Lubricant Viscosity Explained

A lubricant’s viscosity and how it changes under different temperatures and operating conditions is one of the most important properties that determines lubricant performance and protection.

Viscosity can be viewed in two ways:

• Kinematic viscosity
• Dynamic (or absolute) viscosity

Pouring_oilKinematic viscosity is defined by the lubricant’s resistance to flow and shear due to gravity. To illustrate, imagine pouring two containers, one filled with water and the other with honey. Each fluid’s Kinematic viscosity governs the rate at which it flows. Since the Kinematic viscosity of water is lower, it flows faster. Kinematic viscosity, measured using ASTM D445 methodology, determines an SAE oil’s high-temperature viscosity grade (the “30” in 5W-30).

Dynamic viscosity, measured by the Cold Crank Simulator (CCS) test (ASTM D5293), is defined as the lubricant’s resistance to flow as indicated by its measured resistance, best thought of as the amount of energy required to move an object, such as a metal rod, through the fluid. It takes less energy to stir water compared to honey because the Dynamic viscosity of water is lower. Dynamic viscosity determines an oil’s low-temperature grade (the “5W” in 5W-30).

What does it mean to motorists? The lubricant’s viscosity at 40ºC and 100ºC is used to calculate its viscosity index (VI) – a measure of how much the viscosity of the fluid changes due to temperature. As we said, viscosity change due to temperatures and operating conditions influences performance. A lubricant that undergoes little viscosity change will generally perform better. A high VI indicates the fluid undergoes little viscosity change due to temperature fluctuations, while a low VI indicates a relatively large viscosity change.

Synthetic fluids generally have much higher viscosity index numbers compared to conventional fluids, meaning they provide improved protection to critical components over a wide range of temperatures. VI is normally reported on the oil’s product data sheet, like this one.

AMSOIL synthetic lubricants boast high VI numbers, meaning they’re more stable than competitive lubricants, so you can count on them to deliver outstanding protection.

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