VISCOSITY: The state of being thick, sticky, and semifluid in consistency, due to internal friction.
A lubricant’s viscosity is generally considered one of its most important properties, and with good reason. If the viscosity of a lubricant is even slightly different from what is needed for a particular component and application, the lubricant will not be able to lubricate the component effectively. This may lead to significant damage and possible equipment failure.
Understanding viscosity is important for proper lubricant selection, management, and maximizing the performance, efficiency, and life span of your equipment.
Below are answers to some common questions about viscosity that may shed some light on this complex yet vital aspect of industrial lubrication.
Q: What are absolute viscosity and kinematic viscosity, and what is the difference between them?
A: Absolute (or dynamic) viscosity and kinematic viscosity represents the two methods for measuring and reporting viscosity.
Absolute viscosity measures a fluid’s resistance to flow when the fluid is subjected to a controlled force, such as a moving machine part, pressurized air, or pump. The more force required to move the fluid, the greater the fluid’s absolute viscosity.
KEY INSIGHT: The reason it takes more force to mix yogurt in a blender than it does to mix fruit juice is because yogurt has a higher absolute viscosity.
Kinematic viscosity measures a fluid’s resistance to flow under the force of gravity. The slower the fluid’s flow in relation to gravity, the higher its viscosity.
KEY INSIGHT: If you were to tip over open containers of honey and water, the honey would pour out slower than the water because the honey had a higher kinematic viscosity.
Q: Does a fluid’s viscosity change in relation to the rate of shear stress (i.e., how rapidly or slowly the fluid is stirred, pumped, or otherwise forced to move), or is the viscosity constant?
A: The short answer? It depends on the specific fluid.
The long answer: Most fluids maintain constant viscosity regardless of the pace at which the fluid is forced to move. Those fluids are known as Newtonian fluids because they adhere to Sir Isaac Newton’s law of fluid mechanics. For Newtonian fluids, absolute viscosity is constant, as is kinematic viscosity, and the two measurements are relative to one another. Most lubricant oils are considered Newtonian.
With non-Newtonian fluids, the viscosity varies with the rate at which the fluid is forced to move. Mayonnaise is a good example: It has a high viscosity when subjected to the relatively low shear of gravity (i.e., an open jar of mayonnaise does not readily spill out if the jar is turned on its side). But when stirred, mayonnaise yields very easily to the force. Its viscosity decreases as the shear rate increases.
KEY INSIGHT: Shear Rate is the rate at which fluid layers slide past each other, measured as the change in velocity over distance, and is expressed in reciprocal seconds.
Because of this variability, the viscosity of non-Newtonian fluids cannot be measured conventionally. It must be measured in relation to the temperature and the shear rate. This measurement is termed “apparent viscosity.”
Q: If most lubricant oils are considered Newtonian, which lubricants fall into the non-Newtonian category?
A: Greases and multi-grade oils are non-Newtonian, as are most lubricant oils that have any of the following characteristics:
Non-Newtonian fluids are used in a variety of different applications in both everyday life and in manufacturing.
Q: Can a lubricant’s viscosity change due to other factors?
A: Yes. The viscosity of a Newtonian or non-Newtonian fluctuates inversely with temperature, which is why temperature always must be reported with viscosity measurements.
A lubricant’s viscosity also can be altered by contamination, oxidation, thermal degradation, improper mixing the lubricant with another lubricant of a different viscosity (higher or lower), or with a solvent.
Regular oil analysis is critical to monitoring a lubricant’s viscosity and other aspects of its condition.
Q: How is viscosity measured?
A: Various types of instruments called viscometers are used to measure viscosity. Many viscometers measure the time it takes for fluid to pass through a capillary tube. For kinematic viscosity, this is under the force of gravity. For absolute viscosity, this is controlled by an external force.
Kinematic viscosity is usually measured in units called centistokes (cSt). It is common to measure the kinematic viscosity of industrial lubricants in centistokes at 40°C (104°F). That is the basis for the ISO 3448 kinematic viscosity grading system, a globally accepted industry standard established by the International Standards Organization. For some lubricants, especially those designed for automotive use, kinematic viscosity is typically measured in centistokes at 100°C (212°F) in accordance with the SAE J300 grading system, the international standard set by the Society of Automotive Engineers.
Laboratories usually measure and report the kinematic viscosity of a lubricant. If absolute viscosity is measured and reported, the unit of measurement is the centipoise (cP).
Q: What is viscosity index?
A: A lubricant’s viscosity index (VI) is a unitless number that indicates the viscosity’s stability in relation to temperature changes. The viscosity of a lubricant fluctuates inversely with temperature. With some fluids, a slight change in temperature may cause a dramatic shift in viscosity. For other fluids, the viscosity barely changes even with substantial temperature fluctuations. The higher a lubricant’s VI, the more stable its viscosity is over a wide range of temperatures (i.e., the less it changes with temperature).
VI is often an important consideration when selecting lubricants for applications experiencing wide temperature variations.
Q: What problems may occur if a lubricant’s viscosity is too high or too low?
A: If a lubricant’s viscosity is too high, the lubricant may not flow adequately to where it is needed. This may result in:
If a lubricant’s viscosity is too low, the fluid might not sufficiently separate and protect the parts as intended. The consequences may include:
Q: What is the best way to select and maintain proper lubricant viscosity in a system?
A: There are two recommended ways that you can choose from:
DuBois makes lubricant selection and maintenance easier for you. DuBois designs and manufactures industry-leading industrial lubricants that offer unsurpassed performance, protection, and service life.
Our lubrication expertise and unique manufacturing processes allow us to customize formulations to your customers’ specifications, so you can provide them with the exact viscosity and other properties that their systems, applications, and operating conditions demand.
DuBois offers a complimentary oil analysis program that is available to all your customers who use our lubricants. With DuBois’ thorough understanding of viscosity and other lubricant properties, as well as our expert analysis services, you will help your customers maintain proper lubrication and equipment protection.
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