Failure analysis of hydraulic system

Hydraulic oil selection and hydraulic system failure analysis
Date of issue: 2017/10/13
Hydraulic oil is a kind of industrial lubricating oil with large amount and wide application. Hydraulic oil is widely used in metallurgy, mining, construction machinery, automobile, aircraft, transport vehicles, machine tools and other hydraulic systems. The demand for hydraulic oil is about 10 million tons/year, accounting for half of industrial lubricants. At present, the annual consumption of hydraulic oil in China is about 300,000 tons, of which anti-wear hydraulic oil is about 40,000 tons. Hydraulic fluid is a vital component of any hydraulic system. Its function is to efficiently transfer energy, lubricate components and act as a cooling medium. Whether the hydraulic system can work reliably, sensitively, accurately, effectively and economically is closely related to the type and performance of hydraulic oil used. Therefore, the correct selection of hydraulic oil is to ensure the normal and long-term work of the hydraulic system. When the hydraulic system failure, find out the reason in time, take the correct solution is to protect the equipment, avoid causing heavy losses of important measures.

Selection of hydraulic oil

Because the hydraulic transmission has the advantages of small volume, light weight, smooth transmission, reliable work, convenient operation, and easy to achieve stepless transmission, it is used in many industrial departments of the transmission system. Different industrial departments due to the use of requirements, operating conditions, the difference in the application environment, the hydraulic transmission system used is also very different. It is an effective measure to ensure continuous production, save material consumption and improve economic benefit to correctly select hydraulic oil varieties and ensure long-term stable and safe operation of hydraulic system. 

According to ISO 6743/4-1982, classification of Lubricants, industrial lubricants and related products (Part 4), Group H hydraulic systems are divided into hydrostatic system hydraulic oil, hydraulic guide oil, non-combustible hydraulic oil and fluid hydraulic system according to different applications. In 1999, ISO issued the ISO 6743-4:1999 (E) classification, adding a new category of environmentally acceptable hydraulic fluids. China issued GB 7531.2-87 hydraulic oil classification standard in 1987, equivalent to iso6743/4-1982 classification method. Table 1 lists the ISO and China’s hydraulic oil classification, code and corresponding product quality standards.

According to different application occasions should choose different types of hydraulic oil varieties, plus the type of hydraulic pump, working temperature and pressure, operating conditions and the surrounding environment is different, the selection of hydraulic oil is a careful and requires a certain oil knowledge of the work. According to statistics, the majority of the failure of hydraulic system operation is caused by the improper selection and use of hydraulic oil. Therefore, the correct selection and reasonable use of hydraulic oil is of great significance to the reliability of hydraulic equipment operation, prolong the service life of the system and components, ensure the safety of equipment and prevent the occurrence of accidents. In particular, the hydraulic system to reduce the volume, reduce weight, increase power, improve efficiency, increase reliability and environmental friendly direction of development, the correct selection of hydraulic oil is more important.

2.1 Selection principle of hydraulic oil

General hydraulic equipment manufacturers specify the varieties, grades and viscosity levels of hydraulic oil used by the equipment system in the equipment instruction manual or operating manual. Users should first choose hydraulic oil according to the recommendation of the equipment manufacturer. However, there are also many occasions where the working conditions and use environment of the system used by users differ from those specified by the equipment manufacturer, and the hydraulic oil needs to be selected by themselves. Generally, it can be selected according to the following principles:

(1) No type of hydraulic oil should be selected for the system. This should be determined according to the working condition and working environment of the system.

(2) Secondly, we should determine what viscosity level of hydraulic oil the system should choose.

(3) Understand the performance of the selected hydraulic oil.

(4) Know the price of the product.

2.2 Selection of hydraulic oil varieties

(1) Select hydraulic oil varieties according to the working pressure and temperature of the hydraulic system.

(2) Select hydraulic oil varieties according to the working environment of the hydraulic system.

(3) Select hydraulic oil varieties according to special performance requirements.

2.3 Selection of hydraulic oil viscosity

After selecting the right variety, it is also necessary to determine what viscosity level of hydraulic oil to use to enable the hydraulic system to work in a superior state. Although the viscosity is too high for lubricity, but increase the resistance of the system, pressure loss increases, resulting in increased power loss, oil temperature rise, hydraulic action instability, noise. High viscosity will also cause difficulty in oil absorption when starting at low temperature, and even cause interruption of oil supply and equipment failure when starting at low temperature. On the contrary, when the viscosity of the hydraulic system is too low, it will increase the internal and external leakage of hydraulic equipment, the working pressure of the hydraulic system is unstable, the pressure is reduced, and the hydraulic working parts are not placed, which will lead to increased wear and tear of the pump.

The choice of viscosity level first depends on the type of pump, each type of pump has its best range of viscosity: blade pump for 25 ~ 68mm2/S, plunger pump and gear pump are 30 ~ 115mm2/S. The ultra-small working viscosity of vane pumps should not be less than 10mm2/S and the ultra-large starting viscosity should not be greater than 700mm2/S. The ultra-small working viscosity of the plunger pump should not be less than 8mm2/S, and the ultra-large starting viscosity should not be greater than 1000mm2/S. Gear pump requires a large viscosity, ultra-small working viscosity should not be less than 20mm2/S, large starting viscosity can reach 2000mm2/S. 

The choice of viscosity level should also consider the working condition of the pump, the use of high temperature and pressure hydraulic system to choose higher viscosity hydraulic oil, can obtain better lubricity, on the contrary, the temperature and pressure is low, should choose lower viscosity, so as to save energy. In addition, it should be considered that the working viscosity of hydraulic oil at ultra-low temperature of the system should not be greater than the super viscosity of the pump. Table 3 lists viscosity options for various pumps at different pressures and temperatures. 

With the development of hydraulic technology, hydraulic system is increasingly widely used in various industrial sectors. Therefore, it is very important to ensure the normal action of the hydraulic system and increase the reliability of the work. The correct selection of hydraulic oil varieties and viscosity is the first condition to ensure the long-term and reliable work of the system. On the other hand, it is also very important to find the cause and solve the hydraulic system failure quickly. Hydraulic equipment is composed of mechanical, hydraulic, electrical and instrument devices, so in the analysis of hydraulic system failure must first understand the transmission principle of the whole hydraulic system, structural characteristics and components and material configuration. Because the hydraulic system is a sealed pressure system, the flow of oil in the pipeline, the action of the parts inside the hydraulic components and whether the seal is damaged are not easy to observe, so it is difficult to analyze the cause of the failure and determine the location of the failure.

There are many reasons for the failure of the hydraulic system, summed up in three aspects: one is the mechanical failure of the equipment, which includes the unreasonable design of the hydraulic system, the incorrect installation clearance, the quality of the hydraulic components, the improper selection of seals, etc., the hydraulic system failure caused by these problems generally has no relationship with the hydraulic oil. Secondly, the error caused by the hydraulic system failure, this refers to the hydraulic system in normal operation due to the operator caused by improper operation, such as error opening and closing of the valve, suddenly interrupted power supply, high operating temperature and pressure, and the wrong during oil filling oil, fuel tank oil level too high or too low, not from the bottom of the tank in time out the separation of water and so on. Hydraulic system failures caused by operating errors are not caused by hydraulic oil quality problems, most do not involve hydraulic oil, but some can be reflected from the quality change of hydraulic oil. Three is due to the quality of hydraulic oil caused by hydraulic system failure, which is mostly due to improper oil selection or the use of unqualified oil, may also be the use of hydraulic oil for a long time, not to replace the new oil caused by. This paper only from the quality of hydraulic oil and the use of the problems of the hydraulic system fault analysis. 3.1 Automatic shutdown of hydraulic oil system when the oil temperature is too high 3.2 Unstable or insufficient pressure of the hydraulic system 3.3 Influence of water inlet in the hydraulic system 3.4 Influence of air mixing in the hydraulic oil 3.5 Particle pollution in the hydraulic system 3.6 Replacement and cleaning of hydraulic oil mixed with other oil in the hydraulic system The hydraulic oil will certainly deteriorate in use. Continuing to use the changed hydraulic oil will cause the hydraulic system to work improperly, wear and corrosion of parts, and serious accidents will be caused. On the contrary, if the hydraulic oil has not deteriorated on the replacement, not only cause waste, but also need to stop the oil, increase unnecessary expenses. Therefore, it is an important means to protect equipment and reduce costs to understand the quality of hydraulic oil in use and replace it in time when it has obvious deterioration. The deterioration of hydraulic oil includes the change of composition and properties of oil, mainly the oxidative deterioration of hydraulic oil, the volatilization of light fractions, additive loss and oil pollution, all of which will change the performance of hydraulic oil and affect the normal use.

In order to correctly grasp the replacement time of oil products in use, China has formulated the standard and industry standard for oil change index of various commonly used lubricating oils.

Post time: Oct-27-2021