Components used to control fluid pressure, flow and direction in hydraulic drives. Control pressure which is called the pressure control valve, the control flow called flow control valve, control the flow, the flow and the direction is called the directional control valve. Directional control valve is divided into use by one-way valve and the valve. ①check valve: only allow one-way fluid in the pipeline connected, the reverse that cut off. ② reversing valve: to change the different lines between the pass, off relations, according to the spool in the valve body working position number points two, three, etc .; according to the number of channels controlled by two, three, four , Five links, etc .; according to the spool drive sub-manual, motorized, electric, hydraulic and so on. The flow control valve regulates the flow by adjusting the orifice area between the valve core and the valve body and the local resistance it creates to control the speed of movement of the actuator. Flow control valve is divided into 5 uses. ①throttle: adjust the throttle area, can make little change in load pressure and less demanding uniformity of motion of the implementing element's speed of movement remained basically stable. ② governor valve: When the load pressure changes to maintain the throttle valve inlet and outlet differential pressure for the set value. In this way, after adjusting the orifice area, the speed control valve can keep the flow through the throttle valve unchanged, so that the movement speed of the actuator is stable. ③ diverter valve: Regardless of the load size, the same oil source of the two actuators can be equal for equal flow diverter valve or synchronous valve; proportional flow to get the proportion of diverter valve. ④ collector valve: the role of the bypass valve and the opposite, so that the flow into the collection valve in proportion to the distribution. ⑤ shunt manifold valve: both divert valve and current collector valve two functions. Pressure control valve according to the purpose is divided into relief valve, pressure relief valve and sequence valve. ①relief valve: to control the hydraulic system to achieve a constant pressure regulator. The relief valve used for overload protection is called safety valve. When the system fails, the pressure rises to a limit value that may cause damage, the valve will open and overflow to ensure the safety of the system. ② pressure reducing valve: to control the branch circuit to get lower than the main circuit oil pressure stable. Pressure relief valve according to its control of different pressure functions, and can be divided into fixed value pressure relief valve (output pressure is constant value), fixed pressure relief valve (input and output pressure differential for the fixed value) and the proportional pressure relief valve (Input and output pressure to maintain a certain ratio). ③ Sequence valve: can make an actuator (such as hydraulic cylinder, hydraulic motor, etc.) action, and then in order to make the other actuator action. In the late 1960s, electro-hydraulic proportional control valves were developed on the basis of several types of hydraulic control valves. Its output (pressure, flow) can vary continuously with the incoming electrical signal. Electro-hydraulic proportional control valve according to different roles, correspondingly divided into electro-hydraulic proportional pressure control valve, electro-hydraulic proportional flow control valve and electro-hydraulic proportional directional control valve.