Replacing Sprocket Rings on the Caterpillar 931B Track Loader

[MikePhua]

The 931B and Its Mechanical Heritage
The Caterpillar 931B is a mid-sized track loader introduced in the late 1970s as part of CAT’s evolution from the 931A. Designed for versatility in excavation, grading, and material handling, the 931B featured a hydrostatic transmission, a 3204 four-cylinder diesel engine, and a robust undercarriage system. With an operating weight of approximately 16,000 pounds and a bucket capacity near 1 cubic yard, it became a staple in small construction fleets and municipal yards.
The 931B was part of Caterpillar’s broader push to modernize its track loader lineup, offering improved operator comfort, better visibility, and easier service access. Though no longer in production, thousands of units remain in service globally, especially in rural and restoration settings.
Terminology and Component Notes
- Sprocket Ring: The toothed outer section of the drive sprocket that engages with the track links.
- Bolt-On Ring: A replacement sprocket ring designed to be welded or bolted onto the existing hub after the original teeth wear out.
- Vertical Borer: A precision machining tool used to ensure concentricity and flatness when installing new rings.
- Radius Marker: A layout tool used to scribe a circular cut line on the sprocket face for accurate removal.
- Center Punch: A pointed tool used to mark the exact center of a circle, critical for symmetrical layout and alignment.
Cutting and Replacing the Sprocket Ring
When the original sprocket teeth wear down, replacing the entire sprocket assembly can be costly and time-consuming. A more practical solution is to cut off the worn ring and weld on a new bolt-on replacement. This process requires precision to ensure the new ring runs true and does not cause track misalignment or premature wear.
Recommended procedure:

• Use a radius marker with soapstone to scribe a clean cut line around the sprocket face
  
• Center punch the hub to establish a reference point for symmetry
  
• Cut the ring using an oxy-acetylene torch or plasma cutter, following the scribed line
  
• Remove the sprocket from the machine if possible and mount it in a vertical borer to machine the surface flat
  
• Align the new ring using a tight-fitting shaft or jig to maintain concentricity
  
• Weld or bolt the ring in place using high-strength fasteners or continuous weld beads
  
• Inspect for runout and correct with shims or grinding if necessary
  

One technician shared that he used a round steel plate with a spacer to center the punch mark, ensuring the new ring was aligned perfectly before welding. Another noted that placing the new ring over the old one before removal helped visualize fitment and avoid surprises.
Precision and Alignment Considerations
Installing a sprocket ring without proper alignment can lead to:

• Uneven track wear
  
• Increased vibration during travel
  
• Premature failure of track links and bushings
  
• Excessive noise and reduced efficiency
  

Using a vertical borer is considered the gold standard for ensuring the ring is true. However, in field conditions, careful layout and slow, controlled welding can achieve acceptable results. Always check for radial and axial runout before final installation.
Preventative Maintenance and Undercarriage Care
To extend the life of the sprocket and undercarriage:

• Inspect sprocket teeth monthly for signs of hooking or wear
  
• Monitor track tension and adjust according to manufacturer specs
  
• Grease track rollers and idlers regularly
  
• Avoid high-speed turns on hard surfaces, which accelerate wear
  
• Replace track links and bushings as a set to maintain pitch integrity
  

For machines operating in abrasive environments like sand or gravel, consider installing wear plates or guards to protect the sprocket hub and ring.
Caterpillar’s Track Loader Legacy
Caterpillar has produced track loaders since the 1950s, with the 931 series representing a shift toward hydrostatic drive and modular components. The 931B was succeeded by the 931C and later the 939, which introduced electronic controls and improved hydraulics. Despite these advancements, many operators prefer the mechanical simplicity of the 931B, especially for tasks that require reliability over refinement.
Sales of the 931B peaked in the early 1980s, with thousands deployed across North America, Europe, and Australia. Its continued presence in the field is a testament to its build quality and ease of repair.
Conclusion
Replacing the sprocket ring on a Caterpillar 931B is a task that blends fabrication skill with mechanical precision. With the right tools, careful layout, and attention to alignment, operators can restore drive performance and extend the life of their machine. In the world of track loaders, the 931B remains a symbol of rugged engineering—and with a fresh sprocket ring, it’s ready to dig in once again.