If the excavator is not properly maintained after failure, it will not only fail to eliminate the failure, but also cause the excavator to fail to work normally, and even shorten the service life of the excavator and carry out overhaul prematurely. Therefore, when the boom of the excavator is lifted slowly, the fault must be carefully analyzed and eliminated in time.
Cause of failure:
Each action of a Hitachi ex200-5 hydraulic excavator is normal after the engine is started every day. However, after running for 2 ~ 4h, the boom lifting is gradually slow, and the boom lowering and other actions are normal.
Analyze the fault cause:
From the fault phenomenon of the excavator, it can be judged that the boom cylinder, main pump and boom lifting overload overflow valve are all free of fault. According to the fact that the boom lifting speed is normal when the excavator is just started (the hydraulic system temperature is low and the hydraulic oil viscosity is high), but the boom lifting is slow after the excavator operates for a period of time and the hydraulic oil temperature rises, it is judged that the fault is related to the leakage of some components.
Troubleshooting:
Based on the above analysis, it is further determined that the leakage element may be boom I valve or boom crushing II valve. Disassemble and inspect boom I valve and boom crushing II valve, the valve core slides smoothly, and no obvious wear or strain is found. After reinstalling the valve cores of the two valves, the test run was conducted, and all actions returned to normal. The fault reappeared two days later.
According to the last maintenance experience, we judged that the hydraulic oil temperature may have risen, causing a component in boom I valve and boom crushing II valve to be heated and deformed, causing the movement of the valve core to be blocked, resulting in the decrease of oil supply to the boom cylinder and the slow lifting of the boom.
When the excavator boom moves slowly, the pilot pressure (secondary) at the lifting side of boom I valve and boom crushing II valve is measured to be 2.85mpa, which is normal. Operate the boom I valve separately to make the main pump 1 supply oil to the boom cylinder separately, drive the boom to lift, and observe that the pressure rises to 24.5mpa soon (the pressure value is normal). Operate the boom crushing II valve separately to make the main pump 2 supply oil to the boom cylinder separately. It is found that the pressure rises slowly and can only reach about 9.0MPa (the pressure value is abnormal). This indicates that the movement of boom crushing II valve spool is subject to large resistance.
In order to find out the reason why the movement of boom crushing valve II spool is blocked, we disassembled and inspected the boom crushing valve II spool again. During disassembly, it was found that the pilot oil pipe was not installed at the other end of the valve. This oil pipe is the pilot oil pipe of the crushing hammer. See Fig. 3 for the B4 oil port at the upper end of the boom crushing II valve. Connect the adjacent conduits first and then test the machine. The boom is lifted normally and the fault disappears. After one pilot oil pipe was purchased and connected according to the schematic diagram, the excavator returned to normal operation.
The analysis shows that there is a certain clearance between the valve core and the valve hole of boom crushing II valve. The oil leaked from the clearance gradually enters the upper end of boom crushing II valve, causing pressure to form in the closed space opposite to the upper end of boom crushing II valve. When the excavator starts working, the amount of oil entering the upper end cap is very small and the pressure is small. When the excavator works for a period of time, the oil volume in the enclosed space at the upper end of boom crushing II valve increases, and the pressure gradually increases. When boom lifting pilot pressure oil (secondary pressure) enters the lower end of boom crushing II valve spool from A4 oil port and pushes the spool to move upward, the closed pressure oil in the upper end cover of boom crushing II valve will prevent the spool from moving upward. Since the working oil output from the main pump 2 cannot merge with the main pump 1 and supply oil to the boom rodless cavity, the boom is lifted slowly after a period of operation. After parking for a period of time, the closed oil in the upper end cover flows back to the hydraulic oil tank through the gap, so the boom lifting works normally at the beginning of the operation every day, and the boom lifting is slow after working for a period of time.
