1、 Working principle of hydraulic valve

The hydraulic valve comprises a valve core, a valve body and a device for driving the valve core to make relative movement in the valve body. In the hydraulic system, it is used to control the pressure, flow and direction of fluid flow to ensure that the actuator works according to the requirements. Generic control element

Basic working principle of hydraulic valve: use the relative movement of valve core in the valve body to control the on-off of valve port and the size of valve port, so as to realize the control of pressure, flow and direction; The flow through the valve port is related to the pressure difference before and after the valve port and the valve port area, which always meets the pressure flow equation.

2、 Classification of hydraulic valves

In hydraulic transmission, an element used to control fluid pressure, flow, and direction. Among them, those controlling pressure are called pressure control valves, those controlling flow are called flow control valves, and those controlling on, off and flow direction are called directional control valves.

Pressure control valve: it is divided into overflow valve, pressure reducing valve and sequence valve according to its purpose（ 1) Overflow valve: it can control the hydraulic system to maintain a constant state when reaching the set pressure. The overflow valve used for overload protection is called safety valve. When the system fails and the pressure rises to the limit value 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 oil pressure of the main circuit. According to the different pressure functions it controls, the pressure reducing valve can be divided into constant value pressure reducing valve (the output pressure is a constant value), constant difference pressure reducing valve (the difference between input and output pressure is a constant value) and constant ratio pressure reducing valve (a certain proportion is maintained between input and output pressure)（ 3) Sequence valve: it can make one actuator (such as hydraulic cylinder, hydraulic motor, etc.) act, and then make other actuators act in sequence. The pressure generated by the oil pump first pushes the hydraulic cylinder 1 to move, and acts on the area a through the oil inlet of the sequence valve. When the hydraulic cylinder 1 moves completely, the pressure increases. After the upward thrust acting on the area a is greater than the set value of the spring, the valve core rises to connect the oil inlet with the oil outlet to move the hydraulic cylinder 2.

Flow control valve: use the orifice area between the regulating valve core and the valve body and the local resistance generated by it to adjust the flow, so as to control the movement speed of the actuator. Flow control valves are divided into 5 types according to their purposes（ 1) Throttle valve: after adjusting the orifice area, the movement speed of the actuator with little change in load pressure and low requirements for movement uniformity can be basically kept stable（ 2) Speed regulating valve: when the load pressure changes, it can keep the differential pressure between the inlet and outlet of the throttle valve as the fixed value. In this way, after the throttle area is set, no matter how the load pressure changes, the speed regulating valve can keep the flow through the throttle unchanged, so as to stabilize the movement speed of the actuator（ 3) Diverter valve: equal diverter valve or synchronous valve can make two actuator elements of the same oil source obtain equal flow regardless of the load; The proportional diverter valve is obtained by proportional flow distribution（ 4) Collecting valve: the function is opposite to the diverter valve to distribute the flow into the collecting valve in proportion（ 5) Diverter and collector valve: it has two functions: diverter valve and collector valve.

Directional control valve: it is divided into one-way valve and directional valve according to its purpose. One way valve: only one-way connection of fluid in the pipeline is allowed, and the reverse is cut off. Directional valve: change the on-off relationship between different pipelines, and divide it into two positions and three positions according to the number of working positions of the valve core in the valve body; According to the number of controlled channels, it is divided into two-way, three-way, four-way, five-way, etc; According to the valve core driving mode, it is divided into pneumatic, motorized, electric, hydraulic, etc. Figure 2 shows the working principle of three position four-way directional 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 valve core is in the middle position, all oil ports are cut off and the actuator does not move; When the valve core moves to the right position, P and a are connected, and B and o are connected; When the valve core moves to the left position, P and B are connected, and a and o are connected. In this way, the actuator can move forward and backward.