Analyze the reasons why the excavator boom of CAT320B cannot descend

A CAT320B excavator suddenly experienced a malfunction where the boom could only be raised but not lowered during operation. However, at this time, the engine was running normally and all other movements of the machine were also normal. Part of the hydraulic circuit of this mobile arm.

Since the engine was running normally when this malfunction occurred and the machine could perform all actions except for the lowering of the boom, it indicates that there is no problem with the engine. After analysis, it is believed that the fault should lie in the hydraulic system. So, we first inspected the hydraulic oil filter to see if there were any metal shavings or other dirt. As a result, no obvious foreign objects were found. Then, we carried out the following checks based on the possible causes of this malfunction.

(1) Measure the pressure of the main oil circuit. Since only the upper pump supplies oil when the boom descents, it is only necessary to measure the output pressure of the upper pump. At this time, the pressure gauge shows a value of 34.3MPa, indicating that the output pressure of the upper pump is normal. Therefore, it can be ruled out that there is a problem with the main pump.

(2) Measure the pressure of the pilot oil. Insufficient pressure of the pilot oil can also cause abnormal operation of the machine. Since other actions are normal, the pilot oil pressure when the boom descends is directly measured, and it is checked whether the oil passage is unobstructed. After inspection, both the pilot oil pressure and the oil circuit are normal.

(3) Check whether the movement of the valve core of the main control valve 1 of the boom is blocked. If the valve core is stuck by dirt, the pilot oil cannot push this valve core to move, and thus the boom cannot descend. After inspection, it was found that the valve core moves freely within the valve body, so this factor can be ruled out.

(4) Check whether there is any problem with the valve of the boom anti-drift assembly. This assembly valve is set to reduce the automatic settlement of the boom when the boom is already in the lifted position and the boom operating handle has returned to the middle position, or when the engine suddenly stalls. This valve only functions during the lowering of the boom and has no impact on the system when the boom rises.

The working process of the boom hydraulic circuit is as follows: When the boom operating handle performs the "boom lifting" action, the pilot oil pushes the valve core of the main control valve 1 of the boom, allowing the main pump pressure oil to pass through the main control valve of the boom to reach the anti-drift assembly valve of the boom and push the control valve core 2. In this way, the main pump pressure oil can reach the bottom of the boom cylinder. When the boom operating handle is in the neutral position (the boom is in the lifted state), the self-weight of the boom causes a relatively high pressure at the bottom of the boom cylinder. Since the oil circuits at both ends of the piston of the insertable valve 5 in the boom anti-drift assembly valve have already controlled the communication of the valve core 2, and the area of the left end of the piston is larger than that of the right end, under the action of oil pressure, the piston moves to the right, sealing the oil circuit from the bottom of the boom cylinder to the main control valve of the boom Therefore, the flow of hydraulic oil through the main control valve of the boom is reduced, thereby reducing the settlement of the boom. When the boom operating handle performs the "boom descent" action, the pilot oil passing through the pilot control valve is divided into two paths: one path pushes the valve core of the main control valve of the boom, allowing the pressure oil of the main pump to pass through the main control valve of the boom and reach the cylinder rod end of the boom cylinder. Another path leads to the control valve core 2 in the anti-drift assembly valve of the boom. Through the action of this valve core, the oil passage at both ends of the piston of the cartridge valve 5 is cut off. At the same time, the oil in its left chamber is connected to the oil tank through the control valve core. The pressure in the left chamber decreases, the piston moves left, and the channel between the bottom end of the boom cylinder and the main control valve of the boom is opened. The oil at the bottom end of the boom cylinder then returns to the oil tank through the main control valve of the boom. Or when the boom descends rapidly, part of the return oil at the bottom of the boom cylinder returns to the boom end of the boom cylinder through the boom regeneration oil circuit (the boom regeneration oil circuit is set up to accelerate the descent of the boom). If the oil in the left chamber of the cartridge valve cannot communicate with the oil tank through the control valve core, the pressure in the left chamber cannot be reduced. As a result, the piston will always be pressed against the right side, thus blocking the return oil passage at the bottom of the boom cylinder and preventing the boom from descending.

Based on the above oil circuit, we believe that the cause of the machine's malfunction should be within the anti-drift assembly valve of the boom. When the anti-drift assembly valve of the boom was removed and disassembled for inspection, it was found that the throttle hole on the control valve core 2 had been blocked by dirt, causing the oil passage to be blocked. So, the hex socket plug at the end of the valve was unscrewed, the dirt in the oil passage was thoroughly cleaned, and the anti-drift assembly valve was cleaned and assembled. After reassembly, when the machine was tested, the faults of the machine had disappeared.

The troubleshooting process of this fault tells us that many components of the internal hydraulic system of the excavator are very precise and highly sensitive to the contamination of hydraulic oil. Therefore, it is necessary to enhance the daily maintenance of the machine, replace the relevant oil products and oil filters on time and regularly, and keep the interior of the system clean, so as to effectively avoid the occurrence of similar faults.