In hydraulic systems, leakage affects product quality and is a critical issue that must be considered. For example, in hydraulic cylinders, severe leakage not only pollutes the surrounding environment but also reduces the pressure in the working chamber, preventing the hydraulic cylinder from functioning properly. Adopting advanced methods to effectively prevent leakage and achieve "zero leakage" in hydraulic systems has been a long-standing goal of the hydraulic industry. Furthermore, accurately analyzing the root causes of hydraulic system leakage can help us promptly eliminate leakage failures. As mechanical engineering students, through our study of the course "Hydraulic and Pneumatic Transmission" and consultation of relevant materials, combined with our professional internships, engineering training, and observations from daily life, we have analyzed the forms and causes of leakage in hydraulic transmission concerning common leakage failures and proposed measures to control leakage.

Compared to mechanical transmission, hydraulic transmission is a new technology, originating from the principle of hydrostatic transmission proposed by Pascal in 1654. It is a transmission form that uses liquid as the working medium and transmits energy through energy conversion devices. Hydraulic transmission has the following advantages: ① The working fluid can be transported to any location through pipelines; ② The arrangement of the actuator is not limited by the orientation; with the connection of oil pipes, the transmission mechanism can be arranged conveniently and flexibly; ③ Hydraulic transmission can convert the rotary motion of the prime mover into linear motion; ④ Stepless speed regulation can be easily achieved; ⑤ Load control, speed control, and direction control are easy to achieve, and centralized control, remote control, and automatic control are also easy to implement; ⑥ Hydraulic transmission is smooth and vibration-free; ⑦ Good lubrication conditions can improve the reliability and service life of hydraulic components; ⑧ Hydraulic components are conducive to standardization, serialization, and generalization. Therefore, hydraulic transmission has been widely used in various sectors of the national economy.

However, hydraulic transmission also has some disadvantages: ① There are resistance losses in the flow of liquid, oil leakage, and mechanical friction, so the efficiency is relatively low; ② Higher requirements for controlling the operating temperature; ③ Due to the leakage and compressibility of the working fluid, the rigidity of the hydraulic system is poor, making it impossible to guarantee a strict transmission ratio; ④ Very strict requirements for the use and maintenance of the working fluid; ⑤ Higher cost of hydraulic components; ⑥ Fault diagnosis and handling of hydraulic systems are more difficult, requiring higher technical level and professional knowledge of the personnel. Among them, leakage of the working fluid has always been an unavoidable problem, and its solution is also one of the focuses of research in various industries.

Leakage Forms

Leakage can be divided into internal leakage and external leakage according to the flow direction. External leakage mainly refers to the leakage of hydraulic oil from the system to the environment, occurring outside the hydraulic pipelines, hydraulic valves, hydraulic cylinders, and hydraulic pumps (hydraulic motors) of the hydraulic system; internal leakage refers to the leakage of hydraulic oil from the high-pressure side to the low-pressure side within the system due to the pressure difference between the high-pressure and low-pressure sides and the failure of sealing components, such as the leakage of oil from the high-pressure cavity to the low-pressure cavity in hydraulic transmission; leakage from the high-pressure channel of the directional valve to the return oil channel, etc. The main forms of leakage include gap leakage, porous leakage, adhesion leakage, and dynamic leakage.

1. Gap Leakage

There are two main types of gap leakage in the hydraulic systems of engineering machinery: leakage at fixed seals (static mating surfaces) and leakage at moving seals (dynamic mating surfaces). The leakage points at fixed seals mainly include the joint between the hydraulic cylinder cover and the cylinder barrel; the moving seals mainly include the hydraulic cylinder piston and the inner wall of the cylinder barrel, and between the piston rod and the guide bushing of the cylinder cover. The amount of gap leakage is related to factors such as pressure difference and gap.

2. Porous Leakage

In hydraulic components, various cover plates, due to the influence of surface roughness, cannot be in complete contact with each other. In the microscopic recesses where the two surfaces do not contact, many gaps of various cross-sectional shapes and sizes are formed. The cross-sectional size of the gaps is related to the surface roughness. In porous leakage, the liquid needs to flow through many curved gaps. When conducting sealing performance tests, a certain pressure holding time is required before the leakage can be revealed.

3. Adhesion Leakage

Viscous liquids have a certain degree of adhesion to solid arms. After contact, a thin layer of liquid adheres to the solid surface. If the film on the solid surface is thicker, the oil film will be scraped off by the sealing ring due to relative motion, resulting in adhesion leakage. The basic method to prevent adhesion leakage is to control the thickness of the liquid adhesion layer.

4. Dynamic Leakage

On the sealing surface of the rotating shaft, if there are traces of spiral processing, when the shaft rotates, the liquid will flow along the grooves of the spiral traces under the action of the centrifugal force of the rotating shaft. If the direction of the spiral traces is consistent with the direction of rotation of the shaft, due to the "pumping oil" effect of the spiral traces, dynamic leakage will occur. The characteristic of dynamic leakage is that the higher the rotational speed of the shaft, the greater the amount of leakage. To prevent dynamic leakage, avoid the existence of processing marks with a "pumping oil" effect on the sealing surface of the rotating shaft and the lip of the sealing ring, or use the principle of dynamic leakage to pump the leaked oil back using the pumping oil effect of the spiral traces to prevent leakage.

Causes of Leakage

The causes of leakage in hydraulic systems are numerous and complex. This article mainly summarizes and analyzes them from the aspects of seal leakage, working fluid contamination, component manufacturing and assembly precision exceeding tolerances, oil temperature rise and heat generation, and hydraulic system pressure impact.

(1) Seals: In hydraulic cylinders, leaks are first considered to be related to seals, because seals are the main components in hydraulic cylinders that prevent leaks. They mainly include YX-type sealing rings, combination sealing rings, U-type sealing rings, V-type sealing rings, O-rings, gland rings, and PTFE seals, etc. Among them, YX-type sealing rings, combination sealing rings, and U-type sealing rings mainly rely on the pressure of the pressure oil to generate tension, causing the inner and outer lips to open to achieve sealing, and the linear quality requirements for the inner and outer lips are very high; V-type sealing rings, O-rings, gland rings, and PTFE seals mainly rely on compression to seal. The physical and chemical properties and other quality aspects of the seals directly affect the quality of hydraulic cylinder products. The main reasons for seal leakage are as follows: ① Problems with seal quality. The seal material is inferior, and the manufacturing process and precision do not meet the requirements, and there are defects in the mold and trimming, etc. ② Unreasonable selection of seals. The selected seals cannot meet the requirements of working pressure, working speed, and temperature, or the type of seal is improperly selected. ③ Improper design of the seal installation groove, unreasonable selection of installation clearance and compression; the processing precision and surface roughness of mating parts do not meet the requirements; the lip is damaged during installation without protection or foreign objects are mixed in; improper temperature and humidity during storage, deterioration due to oxygen and ozone corrosion, etc. ④ When bubbles in the liquid pass through the sealing lip, the bubbles are compressed to a fraction of their original size. When this bubble reaches the non-pressure side of the seal, it quickly releases energy, damaging the sealing lip. On the other hand, when bubbles containing a certain proportion of oil vapor reach a sufficiently high temperature due to pressure, they will spontaneously ignite, causing the support ring to burn or melt, and even causing local burning and carbonization of the seal.

(2) Oil Contamination: Includes gas contamination, particulate contamination, and water contamination. ① Gas contamination: Under atmospheric pressure, about 10% of air can be dissolved in hydraulic oil. Under high pressure in the hydraulic system, more air or gas will be dissolved in the oil. Air forms bubbles in the oil. If the hydraulic support rapidly changes pressure between high and low pressure in a very short time during operation, it will cause the bubbles to generate high temperature on the high-pressure side and burst on the low-pressure side. If there are dents and damage on the surface of the hydraulic system components, the hydraulic oil will rush to the surface of the components at high speed, accelerating surface wear and causing leakage. This type of corrosion caused by cavitation is called cavitation erosion. ② Particulate contamination: Due to particulate contamination of the test bench, the probability of leakage is very small, but it still exists in practice. During the use of the hydraulic cylinder, the particles generated due to the above reasons, some adhere to the sealing components, and some adhere to the moving surface, scratching the sealing surface of the sealing components, causing the sealing components to fail, and causing the hydraulic cylinder to leak. This fault can be eliminated by removing burrs that have not been cleaned, cleaning each component, and then replacing the sealing components; more serious ones will "pull the cylinder", and the elimination of this fault is more troublesome; even more serious ones can cause the cylinder to be scrapped. ③ Water contamination: Due to factors such as a humid working environment, water may enter the hydraulic system. Water will react with the hydraulic oil to form acidic substances and oil sludge, reducing the lubricating performance of the hydraulic oil and accelerating the wear of parts. Water will also cause the valve stem of the control valve to stick, making the control valve difficult to operate, scratching the seals, and causing leakage.

(3) Exceeding Component Manufacturing and Assembly Precision: All hydraulic components and sealing components have strict requirements for size tolerances, form and position tolerances, etc. If the tolerance is exceeded during manufacturing, for example: the piston radius of the oil cylinder, the depth or width of the sealing groove, the hole for installing the sealing ring and the size exceed the tolerance, or the roundness is lost due to processing problems, there are burrs or depressions, the plating peels off, etc., the seal will deform, scratch, be pressed to death, or not be pressed firmly, causing it to lose its sealing function, and the part itself will have an innate leakage point, causing leakage during assembly or use. Rough system assembly, lack of vibration damping and isolation measures; system overpressure use, failure to check the system in a timely manner according to regulations, and failure to replace easily damaged parts when their service life expires will all cause system leakage.

(4) Excessive Oil Temperature Rise: When the hydraulic system is working, the oil temperature must be controlled. If the temperature is too high, the following phenomena will occur: ① In most cases, when the oil temperature frequently exceeds 60℃, the viscosity of the oil will greatly decrease, not only making the lubricating oil film thinner, increasing friction, and increasing wear, but also causing the sealing rings to expand, age, and fail, resulting in leakage in the hydraulic system. Studies have shown that the life of the seal is halved for every 10℃ increase in oil temperature. ② After the oil temperature rises, the volume expands, and the pressure increases, and the leakage increases with the increase in pressure, thus increasing the leakage. ③ The oil temperature rise causes changes in the gap expansion and contraction. Generally, the materials of the mating parts are different, and the thermal expansion coefficients of the materials are different. When the oil temperature rises, the mating parts will change due to different expansion and contraction, causing the mating clearance to change. When the clearance becomes larger, the leakage increases.

(5) Hydraulic System Pressure Shock: Frequent shifting in the hydraulic system, sudden starting of the oil pump or closing of valves under high pressure, and rapid movement of the cylinder body can all cause instantaneous peak pressures several times higher than the working pressure, sometimes enough to damage the sealing device, pipeline, or other hydraulic components, causing leakage.

Leak Control Measures

The factors causing leakage in the hydraulic system of engineering machinery are the result of multiple comprehensive influences. With existing technology and materials, it is difficult to fundamentally eliminate leakage in the hydraulic system. Only by starting from the above factors affecting hydraulic system leakage and taking reasonable measures can hydraulic system leakage be minimized.

(1) In the design and processing stages, it is necessary to fully consider the important factors affecting leakage, the design and processing of sealing grooves. The Society of Automotive Engineers (SAE) recommends the following two solutions to prevent oil leakage from oil ports. ① SAE straight thread "O"-ring oil port: Sealing relies on the "O"-ring, and connection relies on straight threads; ② SAE 4-bolt flange: Suitable for larger oil ports. In addition, the selection of seals is also very important. The surface roughness of the sealing surface is usually Ra3.2～Ra1.6 for static seals and Ra0.8～Ra0.4 for dynamic seals. If the influencing factors of leakage are not fully considered at the beginning, it will bring immeasurable losses to future production.

(2) In the control of hydraulic oil contamination, it is necessary to start from the source of contamination, strengthen the control of pollution sources, and also adopt effective filtration measures and regular oil quality checks. In order to effectively cut off the pollution of external factors (water, dust, particles, etc.) to the hydraulic cylinder, some protective measures can be added.

(3)Assembly aspects. Before assembling the hydraulic system, the pressure resistance performance of the components should be tested, and the sealing surfaces of the components should be checked for any external leakage. If leakage occurs, corresponding measures should be taken. Only when all components are leak-free can they be officially installed and used in the system. When assembling hydraulic components and hydraulic systems, strict cleaning, deburring, removal of weld spatter, and necessary rust prevention treatment should be carried out. Assembly should be carried out strictly in accordance with the assembly process requirements to prevent gaps distortion and abrasive materials such as burrs, weld spatter, and rust from being generated due to assembly.

(4)Reducing hydraulic impact. Common methods include: ①Minimizing the system's start, stop, and shifting frequency; ②For hydraulic systems that require frequent shifting, using directional valves with dampers as much as possible; ③Setting accumulators in front of the control valves to reduce the distance of shock wave propagation and thereby mitigate hydraulic impact; ④Increasing the pipe diameter, shortening the pipe length as much as possible, or using rubber hoses; ⑤Setting safety valves, which can relieve the load and reduce pressure surges when the system pressure increases. (5)Mechanical leakage is prone to occur during the running-in period. This is because some casting and processing defects are difficult to detect during assembly and commissioning, but due to the vibration and impact during operation, these defects are exposed. Therefore, attention should be paid to leakage during the running-in period, so that it can be detected and dealt with early.

(6)From a technical point of view, new oil seals equipped with sensors for testing the leakage of test seals (rotary seals) are being produced abroad; and main shaft integrated pneumatic seals that combine the advantages of friction seals and labyrinth seals have also emerged; at the same time, combination seals such as "Pan-塞seals" composed of U-shaped rubber parts + V-shaped springs, which can be used in hydraulic systems with a pressure of 45 MPa, are widely used.

Conclusion

Leakage in hydraulic systems has various forms and is a common phenomenon in hydraulic equipment. There are many reasons for leakage, which are difficult to eliminate completely. It should be solved carefully from the aspects of design, manufacturing, installation and use. The above is our analysis of the leakage forms and causes of hydraulic transmission based on course learning, consulting relevant materials, and combining our professional internship, engineering training and observations and thoughts from daily life, and puts forward some measures to control leakage.

Keywords:Hydraulic cylinders，Cylinder Manufacturer，Jacking Cylinders，Prestressed Equipment