Abstract: A novel high-pressure dual-pump pressure holding design is proposed. Analysis and comparison demonstrate its significant advantages in reducing manufacturing costs, improving system efficiency, and extending system service life. Keywords: Pressure holding, leakage, oxidation 1 Introduction Pressure holding circuits are widely used in hydraulic machine tools and equipment, with dual-pump pressure holding circuits being particularly common. This circuit uses a combination of two pumps with different operating parameters as power components: one is a large-displacement, low-pressure pump, and the other is a small-displacement, high-pressure pump. The two pumps coordinate oil supply to achieve the movement and pressure holding of various actuators in the hydraulic system. Using a dual-pump pressure holding circuit can improve system operating efficiency and reduce energy consumption. The design of the pressure holding circuit system should be rationally selected based on the specific working conditions and requirements. The design scheme should be selected according to specific parameters such as the length of pressure holding time, the level of pressure holding, and the operating speed of the actuators, striving for economy, rationality, and efficiency. 2 General Design Methods When designing hydraulic systems, people often first think of applying various commonly used circuits. However, not all hydraulic systems can simply copy commonly used circuits without considering specific functional requirements. The design must consider performance, economic efficiency, and safety factors; otherwise, the design may be unreasonable. Figure 1 is a hydraulic schematic diagram of a simple hydraulic press. Its function is to press down the workpiece, hold the pressure, and eject the workpiece. To complete this task, the machine is set to pressures of 3 MPa for the low-pressure pump and 29 MPa for the high-pressure pump. The rated flow rate of the low-pressure pump is 160 L/min, and the rated pressure is 6.3 MPa; the rated flow rate of the high-pressure pump is 10 L/min, and the rated pressure is 31.5 MPa. From Figure 1 and the technical parameters, we can analyze the operation of the hydraulic system and the high-pressure holding function. When the electro-hydraulic directional valve of the main hydraulic cylinder 1 is energized, the solenoid valves controlling the unloading of the high-pressure and low-pressure pumps are also energized. The two pumps supply oil to the system at a lower pressure, causing the main cylinder to press down. When the main cylinder presses the workpiece into place and holds the pressure, the system pressure rises to the pressure relay's set pressure. The pressure relay sends a signal, energizing the low-pressure pump solenoid valve, and the low-pressure pump unloads. At this time, the high-pressure oil from the high-pressure pump supplies oil to the main cylinder for pressure holding. A portion of the high-pressure oil is used to compensate for system leaks, while the remainder flows back to the oil tank via the high-pressure pump's relief valve. In principle, this system can fully execute all working procedures of the hydraulic press. However, analysis of its technical parameters reveals its inherent flaws: the system requires operation under both high and medium-low pressure conditions. For example, the ascent and descent of main cylinder 1 and ejector cylinder 2 are under low pressure, while the system operates under high pressure during the main cylinder's pressure holding period. Figure 1 shows that during pressure holding, the high-pressure oil is connected to the high-pressure pump's relief valve, unloading solenoid valve, check valve, pressure relay, pressure gauge switch, main cylinder electro-hydraulic directional valve, ejector cylinder electro-hydraulic directional valve, and the two hydraulic cylinders. Therefore, the rated pressure resistance of these components must be selected as 31.5 MPa. 3. Rational Design Methods The price of hydraulic components is related to their technical parameters such as pressure rating and flow rate. Higher pressure and flow rates result in higher prices, and vice versa. Therefore, cost reduction should be rationally considered when designing the system and selecting components. Analysis of Figure 1 shows that the low-pressure pump's relief valve, unloading solenoid valve, and pressure gauge switch operate under low-to-medium pressure conditions, while components other than the pump outlets need to operate under high pressure or be connected to high-pressure oil. So, can a more suitable hydraulic system replace it? The answer is yes. If the above hydraulic press principle is designed as shown in Figure 2, the high-pressure holding circuit will be more rational. In Figure 2, the low-pressure and high-pressure pumps simultaneously supply oil to the hydraulic cylinder during the main hydraulic cylinder's descent. When the main cylinder reaches its lower position, the system pressure increases, the pressure relay sends a signal, and pump 1 unloads. At this time, the high-pressure pump directly supplies high-pressure oil to the main cylinder to implement high-pressure holding. Its characteristic is that the high-pressure oil does not need to pass through the main cylinder's electro-hydraulic directional valve, nor does it communicate with the ejector cylinder and its electro-hydraulic valve. The hydraulically controlled check valve blocks the connection between the high-pressure oil and these components when the system generates high pressure. After the pressure holding is completed, the main hydraulic cylinder is energized via the electro-hydraulic directional valve's rising solenoid (simultaneously, the low-pressure pump unloading solenoid valve is energized), the low-pressure pump resumes oil supply, and the hydraulic control check valve opens. At this time, the high-pressure pump oil returns to the oil tank via the hydraulic control check valve and the electro-hydraulic directional valve. When the main cylinder rises to the limit switch and touches the limit switch, the electro-hydraulic valve is de-energized and returns to the neutral position, and the unloading solenoid valves of both pumps are also de-energized, and both pumps are unloaded. 4. Analysis and Comparison Comparing the two hydraulic systems in Figure 1 and Figure 2, it is easy to conclude that the two systems have basically the same function, but the final effect produced by Figure 2 will be better than that of Figure 1. Specifically, this is reflected in the following aspects: ① Reduced manufacturing costs: The hydraulic valve core needs to move within the valve body, and the hydraulic cylinder piston needs to move within the cylinder body. This relative movement requires clearance, and the size of the clearance depends on the working pressure level of the valve and other components, as well as the processing performance of the components. The higher the working pressure, the smaller the clearance required by the components, the higher the required fitting precision, the higher the material requirements, the greater the processing difficulty, and the higher the manufacturing cost. For components with the same rated flow rate and rated pressure of medium to low pressure (≤6.3MPa), the clearance between the valve core and valve body can be 3 to 4 times larger than that of a high-pressure (31.5MPa) valve. The same applies to the precision requirements for the fit between the hydraulic cylinder piston and cylinder body, and between the piston rod and cylinder guide sleeve. Therefore, components such as high-pressure valves and high-pressure cylinders are significantly more expensive. The system shown in Figure 2 allows the two sets of electro-hydraulic valves and the ejector cylinder in the high-pressure system of Figure 1 to operate outside the high-pressure zone, always under medium to low pressure. This alone can reduce costs considerably. ② Reducing system leakage is an important factor affecting the efficiency of hydraulic systems. Internal and external leakage is particularly prominent in high-pressure oil areas. Taking the electro-hydraulic directional valve in the system as an example, comparing the electro-hydraulic directional valve in the medium to low-pressure system with the high-pressure series electro-hydraulic directional valve in the high-pressure system, the ratio of internal leakage between the two valves is 1:60. For example, in a medium-low pressure system (≤6.3MPa), an electro-hydraulic directional valve with a flow rate of 200 L/min has an internal leakage of ≤30 mL/min. However, in a high-pressure system (e.g., 31.5MPa), the internal leakage of the same flow rate electro-hydraulic valve reaches 1.8 L/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 5 L/min, exceeding half of the oil supply that the high-pressure pump can provide. It is evident that leakage is particularly severe in high-pressure systems. The system in Figure 2 avoids contact between the two sets of electro-hydraulic directional valves and the ejector cylinder and the high-pressure oil, thus significantly reducing system leakage. Reduced leakage also reduces power loss, allowing for a smaller rated flow rate of the high-pressure pump, improving efficiency, and mitigating system overheating. Furthermore, the requirements for seals can be reduced, and the seals are less prone to damage. ③ It facilitates a more rational layout of the hydraulic station's integrated ports. In the system of Figure 1, the high-pressure oil must pass through four integrated blocks to reach the main hydraulic cylinder, while the system in Figure 2 can centrally arrange the components in contact with the high-pressure oil on two integrated blocks. This arrangement reduces the work of sealing high-pressure oil between integrated blocks, simplifies the processing of integrated blocks, and reduces local losses in the hydraulic pump station due to the reduction of intermediate links. ④ Reduced oil oxidation and deterioration: Hydraulic oil is much more likely to oxidize and deteriorate under high pressure than under low pressure. In the system of Figure 1, when holding pressure, high-pressure oil is present not only in the main cylinder but also in the ejector cylinder and its pipelines. In the system of Figure 2, only the main cylinder contains high-pressure oil during pressure holding. Therefore, the amount of oil squeezed by high pressure in the latter is only half that in the former. This plays a positive role in slowing down oil oxidation and deterioration and extending oil service life. 5 Conclusion A well-designed hydraulic system can create good economic benefits. Commonly used circuits are representative and can be used as a reference, but they cannot be copied verbatim. Different locations, requirements, and technical parameters result in different design schemes. Some systems focus on flow design, while others emphasize pressure factors. However, in any case, comprehensive consideration and optimized design are the reasonable approach.