LUBRICANTS AND LUBRICATION
With the movement of two mating parts of the machine, heat is generated. If it is not controlled the temperature may rise resulting in total damage of the mating parts. Therefore a film of cooling medium with high viscosity is applied between the mating parts which is known as a 'lubricant'.
A 'lubricant' is a substance having an oily property available in the form of fluid, semi-fluid, or solid state. It is the lifeblood of the machine, keeping the vital parts in perfect condition and prolonging the life of the machine. It saves the machine and its parts from corrosion, wear and tear, and it minimizes friction.
Purposes of using lubricants:-
► Reduces friction.
► Prevents wear.
► Prevents adhesion.
► Aids in distributing the load.
► Cools the moving elements.
► Prevents corrosion.
► Improves machine efficiency.
PROPERTIES OF LUBRICANTS:-
Viscosity
It is the fluidity of an oil by which it can withstand high pressure or load without squeezing out from the bearing surface.
Oiliness
Oiliness refers to a combination of wettability, surface tension and slipperiness. (The capacity of the oil to leave an oily skin on the metal.)
Flashpoint
It is the temperature at which the vapour is given off from the oil (it decomposes under pressure soon).
Fire point
It is the temperature at which the oil catches fire and continues to be in flame.
Pour point
The temperature at which the lubricant is able to flow when poured.
Emulsification and de-emulsibility
Emulsification indicates the tendency of an oil to mix intimately with water to form a more or less stable emulsion. De-emulsibility indicates the readiness with which subsequent separation will occur.
METHODS OF APPLYING LUBRICANT:-
The following methods are used for efficient lubrication.
► Gravity feed method
► Force feed method
► Splash method
Gravity feed method
There are numerous ways of employing this principle, varying from the simple oil hole to the more elaborate wick and glass-sided drip feed lubricators in which the flow of the oil may be controlled and observed through the glass. A selection of these lubricators is shown in Fig1.
Force (Pressure) feed method
There are various systems of lubrication employing a pressure feed to the lubricant, and the most important of such systems may be classified roughly into the following.
► Continuous feed of oil under pressure to each bearing concerned. In this method an oil pump driven by the machine delivers oil to the bearings and back to a sump from which it is drawn by the pump.
► Pressure feed by hand pump in which a charge of oil is delivered to each bearing at intervals (once or twice a day), by the machine operator.
► OH or grease gun method. The oil hole leading to each bearing is fitted with a nipple and by pressing the nose of the gun against this the lubricant is forced into the bearing. (Figs b, c, d & e)
Splash method
In this method the shaft, or something attached to it, actually dips into the oil and a stream of lubricant is continually splashed round the parts requiring lubrication. This method is
employed for the gears and bearings inside all gear drives, the lower parts of the gears actually
dipping in the oil. (Figs a, b and c)
A common method of employing splash lubrication is known as 'ring oiling."
CLASSIFICATION OF LUBRICANTS
Lubricants are classified in many ways. According to their state, lubricants are classified as:
► Solid lubricants
► Semi-solid or semi-liquid lubricants
► Liquid lubricants.
Solid lubricants
These are useful in reducing friction where an oil film cannot be maintained because of pressure and temperature. Graphite, molybdenum disulphide, talc, wax, soapstone, mica and trench chalk are solid lubricants.
Semi-liquid or semi-solid lubricants
Greases are semi-liquid lubricants of higher viscosity than oil. Greases are employed where slow speed of heavy pressure exists. Another type of application is for high temperature components, which would not; retain liquid lubricants.
Liquid lubricants
According to the nature of their origin, liquid lubricants are classified into:
► Mineral oil
► Animal oil
► Synthetic oil.
According to the product line of Indian Oil Corporation the lubricants are classified as:
- Automotive lubricating oils
- Automotive special oils
- Rail-road oils
- Industrial lubricating oils
- Metal working oils
- Industrial special oils
- Industrial greases
- Mineral oils.
For industrial purposes the commonly used lubricants for machine tools are:
► Turbine oils
► Circulating and hydraulic oils (R & 0 Type)
► Circulating and hydraulic oils (anti-wear type)
► Circulating oil (anti-wear type)
► Special purpose hydraulic oil (anti-wear type)
► Fire-resistant hydraulic fluid
► Spindle oil
► Machinery oils
► Textile oils
► Gear oils
► Straight mineral oils
► Morgan bearing oils
► Compressor oils.
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