Lubrication and Lubricants
The process by which friction between sliding surfaces is reduced is known as lubrication. Lubricants are substances that are applied between two moving and sliding surfaces in order to decrease friction between them.
The friction between metal to metal components in machines occurs as a result of moving surfaces, and the machine encounters resistance, slowing its progress. As a result of friction, a significant quantity of energy is released in the form of heat, reducing the overall efficiency of the machine.
Objective of Lubrication:
Primary Objective of Lubrication:
To reduce friction between moving parts so as to reduce
- Power loss
- Wear of the moving parts
Secondary Objective of Lubrication:
- To provide cooling effect
- To provide cushioning effect
- To provide cleaning action
- To provide sealing action
Purpose of Lubrication:
- To reduce friction
- To cool down moving parts
- To prevent corrosion (component wear from chemical reaction with air and the moist)
- To prevent contamination
Requirements of Lubricants:
- Physical stability
- Chemical stability
- Resistance against corrosion
- Pour point
- Flash point
- Resistance against extreme pressure
According to the following mechanism, the behaviour of lubrication may be explained.
(a) Thick-Film Lubrication (Fluid-Film or hydrodynamic lubrication)
(b) Thin Film Lubrication (also known as Boundary Lubrication) and
(c) Extreme Pressure Lubrication.
Because the moving/sliding surfaces are separated from one another by a thick layer of fluid, there is very little direct surface to surface contact and very little welding of junctions in this configuration. It fills in and conceals the imperfections of moving or sliding surfaces, forming a thick layer between them and preventing direct contact between the materials’ surfaces. As a result, there is less friction on the surface. The lubricant selected should have the lowest viscosity possible (in order to decrease the internal resistance between the particles of the lubricant) under operating circumstances, while also remaining in place and separating the surfaces as much as possible.
In the case of thick-film lubrication, hydrocarbon oils (mineral oils that are lower molecular weight hydrocarbons with about 12 to 50 carbon atoms) are regarded to be acceptable lubricants. Ordinary hydrocarbon lubricants are mixed with chosen long chain polymers in order to preserve the viscosity of the oil throughout the year, regardless of the temperature.
Thin film Lubrication
This kind of lubrication is recommended in situations when a continuous layer of lubricant is not possible to be maintained. In such situations, the clearance area between the moving/sliding surfaces is lubricated with a substance that may be adsorbed on both of the metallic surfaces by either physical or chemical forces, depending on the application. This adsorbed layer contributes to the separation of the metal surfaces from one another, at least up to the height of the peaks present on the surface of the metals.
In this type of lubrication, vegetable and animal oils and their soaps can be used because they can be physically adsorbed or chemically react with the metal surface to form a thin film of metallic soap that can act as a lubricant. Vegetable and animal oils and their soaps can be used because they can be either physically adsorbed or chemically react with the metal surface to form a thin film of metallic soap that can act as a lubricant.
Although these oils have a high oiliness, they have the drawback of degrading at high temperatures, which makes them unsuitable for use in cooking. Mineral oils, on the other hand, are thermally stable, and the addition of vegetable/animal oils to mineral oils may increase their oiliness, which is beneficial in certain applications. Other materials such as graphite and molybdenum di-sulfide are appropriate for thin film lubrication.
Extreme pressure Lubrication
When the moving/sliding surfaces are subjected to very high pressure and speed, a high local temperature is reached. Under these circumstances, liquid lubricants fail to adhere to the moving/sliding surfaces and may degrade, if not evaporate completely. Special additives are added to mineral oils in order to withstand the high pressures encountered in these applications. Extreme pressure additives are what they are referred to as. On metal surfaces, these chemicals help to create more robust coatings (that are capable of withstanding very high loads and high temperatures).
Organic compounds containing active radicals or groups such as chlorine (as in chlorinated esters), sulphur (as in sulphurized oils), or phosphorus (as in phosphoric acid) are important additions (as in tricresylphosphate). When these compounds come into contact with metallic surfaces, they react to produce metallic chlorides, sulfides, or phosphides, depending on the temperature.
Forms of Lubricant:
- Liquid Lubricants (Ex. Petroil for automotive engines)
- Semisolid Lubricants (Ex. Grease)
- Solid (Dry) Lubricants (Ex. Graphite Powder and Molybdenum Disulfide)
- Gas Lubricants (Ex. Pressurized Nitrogen gas)
Oil additives are chemical compounds that improve the lubricant performance of base oil. It is possible to find additives in lubricating oils in concentrations ranging from a few parts per million to several percentage points, and based on the function that these products must do, they may be classified as follows:
- Chemicals that modify the inherent properties of base oils (such as viscosity index modifiers and pour point improvers) are examples of additives.
- Substances that act as lubricants and protecting agents (antioxidants)
- Engine metal surfaces are protected by substances that impart new characteristics to the metal surfaces (detergents, dispersants, friction modifiers, anti-wear/Extreme Pressure additives, rust and corrosion inhibitors).
- Oxidation inhibitors
- Corrosion inhibitors
- Detergents and dispersants
- Anti-scuff additives
- Viscosity index improvers
- Anti-foaming additives
- Extreme pressure additives
- Pour point depressants
Classifications of Oil additives are:
- Extreme pressure additives
- Oxidation Inhibitors
- Detergent Dispersants
- Rust Inhibitors
- Foam Inhibitors
- Pour point Depressants
Types of Lubricants:
- Animal oils
- Vegetable oils
- Mineral Oils
- Synthetic Lubricants
- Solid Lubricants
a) Liquid lubricants:
These are lubricants that are liquid in nature. Animal oils, vegetable oils, petroleum oils, and synthetic lubricants are all included in this category.
- Animal oils include tallow oil, whale oil, and other similar products.
- Castor oil, palm oil, and other types of vegetable oils
- Petroleum fractions include petroleum oils and petroleum fractions.
- Synthetic lubricants include polyglycols, silicones, and other similar substances.
b) Semisolid lubricants (grease):
Semi-solid lubricants are produced by emulsifying oil and fat with thickening agents such as sodium soap, calcium soap, lithium soap, and aluminum soap at a high temperature, and then cooling the mixture.
Classification of Semi-Solid Lubricants
Using soda as a base:
Here, sodium soaps are used to thicken mineral or petroleum oil, which is a kind of thickening agent. Because of this, they are only minimally soluble in water. They are capable of withstanding temperatures of up to 175°C.
A solution of lithium soaps and petroleum oil is used in this application. They are water resistant and may be used in temperatures up to 15°C.
Based on calcium:
Specifically, calcium soaps are emulsifying with petroleum oil in this instance. They are also water resistant and may be used up to 80°C. When exposed to higher temperatures, soap and petroleum oil separate from one another.
c) Solid lubricants:
Solid lubricants are lubricants that are solid at room temperature. Solid lubricants such as graphite, molybdenum disulphide (MoS2), and boron nitride (BN)x are the most often utilized materials. They are used in high-temperature and high-load situations (pressure).
Graphite is a kind of mineral that is found in nature. The most common use for it is as a solid lubricant. It has a layer structure, and the layers are kept together by weak Vander Waals forces, which allow for the simple sliding of one layer onto another. It feels extremely soapy to the touch and is non-flammable. It is used in high-temperature conditions (about 450°C). They may be used either in powder form or combined with oil or water to create a paste.
Molybdenum disulphide (MoS2):
Molybdenum disulphide (MoS2) is a chemical compound that occurs naturally in molybdenum. It is a sandwich-like structure in which a hexagonal layer of molybdenum (Mo) atom is sandwiched between two hexagonal layers of sulfur, creating a molybdenum-sulfur sandwich. Vander Waals forces, which are weak in comparison to graphite, hold each layer together. It is stable up to 400°C. It differs from graphite in that it is utilized under high vacuum, while graphite is combined with water or oil (as opposed to pure graphite). It clings even more firmly to the metal or other surface than it did before.
Viscosity ratings of Lubricants:
Lubricant grade standards are Society of Automotive Engineers (SAE), International Standards Organization (ISO), American Gear Manufacturers Association (AGMA)
- SAE ratings: 0W, 5W, 10W, 20W, 25W, 10, 15, 20, 30, 40, 50, 60, 80, 100, 150, 200, 300
- API ratings:
- Petrol Engines – SA, SB, SC,…… to SM
- Diesel Engines – CA, CB, CC,…… to CH
- Gear Oil – GL-1, GL-2, GL-3, GL-4, GL-5, MT-1
Methods of Applying Lubricants:
There are Six main methods of applying lubricants:
- Manual Methods – such as manual grease gun
- Drip Methods – use gravity to supply oil drops
- Automatic Grease Lubricators – A reservoir is used to supply grease. The grease is put under pressure by the action of a spring to supply grease at a pre-calculated amount
- Bath method – such as in lubricating chains
- Splash and submersion methods – such as in gearboxes, where splashing is performed by the rotating gears
- Pressurized lubrication systems – such as in automotive engines
Oil Sump Lubrication
It is the most often utilized type of lubricating, particularly for low and medium speed activities. When placing the oil surface, it is preferable to be able to check the position of the oil surface by utilizing the oil gauge rather than guessing where it should be put on the rolling element.
Drip Feed Lubrication
In this technique, oil supply is regulated by changing the amount of oil drop, and it is often used for tiny bearings that run at a rapid rate.
In order to feed oil to bearings, this technique makes use of a gear or a circular flow ring. A common use for this material is in automobile transmissions or gearboxes.
It is commonly employed when it is required to cool the bearing components of a machine that rotates at a fast rate or in an environment with a high ambient temperature. Oil is supplied into the system via the feed pipe and collected through the recovery pipe, which is then cooled and re-fed. To prevent back pressure from developing in the oil within a bearing, the recovery pipe’s diameter should be greater than the feed pipe’s diameter.
In high-speed revolution bearings, it is extensively employed, and oil is jet-sprayed into the inside of the bearing via one or more nozzles under continuous pressure into the bearing’s interior. According to general rule, the jet stream speed must exceed 1/5 of the circumferential speed of the inner ring outer surface because the air wall produced by surrounding air rotating with the bearing has the tendency to decrease the jet stream in general. Assuming that the total amount of lubricant remains constant, the larger the number of nozzles, the smoother and more significant the cooling effect becomes.
Lubrication by spray
Spray lubrication is a technique of lubricating a bearing that vaporizes the lubricant by forcing air into the bearing before spraying it into it. It has the following advantages.
– Because of the tiny amount of lubrication needed, its whirling resistance decreases, making it ideal for high-speed revolution bearings with low whirling resistance.
– Because it reduces the amount of lubricant that is released, it may also reduce the amount of pollution that is generated by the equipment.
– Because new lubrication is continuously supplied into the bearing, the bearing’s life may be prolonged. Thus, it is extensively used in a variety of machining applications.
Testing of Lubricants:
- Viscosity test
- Flash point and fire point test
- Loss due to evaporation
- Cold test or Pour point test
- Specific gravity test
When lubricating oil reaches its cloud point, the temperature at which it begins to look foggy is referred to as cloud point.
The lowest temperature at which lubricating oil becomes semi-solid and stops to flow is referred to as the pour point of the oil. It shows the appropriateness of lubricants when they are utilized in a cold temperature. Low pour point is an important characteristic of a good lubricant.
The flash point of a volatile substance is the lowest temperature at which vapors of the material may spontaneously ignite for a brief period of time when an ignition source is placed close to the material. The flash point of the lubricating oil should be a reasonable amount higher than its operating temperature.
The fire point of a fuel is defined as the lowest temperature at which the vapour of that fuel will continue to burn for at least 5 seconds when an ignition source is placed close to the fuel. The difference between fire point and flash point is about 10°C.
Flow resistance is determined by the viscosity of a fluid, which is a physical characteristic of the fluid. It serves as an indication of the lubricating oil’s capacity to flow. The lower the viscosity, the greater the ability of the fluid to flow. Increases in temperature result in a reduction in viscosity of the lubricating oil, whereas increases in pressure result in an increase in viscosity of the lubricating oil. For the sake of simplicity, we may state that excellent lubricating oil is one whose viscosity does not vary with temperature.
The viscosity index of a liquid is defined as the change in viscosity of a liquid as a function of temperature. A high viscosity index indicates a relatively small change in viscosity as a function of temperature, while a low viscosity index indicates a relatively significant change in viscosity as a function of temperature
- Acid value test
- Saponification value test
- Insoluble residue test
- Moisture and Emulsification test
Effect of Engine conditions or Lubricating oil:
- Sludge formation
- Lacquer formation
- Oil dilution
Sources of Oil consumption in the engine:
- Loss through leakage
- Loss through crankcase ventilation
- Loss on account of wear of engine parts
- Loss due to excessive vehicle speed
Main engine parts requiring lubrication:
- Main bearings
- Big end bearings
- Gudgeon pin bearings
- Piston rings and cylinder walls
- Timing gears
- Camshaft and camshaft bearings
- Valve mechanism
- Electrical equipment like distributor drive, generator, starting motor etc
Systems of Engine Lubrication:
- Petroil system
- Splash system
- Pressure system
- Dry sump system
Components of pressure system of lubrication:
- Oil strainer
- Oil pump
- Oil filter
- Oil level indicator
- Oil Pressure gauge
- Oil pressure warning light
Types of Oil pumps:
- Gear pump
- Crescent type gear pump
- Rotor pump
- Plunger pump
- Vane pump
Oil Filtering systems:
- By-pass system
- Full flow system
Types of Oil filters:
- Cartridge type
- Edge type or Stack type
- Centrifugal type
Lubrication system service:
- Checking the oil level
- Oil change
- Checking the oil pump
- Checking the Oil filter
Lubrication system troubles:
- Low oil pressure
- High oil pressure
- Excessive oil consumption
Lubricants are used in a variety of applications
1) Lubricants are mainly used to decrease friction between two surfaces that move relative to one another.
ii) Anti-rust and anti-corrosion agents
iii) It is used in the soap and paint manufacturing sectors.
iv) Liquid lubricants are utilized in the production of medications.
v) Lubricants are also utilized as cutting fluids in the metalworking industries, such as cutting, grinding, and drilling.
vi) Anti-wear, antioxidant, and anti-foaming agents are some of the functions of these compounds.