In order to solve the problem of vehicle steering resistance and reduce the driver's labor intensity, the hydraulic technology was first introduced in construction machinery Z, and hydraulic booster was introduced in the steering system. Due to the outstanding advantages of hydraulic booster in the application process and people's in-depth research on hydraulic components and hydraulic systems, it is found that hydraulic transmission also has the advantages of high power density, easy to realize linear motion, high speed stiffness, flexible configuration, convenient power transmission and control, so hydraulic technology has been widely used in construction machinery.

However, there are still some problems in the hydraulic system of domestic construction machinery, which affect the construction operation. The main problems are as follows:;

1. Serious leakage

Hydraulic system leakage not only causes oil resource loss, environmental pollution, shutdown loss, but also reduces system power. According to the statistics of the 1980s in Japan, oil leakage (only external leakage) accounted for 20%~30% of all failures of construction machinery, including 33% of hydraulic cylinder oil leakage, 23% of piping and 20% of hydraulic devices. Compared with foreign construction machinery, the oil leakage of construction machinery in China is more serious. "One line, one stop, one large area" vividly describes the current situation of construction machinery in China.

The main reasons for oil leakage are: during the operation of construction machinery, various parts of the piping often bear the periodic vibration caused by the rotation of the engine and pump, as well as the impact and vibration of external loads on the machine, resulting in the loosening or fatigue fracture of the pipe joints and oil leakage; in addition, the poor working environment of construction machinery makes the piston rod frequently exposed to dust, soil, wind and rain, salt spray, etc, This causes premature wear of the sealing surface of the hydraulic cylinder and oil leakage. In the future, construction machinery will develop towards large-scale and high-pressure, so leakage prevention and control is still the main problem to be solved in the hydraulic system of construction machinery in the future.

2. High noise

Noise makes people feel uncomfortable, even upset. As a kind of pollution, noise has been paid more and more attention. The high pressure of the hydraulic system will inevitably lead to noise, and become the main factor hindering the further improvement of the power density of the hydraulic system of construction machinery.

Hydraulic system noise is divided into fluid noise and mechanical noise, of which fluid noise accounts for a considerable proportion.

Fluid noise is caused by sudden changes in oil flow rate, pressure and cavitation. For example, the conversion between the working chamber and the suction chamber of the hydraulic pump causes the noise caused by the sharp change of the pressure in the chamber; When the system pressure of the air dissolved in the hydraulic oil is lower than the air separation pressure, a large number of bubbles are rapidly separated to form. When these bubbles encounter high pressure, they are crushed to produce a strong noise caused by hydraulic impact; In addition, the high-pressure fluid ejected from the valve port can produce high-frequency sound.

Mechanical noise is mainly caused by contact, impact and vibration between parts. If the hydraulic pump and hydraulic motor rotate unbalanced, periodic unbalanced force will be generated, which will cause bending vibration of the shaft and noise. This vibration will make a loud noise when it is transmitted to the oil tank and pipeline.

Up to now, the vibration and noise problems caused by increasing the working pressure of the hydraulic system of construction machinery have not been fundamentally solved, making it difficult to further improve the power density of the hydraulic system.

3. The hydraulic system is seriously polluted

According to statistics, 70%~80% of the hydraulic mechanical failures are caused by the hydraulic system, and 70%~

85% is caused by dirty hydraulic oil. Therefore, since the mid 1970s, people have been taking reducing the pollution of the hydraulic system of construction machinery and improving the reliability of the system as a major research topic.

The pollutants in the hydraulic system of construction machinery are divided into three categories: assembly pollutants, invasion pollutants and generated pollutants. The assembly pollutants can be controlled during the manufacturing, assembly and commissioning of the manufacturer; Invasive pollutants and generated pollutants are mainly generated in the process of equipment use, which depends on the operating environment, maintenance and servicing level of construction machinery. For example, wear of hydraulic component movement pairs and gearbox friction plates, aging and damage of seals will produce pollutants of different shapes, causing pollution to the hydraulic system; In addition, because construction machinery has been working in the harsh field environment for a long time, and many maintenance is also carried out in this environment, sediment, water, dust, etc. in the environment invade the hydraulic system and cause pollution. Therefore, the generation and invasion of pollutants are the main reasons for the pollution of the hydraulic system of construction machinery.

4. Frequent hydraulic impact

The load of construction machinery changes frequently when working, sometimes with large changes. The large changes in the load cause the fluid flow in the hydraulic system to quickly turn or stop, which will cause the pressure in the system to crack dramatically, forming an instantaneous pressure peak and producing hydraulic impact. For example, the vibration of the vibratory roller, the sudden reversal of the hydraulic cylinder of the cement concrete pump truck, and the braking of the rotary hydraulic motor of the hydraulic excavator will produce hydraulic shocks.

The peak pressure of hydraulic impact is often several times higher than the normal working pressure, and is often accompanied by huge vibration and noise, which causes some hydraulic components (such as pressure relay, hydraulic control valve, etc.) to misoperate, leading to equipment damage. More commonly, it breaks through the oil circuit of hydraulic seals, causing leakage, making the system unable to work normally.

The leakage, pollution and hydraulic impact of the hydraulic system of construction machinery are important factors affecting the reliability and performance stability of the hydraulic system. In terms of leakage prevention, new materials and processes can be introduced, such as engineering plastic, composite materials, fine ceramics, low resistance wear-resistant materials, high-strength light alloys, memory alloys and other new generation materials, to improve the quality of hydraulic components and sealing components, and reduce component damage caused by adhesion, scratches, cavities, and turbidity. The pollution of the hydraulic system of construction machinery has a great relationship with the use environment, so it is very important to strengthen the daily maintenance management of construction machinery while continuously strengthening and improving the filtering technology. To reduce the hydraulic impact, the first thing is to reasonably select and match the components in the system design, so as to avoid the hydraulic impact of the system as much as possible. For example, throttle valves are used to limit the flow rate of the pipeline, and buffer devices such as accumulators are added in the pipeline. For systems where hydraulic shock is unavoidable, a quick response relief valve should be selected to limit the peak pressure. Only by solving the above problems step by step, can the hydraulic system play a better role in construction machinery.