The present disclosure relates to a safety system for a construction machine, and the safety system for a construction machine according to the exemplary embodiment of the present disclosure includes: a pilot pump which supplies a pilot working fluid; multiple spools which are operated by the pilot working fluid to control a supply of a main working fluid and each have a signal unit formed in one region thereof; a signal line which sequentially connects the pilot pump and the signal units of the multiple spools and discharges the pilot working fluid, which is supplied by the pilot pump, to a hydraulic tank; and a pressure sensor which measures pressure in the signal line.

A forklift according to an exemplary embodiment of the present disclosure includes: a first hydraulic pump and a second hydraulic pump each of which generates a working fluid; a lift cylinder which raises or lowers a carriage; a first hydraulic line which supplies the working fluid from the first hydraulic pump to the lift cylinder and a second hydraulic line which supplies the working fluid from the second hydraulic pump to the lift cylinder; a first lift spool which controls the working fluid to be supplied to the lift cylinder through the first hydraulic line and a second lift spool which controls the working fluid to be supplied to the lift cylinder through the second hydraulic line; a first pilot line which transmits a pilot signal for operating the first lift spool and a second pilot line which transmits a pilot signal for operating the second lift spool; and an opening/closing valve which selectively closes any one of the first pilot line and the second pilot line.

A hydraulic system includes first and second valve assemblies connected to a common pump. The first valve assembly includes a main valve housed inside a manifold. A pressure compensator valve maintains a constant pressure drop across a variable orifice of the main valve. A pressure limiter valve is in communication with the main valve and the pressure compensator valve, and allows an actuator connected to the first valve assembly to operate independently of the second valve assembly so that fluid flow to a work port of the first valve assembly is not interrupted by operation of the second valve assembly.

The invention provides a construction machine in which the load torque at the time of engine start-up can be reduced even when the engine stops against the will of the operator. A hydraulic excavator includes a control device (35) having a pump displacement control section (37) and an unload control section (38). The pump displacement control section (37) makes the displacement of a hydraulic pump (16) variable to a minimum displacement by controlling a regulator device (20) when the speed of an engine (14) detected by a speed sensor (41) becomes equal to or less than a preset low speed N3 at the time of driving of the engine (14). The unload control section (40) controls an unloading valve (24) to the open position at the time of start-up of the engine (14).

A hydraulic system includes a hydraulic pump to discharge an operation fluid, a hydraulic device to be operated by the operation fluid, a plurality of hydraulic pressure controllers each to change an operation state of the hydraulic pressure controller based on a pressure of the operation fluid so as to control the hydraulic device, and a proportional valve connected to the plurality of hydraulic pressure controllers. The proportional valve is configured to set a pressure of the operation fluid supplied to the hydraulic pressure controllers.

A hydraulic drive system includes control valve and operating devices, a variable displacement pump, and a flow regulator. When an operating lever inclination angle becomes a value, a control valve opening area becomes a reference. When the operating lever inclination angle maximizes, the opening area maximizes. The flow regulator: until the operating lever inclination angle becomes the value, increases the pump discharge flow rate with the inclination angle, so a differential pressure between pump discharge and actuator load pressures is constant; when the operating lever inclination angle becomes the value, controls the pump discharge flow rate, so a control valve passing flow rate is an actuator maximum flow rate when the differential pressure is constant; and when the operating lever inclination angle is between the value and the maximum, defines a maximum pump discharge flow rate, so the pump discharge flow rate is kept to the actuator maximum flow rate.

An optional-attachment-operating direction switching valve is provided in a part of an unloading passage which part is on the upstream of an arm-operating direction switching valve and a bucket-operating direction switching valve. The hydraulic circuit is arranged such that the optional-attachment-operating direction switching valve receives pressure oil from a pressure oil supply passage which branches from the part of the unloading passage on the upstream of the optional-attachment-operating direction switching valve, the arm-operating direction switching valve receives pressure oil from a pressure oil supply passage which branches from a part of the unloading passage which part is between the optional-attachment-operating direction switching valve and the arm-operating direction switching valve, and the bucket-operating direction switching valve receives the pressure oil from a pressure oil supply passage which branches from a part of the unloading passage which part is between the optional-attachment-operating direction switching valve and the bucket-operating direction switching valve.

A hydraulic working machine is provided that uses a variable displacement pump and open center type flow rate control valve for a controlling a hydraulic actuator, and has a negative control throttle disposed in a center bypass oil passage to generate a negative control pressure. A hydraulic pump control system performs virtual bleed-off control for reducing the bleed-off flow rate of the center bypass oil passage, and can operate the hydraulic actuator with the same performance as an open center control. The control system includes a bypass cut valve disposed upstream from the negative control throttle to reduce flow through the center bypass oil passages, and a negative control pressure output valve that outputs a virtual negative control pressure. The control system is configured to reduce the bleed-off flow rate by operating the bypass cut valve when virtual bleed-off control is performed, and to reduce the pump flow rate by the bleed-off reduction flow rate.

A hydraulic unit includes two pressure connections for supplying devices, in particular hydraulic rescue devices, including a first hydraulic circuit with a first pump arrangement and a first pressure connection and a second hydraulic circuit with a second pump arrangement and a second pressure connection, wherein the pump arrangements are driven at the same time by a common drive, and wherein by a first valve switch the first hydraulic circuit can be connected to the second hydraulic circuit and by a second valve switch the second hydraulic circuit can be connected to the first hydraulic circuit. The valve switches include a spring acting in the direction of an initial position and from the first hydraulic circuit or from the second hydraulic circuit a first control line runs to the first valve switch and from the second hydraulic circuit or from the first hydraulic circuit a second control line runs to the second valve switch.

Pressure compensating valves not fully closing at the stroke end are employed, and upon the operator's operation for the traveling, pilot primary pressure is reduced and supplied to remote control valves 34c-34h of non-travel operating devices. Thus, the inflow of the hydraulic fluid into non-travel actuators is suppressed and a necessary amount of hydraulic fluid for travel motors is secured in travel combined operation. Accordingly, when saturation occurs in a construction machine's hydraulic drive system performing the load sensing control due to combined operation with a great load pressure difference between two actuators, deceleration/stoppage of an actuator on the low load pressure side is prevented by preventing full closure of the pressure compensating valve on the low load pressure side, while also preventing deceleration/stoppage of a high load pressure actuator by securing a necessary amount of hydraulic fluid for the high load pressure actuator.