Lubricant viscosity and ISO viscosity grade
author: Leila
2024-08-27
It is said that in the selection of lubricants, viscosity is the primary consideration, and appropriate viscosity is the premise of lubricant selection. So, what is the viscosity of lubricants? How is the viscosity of lubricants graded? What is the graded standard?
1. What is the viscosity of lubricants?
The viscosity of lubricants characterizes the force inside the lubricant fluid that prevents the flow of oil. The greater the viscosity, the greater the resistance of the oil itself to prevent flow. The corresponding daily concept is "thicker", and vice versa. Simply put, lubricants are squeezed between machines. If the viscosity of the lubricant is small, it is easy to be squeezed out, and if the viscosity is large, it is relatively difficult to squeeze out.
The liquid with the lowest viscosity is water, which is only 1 centistoke. We have all seen water skiing. When athletes stand on water skis and slide across the water, we can see a lot of water splashing. In another scene, if the same sport is carried out on an "oil lake" filled with gear oil with a viscosity of 220, the splashing will not be so severe.
It is not difficult to imagine from here that if the mechanical load is high, then we need a lubricant with a higher viscosity. If the machine speed is high, then a low-viscosity lubricant is needed to reduce the viscous resistance of the lubricant to the machine's forward movement when the machine rolls up, and can quickly flow back to its original position when the machine leaves (imagine water skiing. If a high-speed water skier slides on the oil, he will be stuck and cannot slide as freely as on the water).
2. How to classify the viscosity of lubricants:
Speaking of the classification of lubricant viscosity, various classification standards are dazzling: SAE viscosity (Society of Automotive Engineers), AGMA viscosity (American Gear Manufacturers Association), SUS viscosity (Saybolt Universal Seconds), centistokes (unit of cSt kinematic viscosity), and absolute viscosity... In order to prevent confusion, we can divide lubricants into two categories: industrial lubricants and automotive lubricants. Automotive lubricants (engine oil, automotive gear oil) commonly use SAE viscosity, and industrial lubricants can look at ISO viscosity.
3. What is ISO viscosity?
ISO stands for International Organization for Standardization, which means "International Organization for Standardization" in Chinese. Industrial lubricants are used in various machines, such as gear oil used in industrial gears, hydraulic oil of hydraulic equipment, turbine oil, compressor oil, etc. Given the diversity and complexity of industrial machines, the viscosity classification is dazzling. Since viscosity is so important, can there be a unified classification standard so that people can see the viscosity information at a glance? Therefore, ISO viscosity was created.
In 1975, the ISO organization joined hands with the American Society for Testing and Materials (ASTM), the American Friction and Lubrication Association (STLE), the British Standards Institute (BSI) and the German Standardization Association (DIN) to determine a universal viscosity grading method to prevent the complexity and confusion caused by various classification methods. The "ISO viscosity grade" is the viscosity grade certified by the International Organization for Standardization. The ISO viscosity is marked as ISO VG, where VG stands for Viscosity Grade. Such as ISO VG 220, ISO VG 320, etc.
4. ISO viscosity grade classification standard:
Since the viscosity of lubricating oil changes with temperature: when the temperature rises, the viscosity of lubricating oil decreases (becomes thinner), and when the temperature drops, the viscosity increases (becomes thicker). Therefore, when formulating ISO viscosity grades, various factors of industrial application are fully considered, and the kinematic viscosity measured at 40°C is used as the benchmark of ISO viscosity.
ISO viscosity grades have been developing. The latest table was released in 1992, divided into 20 viscosity grades, with the lowest grade being 2 and the highest being 3200. The higher the viscosity grade, the greater the viscosity.
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