When the control handle is placed in the swing position, control oil reaches the control valve, pushing the swing valve spool so that high-pressure oil from the rear working pump enters the swing motor; at the same time, the oil return circuit from the control oil to the tank is cut off, increasing the pressure of the brake release valve, pushing its spool so that another auxiliary pressure oil enters the brake piston, releasing the swing brake device and causing the swing motor to operate; the swing motor outputs power through the pinion of the transmission, and the pinion meshes with the swing ring gear to produce the swing action.

    1. Fault Cause Analysis and Inspection

    From the working principle of the swing mechanism, the causes of the fault can be in three aspects: the main working oil circuit, the mechanical transmission, and the control oil circuit.

    (1) Inspection of the main working oil circuit

    If the pressure in the main swing oil circuit is low, the swing action cannot be produced. Measure the pressure of the main working oil circuit system. There is an internal hex plug at the pressure outlet of the rear working pump. Remove it and install a pressure gauge. Operate the swing handle and accelerate the diesel engine. The measured pressure reaches 23.5 MPa, which is the normal working pressure of the system, indicating that all hydraulic components in the main working oil circuit are normal.

    (2) Inspection of the mechanical transmission part

    The swing motor outputs power through the pinion of the transmission, and the pinion rotates around the swing ring gear. During this power output process, if the meshing gears are jammed, the swing action will also not occur.

    Generally, abnormal gear meshing will produce abnormal noise. The operator did not hear any abnormal noise during swing operation, so the mechanical transmission fault can be temporarily ruled out.

    (3) Inspection of the control oil circuit

    Since other actions of the excavator are normal, the auxiliary oil pump that provides control oil pressure is working normally. There are three control oil circuits for the swing action: the circuit controlling the swing valve spool, the brake release valve, and the circuit entering the parking brake piston. The specific analysis is as follows.

    When operating the swing handle, a strong oil flow sensation was felt when touching the pressure oil pipe of the swing motor, indicating that the main working pressure oil has reached the swing motor, the swing valve spool of the control valve can open normally, and the control oil circuit works normally. Measure the oil pressure to the parking brake piston. Open the pipe joint of this circuit to the swing motor, install a tee joint and a pressure gauge. The measured pressure reaches 4 MPa, indicating that the pressure of this circuit is normal.

    Measure the oil pressure to the brake release valve. Open the pipe joint of this circuit to the swing motor, install a tee joint and a pressure gauge. The measured pressure is 0.4-0.6 MPa, which is obviously too low (the normal working pressure should be 4 MPa). The brake release valve cannot be opened, and the control oil of the other circuit cannot flow into the brake piston, let alone open the swing brake device. Therefore, the swing has no action. There are two reasons for the low pressure in this circuit: first, the control valve leaks, causing the oil pressure entering the brake release valve to become low (connect the pressure gauge to the pipe joint of the control oil pipe from the control valve to the brake release valve; the measured oil pressure is only 0.4-0.6 MPa, indicating that the judgment of control valve leakage is correct); second, the brake release valve leaks due to wear causing increased fit clearance (after disassembly and inspection, its valve spool and valve hole fit well, and there is no jamming).

    2. Fault Treatment

    From the fault cause analysis and inspection, the main cause of the fault is leakage in the control valve. Since repair is relatively difficult, the control valve was replaced. After repair, the swing action returned to normal.