Diagnosing and Troubleshooting Slow Lifting of Excavator Boom: Causes and Solutions

The slow lifting of the boom of an excavator can be attributed to several factors that affect the output power of the oil cylinder, the pressure difference on both sides of the piston, and the output of the hydraulic oil pump. By understanding these reasons, we can diagnose and address the issue effectively, ensuring smooth construction operations. Let's explore these reasons in detail:

 

1. Reduction of Output Power of the Oil Cylinder:

The boom of an excavator is lifted by the pressure oil output from the oil pump, which enters the rear chamber of the boom cylinder through the pipeline and valve. The piston within the cylinder moves under the working oil pressure, causing the boom to lift. The lifting speed is influenced by the pressure and flow of the oil entering the cylinder. If the power input to the cylinder decreases, the boom lifting speed will slow down. The reduction of power input can be caused by the following factors:

 

(a) Influence of the Oil Pump: The speed of the piston in the oil cylinder is directly proportional to the working flow of the oil entering the cylinder. If the output flow of the hydraulic oil pump decreases, it affects the lifting speed of the boom cylinder. The total efficiency of the oil pump, which is the sum of its volumetric efficiency and mechanical efficiency, plays a significant role. If the total efficiency decreases due to increased oil pump leakage or mechanical friction loss, the actual flow rate of the oil pump decreases, leading to slow lifting of the boom.

 

(b) Impact of the Oil Cylinder: Damaged piston seal rings, strained oil cylinders, or damaged cylinder head seal rings can result in oil leakage or seepage, reducing the volumetric efficiency of the oil cylinder. Consequently, the output power of the cylinder decreases, causing the boom to lift slowly.

 

(c) Impact of Oil Leakage: Poorly sealed sections of the oil circuit, such as the suction section (from the oil pump to the oil tank) and the pressure section (from the oil pump to the oil cylinder through the control valve), can lead to air suction or oil leakage. This affects the pumping efficiency and reduces the oil flow into the cylinder, resulting in decreased pressure and slow boom lifting.

 

(d) Effect of Oil Circuit Blockage: Mechanical impurities present in the hydraulic system's working oil can block oil flow in narrow passages, reducing the oil flow into the cylinder and causing a drop in pressure. Consequently, the output power of the oil cylinder decreases, resulting in slow lifting of the boom. The blockage of the oil filter at the oil suction port can increase the vacuum degree, leading to air suction and affecting the output power of the oil pump. Similarly, blockage in the oil return circuit's filter screen or radiator can increase back pressure on the oil cylinder, reducing the pressure difference on both sides of the piston and slowing down the boom lifting speed.

 

2. Influence of the Valve:

Several valve-related issues can contribute to the slow lifting of the boom:

 

(a) Improperly Set Safety Valve: If the set pressure of the safety valve (overload valve or overflow valve) is too low or the valve does not close tightly, the working oil pressure in the system decreases, resulting in slow boom lifting due to low pressure.

 

(b) Faulty One-Way Valve: Mechanical impurities or improper closure of the one-way valve and its seat can cause oil flow back to the oil tank, similar to the previously mentioned scenarios, leading to slow boom lifting.

 

(c) Excessive Gap in the Reversing Valve: When the spool of the reversing valve has a large gap with its matching pair, some pressure oil flows back to the oil tank during operation. This results in slow boom lifting.

 

3. Incorrect Oil Pump Inlet and Outlet Connections:

If the inlet and outlet oil pipe joints of the oil pump are reversed, the oil cylinder will not operate correctly, causing slow boom lifting.

 

Diagnosis and Troubleshooting:

 

To diagnose and address the slow lifting of the excavator boom, follow these steps:

 

1. Visual Inspection of Oil Leakage: Check for any oil leakage from the main oil circuit. Oil leaking out of the system indicates a fault that prevents the boom from lifting properly. Identify the cause of the leakage and address it accordingly.

 

2. Check Oil Level and Quality: Ensure that the oil level is sufficient in the oil tank. If it is low, add oil to meet the specified height requirements. Additionally, check for air bubbles in the oil. Excessive air bubbles may indicate a connection between the oil circuit and the gas circuit, requiring replacement of the rotary joint's sealing ring. Dark brown, oxidized, and foul-smelling oil indicates oil deterioration and necessitates replacement.

 

3. Monitor Oil Temperature: High oil temperature, exceeding 80°C, can contribute to slow boom lifting. Ensure adequate cooling, and if the fault disappears after cooling down, it indicates poor radiator heat dissipation or prolonged heavy load operation. Conversely, if slow lifting occurs only at high oil temperatures, it suggests deteriorated oil quality or worn hydraulic components, reducing system efficiency.

 

4. Check the Main Safety Valve: Adjust the main safety valve to ensure it is set correctly. If adjusting the valve increases pressure to the specified value and the boom lifting speed returns to normal, it indicates an issue with the main safety valve adjustment. If adjustment does not resolve the problem, inspect the valve for a stuck open position or inadequate closure. Replace or repair the valve accordingly. Replace a broken pressure regulating spring.

 

5. Inspect the Oil Cylinder: If the boom automatically descends noticeably when the control valve is in the middle position after the oil cylinder is lifted, it indicates poor seals, improper closure of the oil cylinder's safety valve, or oil pipe leakage. Identify the cause and address it accordingly. Damaged seals during cylinder assembly, mechanical impurities causing spool or one-way valve blockage, or loose pipe joint connections can all lead to reduced pressure in the hydraulic system, resulting in slow boom lifting. Find and eliminate the root cause of the problem.

 

6. Adjust the Boom Cylinder Safety Valve: If the setting pressure of the boom cylinder's safety valve is lower than the specified value, adjust it accordingly. Increase the pressure by adding a thicker pad to the spring (increasing the pad thickness by 1mm results in a pressure increase of 2.45MPa).

 

By following these diagnostic and troubleshooting steps, operators and maintenance personnel can effectively identify and resolve issues causing slow lifting of the excavator boom. Regular maintenance, including oil quality checks, filter replacements, and seal inspections, can help prevent such issues from arising, ensuring optimal performance of the excavator during construction operations.

 

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