Why Lubrication Matters
Lubrication is the single most controllable factor in bearing service life. Studies consistently show that 30–50% of premature bearing failures are lubrication-related — wrong grease, wrong quantity, wrong interval, or contamination of the lubricant.
Getting lubrication right is not complicated, but it requires following engineering principles rather than “the way we’ve always done it.”
Grease Selection Criteria
The grease specification for a spherical roller bearing must consider five factors:
1. Base Oil Viscosity
The most important property is the base oil viscosity at operating temperature. The required viscosity depends on bearing speed and size:
- For normal speeds (n × dm < 500,000): viscosity ratio κ ≥ 2
- For high speeds: κ ≥ 1
- For very low speeds or high loads: κ ≥ 4
The viscosity ratio κ is the actual viscosity divided by the required viscosity for adequate oil film formation. Below κ = 1, boundary lubrication conditions exist and bearing life is reduced.
2. Thickener Type
| Thickener | Max Operating Temp | Water Resistance | Notes |
|---|---|---|---|
| Lithium | 120°C | Good | Most common, general purpose |
| Lithium Complex | 150°C | Good | Higher temp, better mechanical stability |
| Polyurea | 160°C | Excellent | Long life, good oxidation stability |
| Calcium Sulfonate | 180°C | Excellent | Extreme pressure, corrosion protection |
3. NLGI Grade
- NLGI 2: Standard for most spherical roller bearing applications
- NLGI 1: For low-temperature or central lubrication systems
- NLGI 3: For vertical shaft applications or high-temperature where a stiffer grease is needed
4. EP (Extreme Pressure) Additives
For spherical roller bearings in heavy-load applications (steel mills, mining, crushers), EP additives are recommended. These additives form a protective layer on the metal surfaces that prevents welding under boundary lubrication conditions.
5. Base Oil Type
- Mineral oil: Standard for most applications. Good performance-to-cost ratio.
- Synthetic (PAO): Wider temperature range, longer life. Recommended for high-temperature or extended-interval applications.
- Synthetic (Ester): For extreme temperatures. Higher cost.
Relubrication Interval Calculation
The relubrication interval depends on bearing size, speed, and operating temperature:
For spherical roller bearings on horizontal shafts at normal temperatures (< 70°C):
tf = K × [ (14 × 10⁶) / (n × √d) − 4d ]
Where:
- tf = relubrication interval in hours
- K = factor based on bearing type and conditions
- n = speed in RPM
- d = bore diameter in mm
For temperatures above 70°C, the interval is halved for every 15°C increase.
Practical guidelines:
| Operating Temperature | Relubrication Interval Adjustment |
|---|---|
| < 70°C | Use calculated interval |
| 70–85°C | Halve the interval |
| 85–100°C | Quarter the interval |
| > 100°C | Special high-temperature grease and very frequent relubrication |
Grease Quantity
Initial fill: 30–50% of the bearing’s free internal space. Never fill completely — overfilled bearings overheat.
Relubrication quantity: G = 0.005 × D × B grams, where D = outer diameter (mm), B = bearing width (mm).
Relubrication procedure: Pump fresh grease slowly while the bearing is rotating. Continue until fresh grease is observed purging from the seals or drain plug. This ensures old, contaminated grease is expelled.
Grease Compatibility
Mixing incompatible greases is a common and costly mistake. When switching grease types:
- Lithium and lithium complex: Usually compatible
- Polyurea and lithium: Incompatible — the mixture liquefies
- Clay and lithium: Usually compatible
When in doubt, purge the old grease completely before adding the new type. Run the bearing at low speed and pump the new grease until it appears clean at the purging point.
Storage and Handling
- Store grease cartridges and containers in a clean, dry environment
- Keep containers sealed until use
- Do not mix old and new grease in the bearing
- Clean grease fittings before connecting the grease gun
- Use dedicated grease guns for different grease types — or thoroughly clean between types