Properties of Lubricant

The properties required in a lubricant are decided by the particular purposes for which it is to be used. However, every lubricant must have following properties.
  • It should wet the surfaces, carry away heat and protect surfaces against corrosion.
  • It should possess suitable viscosity.
  • It should be chemically stable during use, having no tendency to deposit gum, varnish, sludge or other materials, which may interfere with its correct performance.
Viscosity. The viscosity is a measure of resistance to flow due to the internal friction of fluid. The viscosity of fluid is usually specified as the time in seconds that is takes for a given amount of the fluid to flow by gravity through a standard sized orifice at a given temperature. So we can say if a fluid flows freely is said to have a low viscosity and one which is sluggish, posses a high viscosity. The instrument used to measure the viscosity is called viscosity meter. The viscosity is inversely proportional to temperature i.e. it decreases as the temperature rises and increases as temperature falls.

S.A.E Numbers. The Society of Automotive Engineers has recommended the S.A.E viscosity number for lubricating oils. The range of viscosity within which it falls at the given temperature determines the viscosity number of any lubricating oil.
  • 5W, 10W and 20W SAE number lubricating oils are for winter use and the viscosity tests are made at 0o F.
  • 20W, 30W, 40W and 50W SAE number lubricating oils are for summer use the viscosity tests are made at 210o F.
Viscosity Index. Oil, which is very thick at low temperature and becomes very thin at high temperature is said to have a low viscosity index. If it changes relatively little in viscosity with temperature change, it is said to have a high viscosity index, which is desirable for good lubrication.

Additives Small quantities of substances are added to lubricating oils to give the oils some desirable properties, which it would not otherwise posses, are known as additives. These Additives can be divided into three classes.
  • Extreme Pressure Additives. It contains sulphur, phosphorous chlorine and other chemicals, the oil being known as extreme pressure oil. Different types of extreme pressure oil should not be mixed since harmful chemical reactions may occur between the different additives.
  • Anti Corrosive Additives. Added to protect some particular part of the engine from corrosion e.g. bearing metals. These should not be confused with inhibitors, which slow down the formation of oxidation products.
  • Detergent Additives. Added to reduce formation of lacquer and other carbon deposits on pistons and rings and to keep in soluble impurities in suspension. The oils are usually known as HD (Heavy Duty) oil.
Lubrication System of an Engine The purpose of the lubrication system is to ensure that a film of oil is always present between moving parts, thus preventing metal to metal contact with each other. Lubricating oil for engine may be regarded as the blood in human body. The film of oil is all that stands between the various moving parts.

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