The hydraulic transmission system has the advantages of small size, light weight, large specific power, smooth operation, stepless speed regulation, etc. It is widely used in various cranes. However, the transmission efficiency of the hydraulic transmission system is lower than other transmission systems, and it is easy to produce faults and it is difficult to analyze and eliminate the faults. Therefore, the maintenance personnel are required to have certain basic knowledge of the hydraulic system.


There are two main types of leakage in the hydraulic system of construction machinery. The leakage at the fixed seal and the leakage at the moving seal. The leakage at the fixed seal mainly includes the bottom of the cylinder and the connection of various pipe joints. The movement seal mainly includes the piston rod of the oil cylinder. Multi-way valve stem and other parts. The leakage of oil can also be divided into external leakage and internal leakage. External leakage mainly refers to the leakage of hydraulic oil from the system to the environment. Internal leakage refers to the existence of the pressure difference between the high and low pressure sides and the failure of seals. Make the hydraulic oil flow from the high pressure side to the low pressure inside the system


All hydraulic components and sealing components have strict dimensional tolerances, surface treatment, surface finish and geometric tolerances. If it is out of tolerance in the manufacturing process, such as: the piston radius of the cylinder, the depth or width of the sealing groove, the hole size of the sealing ring is out of tolerance or due to processing problems, it is out of round, has burrs or pits, chrome plating, etc., seal There will be deformation, scratching, crushing or pressing, which will make the seal lose its function.

The hydraulic components should be free from brutal operation during assembly. If excessive force will be used to deform the parts, especially knocking the cylinder block, sealing flange, etc. with copper rods, etc.; the parts should be carefully checked before assembly. The oil is lightly pressed in, and diesel oil is used for cleaning, especially rubber components such as sealing rings, dust rings, O-rings, etc. If gasoline is used, it will easily age and lose its original elasticity, thus losing its sealing function. The reliability of the hydraulic system depends to a large extent on the design of the hydraulic system seal and the selection of seals. Since the design of the seal structure is unreasonable in the design, the selection of the seals is not in accordance with the specifications. Compatible types of sealing materials, load conditions, ultimate pressure, working speed, changes in ambient temperature, etc. These all directly or indirectly cause hydraulic system leakage to varying degrees. In addition, due to the dust and impurities in the use environment of construction machinery, appropriate dust seals should be selected in the design to prevent dust and other dirt from entering the system to damage the seal and contaminate the oil, thereby causing leakage. The design considers that the geometric accuracy and roughness of the moving surface are not comprehensive, and the strength check of the connection part is not carried out in the design, which will cause leakage in the mechanical work.




At atmospheric pressure, about 10% of the air can be dissolved in the hydraulic oil, and under the high pressure of the hydraulic system, more air or gas will be dissolved in the oil. Air forms bubbles in the oil. If the hydraulic support changes rapidly between high and low pressure in a very short time during operation, the bubbles will generate high temperature on the high pressure side and burst on the low pressure side. If the components of the hydraulic system When there are pits and damage on the surface, the hydraulic oil will rush to the surface of the component at high speed to accelerate the surface wear and cause leakage. As the main actuator of some engineering machinery hydraulic systems, the hydraulic cylinder is directly exposed to the environment due to the exposed piston rod. Although the guide sleeve is equipped with a dust ring and seals, it is inevitable to bring dust and dirt. Into the hydraulic system, accelerate the scratches and wear of seals and piston rods, etc., causing leakage, and particle contamination is one of the fastest damaging factors for hydraulic components.


The seal is made of oil-resistant rubber and other materials. Due to long-term use, aging, cracking, damage, etc. will cause system leakage. If the part is damaged by collision during work, it will scratch the sealing element and cause leakage.