The excavator can rotate infinitely without being affected by the chain track. This is all due to the "central swivel joint". The excavator powertrain is in the upper part of the frame and is responsible for providing power, while the chassis travel motor is driven by the powertrain in the upper part of the frame. The power is transmitted between the upper and lower frames through hydraulic pipelines.
The excavator is fully hydraulically driven. The engine output directly drives the hydraulic pump, which provides power for the entire vehicle. All movements of the vehicle are hydraulically driven, including the telescopic cylinder, slewing motor, and travel motor. The device that controls the direction of the hydraulic oil is called a valve group and a distributor, which is controlled by the control handle (pilot handle) in the cab. The handle drives the valve group to move, and the valve group controls the direction and flow of the hydraulic oil in and out, which also controls the movement and speed. Every movement of the excavator is completed by the handle driving the valve group. Except for walking, walking is controlled by foot, which is the same principle as the handle.
The upper frame is easy to understand, because the engine hydraulic pump valve group and other components are on the frame, and the direction of the hydraulic pipelines of each cylinder motor can be seen intuitively. Just like the circuit diagram, it is simple and clear. The more abstract one is the so-called 360-degree rotation, which is the problem of the travel motor drive. If the hydraulic pipe connects the upper and lower frames, it may not be able to make a 360-degree rotation, and the pipe will be torn. Therefore, a central swivel joint is used here to complete the 360-degree rotation instead of using hydraulic pipes.
The picture above is the central swivel joint. The valve group output is connected to the upper part of the swivel joint through a pipeline, and there is a corresponding output joint under the frame, which is connected to the travel motor through a pipeline. Therefore, no matter how the frame rotates, the motor oil pipe below will not rotate.
The above picture is a cutaway swivel joint, and the internal structure can be clearly seen.
The swivel joint consists of a shell and a shaft. The shell is the input part, and the shaft is the output part. There are oil grooves in the shell, and corresponding oil holes in the shaft. There are seals between each set of oil grooves to ensure that the hydraulic oil does not leak or crosstalk.
The principle may be difficult to see clearly, but it is easy to understand by referring to the principle of the headphone plug in the picture above! After the headphone is plugged in, it can rotate 360 degrees. Just like the principle of a slip ring, no matter how it is rotated, the reed in the socket always maintains good contact with the headphone plug. Therefore, no matter how the headphone is rotated, it will not affect the sound of the headphone. The same principle applies to the central swivel joint.
