Viscosity
Viscosity is a fluid's resistance to flow, arising from internal friction between adjacent layers moving at different velocities. Dynamic (absolute) viscosity is expressed in pascal-seconds or centipoise (cP); kinematic viscosity, which is dynamic viscosity divided by density, is expressed in mm²/s or centistokes (cSt). Liquid viscosity falls markedly as temperature rises, a behaviour quantified for lubricants by the viscosity index. Viscosity governs lubricant film thickness in bearings and gears, pump selection and performance derating, pipeline pressure drop and hydraulic system response. Industrial lubricants are graded by kinematic viscosity at 40 °C under ISO 3448 (ISO VG grades), and kinematic viscosity is measured by methods such as ASTM D445. High-viscosity liquids often favour positive-displacement pumps over centrifugal pumps.
Examples
- ISO VG 46 hydraulic oil
- ISO VG 220 industrial gear oil
- Heavy fuel oil heated before pumping