What Are the Properties of Lubricants? - Viscosity

From ultra-light oils to heavy-duty greases, lubricants come in an array of forms. Viscosity is one of the most important physical properties of lubricants to consider when choosing a product. Internal resistance to flow determines how a lubricant will behave when in use, if it can achieve specific goals and ultimately, if it’s suitable for the application.

Below, we take a closer look at the importance of viscosity and why it’s one of the top physical properties of lubricants considered by operators.

What is viscosity?

Also known as internal friction, viscosity describes how resistant a lubricant is to flow. Viscosity plays an important role in determining the thickness of the protective film created by the lubricant, as well as factors like load capacity and ability to reduce friction. As a result, viscosity is one of the most important physical properties of lubricants considered when developing mechanical maintenance strategies.

High viscosity lubricants are thicker and more resistant to flow, making them suitable for heavy-duty applications. In comparison, low viscosity lubricants are thin and fast-flowing. They’re generally used for smaller, faster moving components.  

Kinematic viscosity vs dynamic viscosity

When describing viscosity, two parameters are used. Kinematic viscosity describes the rate at which a lubricant is flowing when exposed to a certain level of force. Dynamic viscosity measures the force needed for the lubricant to achieve a certain flow rate.

Viscosity Index

Viscosity index (VI) is used to measure how dependent on temperature the kinematic viscosity of a lubricant is. A number is assigned by comparing the kinematic viscosity of the lubricant at 40°C with the kinematic viscosity of two benchmark oils, one with a VI of 100 and the other with a VI of 0. At 100°C both benchmark oils should have the same viscosity as the lubricant being tested. ASTM D2270 is one of the most widely used methods used to calculate viscosity index.

Factors that affect viscosity

Viscosity is generally calculated using a clean and high quality sample, in stable operating conditions at a temperature of 40°C. However, a variety of factors can affect viscosity and compromise a lubricant’s ability to achieve goals like reducing friction and preventing wear. These can include:

• Water contamination
• Particulate contamination
• Age-related degradation
• Operating temperature

Monitoring viscosity is one of the most importance elements of any lubricant analysis strategy. Even minor changes can impact performance and increase the risk of unplanned downtime and in a worst case scenario, critical failure.

Contaminants such as H20 and particulate matter don’t just compromise the efficacy of lubricants. The hydrogen market is also concerned with impurities that can contaminate gas and damage fuel cells. Find out more about the latest technologies being used to detect hydrogen contaminants in ‘Combination of OFCEAS Spectroscopy and Low Pressure sampling, for Impurities Measurement in Hydrogen Production and Storage.’