Before purchasing oil from leading brands like Fuchs and Kluber, it is advisable to examine the ISO number.
The International Standards Organisation Viscosity Grade, or ISO VG for short, is a standardised method to classify viscosity. In lubrication, the term viscosity refers to a lubricant’s resistance to flow. The thicker an oil is, the more viscous it is and harder to pump.
ISO VG states viscosity in numbers on a scale that starts at two and reaches 1,500. An essential resource for selecting a lubricant, it ensures that the product chosen is the correct option for a specific type of machinery, application or operating condition.
With equipment built in various places around the world, the system was created to standardise information on viscosity and help equipment users find the right product.
How the ISO VG system works
ISO VG is measured using kinematic viscosity in centistokes (cSt). It measures the flow of a fluid while subject to gravity. The Capillary Tube Viscometer Test is among the most common testing tools. This test is performed by placing the lubricant in a glass capillary tube at a controlled temperature of 40°C.
The lubricant is drawn to one of the tube’s ends via suction and then released, allowing gravity to pull it. The time it takes to flow between the two points is listed in cSt and forms the basis for the ISO VG number. The more viscous the oil, the more slowly it flows, giving it a higher number.
Why is the ISO VG number important?
How thick the oil film is between working parts is critical to mechanical longevity and achieving optimum performance. When lubricating oil is too thin, surface-to-surface contact occurs, creating premature wear and tear and elevated temperatures that cause lubricants to oxidise. However, if the oil is too thick, it can cause internal friction, inefficient processes, poor pumpability and inadequate heat dispersion.
While ISO VG numbers can aid in choosing a lubricant, they are just one consideration. Whether you are ordering gear oil or grease, always consider your work rates, operating conditions and your materials’ compatibility to tailor your selection.
