The boom can only be raised but not lowered suddenly during excavator operation. After inspection, the engine operates normally and other actions of operating the machine are also normal. This article will analyze the causes of the faults and describe how to eliminate the faults for reference only.
1. Cause analysis
Since the engine operates normally and the machine can complete other actions except boom lowering, it indicates that there is no problem with the engine and electronic control system. It is preliminarily judged that the boom hydraulic system is faulty.
When the boom handle is operated to lift the boom, the pilot pressure oil pushes the valve core of the boom main control valve to slide downward, and the main pump pressure oil reaches the holding valve assembly through the boom main control valve. At this time, the oil circuits in the left and right chambers of the logic valve are connected. After the main pump pressure oil pushes the piston of the logic valve to the left, it can reach the rodless chamber of the boom cylinder to raise the boom.
When the boom handle is in the middle position, the pressure in the rodless cavity of the boom cylinder is high under the action of the boom's own mass. Since the oil circuits of the left and right chambers of the logic valve are connected through the pressure control valve, and the cross-sectional area of the left end of the piston is greater than that of the right end, the piston moves to the right under the action of oil pressure difference and spring force, which seals the oil circuit from the rodless chamber of the boom cylinder to the boom main control valve, so the settlement of the boom is reduced.
When the boom handle is operated to lower the boom, the pilot pressure oil is divided into two circuits through the pilot oil pipe, one of which enters the boom main control valve to push the valve core of the main control valve upward, so that the main pump pressure oil reaches the rod cavity of the boom cylinder through the boom main control valve. The other way flows to the pressure control valve to push the action of the valve core of the pressure control valve and cut off the oil circuit in the right chamber of the logic valve. At the same time, the pressure oil in the left chamber is connected with the oil tank through the pressure control valve to reduce the pressure in the left chamber, and the piston moves left in the throttling state, opening the channel between the rodless chamber of the boom cylinder and the boom main control valve. The oil in the rodless chamber of the boom cylinder flows back to the oil tank through the boom main control valve.
According to the analysis of the above boom hydraulic system principle, the cause of the fault may be insufficient oil pressure of the main pump, insufficient pilot oil pressure, failure of the boom main control valve, failure of the holding valve assembly or boom cylinder.
2. Troubleshooting
1) Detect the pressure of main oil circuit
Since only the main pump supplies oil when the boom is lowered, it is sufficient to detect the output pressure of the main pump. During the test, the value displayed on the pressure gauge is 34.5 MPa, indicating that the main pump and main safety valve are normal.
2) Detect pilot pressure
Since other actions are normal when operating the machine, the secondary pilot pressure during boom lowering shall be measured directly, and the pilot oil circuit shall be checked to see whether it is unblocked. After detection, the secondary pilot pressure is 2.6 MPa and the oil circuit is unblocked.
3) Detect the boom main control valve
Disassemble and inspect the boom main control valve. The valve core is smooth without scratches and can move freely in the valve body, eliminating the factor that the boom cannot fall due to the stagnation of the valve core.
4) Test and hold valve assembly
When removing the retaining valve assembly, it was found that the orifice on the pressure control valve had been blocked by dirt, resulting in blocked oil circuit. Therefore, the pressure control valve was thoroughly cleaned, the machine was installed and retested, and the fault disappeared. The function of the holding valve assembly is to reduce the automatic settlement of the boom when the boom is lifted and the operating handle is in the middle position, or when the engine suddenly stops, and the pressure control valve plays a throttling role in the process of lowering the boom.
