Cladding Repair Process for Ball Mill Trunnion Bearings

Literature Overview

Ball mill trunnion bearings are critical rotating components in cement, mining, and mineral processing operations. These bearings support the mill shell and rotate at low speeds (typically 0.5–3.0 rad/min) under enormous radial loads. The bearing surface is subject to fretting wear, adhesive wear, and in some cases, corrosion from process chemicals. When the bearing surface is damaged beyond the allowable dimensional tolerance, overlay welding repair becomes the preferred restoration method. The literature reviewed here details the cladding repair process for ball mill trunnion bearings, addressing material selection, welding procedure, dimensional control, and post-weld machining.

Core Technical Points

The repair of ball mill trunnion bearings presents several unique challenges that distinguish it from routine cladding applications:

Geometric constraints: The bearing surface is a cylindrical or spherical surface with tight dimensional tolerances (typically IT6–IT7 grade). The cladding must be applied with sufficient excess for subsequent machining while maintaining the original geometry.
High load capacity: The repaired bearing must withstand radial loads of 500–5000 kN depending on mill size. The cladding layer must have adequate compressive strength and fatigue resistance.
Minimal distortion: Any distortion of the bearing surface during welding can cause misalignment of the mill shell, leading to premature bearing failure. The maximum allowable distortion is typically 0.05 mm per meter of bearing length.
Rapid turnaround: Ball mills represent high-value production assets, and downtime costs can exceed 50,000 CNY per day. The repair process must be optimized for minimum repair time.

Material Selection and Welding Procedure

The selection of cladding material for trunnion bearing repair depends on the failure mechanism and the service environment:

Failure Mode	Recommended Cladding Material	Hardness (HRC)	Key Properties
Fretting wear	Stellite 6 or Stellite 6B	40–45	Excellent fretting resistance, good hot hardness
Adhesive wear	Ni-Cr-Mo alloy (e.g., Ni-8Cr-4Mo)	45–50	Low friction coefficient, good anti-galling
Corrosive wear	316L stainless steel or Alloy 625	25–35	Excellent corrosion resistance
General wear	Cr-C hardfacing	55–60	High hardness, good abrasion resistance
Combined wear	Multi-layer: 316L root + Stellite 6 cap	40–55	Combined corrosion and wear resistance

The welding procedure typically follows a multi-layer approach:

Root layer: GTAW (TIG) with 316L or similar austenitic filler, wire diameter 1.6 mm, current 120–160 A, travel speed 5–8 cm/min. This layer ensures good fusion with the base metal and provides a corrosion-resistant interface.
Filler layers: GMAW or SAW with the selected cladding alloy, 2–3 passes, wire diameter 1.2–2.4 mm, current 200–350 A, travel speed 3–6 cm/min.
Cap layer: Final pass with precise profile control, wire diameter 1.2 mm, current 180–250 A, travel speed 4–7 cm/min. The cap layer is machined to final dimensions.

The total cladding thickness is typically 3–8 mm, providing 1.5–3 mm of excess for machining to the final bearing dimensions.

Process Parameters and Thermal Management

Thermal management is the most critical aspect of trunnion bearing repair. The following parameters are essential for controlling distortion:

Parameter	Target Value	Control Method
Preheat temperature	150–250 °C	Induction heating or gas torch
Interpass temperature	150–250 °C	Infrared pyrometer monitoring
Maximum heat input	3.0 kJ/mm	Current and travel speed control
Welding sequence	Symmetric, spiral pattern	Pre-programmed path
Post-weld cooling rate	< 50 °C/min	Insulation blanket or controlled cooling
Stress relief temperature	550–650 °C	Furnace or induction heating

The welding sequence for cylindrical bearings follows a spiral pattern starting from the center and proceeding outward in both directions. This symmetric approach minimizes net thermal expansion and prevents eccentricity. For spherical bearings, a similar symmetric approach is used, starting from the equator and welding poleward.

Defect Analysis and Quality Control

The following defects are most commonly encountered in trunnion bearing overlay repair:

Defect	Root Cause	Detection Method	Acceptance Criteria
Cracking	High carbon equivalent of base metal, inadequate preheat	MT, PT	No cracks longer than 3 mm in cladding layer
Porosity	Inadequate shielding, contaminated base metal	UT, RT	No porosity > 1 mm diameter, no clustered porosity
Lack of fusion	Insufficient current, poor surface preparation	UT, MT	No lack of fusion at any depth
Excessive hardness	Too high carbon content, rapid cooling	Hardness test	Maximum 55 HRC for Stellite; maximum 35 HRC for stainless
Surface roughness	Inadequate machining	Surface profilometer	Ra ≤ 0.8 μm for bearing surface
Dimensional deviation	Welding distortion, inadequate machining	Coordinate measuring machine	Within IT6 tolerance for bearing diameter

The quality control plan follows a systematic approach: visual inspection of all welds, magnetic particle testing of 100% of the cladding surface, ultrasonic testing of critical areas, hardness testing at specified intervals, and dimensional verification after machining.

Integration with Engineering Practice

In practice, ball mill trunnion bearing repair is often performed as an in-situ operation to avoid the cost and complexity of removing the bearing from the mill. This requires portable welding equipment, temporary support fixtures, and careful management of the repair area to prevent contamination from mill dust and chemicals.

The economic analysis typically shows that overlay welding repair costs 30–50% of the cost of replacing the bearing, with a repair time of 24–72 hours depending on bearing size and damage extent. For large ball mills with bearing diameters exceeding 500 mm, the repair cost can exceed 200,000 CNY, making the economic case for repair even stronger.

A practical case study from a cement plant involved the repair of a 1600 mm diameter trunnion bearing with a worn surface area of approximately 2.5 m². The repair used a multi-layer approach with 316L root layer and Stellite 6 cap layer, totaling 6 mm of cladding. The repair was completed in 48 hours with a total material cost of 180,000 CNY. The bearing returned to service and operated for 18 months before the next repair was required, compared to 6 months for the original bearing before failure.

Study Insights and Implications

The trunnion bearing repair application demonstrates that overlay welding can be a highly effective restoration technology for large, high-value rotating components. The key to success lies in careful thermal management, appropriate material selection, and rigorous quality control. The multi-layer approach, combining a corrosion-resistant root layer with a wear-resistant cap layer, provides the best balance of properties for most service conditions.

An important practical insight is the value of pre-planning the repair. A detailed welding procedure specification (WPS) developed before the repair begins, including welding sequence, thermal management plan, and inspection schedule, significantly improves repair quality and reduces the risk of rework. The use of induction heating for preheating and stress relief has become increasingly common due to its precision and speed compared to traditional gas torch methods.

The literature also highlights the importance of post-repair monitoring. After the bearing is returned to service, periodic inspection of the bearing surface should be conducted to detect early signs of wear or cracking. This can be accomplished through vibration analysis, temperature monitoring, and periodic visual inspection during scheduled maintenance.