Reasons for hydraulic system leakage and solutions for hydraulic pump maintenance

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During the maintenance of hydraulic pumps, it is important to understand that leaks in the hydraulic system can affect the quality of the product, and this is an issue that must be taken into consideration. For example, in hydraulic cylinders, severe leakage not only contaminates the environment around the equipment but also reduces the pressure in the cylinder’s working chamber, preventing the cylinder from functioning properly. Adopting more advanced methods to effectively prevent leaks and achieve a \"zero-leak\" hydraulic system has been the goal pursued by the hydraulic industry for many years. Additionally, accurately analyzing the root causes of leaks in hydraulic systems can help us promptly rectify any leakage problems in such systems. As mechanical engineering students, through studying the course on \"Hydraulic and Pneumatic Transmission\" as well as consulting relevant materials, and by combining our professional experiences, engineering training, and what we observe and think about in daily life, we have analyzed the types and causes of leakage in hydraulic transmission systems regarding common leakage problems, and proposed measures to control such leakage. Compared to mechanical transmission, hydraulic transmission is a new technology that originated from the principle of hydrostatic transmission proposed by Pascal in 1654. It is a type of transmission that uses a liquid as the working medium and relies on an energy conversion device to transfer energy. Hydraulic transmission has the following advantages: ① The working fluid can be delivered to any location through pipes; ② The arrangement of the actuating elements is not restricted by orientation, and the transmission mechanism can be arranged conveniently and flexibly with the help of oil pipes; ③ Hydraulic transmission can convert the rotational motion of the prime mover into linear motion; ④ Stepless speed control can be easily achieved; ⑤ Load control, speed control, and direction control are easy to implement, and centralized control, remote control, and automatic control are also feasible; ⑥ Hydraulic transmission is smooth with no vibrations; ⑦ Good lubrication conditions improve the reliability and service life of hydraulic components; ⑧ Hydraulic components facilitate standardization, serialization, and generalization. Therefore, hydraulic transmission is widely used in various sectors of the national economy. However, hydraulic transmission also has some disadvantages: ① There are losses due to fluid flow resistance, oil leakage, and mechanical friction, resulting in lower efficiency; ② High requirements are placed on controlling the operating temperature; ③ Due to oil leakage and compressibility, the hydraulic system lacks rigidity, which prevents it from maintaining a precise transmission ratio; ④ Strict requirements exist regarding the use and maintenance of the working fluid; ⑤ Hydraulic components are costly; ⑥ It is difficult to diagnose and address faults in hydraulic systems, requiring high technical skills and specialized knowledge from the operators. Among these, the leakage of working fluid has always been an inevitable problem, and finding solutions to it is also one of the key areas of research in various industries. Types of leakage: Based on the direction of flow, leakage can be classified into internal leakage and external leakage. External leakage refers to the leakage of hydraulic oil from the system into the surrounding environment, occurring outside the hydraulic system’s pipelines, valves, cylinders, and pumps (motors). 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; for example, in hydraulic systems, this can be the leakage of oil from the high-pressure chamber to the low-pressure chamber, or leakage from the pressure channels in the directional control valves to the return oil channels. The main forms of leakage include gap leakage, porous leakage, adhesion leakage, and dynamic leakage. 1. Leakage through gaps in construction machinery hydraulic systems can occur in two main ways: leakage at fixed seals (static mating surfaces) and leakage at moving seals (dynamic mating surfaces). The areas where leaks occur at fixed seals mainly include the junction between the cylinder head and the cylinder barrel of the hydraulic cylinder, etc.; the areas where leaks occur at moving seals mainly include the interface between the piston of the hydraulic cylinder and the inner wall of the cylinder barrel, as well as between the piston rod and the guide sleeve of the cylinder head. The amount of leakage through the gap is related to factors such as pressure difference and the gap size. 2. In porous and leaky hydraulic components, various covers have surfaces on which complete contact between them is not possible due to surface roughness. In the microscopic recesses where these surfaces do not make contact, numerous voids are formed, with various cross-sectional shapes and sizes; the dimensions of these voids are related to the surface roughness. Due to multiple gaps for leakage, the liquid must flow through numerous curved gaps; during seal performance tests, a certain pressure retention time is required before leakage becomes apparent. 3. Adhesive leakage: There is a certain degree of adhesion between viscous liquids and solid arms; upon contact, a thin layer of liquid adheres to the surface of the solid. If the film on the solid surface is thick, the oil film may be scraped off by the sealing ring due to relative movement, resulting in adhesive leakage. The basic method to prevent adhesive leakage is to control the thickness of the liquid adhesive layer. 4. Power leakage occurs on the sealing surface of the rotating shaft; if there are traces of helical machining, as the shaft rotates, the liquid flows along the grooves formed by these helical traces under the force generated by the shaft’s rotation. When the direction of the spiral trace is consistent with the rotation direction of the shaft, power leakage occurs due to the \"oil pumping\" effect of the spiral trace. The characteristic of power leakage is that the higher the shaft speed, the greater the leakage amount. To prevent power leakage, it is necessary to avoid any machining marks that could cause \"oil pumping\" on the sealing surface of the rotating shaft and on the lip of the seal. By taking advantage of the principle of power leakage, and utilizing the oil-pumping effect of these spiral marks, the leaking oil can be pumped back, thereby preventing leakage. The reasons for leaks in hydraulic systems are numerous and complex. This article mainly summarizes and analyzes them from aspects such as seal leakage, contamination of the working fluid, deviations in the manufacturing and assembly precision of components, temperature rise and heat generation of the fluid, and pressure surges in the hydraulic system. (1) Seals: In hydraulic cylinders, when leakage is mentioned, seals are the first thing that comes to mind, as they are the primary components used to prevent leakage in such cylinders. It mainly includes YX-type seals, combined seals, U-type seals, V-type seals, O-rings, PTFE seals, and Stellite seals. Among them, the YX-type seals, combined seals, and U-type seals rely mainly on the tension generated by the pressure of the hydraulic fluid to open the inner and outer lips and thus achieve sealing; high requirements are placed on the linear mass of these inner and outer lips. The V-type seals, O-rings, ring seals, and thrust seals, on the other hand, rely primarily on compression to achieve sealing. The quality of sealing elements, in terms of their physical and chemical properties, directly affects the quality of hydraulic cylinder products. The main reasons for seal leakage are as follows: ① There are issues with the quality of the seal. The seal material is of poor quality, the manufacturing process and precision fail to meet requirements, and there are defects in the molds and trimming as well. ②The seal was not selected appropriately. The selected seal does not meet the requirements regarding working pressure, operating speed, temperature, etc., or the type of seal chosen is inappropriate. ③The installation grooves for the seals are poorly designed, with inappropriate selection of installation clearance and compression amount; the machining precision and surface roughness of the mating parts do not meet the required standards; the seal lips are damaged due to a lack of protection during installation, or foreign debris gets trapped inside; improper temperature and humidity conditions during storage lead to degradation as a result of exposure to oxygen and ozone. ④As the bubbles in the liquid pass by the sealing lip, they are compressed to a fraction of their original size. When such bubbles reach the non-pressure side of the seal, they rapidly release energy, causing damage to the seal lip. On the other hand, when bubbles containing a certain proportion of oil vapor reach a sufficiently high temperature due to pressure, they will catch fire, thereby damaging or melting the support rings; local burning and carbonization of the seals may also occur. (2) Fluid contamination: includes gas contamination, particle contamination, water contamination, etc. ①Gas contamination: At atmospheric pressure, about 10% of air can dissolve in hydraulic oil; under the high pressures in hydraulic systems, more air or gases will 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 reach high temperatures on the high-pressure side and to burst on the low-pressure side. When there are pits or damages on the surfaces of the components in the hydraulic system, the hydraulic oil will rush towards these surfaces at high speed, accelerating surface wear and leading to leaks. This type of corrosion caused by cavitation is known as cavitation erosion. ②Particle contamination: Due to the particle contamination of the test bench, the likelihood of leakage is very low, but it still exists in practice. During the operation of hydraulic cylinders, the particles generated for the reasons mentioned above stick to some sealing elements and to the moving surfaces; these particles scratch the sealing surfaces of the sealing elements, causing them to fail and resulting in leaks in the hydraulic cylinder. This type of fault can be resolved by removing any remaining burrs, cleaning the various components, and replacing the sealing elements; in more severe cases, it may cause \"cylinder scoring\", and fixing such faults is more complicated; still more serious situations can lead to the destruction of the cylinder barrel. ③Water contamination: 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 it can damage seals leading to leaks. (3) Excessive manufacturing and assembly tolerances of components: All hydraulic components and sealing parts are subject to strict requirements regarding dimensional tolerances, form and position tolerances, etc. 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 grooves, or 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 on their surface, or if the coating peels off – then the seals will suffer from deformation, scratches, being crushed, or not being compressed properly. As a result, they lose their sealing function, and the parts themselves end up having inherent leakage points that cause leaks after assembly or during use. Rough system assembly, the lack of vibration reduction and isolation measures; operating the system under overpressure, failing to conduct timely inspections as required, and not replacing worn-out components when their lifespan expires – all of these can lead to system leaks. (4) Excessively high oil temperature: When the hydraulic system is in operation, it is necessary to control the oil temperature. If the temperature is too high, the following problems may occur: ① In most cases, when the oil temperature frequently exceeds 60°C, the viscosity of the oil **decreases. This not only results in a thinner lubricating film, thereby increasing friction and wear, but it also causes the sealing rings to expand, age, and fail, leading to leaks in the hydraulic system. Research shows that for every 10°C increase in oil temperature, the lifespan of seals is reduced by half. ②As the oil temperature rises, its volume expands and the pressure increases; leakage also increases as pressure rises, thereby leading to more leakage. ③An increase in oil temperature causes changes in the expansion and contraction of the gap. Generally, due to differences in the materials of the mating parts, their thermal expansion coefficients vary. As the oil temperature rises, these parts expand and contract at different rates, which causes changes in the clearance between them. When this clearance increases, leakage tends to increase as well. (5) Pressure surges in the hydraulic system: Frequent direction changes in the hydraulic system, as well as the sudden start of the oil pump under high pressure or the rapid closing of valves and cylinders, can cause instantaneous peak pressures several times higher than the operating pressure. Sometimes these pressures are sufficient to damage sealing devices, pipes, or other hydraulic components, leading to leaks. The factors causing leaks in the hydraulic systems of construction machinery due to leakage control measures are the result of multiple interrelated influences. With current technologies and materials, it is very difficult to completely eliminate leaks in hydraulic systems. Only by considering the above factors that affect hydraulic system leakage and taking reasonable measures to minimize it can we achieve this. (1) In the design and manufacturing stages, due consideration must be given to the important factors affecting leakage, namely the design and manufacturing of the sealing groove. The Society of Automotive Engineers (SAE) recommends the following two solutions to prevent oil port leaks. ①SAE straight-thread \"O\"-ring oil port: Sealing is achieved through the \"O\"-ring, while connection is accomplished via straight threads; ② SAE4 bolt flange: Suitable for larger oil ports. Additionally, the selection of seals is also very important. The roughness of sealing surfaces is typically Ra3.2–Ra1.6 for static seals, and Ra0.8–Ra0.4 for dynamic seals. Failing to comprehensively consider the factors affecting leaks from the outset will result in immeasurable losses in future production. (2) In terms of pollution control for hydraulic oil, it is necessary to address the sources of pollution, 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 the hydraulic cylinder, certain protective measures can be taken. (3) Assembly aspect. Before assembling the hydraulic system, it is necessary to test the pressure resistance of the components and check for any external leaks at their joints; if leaks are detected, appropriate measures must be taken. Only when all components are leak-free and airtight can they be officially installed in the system for use. When assembling hydraulic components and hydraulic systems, thorough cleaning is required, including the removal of burrs and welding slag, as well as appropriate anti-rust treatment. Assembly must be carried out in strict accordance with the specified procedures, to prevent gap distortions caused by assembly, as well as abrasive particles resulting from burrs, welding slag, and rust. (4) Reduce hydraulic shock. Common methods include: ① Minimizing the frequency of system startup, shutdown, and direction changes; ② For hydraulic systems that require frequent direction changes, using directional control valves equipped with dampers whenever possible; ③ Installing accumulators in front of the control valves to reduce the distance over which shock waves travel, thereby mitigating hydraulic shocks; ④ Increasing the diameter of the pipes, shortening their length as much as possible, or using rubber hoses; ⑤ Installing safety valves that can relieve pressure when it rises, thus reducing pressure shocks. (5) Machinery is prone to leakage during the running-in period, as some casting and machining defects are difficult to detect during assembly and debugging. However, due to vibrations and impacts during operation, such defects become apparent; therefore, attention should be paid to leakage during this period, so that it can be detected early and addressed promptly. (6) Technically speaking, foreign countries are producing new types of oil seals equipped with sensors for measuring leakage from rotary seals; pneumatic seals that combine the advantages of friction seals and labyrinth seals have also been developed for spindles. At the same time, combined seals such as the “Panseal” – which consists of a U-shaped rubber component and a V-shaped spring – and can be used in hydraulic systems with pressures up to 45 MPa, are becoming very popular. Conclusion: There are various forms of leakage in hydraulic systems, and this is a common phenomenon in hydraulic equipment. The causes are numerous, and it is difficult to eliminate such leakage; therefore, it is necessary to address the issue carefully and thoroughly from aspects such as design, manufacturing, installation, and operation. The above is our analysis of the forms and causes of leakage in hydraulic transmission, based on what we have learned from courses and by consulting relevant materials, as well as on our own professional experience, engineering training, and observations and thoughts from daily life; we have also proposed some measures to control leakage.