Excavator is an earth moving machine that uses a bucket to excavate materials that are higher or lower than the bearing surface, and loads them into a transport vehicle or discharges them to a stockyard. There are many types of hydraulic systems for excavators, which can be classified according to different aspects. From the form of pump, there are quantitative pump system and variable pump system.
The quantitative pump system is still used in small hydraulic excavators. The excavator with a machine weight of about 13 tons adopts a quantitative system of about 50%. This system mostly uses an external gear pump, and the working pressure of the system is about 16MPa.
The main reasons for the application of the quantitative system in small and medium-sized excavators are as follows:
1) The quantitative pump has the advantages of simple structure, low price and reliable operation.
2) Because the hydraulic pump often works under non full load, the service life of the pump is longer than that of the variable displacement pump.
3) Since the flow is fixed, the speed of the actuator is also stable, and the trajectory of the working device is easy to control, the excavation surface is better.
In the quantitative system, because the flow is fixed, the flow cannot be changed correspondingly when the load changes. When the load is small, the operation speed cannot be improved and the power cannot be fully utilized. In order to meet the operation requirements, the engine of the quantitative system shall be determined according to the maximum external load and operation speed.
Generally, the average load of single pump quantitative system is about 60% of the maximum load. Therefore, the average engine power of the system is only about 60%. The movement of each mechanism of single bucket hydraulic excavator needs to change speed due to operation requirements. In order to obtain different speeds, the valve is often used to adjust the speed, resulting in high heating and serious waste.
The variable system retains the traditional system circuit and directional valve, and has the characteristics of automatically changing the displacement of the hydraulic pump with the change of load. Since its application, it has rapidly replaced the constant displacement pump. At present, the main oil circuit of the hydraulic system of large and medium excavators basically uses variable displacement pump for oil supply.
In order to make full use of the diesel engine power without overloading the diesel engine, large and medium-sized hydraulic excavators usually use a variable system composed of constant power variable pump and quantitative motor. It can automatically change the displacement of the hydraulic pump with the load change, so that the diesel engine often works close to its design power.
With the development of hydraulic technology, in order to improve the accuracy of compound actions in the operation cycle of excavators, the later medium tonnage excavators generally adopted the dual pump dual circuit variable system.
