Estimated reading time: 20 minutes
The directional control valve is essentially a kind of on-off valve. The so-called directional control is to make the oil circuit on or off or to converge and diverge the flow. It is mainly used to control the reversing, start, and stop of the actuators of the hydraulic system. The directional control valve can be divided into two types according to its purpose: one-way valve and reversing valve; according to the operation method, there are manual valves, motorized valves, and electric valves.
Question 1: The Problem of Selecting The Neutral Function of The Directional Control Valve
The hydraulic system is shown in Figure 1-1(a) found in actual work that when the reversing valve is in the neutral position, the load will slowly slide down. Checking the various components did not find any leakage, and there was no internal leakage of the hydraulic cylinder. After analysis, it is confirmed that the cause of this phenomenon is the internal leakage of the reversing valve. When the reversing valve is in the neutral position because the valve core and the valve sleeve are sealed by a metal gap, the pressure oil will leak from the P port to the A and B ports, and the oil from the A and B ports will also leak to o port. In this way, the hydraulic cylinder and the “o” type neutral function reversing valve actually constitute a differential circuit, when the generated thrust is sufficient to overcome the load, it will make the hydraulic cylinder move.
In view of this situation, one of the solutions is to change the neutral function of the reversing valve to a “Y” type, so that both chambers of the hydraulic cylinder can be at zero pressure and no thrust can be formed. In addition, in order to prevent gravity from pulling the hydraulic cylinder, a hydraulic control check valve is added to the oil return path where the hydraulic cylinder descends to further ensure the reliable stop of the hydraulic cylinder. The improved system is shown in Figure 1-1(b).
Question 2: Manual And Maneuvering Methods
Manual Reversing Valve
The method of manipulating the spool valve to change direction is controlled by the action of a manual lever, which is the manual reversing valve; the manual reversing valve is simple in operation and reliable in work. It can be used in occasions where there is no power supply and is widely used in construction machinery.
- In a complex system, especially when the actions of various actuators need to be linked, interlocked, or the working cycle needs to be strictly controlled, manual reversing valves should not be used.
- Even threaded valves should be fixed on the processed mounting surface with screws, and it is not allowed to use pipes to support the valve in the air.
- The external drain port should be directly connected back to the fuel tank. If the external oil drain pressure increases, the operating force increases and the external oil drain port is blocked, and the spool valve cannot work.
Motorized Reversing Valve
The motorized reversing valve is also called the stroke reversing valve, which can change the direction of oil flow by pushing the spool to move by the stopper or cam installed on the actuator. It generally only has a two-position working mode, that is, an initial working position and a reversing working position. At the same time, when the stopper or cam is released from the roller at the end of the spool, the spool is automatically reset by the spring.
- Because the stroke switch can be matched with solenoid valve or electro-hydraulic reversing valve, it is easy to realize stroke control (reversing), instead of motorized reversing valve, that is, travel reversing valve, and the motorized reversing valve is difficult to piping, and it is not easy to change the control position. , So there is currently little domestic production of motorized reversing valves.
- For the stroke valve, it is fastened on the machined base surface with screws, and the installation direction depends on the needs. The cam or bumper on the moving part can be used to press or leave the roller of the stroke valve to move the slide valve to realize the reversal of the oil circuit. The stroke of the bump or cam should be controlled within the stroke specified by the corresponding stroke valve model. When using bumpers, it is recommended that the inclination angle of bumpers is 30°, not more than 35°.
Electromagnetic Directional Valve And Electro-hydraulic Directional Valve
An electromagnetic reversing valve, also called a solenoid valve, is a conversion element between hydraulic control system and electrical control system. It uses the suction force of the electromagnet to push the spool valve core to move and change the oil flow to realize the reversal, start and stop of the actuator.
Question3: Use Occasions
Where The Switching Time is Required
Once the solenoid type (DC type, AC type, etc.) of the solenoid valve and the valve structure is determined, the valve commutation time is determined; the electro-hydraulic directional valve can be adjusted to control the opening of the throttle on the oil circuit To adjust its commutation time. In this example, the commutation stability is required to be high, and an electro-hydraulic directional valve with adjustable commutation time should be used. However, due to the electromagnetic directional valve, the hydraulic shock in the commutation process is strong, and the equipment flutters, which affects the quality of the product.
Occasions That Require High Commutation Stability
Although the electro-hydraulic directional valve is generally only used in large-flow systems, when the system has higher requirements for the commutation stability of the actuator, you can consider using the electro-hydraulic directional valve to replace the ordinary electromagnetic directional valve.
The action of the electromagnetic reversing valve is relatively blunt, and the switching time is very short, and it is easy to stop the work after the hydraulic cylinder suddenly changes.
The main spool of the electro-hydraulic directional valve is driven by a small-diameter solenoid valve, and the damping of its driving oil circuit can be adjusted, which makes the action speed of the main spool controllable within a certain range, that is to say, it can be controlled to a certain extent. The upper makes the main spool of the electro-hydraulic directional valve open or close at a slower speed. This reduces the impact of the hydraulic system and improves the commutation stability of the system.
Where The Flow Rate is Required
For electromagnetic reversing valves, the maximum flow rate should generally be within the rated flow rate and not exceed 120% of the rated flow rate, otherwise, it will easily lead to excessive pressure loss, causing heat and noise. If there is no suitable reversing valve, larger pressure and flow can also be used, but the economy is less. When the flow exceeds 63L/min, the electromagnetic reversing valve cannot be used. The electro-hydraulic directional valve can be selected for the large flow system.
Question 4: Power Supply Problem
The Electromagnetic Reversing Valve Should Pay Attention to The Control Power Parameters That Are Consistent With The Solenoid Valve
The electromagnetic reversing valve relies on the thrust of the electromagnet to achieve the purpose of reversing. The structure of the electromagnet is: AC type, DC type, and local rectification type; working power specifications are AC 110V, 220V, 380V, DC 12V, 24V, 36V, 110V, etc.; the armature of the electromagnet has two types, wet type, and dry type. Before use, be sure to figure out the type and rated voltage of the electromagnet used on the solenoid valve, and connect the power supply as required, otherwise, it may cause commutation The valve does not work normally or the electromagnet is burned out, as shown in Figure 1-2.
Precautions For The Use of The Power Supply of The Electro-hydraulic Directional Valve
- AC is 220V, 1.2A, and the model is represented by D; DC is 24V, 1.9A, and the model is represented by E. The voltage fluctuation value is ±15%. There must be a back pressure greater than 0.3 MPa in the oil inlet or return path for the spool valve to work normally.
- Pay attention to the type and rated voltage of the power supply used by the electromagnet in order to supply power to the electromagnet correctly.
- The electro-hydraulic directional valve with two electromagnets should avoid the situation that the two electromagnets are energized at the same time.
- Check whether the function of the slide valve of the electro-hydraulic directional valve and hydraulic-controlled directional valve meets the requirements.
- Check whether the oil inlet mode and oil return mode of the control oil circuit is in the required state.
Question 5: Installation of Electromagnetic Directional Valve
The axis of the solenoid valve must be installed in the horizontal direction. For example, if it is installed in the vertical direction, it will be affected by the weight of the valve core, armature, and other parts, which will cause abnormal reversing or resetting.
Plate-type connection electromagnetic directional valve, the surface of the mounting base plate should be ground, and there should be flatness requirements, and no protrusions should be allowed. The mounting screws cannot be tightened too loosely, otherwise, the oil will leak at the joint between the plate-type connection solenoid directional valve and the bottom plate.
When overhauling the directional valve, pay attention to the assembly sequence of the valve core as shown in Figure 1-3. Generally, the shoulders on both sides of the reversing valve body are symmetrical, and the distance between the shoulders on both sides of the valve core is asymmetrical in most cases, so it should be directional during assembly. It is best to arrange them in order during maintenance. On the contrary, it will cause malfunction.
When installing various valves, pay attention to the orientation of the oil inlet and return port. For some valves, if the oil inlet and return port are installed reversely, it will cause an accident.
In order to prevent air from penetrating into the valve, the connection should be well sealed.
Some valves often have two holes with the same function for easy installation, and the unused ones should be blocked after installation.
Flange-installed valves should not be over-tightened, because sometimes over-tightening may cause poor sealing. When it must be tightened, if the original seal or material cannot meet the sealing requirements, the form or material of the seal should be replaced.
For general-adjusted valves, when rotating in a clockwise direction, the flow rate and pressure will be increased, and when rotating in a counterclockwise direction, the flow rate or pressure will be reduced.
Question 6: The Use of Electromagnetic Reversing Valve
- Pay attention to the cleanliness of the oil used. Since the matching gap between the valve core of the solenoid valve and the valve body hole is small, usually only 0.006～0.015mm, if large impurities enter this gap, the valve core may be stuck. Therefore, the filtering accuracy of the oil must be strictly controlled.
- Solenoid valves are divided into the wet type and dry type according to whether the armature of the electromagnet is immersed in the oil. The solenoid valve adopts the wet type electromagnet. As the oil enters the electromagnet, the magnetic material particles in the oil will be adsorbed on the iron core Above, which affects the pull-in action of the electromagnet, so the system with a wet solenoid valve should pay attention to the filtration of magnetic material particles.
- Check whether the function of the solenoid valve slide valve meets the requirements. Solenoid valves have many spool valve functions, and there is a difference between normal installation and reverse installation when they leave the factory, so be sure to check whether the function of the spool valve is consistent with the requirements when using it, so as not to affect the normal operation of the system.
- Do not make the pressure of the oil return port T of the solenoid valve exceed the allowable oil return backpressure. Because the working pressure of the T port is limited, when the four-way solenoid valve blocks one or two ports, it is used as a three-way valve or
- When the two-way solenoid valve is used, if the system pressure exceeds the allowable back pressure value of the solenoid directional valve, the T port cannot be blocked. At present, the electromagnetic directional valve with rated pressure of 21MPa and 31.5MPa, the allowable oil return backpressure at the oil return port T is usually only 6.3～7.0MPa. When this value is exceeded, the reversing and resetting performance of the solenoid valve will be affected.
- Generally, the oil cavity at both ends of the solenoid valve is the drain cavity or the oil return cavity. Check whether the pressure in the cavity is too high. If the drain or return pipelines of multiple solenoid valves in the system are connected together to cause excessive back pressure, they should be separately connected to the fuel tank.
- Do not energize the two electromagnets of the double electromagnet solenoid valve at the same time. If the two electromagnets are energized at the same time, they will not be attracted well. For AC electromagnets, the two electromagnets are energized at the same time, causing the coil to heat up and even burn out. For the DC electromagnet, although it will not burn out, the position of the valve core is not fixed, which will cause the system to malfunction. Therefore, the energization time of the two electromagnets should be interlocked to avoid simultaneous energization.
Question 7: Pilot Control Oil For Electro-hydraulic Directional Valve
The pressure oil is used to push the spool to change direction to realize the control of the large flow change. This is a hydraulic reversing valve. The flow of oil used to drive the spool to change direction does not need to be large. An ordinary small-sized electromagnetic directional valve can be used as the pilot control valve, which is installed together with the hydraulic directional valve to achieve a small flow electromagnetic directional valve. Control the reversal of the large-diameter hydraulic directional valve, this is the electro-hydraulic directional valve. Electro-hydraulic directional valves and hydraulic directional valves are mainly used in occasions with the large flow (more than 60L/min), generally used in high-pressure and large-flow systems, and their functions and applications are the same as electromagnetic directional valves.
Control Oil Supply Method
- The pilot oil supply mode of the electro-hydraulic directional valve has two modes: internal oil pilot control and external oil pilot control, which are referred to as internal control and external control for short.
- External oil pilot control mode. The external oil control method means that the oil supply to the pilot solenoid valve is supplied by another control oil circuit system, or in the same hydraulic system, through a branch pipeline as the control oil circuit. The former can be used as a control oil source with a separate auxiliary hydraulic pump; the latter can be separated from the main oil circuit of the system through a pressure-reducing valve, etc.
- External control form, because the minimum control pressure of the electro-hydraulic directional valve spool is generally designed to be relatively small, most of which is below 1MPa, so the control oil pressure does not need to be too high, and a low-pressure hydraulic pump can be used.
- To add a set of the auxiliary control system.
Internal Oil Pilot Control Mode
After the pressure oil of the main oil circuit system enters the oil circuit of the electro-hydraulic directional valve, a part of it is separated as the control oil and directly communicates with the oil inlet cavity of the upper pilot valve through the internal hole of the valve body. The characteristic is that no auxiliary is required. The control system eliminates the need for control oil pipes and simplifies the layout of the entire system.
- The control pressure is the oil pressure entering the main oil circuit system of the valve. When the working pressure of the system is high, the loss of this part of the high-pressure flow should be considered, especially in the electro-hydraulic directional valve. When the entire high-pressure flow distribution is restricted, the energy loss caused by this control method should be considered.
- The internal control method is generally selected to simplify the layout of the system when the number of electro-hydraulic directional valves in the system is small, and the total high-pressure flow is surplus.
- In order to unload the hydraulic pump, the middle position of the electro-hydraulic directional valve should be H type, M type, K type, X type, but because the hydraulic pump is in the unloading state, the system pressure is zero, and the main spool cannot be used. Reversing.
- Measure 1: When the internal oil control method is used and the main valve is unloaded in the neutral position, a back pressure valve must be added to the oil return line to keep the system at a certain pressure. The backpressure should be at least greater than the minimum control pressure of the main valve of the electro-hydraulic directional valve.
- Measure 2: Install a pre-pressure valve in the oil inlet P of the electro-hydraulic directional valve. It is actually a plug-in check valve with a large opening pressure. When the electro-hydraulic directional valve is in the middle position, the oil flow Passes through the pre-pressure valve first, and then returns to the oil tank from port T through the inner flow channel of the electro-hydraulic directional valve, so as to establish the required control pressure before the pre-pressure valve.
- The electro-hydraulic directional valve adopts several spool valve functions that cannot be used in the internal control form, as shown in Figure 1-4. When the electro-hydraulic directional valve adopts the internal control mode, the main valve P cavity provides pressure to the pilot valve. When the main valve spool neutral function will cause the main valve P cavity to lose pressure, the pilot valve will lose control of the main valve. Pressure, the valve will not work. For example, Rexroth series electro-hydraulic valves whose neutral function codes are F, G, H, P, S, T, V, etc. will cause the main valve P cavity to lose pressure, so the above neutral function valves are not suitable for internal control. If it is necessary to install the above-mentioned neutral function valve, a pre-pressure valve must be installed in the main valve body, or a back pressure valve must be added to the valve return line to ensure that the main valve spool can be reliably switched when using internal control.
Control Oil Return Method
Corresponding to the pilot oil supply mode of the electro-hydraulic directional valve, the pilot oil return method also has two types of internal oil return and external oil return referred to as internal drain type and external drain type for short.
- The internal oil return of the control oil. The internal return of the control oil means that the pilot control oil communicates with the return cavity of the main oil circuit of the hydraulic valve through the internal passage, and returns to the tank together with the return oil of the main oil circuit. Figure 1-5 is a schematic diagram of the internal oil return of the control oil. The feature of this form is that the control oil return pipeline is omitted, which simplifies the system.
- The internal oil return is affected by the oil return back pressure of the main oil circuit. Since the oil return back pressure of the solenoid pilot valve is subject to certain restrictions when the internal oil return form is used, the return pressure of the main oil circuit must be less than the allowable back pressure value of the solenoid pilot valve, otherwise, the normal operation of the solenoid pilot valve will be affected. affected.
- External return of control oil. The external oil return of the control oil refers to the oil discharged from the control chambers at both ends of the electro-hydraulic directional valve, which is directly returned to the oil tank through the return cavity of the pilot solenoid valve (threaded or flanged electro-hydraulic directional valves generally use this. This method can also be connected back to the oil tank through the specially processed oil return hole on the lower hydraulic valve (plate connection type-this method is generally used). Figure 1-6 shows the external oil return structure of the control oil of the plate connection type electro-hydraulic directional valve. The characteristic is that the control oil return back pressure is not affected by the main valve return back pressure. It can be directly connected to the fuel tank or connected to the main oil pipeline whose backpressure is not greater than the allowable back pressure of the solenoid pilot valve and then connected back to the fuel tank together. More flexible.
- There is an additional oil return pipeline, which uses more complex systems for electro-hydraulic directional valves and increases the pipeline layout.
- The electro-hydraulic directional valve should pay attention to the control form of the pilot valve and the valve function. The pilot valve control mode is divided into internal control, external control, internal leakage, and external leakage. There can be four types after combining respectively. If the valve itself is a control mode, and the actual piping is completed in another control mode, the valve will not work or cannot work reliably.
- When using the internal oil return method, the oil return back pressure of the oil return port of the electro-hydraulic directional valve cannot exceed the maximum oil return back pressure allowed by the pilot solenoid valve.
Control Oil Pressure Problem
- For all types of valves with a pressure of 20 MPa, the control oil pressure shall not be lower than 0.35 MPa.
- For all types of valves with a pressure of 31.5MPa, the control oil pressure shall not be lower than 1.0MPa.
- The electro-hydraulic reversing valve (such as M, H type, etc.) whose spool valve function can unload the hydraulic pump in the middle position should be placed on the return line of the main valve with a back pressure greater than the control oil pressure to ensure the reversing action of the main valve. , And the control oil return should take the form of external oil return. The function of the spool valve of the two-position valve refers to the situation in the transitional position.
- The pilot control pressure of the electro-hydraulic reversing valve should conform to the scope of use in the product catalog. The application range of the pilot control pressure of the electro-hydraulic directional valve is based on the premise that the electro-hydraulic directional valve can work reliably. If it is less than the minimum pilot control pressure, the electro-hydraulic directional valve cannot work or cannot work normally and reliably.
- Pay attention to whether the pressure of the control oil can meet the reversing requirements of the electro-hydraulic reversing valve or hydraulic control reversing valve. Electro-hydraulic directional valves and hydraulic-controlled directional valves with spool valve functions such as M, H, K, X, etc., if the control oil is provided by the main oil circuit, measures should be taken to keep the main oil circuit changing when the valve is in the middle position To the required pressure.