Fault Analysis and Elimination of Slow Lifting of EX300-3 Excavator Boom

One of our company's EX300-3 excavators suddenly encountered a problem of slow lifting of the boom cylinder during construction and was unable to work normally at all. Based on this phenomenon, we conduct an analysis and diagnosis.

First, check the oil pressure of the hydraulic system. The hydraulic system of the EX300-3 excavator is supplied with oil by a double variable main pump. The malfunction of the working device is very likely related to the system oil pressure. However, only the boom hydraulic cylinder suddenly lifted slowly, while the middle boom and bucket cylinders were working normally. It is estimated that this is not a fault of the main pump. If there is a problem with the main pump, the oil pressure in the main hydraulic circuit will definitely decrease, which will cause the normal operation of other actions. Sure enough, when we measured the oil pressure, the system oil pressure was 350kgf/cm ², which was normal. Therefore, the fault of the main pump could be ruled out.

Second, we test the oil pressure of the pilot hydraulic system. Because there is a main pump oil volume control valve at the top of the double variable main pump, it controls the oil volume adjustment swing frame in the main pump through the pilot oil pressure, thereby changing the oil volume output of the main pump. When an excavator is in operation, the boom cylinder is under heavy load. Insufficient oil supply can also cause slow movement. However, we measured the oil pressure of the pilot system to be 40kgf/cm ², which is also within the normal range. And the normal operation of other actions indicates that the pilot hydraulic system is working properly. Therefore, the fault of the pilot oil circuit can be ruled out.

Next, let's check the main circuit relief valve. If the main circuit relief valve leaks, it will inevitably cause a decrease in oil pressure, thus making the operation slow. When the main relief valve was opened, no signs of dirt blockage or valve jamming were found, and no signs of wear were observed on the surface of the valve stem. After installing the relief valve, the system oil pressure can also be adjusted normally, so there is no problem with the main relief valve.

Fourth, we conducted another test and found that the movement of the boom was not operated, and the oil pressure of the measurement system was normal. However, as soon as the boom cylinder was operated, the system oil pressure suddenly dropped from 350kgf/cm2 to 200kgf/cm2. It can be seen that there is an oblique leakage in the control valve of the boom cylinder. Following the hydraulic pipeline, we found the single control valve that controls the action of the brake arm cylinder in the multi-link control valve. There is an oil replenishment valve at the lower end of the single control valve of the boom cylinder. We opened the oil replenishment valve for inspection and found that there was dirt blocking the valve, the spring was stuck, and the surface of the valve stem of the oil replenishment valve was also worn. We cleaned the valve core, ground and repaired the valve stem, and replaced the spring. (Note: The spring of the oil replenishment valve has lost its elasticity due to being stuck.)" After reassembly, the excavator boom operated normally and the work was also normal. The problem has been satisfactorily resolved.

We have summarized that the cause of the malfunction is as follows: when the boom cylinder is working, the oil replenishment valve cannot replenish oil in time, and it causes leakage in the system oil circuit, reduces the system oil pressure, and thus makes the lifting of the excavator boom slow. Subsequently, we promptly replaced the hydraulic filter, pilot filter and hydraulic oil. The excavator has been working normally so far.