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Wendy Xu
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Aug 18,
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Figure 1'1 shows the hydraulic system of a 600-ton pipe expander used in a factory. It can realize the action cycle of vertical cylinder fast down, sealing control cylinders on both sides synchronously fast forward, mold sealing one vertical, side cylinder pressurization-two cylinders synchronously rewind the mold opening and the vertical cylinder quickly retreats the mold opening action cycle.
When the solenoid 2DT is energized, the electromagnetic directional valve 10 is switched to the left position, and the hydraulic pump 1-the left position of the electro-hydraulic directional valve 8-the left position of the valve 10-the double hydraulic control check valve 13-the rodless cavity of the vertical cylinder 19, The vertical cylinder 19 quickly descends and seals the mold.
When the mold sealing pressure reaches the specified value, the electric contact pressure gauge 14 sends out a signal, the solenoid 2DT is cut off, 1DT, 5DT, 6DT are energized, and the hydraulic oil passes through the valve 8 to the right and the shunt valve 9-the electromagnetic reversing valve 11 to the right- The left position of the electromagnetic reversing valve 12-the double hydraulic control check valves16 and 18 enter the rodless chambers of the side cylinders 20 and 21 respectively, and the cylinders on both sides fast forward and seal the mold synchronously.
When the mold sealing pressure reaches the specified value, the electric contact pressure gauge 17 sends a signal, the ultra-high pressure hydraulic-pressure 15 and 2 move backward at the same time, and respectively inject high-pressure oil into the rodless cavity of the vertical cylinder 19 and the side cylinders 20 and 21 to achieve Output 6MN (600tf) pressing force.
When the electromagnets 1DT, 4DT, 7DT are energized, the cylinders on both sides will retreat and open the mold synchronously. When the end of the stroke is reached, the stroke will open and a signal will be sent. The electromagnet 1DT is de-energized and the 3DT is energized, and the vertical cylinder quickly moves upward to open the mold.
In order to improve work efficiency and reduce power consumption, the system adopts pressure compensation variable axial piston pump 1 to achieve low-pressure and high-speed operation of vertical cylinders and side cylinders, and high-pressure and low-speed operation during mold sealing. The specifications of the three hydraulic cylinders are the same.
The vertical cylinder and the side cylinder move slowly. The vertical cylinder moves down quickly for 70 seconds, and the side cylinder moves forward quickly for 60 seconds. The return time is longer, resulting in extremely low production efficiency.
It can be seen from the working principle of the system that the vertical cylinder's rapid downward movement and the side cylinder's rapid forward movement are performed independently. A hydraulic source provides flow, and its operating speed is determined by the output flow of the hydraulic pump 1. When the vertical cylinder and the side cylinder are running quickly, because there is no load, the working pressure of the system is very low at this time, and the pressure compensation variable hydraulic pump outputs at full flow.
The hydraulic cylinder moves slowly and the running time is too long, indicating that the actual flow input to the hydraulic cylinder is too small. This may have the following two situations:
It is verified by inspection that the volume loss of hydraulic pump 1 is not large, and the volumetric efficiency has not decreased. It is inferred from this that the latter may cause problems in the system.
From the above analysis, it can be seen that when the hydraulic cylinder is running fast, the total partial pressure loss caused by the oil passing through the hydraulic valves is very large, especially the pressure loss through the electromagnetic reversing valve and the two-way hydraulic lock is the largest, which greatly exceeds their rated pressure Loss value. The total partial pressure loss of the hydraulic valve is combined with the pressure loss along the pipeline and the partial pressure loss, which will inevitably increase the working pressure of the system and reduce the actual output flow of the pump, thereby reducing the operating speed of the hydraulic cylinder. Slow, the running time becomes longer.
Conclusion: The main cause of system problems is improper selection of the specifications of the electromagnetic reversing valve and the two-way hydraulic lock.
The best way to solve this problem: select the electromagnetic directional valve and two-way hydraulic lock with appropriate specifications, and replace the corresponding components in the system, thereby reducing the partial pressure loss of the hydraulic valve.
When selecting a hydraulic valve to form a hydraulic system, in addition to considering the proper function of the selected hydraulic valve, it is also necessary to consider that its specifications should meet the performance requirements of the system. The improper selection of component specifications in this example will cause slow movement of the main and side cylinders and affect the normal working performance of the system, but will also cause excessive resistance loss, resulting in large energy loss and increased oil temperature.
Figure 1'4 shows the hydraulic system of a stretch-bending machine designed and manufactured by a certain factory, which can realize the stretching and bending of the profile. Operate the manual reversing valve 2. The oil enters the rodless cavity of the stretching hydraulic cylinders 4 and 5 through the left position, and the pistons of the stretching hydraulic cylinders 4 and 5 move forward to carry out the stretching process of the profile. When the required stretch rate is reached, the manual reversing valve 3 is then operated to connect to the circuit in its left position, and the oil enters the rodless cavity of the bending hydraulic cylinder 6 to bend the profile. After bending and shaping, the workpiece is unloaded, and the bending hydraulic cylinder 6 and the stretching hydraulic cylinders 4 and 5 are retracted, thereby completing a profile stretching and bending action cycle. The system pressure is set by overflow valve 1. After improvement, the hydraulic system adopts an electromagnetic directional valve and electromagnetic overflow valve, as shown in Figure 1'4(b).
Existing problems: The system adopts an electromagnetic overflow valve, which has obvious effects on reducing power consumption and reducing oil heating and leakage. However, when the stretching and bending actions begin, there is obvious hydraulic shock, accompanied by equipment flutter, which affects the quality of the product.
The electromagnetic reversing valve selected has the same specifications as the original manual reversing valve. The only difference is the reversing operation method. What is the problem?
The electromagnetic reversing valve changes the working position of the valve by using the suction force of the electromagnet to push the spool relative to the valve body. It switches quickly and the reversing time is short. Therefore, hydraulic shock will inevitably occur when the reversing valve is switched. The manual reversing valve is used to manipulate the lever and push the spool to move relative to the valve body to change the position. It does not switch as quickly as the former. It gradually opens or closes the valve port. It has the effect of throttling and damping, so it has an impact on the hydraulic pressure. Has a buffering effect. Therefore, the commutation stability of the modified system is poor.
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Using the same specification electro-hydraulic reversing valve instead of the electromagnetic reversing valve can eliminate the hydraulic shock. At the same time, it also has the advantages of flexible operation of the electromagnetic reversing valve and easy automatic program control. The hydraulic system after further transformation is shown in Figure 1- As shown in 2(c). Adjusting the electro-hydraulic reversing valve controls the damper in the pressure oil circuit to adjust the reversing time, thereby improving the stability of the reversing.
This example illustrates: For hydraulic equipment that requires high commutation stability, it is advisable to use an electro-hydraulic directional valve or a controllable commutation time.
The manual reversing valve constitutes the hydraulic system.
Figure 1'6 shows the hydraulic system of a lifting platform made by a factory, which can realize equipment lifting, lowering, and stopping at any position.
When 2DT is energized, oil-electromagnetic reversing valve 5 right position-hydraulically controlled check valve 7-plunger cylinder 8 lower cavity-plunger moves upwards-the weight is lifted. When the 1DT is energized, the oil-hydraulic control check valve 7-electromagnetic reversing valve 5 left position-throttle valve 4-oil tank, the plunger moves downwards-the weight drops. The descending speed of the weight is controlled by the throttle valve 4, and the descending stop position is controlled by the hydraulically controlled check valve 7. When the 3DT is energized, oil-hydraulic pump 1-relief valve 2-oil tank (unloading).
There is a problem: when the weight is descending, it emits beat-like vibration and noise.
All hydraulic components in the detection system are normal, so the problem of the system is not caused by the failure of the component itself. In the system, a throttle valve is used to adjust the descending speed of the plunger cylinder, so when the oil flows in the reverse direction, the hydraulic control check valve outlet pressure is backpressure. The control oil pressure is still the setting value of the overflow valve 6, so the force balance of the valve core is broken, the valve core drops to close the valve port, and the pressure in the oil outlet cavity becomes zero. At this time, the control oil pressure set by the overflow valve 6 opens the hydraulic control check valve again, the oil flows in the reverse direction, and backpressure is generated in the reverse oil outlet cavity, and the hydraulic control check valve is closed again. This was repeated so that the plunger cylinder was lowered and stopped intermittently, and vibration and noise were emitted at the same time.
Solution method
Directional Control Valves (DCVs) form the backbone of hydraulic systems, wielding the power to regulate fluid flow and channel it precisely to desired locations. In this comprehensive guide, we embark on a journey to explore the fundamental aspects, working principles, and applications of these crucial valves, enabling industries to harness the potential of fluid power.
A directional control valve is a mechanical device that controls the direction of fluid flow within hydraulic systems. These valves act as gatekeepers, determining which actuators or hydraulic cylinders receive pressurized fluid and in which direction it flows. The significance of a directional control valve lies in its ability to provide precise and efficient control over hydraulic systems. By regulating fluid flow, these valves allow machinery to execute diverse tasks with accuracy, whether it's the smooth movement of heavy construction equipment or the intricate operations of aerospace systems. The proper functioning of DCVs is vital for optimizing performance, reducing energy consumption, and ensuring safety in hydraulic applications.
The roots of directional control valve technology can be traced back to ancient civilizations that harnessed the power of water to operate watermills and irrigation systems. However, it was in the 20th century that significant advancements in hydraulics and control systems spurred the development of modern DCVs. The adoption of hydraulic machinery in various industries, such as manufacturing, construction, and transportation, propelled the demand for more sophisticated and precise control over fluid flow, leading to the evolution of today's highly efficient directional control valves.
Spool valves are among the most common types of DCVs, characterized by a cylindrical spool that slides within a valve body. The position of the spool determines the fluid pathway and, consequently, the direction of fluid flow. There are several configurations of spool valves, including:
Poppet valves utilize a disc or cone-shaped poppet to control fluid flow. They can be either direct-acting or pilot-operated, depending on the method of actuation. Poppet valves offer advantages in high-pressure applications and exhibit minimal leakage, making them suitable for critical systems.
Rotary valves utilize rotary motion to control fluid flow, typically achieved through a rotating spool or cylinder. They offer precise control over flow paths and are commonly used in specialized applications.
A directional control valve can be actuated through different methods, each offering unique benefits for specific applications:
Directional Control Valves are symbolically represented in hydraulic diagrams, where specific symbols denote different valve positions. The most common valve positions include:
Single Solenoid vs. Double Solenoid Valves: Directional Control Valves can be classified as single solenoid or double solenoid, based on the number of solenoids used to control their positions. Single solenoid valves have one solenoid for actuation, while double solenoid valves employ two solenoids'one for each direction.
Series Circuit Design: In series circuit configurations, multiple directional control valves are connected in sequence to control fluid flow step-by-step, allowing precise control over actuator movements.
Parallel Circuit Design: Parallel circuit configurations involve multiple directional control valves arranged in parallel, offering simultaneous control over multiple actuators, increasing system efficiency.
Combination Circuit Design: Combination circuit designs combine series and parallel configurations, providing versatility and flexibility in controlling fluid flow for complex applications.
When selecting the appropriate directional control valve for a specific application, several factors need to be taken into account:
Safety is paramount when dealing with hydraulic systems and directional control valves. Implementing proper safety measures is crucial to prevent accidents and ensure the well-being of operators and maintenance personnel. Some key safety precautions include:
Lockout/Tagout Procedures: Isolating and securing the hydraulic system during maintenance or repair to prevent unintended activation. Performing proper LOTO procedures ensures the safety of workers.
Personal Protective Equipment (PPE): Wearing appropriate PPE, such as gloves and eye protection, when handling hydraulic components.
Avoiding Hydraulic Hazards: Being aware of potential hydraulic hazards, such as high-pressure fluid leaks or sudden movements of actuators.
Directional Control Valves find applications in a wide range of industries, facilitating essential processes and operations:
Industrial Automation: In industrial automation, DCVs control the movement of robotic arms, conveyor belts, and other automated equipment, enabling efficient production and assembly processes.
Construction and Heavy Machinery: Directional Control Valves play a pivotal role in heavy machinery, such as excavators and loaders, providing precise control over hydraulic cylinders for digging, lifting, and maneuvering.
Agricultural Equipment: In agricultural machinery, DCVs regulate the movement of tractor attachments, ensuring precision in plowing, harvesting, and other farming operations.
1. How do I identify a directional control valve?
To identify a directional control valve, look for standardized valve symbols, check for labels or markings, observe the valve type, count the ports and positions, note the actuation method, and consult manuals or seek expert advice if needed.
2. How do proportional directional control valves differ from on/off directional control valves?
Proportional directional control valves provide variable control over fluid flow, allowing for precise regulation of speed and position, whereas on/off directional control valves only offer binary control, i.e., fully open or fully closed. Proportional valves are commonly used in applications requiring smooth and proportional control, such as in precision automation and motion control systems.
3. Can directional control valves be used in conjunction with other types of valves in hydraulic circuits?
Yes, directional control valves can be used in combination with other types of valves, such as pressure control valves, flow control valves, and check valves, to create complex hydraulic circuits. This allows for more sophisticated and efficient control over fluid flow, pressure, and direction in various hydraulic systems.
4. How can I ensure the proper maintenance and longevity of directional control valves?
To ensure optimal performance and longevity of directional control valves, regular maintenance is crucial. This includes periodic cleaning, inspection for wear and tear, checking for leaks, and replacing worn-out seals and O-rings. Following manufacturer's guidelines and adhering to proper installation procedures are also essential for extending the life of the valves.