Resolving Swing Pin Wear in a CAT 931B Backhoe Attachment

[MikePhua]

The CAT 931B and Its Backhoe Integration
The Caterpillar 931B track loader, introduced in the 1980s, was designed for versatility in earthmoving, grading, and light excavation. With an operating weight around 16,000 lbs and powered by a 70–80 horsepower diesel engine, the 931B was often paired with a backhoe attachment—commonly referred to as the 5G2150 backhoe. This add-on transformed the loader into a hybrid machine capable of trenching and material handling in confined spaces.
Unlike purpose-built backhoes, the swing mechanism on the 931B’s backhoe is mounted to a cast steel swing frame, which is subject to significant side loading during operation. Over time, wear in the swing pin and bushing assembly can lead to misalignment, chatter, and accelerated component failure.
Terminology annotation:
- Swing Pin: A pivot shaft that allows the backhoe boom to rotate left and right. - Swing Frame: The structural component that houses the swing pin and supports the backhoe boom. - Chatter: A vibration-induced wear pattern caused by loose or misaligned components under load.
Symptoms of Excessive Wear and Powdered Metal Debris
Operators have reported swing pins wearing deeply—up to 3/16"—on the bucket-facing side, accompanied by black or grey powdered metal residue. This material is often mistaken for dirt or corrosion but is actually the result of metal-on-metal abrasion, sometimes referred to as galling.
In one case, the pin was fabricated from a chromed hydraulic cylinder rod. While chrome offers surface hardness, the underlying steel may lack the toughness required for side-load applications. Once the chrome layer fails, the exposed base metal can deform, crack, and shed particles that act as grinding compound against the bushing.
Terminology annotation:
- Galling: A form of wear caused by adhesion between sliding surfaces, leading to material transfer and surface damage. - Hydraulic Cylinder Rod: A hardened steel shaft used in hydraulic actuators, often chrome-plated for corrosion resistance. - Spring Tension Bushing: A flexible bushing designed to conform to slightly irregular bores, often used in articulation joints.
Material Compatibility and Greasing Practices
The decision to use spring tension bushings was based on the bore being slightly out of round. These bushings are marketed as self-adjusting and often advertised as “maintenance-free.” However, in high-load swing applications, lack of lubrication can accelerate wear—especially when paired with mismatched materials.
Recommendations include:

• Always grease swing bushings daily, regardless of manufacturer claims
  
• Avoid pairing chrome-plated pins with soft bushings unless both materials are matched for hardness
  
• Use OEM-grade pins or case-hardened steel with known wear characteristics
  
• Inspect bore roundness with a dial bore gauge before selecting bushing type
  

Terminology annotation:
- Case-Hardened Steel: Steel that has been surface-hardened while retaining a tough core, ideal for wear-resistant pins. - Dial Bore Gauge: A precision tool used to measure internal bore diameter and roundness. - OEM-Grade: Components manufactured to original equipment specifications, ensuring compatibility and durability.
One technician noted that pins made from old hydraulic rods performed well in standard steel bushings but failed when paired with softer materials. Another emphasized that powdered metal is a sign of chattering, not just wear—indicating that the pin is moving unevenly within the bushing.
Repair Strategies Without Line Boring
When line boring is not feasible, alternative methods include:

• Welding the bore partially and using a hole saw to restore diameter
  
• Cleaning up the bore with a die grinder and finishing with a flap disc
  
• Installing custom bushings with oversized outer diameters to match the bore
  
• Using dry ice or liquid nitrogen to shrink bushings for press-fit installation
  

Terminology annotation:
- Line Boring: A precision machining process that restores bore alignment and diameter using a portable boring bar. - Hole Saw: A circular cutting tool used to enlarge or restore bore diameter, often guided by a pilot bit. - Press Fit: A tight mechanical fit achieved by shrinking one component before insertion, ensuring secure seating.
One operator successfully used the hole saw method on a D4C dozer’s C-frame, achieving a tight fit with minimal flap wheel cleanup. Another fabricated a line boring rig from scratch to restore a swing tower on a Ford 4500 TLB, welding material and machining the bore to accept custom bushings.
Preventative Measures and Long-Term Solutions
To prevent future swing pin failures:

• Match pin and bushing hardness ratings
  
• Maintain strict greasing schedules, especially in dusty or wet environments
  
• Avoid chrome-plated pins unless paired with compatible bushings
  
• Inspect swing frame bore for roundness and surface condition before installation
  
• Consider sacrificial wear pads or expandable pins if bore damage is extensive
  

Terminology annotation:
- Sacrificial Wear Pad: A replaceable component designed to absorb wear and protect structural parts. - Expandable Pin: A pin that can be mechanically expanded to compensate for bore wear, often used in field repairs.
In severe cases, replacing the swing frame may be necessary. Cast steel frames can sometimes be welded and re-machined, but cast iron frames are more brittle and difficult to repair.
Conclusion
Excessive swing pin wear in a CAT 931B backhoe attachment is often the result of material mismatch, inadequate lubrication, and bore irregularities. While spring tension bushings offer convenience, they must be paired with proper greasing and compatible pins. With careful diagnosis and creative repair strategies, operators can restore swing function and extend the life of their machines. In the world of field repairs, precision and pragmatism go hand in hand—and every pin tells a story of force, friction, and resilience.