Kobelco SK 250 Stick Arm Cylinder Leaks Down: Causes and Solutions

[MikePhua]

Introduction
The Kobelco SK 250 is a powerful and efficient excavator, used widely for various construction tasks. However, like any complex piece of machinery, it is prone to issues that require troubleshooting and maintenance. One of the common problems faced by operators of the SK 250 is when the stick arm cylinder leaks down, causing the arm to slowly drop under load. This issue can be problematic and should be addressed promptly to avoid further damage. In this article, we’ll explore the potential causes of this problem and provide solutions to fix the leaking stick arm cylinder.
Causes of Stick Arm Cylinder Leaks

1. Worn or Damaged Seals:
   The most common cause of a leaking stick arm cylinder is worn or damaged seals. These seals are responsible for preventing hydraulic fluid from leaking out of the cylinder. Over time, the seals can deteriorate due to wear, age, or exposure to extreme temperatures and pressure.
   Solution: Inspect the seals on the cylinder. If they show signs of wear, cracks, or deterioration, they should be replaced. It’s important to use genuine Kobelco parts to ensure compatibility and reliability.
   
2. Hydraulic Fluid Contamination:
   Contaminated hydraulic fluid can cause damage to the seals and internal components of the stick arm cylinder. Contaminants such as dirt, water, or metal shavings can compromise the integrity of the hydraulic system, leading to leaks.
   Solution: Check the hydraulic fluid for signs of contamination. If the fluid is dirty or contains debris, flush the system and replace the fluid with the correct type. Be sure to use clean equipment when refilling the hydraulic system to prevent further contamination.
   
3. Damaged Cylinder Barrel:
   A bent or damaged cylinder barrel can cause the stick arm cylinder to leak. This may happen if the excavator is subjected to excessive stress or impact, such as striking hard surfaces or lifting excessive loads. A damaged barrel will not maintain the proper seal and will lead to hydraulic fluid leakage.
   Solution: Inspect the cylinder barrel for any signs of deformation or damage. If the barrel is bent or severely damaged, it may need to be replaced or repaired by a professional. Be cautious when using the machine to avoid putting excessive stress on the cylinder.
   
4. Valve or Control Issues:
   In some cases, a faulty valve or control system can cause the cylinder to leak down. If the valve is not properly sealing, hydraulic fluid may leak past the valve and cause the stick arm to lower unintentionally. This could be a result of worn valve seals or internal wear.
   Solution: Check the valve controlling the stick arm for any issues. Inspect the valve seals and internal components for wear or damage. If the valve is the problem, it may need to be cleaned, rebuilt, or replaced.
   

Steps to Diagnose the Issue

1. Inspect the Cylinder and Seals:
   Begin by visually inspecting the stick arm cylinder for any obvious signs of leaks. Check around the seals, as well as the rod, barrel, and end caps. If you notice any fluid around the seals, it is a clear indicator of a seal problem.
   
2. Check Hydraulic Fluid Levels and Quality:
   Check the hydraulic fluid levels to ensure they are within the recommended range. Low fluid levels can indicate a leak in the system. Additionally, examine the fluid quality to ensure it is clean and free from contamination. If the fluid is contaminated, flush the system and replace the fluid.
   
3. Test the Cylinder's Performance:
   Operate the excavator and monitor the performance of the stick arm cylinder. If the cylinder leaks down under load or if you notice a drop in the arm’s holding capacity, this confirms that the issue is related to the cylinder seals or hydraulic system.
   
4. Check for Valve Malfunctions:
   If the cylinder is operating normally but still leaks down, inspect the valve and control system. A malfunctioning valve can cause the fluid to bypass and lead to cylinder leakage. Test the valve for proper operation and replace it if necessary.
   

How to Fix the Leaking Stick Arm Cylinder

1. Replacing the Seals:
   If the seals are worn or damaged, the only solution is to replace them. This process involves disassembling the stick arm cylinder, removing the old seals, and installing new ones. It’s important to follow the manufacturer’s instructions and use the correct seals to ensure proper sealing and functionality.
   
2. Flushing the Hydraulic System:
   If contamination is the cause of the issue, you will need to flush the hydraulic system. This involves draining the old fluid, cleaning the system to remove contaminants, and refilling it with fresh, clean hydraulic fluid. Be sure to replace the hydraulic filter as well to ensure the fluid remains clean.
   
3. Repairing or Replacing the Cylinder Barrel:
   If the cylinder barrel is damaged, it may need to be straightened or replaced. This requires specialized equipment and should be handled by a qualified technician. Attempting to repair a damaged barrel without the proper tools and knowledge can result in further damage.
   
4. Fixing Valve Issues:
   If the problem lies with the valve, you will need to clean, rebuild, or replace the faulty valve. Ensure that the valve seals are in good condition and replace any worn or damaged components. Proper valve function is essential for the cylinder to perform correctly.
   

Conclusion
A leaking stick arm cylinder on the Kobelco SK 250 excavator is a problem that can lead to reduced performance and increased operational costs if not addressed promptly. By understanding the common causes of this issue—such as worn seals, hydraulic fluid contamination, damaged cylinders, or valve malfunctions—operators can take the necessary steps to diagnose and fix the problem. Regular maintenance, including hydraulic fluid checks and seal inspections, will help prevent leaks and extend the life of the machine. If you are unsure about the repair process, it is always a good idea to consult a professional technician to ensure that the job is done correctly.