Diagnosing Coupling Alignment and Puck Failures on the Caterpillar 955 Track Loader

[MikePhua]

The Legacy of the Caterpillar 955 Series
The Caterpillar 955 track loader was a staple of mid-20th century earthmoving, combining the ruggedness of a dozer with the versatility of a loader. Introduced in the 1950s and refined through the 1980s, the 955 series—particularly the 955K and 955L—was widely used in construction, demolition, and quarry operations. With a powertrain built around a torque converter and powershift transmission, the machine offered smooth operation and high breakout force. Caterpillar sold tens of thousands of units globally, and many are still in service today due to their mechanical simplicity and rebuildable components.
One critical element in the drivetrain is the rubber puck coupling between the engine flywheel and transmission input. These pucks absorb vibration and allow for minor misalignment, but when they fail, they can cause severe vibration, loss of drive, and damage to surrounding components.
Terminology Notes

• Puck Coupling: A set of rubber or elastomeric discs that connect the engine flywheel to the transmission, allowing for torque transfer and vibration damping.
  
• Runout: The deviation of a rotating component from true center, measured with a dial indicator.
  
• Shims: Thin metal spacers used to adjust alignment between components.
  
• Trunnion: A pivoting mount that allows limited movement of the transmission or engine to accommodate flex.
  

Symptoms of Puck Failure and Misalignment
Operators may observe:

• Visible separation or cracking of pucks, especially between 6 and 10 o’clock positions when viewed from below
  
• Excessive vibration during startup or under load
  
• Difficulty engaging gears or maintaining consistent drive
  
• Uneven wear on mounts or coupling bolts
  
• Audible thumping or knocking from the bellhousing area
  

In one case, a loader in Southern California showed puck separation after sitting idle for over a decade. The operator noted that adding a 1/8" washer to the left rear mount reduced the separation, indicating misalignment as the root cause.
Root Causes and Diagnostic Pathways
Puck failure is often a symptom of deeper alignment issues. Key diagnostic steps include:

• Check Engine and Transmission Mounts
  • Worn rubber mounts can allow sagging or tilt
    
  • Solution: Replace all mounts before attempting alignment
    
• Measure Flywheel Runout
  • Use a dial indicator to check outer runout and face deviation
    
  • Acceptable values are typically under 0.030" for outer and 0.020" for face
    
• Shim Adjustment
  • Use calibrated shims to correct angular misalignment
    
  • Common shim thicknesses include 0.065", 0.090", and 0.125"
    
• Trunnion Movement
  • Verify axial and vertical play in the transmission trunnion
    
  • Solution: Lubricate or rebuild trunnion if seized or worn
    
• Coupling Bolt Torque
  

• Uneven torque can distort puck alignment
  
• Solution: Torque bolts in a star pattern, recheck after warm-up
  

A technician in Texas rebuilt a 955’s coupling by installing two 0.065" shims on each front mount and a 1/8" washer on the rear left. Final dial readings showed 0.030" outer runout and 0.022" face deviation—within acceptable limits.
Preventive Maintenance and Upgrade Options
To extend puck life and maintain alignment:

• Inspect mounts and coupling annually
  
• Replace pucks every 5–7 years or 2,000 hours, whichever comes first
  
• Use anti-vibration washers and torque-lock fasteners
  
• Clean mating surfaces before reassembly
  
• Document shim configurations for future reference
  

Some owners retrofit their 955s with polyurethane pucks for improved durability. A crew in Argentina added alignment pins to the bellhousing to simplify future rebuilds and reduce setup time.
Operator Anecdotes and Field Wisdom
A retired operator in Montana recalled rebuilding a 955 coupling in freezing weather. He used a propane torch to warm the bellhousing and ease puck installation. After aligning with a dial indicator and shimming the mounts, the machine ran smooth for another decade.
In British Columbia, a demolition team used a 955L with reinforced pucks and upgraded mounts to clear a collapsed warehouse. Despite heavy shock loads, the coupling held firm thanks to precise alignment and regular inspection.
Recommendations for Technicians and Fleet Managers
When servicing puck couplings:

• Begin with mount inspection and replacement
  
• Use dial indicators to measure runout before disassembly
  
• Shim strategically based on measured deviation
  
• Torque bolts evenly and recheck after thermal cycling
  
• Keep spare pucks, shims, and washers in inventory
  

A fleet manager in Georgia created a coupling service checklist including mount condition, shim thickness, runout readings, and torque specs. This reduced vibration complaints and extended drivetrain life across his legacy loader fleet.
Conclusion
Puck coupling issues in the Caterpillar 955 are often the result of misalignment, worn mounts, or improper installation. With careful diagnostics, strategic shimming, and preventive care, technicians can restore smooth operation and protect the drivetrain from long-term damage. In vintage loaders, precision matters—and the puck is the pivot point where power meets reliability.