Lubrication science
Grease compatibility: when two greases can and cannot mix
Quick answer
Why mixing the wrong greases softens, hardens or bleeds, and a general guide to which thickeners to avoid mixing.
What incompatibility means
Two greases are compatible when a mixture keeps the properties of the two on their own. When they are not, the mixture can soften and run out, harden and cake, or lose oil. The bearing then runs hot or starves, and the damage looks like lubrication failure.
What decides compatibility
- Thickener type: the biggest factor.
- Base oil type: mineral, PAO, ester and silicone do not all mix.
- Additive package.
- Consistency (NLGI grade) and operating temperature.
General guide to thickener mixing
| Mixing | General result | Advice |
|---|---|---|
| Lithium with lithium complex | Usually compatible | Safe to top up between the two in most cases |
| Lithium with calcium | Usually compatible | Check consistency after mixing |
| Lithium with calcium sulfonate complex | Borderline | Test before relying on it |
| Lithium or lithium complex with polyurea | Often incompatible | Test, or purge fully before changing |
| Lithium with clay (bentonite) | Incompatible | Do not mix |
| Polyurea with clay | Incompatible | Do not mix |
| Calcium sulfonate complex with polyurea | Borderline | Test first |
Base oil cautions
- PAG does not mix with mineral oil or PAO.
- Silicone greases do not mix with others.
- Ester greases can swell some seals.
Signs of incompatibility
- Grease becomes soft or runny.
- Grease becomes hard or lumpy.
- Oil bleeds from the grease.
- Colour or smell changes.
- Bearing temperature rises after re-greasing.
Test methods
| Test | Checks |
|---|---|
| Visual and feel | Colour, texture, smoothness |
| Worked penetration, ASTM D217 | Change in consistency |
| Dropping point, ASTM D566 or D2265 | Change in heat resistance |
| Oil separation, ASTM D1742 | Oil bleed |
| Four-ball wear, ASTM D2266 | Wear protection of the mix |
| ASTM D6185 guide | How to evaluate compatibility of grease mixtures |
When you change greases
- Same grease: re-grease directly.
- Different grease of known compatibility: re-grease and watch the first intervals.
- Unknown compatibility: remove the old grease thoroughly before applying the new one, or purge with several shots and check.
- Critical, hot or heavily loaded bearings: use only proven compatible greases, and record the change.
Takeaway. Never assume two greases mix. Check the thickener and base oil, test where it matters, and purge or clean out when in doubt. This chart is a general guide, so confirm with the grease maker.
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