A PC220-5 Komatsu excavator in our unit, after 8500 hours of operation, developed weak operation of the bucket cylinder and left travel motor, while swing and right travel were normal, and other actions were slightly sluggish.

    2. Fault Analysis

    (1) Possible causes of weak bucket cylinder operation

    ① Fault in the pilot oil circuit controlling the bucket.

    ② Control valve spool stuck or severely worn.

    ③ Bucket circuit replenishing valve stuck.

    ④ Bucket cylinder, piston, or oil seal severely damaged.

    ⑤ Main unloading valve stuck.

    ⑥ Fault in the rear pump or its control system.

    (2) Possible causes of weak left travel motor operation

    ① Fault in the pilot oil circuit controlling left travel.

    ② Control valve spool stuck or severely worn.

    ③ Travel motor faulty.

    ④ Severe oil leakage at the center swivel joint.

    ⑤ Main unloading valve stuck.

    ⑥ Fault in the rear pump or its control system.

    According to the hydraulic system schematic of this machine, the bucket cylinder and left travel motor are both supplied separately by the rear pump. Therefore, when both the bucket cylinder and left travel motor are weak simultaneously, the cause is most likely in the main unloading valve or the rear pump and its control system. Thus, the high-pressure oil lines of the front and rear pumps were swapped. Upon testing, the bucket cylinder and left travel motor worked normally, while the swing motor and right travel motor became weak. This indicates that the bucket cylinder, left travel motor, and their control systems are normal; the fault lies in the rear pump, which supplies oil separately to the bucket cylinder and left travel motor, or in its control system.

    (3) The rear pump control system was checked according to the attached figure, and the results were analyzed.

    ① Since the front pump works normally, it proves that the control pilot pump and TVC valve shared by the front and rear pumps work normally.

    ② A pressure gauge with a range of 6 MPa was installed at the NC valve outlet, and the pressure there was measured as Pi (there is no measuring point at the CO valve outlet). When the CO valve adjusting bolt was tightened 2-4 turns, Pi increased; when the adjusting bolt was returned to its original position, Pi dropped back to the original value. The test results conform to the CO valve operating characteristics, indicating that the CO valve works normally.

    ③ When the NC valve adjusting bolt was tightened 2-4 turns, Pi increased; when the adjusting bolt was returned to its original position, Pi dropped back to the original value. The test results conform to the NC valve operating characteristics, indicating that the NC valve works normally.

    ④ After disassembling and inspecting the servo mechanism, it was found that the return spring was not broken and had good elasticity, the linkage mechanism was not disconnected, and the valve spool was not stuck or worn. This indicates that the servo mechanism works normally.

    From the above inspection results, the rear pump control system works normally. The weak operation of the bucket and left travel motor can only be caused by a fault in the rear pump itself.

    The hydraulic pump assembly was removed and disassembled for inspection. The hydraulic components of the front pump were intact, but the rear pump was severely damaged: there were several deep grooves on the sealing belt of the valve plate, and the end face of the cylinder block was slightly scratched; other hydraulic components showed no obvious wear. Obviously, the rear pump could not work normally because the contact surface between the cylinder block and the valve plate was severely worn, causing serious leakage of hydraulic oil, which prevented oil pressure from building up, thus leading to weak operation of the bucket cylinder and left travel motor.

    3. Troubleshooting

    In view of the slight damage to the end face of the cylinder block and the severe damage to the valve plate, we adopted a repair scheme of grinding the cylinder block and replacing the valve plate. First, the worn end face of the cylinder block was precision-ground with a surface grinder, then polished with chromium oxide, and finally manually lapped with the valve plate to ensure a contact area of more than 95%.

    After installing the repaired cylinder block and valve plate, the excavator was tested and resumed normal operation. It has been in use for over a year without any problems.