ระดับความหนืดของน้ำมัน — ISO VG

ISO VG (Viscosity Grade) is a classification system for industrial lubricants defined by ISO 3448. Each grade corresponds to a nominal kinematic viscosity at 40 °C in centistokes (cSt). The grade number equals the midpoint viscosity — for example, VG 46 means 46 cSt ± 10%.

There are 18 grades from VG 2 to VG 1500, each step being roughly 50% higher than the previous.

The optimal viscosity depends on the application and operating conditions:

• VG 10–22 — High-speed spindles, light loads
• VG 32–46 — Hydraulic systems (most common)
• VG 68–150 — Industrial gearboxes
• VG 220–460 — Heavy-duty enclosed gears
• VG 680–1500 — Very heavy/slow machinery

Always consult the equipment manufacturer's recommendations. Using too thin an oil leads to wear; too thick increases energy losses and heat.

ISO VG grades classify industrial lubricants by kinematic viscosity at 40 °C. SAE grades are used for automotive oils: SAE crankcase grades (e.g. 10W-30) and SAE gear grades (e.g. 75W-90) use different temperature references and scales.

The two systems do not directly correspond. Approximate cross-references exist — for example, ISO VG 46 ≈ SAE 20, ISO VG 150 ≈ SAE 40 — but these are only rough equivalents.

Oil viscosity decreases as temperature increases. ISO VG grades specify viscosity at 40 °C only. The rate of viscosity change with temperature is described by the Viscosity Index (VI).

A higher VI means less viscosity change with temperature. Typical mineral oils have VI 80–100; synthetic oils can reach VI 150+. For critical applications, always verify viscosity at the actual operating temperature.

cSt (centistokes) is the unit of kinematic viscosity, equal to mm²/s. It measures how quickly a fluid flows under gravity. The related unit cP (centipoise) measures dynamic viscosity.

Conversion: cP = cSt × density (density in g/cm³). For typical mineral oil at ~0.87 g/cm³, cP ≈ cSt × 0.87.