Hydraulic control valves are components used to control the pressure, flow, and direction of fluid flow in hydraulic systems. They are crucial components that influence the performance and reliability of hydraulic systems. This article introduces the typical structure and operating principles of hydraulic control valves and summarizes and analyzes some common issues with them, aiming to master the theory and methods of analyzing hydraulic control valves.

Hydraulic control valves can be classified in numerous ways, so much so that the same valve may have different names depending on its intended purpose in different applications. The following describes several different classification methods.

1. Classification based on function in hydraulic system

(1) Pressure control valve

Function: Control the fluid pressure in the hydraulic system or use pressure to achieve control.

① Overflow valve

Control the inlet pressure of the control valve. The safety valve is used to prevent the system from overloading and plays an emergency protection role. The constant pressure valve is used to maintain the system pressure constant and overflow the excess oil back to the tank.

② Pressure reducing valve

Control the outlet pressure of the valve lower than the inlet pressure. The fixed value pressure reducing valve is to ensure that the outlet pressure is a fixed value (referred to as the pressure reducing valve). The differential pressure reducing valve is to ensure that the pressure difference between the outlet and the inlet is a fixed value; if it is used to control the pressure difference of other valves (such as throttle valves), it is called an externally controlled differential pressure reducing valve. The ratio pressure reducing valve is to ensure that the ratio of the outlet and inlet pressure is a fixed value.

③ Sequence valve

When the control pressure reaches the set value, the valve port is opened to allow the fluid to flow through. According to the control method and purpose, it is divided into sequence valve, unloading valve, back pressure valve, balancing valve, hydraulic switch, etc.

(2) Flow control valve

Function: Control the flow of fluid in the hydraulic system.

① Throttle valve

A valve composed of adjustable fluid resistance.

② Speed control valve

Composed of a throttle valve and a pressure compensation mechanism, the flow is not affected by changes in inlet or outlet pressure.

③ Diverter valve

Divides the incoming oil into two streams at a fixed ratio and is not affected by load changes. The combining valve combines the two proportional flows into one.

(3) Directional control valve

Function: Control the flow direction of fluid in the hydraulic system.

① Check valve

Only allows fluid to flow in the forward direction and blocks it in the reverse direction.

② Directional valve

Changes the direction of fluid flow by connecting or closing the oil port.

(4) Other valve types

① General valve: The above are general valves with a single function.

② Special valve: Such as the flow stabilizing valve used in engineering machinery.

③ Composite valve: A combination valve with multiple functions, such as a one-way pressure reducing valve.

2. Classification by control method

(1) On-off fixed value control valve

① Control method:

Open and close the liquid flow path manually, mechanically, by electromagnet or by controlling pressure oil, and control the liquid flow parameters.

② Features:

Widely used, also known as ordinary hydraulic valve. Simple structure, low cost, but low control accuracy and dynamic response.

(2) Servo control valve (follow-up valve)

① Control principle:

Continuously and proportionally control the liquid flow parameters output by the system according to the input signal (electrical, mechanical, pneumatic, etc.) and feedback.

② Features:

Fast dynamic response, high static performance, but expensive and poor anti-pollution ability. Mainly used in high-precision control occasions (such as aerospace, precision machine tools).

(3) Electro-hydraulic proportional valve

① Performance positioning:

between on-off valve and servo valve, taking into account both control accuracy and cost.

② Control method:

Continuously and proportionally control the liquid flow parameters according to the input electrical signal. Meet general industrial control needs, simpler structure than servo valve, lower price and easy to maintain.

③Categories:

Conventional electro-hydraulic proportional valve: Based on an on-off valve, this valve adopts a proportional solenoid drive. Electro-hydraulic proportional valve with internal feedback: A new design with an integrated feedback mechanism improves control accuracy.

3. Classification by structural type

(1) Sliding valves

The valve core of the sliding valve is cylindrical. The valve core slides relative to the valve body hole to change the size of the liquid flow path opening to control the liquid flow parameters.

(2) Cone valves

The valve core of the cone valve is conical. The displacement of the valve core relative to the valve seat hole changes the size of the liquid flow path opening to control the liquid flow parameters. Ball valves with spherical valve cores can be classified as cone valves because their performance is similar to that of cone valves.