If the excavator fails to maintain after improper maintenance, it not only cannot eliminate the fault, but also can cause the excavator to not work normally, and even lead to shortened service life of the excavator and premature overhaul. Therefore, when the excavating mobile arm is gradually lifting, the fault must be carefully analyzed and eliminated in time.
Causes of failure:
A Hitachi EX200-5 hydraulic excavator starts every day after the engine is in normal operation, but after 2 to 4 hours of operation, the boom raises gradually, the boom lowers and other actions are normal.
Analyze the cause of the failure:
From the failure phenomenon of the excavator, it can be judged that the boom cylinder, the main pump and the boom lifting overload relief valve are all free of failure. According to the boom raising speed when the excavator is just started (the hydraulic system temperature is low and the hydraulic oil viscosity is large), and the excavator works for a period of time, after the hydraulic oil temperature rises, the boom raises slowly, judging the The fault is related to the leakage of some components.
Troubleshooting:
After the above analysis, it is further determined that the leaking component may be the boom I valve or boom crush II valve. Dismantle and inspect the boom I valve and boom crush II valve, the spool slides smoothly, no obvious wear or strain is found. After reinstalling the spools of the two valves and testing the machine, all actions returned to normal, and the fault reappeared 2 days later.
According to the previous maintenance experience, we judged that the temperature of the hydraulic oil may have increased, which caused a certain component of the boom I valve and boom crush II valve to be heated and deformed, which caused the movement of the spool to be blocked, resulting in a decrease in the oil supply of the boom cylinder. , Slower boom lift.
When the excavating manipulator moves slowly, measure the pilot pressure (secondary) on the lifting side of the boom I valve and boom crushing II valve (secondary), which is normal. Operate the boom I valve separately, so that the main pump 1 supplies oil to the boom cylinder separately, and drive the boom to lift. It is observed that the pressure soon rises to 24.5MPa (the pressure value is normal). Operate the boom crusher II valve separately to make the main pump 2 supply oil to the boom cylinder separately, and find that its pressure rises slowly and can only reach about 9.0MPa (this pressure value is abnormal). This shows that the movement of the spool of the boom crush II valve is subject to greater resistance.
In order to find out the reason why the movement of the spool of the boom crush II valve is blocked, we again dismantle the spool of the boom crush II valve. During the disassembly, it was found that the other end of the valve was not equipped with a pilot oil pipe. The oil pipe is the pilot oil pipe of the hammer, see the b4 port of the upper end of the boom crush II valve in Figure 3. Connect the adjacent pilot pipes and test the machine. The boom is raised normally and the fault disappears. Purchase a pilot oil pipe and connect it according to the schematic diagram, then test the machine, and the excavator works normally.
Analysis believes: there is a certain gap between the spool and valve hole of the boom crush II valve. The oil leaking from the gap gradually enters the upper end of the boom crushing II valve, so that the closed space opposite the upper end of the boom crushing II valve forms a pressure. When the excavator starts to work, the amount of oil that enters the upper end cover is very small, and the pressure is small. When the excavator works for a period of time, the amount of oil in the upper closed space of the boom crush II valve increases, and the pressure gradually increases. When the boom lifting pilot pressure oil (secondary pressure) enters the lower end of the boom crush II valve spool from the a4 port and pushes the spool upward, the closed pressure oil in the upper end cap of the boom crush II valve will obstruct the spool Move up. Because the working oil output from the main pump 2 cannot merge with the main pump 1 and supply oil to the rodless cavity of the boom, the boom rises slowly after working for a period of time. 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 work is normal at the beginning of the work every day, and the boom lifting will appear slow after a period of work.
