Properties of lubricants
The essential properties of lubricants are given below.
Viscosity: Viscosity of lubricant is the resistance offered by it to the deforming stress i.e., it is the resistance offered to the flow of the lubricant. Viscosity of the oil decreases with the increase in temperature. Viscosity of the lubricating oil should not go down below a certain specified value at the highest operating temperature of the bearings. If too thick oil is used, it will lead to power loss, higher operating temperature and excessive wear and tear of the parts. If the oil is too thin, it cannot lubricate properly and lead to rapid wear of moving parts.
Oiliness: It is the property of an oil to spread and attach itself firmly with the bearing surfaces. Oilness of the lubricating oil should be high for better lubrication.
Flash and fire point: Flash point og an oil is the minimum temperature at which it gives off enough vapour so that a momentary flame is obtained when a naked flame is brought near the oil surface. Fire point is the minimum temperature at which an oil continuously burns. Fire point is always greater than the flash point. The flash point of the lubricating oil should be higher than the operating temperature of the bearing.
Volatility: When the lubricating oil is exposed to a high temperature for a long time, it may evaporate. This property is known as volatility. The loss of lubricating oil is known as loss of evaporation. The lubricating oil should have low volatility at the operating temperature. If it has high volatility, oil consumption will be more.
Detergency: The lubricating oil should carry away small particles to keep the interior of the engine clean. This property of lubricating oil is known as delergency.
Demulsibility (water separation): The lubricating oil should not form an emulsion when brought in contact with water. The property of resisting emulsification is known as demulsibility. The emulsion leads to collection of dust, dirt etc., and will increase friction, wear and oxidation. Hence it should be avoided.
foaming: It is the condition in which minute air bubbles are held in the oil. This will reduce mass flow of oil and also increase oxidation. Hence the lubricating oil should be free from foaming trouble.
Corrosiveness: The lubricating oil should not attack the engine materials chemically. The oil should prevent corrosion and it should not contain sulphur.
In addition a lubricating oil should have a high film strength to withstand loads. It should be non-toxic and cheaply available.
Purposes of lubrication (or) Functions of lubrication
1. It reduces friction between moving parts.
2. It reduces wear and tear of the moving parts.
3. It minimizes power loss due to friction.
4. It provides cooling effect – During circulation, it carries heat from the hot moving parts and delivers it to the surroundings through crankcase.
5. It provides cushioning effect – It serves as a cushion against the shocks of the engine.
6. It provides cleaning action – Impurities such as carbon particles are dissolved during its circulation.
7. It provides a sealing action – It helps the piston rings to provide an effective seal against high pressure gases in the cylinder from leaking out.
8. It reduces noise.