During the long-term construction of the excavator, common actions such as walking and slewing often have problems, especially external factors such as long years and poor construction environment. Moreover, such problems cannot be solved, and they must be repaired in time.
Fault phenomenon
A Kobelco sk250-8 excavator failed to work properly after 6500h of operation. However, the slewing action and right travel were normal, and the rest were slightly slow.
Fault analysis:
(l) Possible causes of bucket cylinder failure
① There is a problem with the pilot oil circuit that controls the bucket.
② The control valve spool is stuck or severely worn.
③ The makeup valve for the bucket circuit is stuck.
④ The bucket cylinder, piston or oil seal are severely damaged.
⑤ The main unloading valve is stuck.
⑥ The rear pump or its control system is faulty.
(2) Possible causes of weak operation of left travel motor
① The pilot oil circuit that controls left travel is faulty.
② The control valve spool is stuck or severely worn.
③ The propel motor is faulty.
④ The center swivel joint has serious oil leakage.
⑤ The main unloading valve is stuck.
⑥ The rear pump or its control system is faulty.
According to the hydraulic system principle of the machine, both the bucket cylinder and the left travel motor are supplied by the rear pump separately, so the bucket cylinder and the left travel motor are unable to work at the same time. The most likely cause is the main unloading valve or the rear pump and its control system. Therefore, the high-pressure oil pipes of the front pump and the rear pump were exchanged with each other. When trying the machine again, it was found that the bucket cylinder and the left travel motor were working normally. On the contrary, the swing motor and the right travel motor were unable to work. This indicates that the bucket cylinder, left travel motor and their control systems are normal. The fault should be on the rear pump or its control system that supplies oil to the bucket cylinder and left travel motor separately.
3. Troubleshooting
In view of the small damage on the end face of the piston cylinder and the serious damage on the valve plate, we adopt the maintenance scheme of grinding the piston cylinder and updating the valve plate. That is, the worn end face of the piston cylinder is finely ground with a surface grinder, then polished with chromium oxide, and finally the piston cylinder and valve plate are manually ground to ensure that their contact area reaches more than 95%.
After the repaired plunger cylinder and valve plate were installed and tested, the excavator returned to normal. It has been used for more than a year without any problems.
