Preface In recent years, the widespread application of a certain miniature special pump in aviation, aerospace, modern main battle tanks, and shipbuilding has attracted widespread attention both domestically and internationally. Due to its small size, light weight, and high rotational speed, this miniature special pump features a compact axial structure, small radial dimensions, and few shaft parts, and is characterized by its ultra-miniature, ultra-high speed, and ultra-lightweight nature. Some experts predict that in the 21st century, automobiles may be powered by this miniature special pump, replacing piston engines. This article will briefly introduce the working principle of this miniature special pump and analyze and discuss several key issues in its development and design for application in a certain aerospace servo system. 1. Working Principle A simplified structural diagram of the miniature special pump is shown in Figure 1: [align=center] Figure 1 Simplified structural diagram of the miniature special pump[/align] The turbine rotor 2 of this pump, driven by a gaseous medium (such as helium, high-temperature fuel gas, etc.), drives the coaxially connected radial impeller 1 to run at an ultra-high speed of tens of thousands of revolutions per minute. Through centrifugal force, the liquid medium (usually hydraulic oil) entering the impeller cavity axially is accelerated and pressurized. After the kinetic energy of the liquid medium is converted into high-pressure energy through the deceleration and pressurization pipe at the impeller output end, it provides driving force for the hydraulic servo system. 2. Miniaturization and Optimization Design As a core power component of a spacecraft servo control system, this miniature special pump requires small size and light weight in its structural design. Therefore, an optimized design scheme must be adopted during the development process to control its structural dimensions within the range of 80 x 80 mm. The miniaturization design of the miniature special pump cannot achieve the design performance indicators by simply scaling down proportionally. During the development process, we mainly adopted structural optimization technology by merging parts, reducing connection links, and increasing the overall structural form. Low-density, lightweight materials were extensively used for components where structural strength allowed. Specific optimization schemes are as follows: 1) The miniature special pump adopts a centrifugal radial impeller and a single-stage axial flow turbine, with the turbine blades, turbine disk, and turbine shaft as a single integrated structure. 2) The turbine rotor uses an integral forged blank, with the blades formed by electrical discharge machining and then polished by abrasive flow. This machining method is technically mature, has high machining accuracy, and the quality of the parts is easy to control. 3) The shaft system adopts a two-support scheme, with a cantilevered layout for the turbine disk. 3. Vibration Reduction Methods for Shaft System Because the turbine disk of the miniature special pump adopts a cantilever structure, the two support points of the shaft system are located between the turbine disk and the centrifugal radial impeller. The design operating speed is between the first and second critical speeds of the shaft system (tens of thousands of revolutions per minute). From start-up to reaching the operating speed, the turbine must quickly pass through the first critical speed in a very short time, resulting in a certain degree of vibration in the shaft system. Excessive vibration can lead to the failure of the miniature special pump. Therefore, it is essential to minimize the vibration of the shaft system of the miniature special pump during operation. Vibration reduction can be achieved through two approaches. 3.1 Analysis of the Dynamic Characteristics of the Miniature Special Pump Shaft System The main factor affecting shaft vibration is the turbine speed. The designed operating speed of the miniature special pump must be greater than 1.4 times the first-order critical speed and less than 0.7 times the second-order critical speed. To understand the mechanism of the turbine speed's influence on shaft vibration, the dynamic characteristics of the shaft system must be studied. Given the significant deviations in general analysis and calculation methods, the finite element method was used here, yielding the following conclusions: When the second-order critical speed of the shaft system is close to the operating speed, if overspeeding occurs during turbine operation, its speed can easily approach or even reach the second-order critical speed, causing synchronous resonance in the shaft system, leading to instability or even damage to the miniature special pump. By adjusting the distances between the turbine disk's center of mass, the radial impeller's center of mass, and the two support points of the shaft system, as well as the individual masses of the turbine disk and the radial impeller, the first and second-order critical speeds of the shaft system can be adjusted, thereby improving the shaft system's dynamic characteristics and achieving vibration reduction. 3.2 Improving the Dynamic Balancing Accuracy of the Shaft System The dynamic balancing accuracy of the shaft system and turbine rotor has a significant impact on its vibration. Appropriate measures must be taken to improve this accuracy during production. After the turbine rotor is initially balanced according to the G1 standard in ISO 1940, it is assembled with the shaft system and dynamically balanced according to G2.5 standards. The residual imbalance at the maximum outer diameter of the entire rotor shaft system is controlled to be less than 25.6 mg•mm. Simultaneously, during the assembly of the miniature special pump, the radial runout at the cantilever end of the turbine in its free state is strictly controlled to ensure it does not exceed 0.02 mm. 4. Pre-tightening of Shaft Components and Lubrication of Bearings If axial clearance occurs between the shaft components of the special pump under ultra-high-speed rotation, it will lead to instability in the rotor and shaft system, affecting the safety and reliability of the pump operation. Furthermore, due to the ultra-high-speed operation of the shaft system, the extremely high heat generated by friction in the bearings, a key component, will cause rapid pump failure. Therefore, measures must be taken to eliminate clearance between shaft components and to lubricate the bearings. 4.1 Preload of Shaft Components The control of clearance between shaft components is achieved by applying preload. A predetermined load is applied to the turbine shaft at one end of the shaft system to maximize the axial interaction force between the various parts of the shaft system, thereby ensuring that there is no axial positional change between the various parts of the shaft system during operation. For the bearings, a key component of the shaft system, a disc spring with an axial dimension of only 3 mm is installed. During assembly, the disc spring is axially compressed to a certain position, and the resulting expected axial preload can eliminate the radial and axial clearance inside the ball bearing, thereby ensuring that the rotor has high rotational accuracy and working stability under ultra-high speed operation. 4.2 Lubrication of Bearings The lubrication scheme for the two ultra-high speed ball bearings in the shaft system is shown in Figure 2. When the micro special pump is working, the radial impeller provides a portion of the hydraulic oil, which flows into the bearing cavity through the AB oil path in the figure. After lubricating the bearing, the special structure inside the bearing cavity pressurizes the hydraulic oil, which returns to the radial impeller inlet through the CD oil path for recycling. At the same time, due to the circulating flow of the lubricating hydraulic oil, the bearing can also be adequately cooled during the lubrication process. Experiments show that this lubrication scheme is ingeniously conceived, simple in structure, rationally laid out, and reliable in operation. [align=center] Figure 2 Schematic diagram of lubrication oil circuit of shaft system of miniature special pump[/align] 4.3 Application of fluid self-lubricating hydrodynamic film bearing in miniature special pump In order to solve the problems of power loss and lubrication and cooling caused by friction between the inner ring and the balls of high-speed rotating ball bearings, radial and axial thrust fluid self-lubricating hydrodynamic film bearings have been developed abroad. Taking the radial bearing as an example, the structural schematic diagram of the fluid self-lubricating hydrodynamic film bearing is shown in Figure 3. [align=center] Figure 3 Schematic diagram of hydrodynamic film bearing[/align] The bearing consists of an outer ring for support, an inner ring, and several curved foils installed on the inner ring. The other end of the foils is suspended freely. The shaft system of the miniature special pump is installed at the center of the foil group. In the non-working state, the bearing is filled with hydraulic oil at normal temperature and pressure. When the pump starts working, the foil connects with the shaft system and supports the shaft system. As the pump speed increases, a wedge-shaped oil film with a certain pressure is gradually formed between the shaft system and the foil. The pressure of the wedge-shaped oil film increases with the increase of the shaft speed. When the speed increases to a certain value, the shaft system is separated from the foil under the action of the wedge-shaped oil film pressure and runs suspended above the wedge-shaped oil film. There is almost no obvious friction loss between the shaft system and the foil, and the internal power consumption of the shaft system is negligible. Therefore, the shaft dynamic characteristics of the micro special pump after using the fluid self-lubricating dynamic pressure oil film bearing are quite perfect, and the working speed is no longer affected by the critical speed. 5 Conclusion As a special power element, the micro special pump introduced in this paper has been successfully applied in aerospace hydraulic systems after our research and development. With the solution of some key problems (such as pump working efficiency), this pump, with its excellent advantages: compact structure and light weight, can be applied to other fields. References: [1] Thick Yu Turbine Parts Structure and Strength Measurement [M] Beijing: Machinery Industry Press, 1987. [2] Xu Huafang. Fundamentals of Aerodynamics EM3. Beijing: Beijing University of Aeronautics and Astronautics Press, 1987.