[Abstract] This paper analyzes the causes of leakage in hydraulic system from three aspects and proposes corresponding solutions. [Key words] hydraulic system; oil leakage; solution

CLC number: TH137 Document code: A Article ID: 1009-914X(2015)44-0028-02

In general, the leakage of hydraulic systems is divided into internal leakage and external leakage. The flow of oil from the high pressure chamber of the hydraulic component to the low pressure chamber is referred to as internal leakage; the flow of oil from the interior of the hydraulic system to the outside of the system is referred to as external leakage. The external leakage will affect the safety and reliability of the system work, resulting in waste of oil, equipment stoppage, reducing productivity and increasing costs. Therefore, we must control and prevent leakage of the hydraulic system. The causes of leakage in the hydraulic system are complicated. This paper mainly analyzes the common leakage phenomena of hydraulic systems from the aspects of external leakage caused by hydraulic cylinders, pipelines and seal failures.

1 Hydraulic cylinder leakage

Hydraulic cylinders are used as actuators in hydraulic systems for easy reciprocating linear motion and are therefore widely used in rolling mill production lines. Due to the current dust, high temperature, vibration and other reasons, the working environment of the on-site hydraulic cylinder is bad, which often causes the hydraulic system to malfunction when working.

1.1 Oil leakage from the seal of the cylinder liner and cylinder head (or guide sleeve). The cylinder liner and the cylinder head of the production site hydraulic cylinder are mostly threaded, and then an O-ring or a combination pad is added at the threaded port. When oil leakage occurs here, most of them are caused by damage or aging of the O-ring or the combination pad. It is possible to solve the problem by replacing the appropriate O-ring or combination pad in time.

1.2 The relative movement between the piston rod and the guide sleeve leaks outward. In general, the piston rod here is sealed with an O-ring or a Y-ring. Since there is no lubrication between the piston rod and the seal, it is dry friction, and the frictional resistance is large, which tends to aggravate the wear of the seal. In addition, the outer surface of the piston rod is in contact with various types of impurities, which often causes scratches, grooves, pitting or corrosion on the surface of the piston rod, which exacerbates the wear of the seal. When the oil leakage problem occurs here, in addition to replacing the seal, the piston rod should be inspected for damage. If there is damage to the piston cylinder, it can be cleaned with gasoline. After drying, apply it to the damaged part with metal glue. Then use the piston rod oil seal to move back and forth on the piston rod, scrape off the excess glue, and then put it into use after the glue is fully cured.

1.3 The oil cylinder pipe joint is not tightly sealed and causes oil leakage. Almost all hydraulic systems are loosened due to shock and vibration during use. The oil leakage of the dry joint is mainly related to the processing precision, fastening strength and burr of the joint. Therefore, when there is oil leakage in the pipe joint, in addition to checking the sealing condition of the sealing ring, it should also be checked whether the joint is assembled correctly, whether it is reliably tightened and whether the contact surface is scratched, and if necessary, it should be replaced and repaired.

2 External leakage of pipes

There are many pipelines used in the hydraulic system. According to different working pressures and installation positions, we usually use carbon steel pipes, stainless steel pipes and rubber pipes. Pipeline oil leakage accounts for a large proportion of hydraulic system oil leakage failures, which is mainly due to improper installation of pipes. First of all, when the pipe joint is installed, if the tightening torque seriously exceeds the specified value, the joint bell mouth will be broken, the thread will be pulled and tripped, and oil leakage will occur. Secondly, when the hydraulic pipeline is installed, the inferior pipeline or the hose with insufficient pressure is selected, which is bound to cause pipeline leakage during the production process.

2.1 Poor bending of the pipeline

During the assembly process of the hard pipe, the pipe should be bent according to the bending radius, otherwise the pipe will have different bending internal stresses, and leakage will gradually occur under the action of oil pressure. Normally, the radius of the bend of the hard pipe should be 3-5 times larger than the outer diameter of the pipe. If the bending radius is too small, the outer wall of the pipe will become thinner, and the inner pipe wall will have wrinkles, which will cause a large internal stress in the pipe, and the strength will be greatly weakened. In the case of severe vibration and high pressure impact, the pipeline It is easy to produce transverse cracks and oil leakage. If the bending radius is too large, longitudinal cracks are easily generated and oil leakage occurs when the oil pressure is pulsating.

2.2 Pipeline environment is poor

During the working process of the hydraulic system, the pipeline is extremely susceptible to deformation due to environmental influences, resulting in oil leakage. Therefore, the hose should be installed with a 30% margin to accommodate the change in residual temperature, the tension and vibration of the hose. Rubber hoses should avoid high temperatures and corrosive gases and should be replaced as soon as they are found to be severely cracked, hardened or bubbling.

2.3 Unreasonable fixed installation causes pipeline leakage

In the process of installing the tubing, if the length of the pipeline, the angle, and the thread are not properly considered, the pipeline will be deformed and the installation stress will be generated. At the same time, it is easy to hit the pipeline and the strength is lowered. During installation and routine maintenance, pay attention to the fixing of the pipeline. Pipe clamps should be fixed properly. If it is too loose, friction will occur between the pipe and the pipe clamp, and the vibration will be strengthened, so that the pipe weld crack will cause oil leakage. If it is too tight, the surface of the pipeline will be pinched and deformed, causing damage to the pipeline.

3 Seal failure causes system oil leakage

There are many reasons for the failure of the seal. The structural design of the seal, the selection of the seal, the installation, the contamination of the system, and the increase in the temperature of the system oil can all cause failures in the sealing system. Common faults in hydraulic systems are mainly caused by the following reasons, see Table 1.

3.1 The sealing groove is too deep or too shallow, and the ellipticality and the taper of the sealing contact surface of the moving part are so poor that it is difficult to obtain a uniform and accurate compression amount after the O-ring is installed. For example, on the cylindrical surface, the annular groove of the shaft and the hole are easy to be deep or dig shallow so that the amount of compression of the O-ring is insufficient, so that oil leakage occurs. In the case of flat sealing, if the O-ring sealing groove is too shallow, it will cause the groove to not catch the O-ring during installation, and it is easy to escape, and it is even possible to squeeze the sealing ring between the joint faces, thereby causing oil leakage.

Generally, we follow this principle in the process of selecting the hydraulic system seal. The high pressure is large, and the low pressure is small. The static seal has a large amount of compression, and the dynamic seal has a small amount of compression. At the same time 3.2 seal failure caused by excessive oil temperature.

The hydraulic system inevitably causes a certain energy loss during the energy transfer process, which causes the system oil temperature to rise. Generally, the oil temperature control is ideal in the range of 30-50 °C. When the oil temperature reaches a high temperature of 80 ° C, the seal will age and fail early.

3.2.1 Reasons for high system oil temperature

(1) When the fuel tank area is small, the heat dissipation conditions are poor, and the heat dissipation system fails, only the question is raised.

(2) Improper pressure adjustment causes system oil temperature to rise

(3) The incorrect selection of the oil brand number causes the system oil temperature to be too high.

(4) The system oil temperature is high due to the friction heat generated by the actuator

3.2.2 Solution for excessive oil temperature in hydraulic system

(1) Reasonably design the fuel tank. In general, the effective volume of the starting hydraulic system tank is 3-5 times the pump flow per minute. In the case of inconvenient improvement of the tank volume, the system's circulating cooling capacity can be increased to reduce the system oil temperature.

(2) According to the system work requirements, adjust the system pressure reasonably.

(3) Reasonable selection of hydraulic oil.

(4) Reasonably design the unloading circuit.

3.3 Oil contamination causes seal hydrolysis or wear

3.3.1 Hydraulic systems are often contaminated in an open working environment.

(1) Product contamination

1 hydraulic oil oxidation metamorphic precipitate

2 hydraulic oil mixed with moisture and air

3 components wear, damage and generate pollutants

(2) Intrusion pollution

Mainly refers to the pollutants in the surrounding environment entering the hydraulic system through some possible intrusion points; in the maintenance process, care is taken to bring the pollutants into the cleaning process; during the refueling process, the pollutants are directly introduced without filtering.

(3) Mixing other oils into the hydraulic oil

Different types and different grades of hydraulic oil have different chemical compositions. When hydraulic oil is mixed with other oils, its chemical composition is changed, and its properties are changed.

3.3.2 Methods and measures for controlling oil pollution

In order to extend the service life of the seal, the degree of contamination of the hydraulic oil should be controlled within a certain range. Under normal circumstances, take the following measures to control pollutants

(1) Improve the quality of the seal, reduce the damage caused by the seal, and let the cooling water enter the hydraulic system through the hydraulic cylinder, while reducing the seal wear into the system.

(2) Improve the filtration accuracy of the filter element according to the working requirements of the hydraulic system. The operation of the filter element is tested, and the blockage is found and replaced in time.

(3) Regularly check the cleanliness of oil and control the cleanliness of oil

 4 Conclusion 

In industrial production, the external leakage of hydraulic system is a common phenomenon in hydraulic equipment, which directly affects the stability and reliability of hydraulic equipment. For the cause of the leakage, corresponding measures can be taken to reduce or eliminate the leakage to ensure stable operation of the equipment for a long period of time.

references 

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