Requirements and characteristics of lubricants
The main requirement for lubricants is the ability to:
Keep surfaces separate under all loads, temperatures and speeds, thus minimising friction and wear;
Act as a cooling fluid removing the heat produced by friction or from external sources;
Remain adequately stable in order to guarantee constant behavior over the forecasted useful life;
Protect surfaces from the attack of aggressive products formed during operation;
Show cleaning capability and dirt holding capacity in order to remove residue and debris that may form during operation.
The properties of lubricants:
The main properties of lubricants, which are usually indicated in the technical characteristics of the product, are:
- Viscosity index
- Pour point
- Flash point
Viscosity describes the flow behavior of a fluid. The viscosity of lubricating oils diminishes as temperature rises and consequently is measured at a given temperature (e.g. 40ï¿½C).
The viscosity of a lubricant determines the thickness of the layer of oil between metallic surfaces in reciprocal movement. The most widely used unit of measurement of viscosity is the centistoke (cSt).
Viscosity indexbetter engine start up at low temperatures (lower internal friction);a higher stability of the lubricating film at high temperatures.
The viscosity index is a characteristic used to indicate variations in the viscosity of lubricating oils with changes in temperature. The higher the level of the viscosity index, the lower the variation in viscosity at temperature changes.
Consequently, if two lubricants with the same viscosity are considered at a temperature of 40 C, the one with the higher viscosity index will guarantee:
There are a number of viscosimetric classification systems that indicate, usually with a number, a more or less limited viscosity range.
The aim is to provide, along with the viscosity index, a rapid indication of the most appropriate choice of lubricant for a specific application.
ISO VG degrees are widely used to classify industrial oils. Each degree identifies a kinematic viscosity gap measured at 40 C.
SAE degrees are used in the field of engine oils and gear oils.
The pour point refers to the minimum temperature at which a lubricant continues to flow. Below the pour point, the oil tends to thicken and to cease to flow freely.
The flash point is the minimum temperature at which an oil-vapour-air-mixture becomes inflammable. It is determined by progressively heating the oil-vapour-air-mixture in a standard laboratory receptacle until the mixture ignites.