Track Tension Failure After Sudden Impact on a JD 790 Excavator

[MikePhua]

The JD 790 and Its Undercarriage Design
The John Deere 790 excavator was introduced in the late 1980s as part of Deere’s expansion into full-size hydraulic excavators. Built for general earthmoving, trenching, and forestry work, the 790 featured a robust undercarriage with grease-adjusted track tensioning, a recoil spring assembly, and a floating idler system. Its design prioritized simplicity and field serviceability, making it a favorite among contractors and municipalities.
With thousands of units sold across North America, the 790 remains in use today, often in secondary roles or owner-operator fleets. However, age and wear can expose vulnerabilities in the track tensioning system—especially when subjected to sudden shock loads.
Symptoms of Sudden Track Loosening
A common failure scenario involves a loud mechanical “bang” followed by immediate slack in one track. The machine may still move, but the affected track sags visibly, and attempts to re-tension it using the grease fitting yield no results. This points to a failure in the recoil spring assembly or misalignment of the idler piston.
Typical symptoms include:

• Audible impact followed by track slack
  
• Grease tensioner unable to accept more grease
  
• Idler visibly displaced or misaligned
  
• No visible hydraulic leaks or broken links
  

In one case, the piston had physically slipped off the idler, rendering the tensioning system inoperative. This type of failure is rare but can occur when the idler frame has excessive play or the recoil spring is fatigued.
Terminology Clarification

• Recoil Spring: A heavy-duty coil spring that absorbs shock and maintains track tension.
  
• Grease Cylinder: A hydraulic-free tensioning mechanism that uses injected grease to push the idler forward.
  
• Idler: The front wheel of the track system that guides and tensions the track.
  
• Track Frame: The structural housing that supports rollers, idlers, and the recoil assembly.
  

Root Causes and Contributing Factors
Several factors can lead to this type of failure:

• Fatigue in the recoil spring after thousands of hours of operation
  
• Excessive play in the idler mounting due to worn bushings or cracked welds
  
• Misalignment caused by uneven wear or impact loading
  
• Grease cylinder seal failure allowing pressure loss
  

In older machines, the idler may begin to tilt or shift under load, especially during aggressive turning or when climbing over debris. If the piston slips off its seat, the spring can no longer maintain tension, and the track goes slack instantly.
A technician in British Columbia reported a similar incident on a Hitachi EX200, where the idler frame had cracked near the weld seam. The piston slipped during a side-load maneuver, causing the track to derail. Welding and realignment restored function, but the recoil spring had to be replaced due to deformation.
Inspection and Repair Strategy
To address the issue:

• Remove track guards and inspect the idler alignment
  
• Check for cracks or deformation in the track frame
  
• Measure recoil spring length and inspect for binding or rust
  
• Remove grease fitting and test cylinder movement manually
  
• Realign piston and idler using a pry bar and hydraulic jack if necessary
  

If the recoil spring is broken, replacement is mandatory. These springs are under extreme compression and must be handled with caution. Use a press or follow OEM procedures to avoid injury.
Recommendations:

• Replace recoil spring and grease cylinder as a matched set if either fails
  
• Weld reinforcement plates on the idler frame if excessive play is observed
  
• Use high-pressure grease rated for track tensioning systems
  
• Inspect track links and bushings for uneven wear that may contribute to misalignment
  

Preventive Maintenance and Long-Term Reliability
To prevent future failures:

• Inspect track tension monthly, especially after working in rocky terrain
  
• Replace grease seals every 1,000 hours or during undercarriage service
  
• Monitor idler alignment during visual walkarounds
  
• Avoid sudden directional changes under full load
  
• Keep track frames clean and free of packed debris that may stress components
  

For machines operating in forestry or demolition, consider installing reinforced idler brackets and using upgraded recoil springs with higher fatigue resistance.
Conclusion
A sudden track loosening event accompanied by a loud bang is often the result of recoil spring or idler piston failure. On machines like the JD 790, this can be traced to age-related wear, misalignment, or impact loading. By inspecting the tensioning system thoroughly and addressing root causes—not just symptoms—operators can restore track integrity and prevent costly downtime. In the world of excavators, a tight track isn’t just about performance—it’s about safety and control.