What is Grease?: Your Guide to Grease Basics | GEM

In today's digital world, it is easy to forget that mechanical parts need some kind of lubrication. Humans have used grease as a lubricant for thousands of years and so it is unsurprising that modern greases are highly sophisticated, with many varieties. This introduction looks at what grease does, its ingredients and how it fails.

Understanding Grease vs Oil

Grease is not the same as oil, although oil tends to be one of its ingredients.

Grease comprises a lubricant (usually the base oil) that is suspended in a thickener. Without this thicker, the oil would simply drip off the component it should protect.

Grease works by releasing lubricating oil as it is squeezed. As the pressure is released, the oil is reabsorbed back into the thickener.

Like other types of automotive oil, the base oil that is used in grease possesses a complex chemical structure. It is down to the grease manufacturer whether to use relatively low-quality mineral base oils, or more sophisticated synthetics. This choice (and more) depends on the intended use and also the grease's price point.

Additives might also be included, to enhance the grease's properties. Anti-oxidant chemicals, for instance, increase the base oil's life and boost its high-temperature resistance. Extreme Pressure additives might also be employed, indicated by the grease being advertised as being suitable for 'EP' applications.

What is grease made from?

Aside from oil, grease comprises a thickener. Most thickeners are based on soap, which can be animal fat derived, although vegetable oils are more common. These thickeners can be enhanced with a 'complex' chemical structure and fortified with other ingredients, such as calcium and lithium, among others, to give enhanced high-temperature stability, or improved water resistance, if required.

The alternative is to thicken the oil itself with a solid ingredient. Clay-based thickeners tend to be tolerant of high temperatures; anti-seize copper grease, for instance, possesses a clay base.

Anti-corrosion additives might feature. In cases, where the grease's ingredients may attack the metals within the component it is supposed to protect, Metal passivators can be added, to coat the metal with a strong anti-corrosive layer.

Other additives include those that improve tackiness, to reduce the chance of the grease being flung from the machinery it is supposed to protect. Solid lubricants might be added to enhance the grease's lubrication quality over that provided by the base oil.

NLGI Grease Classifications

Other than the necessary ingredients that match the grease to its intended application, the other consideration is its thickness ('viscosity'). The NLGI (National Lubricating Grease Institute) defines grease constancy from 000 (being a fluid) and 6 (being hard). Unlike engine oils, for instance, the NLGI grade tends not to be printed on the packaging.

How does grease fail?

Grease does not last forever. Typically, failure occurs when the grease overheats; the thickener fails, the oil drains away and the component becomes starved of lubrication. Should you notice oil seeping from a greased component, the part will need the old remnants cleaned out and repacked with fresh grease.

Water can also wash grease away from the component it is supposed to protect. Typical examples include boat trailer wheel bearings, or tow hitches, which are reversed into the sea and exposed to salt water.

Grease can also expire over time - especially if its structure is plant-based, such as rubber grease. The oil can also separate from the grease structure; should you open an old can of grease and discover a layer of oil floating on the top, the product has expired and should be discarded.

The base oil can also react with atmospheric oxygen and this oxidisation prejudices its lubrication properties. This is a good reason why you should not use old grease.

Choosing the right grease

There are many different types of grease, because there are many applications. Using the wrong grease can cause the component to wear and fail prematurely.

Grease compatibility

Typical examples of this include:

Copper grease - An anti-seize grease for high-temperature applications

Ceramic Grease - for modern brakes that provides lubrication (unlike copper grease)

Rubber grease - This vegetable-based grease does not attack rubbers

Extreme Pressure grease - For low-temperature applications, where high shock load protection is required.

PTFE greases - Where grease is designed to be dry but still proves a lubrication function.

Our accompanying blog on this topic looks at different types of grease in more detail.

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