Basic Concepts of Lubricants

Basic Concepts of Lubricants

Density

Density means the volume weight of a substance. In oils, it is usually indicated in the temperature of +15°C or +20°C, in units kg/m3. Lubricant densities range between about 700 and 950 kg/m3, depending on the quality, viscosity and additive content of the lubricant.

Viscosity

The thicker a fluid is, the greater its viscosity. Nowadays, lubricant viscosity is usually expressed with the units centistoke (mm2/s) and centipoise (mPas).

Centistoke (cSt)

Centistoke is a unit of kinematic viscosity, based on the amount of force required to beat the internal friction of fluid.

Centipoise (cP)

Centipoise is a unit of dynamic viscosity, often used for expressing the internal friction of oil in low temperatures. The connection of cSt and cP is cP = cSt x fluid density.

The temperature must always be given when expressing viscosity with any unit. All oils become much thinner as the temperature rises. A typical viscosity of motor oil SAE 10W at a temperature of -20 °C may be 2,000 cP, but if it heats up to a temperature of +100 °C the viscosity is only 5.2 cSt.

Kinematic viscosity is measured by the pictured Ubbelohde viscometer. It measures the time the oil requires to flow from point m1 to point m2.

Viscosity index

Viscosity index (V.I.) describes the fluid’s tendency to thin as the temperature rises. The stronger the fluid thinning the smaller the viscosity index.

The V.I. of single-grade motor oils is about 95-110, and that of multi-grade motor oils even higher than 200.

Flash point

The flash point expresses the flammability of a fluid. Flash point is the temperature at which, measured from the fluid with a certain method, flammable gases are vaporized so much that they flame up when ignited with a naked flame, but the fluid does not carry on burning.

Ignition point

Ignition point is the temperature at which the gases vaporized from the fluid when heated in an open cup burn for at least five seconds when ignited with a...