What Makes Greases Incompatible
Grease consists of a base oil (typically mineral or synthetic) held in a thickener matrix (the soap structure). Compatibility problems arise when two different thickener types are mixed — the thickener structures interact chemically or physically, causing:
- Softening: The mixed grease drops to a lower NLGI consistency grade — flows out of the bearing under operating centrifugal force
- Hardening: Some combinations cause the grease to harden, blocking lubrication channels
- Oil separation: The base oil separates from the thickener, flooding the bearing initially then leaving a dry thickener residue
- Reduced high-temperature performance: Drop point of the mixture may be lower than either component alone
Compatibility Matrix
Legend: ✓ Compatible | ✗ Incompatible | ? Borderline (test before use)
| Thickener A \ Thickener B | Lithium | Lithium Complex | Calcium | Calcium Complex | Sodium | Polyurea | Aluminium Complex | Calcium Sulfonate |
|---|---|---|---|---|---|---|---|---|
| Lithium | ✓ | ✓ | ✗ | ? | ✗ | ✗ | ? | ? |
| Lithium Complex | ✓ | ✓ | ✗ | ? | ✗ | ? | ? | ✓ |
| Calcium | ✗ | ✗ | ✓ | ✓ | ✗ | ✗ | ✗ | ? |
| Calcium Complex | ? | ? | ✓ | ✓ | ? | ? | ✓ | ✓ |
| Sodium | ✗ | ✗ | ✗ | ? | ✓ | ✗ | ✗ | ✗ |
| Polyurea | ✗ | ? | ✗ | ? | ✗ | ✓ | ✗ | ? |
| Aluminium Complex | ? | ? | ✗ | ✓ | ✗ | ✗ | ✓ | ✓ |
| Calcium Sulfonate | ? | ✓ | ? | ✓ | ✗ | ? | ✓ | ✓ |
Note: Compatibility can also depend on base oil type (mineral vs. PAO vs. ester) and additive packages. Where borderline, conduct a bench test before committing to a change across a large population of bearings.
Common Industrial Grease Types
| Thickener | Typical applications | Max continuous temp | Notes |
|---|---|---|---|
| Lithium (simple) | General purpose bearings, light industry | ~120°C | Most common type; multi-purpose |
| Lithium Complex | Electric motors, rolling element bearings, high-temp applications | ~150–175°C | Preferred for motors; widely available |
| Polyurea | Electric motors (OEM factory fill in many SKF/NSK bearings), clean environments | ~150°C | Very common as factory fill; DO NOT mix with lithium |
| Calcium Sulfonate Complex | Heavy-duty, wet environments, marine, offshore | ~160°C | Excellent water resistance; compatible with lithium complex |
| Aluminium Complex | Wet/humid environments, food industry | ~150°C | Good water resistance and adhesion |
| Sodium | Legacy applications — largely superseded | ~120°C | Poor water resistance; soluble in water |
How to Change Grease Type Safely
When you need to switch from an incompatible grease type (e.g., moving from polyurea to lithium complex across a motor fleet):
- Identify all affected bearings — document the current grease type in use (check maintenance records, look at the last grease used, or take a small sample for testing)
- Open the bearing where possible (sealed bearings cannot be flushed — replace rather than re-grease)
- Remove as much old grease as possible using clean lint-free rags and a wooden or plastic spatula (avoid metal tools that can score bearing raceways)
- Flush with flushing oil or low-viscosity mineral oil compatible with both grease types — pack, run for 15 minutes, drain, repeat until the purged grease runs clean
- Pack with new grease type to the correct fill level (30–50% of housing free volume for most rolling element bearings)
- Run and monitor: Check bearing temperature and ultrasound level after 1 hour, 4 hours, and 24 hours of operation
Grease Gun Management
Mixing at the point of application — wrong gun to wrong bearing — is the most common cause of compatibility issues in practice. Implement:
- Colour-coded grease guns per grease type — one gun per grease, dedicated
- Clearly labelled storage of grease cartridges and bulk containers
- Zerk fitting covers or coloured nipples to match the required grease
- Grease type recorded on machine asset register and maintenance route sheets