Pressure control valves are divided into relief valves, pressure reducing valves and sequence valves according to their application.

(1) Relief valve: It can control the hydraulic system to maintain a constant state when it reaches 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 fixed value pressure reducing valve (the output pressure is a constant value), fixed difference pressure reducing valve (the difference between input and output pressure is a fixed value) and fixed ratio pressure reducing valve (the input and output pressure maintain a certain ratio).

(3) Sequence valve: After one actuator (such as hydraulic cylinder, hydraulic motor, etc.) is activated, other actuators are activated in sequence. The pressure generated by the oil pump first pushes the hydraulic cylinder 1 to move, and at the same time acts on the area A through the oil inlet of the sequence valve. When the hydraulic cylinder 1 moves completely, the pressure rises. 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 and the oil outlet, 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, so as to control the movement speed of the actuator. Flow control valves are classified into 5 types according to their application.

(1) Throttle valve: After adjusting the area of the orifice, the movement speed of the actuator that can make the load pressure change little and the motion uniformity is not high is basically kept stable.

(2) Speed regulating valve: When the load pressure changes, the pressure difference between the inlet and outlet of the throttle valve can be kept at a constant value. In this way, after the area of the orifice is adjusted, the speed regulating valve can keep the flow through the throttle constant regardless of the change of the load pressure, so that the movement speed of the actuator is stable.

(3) Diverter valve: Regardless of the load, the equal flow diverter valve or synchronous valve can make the two actuators of the same oil source get equal flow; the proportional diverter valve can obtain the proportional distribution flow.

(4) Converging valve: The function is opposite to that of the diverting valve, so that the flow flowing into the collecting valve is distributed in proportion.

(5) Diverting and collecting valve: It has two functions of diverting valve and collecting valve.

Directional control valves are divided into one-way valves and reversing valves according to their uses.

One-way valve: only allows the fluid to be connected in one direction in the pipeline, and the reverse direction is cut off.

Reversing valve: change the on-off relationship between different pipelines, according to the number of working positions of the valve core in the valve body, it can be divided into two positions, three positions, etc.; according to the number of channels controlled, it can be divided into two-way, three-way, four-way, five-way, etc.; according to the driving mode of the valve core, it can be divided into manual, motor, electric, hydraulic, etc. When the spool is in the middle position, all oil ports are cut off, and the actuator does not move; when the spool moves to the right position, P connects with A, and B connects with O; when the spool moves to the left position, P connects with B, and A connects with O. In this way, the actuator can move forward and backward.

Several considerations for selecting hydraulic control valves:

The specifications and models of various hydraulic control valves can be selected from product catalogs or catalogs based on the maximum system pressure and the actual flow rate of the valve (checked from the working condition diagram and system diagram) and considering the valve control characteristics, stability, oil port size, external dimension, installation connection method, and operation method. The precautions for selecting various hydraulic control valves are as follows:

1. The actual flow rate of the hydraulic control valve:

The actual flow rate of the hydraulic control valve is related to the series and parallel connection of oil circuits: the flow rate of each oil circuit in series is equal; the flow rate of parallel oil circuits working at the same time is equal to the sum of the flow rates of each oil circuit. In addition, when using a single-piston rod hydraulic cylinder system, it is necessary to pay attention to the difference in the oil return flow when the piston is extended and retracted: the ratio of the oil return flow of the rodless chamber when retracted to the oil return flow of the rod chamber when extended is equal to the ratio of the two chamber areas.

2. Rated pressure and rated flow of hydraulic valve:

The rated pressure and rated flow of each hydraulic control valve should generally be used with pressure and flow close to each other. For systems with high reliability requirements, the rated pressure of the valve should be higher than its working pressure. If the rated pressure and rated flow rate are lower than the operating pressure and flow rate, it will easily cause hydraulic clamping and hydraulic power and have an adverse effect on the working quality of the valve; the flow rate of the sequence valve and pressure reducing valve in the system should not be much lower than the rated flow rate, otherwise vibration or other unstable phenomena will easily occur. Flow valve, should pay attention to its minimum stable flow.

3. Installation and connection method of hydraulic control valve:

The valve installation and connection method has a decisive influence on the subsequent design of the hydraulic device structure. When selecting a hydraulic valve, the integration method of the hydraulic control device should be well known. For example, the plate-type connection hydraulic valve is used, because the valve can be installed on the oil circuit board or the oil circuit block, which is convenient for system integration and rational design of the hydraulic device. In addition, when replacing the hydraulic valve, it is not necessary to dismantle the oil pipe, so the installation and maintenance are more convenient; if the superimposed valve is used, it is necessary to study the pressure and flow of the superimposed valve series spectrum for selection, etc.

4. Selection of directional control valve:

Ordinary one-way valve with simple structure, the main attention should be paid to the reasonable selection of its opening pressure: lower opening pressure can reduce the resistance loss of liquid flow one-way valve; if a one-way valve is used as a back pressure valve, its opening pressure is higher to ensure sufficient back pressure. For hydraulic control valves, in order to avoid abnormal vibration and noise in the system, attention should also be paid to the reasonable selection of its pressure relief method: when there is back pressure at the outlet of the hydraulic control check valve, the external leakage type should be selected, and the internal leakage type should be selected in other cases.

As for the reversing valve, attention should be paid to satisfying the requirements of the system for automation and operation cycle, and its operation type should be reasonably selected from manual, mechanical, electromagnetic, electro-hydraulic and other types. Correctly select the mid-position function of the spool valve and grasp its transition quasi-state function. The hydraulic lock (double hydraulic control check valve) is used to lock the hydraulic actuator system, and the "H" and "Y" shape mid-position function spool type reversing valve should be selected, so that when the reversing valve is in the middle position, the control chambers of the two hydraulic control check valves are connected to the oil tank to ensure the reliable reset of the hydraulic control check valve and the good locking state of the hydraulic actuator. The mid-position function of the selected spool valve can change to the transition position, and the oil circuit should not be completely blocked, otherwise it will cause the instantaneous pressure of the system to be infinite and cause accidents such as pipeline explosion.