Introduction to the knowledge of hydraulic valves

- Jul 22, 2019-

hydraulic valves

1,Working principle of hydraulic valve

The hydraulic valve includes a spool, a valve body, and a device that drives the spool to move relative to each other within the valve body. It is used in the hydraulic system to control the pressure, flow and direction of the flow, ensuring that the actuators work as required. Control element

The basic working principle of the hydraulic valve: the relative movement of the valve core in the valve body to control the opening and closing of the valve port and the size of the valve port to achieve pressure, flow and direction control; and the flow through the valve port and the differential pressure before and after the valve port It is related to the valve port area and always satisfies the pressure flow equation.


2,Classification of hydraulic valves

A component used in hydraulic drives to control fluid pressure, flow, and direction. The pressure control valve is called a pressure control valve, the flow control valve is called a flow control valve, and the directional control valve is called to control the on, off and flow directions.

Pressure control valve: 

flow control valve

Divided into overflow valve, pressure reducing valve and sequence valve according to the purpose. (1) Overflow valve: It can control the hydraulic system to maintain a constant state when the set pressure is reached. The relief valve for overload protection is called a safety valve. When the system fails and the pressure rises to a limit that may cause damage, the valve port will open and overflow to ensure the safety of the system. (2) Pressure reducing valve: It can control the branch circuit to obtain a stable pressure lower than the main circuit oil pressure. The pressure reducing valve can be divided into fixed value pressure reducing valve (output pressure is constant value), differential pressure reducing valve (input and output pressure difference is fixed value) and proportional pressure reducing valve according to the pressure function controlled by it. (Proportion is maintained between input and output pressure). (3) Sequence valve: After an actuator (such as a hydraulic cylinder, a hydraulic motor, etc.) can be actuated, the other actuators are operated in sequence. The pressure generated by the oil pump first pushes the hydraulic cylinder 1 to move, and at the same time, the oil inlet of the sequence valve acts on the area A. When the hydraulic cylinder 1 is completely moved, the pressure rises, and the upward thrust acting on the area A is greater than the setting of the spring. After the value, the spool rises to make the oil inlet and the oil outlet communicate, so that the hydraulic cylinder 2 moves.


Flow Control Valve: The flow rate is adjusted by adjusting the orifice area between the spool and the valve body and the local resistance generated by it, thereby controlling the speed of movement of the actuator. The flow control valves are divided into five types according to their purposes. (1) Throttle valve: After setting the throttle opening area, the moving speed of the actuator which can make the load pressure change little and the motion uniformity requirement is not high is basically kept stable. (2) Speed control valve: The pressure difference between the inlet and outlet of the throttle valve can be kept constant when the load pressure changes. In this way, after the orifice area is set, regardless of the change in the load pressure, the speed regulating valve can maintain the flow rate through the throttle valve, thereby stabilizing the moving speed of the actuator. (3) Diverter valve: Regardless of the size of the load, the two actuators of the same oil source can be equal-flowing valves or synchronous valves with equal flow rate; the proportionally distributed flow rate is obtained as a proportional diverter valve. (4) Collector valve: The action is opposite to that of the diverter valve, so that the flow into the collector valve is proportionally distributed. (5) Split flow collecting valve: It has two functions of diverter valve and collector valve.

Directional control valve: divided into check valve and reversing valve according to the purpose. One-way valve: only allows the fluid to be unidirectionally connected in the pipeline, and the reverse is cut off. Directional valve: change the on-off relationship between different pipelines, according to the working position of the valve core in the valve body, two, three, etc.; according to the number of channels controlled, two-way, three-way, four-way, Five-way, etc.; according to the spool drive mode, manual, motorized, electric, hydraulic, etc. Figure 2 shows the working principle of the three-position four-way reversing valve. P is the oil supply port, O is the oil return port, and A and B are the output ports to the actuator. When the spool is in the neutral position, all ports are cut off and the actuator is not moved; when the spool moves to the right position, P and A pass, B and O pass; when the spool moves to the left, P and B Pass, A and O pass. In this way, the actuator can move in both forward and reverse directions.


3,Hydraulic valve features

1). The action is sensitive, the work is stable and reliable, and the impact, vibration and noise are as small as possible.

2). The resistance loss when the oil flows through the valve is small.

3). The sealing is better, and the leakage is small.

4). The structure should be simple and compact, small in size, large in versatility and long in life.


4,The role of hydraulic valves

The hydraulic valve is used to control the flow direction of the oil in the hydraulic system or to adjust its pressure and flow, so it can be divided into three categories: directional valve, pressure valve and flow valve. A valve of the same shape can have different functions depending on the mechanism of action. The pressure and flow valves use the throttling of the flow section to control the pressure and flow of the system, while the directional valve uses the replacement of the flow passage to control the flow direction of the fluid. That is to say, although there are many different types of hydraulic valves, they still maintain some basic commonalities.