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Requirements and the properties for lubricants

REQUIREMENTS FOR 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
Fulfil detersive and dispersive functions 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

Viscosity is the resistance of a fluid to the reciprocal slippage of its particles. The viscosity of lubricating oils diminishes as temperature rises and consequently is measure 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, expressed on a conventional scale, used by the oil industry to indicate variations in the viscosity of lubricating oils with changes in temperature.
In other words, the viscosity index measures the variations in the viscosity with changes in temperature; the higher the level of the viscosity index, the lower the variation in viscosity at temperature.
Consequently, if two lubricant with a viscosity of 40°C are considered, the one with the higher viscosity index will guarantee:

  • better engine start up a low temperatures (lower internal friction)
  • a higher level of surface separation (the density of the layer of lubricant) 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.
AGMA numbers, defined by the American Gear Manufacturers Association, are frequently used to classify oils used for industrial gears and are also based on a kinematic viscosity gap measured at 40°C.
In both cases, a higher number refers to a higher level of viscosity.

SAE degrees are used in the field of traction oils.
SUS (Sayboldt Universal Seconds) viscosity, are used increasingly rarely; essentially for the classification of various possible combinations of base oils. The number indicates the seconds used by a given quantity of oil to flow through a measured gap at a given temperature (typically 40°C or 100°C or 100°F or 210°F)

 

Pour Point
The pour point refers to the minimum temperature at which a lubricant continues to flow when cooled. 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 vapours from lubricants mixed with air and progressively heated in a standard laboratory receptacle, becomes inflammable.
There is a closed flashpoint (Penskin Martens) and an open flashpoint (Cleveland).
In addition to defining the limits of use and the precautions to be taken for moving or storage it is also useful as an indication of possible contamination from fuels (which have lower flashpoints than lubricants).


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SAE degrees

SAE degrees

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Last updated on 29/03/11