Most of us working in the lubricating grease industry know the NLGI mantra by now:
A lubricating grease is a solid to semi-fluid dispersion of a thickening agent (thickener) in a liquid.
This is directly from the NLGI website glossary. But what does this mean?
Let’s start at the end of the definition and work our way backwards. The liquid is the lubricating oil and grease is around 85% to 90% oil. This oil is the stuff that lubricates the machine elements. It has a type, mineral, synthetic, etc., and a viscosity. The viscosity is the important property and is different from the apparent viscosity of the grease.
Grease has a thickening agent. These are soaps, detergents, polymers, or solids. Soaps are the most common. To get an idea of what these materials are just think, “bath” bar soap. Those usually contain the sodium salts of fatty acids; i.e. definition of soap. However the property that makes bath soaps useful like being soluble in water are not good for grease. Greases then use metals other than sodium like lithium, calcium, aluminum, and others. Lithium is the metal of choice. The fatty acid component is an organic acid with an 18-carbon chain. Other organic monoacids and diacids may be added to form a complex soap.
These thickeners are dispersed in the liquid. They are not dissolved in the oil but are suspended by the fluid. If you let grease stand undisturbed the oil and thickener will separate. If that happens just remix the oil and thickener to reform the grease.
This dispersion of thickener in oil can lead to a solid or semi-fluid mixture. Solid grease, the so-called brick grease, is solid and lubricating grease is picked up by mechanical action of rubbing against the grease. The fluidity of the grease is indicated by the NLGI grade, which ranges from grade 6 (the solid grease) to grade 000 (fluid like vegetable oil). Grade 2 is normally used has a consistency like that of peanut butter.
The next time you are buying grease you know what a synthetic grade 2 lithium complex grease consists. It is a lithium soap thickener dispersed in a synthetic base oil to give a consistency of peanut butter.
So what can go wrong? Not all grease thickeners are easily identified on the packaging. Thickeners may not be compatible so if you mix two incompatibly thickened greases together the thickening action may be destroyed and the mixture is no longer grease.
How does grease lubricate? That is a more difficult question and the answer will come in future blogs.