Zhuojin Pipe Fitting Co., LtdOverlay Welding Repair Process for Ball Mill Journal Bearings
Literature Overview
This 1994 paper by Wang Zhengjian from Yongchuan Phosphate and Cement Plant, published in "Machinery" (Volume 21, Issue 3, p. 41), documents the development and implementation of an overlay welding repair process for a damaged journal bearing in a cement clinker ball mill. The paper is notable for describing an emergency repair performed without prior precedent, completed in only 4 hours, with subsequent validation through 5 years of successful service.
Background and Emergency Situation
The case involves a clinker ball mill at a phosphate and cement plant that experienced unexpected damage to a journal bearing during operation. The urgency of the situation was compounded by the lack of prior repair precedent for this specific component, requiring rapid development of a suitable repair process.
Ball Mill Operating Conditions
| Parameter | Typical Value | Notes |
|---|---|---|
| Mill diameter | 2.7–3.6 m | Medium to large size |
| Mill length | 4.0–6.0 m | Variable |
| Operating speed | 20–30 rpm | Low speed, high torque |
| Bearing load | 50–150 tons | Extremely heavy loading |
| Operating temperature | 80–120°C | Elevated due to grinding heat |
| Service environment | Cement dust, moisture | Corrosive and abrasive |
Journal Bearing Characteristics
The journal bearing in this application is subjected to:
- Extremely heavy radial loads from the mill shell and grinding media
- Continuous rotation at low speed
- Elevated temperatures from frictional heating
- Exposure to cement dust and moisture
- Occasional shock loads during mill start-up and stoppage
Overlay Welding Repair Process Development
Given the emergency nature of the repair and the lack of prior precedent, the process development followed a systematic approach:
Step 1: Damage Assessment
The damaged bearing surface was carefully inspected to determine:
- Extent of damage (area and depth)
- Type of damage (wear, spalling, cracking, corrosion)
- Remaining sound material thickness
- Structural integrity of the bearing housing
Step 2: Repair Strategy Formulation
Based on the damage assessment, the repair strategy was developed:
- Removal of all damaged material to sound metal
- Creation of a suitable preparation geometry for overlay welding
- Selection of appropriate overlay welding consumable
- Determination of welding sequence and parameters
- Planning of post-weld machining and finishing
Step 3: Consumable Selection
The overlay welding consumable was selected based on the following requirements:
- Hardness compatible with the bearing application
- Adequate toughness to resist impact loading
- Good weldability with the bearing steel substrate
- Resistance to wear under heavy loading conditions
- Availability in the local market
Step 4: Process Execution
The actual repair was executed in approximately 4 hours, including:
| Step | Duration | Activity |
|---|---|---|
| 1 | 0.5 h | Surface preparation and cleaning |
| 2 | 0.5 h | Preheating and temperature control |
| 3 | 2.0 h | Overlay welding (multiple layers) |
| 4 | 0.5 h | Cooling and stress relief |
| 5 | 0.5 h | Post-weld machining and finishing |
Step 5: Quality Verification
After the overlay welding repair, the bearing surface was verified through:
- Visual inspection for surface defects
- Dimensional measurement to verify geometric accuracy
- Hardness testing of the overlay layer
- Functional testing during mill re-commissioning
Process Parameters
The overlay welding process parameters used for the journal bearing repair were:
| Parameter | Value | Rationale |
|---|---|---|
| Welding process | SMAW (SMAW) | Available equipment, suitable for repair |
| Electrode type | Hard facing electrode | Wear resistance requirement |
| Electrode diameter | 4.0 mm | Balance of deposit rate and control |
| Current | 140–180 A | Adequate penetration, controlled heat input |
| Arc voltage | 24–28 V | Stable arc, good deposition |
| Travel speed | 60–80 mm/min | Control of heat input and dilution |
| Number of layers | 2–3 layers | Achieve required thickness |
| Layer thickness | 3–4 mm | Uniform deposition |
| Preheat temperature | 200–250°C | Reduce cracking risk |
| Interpass temperature | 200–250°C | Maintain ductility |
Performance Validation
The overlay welded journal bearing demonstrated excellent performance over a 5-year service period:
- Service life — The repaired bearing operated reliably for 5 years, significantly exceeding the expected life of an emergency repair.
- Wear characteristics — The overlay layer showed uniform wear without spalling, cracking, or delamination.
- Dimensional stability — The bearing maintained its geometric accuracy throughout the service period, indicating good dimensional stability of the overlay layer.
- Load capacity — The repaired bearing adequately supported the full operating load of the ball mill without evidence of excessive deformation or failure.
- Economic benefit — The 4-hour emergency repair avoided significant production downtime and the cost of bearing replacement, providing substantial economic benefit to the plant.
Engineering Practice Insights
This case study provides several valuable lessons for industrial maintenance welding:
- Rapid process development — The ability to develop and implement a suitable repair process in an emergency situation demonstrates the importance of comprehensive welding knowledge and experience.
- Systematic approach — Even in an emergency, following a systematic approach to process development (assessment, strategy, execution, verification) ensures a reliable repair.
- Quality assurance — The 5-year successful service life validates the importance of proper process execution and quality verification, even for emergency repairs.
- Documentation — The detailed documentation of the repair process creates a precedent that can be referenced for future similar repairs, reducing the time required for process development in subsequent emergency situations.
- Preventive maintenance implications — The failure of the journal bearing suggests the need for improved monitoring and preventive maintenance procedures to detect bearing degradation before catastrophic failure occurs.
Key Technical Reflections
This paper illustrates the practical application of overlay welding technology in an emergency maintenance scenario. The successful repair of a heavily loaded journal bearing in only 4 hours, with 5 years of subsequent reliable service, demonstrates the versatility and effectiveness of overlay welding as a repair technology.
The technical success of this repair depended on several critical factors:
- Proper assessment of the damage extent and remaining material condition
- Appropriate selection of overlay welding consumable for the specific service conditions
- Careful control of welding parameters to minimize thermal distortion and cracking
- Thorough surface preparation to ensure good metallurgical bond between the overlay layer and the base metal
- Precision post-weld machining to restore the bearing surface geometry
The economic implications of this repair are substantial. A journal bearing replacement for a large ball mill would require significant downtime, specialized spare parts procurement, and substantial direct costs. The overlay welding repair approach achieved rapid restoration of production at a fraction of the replacement cost, while providing comparable or superior long-term performance.
This case also highlights the importance of maintaining welding expertise and capability within industrial maintenance organizations. The ability to develop and execute a successful repair process under time pressure requires not only technical knowledge but also practical experience and sound engineering judgment. Organizations that invest in developing these capabilities are better positioned to respond effectively to equipment failures and minimize production losses.