Overflow valve, pressure reducing valve and sequence valve are valves that control pressure in hydraulic system, collectively referred to as pressure control valves. The common feature of these three valves is that they work according to the principle of balancing the hydraulic force and spring force acting on the valve core.

1. Working principles of different valves

(1) Overflow valve

It controls the opening and closing of the valve core based on the principle that the force of the oil at the oil inlet in the system acting on the valve core is balanced with the spring force to control the oil pressure at the oil inlet.

(2) Pressure reducing valve

It is a pressure control valve that makes the outlet pressure (secondary pressure) lower than the inlet pressure (primary pressure). When it works, it uses the balance between the hydraulic pressure and spring force of the oil at the oil outlet to control the movement of the valve core and keep the outlet pressure constant.

(3) Sequence valve

It is a hydraulic component used to control the order of action of each component in the hydraulic system. According to the different control methods, sequence valves can be divided into two categories: internal control type and external control type. The internal control type uses the inlet pressure of the valve to control the opening and closing of the valve core; the external control type uses external control pressure oil to control the opening and closing of the valve core.

2.Graphic symbols for direct-acting overflow valves, pressure reducing valves and sequence valves

The graphic symbols of the overflow valve, pressure reducing valve and sequence valve indicate the functions, control methods and external connection ports of these pressure control valves, but do not indicate their specific structures and parameters, the actual positions of the connection ports and the installation positions of the components.

When the hydraulic system is represented by graphic symbols, it can make the system diagram more convenient to draw, simpler and clearer. According to regulations, the graphic symbols of hydraulic components are all represented by the static position or zero position (unstimulated state) of the component.

The graphic symbols of the overflow valve, pressure reducing valve and sequence valve are similar in appearance, but their functions are different, which makes it difficult for beginners to master and distinguish.

(1) Graphical representation of valve bodies and main oil inlet and outlet of three types of valves

The valve bodies and main oil inlet and outlet of the three types of valves, namely, overflow valve, pressure reducing valve and sequence valve, are represented in the same way in the graphic symbols.

(2) Diagrammatic representation of valve cores of three types of valves

The arrows in the boxes represent valve cores. When the valve cores of the relief valve and sequence valve are not working, their main oil inlet and outlet ports are blocked, and the arrows in the diagrams are staggered from the inlet and outlet oil paths; when the valve core of the pressure reducing valve is not working, its main oil inlet and outlet ports are connected, and the arrows in the diagrams connect the inlet and outlet oil paths.

(3) Graphical representation of the operating force of the valve core movement driven by three valves

The relief valve, pressure reducing valve and sequence valve all work based on the principle of balancing the hydraulic force acting on the valve core and the spring force. The expression method in the graphic symbols is the same. The upper side of the box is the hydraulic control force, represented by a dotted line; the other side is the spring force. The control pressure value of the three valves can be adjusted by adjusting the pre-compression amount of the spring. The inclined arrow on the spring indicates that the control pressure is adjustable and can be adjusted by adjusting the spring.

(4) Diagram of the source of hydraulic control force

① The overflow valve controls the pressure before the valve. When it works, it uses the balance between the hydraulic pressure and spring force of the oil at the oil inlet acting on the valve core to control the movement of the valve core. The hydraulic control force is drawn from the valve inlet.

② The pressure reducing valve controls the pressure after the valve. When it works, it uses the balance between the hydraulic pressure and spring force of the oil at the oil outlet acting on the valve core to control the movement of the valve core. The hydraulic control force is drawn from the valve outlet.

③ The internal control hydraulic control force of the sequence valve is drawn from the valve inlet; The external control hydraulic control force uses external control pressure oil to control the opening and closing of the valve core.

(5) Diagram of leaking oil circuit

① The oil outlet of the overflow valve is connected back to the oil tank, and the oil leaking into the spring chamber is connected to the oil outlet through the internal channel, and no oil drain port is required.

② The oil outlet of the pressure reducing valve is a pressure oil circuit and is not connected back to the oil tank, so the oil leaking into the spring chamber needs to be connected back to the oil tank separately.

③ For the sequence valve, if the oil outlet is connected back to the oil tank, the oil leaking into the spring chamber can be connected to the oil outlet through the internal channel, and no external oil drain port is required; If the oil outlet is not connected back to the oil tank, the oil leaking into the spring chamber needs to be connected back to the oil tank separately.

3. Graphic symbols of pilot-operated relief valves, pressure reducing valves and sequence valves

The difference between the pilot-operated relief valves, pressure reducing valves and sequence valves and the above direct-acting graphic symbols is that a triangle symbol is added to indicate hydraulic pilot control.

4. Conclusion

To learn and understand the basic structure, working principle, function, and characteristics of hydraulic systems, to be familiar with basic circuits and typical systems, and then to apply hydraulic circuits and systems in actual production, we must first recognize and understand the graphic symbols of hydraulic components. When identifying graphic symbols, combining the working principle and function of the components to identify graphic symbols will achieve twice the result with half the effort.