Servicing a Steerable Rockwell Rear Axle on Vintage Earthmoving Tractors: Seal Replacement and Structural Insights

[MikePhua]

Introduction: A Glimpse into Mid-Century Earthmoving Engineering
The 1964 Mississippi Road Service tractor, designed for pulling rear pans in large-scale dirt loading operations, features a unique steerable Rockwell rear axle—specifically the PS 251 HX 12 model. This axle design includes a spherical knuckle joint allowing the rear wheels to pivot, enhancing maneuverability in tight jobsite conditions. While robust, these axles present challenges when seals deteriorate, especially for operators unfamiliar with planetary hub layouts and constant velocity (CV) joints.
Understanding the Steerable Rockwell Rear Axle
This axle configuration integrates several key components:

• Spherical knuckle joint for steering articulation
  
• Planetary reduction hub (3- or 4-gear series)
  
• CV joint for torque transmission during steering
  
• Dust and oil seals to protect internal components
  
• Trunnion caps and tie rods for structural support
  

Unlike fixed rear axles, the steerable design allows the rear wheels to pivot independently, improving turning radius and load distribution when towing pans or scrapers.
Terminology Note: Trunnion Cap and Planetary Hub
- Trunnion Cap: A bolted plate securing the steering knuckle to the axle housing, typically located at the top and bottom of the joint.
- Planetary Hub: A gear reduction system housed within the wheel hub, using multiple planetary gears to multiply torque and reduce speed.
Seal Identification and Misconceptions
Initial inspection revealed a damaged rubber seal near the knuckle. While it appeared to be an oil seal, further analysis confirmed it was a dust seal—designed to keep contaminants out, not fluid in. The actual oil seal resides deeper within the axle tube, behind the planetary assembly and CV joint.
This distinction is critical. Replacing only the dust seal without addressing the internal oil seal may result in continued leakage and contamination of the brake area.
Suggested Parameters for Seal Inspection and Replacement

• Dust seal diameter: ~8–10 inches (varies by model)
  
• Oil seal location: Behind planetary hub, inside axle tube
  
• Brake cavity: Should remain dry; presence of oil indicates seal failure
  
• CV joint spline engagement: Must align precisely during reassembly
  
• Torque spec for trunnion cap bolts: 120–150 ft-lbs (check OEM manual)
  

Disassembly Strategy for Seal Access
To access the oil seal, technicians must:

• Remove upper and lower trunnion caps
  
• Disconnect tie rod and steering cylinder
  
• Slide out the entire knuckle and hub assembly
  
• Disassemble planetary gears and CV joint
  
• Extract the seal from the axle tube using a puller or hook tool
  

This process requires careful alignment during reassembly, especially when reinserting the CV joint splines and ensuring the steering geometry remains intact.
Anecdote: The Mystery Leak on a Pettibone Loader
A similar axle design appeared on a 1970s Pettibone loader, where oil leaked onto the tire from the spherical knuckle. The operator initially replaced the outer seal, but the leak persisted. Only after removing the planetary hub did they discover a torn oil seal deep inside the axle tube. The repair required custom seal sourcing and a hydraulic press to reinstall the hub—highlighting the importance of full disassembly when diagnosing axle leaks.
Recommendations for Vintage Axle Maintenance

• Use OEM or high-quality aftermarket seals rated for gear oil exposure
  
• Inspect planetary gears for wear or pitting during disassembly
  
• Replace CV joint boots if cracked or brittle
  
• Clean brake components thoroughly if oil contamination is present
  
• Apply anti-seize to trunnion bolts to ease future service
  
• Document seal dimensions and part numbers for future reference
  

Alternative Solutions for Hard-to-Find Parts
If OEM seals are unavailable:

• Measure the seal cavity and shaft diameter precisely
  
• Source industrial seals from hydraulic or bearing suppliers
  
• Consider machining a custom seal retainer if tolerances differ
  
• Use Viton or nitrile rubber for oil resistance and longevity
  

Conclusion: Preserving Functionality Through Precision
Servicing a steerable Rockwell rear axle on a vintage earthmoving tractor demands mechanical insight, patience, and a clear understanding of seal placement. While the design is rugged, its layered construction means that superficial repairs often miss deeper issues. By committing to full disassembly and careful reassembly, operators can restore steering performance, prevent brake contamination, and extend the life of these historically significant machines. In the world of legacy iron, every seal tells a story—and every leak deserves a closer look.