What is grease?

In today's electrically-dominated world, it is easy to forget that mechanical parts need some kind of lubrication. Grease has been utilised for thousands of years and so it is unsurprising that modern greases are very sophisticated. This introduction looks at what grease does, its ingredients and how it fails.

What is grease made from?

Grease comprises a lubricant (usually oil), suspended in a thickener. With many greases, as it is squeezed, the oil is released, thus lubricating the component. As the pressure is released, the oil is reabsorbed.

The oil possesses a complex chemical structure. Its quality also varies and it is down grease manufacturer choice whether to use relatively low-quality mineral base oils, or more sophisticated synthetics.

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

Just as there is no single type of oil, a variety of thickeners is used. Most thickeners are based on soap, which can be animal fat derived, although vegetable oils are more common, today. These thickeners can be enhanced with a 'complex' chemical structure and fortified with calcium and lithium, for instance, to give enhanced high-temperature stability, or improved water resistance, if required.

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

Anti-corrosion additives might also be included. In cases, where the grease's ingredients may attack the metals within the component it is supposed to protect, metal passivators protect the material, by coating it with a strong protective 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, and solid lubricants to enhance the grease's lubrication quality over that provided by the base oil alone.

How does grease fail?

Grease does not last forever. Typically, this 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 grease remnants to be 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.

What grease should I use?

For these reasons, there are many different types of grease. Our next blog on the topic will look at greases you can buy, how they differ from each other and the application for which they are intended.

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