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Thursday, 28 November 2013

How lubricants are affected by temperature.

It’s common knowledge that lubricants are affected by temperature – in extreme cases negatively influencing performance.

It’s obvious that lubricants are affected by temperature – that’s why multigrade engine oils have a winter rating and an operating temperature rating. But what in the oil chemistry changes with a change in temperature.

Lubricants are affected by temperature changes in several ways.


The viscosity of an oil is its ability to flow or its internal resistance to flow. The viscosity of an oil is probably the most important property to consider when selecting a lubricant.

When an oil film forms between a bearing and a rotating shaft, some of the oil's molecules are attracted to the surface of the shaft, while other oil molecules are attracted to the bearing surface. The relative movement causes the oil layer to shear - called the shear rate and is directly affected by the oil's viscosity and operating temperatures. A multi-grade oil with a lower viscosity will generally have a higher potential shear rate, while a single viscosity oil will generally have a lower potential shear rate.

 Since oil with a lower viscosity and high potential shear rate must still maintain a sufficient oil film, it is quite apparent that the lubricants affected by temperature could fail resulting in metal-to-metal contact. If the oil's viscosity is too high with a low potential shear rate, the internal resistance to flow will increase the temperature dramatically, causing an overheated condition, which can also cause a breakdown of the oil film and may cause oxidation of the oil. Therefore, it is critical that oils be selected by always taking the operating temperature of the equipment into account.

 Pour Point 

The pour point of an oil is defined as the lowest temperature at which a lubricant will flow. It is frequently and erroneously used as the oil viscosity selection criteria.

For example, let's say an oil has a pour point of minus 30 degrees C. Most people assume that this means that the oil will flow to the bearings of the equipment even when the ambient temperature is at minus 30 degrees C. This is a fallacy. At best, this oil with a pour point of minus 30 degrees C and operating in an ambient temperature of minus 30 degrees C will merely churn at the oil pump until the churning of the lubricants are affected by temperature increase. This in turn allows the oil's viscosity to thin sufficiently so that it slowly begins to flow through the oil passages to the lubricated components.

Viscosity Index 

The viscosity index (VI) of an oil is the term used to express an oil's "resistance to viscosity change as the lubricants are affected by temperature changes." For example, an oil that thins out (reduced viscosity) significantly as its temperature increases is said to have a low VI. An oil whose viscosity does not change significantly as it is heated up is said to have a high VI.

Most industrial mineral lubricating oils that might be used in a manufacturing plant or production facility with controlled temperatures need only have VIs of 55B100.

Base Oil

Mineral-based (non-synthetic) oils have various bases depending upon their molecular and chemical structure. Mineral Base oils can be paraffinic, naphthenic or aromatic, and these lubricants are affected by temperature more than synthetic oils.

For example, naphthenic base oils have low natural VIs and may be selected for equipment where extreme temperatures do not affect operation. On the other hand, paraffinic base oils have natural VIs that are considerably higher than naphthenic types, making them desirable base stocks for lubricants used in outdoor applications.

Synthetic base stock is very stable at extreme temperatures and do not oxidise like regular base oils.

When lubricants are affected by temperatures that are extremely high they may degrade.

The reason for this is that oxidation takes place which cannot be reversed. And it’s not only the oxidation of the oil that’s a problem the additives are also depleted at a higher rate.

Synthetic lubricants are affected by temperature to a much lesser degree and are therefore highly recommended for extreme operating temperatures. Habot Synthetic Lubricants produce a range of quality oils that operate under all temperature conditions. Contact us now to find out which oil is best suited to your application.