ISO Viscosity Grades

Certainly! ISO Viscosity Grades play a crucial role in the world of lubrication. Let’s dive into it.

What are ISO Viscosity Grades?

• ISO Viscosity Grades, also known as ISO VG, provide a standardized method for classifying industrial liquid lubricants based on their viscosity. These grades help lubricant suppliers, equipment designers, and users communicate effectively about lubricant requirements.
• The ISO VG system ensures consistency by defining viscosity ranges for each grade. The midpoint of each range represents the nominal viscosity value, and the actual viscosity of a lubricant can vary by ±10% from this midpoint.

Understanding Viscosity:

• Viscosity measures an oil’s resistance to flow (shear stress) under specific conditions. It reflects how well the oil stays put when subjected to mechanical forces.
• Imagine a water-skier cutting through water. Water has a viscosity of 1 centistoke (cSt), which is at the lower end of the viscosity scale. If the skier were skiing on a lake of SAE 90/ISO 220 gear oil, the spray generated would be considerably less because the oil resists the ski’s force more effectively.

Absolute Viscosity:

• Absolute viscosity (also called dynamic viscosity) is a measure of how a fluid behaves under pressure. It is expressed in centipoises (cP). Machine designers are interested in this property, especially for pressurized hydraulic lines.

ISO Viscosity Grade Numbers:

• Each ISO viscosity grade corresponds to a specific viscosity bracket. For example:
  • ISO VG 32 relates to a viscosity bracket of 28.8 to 35.2 mm²/s, with a midpoint of 32.0 mm²/s.
  • The ISO VG system covers a wide range of viscosities, allowing users to select the right lubricant for their equipment.

In summary, ISO Viscosity Grades provide a common language for lubrication professionals, ensuring clear communication and effective lubricant selection.

This classification defines 20 viscosity grades in the range of 2 to 3200 square millimeters per second (1 mm2/s = equals 1 cSt) at 40ºC (104ºF). For petroleum-based liquids, this covers approximately the range from kerosene to cylinder oils.

Each viscosity grade is designated by the nearest whole number to its midpoint kinematic viscosity in mm2/s at 40ºC (104ºF), and a range of +/- 10 percent of this value is permitted. The 20 viscosity grades with the limits appropriate to each are listed in Table 1.

The classification is based on the principle that the midpoint (nominal) kinematic viscosity of each grade should be approximately 50 percent greater than that of the preceding one. The division of each decade into six equal logarithmic steps provides such a system and permits a uniform progression from decade to decade. The logarithmic series has been rounded off for the sake of simplicity. Even so, the maximum deviation for the midpoint viscosities from the logarithmic series is 2.2 percent.

Table 2 pulls together some popular viscosity measurement methods into one table. If the practitioner is comfortable with one particular measure but would like to see the correlating viscosity range in another measure, all he must to do is place a straight horizontal line through his chosen viscosity type and see its correlation within the other types of measures.

While it is true that some viscosity grades will be left out of the mix as companies move toward adopting the ISO designation, it is not necessary that the users of those products have to move away from them. Further, there is no intention to offer quality definition of lubricants with this scale. That a product has an ISO VG number associated with it has no bearing on its performance characteristics.

The ISO designation has been under development since 1975. The most recent release in 1992 (ISO 3448) contains 20 gradients. This covers nearly every type of application that the lubricant practitioner can expect to encounter. The lubricant manufacturing community has accepted the recommended ISO gradients and has devoted appreciable effort and energy to conform to the new grading approach with old and new products.

It is unlikely that all of us who learned about the use of oil from our mentors or friends under the hood of a car will ever abandon the SAE grading system. We don’t have to. At least for automotive oils, we can expect to continue to see the 10- 20- 30- 40- 50- values used. It is likely, however, that in the industrial lubrication world there will be more ISO dependence in the future.