Diagnosing and Resolving Jerky Track Movement in the Kobelco SK120-5

[MikePhua]

Introduction
The Kobelco SK120-5 is a mid-sized hydraulic crawler excavator renowned for its reliability and performance in various construction and excavation tasks. However, like all heavy machinery, it can experience operational issues over time. One such problem reported by operators is jerky or erratic track movement, particularly when the machine is warmed up. This article delves into the potential causes of this issue and offers practical solutions to address it.
Understanding the Problem
Operators have reported that after the machine warms up, one of the tracks exhibits jerky movement when attempting to move forward slowly. The jerking motion is often noticeable when the control is eased forward gently. Interestingly, applying a sudden, firm push on the control lever can sometimes result in smoother operation. While the track motor produces a slight whirring noise when free-spinning off the ground, there are no visible signs of oil leakage, and the rubber boot around the track control is missing on that side.
Potential Causes
Several factors could contribute to jerky track movement in the Kobelco SK120-5:

1. Contaminated Hydraulic Fluid: Over time, hydraulic fluid can become contaminated with dirt, metal particles, or degraded additives. This contamination can lead to internal wear in hydraulic components, causing erratic movements.
   
2. Worn or Damaged Seals: Seals within the hydraulic system can degrade due to age, heat, or contamination. Worn seals may allow hydraulic fluid to bypass, leading to inconsistent track movement.
   
3. Faulty Control Valves: The control valves regulate the flow of hydraulic fluid to various components. If these valves become sticky or malfunction, they can cause jerky or unresponsive movements.
   
4. Issues with the Final Drive: The final drive is responsible for transmitting power from the hydraulic motor to the tracks. Problems such as low oil levels, contamination, or internal wear can result in jerky track movement.
   
5. Center Joint Problems: The center joint allows the upper and lower parts of the excavator to rotate independently. If the center joint experiences issues, such as internal wear or seal failure, it can affect track movement.
   

Diagnostic Steps
To accurately diagnose the cause of jerky track movement, consider the following steps:

• Inspect Hydraulic Fluid: Check the hydraulic fluid for signs of contamination or degradation. If the fluid appears dirty or has an unusual odor, it may need to be replaced.
  
• Examine Seals and Hoses: Look for any visible signs of leaks or damage in the seals and hoses. Pay particular attention to areas where the rubber boot is missing, as this can expose components to dirt and debris.
  
• Test Control Valves: Operate the control levers and observe the response of the tracks. If the tracks hesitate or jerk, the control valves may be sticking or malfunctioning.
  
• Check Final Drive: Inspect the final drive for proper oil levels and signs of wear. Listen for unusual noises that could indicate internal issues.
  
• Assess Center Joint: If accessible, inspect the center joint for signs of wear or damage. Any issues here can affect the overall movement of the tracks.
  

Recommended Solutions
Based on the diagnostic findings, the following solutions may be appropriate:

• Replace Contaminated Hydraulic Fluid: Drain the old hydraulic fluid and replace it with fresh, clean fluid that meets the manufacturer's specifications.
  
• Repair or Replace Worn Seals: If seals are found to be damaged or worn, they should be replaced to prevent further issues.
  
• Service Control Valves: Clean or replace faulty control valves to ensure smooth operation.
  
• Repair Final Drive: If the final drive is found to be faulty, it may need to be repaired or replaced.
  
• Address Center Joint Issues: Repair or replace the center joint if it is found to be the source of the problem.
  

Conclusion
Jerky track movement in the Kobelco SK120-5 can be caused by various factors, including contaminated hydraulic fluid, worn seals, faulty control valves, issues with the final drive, or problems with the center joint. By systematically diagnosing the issue and addressing the underlying cause, operators can restore smooth and reliable track movement, ensuring optimal performance of the excavator. Regular maintenance and timely repairs are essential to prevent such issues and extend the lifespan of the equipment.