What Is the Difference Between ISO Grades of Oil?

What Is the Difference Between ISO Grades of Oil
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Industrial machines and even hand tools rely on lubricants, or oils, to continue to function properly. This material ensures that parts can move freely without incurring damage. Hydraulics frequently used mineral oil-based fluids to transfer power or heat to elements of various machinery, including excavators. A perhaps more common use of hydraulic oils is the oil that automobiles use for braking (brake fluid). This fluid is one of many to which the ISO viscosity gradient scale can be applied.


The International Standards Organization Viscosity Grade, or ISO VG, is a numerical rating of the viscosity of oils and lubricants as established by several organizations in 1975. The International Standards Organization (ISO), American Society for Testing and Materials (ASTM), Society for Tribologists and Lubrication Engineers (STLE), British Standards Institute (BSI), and Deutsches Institute for Normung (DIN) established the ISO VG to help standardize the industry. Lubricant and oil supplies and manufacturers, as well as manufacturers of machinery that use lubricant, use this ranking in their work as it describes the material's resistance to flow.


As the viscosity of the oil increases, so does the density of the material, as a higher density results in oil that is less likely to respond to flow or other movement. Thus, an oil or lubricant with a viscosity grade of 220 is thicker and more solid-like than an oil with a VG of 100 or 68. The grade is a literal measurement of the oil's ratio of absolute viscosity in centipoise (a unit of measurement) to the density, also known as centistoke.


Since its inception in 1975, the organizations have developed 20 viscosity gradients to cover the range of oils and lubricants that are common in hydraulic application. The lowest common ISO grade is 32 and the scale ranges up to 220. The scale also includes grades 46, 68, 100 and 150.


Because the viscosity of oil and other liquids is dependent upon temperature, the ISO grade is only applicable at a specific temperature. Base ISO grades are calculated when the oil is at a temperature of 40 degrees C (104 degrees F) and raising or lowering the temperature of the material will alter the oil's resistance to movement such as flow. For example, raising the temperature to 100 degrees Celsius will change the number of centistokes from a grade to just 5.4 centistokes, in comparison to the 32 centistokes at 40-degrees Celsius. At this temperature, the oil is more likely to be effected by flow.

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