The Most Important Property
by Ambrose Hughey
Business Manager
, Northwest

Viscosity is often referred to as the most important property of a lubricant. This article will help provide a better understanding of the importance of a lubricant’s viscosity and some factors that can affect a lubricant’s viscosity.

Proper lubrication entails reducing friction and preventing metal to metal contact, which are two of a lubricant’s chief functions. A reduction in friction allows for a smoother and prolonged equipment life with less heat generation. Prevention of metal to metal contact will also reduce heat formation, but potentially more important, the prevention of metal to metal contact will control the amount of wear being generated in the system. Having the lubricant provide a sufficient film thickness is necessary to achieve both of these functions. Viscosity plays a major role in the lubricant’s ability to perform these vital functions.

Another chief function of a lubricant is to cool parts by transferring away heat however, if the lubricant’s viscosity is too thin or too thick the opposite can actually occur. If the lubricant’s viscosity is too low then excessive heat can generated from metal-to-metal contact due to an insufficient film thickness. If the lubricant’s viscosity is too high, heat can be generated, due to the internal fluid friction created within the oil. Therefore, for the lubricant to effectively carry away heat the proper viscosity is needed.

The above vital functions of proper lubrication are dependant on having the proper viscosity. Selecting the proper viscosity is critical to efficient operation of equipment. Many factors such as equipment design, speed, load, ambient conditions, etc need to be considered in order to select the proper viscosity. Most equipment has OEM recommendations for the proper viscosity grade. Once the lubricant is put into service, monitoring its viscosity is important to maximize equipment life and reducing costs. Several factors can change the lubricant’s viscosity and the change can go either way resulting in an increase or decrease in viscosity.

Diesel fuel dilution – As diesel fuel possesses a much lower viscosity than lubricating oils, any fuel contamination will lower the viscosity.

VI (viscosity index) improver shearing – This is effected by the amount of VI improver, operating temp, and shear rate. VI improver additives are polymer additives that can have bond breakage under high shear that results in a reduction in viscosity at higher temps and VI.

Wrong makeup oil – The addition of the wrong makeup oil can result in either a reduction or increase in viscosity depending on the viscosity of the wrong make up oil and any subsequent reactions that occur.

Thermal cracking – Lubricants subjected to extremely high temps can have their molecular bonds broken resulting in smaller molecules, which subsequently results in a reduction in lubricant’s viscosity.

Contamination – Many contaminants can increase the viscosity such as soot loading and water and oil emulsions.

Oxidation – Oxidation byproducts generally cause the viscosity of lubricants to increase.

High operating temperature – A high operating temperature can lead to an increase in oxidation and thermal degradation both of which can result in an increase in viscosity.


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