Fault analysis of Electro-hydraulic Directional Valve
Fault analysis of Electro-hydraulic Directional Valve
In the high-pressure and large flow system, due to the large steady-state hydraulic force acting on the reversing valve core, the electromagnetic force of the conventional electromagnetic reversing valve is generally relatively small, which can not ensure its reliable reversing. In this hydraulic system, the electro-hydraulic reversing valve should be used for direction switching. The electro-hydraulic reversing valve belongs to a secondary structure. At first, the pilot stage is used to change the direction of the hydraulic directional valve to control the pressure oil. Because the flow of control pressure oil is very small, the specification of electromagnetic directional valve can be small (common is 6-diameter variety). As the main valve, the working position of hydraulic directional valve is determined by the working position of electromagnetic directional valve. At present, there are 32 diameter Electro-hydraulic Directional Valves in China, with the maximum working pressure of 35.0mpa and the maximum flow of 1100L / min.
2. Improper selection of Electro-hydraulic Directional Valve will also lead to failure of reversing
A unit reformed a hydraulic system of a press by itself. Its oil source control circuit is shown in Figure 186. Its working pressure is 31.5Mpa and flow is 100L / min. Rexroth 4wehl6c50 / 6ag24net24 / D1 Electro-hydraulic Directional Valve produced by a domestic manufacturer is used for oil circuit control. The valve is internally controlled and internally discharged. In this structure, the control oil is introduced from the P cavity of the main valve, And discharged back to the oil tank through the T cavity of the main valve. Due to the limitation of the strength of the end cover and screw of the main valve, the maximum control pressure can not exceed 25.0mpa. Therefore, a constant ratio pressure reducing valve is installed between the pilot valve and the main valve, and the pressure reducing ratio is 1:0.66. See Figure 187 for detailed symbols of this electro-hydraulic directional valve.
The system breaks down in the commissioning stage, which shows that the commutation is unreliable. When the electro-hydraulic directional valve is in an initial position (such as the electromagnet is in the energized or de energized state), the system pressure can rise to the rated pressure after the main motor is started. When the electro-hydraulic directional valve is switched (such as the electromagnet changes from the energized state to the de energized state or the electromagnet changes from the de energized state to the energized state), the system pressure suddenly drops to the low-pressure state (about 0.3MPa). After shutdown again, restart the motor, and the system pressure can rise to the rated pressure again. When the electro-hydraulic directional valve is switched, the system pressure suddenly drops to the low-pressure state. After checking the relevant components in the system one by one, it is found that the fault lies in the unreliable reversing of the electro-hydraulic directional valve. Generally, there may be the following reasons: the electromagnet fails; The return spring force of pilot valve or main valve is insufficient; The guide valve core is stuck; The main valve element is stuck.
After inspection, the above reasons are eliminated. It is analyzed that the pilot valve of the electro-hydraulic directional valve is 4we6d electromagnetic directional valve, and its main valve core is refitted on the three position valve core H-type function. The symbol of the directional function is shown in Figure 188. The stroke of the main valve core from the initial position to the reversing position is relatively long (20mm), so when the main valve core reverses from the left end to the right end or from the right end to the left end, it must go through the middle transition function transition.
Now analyze the reversing process. Assuming that the system motor is in the shutdown state and the pilot valve electromagnet of the electro-hydraulic reversing valve is in the power-off state at the initial position, start the motor, adjust the system pressure to the rated pressure of 31.5Mpa, then energize the solenoid iron of the pilot valve, control the oil to act on the right end of the main valve core through the constant ratio pressure reducing valve and solenoid valve, and push the main valve core to move to the left, When the main valve core passes through the middle function, the P, a, B and t cavities of the main valve are connected, and the system returns oil directly through the T cavity, resulting in a sudden drop in the system pressure. In addition, the pilot valve control oil is internally supplied and internally discharged, that is, the control oil is directly connected to the P cavity of the main valve, and the return oil of the pilot valve is directly connected to the return oil cavity of the main valve, so the pressure difference at both ends of the main valve core is very small, It is not enough to push the main valve core to continue to move to the left. Under the combined action of the control oil force at the left and right ends of the main valve core, the friction between the main valve core and the main valve body, clamping force, hydraulic force and the force of the spring at the left end of the main valve core, the main valve core can only be in a balanced position near the middle, so that the system is in the unloading state. Therefore, in order to enable the main valve core to continue reversing, like the three position four-way electro-hydraulic reversing valve with medium unloading function and internal supply and internal discharge of control oil, a pre control pressure valve must be installed in the P cavity of the main valve, so that sufficient pressure difference can be generated at both ends of the main valve core to ensure the reliable reversing of the main valve core. The pre control pressure valve is essentially a plane sealed, When there is no constant ratio pressure reducing valve between the main valve and the pilot valve, the opening pressure of the one-way valve is 0.45MPa. When a certain ratio pressure reducing valve is installed between the main valve and the pilot valve, the opening pressure of the one-way valve is 0.7MPa. Due to the low pilot control pressure, the maximum flow of the slide valve of this function is only 160L / min.
It is understood that the reason for the above phenomenon is that the samples provided by the manufacturer do not prompt the matters that should be paid attention to in the selection of the two position four-way Electro-hydraulic Directional Valve with this function. Because the users do not know the specific structure of the Electro-hydraulic Directional Valve very well, they ignore its transition function. It can be seen that manufacturers should not only provide users with high-quality products, but also provide complete product samples.