Why temperature destroys a grease
A grease is not a liquid: it is a thickener that retains the base oil in its structure. Heat causes two distinct phenomena. The first is thickener melting: when the drop point is reached, the grease loses its structure and the oil separates. Without structure, there is no lubricating film. The second is base oil oxidation: at high temperature, the base oil oxidises, forming varnishes and hard deposits that can seize the bearing.
The most common mistake is confusing the drop point with the maximum operating temperature. They are not the same. As a practical rule, the maximum operating temperature should be at least 50°C below the thickener drop point.
The four temperature factors to define
Before selecting any grease, the application must be characterised using four temperature data points:
- Continuous operating temperature: the temperature the lubrication point experiences during normal operation.
- Peak temperature: the maximum temperature that can be reached during starts, high-load cycles, or proximity to heat sources.
- Minimum ambient temperature: critical for cold starts. A grease that hardens excessively may prevent the bearing from turning.
- Thermal cycling: repeated heating-cooling cycles accelerate oil bleeding and thickener degradation.
The thickener: the component that most limits temperature
The thickener type determines the thermal ceiling of the grease. Here is the practical guide by thickener:
- Lithium soap: up to 120–130°C continuous. The most common, economical, and versatile for normal conditions.
- Lithium complex soap: up to 160–180°C continuous (drop point >260°C). The natural evolution of lithium for more demanding applications.
- Polyurea: up to 180–200°C. Excellent for sealed-for-life bearings and electric motors.
- Calcium complex soap: up to 150–160°C with water resistance superior to lithium.
- Bentonite (clay): no defined drop point. Valid up to 280–300°C. The only option for industrial ovens and chains in steelworks.
Golden rule: if the operating temperature continuously exceeds 130°C, conventional lithium is not sufficient. It is necessary to switch to lithium complex, polyurea, or bentonite.
The lubricating base and its role in thermal stability
The base oil retained by the thickener also matters. A lithium complex grease with a low viscosity index mineral base will perform worse at high temperature than the same grease formulated with PAO (poly-alpha-olefin) or synthetic ester.
PAO base offers a viscosity index above 140, maintains its viscosity much more stably with temperature, and resists oxidation better. For applications with temperatures continuously above 150°C, synthetic base is not a luxury: it is a technical requirement.
The NLGI grade: how temperature affects consistency
At elevated temperature, a grease softens. At low temperature, it hardens. An NLGI 2 grease may behave like NLGI 1 at 80°C, making it difficult to retain on vertical guideways. In extreme cold, it may behave like NLGI 3, seizing the bearing on start-up.
In applications with wide thermal cycles (-20°C to +150°C), the NLGI grade selection must account for the full range, not just the nominal operating temperature.
There is no one-size-fits-all grease. The combination of thickener + base + NLGI grade must be evaluated against the four temperature parameters of your application. If in doubt, an oil analysis at the lubrication point after 500 hours of operation will reveal whether the grease is degrading before the bearing fails. At LUBESOLUT, we carry out this selection free of charge as part of our technical advisory process.
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