Fault analysis and Elimination of weak operation of the bucket cylinder and left walking motor of the excavator

1. Fault phenomenon

One of the PC220-5 Komatsu excavators in our company has been in operation for 8,500 hours. After that, the bucket cylinder and the left travel motor were found to be weak in operation. However, the rotation and right travel were normal, while the other movements were slightly sluggish.

2 Fault analysis

(1) Possible causes of the weak operation fault of the bucket cylinder

There is a fault in the pilot oil circuit controlling the bucket.

② The valve core of the control valve is stuck or severely worn.

③ The oil replenishment valve of the bucket circuit is stuck.

④ The bucket cylinder, piston or oil seal is severely damaged.

⑤ The main unloading valve is stuck.

⑥ There is a fault with the rear pump or its control system.

(2) Possible causes of the left walking motor's weak operation fault

There is a fault in the pilot oil circuit that controls the left movement.

② The valve core of the control valve is stuck or severely worn.

③ There is a fault with the walking motor.

④ The center rotary joint has severe oil leakage.

⑤ The main unloading valve is stuck.

⑥ There is a fault with the rear pump or its control system.

From the principle of the hydraulic system of this machine, it is known that both the bucket cylinder and the left traveling motor are supplied with oil separately by the rear pump. Therefore, if both the bucket cylinder and the left traveling motor experience weak operation simultaneously, the most likely cause lies in the main unloading valve or the rear pump and its control system. So the high-pressure oil pipes of the front pump and the rear pump were swapped. When the machine was tested again, it was found that the bucket cylinder and the left traveling motor were working normally, while the rotary motor and the right traveling motor were weak. This indicates that the bucket cylinder, the left traveling motor and their control system are all normal. The fault should lie in the rear pump or its control system that supplies oil separately to the bucket cylinder and the left traveling motor.

(3) Check the control system of the rear pump according to the attached diagram and analyze the results

① As the front pump is operating normally, it proves that the control pilot pump and TVC valve shared by the front pump and the rear pump are working properly.

② Install an oil pressure gauge with a range of 6MPa at the outlet of the NC valve. The oil pressure at this location was measured to be Pi (as there is no measurement point for the outlet pressure of the CO valve). When the adjusting bolt of the CO valve was tightened by 2 to 4 turns, it was found that the Pi value rose. When the adjusting bolt was adjusted back to its original position, the Pi dropped back to its original value. The test results are in line with the working characteristics of the CO valve, indicating that the CO valve is operating normally.

When the adjusting bolt of the NC valve was tightened by 2 to 4 turns, it was found that the Pi value rose. When the adjusting bolt was returned to its original position, the Pi dropped back to its original value. The test results are in line with the working characteristics of the NC valve, indicating that the NC valve is operating normally.

After disassembling and inspecting the servo mechanism, it was found that the return spring was not broken and had good elasticity, the connecting rod mechanism did not fall off, and there was no jamming or wear on the valve core. This indicates that the servo mechanism is working normally.

From the above inspection results, it can be known that the control system of the rear pump is working normally. The weak operation of the bucket and the left walking motor can only be caused by a fault in the rear pump itself.

The hydraulic pump assembly was removed. After disassembly and inspection, it was found that all the hydraulic components of the front pump were intact, while the rear pump was severely damaged. There were several deep grooves at the oil sealing band of the distribution plate, and the end face of the plunger cylinder was slightly scratched. There was no obvious wear on the other hydraulic components. Obviously, the malfunction of the rear pump is due to the severe wear of the contact surface between the plunger cylinder and the distribution plate, which causes severe leakage of hydraulic oil, resulting in the inability to establish oil pressure and thus leading to weak operation of the bucket cylinder and the left walking motor.

3 Troubleshooting

Given that the end face of the plunger cylinder is not severely damaged while the distribution plate is severely damaged, we adopt a maintenance plan of grinding the plunger cylinder and replacing the distribution plate. First, the worn end face of the plunger cylinder is precisely ground with a surface grinding machine, then polished with chromium oxide, and finally the plunger cylinder and the distribution plate are manually ground to ensure that their contact area reaches more than 95%.

After reassembling the repaired plunger cylinder and distribution plate and conducting a test run, the excavator resumed normal operation. It has been in use for over a year without any problems.