All lubricants start with a base oil. There are three types: mineral, synthetic, and vegetable. In industrial applications, we mostly deal with mineral and synthetic, so I would like to focus on these. Mineral oil comes from crude oil and the quality depends on the refining process. There is a grading scale for oil and different applications require different oil quality. Mineral oil is mainly made up of four different types of molecules – paraffin, branched paraffin, naphthene, and aromatic. Paraffinic oils have a long, straight chained structure, while branched paraffinic oils are the same with a branch off the side. These are used mainly in engine oils, industrial lubricants, and processing oils. Naphthenic oils have a saturated ring structure and are most common in moderate temperature applications. Aromatic oils have a non-saturated ring structure and are used for manufacturing seal compounds and adhesives. Synthetic oils are man-made fluids that have identical straight chained structures, much like the branched paraffinic oils. One of the benefits of a synthetic is that the molecular size and weight are constant while mineral oils vary greatly; therefore the properties are very predictable.

So why don’t we use synthetic oils all the time if we know exactly what it’s going to do? While there are many advantages to using a synthetic, there are almost as many reasons to not use it. The best quality mineral oil is mostly made up of paraffinic oils, like those in synthetic oil. So, in many applications, mineral oil is just as good as synthetic, and in these applications is most likely the preferred base due to synthetic’s high cost, toxicity, solubility, incompatibility, and hazardous disposal. However, in extreme applications where a high flash point, low pour point, fire resistance, thermal stability, high shear strength, or high viscosity index is needed, a synthetic may be just what’s required.

We briefly discussed a couple of the additives that are used with a base oil in order to improve performance, but I’d like to expand on the most common additives now. The most important property to look at when choosing a lubricant is its viscosity. This is the oil’s resistance to shear and flow. The simplest way to describe viscosity is to relate it to substances that we are familiar with. The higher an oil’s viscosity, the slower it flows. Molasses, for example, has a very high viscosity while baby oil has a very low viscosity. The viscosity required for an application depends on the speed, operating temperature, and type of bearing as well as the type of component, like a gearbox versus a motor. Working hand in hand with viscosity is the viscosity index, which relates change in viscosity due to temperature. The higher the viscosity index, the less viscosity is affected by temperature. This property can be improved with a viscosity index additive. Rust inhibitors protect surfaces against rust by forming a thin water repelling film on the metals surface. Dispersants help protect components against abrasion from wear products by enveloping particles and suspending them in the oil so that they may be easily flushed and removed from the system. Antiwear and extreme pressure (EP) additives react with a component’s surfaces to form a thin protective layer to prevent metal-to-metal contact. This is especially helpful in situations where there is high pressure or a lot of stop and start evolutions. Detergents work to neutralize acids and clean surfaces where deposits may be detrimental. Finally, defoamants weaken the surface tension of bubbles so that they may break easily and minimize foaming.

For any given oil, the ingredients are the base oil and the additives. The only difference for grease is that it also has a thickener. This is most commonly described as “the sponge that holds the lubricant.” Up to thirty percent of grease is made up of the thickener which is either a simple or complex soap. Simple soap is made up of long fibers and has a smooth, buttery texture. Examples of simple soaps are lithium, polyurea, calcium, and silica. Complex soap is made up of short and long fibers and has a more fibrous texture. Some examples are aluminum, sodium, and barium.

There are benefits of using a grease as opposed to oil in certain applications. Grease seals out contaminants, is better suited for insoluble solid additives like molybdenum disulfide and graphite, and has better stop-start performance because it doesn’t drain away like oil, for a lower chance of a dry start. However, the thickness of grease limits bearing speed, reduces cooling of components, makes for difficult sampling and analysis, and makes it difficult to determine the proper amount of grease that needs adding. This is something that must be taken into consideration when deciding if oil or grease would be better suited for the application.

With a basic understanding of lubrication, you can see there are quite a few advantages of using the proper lubrication in your machines. Higher efficiency, longer life, better reliability, and less money spent on maintenance are goals that every company strives to achieve.