Ball Mill Bearing

Detailed Introduction to Ball Mill Bearings and Their Manufacturing & Inspection Processes

I. Functions and Types of Ball Mill Bearings

• High load capacity: Capable of bearing large radial loads (primary) and axial loads (from cylinder 窜动);

• Impact resistance: Adaptable to transient overloads caused by uneven feeding;

• Wear resistance: Low friction coefficient with the journal (≤0.001) and a service life ≥10,000 hours;

• Maintainability: Easy to install, lubricate, and replace.

1. Spherical Roller Bearings

• Structure: Double rows of rollers with a spherical raceway in the outer ring, allowing the inner ring to align relative to the outer ring (±2° deflection tolerance);

• Characteristics: Mainly bear radial loads and minor axial loads, suitable for small to medium ball mills (cylinder diameter ≤3m).

2. Double-Row Tapered Roller Bearings

• Structure: Two rows of tapered rollers, capable of bearing both radial and axial loads (strong axial load capacity);

• Characteristics: Must be installed in pairs (symmetric arrangement), suitable for medium to large ball mills (cylinder diameter 3-5m).

3. Sliding Bearings (Babbitt Metal Bearings)

• Structure: Babbitt metal (tin-based or lead-based) is cast on the inner surface of the bearing shell, lubricated by an oil film;

• Characteristics: Extremely high load capacity (suitable for extra-large ball mills with cylinder diameter ≥5m) but require a forced lubrication system and complex maintenance.

II. Manufacturing Process of Ball Mill Bearings (Taking Spherical Roller Bearings as an Example)

1. Manufacturing of Inner and Outer Rings (Material: High-Carbon Chromium Bearing Steel GCr15)

• Blank Preparation:

• Forging: Round steel is heated to 850-900℃ and forged into ring blanks (with a 3-5mm allowance on the large end). After forging, normalizing (860℃ for 2h, air cooling) is performed to eliminate network carbides;

• Inspection: Ultrasonic testing (UT) for internal defects (no inclusions ≥φ2mm allowed).

• Rough Machining:

• Turning: CNC lathes machine the inner hole, outer circle, and end faces, leaving a 1-2mm grinding allowance;

• Drilling (outer ring): Lubrication holes (5-8mm diameter) are drilled with a position tolerance of ±0.5mm.

• Heat Treatment:

• Quenching: Heating to 830-860℃, oil cooling to ≤200℃, resulting in a hardness of 61-65HRC;

• Tempering: Holding at 150-180℃ for 2-3h to eliminate quenching stress and stabilize the structure.

• Finish Machining:

• Inner ring: Internal grinders machine the inner hole (tolerance IT5, surface roughness Ra≤0.4μm); centerless grinders machine the outer raceway (spherical accuracy 0.005mm);

• Outer ring: Cylindrical grinders machine the outer circle (tolerance IT6); special grinders machine the inner spherical surface (curvature radius deviation ±0.01mm);

• Grinding:

• Superfinishing: Superfinishing the raceway surface (Ra≤0.1μm) to reduce friction coefficient.

2. Manufacturing of Rollers (Material: GCr15SiMn, with better wear resistance than GCr15)

• Blank: Cold heading to form the spherical head, removing oxide scale;

• Heat Treatment: Quenching + low-temperature tempering (same as inner ring, hardness 60-64HRC);

• Grinding: Centerless grinders machine the outer circle (cylindricity ≤0.002mm); spherical grinders machine the spherical ends (≥90% fit with the raceway).

3. Manufacturing of Cages (Material: Low-Carbon Steel 20# or Brass H62)

• Stamping: Steel plates are stamped into pocket structures (for 20# steel), or brass is forged and then turned to form pockets;

• Shaping: Pocket size tolerance ±0.1mm to ensure flexible rotation of rollers in pockets (clearance 0.1-0.3mm).

4. Assembly Process

• Cleaning: All parts are cleaned with kerosene (removing iron chips and oil);

• Group Matching: Inner rings, outer rings, and rollers are grouped by dimensional tolerance (±0.001mm per group) to ensure radial clearance (0.1-0.3mm);

• Riveting/Welding: After assembling the cage and rollers, fix them by riveting (steel cages) or welding (brass cages);

• Final Inspection: Check rotational flexibility (no jamming), apply anti-rust oil, and package.

III. Inspection Process of Ball Mill Bearings

1. Raw Material and Blank Inspection

• Material Inspection:

• GCr15: Spectral analysis to verify Cr (1.4-1.65%) and C (0.95-1.05%) content;

• Babbitt metal (sliding bearings): Chemical analysis to verify tin-based alloy (Sn≥83%) and lead-based alloy (Pb≥78%).

• Forging Quality:

• Macrostructure inspection (etching method): No shrinkage cavities or cracks, network carbide grade ≤2.

2. In-Process Inspection (Key Nodes)

• After Heat Treatment:

• Hardness: Rockwell hardness tester measures inner/outer rings (61-65HRC) and rollers (60-64HRC);

• Metallographic Structure: Martensite grade ≤2, retained austenite ≤15% (to avoid dimensional changes).

• After Finish Machining:

• Dimensional Accuracy: CMM measures inner ring inner diameter (IT5) and outer ring outer diameter (IT6);

• Geometric Tolerance: Inner ring radial runout ≤0.003mm, outer ring end face perpendicularity ≤0.002mm/100mm;

• Surface Quality: Raceway roughness (Ra≤0.4μm), no scratches or bruises (checked with a 10x magnifier).

3. Finished Product Inspection

• Rotational Accuracy:

• Inner ring radial runout (Fr) ≤0.01mm, outer ring radial runout (Fa) ≤0.015mm;

• Radial Clearance: Measured with feelers or clearance gauges (0.1-0.3mm, adjusted by bearing size);

• Life Test: Sampling for accelerated life testing (1000r/min, 1.2x rated load), with a life ≥1000 hours (equivalent to ≥10,000 hours in actual use);

• Sealing Performance (for sealed bearings): Water immersion test (50mm water depth, 500r/min, no water ingress for 30min).