Welding to Repair Worn Bores in Heavy Equipment

[MikePhua]

Why Bore Repair Matters in Field Equipment
In the world of heavy machinery—excavators, loaders, dozers, and forestry rigs—pivot bores are among the most abused structural features. These circular openings house bushings, pins, and bearings that allow controlled articulation of arms, buckets, and linkages. Over time, due to vibration, load cycling, and poor lubrication, bores can elongate, oval out, or crack. Left unchecked, this wear leads to misalignment, excessive pin play, and ultimately mechanical failure.
Repairing worn bores is not just about restoring geometry—it’s about preserving the integrity of the entire linkage system. Welding is one of the most common and effective methods for bore restoration, especially when machining or sleeving is impractical due to cost, location, or urgency.
Common Bore Damage and Its Root Causes
Bore wear typically results from:

• Pin rotation due to seized bushings
  
• Lack of grease or contaminated lubrication
  
• Overloading beyond design limits
  
• Corrosion from water ingress or chemical exposure
  
• Improper fitment during previous repairs
  

In one case, a forestry contractor in British Columbia noticed excessive slop in the boom-to-stick joint of a 1990s excavator. Inspection revealed the bore had worn 3 mm out-of-round, with deep scoring from a seized pin. The machine had been operating in wet clay without daily greasing—a recipe for accelerated wear.
Welding Techniques for Bore Restoration
There are several welding approaches to rebuild worn bores, each suited to different scenarios:

• Stick Welding (SMAW): Common in field repairs. Offers deep penetration and good control but requires post-weld machining.
  
• MIG Welding (GMAW): Faster deposition rate, ideal for filling large areas. Less spatter and easier cleanup.
  
• TIG Welding (GTAW): Precise and clean, but slower. Used for thin materials or when distortion must be minimized.
  
• Submerged Arc Welding (SAW): High deposition and low spatter, but typically reserved for shop environments.
  

The process generally involves:

• Cleaning the bore thoroughly, removing grease, rust, and contaminants
  
• Preheating the area to reduce thermal shock and prevent cracking
  
• Welding in layers, building up material concentrically
  
• Allowing controlled cooling to avoid hardening or brittleness
  
• Machining the bore back to spec, either with a line boring tool or mill
  

For bores with cracks or structural damage, gouging out the defect before welding is essential. In some cases, a full sleeve insert may be welded in after buildup to ensure long-term durability.
Material Selection and Filler Rods
Choosing the right filler metal is critical. For mild steel bores, ER70S-6 wire or 7018 rods are commonly used. For high-strength applications, low-hydrogen electrodes or specialty alloys may be required.
Parameters to consider:

• Base metal composition: Match filler to parent material
  
• Hardness requirements: Avoid overly hard welds that resist machining
  
• Corrosion resistance: Use stainless or nickel alloys in marine or chemical environments
  
• Heat input: Control amperage to minimize distortion
  

A technician in Queensland once used a high-hardness rod to rebuild a loader bore. The result was a surface too hard to machine, requiring carbide tooling and excessive time. Lesson learned: match weldability with machinability.
Line Boring and Post-Weld Machining
After welding, the bore must be machined to restore concentricity and diameter. Line boring is the preferred method, using a portable boring bar mounted across the bore axis. These tools can cut with precision down to ±0.001", ensuring proper pin fitment.
Steps include:

• Setting up the boring bar with centering cones or bushings
  
• Using carbide or HSS cutters to machine the weld buildup
  
• Measuring with telescoping gauges or bore micrometers
  
• Final polishing or honing for smooth pin engagement
  

Some operators use adjustable boring heads with digital readouts, while others rely on manual feed and feel. Either way, patience and accuracy are key.
Alternative Methods and When to Use Them
In some cases, welding may not be the best option. Alternatives include:

• Sleeving: Installing a pre-machined steel sleeve, welded or press-fit
  
• Epoxy-based bore repair: Temporary fix using metal-filled compounds
  
• Oversize bushings: Machining the bore to a larger diameter and fitting custom bushings
  
• Bore welding robots: Automated systems that weld and machine in one setup
  

These methods are useful when time is limited or when welding risks distortion. However, they may lack the longevity of a full weld-and-bore repair.
Field Anecdote and Practical Insight
In 2021, a mining operation in Nevada faced downtime due to a worn bore on a haul truck’s steering knuckle. The bore had elongated by 5 mm and cracked along the lower edge. A mobile welding crew arrived with a diesel-powered MIG unit and a line boring rig. Over 36 hours, they rebuilt the bore, machined it to spec, and installed a new bushing. The truck returned to service, saving the company an estimated $80,000 in lost productivity.
Preventative Measures to Reduce Bore Wear
To extend bore life and reduce repair frequency:

• Grease daily with high-pressure lithium or moly-based grease
  
• Inspect pins and bushings for rotation or scoring
  
• Use sealed bushings in abrasive environments
  
• Avoid side loading or misalignment during operation
  
• Replace worn pins before they damage the bore
  

Some operators install grease fittings with check valves to prevent backflow and contamination. Others retrofit wear indicators to monitor pin movement.
Conclusion
Welding to repair bores is a time-tested solution that blends metallurgy, precision machining, and field ingenuity. Whether restoring a loader arm, excavator boom, or dozer pivot, the process demands attention to detail and respect for material behavior. With the right technique, tools, and mindset, even severely worn bores can be brought back to life—keeping machines working and downtime at bay.