Unlike piston engine oils (PEOs) and turbine engine oils (TEOs), greases tend to be thicker, taking on a solid or semi-solid form. They are a thickened lubricant oil, with the main benefit being that they remain in place, available for use at the most critical parts of a mechanical operation. Because of this thickener, a grease acts like a sponge, releasing lubricant when necessary and then reabsorbing it once the pressure has been relieved. While greases benefit from having less flow, this does mean that they are ineffective at removing heat and other contaminants from the process.
Greases are generally made up of three components: base fluid, thickener and additives.
When selecting a grease, the base fluid can have a strong effect on the performance in certain circumstances. A mineral oil works well with seals, but is generally ineffective at extreme temperatures. On the other hand, synthetic fluids work well in extreme temperatures but are more expensive than mineral oils. The viscosity of the fluid can also impact on performance. A low viscosity fluid works best in low temperatures or high speed environments. Higher viscosity fluids prefer higher temperatures and slower equipment speeds.
Different thickening agents have different characteristics, including variable melting points, additive carrying performance and water resistance.
Additives are common in greases to improve their performance: • Dyes – change the colour or appearance • Stabilisers – ensure the base fluid and thickener form a consistent mix • Antioxidants – slow down base fluid oxidation • Corrosion inhibitors – protect the metal surface • Anti-wear and load carrying – improve the performance of a grease in extreme pressures • Solid film lubricants – increased high temperature performance
As mentioned previously, the main characteristic of a grease is its consistency; however, this can vary depending on the amount of thickener used. Consistency is measured in NLGI (National Lubricating Grease Institute) ratings, ranging from 000 to 6, although anything greater than 3 is less common. Each NLGI rating can be compared to an everyday product as outlined below.
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