According to the purpose is divided into one-way valve and reversing valve. One-way valve: only allow the fluid in the pipeline in one direction, the reverse is cut off. Reversing valve: change the on - off relationship between different pipelines. According to the working position of the valve core in the valve body, the number of two, three, etc. According to the number of controlled channels into two, three, four, five, etc.; According to the drive mode of the valve spool separation, motor, electric, hydraulic, and so on. Figure 2 shows the working principle of three-position four-way reversing valve. P is the oil supply port, O is the oil return port, and A and B are the outlets leading to the execution element. When the valve core is in the middle position, all the oil ports are cut off and the executive components are not moved. When the spool is moved to the right, P and A pass, B and O pass; When the spool is moved to the left, P is connected to B and A to O. Thus, the executive element can move forward and backward.
In the late 1960s, the electro-hydraulic proportional control valve was developed on the basis of the above hydraulic control valves. Its output (pressure, flow) can change continuously with the input electrical signal. Electro-hydraulic proportional control valve according to the role of different, correspondingly divided into electro-hydraulic proportional pressure control valve, electro-hydraulic proportional flow control valve and electro-hydraulic proportional direction control valve.