Introduction of Hydraulic Circuits
Pressure Control Circuit Edition

Power Reduction circuit

YUKEN KOGYO CO., LTD.

Power Reduction Circuit
Circuit composed of a double pump and a relief valve
The circuit shown in Fig. 1-5 comprises a double pump with large and small capacity, a relief valve forhigh pressure setting, and an unloading valve. At low pressure, the flow rate of the large-capacity andsmall-capacity pumps are combined, resulting in a large flow rate. When the pressure reaches the set
pressure of the unloading valve, the large-capacity pump unloads, and the small-capacity pump operates
an actuator. The graph of the pressure delivered to the actuator and the flow rate is shown in Fig. 1-6. At
high pressure, the flow rate decreases, enabling a reduction in pump shaft input and resulting in power
reduction.

Relief valve

Low pressure/large
flow

Unloading valve

Flow rate

Highpressure/smallcapacity pumpLowpressure/large
capacity pump
Low pressure, large
flow operation

S1002-E

Fig. 1-5

High pressure, smallflow operation

High pressure/small flowPressureFig. 1-6Ⓒ YUKEN KOGYO CO., LTD. All Rights Reserved.

Unloading Circuit
Circuit using a two-pressure two-flow control type pump
The circuit shown in Fig. 1-7 uses a pump of two-pressure two-flow control type by system pressure. This
single variable displacement pump operates as both a low pressure/large flow pump and a highpressure/small flow pump, keeping the electric motor capacity small. As the pump discharge pressure
approaches the preset pressure PL by increasing load, the discharge rate automatically decreases to QL
as shown in Fig. 1-8. Hydraulic power is proportional to the pressure and flow rate. Therefore, setting asmall flow rate at high pressure can keep the electric motor capacity small.

Pump of two-pressure
two-flow control type bysystem pressure

Low pressure/large flow

High pressure/small flow

Pressure
Fig. 1-7
S1002-E

Fig. 1-8Ⓒ YUKEN KOGYO CO., LTD. All Rights Reserved.

Unloading Circuit
Circuit using an accumulator
The circuit shown in Fig. 1-9 reduces the power by using an accumulator. When the circuit pressure
reaches the set value of the pressure switch (PS), the solenoid operated directional valve turns OFF, andthe pump unloads. However, the circuit pressure is maintained by the accumulator. When the circuit
pressure drops and reaches the lower limit value of the PS, the solenoid operated directional valve turns
ON.
The circuit shown in Fig. 1-10 uses an accumulator and an unloading relief valve. When the circuit
pressure reaches the set pressure of the unloading relief valve, the pump unloads. However, the circuit
pressure is maintained by the accumulator.
PS

Solenoid operated
directional valve OFFUnloadUnloading
relief valve

Fig. 1-9S1002-EFig. 1-10
Ⓒ YUKEN KOGYO CO., LTD. All Rights Reserved.

The hydraulic circuits shown in this document are limited to typical ones.
Refer to this document when creating hydraulic circuits.
If you require assistance with hydraulic circuit considerations or any other hydraulics-related questions,please contact us.

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