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This article will give you a detailed guide on Lubrication.
Let’s start with the introduction first…
In engineering equipment, machines and mechanisms, wherever metal surfaces are in contact and a relative motion takes place, the frictional force comes into existence.
The frictional force developed in machines, which consists of many sliding and rotating components like bearings, valves, gears, pistons, etc may be so large that it may cause excessive wear and tear which may require the replacement of the components.
The frictional force consumes a large amount of power. Also, the heat produced during different motions may increase the temperature of various components, which may cause serious damage to the components.
These difficulties are overcome by interposing a suitable thin film of a lubricant between the surfaces of two metal parts in contact.
After the introduction, Let’s dive into the mainstream…
What is Lubrication?
An application of thin film between sliding or rotating or moving objects, to reduce wear and tear and increase overall efficiency, Called lubrication.
Here, The lubricants can be Liquid, solid, or gas.
The thin film of lubricant reduces the friction considerably by keeping the two metal surfaces apart from each other, thus reducing the wear and tear and the temperature developed.
Proper lubrication with timely addition or replacement plays a vital role in maintaining the machine’s accuracy and increasing its working life.
Principle of Lubrication-
The value of frictional resistance between two metallic surfaces due to direct contact is very high. This resistance causes a loss of energy when relative motion takes place between them.
In lubrication, a material having a low coefficient of friction i.e. lubricant is introduced between metallic surfaces.
Therefore the value of frictional resistance reduces and energy loss also reduces when relative motion takes place.
This is the principle of lubrication.
Objectives of Lubrication-
Lubrication fulfils one or more of the following objectives:
- To reduce friction between moving parts to the minimum value, thereby reducing power loss due to friction.
- To form an effective seal between the piston rings and cylinder walls and thus prevent the escape of gases from the cylinder, and avoid power loss.
- To provide a cooling effect by carrying away heat generated by friction.
- Absorb shocks between bearings, clutches, and other machine parts, thus reducing machine noise.
- To minimize wear of moving parts.
- To carry away dust and other foreign particles by washing the parts and acting as a cleaning agent.
Classification of Lubricants-
The types of Lubricants used for engines/machines may be classified on the basis of their physical conditions into four classes namely:
- Solid lubricants
- Semi-solid lubricants
- Liquid lubricants or lubricating oils
- Gas lubricants
Their brief description is given below-
1. Solid Lubricants-
The most commonly used solid lubricants are graphite, talc, mica, wax, etc.
Out of these, graphite is mostly used as a solid lubricant. All solid lubricants are mineral substances mined from the earth.
The solid lubricants are mixed with semi-solid lubricants and the mixture is applied to the bearings. Solid lubricants have the property of filling the depressions in the bearing surfaces and cannot be easily squeezed out from the bearings.
Solid lubricants are used on bearings and parts of machinery that operate at low pressure and low speeds.
2. Semi-Solid Lubricants-
These lubricants have a paste form of physical conditions and are commonly known as ‘grease‘. They do not flow at ordinary room temperature.
Most semi-solid lubricants are made from mineral oils and fatty oils. Grease is a semi-solid lubricant.
The thickness of these lubricants (greases) depends upon the percentage of soap used. Graphite or powder mica is sometimes added to greases.
Cheap greases contain clay, wax, resins, and other substances which thicken the grease but have no lubricating value.
Greases are used on heavy, slow-moving machinery, gear teeth, bearings of trucks, automobiles other parts of machinery where oil cannot reach successfully.
Application of Grease:
- Where pressure and slow, speeds are used.
- In bearings and gears that work at high temperatures.
- In situations where it is necessary to prevent the oil from being thrown out from the bearings, as in textile mills.
- It is desirable to lessen the attention that bearings require.
3. Liquid Lubricant or Lubricating Oil-
Liquid lubricants include all types of mineral oils, and animal and vegetable oils. Mineral oils are in general used for the lubrication of machines.
The mineral oils are obtained from the partial distillation of crude oil. They consist of a mixture of paraffin naphthalene and hydrocarbons etc.
They have a wide range of viscosity but are less viscous and more oily than fatty oils.
The mineral oils are free from acid and cheaper and have other desirable qualities which make them suitable for use as machine lubricants.
Small amounts of vegetable or animal oils are also added to the mineral oil to impart some special properties such as cleaning properties and increase oiliness. Such mixed oils are called blended oils or compound oils.
Vegetable and animal oils have a tendency to decompose in the presence of heat. Some of the best-known animal oils are lard oil and sperm oil.
Vegetable oils are olive oil, cotton seed oil, linseed oil, etc.
Classification of Liquid Lubricants-
Liquid lubricants popularly known as lubricating oils can be catagorised as follows:
- Vegetable oil
- Animal oil
- Mineral oil
- Blended oil
A. Vegetable Oil-
Some important vegetable oils are described below:
- Olive oil
- Rape seed oil
- Castor oil
- Hazel nut oil
- Palm oil
It is obtained from olive fruits by pressing the oil pulp. The colour of oil varies from colourless to golden yellow. Lower grades of oil are used for burning, lubrication and Soap Making.
Rape seed oil:
It is obtained from the seeds of the rape plant, which is a plant of the cabbage family. It is pale yellow in colour.
It is extracted from the seed of the castor oil plants. It is the heaviest natural oil. Its specific gravity is 0.96. It is colourless to pale green.
It is obtained by pressing the kernel of nuts of the hazel tree which belongs to oak family. It is a good lubricant.
It is extracted from palm fruit. It has a pleasant smell and taste. Its specific gravity is 0.91. It is a white or pale yellow lubricant.
B. Animal Oil-
Following are the important types of animal oils and fats, used as lubricants:
- Cattle foot oil
- Whale oil or sperm oil
- Lard oil
- Tallow oil
Cattle foot oil:
It is obtained by boiling with water the feet of the cattle. The oil rises to the surface. It is pale yellow in colour. It is a useful lubricant for delicate mechanisms such as sewing machines, clocks etc.
Whale oil or sperm oil:
It is derived from the sperm whale. It is thin and pale yellow in colour. It is useful for light machinery and spindles.
It is a fat derived from the pig. It is pure white. Lard oil is obtained from lard by hydraulic pressure. It is a pale yellow or colourless liquid. It is a good lubricating oil.
It is animal fat derived from cattle. It is used for soap making and also as a lubricant.
C. Mineral Oil-
The mineral oils used for lubrication are derived from petroleum, shale or coal tar.
These are the most widely used lubricants because they are cheap, available in abundance and quite stable under service conditions.
D. Blended Oil-
Mineral oils when mixed with animal oils or vegetable oils produce a large range of lubricating oils, called blended oils.
4. Gas Lubricants-
Mainly air, hydrogen, nitrogen gas, etc are used as gas lubricants.
Sometimes to find out the required characteristics, two or more lubricants of different types are mixed up and used.
Types of Lubrication Systems-
There are mainly seven types of lubrication systems or methods of lubrication, and they are-
- Wet Sump Lubrication System
- Dry Sump Lubrication System
- Gravity feed system
- Wick Lubrication system
- Ring Lubrication system.
- Splash Lubrication system.
- Pressure Lubrication systems.
The above systems are described below:
1. Wet Sump Lubrication System-
In this wet sump lubrication system, oil is pumped from the sump to different lubricating zones.
Lubricating oil stored in a sump goes to the main gallery through a strainer and filter under pressure.
Pipes carry oil from the main oil gallery to different lubricating zones. After lubrication oil falls itself into the sump and again circulated on the same path.
The oil pressure is measured by a pressure gauge fitted at the main gallery.
2. Dry Sump Lubrication System-
In this Dry sump lubrication system, lubricating oil is first sent to the main oil gallery by a pump to lubricate different parts, and then it is sent to a supply tank by another pump for cleaning.
After cleaning, the oil is again pumped by another pump.
First, oil is filled in the dry sump in sufficient quantity. This oil, from the sump, comes in the oil pump through the strainer.
The oil pump raises the pressure of oil and sends it to the supply tank through a filter.
At the exit point of the supply tank, there is an oil pump that raises the pressure of lubricant to the level that it passes through the oil coolant and reaches different parts of the engine.
The function of oil coolant is to cool the oil before sending it to the engine parts. The heat of the oil is carried away in coolant by circulating water or air.
The function of the pressure regulating valve is to reduce the pressure of oil which has been raised by the oil pump.
After lubricating the parts, the oil in its final condition (warm and filthy) comes back in a dry sump. The cycle of lubrication is repeated several times.
A dry sump lubricating system is generally used in large stationary engines and engines of ships.
3. Gravity Feed System-
This is a system in which oil is not supplied to bearings under direct pressure. An oil head feeds oil to the bearings due to gravity.
It is used for lubricating the main bearing parts of the engine, crank pins, crossheads, etc. For supplying oil in this system, drop feed oilers are used.
Drop feed oiler:
It consists of a glass cup with a metal bottom that has a drip hole at the centre.
The hole is closed by a needle valve which may be opened or closed by means of a snap lever. The rate of feed can be adjusted by means of a screw, which raises or lowers the needle and can be seen through a glass window.
Drop feed oilers are most widely used because of-
- Their large oil reservoir capacities, and
- Close control over oil flow
4. Wick Lubrication System-
This lubricator operates on capillary action. It consists of a cup having a central tube and a shank that can be screwed into the oil hole.
Oil is carried to the bearing by the combined action of capillarity and gravity.
Wick oil lubricators are used for the lubrication of guides, slides, spindles, and plain bearings in a wide variety of equipment.
5. Ring Lubrication System-
In the ring oiling system, The oil ring rests on the journal where a portion of the bearing shell is cut away.
The bearing housing forms an oil reservoir in which oil is maintained at a level sufficiently high to have the lower end of the rings submerged in an oil bath.
One or two rings rest on the top of the journal and their bottom dips in the oil. When the journal rotates, the oil ring rotates with the journal and carries oil from the bath to the top of the bearing.
The oil is then distributed to the bearings by the oil grooves. The surplus oil flows to the ends of the bearings and drops back into the oil reservoir.
6. Splash Lubrication System-
The splash lubrication system is generally used with vertical and horizontal high-speed engines of smaller power and a closed crank chamber.
The closed crank chamber contains oil at such a level that the crank pin dips into the oil at each revolution and the splash takes place.
The oil settles on the parts inside the chamber and chamber walls, oil wells, or pockets, cast on the inside of the casting.
They collect the oil and lead it through various channels to the parts requiring lubrication. Surplus oil returns to the oil chamber.
7. Pressure Lubrication System-
In this system lubricating oil is supplied to the various engine parts under pressure developed by a suitable pump.
The basic components of a pressure lubricating system are:
- Pressure regulator
Oil is drawn from the sump through a strainer by a gear oil pump. The strainer is usually a fine mesh screen that prevents foreign matter from entering the oil circulating system.
An oil pressure relief valve is provided to prevent the build-up of excessive oil pressure.
Most of the oil from the pump goes directly to the engine and a portion of the oil passes through a filter which removes the solid particles from the oil.
Oil is supplied to the crank bearings from where the oil flows to the connecting rod’s big end bearings through the diagonal holes.
From the big end bearings, some oil is led to the small end bearings. From small end bearings, some oil flows to the gudgeon pin bearings and pistons.
There are two types of lubrication mechanisms and they are-
- Fluid Lubrications
- Boundary Lubrication
1. Fluid Lubrication-
When two moving parts in contact are completely separated by a thick film of fluid lubricant and never come in contact with each other, then it is called fluid lubrication.
During the operation, metal surfaces remain in contact with a thick layer of lubricant in between them. Now the friction between these surfaces acts on two layers of lubricant.
The friction will be more if the lubricant is thick and less if the lubricant is thin.
2. Boundary Lubrication-
Metal surfaces sometimes come in direct contact with each other due to lack of lubricant or excessive wear of surfaces, then it is called boundary lubrication.
Functions of a Lubricant-
The main functions of a lubricant are:
- To minimise wear of parts in contact.
- To facilitate the movement of the parts in contact.
- To prevent loss of energy in the form of heat developed.
- To control corrosion.
- To absorb vibrations and reduce noise.
- To prevent leakage, past the piston in the I.C. engine cylinder.
- To sweep out metal particles, dust and carbon particles from the engine cylinder.
Selection of Lubricants-
To select a good lubricant, the following factors are to be considered:
- In actual working conditions, on the basis of required speed, temperature and pressure, the selected lubricant should provide an oil film between bearing surfaces. For this purpose lubricant of proper viscosity should be selected.
- Selected lubricant should have the adequate property of adhesion so that the capability to keep the bearing surface wet should maintain,
- It should have low volatility.
- In the presence of air, it should have minimum oxidation.
- It should have low emulsification properties in wet conditions such as steam engines etc.
- Lubricants should have a high flash point and fire point.
- It should have a low freezing point.
- It should not have corrosive properties.
- It should have an adequate property of surface tension and slipperiness.
Essential Qualities of a Good Lubricant-
The following are the essential qualities of a good lubricant:
The viscosity of a lubricant is the resistance offered by it to its motion. The resistance to motion increases as the viscosity increases.
This property of the fluid is termed viscosity. It is affected by temperature, the higher the temperature the lower will be the viscosity of the oil.
So the lubricant of proper viscosity should be selected on the basis of temperature and pressure prevailing at the place of work.
This property of the lubricant is essential for its proper flow between the surfaces of the bearing.
It is defined as the power of oil to maintain a continuous film between the metal parts when used as a lubricant Animal and vegetable oils have greater oiliness than mineral oils.
Oiliness is important in the lubrication of such parts where the incomplete film is formed as in bearings.
Flashpoint is the temperature at which an oil gives off sufficient vapour to flash for a few seconds on the application of a small flame. It is much lower than the ignition point.
The temperature at which the oil will bum continuously is called a fire point.
During oxidation, the oil takes oxygen from the atmosphere and becomes thick. So its flowing capacity is reduced. A good lubricant, should not oxidise at high temperatures.
The ideal lubricant should not have any tendency to start rusting or corroding and so it should be chemically neutral.
The lubricant should not react with water or form an emulsion with it.
The ability of a liquid to vaporize at a temperature is called its volatility at that temperature. It should not evaporate.
The lubricant should have high adhesiveness so that it may stick to the metal surface to prevent metal-to-metal contact
A lubricant should be chemically neutral and not react with bearing surfaces:
It is the temperature at which an oil ceases to flow when it is cooled.
The cloud point is the temperature at which the water, wax and some other substances in the oil begin to crystalise and separate out.
Essential Requirements of a Good Lubricating Oil-
A good lubricating oil must have:
- High boiling point
- A low freezing point
- Adequate viscosity
- Heat stability
- High oxidation resistance
- Non-corrosive properties
- Stability to decomposition at the operating temperature
Selection of Lubricants for a Particular Job-
The selection of lubricants for a particular job is described below:
Lubricants for internal combustion engines:
In an I.C. engine, the lubricant is to undergo high temperature. Therefore, the lubricant should possess high viscosity index and high thermal stability.
Petroleum oils containing additives are used as lubricants for I.C. engines.
Lubricants for steam engine cylinders:
Lubrication of the steam engine cylinder is done by injecting the lubricating oil dropwire into the steam line. Paraffin base mineral oil to which vegetable oils have been added is used for lubricating steam engine cylinders.
Lubricants for steam turbines:
In steam turbines lubricants have to undergo high temperature and oxidising conditions. So lubricants used are compounded oils containing additives which provide high oxidation stability, and corrosion resistance characteristics.
Lubricants for cutting tools:
In machining processes like cutting, turning and grinding of metals, cutting fluids are used, commonly known as cutting oils.
The cutting oils are essentially mineral oils of low viscosity containing additives like fatty oils, sulphurized fatty oils and chlorinated compounds.
Lubricants for gears:
Gears are subjected to extreme pressures. So thick mineral lubricating oils containing additives like soaps, chlorine, sulphur or phosphorus compounds are employed.
This is it for Lubrication. I hope, you enjoyed being here and got your desired information from this massive guide.
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Abhishek Tiwary is a blogger by passion and a Quality Engineer by profession. He completed his B.Tech degree in the year 2017. Now working in a reputed firm. He loves to share his knowledge with others.