Hydraulic system for construction machinery

A hydraulic system for construction machinery, comprising:

• a hydraulic pump;
• a cylinder control valve adapted to control the flow of hydraulic fluid from the hydraulic pump to the hydraulic cylinder;
• an operating device adapted to operate the cylinder control valve;
• a regeneration motor adapted to be driven by return fluid from one hydraulic fluid port of the hydraulic cylinder;
• a regeneration electric motor adapted to convert rotary power of the regeneration motor into electric energy;
• a pilot hydraulic pressure source;
• at least one return fluid control valve adapted to control the flow of the return fluid according to pilot pressure that is led from the pilot hydraulic pressure source in conjunction with an operation on the operating device;
• at least one solenoid valve for return fluid control valve adapted to control the pilot pressure for driving the return fluid control valve;
• a malfunction prevention device adapted to be capable of interrupting the delivery of the pilot pressure from the solenoid valve for return fluid control valve to the return fluid control valve;
• a flow control valve which is arranged in a line connecting the one hydraulic fluid port to the cylinder control valve and a regeneration valve which is arranged in a regeneration line connecting the line and the regeneration motor are provided as the return fluid control valves;
• a solenoid valve for flow control valve adapted to control pilot pressure for driving the flow control valve and a solenoid valve for regeneration valve adapted to control pilot pressure for driving the regeneration valve are provided as the solenoid valves for return fluid control valve;
• and
• a quick acceleration prevention valve which is arranged in parallel with the solenoid valve for flow control valve, and a shuttle valve adapted to select higher pressure from pilot pressures led from the solenoid valve for flow control valve and the quick acceleration prevention valve, the shuttle valve adapted to lead the selected higher pressure to a pilot pressure port of the flow control valve are provided as the malfunction prevention device;
• wherein when the pressure in the regeneration line between the regeneration valve and the regeneration motor falls below a preset value, the quick acceleration prevention valve is configured to
• connect the solenoid valve for regeneration valve to a tank,
• connect an output line of the operating device to the shuttle valve, and
• lead output pilot pressure of the operating device to the pilot pressure port of the flow control valve by bypassing the solenoid valve for flow control valve.