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The 70D and Its Radial Piston Drive Motor
The John Deere 70D excavator, produced around 1989, was part of Deere’s collaboration with Hitachi, sharing many design elements with the EX60 series. While the engine differs, the hydraulic architecture and undercarriage layout are strikingly similar. One notable distinction is the use of radial piston motors in the final drives, rather than the more common axial piston motors found in larger machines.
Radial piston motors offer high torque at low speeds and are often used in applications requiring compact packaging and strong starting torque. However, they tend to be more mechanically complex and less robust under high-load conditions compared to axial designs. In the 70D, this choice may have been influenced by the machine’s blade-equipped configuration, which demands higher torque during grading and pushing.
Failure of the Final Drive Output Shaft
A common issue in aging 70D units is the failure of the final drive output shaft. In one case, the excavator could only rotate in circles due to one drive motor failing entirely. Upon disassembly, the root cause was revealed: the output shaft had been stripped by the planetary gears, which had dropped due to broken roll pins in the gear hubs.
The damage resembled lathe-like wear, where the unsupported gears chewed into the shaft splines until they were completely eroded. The sun gear remained intact, but the second planetary stage showed severe wear. The roll pins, meant to retain gear alignment, had all fractured, allowing the gears to misalign and destroy the shaft.
Disassembly and Repair Challenges
Removing the damaged shaft required full disassembly of the final drive from the opposite side to access the internal snap ring. This process is labor-intensive and demands careful handling of planetary gear components, brake mechanisms, and bearing assemblies.
Key observations during teardown:
Can the Shaft Be Repaired by Welding and Machining
Rebuilding the shaft by welding and re-hobbing is theoretically possible but requires precision and metallurgical knowledge. The shaft must be restored to its original spline profile and hardness to withstand torque loads and gear engagement.
Recommendations for repair:
Sourcing Used or Aftermarket Parts
Given the age of the 70D, sourcing used parts from salvage yards or dismantlers is a common strategy. However, used final drives may suffer from the same internal failures, especially if roll pins or planetary carriers were neglected.
Tips for sourcing:
Lessons from the Field and Operator Wisdom
One operator shared that he towed his 70D into a garage for repairs, only to discover the final drive failure during disassembly. He had assumed the issue was hydraulic—possibly the rotary manifold or motor—but the mechanical damage was the true culprit.
Another technician noted that planetary gear failures often begin with small components like roll pins. Once these fail, cascading damage follows. Regular inspection and preventive replacement of these pins can avoid catastrophic shaft wear.
Conclusion
The final drive failure on a John Deere 70D excavator highlights the importance of mechanical integrity in radial piston motor systems. While hydraulic components often receive attention, planetary gear alignment and shaft condition are equally critical. Whether rebuilding with weld and machine or sourcing used parts, the key is precision and patience. In vintage iron, every repair is a lesson—and every shaft tells a story.
The John Deere 70D excavator, produced around 1989, was part of Deere’s collaboration with Hitachi, sharing many design elements with the EX60 series. While the engine differs, the hydraulic architecture and undercarriage layout are strikingly similar. One notable distinction is the use of radial piston motors in the final drives, rather than the more common axial piston motors found in larger machines.
Radial piston motors offer high torque at low speeds and are often used in applications requiring compact packaging and strong starting torque. However, they tend to be more mechanically complex and less robust under high-load conditions compared to axial designs. In the 70D, this choice may have been influenced by the machine’s blade-equipped configuration, which demands higher torque during grading and pushing.
Failure of the Final Drive Output Shaft
A common issue in aging 70D units is the failure of the final drive output shaft. In one case, the excavator could only rotate in circles due to one drive motor failing entirely. Upon disassembly, the root cause was revealed: the output shaft had been stripped by the planetary gears, which had dropped due to broken roll pins in the gear hubs.
The damage resembled lathe-like wear, where the unsupported gears chewed into the shaft splines until they were completely eroded. The sun gear remained intact, but the second planetary stage showed severe wear. The roll pins, meant to retain gear alignment, had all fractured, allowing the gears to misalign and destroy the shaft.
Disassembly and Repair Challenges
Removing the damaged shaft required full disassembly of the final drive from the opposite side to access the internal snap ring. This process is labor-intensive and demands careful handling of planetary gear components, brake mechanisms, and bearing assemblies.
Key observations during teardown:
- The hub showed wear marks from gear misalignment
- The planetary carrier had fractured roll pins and displaced gears
- The output shaft splines were completely stripped
- The sun gear remained serviceable, suggesting localized failure
Can the Shaft Be Repaired by Welding and Machining
Rebuilding the shaft by welding and re-hobbing is theoretically possible but requires precision and metallurgical knowledge. The shaft must be restored to its original spline profile and hardness to withstand torque loads and gear engagement.
Recommendations for repair:
- Use high-strength welding rod (e.g., 4140 or equivalent alloy)
- Preheat and post-heat the shaft to prevent cracking
- Machine splines to OEM dimensions using a gear hob or CNC mill
- Perform hardness testing to ensure durability (Rockwell C scale target: 50–55)
- Inspect concentricity and runout to avoid vibration or misalignment
Sourcing Used or Aftermarket Parts
Given the age of the 70D, sourcing used parts from salvage yards or dismantlers is a common strategy. However, used final drives may suffer from the same internal failures, especially if roll pins or planetary carriers were neglected.
Tips for sourcing:
- Request internal photos or teardown reports before purchase
- Verify part numbers and compatibility with Hitachi EX60 components
- Consider aftermarket suppliers specializing in hydraulic drive motors
- Cross-reference with rebuilders who offer remanufactured planetary assemblies
Lessons from the Field and Operator Wisdom
One operator shared that he towed his 70D into a garage for repairs, only to discover the final drive failure during disassembly. He had assumed the issue was hydraulic—possibly the rotary manifold or motor—but the mechanical damage was the true culprit.
Another technician noted that planetary gear failures often begin with small components like roll pins. Once these fail, cascading damage follows. Regular inspection and preventive replacement of these pins can avoid catastrophic shaft wear.
Conclusion
The final drive failure on a John Deere 70D excavator highlights the importance of mechanical integrity in radial piston motor systems. While hydraulic components often receive attention, planetary gear alignment and shaft condition are equally critical. Whether rebuilding with weld and machine or sourcing used parts, the key is precision and patience. In vintage iron, every repair is a lesson—and every shaft tells a story.
