Hitachi EX120-2 Swing Motor Hydraulic Contamination and Repair

[MikePhua]

The EX120-2’s Role in Excavator Development
The Hitachi EX120-2 is a mid-sized hydraulic excavator introduced in the early 1990s as part of Hitachi’s second-generation EX series. Designed for general construction, utility trenching, and light forestry work, the EX120-2 offered a balance of reach, power, and fuel efficiency. With an operating weight of approximately 12 metric tons and a bucket breakout force exceeding 90 kN, it became a popular choice for contractors seeking reliability without the bulk of larger machines.
Hitachi Construction Machinery, founded in 1970, quickly gained global recognition for its hydraulic innovation and durable undercarriage systems. By the time the EX120-2 entered production, Hitachi had already established joint ventures with Deere and Fiat, expanding its reach across North America and Europe. Tens of thousands of EX-series machines were sold worldwide, and many remain in active service today.
Understanding the Swing Motor System
The swing motor on the EX120-2 is a hydraulic-driven unit responsible for rotating the upper structure of the excavator. It receives pressurized fluid from the main pump and converts it into rotary motion via a hydraulic motor and planetary gear reduction. The swing motor is mounted beneath the cab and interfaces with the swing bearing, allowing smooth 360-degree rotation.
Terminology note:

• Swing Motor: A hydraulic motor that powers the rotation of the excavator’s upper frame.
  
• Planetary Gearbox: A gear system that multiplies torque while reducing speed, used in swing and travel drives.
  
• Case Drain Line: A low-pressure return line that allows excess fluid and leakage from the motor housing to return to the tank.
  

The swing motor is lubricated and cooled by hydraulic fluid, which also powers its internal components. Any contamination or overfilling can disrupt this balance, leading to performance issues or mechanical damage.
Symptoms of Hydraulic Fluid Intrusion
Operators may notice several signs when hydraulic fluid enters the swing motor housing or gearbox:

• Milky or foamy fluid in the swing motor case
  
• Excessive pressure in the case drain line
  
• Leaking seals around the swing motor flange
  
• Jerky or delayed swing response
  
• Audible whining or grinding during rotation
  

In one case, a contractor in Queensland found hydraulic fluid overflowing from the swing motor’s fill port. After draining and inspecting the unit, he discovered that the motor’s internal seal had failed, allowing high-pressure fluid to bypass into the gear housing. This not only diluted the gear oil but also increased case pressure, risking seal blowout.
Diagnostic Steps and Inspection Tips
To diagnose swing motor contamination:

• Drain the swing motor case and inspect fluid color and consistency
  
• Check for metal particles or sludge in the drained oil
  
• Remove the case drain line and measure backpressure during operation
  
• Inspect the motor’s internal seals and O-rings for wear or damage
  
• Verify that the hydraulic fluid is not entering through the rotary manifold or center joint
  

If the swing motor is overfilled or pressurized, it may indicate a failed shaft seal or internal bypass. A pressure test can confirm whether fluid is leaking past the motor’s internal components.
Repair Procedures and Preventive Measures
If contamination is confirmed:

• Disassemble the swing motor and gearbox carefully
  
• Replace all internal seals, including shaft seals and case seals
  
• Flush the gearbox and motor housing with clean hydraulic-safe solvent
  
• Refill with manufacturer-recommended gear oil (typically SAE 80W-90 or ISO 150)
  
• Test the motor under load and monitor case drain pressure
  

Preventive tips:

• Inspect swing motor seals every 1,000 hours
  
• Replace gear oil annually or every 2,000 hours
  
• Use magnetic drain plugs to catch early wear particles
  
• Avoid overfilling the swing motor case
  
• Monitor hydraulic fluid condition and change filters regularly
  

One operator in British Columbia added a small sight glass to his swing motor fill port, allowing visual inspection of fluid level and condition without removing the plug. This helped him detect early signs of contamination and avoid a costly rebuild.
Design Evolution and Manufacturer Insights
Later models like the Hitachi ZX120 introduced improved swing motor sealing and electronic monitoring. These upgrades reduced the risk of hydraulic intrusion and allowed real-time diagnostics via onboard displays. However, the EX120-2 remains popular for its mechanical simplicity and field-serviceable components.
Hitachi’s swing motor design has influenced other brands, including Deere and Bell, through shared engineering platforms. Sales data from 2023 indicates that over 18,000 EX120-series machines are still operational globally, with strong demand for parts and rebuild kits.
Conclusion
Hydraulic fluid intrusion into the swing motor of a Hitachi EX120-2 is a serious but solvable issue. With careful diagnostics, seal replacement, and fluid management, operators can restore full swing function and prevent long-term damage. The EX120-2’s enduring popularity is a testament to its robust design—and with proper care, its swing motor will keep turning for years to come.