Rational design of the hottest high pressure doubl

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Reasonable design of high-pressure double pump pressure maintaining hydraulic system

1. Preface

pressure maintaining circuit is widely used in hydraulic machine tools and equipment, among which double pump pressure maintaining circuit is more used. The circuit uses two pump combinations with different working parameters as power components, one of which is a large displacement low-pressure pump and the other is a small displacement high-pressure pump. The two coordinate the oil supply to realize the movement and pressure maintaining of the actuating elements of the hydraulic system. Using double pump pressure maintaining circuit can improve the operation efficiency of the system and reduce energy consumption. The design of the pressure maintaining circuit system should be reasonably selected according to the specific working conditions and requirements, and the design scheme should be selected according to the specific parameters, such as the length of pressure maintaining time, the level of pressure maintaining pressure, and the operating speed of the actuator, so as to achieve economy, rationality and efficiency

2. General method design

when designing hydraulic systems, people often think of applying various common circuits first. However, not all hydraulic systems can copy common circuits without distinguishing specific functional requirements. The design should consider not only the use performance, but also the economic performance and safety factors, otherwise the design may be unreasonable. Figure 1 is the hydraulic schematic diagram of a simple hydraulic press. Its function is to press the workpiece - maintain the pressure - eject the workpiece. To complete this work, the set pressure of the machine is 3Mpa for low-pressure pump and 29 MPa for high-pressure pump. The rated flow of the low-pressure pump is 160L/min, and the rated pressure is 6.3MPa; The rated flow of the high-pressure pump is 10L/min and the rated pressure is 31.5Mpa

from Figure 1 and technical parameters, we can analyze the operation of the hydraulic system and the high-pressure pressure pressure maintaining function. When the electro-hydraulic reversing valve of the main hydraulic cylinder 1 is powered on for reversing, the solenoid valves that control the unloading of the high and low pressure pumps are powered on for reversing, and the two pumps supply oil to the system together at a lower pressure to make the main cylinder move downward. When the master cylinder presses the workpiece in place and maintains the pressure, the system pressure rises to the set pressure of the pressure relay, the pressure relay sends a signal, the solenoid valve of the low-pressure pump is energized, and the low-pressure pump is unloaded. At this time, the high-pressure oil of the high-pressure pump is supplied to the master cylinder for pressure maintaining. The high-pressure oil flows back to the oil tank through the overflow valve of the high-pressure pump, except that part of the high-pressure oil is used to make up for the system leakage in an interview with plastics in 2014

Figure 1 general design of hydraulic press (1. Main hydraulic cylinder 2. Ejector cylinder)

from the perspective of principle, the system can completely execute all working procedures of hydraulic press, but if combined with the analysis of its technical parameters, it is not difficult to see the irrationality of this principle: the system requires both high pressure and medium and low pressure. For example, the rising and falling of master cylinder 1 and ejector cylinder 2 are in a low-pressure state, while the system is in a high-pressure state during the pressure maintaining period of master cylinder. It can be seen from Figure 1 that during pressure maintaining, the high-pressure oil is connected with the overflow valve, unloading solenoid valve, one-way valve, pressure relay, pressure gauge switch of the high-pressure pump, the electro-hydraulic reversing valve of the master cylinder, the electro-hydraulic reversing valve of the ejector cylinder and the two hydraulic cylinders. Therefore, the rated withstand voltage grade of these components must be selected as 31.5 MPa

3. The price of hydraulic components designed in a reasonable way is related to its technical parameters, such as pressure grade, flow grade, etc. If the pressure and flow level are high, the price is high, otherwise, it is low. Therefore, the cost should be reasonably reduced when designing the system and selecting components. From the analysis of Figure 1, the overflow valve, unloading solenoid valve and pressure gauge of the low-pressure pump work under medium and low pressure, and the components outside the oil outlet of the two pumps should work under high pressure, or they should be connected with high pressure to promote the full independent supply of compressed oil for aviation aluminum. So, can there be a more suitable hydraulic system to replace it? The answer is yes. If the principle of the above hydraulic press is designed as shown in Figure 2, the high-pressure pressure maintaining circuit will be more reasonable. In Figure 2, the low-pressure and high-pressure pumps supply oil to the hydraulic cylinder at the same time when the main hydraulic cylinder moves downward. When the master cylinder drops in place, the system pressure rises, the pressure relay sends a signal, and pump 1 unloads. At this time, the high-pressure pump directly supplies high-pressure oil to the master cylinder to implement high-pressure pressure pressure maintenance. Its characteristic is that the high-pressure oil does not need to pass through the master cylinder electro-hydraulic directional valve, and it will not be connected with the ejector cylinder and its electro-hydraulic valve. The hydraulic control check valve analyzes and points out that the high-pressure oil is blocked from connecting with these components when the system generates high pressure. When the pressure maintaining is completed, the main hydraulic cylinder is energized through the rising solenoid of the Electro-hydraulic Directional Valve (at the same time, the unloading solenoid valve of the low-pressure pump is energized), the low-pressure pump restores oil supply, and opens the hydraulic control check valve. At this time, the high-pressure pump oil returns to the oil tank through the hydraulic control check valve and the electro-hydraulic directional valve. When the master cylinder rises to the position and touches the travel switch, the electro-hydraulic valve will be powered off and reset to the middle position, then the unloading solenoid valves of the two pumps will also be powered off, and both pumps will be unloaded

Figure 2 reasonable design of hydraulic press (1. Main hydraulic cylinder 2. Ejector cylinder)

4. Analysis and comparison

compare the two hydraulic systems in Figure 1 and Figure 2. It is not difficult to conclude that the functions of the two systems are basically the same, but the final effect produced by Figure 2 will be better than figure 1. The specific performance is in the following aspects:

① it can reduce the manufacturing cost

the hydraulic valve core should move in the valve body, and the hydraulic cylinder piston should move in the cylinder body. This relative movement should have a gap, and the size of the gap depends on the working pressure level of components such as valves, and also depends on the processing performance of components. The higher the working pressure is, the smaller the clearance of components is required, the higher the fitting accuracy is required, the higher the material requirements are, the corresponding processing difficulty is also increased, and the manufacturing cost is also increased. For example, for components with the same rated flow, the rated pressure is medium and low pressure (≤ 6.3MPa), and the fitting clearance between the valve core and the valve body can be 3 ~ 4 times larger than that of the high-pressure (31.5Mpa) valve. The same is true for the fitting accuracy of the hydraulic cylinder piston and the cylinder body, as well as the fitting accuracy requirements of the piston rod and the cylinder guide sleeve. Therefore, the cost of components such as high-pressure valve and high-pressure cylinder is much higher. The system shown in Figure 2 can make the two sets of electrohydraulic valves and ejector cylinders in the high-pressure level in the system shown in Figure 1 leave the high-pressure area and always work under medium and low pressure. This alone can reduce a lot of costs

② system leakage can be reduced

an important factor affecting the efficiency of the hydraulic system is the internal and external leakage of the system. The leakage is particularly prominent in the high-pressure oil area. Take the electro-hydraulic valve in the system as an example. Comparing the electrohydraulic change-over valve in the medium and low pressure system with the high-pressure series electrohydraulic change-over valve in the high-pressure system, it can be concluded that the ratio of the internal leakage of the two valves is 1:60. For example, in the medium and low pressure (≤ 6.3MPa) pressure system, the internal leakage of the Electro-hydraulic Directional Valve with a flow of 200 L/min is ≤ 30ml/min, while in the high pressure (such as 31.5Mpa) system pressure, the internal leakage of the Electro-hydraulic Directional Valve with the same flow reaches 1.8L/min. according to the system principle in Figure 1, the total internal leakage of the two Electro-hydraulic Directional Valves and the ejector cylinder will exceed 5L/min, more than half of the oil that the high-pressure pump of the system can provide. It can be seen that the leakage is particularly serious in the high-pressure system. The system in Figure 2 avoids the contact of two sets of electrohydraulic directional valves and ejector cylinder with high-pressure oil, which can greatly reduce the leakage of the system. The leakage is reduced, the power loss can also be reduced, the rated flow of the high-pressure pump can be further reduced, the efficiency can be improved, and the heating condition of the system can also be improved. In addition, the requirements for seals can also be reduced, and the seals are not easy to be damaged

③ it is conducive to the reasonable layout of the integrated port of the hydraulic station

the pump station of the system in Figure 1, whose high-pressure oil can reach the main hydraulic cylinder only after passing through four integrated blocks, while the system in Figure 2 can centrally arrange the components in contact with high-pressure oil on two integrated blocks. Such arrangement reduces the work of sealing high-pressure oil between manifold blocks, and also makes the processing of manifold blocks easier. At the same time, the local loss of hydraulic pump station is also reduced due to the reduction of intermediary links

④ the oxidative deterioration of oil is reduced

the possibility of oxidative deterioration of hydraulic oil under high pressure is much greater than that under low pressure. Figure 1 in the system, when high pressure is maintained, high pressure oil is stored not only in the master cylinder, but also in the ejector cylinder and its pipelines. In Figure 2, when the system is maintaining pressure, only the master cylinder has high-pressure oil, so the amount of oil squeezed by high pressure in the latter is only half of that in the former. This plays a positive role in slowing down the oxidation and deterioration of the oil and prolonging the service life of the oil

5. Summary

the implementation of reasonable design of hydraulic system can create good economic benefits. Common circuits are representative, which can be borrowed for reference, but cannot be copied. Different places, requirements and technical parameters make the design schemes unique. Many manufacturers are choosing tensile testing machines. Some systems pay attention to flow design, while others pay more attention to pressure factors, but in any case, it is reasonable to comprehensively consider and optimize the design. (end)

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