Diagnosing Hydraulic Oil Leaks in JD 70D and Hitachi EX60 Final Drives

[MikePhua]

The JD 70D and Hitachi EX60 Shared Platform
The John Deere 70D excavator shares its core architecture with the Hitachi EX60, a compact hydraulic excavator developed in the late 1980s. These machines were built for trenching, utility work, and light demolition, featuring a robust undercarriage and planetary final drives. The final drive assembly combines hydraulic motor input with gear reduction to power the sprockets and tracks. While reliable, these units are prone to internal seal failures over time, especially in high-hour machines or those exposed to wet, abrasive environments.
Common Symptoms of Final Drive Leakage
Operators often report sudden loss of hydraulic oil from the final drive, typically observed as fluid dripping from the sprocket area or pooling beneath the motor guard. In some cases, the leak is so rapid that the hydraulic tank drains within minutes. The absence of gear oil odor and the presence of clean hydraulic fluid point to internal seal failure rather than a gearbox breach.
Key indicators:

• Hydraulic oil dripping from the lower sprocket housing
  
• No gear oil smell (gear oil has a distinct, pungent odor)
  
• Rapid fluid loss upon refilling the hydraulic tank
  
• Clean undercarriage except for localized leak zone
  

Terminology and Component Breakdown
- Final Drive: A gear reduction unit that receives hydraulic input and powers the track sprocket.
- Brake Piston: A hydraulic-actuated component within the final drive that engages the parking brake.
- Duo-Cone Seal: A mechanical face seal used between rotating and stationary components to retain gear oil and exclude contaminants.
- Motor Shaft Seal: A seal preventing hydraulic oil from leaking along the motor’s rotating shaft.
- Belleville Washer: A conical spring washer used to preload components like brake pistons.
Root Causes and Failure Points
The most common cause of hydraulic oil leakage in these final drives is failure of the brake piston D-ring seals. These seals degrade over time due to heat, pressure cycling, and contamination. When they fail, hydraulic oil bypasses the piston and escapes into the drive housing, eventually leaking out through the sprocket area.
Other potential failure points include:

• Motor shaft seal wear or scoring
  
• Duo-cone seal degradation due to clay or debris packing
  
• O-ring disintegration from age or chemical exposure
  
• Hose abrasion against the frame, causing external leaks
  

In one case, a machine operating in wet clay had its undercarriage packed solid, masking the leak until the unit was pressure-washed. Upon teardown, the duo-cone seals were found to be dry-rotted and the brake piston seals had collapsed.
Disassembly and Repair Strategy
Repairing the final drive does not require full removal of the drive unit. The hydraulic motor can be detached to access the brake piston assembly. Once the motor is removed, the Belleville washer and brake spring are extracted, revealing the piston and its two D-ring seals—one large and one small.
Steps for seal replacement:

• Remove hydraulic motor from final drive housing
  
• Extract Belleville washer and brake spring
  
• Pull out brake piston and inspect for scoring
  
• Replace both D-ring seals with OEM or high-quality aftermarket equivalents
  
• Clean mating surfaces and reassemble with fresh hydraulic oil
  

If gear oil is also leaking, the duo-cone seal must be replaced. This requires removing the sprocket drum and ring gear, then accessing the large retaining nut and locking dowel. The dowel is often hardened and recessed, requiring careful extraction or drilling.
Recommendations and Preventative Measures

• Inspect undercarriage regularly for clay buildup or debris packing
  
• Replace hydraulic oil and gear oil at recommended intervals
  
• Use high-quality seals rated for hydraulic pressure and temperature
  
• Monitor for metal shavings in drained oil, especially brass-colored particles
  
• Avoid aggressive operation in abrasive terrain without undercarriage cleaning
  

One technician noted that the locking dowel in the ring gear must be driven through to release the retaining nut. Fabricating a custom socket for reassembly torque is advised. Disassembling the unit in-frame is possible but more difficult—removal of the entire final drive is recommended for thorough inspection.
Conclusion
Hydraulic oil leaks in JD 70D and Hitachi EX60 final drives are typically caused by brake piston seal failure or motor shaft seal degradation. With careful disassembly and seal replacement, these issues can be resolved without full drive removal. Understanding the internal layout and failure modes of these compact excavators is essential for maintaining uptime and avoiding costly fluid loss. In final drives, the smallest seal can make the biggest mess—but with the right tools and knowledge, the fix is within reach.