Refurbishing the Trunnion Assembly on a CAT 988B Loader

[MikePhua]

The CAT 988B and Its Structural Legacy
The Caterpillar 988B wheel loader was introduced in the late 1970s as part of CAT’s heavy-duty loader lineup, designed for quarrying, mining, and bulk material handling. With an operating weight exceeding 90,000 pounds and a bucket capacity of up to 12 cubic yards, the 988B became a cornerstone of high-production sites worldwide. Caterpillar, founded in 1925, had by then established itself as a global leader in earthmoving equipment, and the 988B reinforced that reputation with its robust frame, torque-rich drivetrain, and durable articulation system.
One of the most critical structural components of the 988B is the rear frame trunnion assembly. This pivoting mount supports the rear axle and allows vertical movement of the axle relative to the frame, absorbing shock loads and maintaining traction on uneven terrain. Over time, the trunnion assembly can wear, crack, or deform—especially in machines subjected to high-impact loading or poor maintenance.
Terminology Notes

• Trunnion: A cylindrical pivot that allows rotation or articulation between two components.
  
• Bearing Cap: A cover that holds the trunnion bearings in place and maintains preload.
  
• Bushing: A replaceable sleeve that reduces friction between the trunnion and its housing.
  
• Preload: The initial force applied to bearings or bushings to eliminate play and ensure tight fitment.
  

Symptoms of Trunnion Wear or Failure
Operators and technicians may notice:

• Excessive rear axle movement or clunking during travel
  
• Uneven tire wear or poor traction on rough terrain
  
• Visible cracking or deformation around the trunnion housing
  
• Grease leakage from bearing caps or seals
  
• Difficulty maintaining alignment during steering or loading
  

In one case, a quarry in Arizona reported that their 988B began drifting during bucket fills. Inspection revealed a worn trunnion bushing and cracked bearing cap, which had allowed the rear axle to shift under load.
Disassembly and Inspection Procedures
Refurbishing the trunnion assembly requires careful disassembly and thorough inspection:

• Block the machine securely and remove the rear wheels
  
• Disconnect the rear axle from the trunnion housing
  
• Remove bearing caps and extract the trunnion shaft
  
• Inspect bushings for scoring, ovality, or metal transfer
  
• Check the housing for cracks using magnetic particle or dye penetrant testing
  
• Measure shaft diameter and housing bore to verify tolerances
  

If the shaft shows more than 0.010" wear or the housing is out-of-round by more than 0.005", replacement or machining is recommended.
Refurbishment Strategies and Component Replacement
To restore the trunnion assembly:

• Replace bushings with OEM or high-grade bronze alternatives
  
• Machine the shaft and housing to restore concentricity
  
• Install new seals and grease fittings
  
• Torque bearing caps to manufacturer specifications
  
• Apply anti-seize compound to mating surfaces to ease future service
  

Some rebuilders opt to fabricate custom bushings from polymer composites for improved wear resistance. A mining crew in Chile reported doubling service intervals after switching to graphite-impregnated bushings in their 988B fleet.
Preventive Maintenance and Long-Term Solutions
To extend trunnion life:

• Grease pivot points every 50 hours in high-impact environments
  
• Inspect bearing caps and seals during quarterly service
  
• Monitor rear axle movement during daily walkarounds
  
• Avoid high-speed travel over uneven terrain with full bucket loads
  
• Replace bushings and seals every 5,000 hours or during major rebuilds
  

A fleet manager in British Columbia added trunnion inspection to his loader maintenance checklist, reducing axle-related failures by 60% over two years.
Operator Anecdotes and Field Wisdom
A retired operator in Georgia recalled his 988B developing a rear-end shimmy during haul cycles. After checking the trunnion, he found the bushing had worn through and the shaft was contacting the housing directly. Replacing the bushing and machining the shaft restored stability and improved tire life.
In Argentina, a crew used their 988B for loading salt in corrosive conditions. They installed stainless steel bushings and added a sealed grease system to prevent contamination. The modification extended trunnion life by 40% and reduced downtime.
Conclusion
The trunnion assembly on the CAT 988B is a vital structural component that governs rear axle articulation and load stability. With proper inspection, precision machining, and quality replacement parts, this system can be restored to full function and continue supporting high-production operations. Whether in a quarry, mine, or bulk yard, maintaining the trunnion is essential to keeping the 988B moving, lifting, and performing at its best.