Types of Viscosity in Lubricating Oils and Their Equivalences
Viscosity is one of the most important factors in selecting lubricating oils, as it directly influences the performance, protection, and efficiency of equipment. Understanding the different types of viscosity and how they relate across various standards allows for informed decisions to optimize the operation of engines, hydraulic systems, and industrial gears.
In this article, we will explore the different ways to measure viscosity, their classifications, and the equivalences between international standards to facilitate the choice of the appropriate lubricant.
Viscosity is the resistance of a fluid to flow. In practical terms, it indicates how thick or thin an oil is. A lubricant with high viscosity is thicker and flows slowly, while one with low viscosity is thinner and flows more easily.
This parameter is essential because it affects the lubricating film that protects the metal surfaces in contact. If the viscosity is too low, the lubricating film can be lost, causing wear and excessive friction. If it is too high, it increases resistance to movement, generating energy losses and overheating.
There are different ways to measure the viscosity of an oil, depending on the standard used and the application of the lubricant:
It is the most commonly used in the industry and is measured in centistokes (cSt) at 40°C and 100°C, according to the ISO 3104 standard. It determines the oil's ability to flow under the influence of gravity.
It is measured in millipascal-seconds (mPa·s) and represents the internal resistance of the oil to flow when an external force is applied. It is crucial in hydraulic lubrication and in high-demand engines.
It measures the variation of the oil's viscosity with temperature. A high viscosity index indicates that the oil maintains its properties better at extreme temperatures, which is vital in applications with wide thermal fluctuations.
It determines the oil's ability to flow at low temperatures, crucial in applications where cold starting is a determining factor.
To facilitate the selection of the appropriate lubricant, different organizations have developed viscosity classification standards. Below are the most commonly used:
The SAE J300 is the most used classification for engine oils. It is divided into monograde and multigrade, where multigrade oils have a viscosity range suitable for both cold and hot temperatures.
Example: SAE 5W-30
- "5W": Indicates the viscosity in cold. The lower the number, the better the flow at low temperatures.
- "30": Indicates the viscosity at 100°C, reflecting the behavior in normal operation.
SAE Viscosity Table for Engines:
| SAE Grade | Viscosity at 100°C (cSt) | Viscosity at -30°C or -35°C (cP) |
|---|---|---|
| 0W-20 | 5.6 – 9.3 | ≤ 6200 |
| 5W-30 | 9.3 – 12.5 | ≤ 6600 |
| 10W-40 | 12.5 – 16.3 | ≤ 7000 |
| 15W-50 | 16.3 – 21.9 | ≤ 7000 |
Used in industrial systems, it defines oils according to their kinematic viscosity at 40°C.
ISO VG Viscosity Table:
| ISO VG | Viscosity at 40°C (cSt) |
|---|---|
| 32 | 28.8 – 35.2 |
| 46 | 41.4 – 50.6 |
| 68 | 61.2 – 74.8 |
| 100 | 90.0 – 110.0 |
| 150 | 135.0 – 165.0 |
| 220 | 198.0 – 242.0 |
Example of application:
- ISO VG 32 and 46: In high-precision hydraulic systems.
- ISO VG 100 and 150: In high-torque industrial gears and piston engines.
- ISO VG 220 and above: For reducers and heavy machinery.
Specialized in industrial gear oils, it considers viscosity and the load supported.
Example of Equivalences:
| AGMA Grade | ISO VG | SAE Gear Oil |
|---|---|---|
| 2 | 68 | 75W |
| 3 | 100 | 80W |
| 4 | 150 | 90W |
| 5 | 220 | 140W |
| 6 | 320 | 250W |
To facilitate conversion between classifications, references such as the following can be used:
| SAE (Motor) | ISO VG | SAE (Gears) |
|---|---|---|
| 5W-20 | 32 | 75W |
| 10W-30 | 46 | 80W |
| 15W-40 | 68 | 85W |
| 20W-50 | 100 | 90W |
Viscosity is the key factor in selecting a lubricant, directly impacting performance, equipment lifespan, and energy efficiency in mechanical and industrial applications.
Understanding the differences between SAE, ISO VG, and AGMA classifications allows for the correct choice of lubricant, avoiding premature failures and maximizing the protection of engines, compressors, gears, and hydraulic systems.
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