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A Brief Discussion on the Causes and Countermeasures of Leakage in Construction Machinery Hydraulic Systems Author: XCMG Research Institute I. Introduction Leakage is a common fault phenomenon in hydraulic machinery, and it is particularly severe in the hydraulic systems of construction equipment. It occurs mainly due to pressure differences that arise as fluid flows through hydraulic components and pipelines, as well as the gaps present between these components. Furthermore, harsh operating conditions can also have an impact on the sealing of construction machinery. Once a leak occurs in the hydraulic system, it prevents the system pressure from being established. Additionally, the leakage of hydraulic oil can cause environmental pollution, disrupt production, and even lead to severe consequences that are difficult to estimate. Below, we will briefly discuss the causes of leakage and corresponding countermeasures for some factors that affect leakage in the hydraulic systems of construction machinery. II. Classification of leaks: There are mainly two types of leaks in the hydraulic systems of construction machinery – leaks at fixed seals and leaks at moving seals. Leaks at fixed seals occur primarily at areas such as the bottom of the cylinder and the connections between various pipes, while leaks at moving seals occur mainly in areas such as the piston rod of the cylinder and the valve rods of multi-way valves. Based on oil leakage, it can also be divided into external leakage and internal leakage. External leakage refers to the leakage of hydraulic oil from the system into the environment, while internal leakage occurs when, due to pressure differences between the high-pressure and low-pressure sides or the failure of seals, hydraulic oil flows from the high-pressure side to the low-pressure side within the system. III. Reasons affecting leakage: (I) Design factors: (1) Selection of seals. The reliability of a hydraulic system depends to a large extent on the design of its sealing elements and the choice of seals. Improper selection of sealing structures in the design, as well as non-compliant selection of seals, can lead to problems; moreover, factors such as the compatibility between hydraulic oil and sealing materials, load conditions, maximum pressure, operating speed, and changes in ambient temperature are not taken into account during the design process. All of these cause hydraulic system leaks, directly or indirectly, to varying degrees. Furthermore, since the operating environment of construction machinery is filled with dust and impurities, appropriate dust-proof seals must be selected during design to prevent such contaminants from entering the system, damaging the seals and contaminating the oil, which could lead to leaks. (2) Other design reasons: The design did not take sufficient account of the geometric accuracy and roughness of the moving surfaces, nor was the strength of the joints checked during the design process; all these factors can lead to leaks during the operation of the machinery. (II) Manufacturing and assembly factors (1) Manufacturing factors: All hydraulic components and sealing parts are subject to strict requirements regarding dimensional tolerances, surface treatment, surface finish, as well as form and position tolerances. If there are deviations during the manufacturing process – such as deviations in the piston radius of the cylinder, the depth or width of the sealing groove, the dimensions of the holes used for installing seals; or if the parts become out of round due to machining issues, have burrs or depressions, or if the chromium plating peels off – then the seals will suffer from deformation, scratches, crushing, or insufficient compression, which renders them unable to perform their sealing function. It will cause the part itself to have inherent leakage points, leading to leaks after assembly or during use. (2) Assembly factors: Rough handling should be avoided during the assembly of hydraulic components; excessive force can cause the parts to deform, especially when using copper rods or similar tools to strike the cylinder block, sealing flanges, etc ； Before assembly, the parts should be carefully inspected. During assembly, the parts should be dipped in a small amount of hydraulic oil and pressed in gently. Diesel should be used for cleaning, especially for rubber components such as seals, dust seals, and O-rings; using gasoline will cause them to age quickly and lose their original elasticity, thereby compromising their sealing capabilities. (III) Oil contamination and component damage (1) Gas contamination: At atmospheric pressure, about 10% of air can dissolve in hydraulic oil; under the high pressures in hydraulic systems, more air or gases can dissolve in the oil. Bubbles form in the oil. If, during operation, the pressure in a hydraulic support changes rapidly between high and low levels over a very short period of time, this can cause the bubbles to generate high temperatures on the high-pressure side and to burst on the low-pressure side. If there are imperfections or damages on the surfaces of the components in the hydraulic system, the hydraulic oil will flow at high speed toward these surfaces, accelerating their wear and leading to leaks. (2) Particle contamination: As the main actuating elements in the hydraulic systems of various construction machinery, hydraulic cylinders have piston rods that are exposed and in direct contact with the environment during operation. Although dust seals and other sealing components are installed on the guide sleeves, it is still inevitable that dust and dirt enter the hydraulic system, accelerating the scratching and wear of sealing components and piston rods, and thus causing leaks. Particle contamination is one of the fastest causes of damage to hydraulic components. (3) Water pollution: Due to factors such as a humid working environment, water may enter the hydraulic system. Water reacts with hydraulic oil to form acidic substances and sludge, which reduces the lubricating properties of the hydraulic oil and accelerates the wear of components. Water can also cause the valve stems of control valves to stick together, making it difficult to operate these valves and damaging seals, leading to leaks. (4) Part damage: Seals are made of materials such as oil-resistant rubber; aging, cracking, and damage due to prolonged use can all cause leaks in the system. If a part is damaged due to impact during operation, it can scratch the sealing elements, resulting in leaks. IV. Main countermeasures for leakage prevention and control The factors that cause leakage in the hydraulic systems of construction machinery are the result of various combined influences, and it is very difficult to completely eliminate leakage from hydraulic systems using current technologies and materials. Only by considering the above factors that affect hydraulic system leakage and taking reasonable measures to minimize it can we achieve this. In the design and manufacturing stages, due consideration must be given to the important factors affecting leakage, namely the design and fabrication of the sealing groove. Furthermore, the selection of seals is also very important; failing to consider all factors related to leakage from the outset can result in immeasurable losses in future production. Choose the correct methods of assembly and repair, drawing on past experience. For example, when installing the seal ring, it is advisable to use specialized tools and apply some lubricant to the seal ring. In terms of pollution control for hydraulic oil, it is necessary to address the sources of contamination, strengthen control over those sources, and also implement effective filtration measures as well as regular inspections of the oil quality. To effectively prevent external factors (water, dust, particles, etc.) from contaminating hydraulic cylinders, certain protective measures can be implemented. In short, preventing and controlling leaks requires a comprehensive approach; only by considering various factors can effective measures be taken.