Swing System Failure on the Yutani 120 LC Excavator

[MikePhua]

The Yutani 120 LC and Its Mechanical Lineage
The Yutani 120 LC excavator was part of a short-lived but respected line of hydraulic machines produced by Yutani Heavy Industries before its merger with Kobelco in the early 1990s. Known for their robust steel construction and straightforward hydraulic systems, Yutani excavators were widely used in Southeast Asia and parts of North America. The 120 LC, with an operating weight of roughly 27,000 pounds and a bucket breakout force exceeding 20,000 pounds, was designed for general excavation, trenching, and site preparation.
Though Yutani ceased independent production decades ago, many of its machines remain in service due to their mechanical simplicity and ease of repair. However, sourcing parts and documentation can be challenging, especially for swing systems and undercarriage components.
Swing Table Behavior and Symptoms of Failure
One of the most critical components in any excavator is the swing system, which allows the upper structure to rotate smoothly on the undercarriage. In the Yutani 120 LC, the swing mechanism consists of:

• A swing bearing (slew ring)
  
• Internal gear teeth
  
• A hydraulic swing motor
  
• Reduction gearbox
  
• Grease cavity and seal system
  

When the swing table begins to emit abnormal noise, becomes jerky, or refuses to rotate, it often indicates internal bearing damage or gear misalignment. In this case, the operator reported increased noise after greasing the swing table, followed by erratic movement and eventual failure to swing.
This sequence suggests that the grease may have displaced debris or exposed worn components, triggering a cascade of mechanical symptoms.
Diagnosing Swing Bearing and Gear Damage
To assess the condition of the swing system:

• Remove the access panel and inspect the swing bearing visually
  
• Check for metal shavings or discolored grease
  
• Rotate the upper structure manually (engine off) to feel for binding or flat spots
  
• Inspect the swing motor and gearbox for leaks or loose mounting bolts
  
• Use a dial indicator to measure bearing play—anything over 2 mm radial or axial movement is excessive
  

If the swing bearing is worn or cracked, it must be replaced. This involves lifting the upper structure, removing hydraulic lines and electrical harnesses, and unbolting the bearing from both the carbody and the upper deck.
Repair Complexity and Feasibility
Replacing a swing bearing is a major repair, typically requiring:

• A crane or gantry system to lift the upper structure
  
• Precise alignment tools to seat the new bearing
  
• Torque wrenches to secure bolts to spec (often 400–600 Nm)
  
• Clean environment to prevent contamination during installation
  

While technically feasible for a skilled operator with access to proper tools, this job is best performed in a shop or with field support from a heavy equipment technician. Attempting the repair without lifting equipment or alignment tools risks damaging the new bearing or misaligning the swing gear.
Swing Motor and Gearbox Considerations
If the swing motor is functioning but the table does not rotate, the issue may lie in the reduction gearbox or internal gear teeth. Common failure modes include:

• Broken gear teeth due to shock loading
  
• Worn planetary gears
  
• Hydraulic motor seal failure causing internal leakage
  

To test the motor:

• Engage swing function and listen for motor response
  
• Check hydraulic pressure at the motor inlet (should exceed 2,500 psi)
  
• Inspect return line for flow—lack of return may indicate blockage
  

If the motor spins but the table does not move, the gearbox may be stripped. Rebuilding the gearbox requires disassembly, gear inspection, and replacement of worn components. Some operators opt to source used swing assemblies from salvage yards or compatible Kobelco models.
Field Anecdotes and Practical Advice
One operator in Malaysia reported similar symptoms on a Yutani 120 LC used for palm plantation trenching. After greasing the swing bearing, the machine began to groan and hesitate during rotation. Upon inspection, he found that the bearing had cracked internally, and the grease had pushed fragments into the gear teeth. Replacing the bearing and cleaning the gear restored full function.
Another technician in Oregon used a dial indicator to measure bearing play on a 120 LC with swing hesitation. The reading showed 3.2 mm axial movement—well beyond tolerance. After lifting the cab and replacing the bearing, the swing returned to smooth operation.
Recommendations for Long-Term Swing Health
To maintain swing system integrity:

• Grease the swing bearing every 100 hours using high-pressure lithium grease
  
• Rotate the upper structure during greasing to distribute lubricant evenly
  
• Inspect bearing play annually with a dial gauge
  
• Avoid shock loading during rotation (e.g., sudden stops or impacts)
  
• Replace seals and clean gear teeth during major service intervals
  

For machines operating in dusty or abrasive environments, consider installing a swing bearing seal guard to prevent contamination.
Yutani’s Excavator Legacy
Yutani Heavy Industries was a respected name in hydraulic machinery before merging with Kobelco. Their excavators were known for thick steel frames, reliable hydraulics, and straightforward mechanical systems. Though parts are scarce, many components are compatible with early Kobelco models, and salvage yards often carry swing motors and gearboxes that fit the 120 LC.
Conclusion
Swing failure on the Yutani 120 LC is often the result of bearing wear, gear damage, or hydraulic motor issues. By diagnosing the problem methodically and respecting the mechanical complexity of the swing system, operators can restore rotation and extend the life of these legacy machines. In the world of excavation, smooth swing isn’t just a luxury—it’s a necessity for precision and productivity.