Hitachi EX120 Final Drive Reliability and Rebuild Strategy

[MikePhua]

The Hitachi EX120 and Its Mechanical Legacy
The Hitachi EX120 excavator, introduced in the late 1980s and continuing into the early 1990s, was part of Hitachi Construction Machinery’s global push to dominate the mid-size hydraulic excavator market. With an operating weight of approximately 12,000 kg and powered by a 4-cylinder Isuzu diesel engine producing around 80 horsepower, the EX120 was designed for trenching, site prep, and utility work. Its compact footprint and robust undercarriage made it a favorite among contractors in Asia, Europe, and North America.
Hitachi, founded in 1910, had by then established a reputation for precision hydraulic systems and durable steel fabrication. The EX120 was built with a focus on mechanical simplicity and field-serviceable components, including its final drive assemblies—critical for travel and torque delivery.
Final Drive Architecture and Common Wear Points
The final drive on the EX120 is a planetary gear reduction unit integrated with a hydraulic travel motor. It converts high-speed hydraulic rotation into low-speed, high-torque output, enabling the machine to crawl over uneven terrain and resist external loads.
Key components include:

• Hydraulic travel motor
  
• Planetary gear set
  
• Output shaft and sprocket hub
  
• Bearings and seals
  
• Case drain and brake circuit
  

Terminology annotation:

• Planetary Gear Set: A gear system where multiple planet gears rotate around a central sun gear, providing torque multiplication.
  
• Case Drain: A low-pressure return line that removes internal leakage from the motor housing.
  
• Sprocket Hub: The flange that connects the final drive output shaft to the track sprocket.
  
• Brake Circuit: A hydraulic loop that engages or releases the travel brake depending on pressure input.
  

In one excavation project in Malaysia, an EX120 began losing travel power on one side. Inspection revealed that the planetary gears had worn unevenly due to low oil level and contaminated lubricant. The machine had operated for months without a final drive oil change, leading to accelerated wear.
Symptoms of Final Drive Failure
Operators and technicians often encounter the following signs when the final drive begins to fail:

• Loss of travel torque or sluggish movement
  
• Grinding or clicking noises from the track motor area
  
• Oil leaks around the sprocket hub or motor flange
  
• Excessive heat buildup during travel
  
• Metal shavings in final drive oil during inspection
  

These symptoms may indicate bearing failure, gear tooth damage, seal degradation, or hydraulic motor malfunction. In older machines like the EX120, age-related wear and lack of preventative maintenance are common culprits.
Rebuild Strategy and Component Sourcing
Rebuilding a final drive on a 1990-era EX120 requires mechanical precision and parts compatibility. Many components are still available through aftermarket suppliers or salvage yards, though OEM parts may be discontinued.
Recommended steps:

• Drain and inspect final drive oil for contamination
  
• Remove track and sprocket to access drive assembly
  
• Detach hydraulic motor and inspect splines and seals
  
• Disassemble planetary gear set and measure backlash
  
• Replace bearings, seals, and worn gears
  
• Clean all mating surfaces and reassemble with torque specs
  
• Refill with 80W-90 gear oil or manufacturer-recommended fluid
  
• Test under load and monitor case drain flow
  

In British Columbia, a contractor rebuilt his EX120 final drive using a mix of OEM seals and aftermarket gears sourced from a dismantled ZX120. The rebuild cost under $2,000 and restored full travel function for another 3,000 hours.
Preventative Maintenance and Longevity Tips
To extend the life of the EX120 final drive:

• Change gear oil every 500 hours or annually
  
• Use magnetic drain plugs to monitor metal wear
  
• Inspect case drain flow monthly for signs of internal leakage
  
• Avoid high-speed travel over rocky terrain
  
• Grease track tensioners and inspect sprocket alignment
  
• Keep hydraulic fluid clean and within spec
  

For machines operating in wet or abrasive environments, consider installing external guards around the final drive to prevent impact damage and debris intrusion. In Florida, a land-clearing crew added bolt-on steel shields and reduced seal failures by 60%.
Conclusion
The final drive on the Hitachi EX120 is a testament to mechanical durability and rebuildable design. While age and wear are inevitable, proper diagnostics, sourcing, and service can restore performance and extend machine life. In the world of compact excavators, the EX120 remains a workhorse—and its final drive, when respected and maintained, keeps that legacy moving forward. Whether in trench or quarry, torque is the heartbeat of motion. And the final drive is where that heartbeat begins.