Zhuojin Pipe Fitting Co., LtdOxyacetylene Surfacing Repair of Ball Mill Sliding Bearing
Literature Overview
This paper by Zhang Qibin, published in Mining Machinery (Vol. 28, Issue 10, 2000), describes the use of oxyacetylene flame surfacing to repair the sliding bearing (bearing shell) of a ball mill at Qidaogou Iron Mine Concentrator. The work addresses a common failure mode in grinding mills where the white metal or bronze bearing shells experience progressive wear, scoring, or spalling due to the extreme loads, high rotational speeds, and lubricant degradation characteristic of ball mill operation. The classification TG455 places this within the broader category of welding metallurgy and surfacing technology.
Core Technical Content
Ball mill sliding bearings operate under hydrodynamic lubrication conditions at nominal speed, but during start-up, shutdown, and transient loading, they experience boundary lubrication and mixed-film conditions that accelerate wear. The bearing shells, typically made of cast iron with a lining of white metal (Babbitt alloy) or copper-based alloy, are vulnerable to adhesive wear, erosive wear from contaminated lubricant, and fatigue spalling from cyclic stress. When the bearing shell surface becomes excessively worn or damaged, the shaft-to-shell clearance increases beyond acceptable limits, leading to vibration, misalignment, and eventual catastrophic failure.
Oxyacetylene Surfacing Process Characteristics
| Parameter | Specification | Engineering Rationale |
|---|---|---|
| Flame type | Neutral or slightly carburizing | Avoids excessive oxidation or carbon pickup |
| Wire composition | Matching alloy (Cu-based or Ni-based) | Ensures compatible metallurgy |
| Wire diameter | 2.0-3.0 mm | Adequate deposit rate for field repair |
| Preheating temperature | 200-400°C | Manages thermal gradient in thick casting |
| Deposit thickness | 2-5 mm per pass | Controls dilution and microstructure |
| Number of passes | 2-4 layers | Achieves required build-up with sound bonding |
| Cooling rate | Controlled by preheat and insulation | Prevents cracking in HAZ |
Metallurgical Considerations
The oxyacetylene process offers distinct advantages for bearing shell repair. The relatively low heat input density and controllable thermal gradient reduce the risk of thermal cracking in the cast iron base material. The flame can be precisely directed to the damaged area, minimizing unnecessary heating of the surrounding sound material. However, the process also presents challenges:
- The carbon content of the flame atmosphere can influence the chemistry of the deposit and the HAZ. A neutral flame minimizes carbon pickup, while a slightly carburizing flame may be acceptable for cast iron substrates.
- The deposit microstructure is influenced by the cooling rate, which is relatively low in the oxyacetylene process due to the preheated base metal and low energy density. This can result in coarser microstructures compared to arc surfacing.
- Dilution between the base metal and the surfacing deposit can alter the hardness and wear resistance of the final deposit. Multiple thin layers are recommended to minimize dilution effects.
Engineering Practice Integration
From a practical standpoint, bearing shell repair by surfacing involves a structured approach:
- Removal and assessment: The bearing shell is removed from the housing, and the extent of damage is assessed using profile gauges and surface roughness measurements. Any cracks must be identified by magnetic particle testing before repair.
- Surface preparation: The worn surface is machined to remove all damaged material, typically 1-3 mm below the original surface. The surface is then cleaned to remove oil, oxide, and contaminants.
- Preheating: The entire shell is preheated uniformly to 200-400°C using a torch or induction heater. Uniform preheating is critical to prevent thermal shock and cracking.
- Surfacing: The deposit is applied in thin layers using the selected alloy wire. Each layer is kept thin (1-2 mm) to control dilution and ensure good fusion.
- Post-weld treatment: Stress relief at 500-600°C for cast iron shells, or appropriate treatment for the specific material.
- Final machining: The repaired surface is machined to the original dimensions and surface finish (typically Ra 0.2-0.4 μm for sliding bearings).
The critical success factors are maintaining interpass temperature below 300°C, ensuring complete fusion between layers, and achieving a deposit hardness that matches or exceeds the original bearing lining specification. For white metal bearings, the deposit hardness should be in the range of 20-40 HB, while for copper-based bearings, 80-120 HB is typical.
Key Questions and Reflections
This paper raises several questions that remain relevant in modern bearing maintenance. First, the long-term durability of oxyacetylene-surfaced bearing shells compared to replacement linings is not clearly established in the literature. In my experience, surfacing repairs are acceptable for short-term restoration but may not match the fatigue life of factory-applied linings due to microstructural differences. Second, the paper does not address the lubricant compatibility with the surfacing alloy, which is critical for long-term bearing performance. Third, the decision to repair versus replace should be based on a cost-benefit analysis that includes the probability of early failure, which is inherently higher for repaired bearings.
The oxyacetylene process, while less commonly used in modern workshops due to the prevalence of arc welding equipment, retains advantages for specific applications where low heat input and precise flame control are essential. For thick cast iron bearing shells, the oxyacetylene process may still be the preferred method for surfacing repair in many mining and processing operations. The fundamental principle remains: successful surfacing repair requires matching the metallurgy of the deposit to the base material, controlling the thermal cycle to prevent cracking, and ensuring the final surface quality meets the tribological requirements of the bearing application.