According to the excavator working device and the transmission requirements of each mechanism, the combination of various hydraulic components and pipes is organically connected, which is called the hydraulic system of the excavator. Its function is to use hydraulic fluid as the working medium, use the hydraulic pump to convert the mechanical energy of the engine into hydraulic energy and transmit it, and then transfer the hydraulic energy back to mechanical energy through hydraulic cylinders and hydraulic motors to realize various actions of the excavator.
The operation of the hydraulic excavator is complicated. The mechanism often starts, brakes, reverses, the load changes greatly, the impact and vibration are frequent, and the field operation, temperature and geographical position change greatly, so according to the working characteristics and environmental characteristics of the excavator, The hydraulic system should meet the following requirements:
1) Make sure that the excavator boom, stick and bucket can be operated separately or together to achieve a composite action.
2) The operation of the working device and the rotation of the turntable can be performed separately or in combination to increase the productivity of the excavator.
3) The left and right crawlers of the crawler excavator are driven separately, which makes the excavator easy to travel, flexible to turn, and can be turned on the spot to improve the flexibility of the excavator.
4) Ensure that all actions of the excavator are reversible and steplessly variable.
5) Ensure that the excavator works safely and reliably, and each actuator (hydraulic cylinder, hydraulic motor, etc.) has good overload protection; the slewing mechanism and the traveling device have reliable braking and speed limit; prevent the boom from falling due to its own weight. And the whole machine speeding down the slope.
To this end, the hydraulic system should:
1) High transmission efficiency to fully utilize the engine's power and fuel economy.
2) The hydraulic system and hydraulic components have sufficient reliability under the influence of large load change and sharp vibration shock.
3) Adjust the light and vibration-resistant cooler to reduce the total heat generated by the system, so that the hydraulic oil temperature does not exceed 80 degrees when the main engine is continuously working, or the temperature rise does not exceed 45 degrees.
4) Due to the dust on the excavator work site, the hydraulic oil is easily contaminated. Therefore, the sealing performance of the hydraulic system is good, the sensitivity of the hydraulic components to oil contamination is low, and the oil filter and dustproof device are required for the entire hydraulic system.
5) Hydraulic or electro-hydraulic servo control device is adopted, so that the excavator can set the automatic control system, thereby improving the technical performance of the excavator and reducing the labor intensity of the driver.
According to the characteristics of hydraulic pump, the hydraulic system used in hydraulic excavators generally has three types: quantitative system, variable system and quantitative and variable system.
1) Quantitative system
In the quantitative system used in hydraulic excavators, the flow rate is constant, that is, the flow does not change with the load, and the throttle is usually used to adjust the speed. According to the quantity and combination of oil pump and circuit in the quantitative system, it is divided into single pump single circuit, double pump single circuit quantitative system, double pump double circuit quantitative system and multi-pump multi-loop quantitative system.
2) Variable system
In the variable system adopted by the hydraulic excavator, the stepless speed regulation is realized by the volume variable, and there are three adjustment modes: variable pump-quantitative motor speed regulation, quantitative pump-variable motor speed regulation, variable pump-variable motor tuning speed.
The variable system used in the hydraulic excavator mostly adopts the combination of the variable pump and the quantitative motor to realize the stepless variable, and both are double pump and double circuit. According to the variables of the two loops, there are two types: the divided power variable system and the full power variable system. Each of the oil pumps of the sub-power variable system has a power regulating machine, and the flow change of the oil pump is only affected by the change of the pressure of the circuit in which it is located, and is independent of the pressure change of the other circuit, that is, the oil pumps of the two circuits are independently performed. Constant power regulation variable, two oil pumps each have a bucket engine output power; two oil pumps in the full power variable system are balanced by a total power adjustment mechanism, so that the swing angles of the two oil pumps are always the same, the synchronous variables and the flow rate are equal . It is the total pressure of the system that determines the flow change. The power of the two pumps is different within the variable range. The adjustment mechanism has two forms: mechanical linkage and hydraulic linkage.