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Lubricants Content

Greases Explained

What is a Grease?

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.

Diagram of grease cleaning process
Greases are generally made up of three components: base fluid, thickener and additives.

Base Fluid

When selecting a grease, the base fluid can have a strong effect on the performance in certain circumstances.

A mineral oil is a reliable choice with seals, but tends to lose its effectiveness under extreme temperature conditions. On the other hand, while synthetic fluids work well in extreme temperatures, they come at a slightly higher cost than mineral oils. The viscosity of the fluid can also impact performance. Under low temperatures or high-speed environments, a low viscosity fluid works best, though higher viscosity fluids prefer higher temperatures and slower equipment speeds.

Additives

Additives are common in greases to improve their performance, with a range of purposes.

Dyes are used to change the colour or appearance of the grease, while stabilisers ensure the base fluid and thickener form a consistent mix. Antioxidants are used to mitigate the oxidation of the base fluid, and corrosion inhibitors are integrated to protect metal surfaces from corrosion. In extreme pressure conditions, anti-wear and load-carrying additives enhance the overall performance of the grease and ensure its durability, while solid film lubricants elevate the grease's high-temperature performance.

Thickeners

Different thickening agents have different characteristics, including variable melting points, additive carrying performance and water resistance.

Thickeners diagram

Consistency

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 to the right.

Grease component diagram


Grease properties and grade chart with examples