Requirements and characteristics of lubricants

Requirements and characteristics of lubricants

The main requirements for lubricants are that they are able 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 behaviour 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
  • Viscosity index
  • Pour point
  • Flash point

Viscosity describes the flow behaviour of a fluid. The viscosity of lubricating oils diminishes as temperature rises and consequently is measured at a given temperature (e.g. 40°C).
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 index
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:

  • better engine start up at low temperatures (lower internal friction)
  • a higher stability of the lubricating film at high temperatures

Viscosimetric classifications
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.

Pour Point
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.

Flash point
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.

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Last updated on 30/08/12