Lubricants perform the following key functions.
Viscosity Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. Viscosity coefficients Viscosity coefficients can be defined in two ways: Dynamic viscosity, also absolute viscosity, the more usual one. The SI physical unit of dynamic viscosity is the pascal-second (Pa·s), (equivalent to N·s/m2, or kg/ms). If a fluid with a viscosity of one Pa·s is placed between two plates, and one plate is pushed sideways with a shear stress of one pascal, it moves a distance equal to the thickness of the layer between the plates in one second. The cgs physical unit for dynamic viscosity is the poise(P), named after Jean Louis Marie Poiseuille. It is more commonly expressed, particularly in ASTM standards, as centipoise (cP). Water at 20 °C has a viscosity of 1.0020 cP or 0.001002 kilogram/meter second. 1 P = 1 g·cm−1·s−1. The relation to the SI unit is 1 P = 0.1 Pa·s, 1 cP = 1 mPa·s = 0.001 Pa·s. Kinematic viscosity is the dynamic viscosity divided by the density. The cgs physical unit for kinematic viscosity is the stokes (St), named after George Gabriel Stokes. It is sometimes expressed in terms of centistokes (cSt or ctsk). In U.S. usage, stoke is sometimes used as the singular form. 1 St = 1 cm2·s−1 = 10−4 m2·s−1. 1 cSt = 1 mm2·s−1 = 10−6m2·s−1. Water at 20 °C has a kinematic viscosity of about 1 cSt. Viscosity Grades Viscosity Classification System for Industrial Fluid Lubricants International Organization for Standardization (ISO)