CLADDING TECHNOLOGY SHANXI CO., LTDCladding Technology for EBZ125 Roadheader Turntable Assembly
Equipment Background and Wear Analysis
This 2009 technical paper by Liu Weibin, Zhang Yawen, and Lei Zhenhua from Shaanxi Construction Machinery Co., Ltd. (specifically their Heavy Copper Equipment Research Institute) addresses the weld-overlay cladding of the left and right turntable assemblies on the EBZ125 roadheader. The EBZ125 is a tunneling machine (roadheader) with a cutting power of 125 kW, widely used in underground mine tunneling and road construction in rock formations.
The turntable assembly is a critical component that transmits torque from the drive motor to the cutting head while allowing the cutting head to rotate and articulate. The turntable consists of a large-diameter ring gear that meshes with pinion gears driven by hydraulic motors. This assembly is subjected to extreme loading conditions:
- High contact stress: The gear teeth experience Hertzian contact stresses of 2000–3500 MPa during operation.
- Impact loading: In hard rock formations, the cutting head transmits impulsive loads to the turntable through the gear mesh.
- Abrasive contamination: Rock dust and cutting debris contaminate the gear mesh, introducing abrasive particles between the mating tooth surfaces.
- Fatigue cycling: The gear teeth undergo millions of load cycles during the service life of the machine.
Wear Mechanisms and Failure Modes
Primary Wear Mechanisms
| Wear Mechanism | Description | Contribution to Total Wear |
|---|---|---|
| Abrasive wear | Rock particles embedded in the gear mesh abrade the tooth surfaces | 40–55% |
| Fatigue spalling | Cyclic contact stress causes subsurface crack initiation and surface spalling | 25–35% |
| Adhesive wear | Localized welding and tearing of metal at asperity contacts | 10–15% |
| Corrosive wear | Moisture and sulfur compounds in mine atmosphere cause surface oxidation | 5–10% |
Typical Failure Modes
The turntable gear teeth fail through a progressive sequence:
- Surface micro-pitting: Initial stage where micro-cracks initiate at surface defects or carbide boundaries under cyclic loading.
- Macro-pitting and spalling: Micro-cracks coalesce and propagate to the surface, causing material removal in the form of pits or spalls.
- Tooth profile degradation: Progressive material removal changes the tooth geometry, leading to increased gear mesh noise, vibration, and eventual loss of meshing accuracy.
- Tooth breakage: In severe cases, spalling leads to tooth root cracking and catastrophic tooth failure.
The typical service life of an uncladded turntable gear is 8,000–15,000 hours in medium-hard rock formations, but this can be reduced to 3,000–5,000 hours in abrasive formations such as quartzite or sandstone.
Cladding Material Selection and Design
Material Selection Rationale
The cladding material for the turntable gear must satisfy several competing requirements:
- High hardness: To resist abrasive wear from rock particles.
- Adequate toughness: To resist impact loading and fatigue spalling.
- Good bond strength: To ensure reliable adhesion to the base gear steel.
- Dimensional stability: The cladding layer must not significantly alter the gear geometry.
- Compatibility with base material: The thermal expansion coefficient and weldability must be compatible with the base gear steel.
Recommended Cladding Materials
| Material System | Electrode Type | Hardness (HRC) | Application |
|---|---|---|---|
| Ni-Cr-Cr₇C₃ | D107/D108 | 55–62 | General purpose, good toughness |
| Cr-Cr₇C₃ cast iron | D256/D257 | 58–65 | High abrasive wear resistance |
| High-speed steel type | D507 | 60–66 | Combined abrasive + impact |
| Ni-based amorphous | D256 | 50–58 | Corrosive-abrasive environments |
| Multi-layer composite | Bond layer + hardfacing | 55–68 | Optimal combination |
Multi-Layer Cladding Design
The recommended cladding design for the EBZ125 turntable gear employs a three-layer system:
- Bond layer (1.0–1.5 mm): A ductile Ni-Cr alloy (e.g., D102) that ensures good metallurgical bonding with the base gear steel and reduces residual stresses.
- Intermediate layer (1.0–1.5 mm): A transition alloy with moderate hardness and good toughness (e.g., D507) that provides a gradual transition in properties.
- Surface layer (1.5–2.5 mm): A hardfacing alloy with high hardness and wear resistance (e.g., D256 or D107) that provides the primary wear protection.
Total cladding thickness: 3.5–5.5 mm, with 1.0–1.5 mm removed during post-weld gear grinding.
Welding Process and Quality Control
Welding Procedure
The cladding of the turntable gear requires careful control of welding parameters to minimize distortion and ensure uniform cladding quality:
| Parameter | Specification | Rationale |
|---|---|---|
| Preheat temperature | 250–350°C | Prevent cold cracking in HAZ |
| Interpass temperature | 200–300°C | Control cooling rate, prevent cracking |
| Welding current (SMAW) | 150–220 A | Balance deposition rate and heat input |
| Welding voltage (SAW) | 28–35 V | Optimize arc stability and penetration |
| Travel speed | 50–80 mm/min | Control bead width and dilution |
| Post-weld stress relief | 550–650°C, 2–4 h | Reduce residual stresses |
Quality Control Procedures
- Visual inspection: All weld beads are inspected for surface defects, undercut, and excessive spatter.
- Magnetic particle testing (MT): 100% of the cladding surface is MT-inspected for surface and near-surface cracks.
- Hardness testing: Hardness is measured at multiple locations on the cladding surface to verify uniformity and compliance with specifications.
- Dimensional inspection: The gear geometry is verified after post-weld grinding to ensure that the cladding has not significantly altered the tooth profile.
- Bond strength testing: A coupon test is performed to verify that the bond strength between the base material and the cladding layer meets the minimum requirement (typically > 200 MPa).
Engineering Practice and Lessons Learned
Distortion Control
The EBZ125 turntable gear is a large, heavy component (typically 2,000–4,000 kg) with a complex geometry. Controlling distortion during the cladding process is a significant challenge:
- Balanced welding sequence: The gear is divided into segments, and welding proceeds in a symmetric pattern to minimize angular and warpage distortion.
- Rigid fixturing: The gear is clamped in a heavy-duty fixture during welding to restrain movement and minimize distortion.
- Controlled heat input: Lower heat input welding parameters are used to minimize thermal gradients and the associated residual stresses.
Gear Geometry Preservation
A critical consideration in cladding gear components is the preservation of the gear geometry. The cladding layer is deposited on the tooth surfaces, and subsequent grinding is required to restore the correct tooth profile. The following considerations are essential:
- Sufficient cladding thickness: The cladding must be thick enough to allow for post-weld grinding without exposing the base material. A minimum of 1.5 mm of hardfacing material must remain after grinding.
- Uniform cladding thickness: The cladding thickness must be uniform across the tooth width to ensure that the ground tooth profile is correct. Variations in cladding thickness result in variations in the ground tooth geometry.
- Grinding allowance: A grinding allowance of 1.0–1.5 mm is typically provided in the cladding design to account for the material removed during post-weld grinding.
Comparative Performance Data
| Parameter | Uncladded Gear | Cladded Gear | Improvement |
|---|---|---|---|
| Surface hardness | 28–32 HRC | 58–65 HRC | +20–33 HRC |
| Service life (medium-hard rock) | 8,000–15,000 h | 25,000–40,000 h | 2.5–3.0x |
| Service life (abrasive rock) | 3,000–5,000 h | 12,000–20,000 h | 3.0–4.0x |
| Maintenance frequency | Every 4,000 h | Every 12,000 h | 3x reduction |
| Repair cost (relative) | 100 (replacement) | 30–40 (cladding) | 60–70% savings |
Study Insights and Reflections
The 2009 publication of this work reflects the maturation of cladding technology in the roadheader and tunneling equipment industry. The EBZ125 is a widely used machine in Chinese underground mining operations, and the systematic documentation of cladding procedures for its turntable assembly provided valuable reference material for maintenance engineers.
A particularly valuable aspect of this study is the emphasis on the interaction between the cladding material properties and the specific operating conditions of the roadheader. The turntable gear operates in a unique environment characterized by high contact stresses, impact loading, and abrasive contamination, and the cladding material must be selected to address this specific combination of loading conditions.
The multi-layer cladding approach described in this study represents a sophisticated engineering solution that balances competing requirements: the bond layer ensures reliable adhesion, the intermediate layer provides a gradual transition in properties, and the surface layer provides the primary wear protection. This layered approach is a common theme in advanced cladding design and reflects the principle that optimal performance is achieved through careful engineering of the entire cladding system, not just the surface layer.
The economic analysis presented in this work is particularly compelling. The 2.5–4.0x improvement in service life, combined with the 60–70% reduction in repair cost, demonstrates that cladding is not merely a technical solution but an economically superior strategy for maintaining roadheader turntable assemblies. The reduced maintenance frequency also has significant operational benefits, including reduced downtime and improved machine availability.
This study remains a valuable reference for engineers working with roadheader and tunneling equipment, and its fundamental principles continue to guide cladding design and implementation in this application area.