of a hydraulic actuator and a discharge oil path connected to the other side; and a regenerative valve which is a poppet-type flow regulating valve that is screwed into and attached to the valve body so as to control the flow of oil in the regenerative oil path according to an operation state of the direction switching valve.

Abstract: A pressure loss reducing circuit reduces a pressure loss of oil returning from an actuator to a direction switching valve with a simple configuration while suppressing an increase in the manufacturing cost and substantially eliminating the need of an additional installation space. The pressure loss reducing circuit includes a bypass valve disposed between an actuator oil path and a tank oil path of a direction switching valve, and opened and closed according to a signal from a controller, and moreover screwed into and attached to the direction switching valve. According to an operation signal of a switching spool when oil returning from an actuator is caused to flow to the actuator oil path, the controller opens the bypass valve so that the actuator oil path and the tank oil path communicate with each other and supplies the returning oil to the switching spool and the bypass valve to reduce a pressure loss.

Abstract: An object of the present invention is to solve such problems associated with a merging circuit using a pair of spool-type direction switching valves, in that the installation space of a hydraulic valve device increases and the processing that requires accuracy becomes complex. A merging circuit includes a direction switching valve that merges oil discharged from first and second pumps to supply the oil to an actuator; and a controller. The direction switching valve includes a central bypass oil path connected to the discharge oil paths of the first and second pumps, a first parallel supply oil path connected to the discharge oil path of the first pump, and a second parallel supply oil path connected to the discharge oil path of the second pump and connected to a first parallel oil path via a poppet-type flow regulating valve. The poppet-type flow regulating valve regulates the flow rate to a predetermined magnitude according to a signal of the controller.