Troubleshooting the Center Joint in a Hitachi EX120-2 Excavator

[MikePhua]

The Hitachi EX120-2 and Its Engineering Legacy
The Hitachi EX120-2 excavator is part of a globally respected lineage of mid-size hydraulic excavators developed during the 1990s. Hitachi Construction Machinery, founded in 1970 as a division of Hitachi Ltd., became known for its precision engineering and durable hydraulic systems. The EX120-2, with an operating weight of approximately 12,000 kg and a 4-cylinder Isuzu diesel engine producing around 90 horsepower, was designed for versatility in urban construction, utility trenching, and forestry work.
Its hydraulic system features a center joint—also known as a swivel joint or rotary manifold—that allows hydraulic fluid to pass between the upper structure and the undercarriage while enabling 360-degree rotation. This component is critical for powering travel motors, blade functions, and swing operations without tangling hoses or interrupting flow.
Terminology Notes

• Center Joint (Swivel Joint): A rotating hydraulic manifold that transfers fluid between stationary and rotating parts of the excavator.
  
• Travel Motor: A hydraulic motor that drives the tracks for forward and reverse movement.
  
• Case Drain Line: A low-pressure return line that relieves excess fluid from hydraulic motors to prevent seal damage.
  
• O-Ring Stack: A series of rubber seals inside the center joint that prevent cross-port leakage and maintain pressure integrity.
  

Symptoms of Center Joint Failure
When the center joint begins to fail, operators may notice:

• One track moving slower or not at all
  
• Hydraulic fluid leaking from the undercarriage
  
• Loss of travel power in one direction
  
• Cross-contamination between hydraulic circuits
  
• Pressure spikes or erratic movement during rotation
  

In one documented case from a contractor in Queensland, the EX120-2 began drifting to one side during travel. After ruling out motor and valve issues, technicians discovered that the center joint’s internal seals had degraded, allowing fluid to bypass between ports. Replacing the O-ring stack restored full travel functionality.
Disassembly and Inspection Strategy
Servicing the center joint requires careful disassembly and attention to seal placement. Key steps include:

• Removing the upper structure access panel
  
• Disconnecting hydraulic lines with proper labeling
  
• Extracting the center joint from its housing
  
• Inspecting each port for scoring or wear
  
• Replacing all internal seals, not just the visibly damaged ones
  

A technician in Ontario noted that skipping the case drain seal during rebuild led to premature failure. The motor began leaking within 100 hours, requiring a second teardown. Always replace the full seal kit and verify port alignment during reassembly.
Preventive Maintenance and Fluid Health
To extend the life of the center joint:

• Use only OEM-grade hydraulic fluid with correct viscosity
  
• Replace filters every 500 hours or sooner in dusty environments
  
• Monitor fluid temperature during extended operation
  
• Inspect hoses and fittings for vibration wear
  
• Avoid overloading the swing circuit during travel
  

Hydraulic fluid contamination is a leading cause of seal degradation. A fleet manager in Texas implemented a fluid sampling program and reduced center joint failures by 60% over two years.
Advanced Diagnostics and Pressure Testing
If symptoms persist after seal replacement, deeper diagnostics may be needed:

• Pressure test each port with the machine off and isolated
  
• Use infrared imaging to detect heat buildup in the joint
  
• Check for internal scoring using a borescope
  
• Compare flow rates between left and right travel motors
  
• Inspect pilot control valves for debris or sticking
  

In one case, a rebuilt center joint continued leaking due to a hairline crack in the manifold body—undetectable without dye penetrant testing. Replacing the entire assembly resolved the issue.
Operator Techniques to Reduce Stress on the Joint

• Avoid sharp turns while traveling at full speed
  
• Let the machine come to a full stop before swinging
  
• Use smooth joystick inputs to reduce pressure spikes
  
• Keep the undercarriage clean to prevent debris intrusion
  
• Warm up hydraulics before heavy-duty operation
  

A forestry operator in Washington reported that his EX120-2 lasted over 8,000 hours without center joint failure by following a strict warm-up and shutdown routine.
Anecdote from the Field
In 2021, a small excavation firm in Ireland faced repeated travel issues with their EX120-2. After replacing both travel motors and flushing the system, the problem persisted. A retired Hitachi engineer volunteered to inspect the machine and found the center joint seals had been installed in reverse order. Once corrected, the machine ran flawlessly, and the firm avoided a costly replacement.
Conclusion
The center joint in a Hitachi EX120-2 is a vital but often overlooked component. Its failure can mimic motor or valve issues, leading to misdiagnosis and unnecessary expense. With proper seal replacement, fluid care, and thoughtful operation, this component can deliver thousands of hours of reliable service. Understanding its role and vulnerabilities empowers owners to maintain performance and avoid downtime.