Lubrication failure accounts for 36% of all premature bearing failures, making it the single biggest cause of bearing damage worldwide. Too much grease is as damaging as too little. This guide covers the science of bearing lubrication and practical relubrication procedures for Indian industrial conditions.
Why Bearings Need Lubrication
Bearing surfaces are never perfectly smooth. Under load, asperities on rolling elements and raceways would contact directly, causing micro-welding, wear particles and rapid failure, without a lubricant film separating them. The lubricant also dissipates heat, protects against corrosion and acts as a seal against contaminants.
Grease Lubrication
Grease is the default choice for 80% of bearing applications because it is simple, fill once, seal, forget. Grease is a base oil (mineral or synthetic) held in a thickener matrix (lithium, calcium, polyurea, etc.).
When to Use Grease
- Speeds up to 70% of limiting speed
- Moderate temperatures (−20°C to +120°C for lithium-based)
- Sealed or shielded housings where an oil bath is impractical
- Applications requiring occasional or infrequent relubrication
Grease Selection
| Application | Grease Type | Temp Range |
|---|---|---|
| General industrial | Lithium-based NLGI 2 | −20°C to +120°C |
| High temperature (above 120°C) | Polyurea or Bentonite NLGI 2 | up to +200°C |
| Low temperature | Synthetic base, low-viscosity | −40°C to +80°C |
| Food processing | NSF H1 approved, white grease | −20°C to +130°C |
| High speed spindles | Low-noise bearing grease | −30°C to +120°C |
Relubrication Intervals
JK Lube (available from Masa Technologies) provides practical relubrication intervals based on bearing size and speed. A rough guide for lithium NLGI 2 grease:
Bearing bore 25, 50mm at 3000 rpm: ~1,500 hours
Bearing bore 50, 100mm at 1500 rpm: ~2,000 hours
High temperature (>80°C): Halve all intervals
How Much Grease to Add
Overgreasing is a common mistake. Excess grease churns inside the bearing, generates heat and pushes out seals. Rule of thumb for standard housings: fill to 30, 50% of free space. For electric motors: 1/3 of bearing cavity. Use a grease gun with a counted-stroke method rather than pumping until grease appears at the seal.
Formula: G = 0.005 Ã, D Ã, B (grams), where D = bearing OD in mm, B = bearing width in mm.
Oil Lubrication
Oil is chosen for high-speed applications, high-temperature environments or where the gearbox/housing already contains oil that can splash onto the bearing.
When to Use Oil
- Speeds above 70% of grease-limiting speed
- Temperature above 120°C continuous
- When heat must be dissipated through circulating oil system
- Gearboxes and reduction drives (splash lubrication)
Oil Viscosity Selection
Viscosity must be high enough to form a full film at operating speed and temperature, but not so high that churning losses overheat the bearing. At 40°C operating temperature and 1500 rpm for a 50mm bore bearing: ISO VG 68 is the typical starting point. At 3000 rpm: ISO VG 32. At 500 rpm heavy load: ISO VG 150.
Critical Mistakes to Avoid
| Mistake | Consequence |
|---|---|
| Mixing different grease types | Thickener incompatibility, grease liquefies and runs out |
| Overgreasing | Heat buildup, seal damage, bearing failure |
| Using used oil or contaminated grease | Abrasive particles accelerate raceway wear |
| Lubricating a seized bearing | Masks the symptom, does not fix the problem |
| Lubricating on a schedule without condition monitoring | Over- or under-lubrication |