1. Definition & Importance

Bearings are mechanical components that support and guide rotating shafts, reducing friction between contact surfaces. The lubrication system provides a film of oil or grease between sliding/rolling surfaces to:

• Reduce friction → minimize energy loss.
• Reduce wear → extend bearing life.
• Dissipate heat → prevent overheating and metal deformation.
• Prevent corrosion → protect surfaces from moisture and chemicals.
• Reduce vibration and noise.

2. Bearing Types and Lubrication Requirements

2.1 Plain Bearings (Journal Bearings)

• No rolling elements; shaft slides directly on the bearing surface.
• Mainly lubricated with oil (liquid lubrication).
• Requires a continuous hydrodynamic oil film to avoid metal-to-metal contact.

2.2 Rolling Element Bearings

• Equipped with balls, rollers, or needles; lower friction than plain bearings.
• Can be lubricated with grease or circulating oil.
• Lubricant film must be thin enough to not hinder rolling, yet protective.

3. Bearing Lubrication Principles

3.1 Lubrication Regimes (Stribeck Curve)

1. Hydrodynamic Lubrication – full fluid film, no metal-to-metal contact.
2. Mixed Lubrication – partial surface contact, partial separation by oil.
3. Boundary Lubrication – very thin oil film; EP/AW additives protect surfaces.

4. Selecting Oils and Greases

4.1 Selection Criteria

• Viscosity: Match speed and load. ISO VG classification (ISO 3448) is common.
• Operating Temperature: Choose mineral or synthetic oil accordingly.
• Load Carrying Capacity: Refer to ASTM D2783 (Four-ball EP test).
• Oxidation & Corrosion Resistance: ASTM D943, ASTM D665.
• Material Compatibility: Must not damage seals or bearing linings.

4.2 Lubricating Oils

• Mineral Oils – economical, widely used.
• Synthetic Oils (PAO, Ester, PAG…) – high temperature resistance, long service life.
• Food-grade Oils (NSF H1) – for food processing equipment.

4.3 Lubricating Greases

• Lithium and Lithium Complex – general-purpose.
• Polyurea – high temperature, long life.
• Calcium Sulfonate Complex – excellent water resistance and corrosion protection.

5. Lubricant Supply Methods

5.1 Oil Supply

• Oil Bath
• Oil Mist
• Oil Circulation
• Drip Feed

5.2 Grease Supply

• Manual Grease Gun
• Automatic Grease Dispenser

6. Related International Standards

• ISO 281 – Rolling bearing life calculation.
• ISO 3448 – Viscosity classification.
• ISO 15243 – Bearing damage classification.
• DIN 51825 – Grease classification.
• ASTM D445 – Kinematic viscosity measurement.
• ASTM D3336 – Grease life test for ball bearings.

7. Common Lubrication-related Failures

8. Maintenance & Condition Monitoring

• Monitor operating temperature (infrared thermometer).
• Regular lubricant analysis (ISO 4406, ASTM D5185).
• Vibration analysis for early fault detection.
• Schedule lubrication according to OEM recommendations.

9. Technical Recommendations

• Always select lubricants as per bearing and machine OEM.
• Maintain lubricant cleanliness to minimize wear.
• Avoid mixing incompatible greases.
• Train maintenance staff on proper lubrication practices.