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Operating Temperature Affects Physical Characteristics of Lubricants

Temperature affects a lubricant’s primary physical characteristics including viscosity, viscosity index, pour point, and base oil. Knowing how it changes these characteristics helps in identifying and choosing the right lubricants to use for specific applications. 

Viscosity

An oil’s viscosity deals with its ability to flow or its internal resistance to flow. It is recognized as one of the most important factors to consider when selecting lubricants. Kinematic viscosity is measured in centistokes (cST) at 40 degrees C and 100 degrees C. These specifications are always seen on oil company data sheets.

When an oil film accumulates between a bearing and shaft, some of the oil’s molecules are attracted to either the shaft surface or the bearing surface. This is known as the shear rate and is positively affected by an oil’s viscosity and operating temperatures. Normally, a multi-grade oil with lower (thinner) viscosity will have a higher potential shear rate. Conversely, a single viscosity oil will most likely have a lower potential shear rate.

A lubricant with a lower viscosity and high potential rate must have enough oil film because if temperature rises, the film may fail therefore leading to potential metal-to-metal contact. The internal resistance to flow will increase temperature noticeably if an oil’s viscosity is too high with a low potential shear rate. This results to an overheated condition which can cause oil film breakdown as well as oil oxidation. This also shows how critical it is to look into an equipment’s operating temperature when selecting lubricants.

Pour Point

An oil’s pour point is the lowest temperature at which a lubricant will flow and is often times mistakenly used as the oil viscosity selection criteria.

If a lubricant has a pour point of -30 degrees C, most people will think that the oil will flow to an equipment’s bearing even if the surrounding temperature is also at -30 degrees C.  This is not true because at best, an oil with a pour point of -30 degrees C and an operating environment temperature of -30 degrees C will just churn at the oil pump until the churning results to an oil temperature rise. This then permits the oil’s viscosity to thin adequately so that it gradually flows through the oil passages to the lubricated components.

Most of the time, this process takes at least 5-10 minutes or more with the possibility of severe damage on various components because oil is too thick to flow. It is not advised to choose lubricants based on pour point alone.

Viscosity Index

The resistance to viscosity change as temperature changes is known as an oil’s viscosity index (VI). An oil that thins out drastically as temperature rises is identified to have low VI while an oil with viscosity that doesn’t change considerably when temperature increases is said to have high VI.

Selecting lubricants that will be utilized in temperatures changing significantly must consider the most critical and important factor: temperature/viscosity relationship.  Viscosity Index is particularly crucial when choosing oils for northern U.S. and western Canadian winters or high arctic operations.

Majority of industrial mineral lubricants that might be utilized in manufacturing plants or production facilities with controlled temperaturesonly need VIs of 55B100. These viscosity-index specifications are seldom listed in oil company data sheets although they should be.

Base Oil

Base oils must also be taken into account when selecting lubricants. Depending on molecular and chemical structure, non-synthetic oils have different bases. These base oils can be paraffinic, naphthenic or aromatic. Naphthenic base oils possess low natural VIs and may be chosen for equipment operation not affected by extreme temperatures. Paraffinic base oils have natural VIs that are noticeably higher than naphthenic types, making these as ideal base stocks for oils used in outdoor applications.

Having full understanding of the relationship between temperature and a lubricant’s primary physical characteristics as mentioned above leads to proper lubricant selection. With appropriate lubricants used, efficient equipment operation is guaranteed. The Industrial Lubricant Store offers a comprehensive range of high quality food grade and non-food grade lubricants best to keep industrial equipment operating smoothly and optimally. Learn more about what we offer by visiting www.theindustriallubricantstore.com/products today!

About the Author
Randy Renick
Randy Renick has a Bachelor's degree from LSU. He is an STLE Certified Lubrication Specialist and has a 29 year work history in Industrial Lubrication. He is currently a Lubricant Consultant at The Industrial Lubricant Store.