Precautions for selection of hydraulic directional control valve
（1）Determine the actual flow through the valve At this time, pay attention to the relationship between the flow through the pipeline and the series and parallel connections of the oil circuit. When the oil circuits work at the same time, the flow rate of the system is equal to the sum of the flow rates of each oil circuit.
（2）Pay attention to the difference of the oil return between the two chambers of the single-piston rod hydraulic cylinder. The oil return flow is different when the piston is extended and retracted. The ratio is equal to the ratio of the two-chamber piston areas.
The above two points 1 and 2 emphasize that the actual flow through the valve (not the flow of the pump) should be used as one of the main parameters for selecting the valve. If the actual flow rate through the valve is selected too small, the specification (capacity) of the valve will be selected too small, and the local pressure loss of the valve will be too large, resulting in excessive oil temperature and other consequences, and in severe cases, the system will not work normally. In some hydraulic systems of tube expanders, the partial pressure loss of each valve is as high as 2.4MPa due to the small selection of electromagnetic reversing valve and hydraulic lock capacity, which is far greater than their respective rated pressure loss of 0.3MPa, resulting in system failure. Can not meet the design requirements.
（3）Pay attention to the reasonable selection of the opening pressure of the check valve. The opening pressure of the check valve depends on the stiffness of the built-in spring. Generally speaking, in order to reduce the loss of flow resistance, check valves with low opening pressure should be used as much as possible. On the other hand, for example, to maintain the necessary control pressure of the electro-hydraulic reversing valve, when the check valve is used as a back pressure valve, in order to ensure sufficient back pressure, a check valve with a high cracking pressure should be selected.
（4）Pay attention to the reasonable selection of the pressure relief method of the hydraulic control check valve. When there is back pressure at the outlet of the hydraulic control check valve, the external discharge type should be selected, and the internal discharge type should be selected in other cases. Some lifting device hydraulic systems, due to poor design considerations, should use external leakage hydraulic control check valves, but internal leakage type is used, which causes strong vibration and noise in the system.
In addition, special attention should be paid to whether there is a "negative pressure effect" on the opening pressure of the hydraulic control check valve in the locking circuit of the two-way hydraulic lock. This "negative pressure effect" will cause jumping movement and vibration of the actuator .
（5）It is necessary to correctly select the mid-position function of the spool valve and to grasp the transition state function of the spool valve. The transition state function of the spool valve refers to the connection status of the oil circuit of the spool valve at the reversing transition position. Grasp the transition state function of the spool valve, so as to know whether the oil circuit is completely blocked during the transition process of the valve, resulting in the phenomenon of infinite instantaneous pressure in the system. The designer generally pays attention to the median function of the slide valve, but often pays little attention to the transition function of the slide valve, resulting in unexpected design mistakes.
（6）Pay attention to the application occasions of electromagnetic reversing valve and electro-hydraulic reversing valve. Once the type of solenoid of electromagnetic reversing valve (DC type, AC type, etc.) and the structure of the valve are determined, the reversing time of the valve is determined; The hydraulic reversing valve can be adjusted to control the oil flow.