Grease is mineral or synthetic oil held in a soap or polymer structure. The thickener does not lubricate — the oil does. Its job is to keep the oil where it is needed and release it slowly. That is why two greases with the same consistency number can behave completely differently at temperature: what matters is the base oil viscosity, the thickener type and the drop point.
Base oil viscosity sets the film thickness: too thin and metal touches at temperature, too thick and the bearing churns. Thickener type sets the temperature ceiling and compatibility. Additives — EP, anti-wear, rust inhibitor — set what the grease can survive under load or in wet conditions. A grease is specified by all three, not by its colour or its NLGI number.
A sealed bearing is filled to about 30% of its free space. Fill it to 80% and the balls have to churn the grease, which shears the thickener, generates heat and collapses the structure. In slow, wet, vertical-shaft applications a higher fill is used deliberately to keep water out — that is the exception, not the rule.
Most grease manufacturers publish a life curve against temperature. Above about 70 °C, each additional 15 °C halves the life. A bearing running at 90 °C with a grease rated for 10,000 hours at 70 °C will give roughly 2,500 hours — about three months of continuous running. That is why the same bearing lasts ten years in one plant and one year in another.
| Thickeners | Lithium, lithium complex, calcium, polyurea, PTFE |
|---|---|
| Base oil | Mineral, PAO, ester, silicone, PFPE |
| Consistency | NLGI 2 is the bearing default (NLGI 1 for low temperature) |
| Fill quantity | 25–35% of free space in a sealed bearing |
| Temperature | Standard lithium to about 120 °C |
| Speed factor | Up to ndm ≈ 350,000 for grease |
| Life rule | Halves for every 15 °C above 70 °C |
| Overfilling | Causes churning, heat and grease collapse |
| Thickener | Temperature | Water resistance | Compatibility | Typical use |
|---|---|---|---|---|
| Lithium soap | −30 to +120 °C | Good | Mixes with most | General purpose |
| Lithium complex | −30 to +150 °C | Very good | Limited mixing | Hot, wet duty |
| Calcium sulphonate | −30 to +140 °C | Excellent | Limited mixing | Washdown, marine |
| Polyurea | −30 to +160 °C | Good | Incompatible with lithium | Electric motors, sealed |
| PTFE / PFPE | −50 to +280 °C | Excellent | Incompatible with others | High temperature, chemical |
It is a consistency grade — roughly the stiffness of peanut butter. NLGI 2 is the bearing default; NLGI 1 is softer and used at low temperature or in centralised lines.
30–50% of the free space for slow duty, less for high speed. In a sealed bearing the factory fill is already correct; do not add more.
No. They are incompatible and can soften into a liquid. Purge the housing completely when changing thickener type.
Oxidation from overheating. The base oil has separated and the thickener has carbonised. Check the running temperature — the grease is reporting a problem, not causing one.
Send the designation, the three dimensions and the quantity — we reply with price, brand options and lead time within 30 minutes on working days.