Excavator main control valve PC200-8 PC220-8 safety valve 723-90-76101

Unloading valve working principle: unloading relief valve is composed of relief valve and check valve. When the system pressure reaches the opening pressure of the relief valve, the relief valve is opened and the pump is unloaded. When the system pressure drops to the closing pressure of the relief valve, the relief valve closes and the pump loads into the system.
The unloading valve is a valve that can make the hydraulic pump unload under certain conditions. The unloading valve is usually an relief valve with two two-way valve (usually a solenoid valve), which is used to set the main pressure of the system (oil pump) when it is not unloaded. When the pressure oil is discharged (by two two-way valve action conversion), the pressure oil is directly returned to the tank, and the pressure of the oil pump is reduced to approximately zero, so as to achieve some loop control and improve the life of the oil pump and reduce power consumption.
Belonging to an incorporated loop in a loop. The pressure reducing valve is used to adjust the pressure required by the actuator, which is in series in the loop and generally cannot be used interchangeably.

Extended information:
Type of unloading valve:
Penetration type
The unloading channel and pressure valve are set up separately. When unloading, each spool is in a neutral position, and the oil from the oil source is discharged back to the tank through a special oil channel through each valve, and the unloading oil channel runs through each reversing valve. When one of the valves is working (that is, the unloading oil passage is cut off), the oil from the oil source enters the controlled actuator from the reversing valve of the road, and the working pressure is limited by the pressure valve in the figure.
Unloading type
The unloading valve and the safety valve are integrated to form a pilot-operated pressure valve, which is both a unloading valve and a safety valve, and sometimes an overflow valve. During unloading, the control oil passage passes through each directional valve, which is the same as the unloading oil passage.