There are more types of systems commonly used in excavators, and different names can be derived from different perspectives. For example, there are quantitative system, variable system; constant power control, variable power control; speed-sensing control, pressure-sensing control; negative flow control, positive flow control, load-sensitive control, etc. Many beginners will be dizzy when they are just exposed to these terms! Excavator hydraulic system in the construction machinery is a complex type, so it is also more technology related to it; this is an important reason for the excavator hydraulic system special terminology. But with the in-depth study of excavator hydraulic technology, we will find that the various technologies are mainly about the "engine - hydraulic pump - load" this energy transfer chain; which negative flow control, positive flow control, load-sensitive control that is from the "hydraulic pump - load "link in the two matching and the resulting terminology.
In the above three excavator systems, the majority of excavators using negative flow hydraulic system. Figure 1 below shows the schematic diagram of the excavator negative flow hydraulic system. The dashed box is the main control valve, to simplify the problem Figure 1 only draws a circuit, in practice there should be more than one circuit, such as the moving arm circuit, bucket circuit, bucket circuit, walking circuit, rotary circuit, etc.. From the figure, we can see that the hydraulic system is a composite application of inlet throttling, return throttling, and bypass throttling speed control circuits. In practice, the three throttle valves A, B and C are linked, and their structure is usually in the form of a three-way flow slide valve, the position of which is controlled by the pilot oil. When the slide valve is in the middle position, the throttle valve C has the largest opening area, A and B are completely closed, and the oil discharged from the main pump is returned to the tank by the bypass. When the slide valve is in the left or right position, that is, the cylinder small or large cavity into the oil, C is almost completely closed, the main pump discharge oil almost all by the working oil return to the tank. According to the throttling characteristics, when the flow through the throttle port D, the greater the flow, the greater the pressure at the Pn point. This negative flow hydraulic system controls the main pump displacement by the pressure Pn, i.e., the flow through the bypass, to control the displacement of the main pump. In order to make the main pump displacement with the load, when Pn increases (i.e., the bypass flow increases), it means that the system has a lot of pressure oil from the bypass unloading that there is a flow surplus, so the main pump displacement should be reduced at this time. On the contrary, when Pn decreases, it means that the system has a shortage of oil supply, and the displacement of the main pump should be increased at this time. Negative flow hydraulic system is based on the above ideas and design. In a sense, the negative flow system is also a load-aware system and operator's will-aware system.