Putting the simple back into viscosity
Written by John Sander
Abstract: Simply stated, viscosity is defined as the internal resistance of a fluid to flow. That doesn't sound too difficult, does it? Unfortunately, new temperature, speed and pressure demands on lubricating fluids have changed over the years, resulting in several different measurements and classifications being created to describe lubricant viscosity. Some examples are SUS, cSt, cP, ISO, SAE engine, SAE gear and AGMA; it's enough to make a person's head start to spin. This paper will summarize some of the more commonly used viscosity standards, describe the tests used to measure viscosity, and eliminate some of the confusion all of these standards may create for the end user.
Introduction
In recent years, some large lubricant marketers have run advertisements on TV that highlight the importance of viscosity breakdown. These advertisements make it seem like viscosity is a complex chemical property of the fluid, when in fact it is a measurement of a physical property. Simply stated, viscosity is a measure of a fluid's internal resistance to flow. A good example of this was provided in one of the TV ads, which showed two oils being cooled until one continued to flow out of the bottle readily, while the second dropped out in blobs. The resistance to flow, or viscosity, of the second oil had increased dramatically with the decrease in temperature. This ad illustrated just how important it is to consider viscosity when choosing the proper lubricant for a specific application.
The presence of viscosity information on almost all lubricant marketers' technical literature is an indication that it also is important in the marketing of lubricants. Original equipment manufacturers often specify the lubricant to be used in their equipment by product type and viscosity. Lubricant marketers usually sell their lubricants according to specific viscosity grades, such as SAE 15W40, ISO 46 and AGMA 3.
It is clear that – for the majority of the players in the lubricant industry – the proper viscosity of a fluid is the most important attribute in proper lubrication. There are several reasons for this, including, but not limited to:
• Viscosity affects fluid film thickness under certain conditions of temperature and load in lubrication applications.
• Viscosity affects heat generation and removal in bearings, cylinders and gears.
• Viscosity determines the ease with which machines can be started in low-temperature conditions or can be kept running in high-temperature conditions.
• Viscosity can be used to control a fluid's sealing ability, which results in lower consumption.
