Kobelco SK120 Final Drive Failure and Hydraulic Troubleshooting

[MikePhua]

The SK120 and Its Drive System Architecture
The Kobelco SK120 excavator is a mid-sized crawler machine designed for urban construction, utility trenching, and forestry work. Introduced in the late 1990s and refined through multiple generations, the SK120 became one of Kobelco’s most widely distributed models across Asia, North America, and the Middle East. With an operating weight around 12 tons and a fuel-efficient Isuzu diesel engine, the SK120 balances power with maneuverability.
Its travel system relies on a dual-path hydraulic circuit, where each track is powered by an independent final drive motor. These motors are fed by the main hydraulic pump and controlled via pilot-operated valves. The final drive itself consists of a planetary gear reduction unit coupled to a hydraulic motor, sealed within a compact housing.
Terminology Annotation

• Final Drive: The assembly that converts hydraulic motor output into torque for track movement, typically including planetary gears and bearings.
  
• Travel Motor: A hydraulic motor mounted to the final drive, responsible for driving the track.
  
• Case Drain Line: A low-pressure return line that allows internal leakage oil from the motor to flow back to the tank.
  
• Pilot Pressure: A low-pressure hydraulic signal used to actuate control valves and direct main flow.
  

Symptoms of Final Drive Failure
Operators may encounter a situation where one track—typically the left or right—stops moving entirely, while the other functions normally. In the SK120, this often points to a failure in the final drive motor, a blockage in the hydraulic circuit, or a mechanical breakdown in the planetary gear set.
Common symptoms include:

• No movement in one track under load
  
• Audible whining or grinding from the drive housing
  
• Excessive heat buildup on the affected side
  
• Hydraulic fluid leakage near the motor flange
  
• Case drain line pressure spike or blockage
  

In one documented case, the left track of an SK120 stopped responding entirely. The machine could pivot using the right track, but the left remained stationary even under full travel command.
Hydraulic Diagnosis and Isolation
To isolate the fault, technicians should begin with hydraulic flow checks:

• Swap Travel Hoses: Connect the left motor’s hoses to the right motor’s ports. If the problem shifts sides, the issue lies upstream in the valve or pump.
  
• Check Pilot Pressure: Use a gauge to confirm that pilot signal reaches the travel valve spool.
  
• Inspect Case Drain Flow: Disconnect the case drain line and measure flow rate. A blocked or restricted line can cause internal pressure buildup and motor seizure.
  
• Monitor Return Line Pressure: Excessive backpressure can prevent motor rotation.
  

If hydraulic flow and pilot signals are normal, the fault likely lies within the motor or final drive assembly.
Mechanical Inspection and Common Failures
Final drives are subject to intense torque and shock loads, especially during turning, climbing, or operating on uneven terrain. Common mechanical failures include:

• Bearing Collapse: Roller or ball bearings inside the planetary gear set may fail, causing misalignment and seizure.
  
• Gear Tooth Shearing: Planet gears may lose teeth due to overload or fatigue.
  
• Seal Failure: Oil seals may rupture, allowing contamination or fluid loss.
  
• Shaft Splines Stripping: The motor shaft may lose engagement with the gear hub.
  

To inspect:

• Remove the track and final drive cover
  
• Drain the gear oil and check for metal shavings
  
• Rotate the motor manually and feel for resistance or grinding
  
• Disassemble the planetary gear set and inspect each component
  

In one repair, a contractor found that the planetary carrier had fractured due to a sudden impact while climbing a rock face. The motor was intact, but the gear set had locked up, preventing rotation.
Case Drain Line Importance and Misdiagnosis
The case drain line is often overlooked but plays a critical role in motor health. It allows internal leakage oil to escape, preventing pressure buildup behind seals. If this line is blocked or pinched, the motor may overheat, stall, or rupture seals.
Technicians should:

• Ensure the case drain line flows freely to the tank
  
• Use transparent tubing to observe flow during operation
  
• Replace damaged or kinked hoses immediately
  
• Avoid routing the case drain into high-pressure return circuits
  

In one incident, a machine was misdiagnosed with a failed motor, but the true cause was a blocked case drain line. Replacing the hose restored full travel function.
Preventative Measures and Long-Term Reliability
To extend final drive life on the SK120:

• Change Gear Oil Every 500 Hours: Use manufacturer-recommended viscosity and monitor for contamination.
  
• Inspect Seals Quarterly: Look for weeping or dirt accumulation around the flange.
  
• Avoid High-Speed Turns on Slopes: These maneuvers stress the planetary gears and bearings.
  
• Flush Hydraulic System Annually: Remove sludge and particulates that can damage motor internals.
  
• Use OEM Filters and Fluids: Ensure compatibility and maintain system integrity.
  

Operators working in abrasive environments like quarries or demolition sites should also consider installing guard plates around the final drive housing to prevent impact damage.
Conclusion
Final drive issues on the Kobelco SK120 excavator require a methodical approach to diagnosis. From hydraulic flow checks to mechanical disassembly, each step reveals critical clues. Whether the fault lies in a blocked case drain line, a failed bearing, or a stripped spline, timely intervention can prevent costly downtime. With proper maintenance and awareness of system behavior, the SK120’s travel system can deliver years of reliable service—even in the harshest conditions.