Unit – 3
Lubricants
Lubricants are substances that are used to reduce the friction and wear of the surfaces of the bodies in relative motion. Lubricants can be in solid liquid and gaseous form, depending on its nature, they are used to eliminate heat, wear debris and supply additives, transmit power, protect and seal
Lubrication refers to the control of friction and wear by introducing a friction-reducing film between moving surfaces that are in contact, lubricant used for such purposes can be a fluid, solid, or plastic substance. Oil and grease are common substances used for lubricating a surface, oils actually help in lubrication while grease acts as a thickening agent to obtain consistency.
Classification of Lubricants
Liquid Lubricants
The liquid lubricants are primarily used to support high speed friction and heat dissipation.
- Free-flowing of constant volume
- Amorphous and difficult to compress
- Easily conformable to shape of containers
Solids
Low speed, High load application regime
- They protect the surface against wear and corrosion damages
- Deformable of constant volume
- The solids are not conformed to the shape of the container
- It Resistant to changes in size and shape.
- Molybdenum disulphide and Graphite are common lubricants, the others include tungsten disulphite, boron nitride and polytetrafluorethylene.
Semi-fluid lubricants (greases)
They are mainly greases that are produced, by emulsification of oils, fats with metallic soap and water at 400-600oF, Vaseline is a typical oil base grease. The properties of grease are determined by the types of oil, type of soap, and additives.
Types of oils: mineral, synthetic, vegetable and animal fat.
Types of soap: lithium, sodium, calcium etc
Additives: antioxidation, corrosion protection and pressure.
Viscosity is a measure of the internal friction of a fluid. Viscosity is the most important physical property in fluids with respect to lubrication. The viscosity of a lubricant varies with pressure temperature and also sometimes by the rate at which it is sheared. It is influenced by several factors such as contamination with water, particles or other lubricants
The Viscosity index (VI) is a unit less arbitrary measurement of a fluids change in viscosity relative to temperature change, it is mainly used to study the viscosity-temperature behaviour of the lubricating oils. The lower the VI, the more the viscosity is affected by changes in temperature. The higher the VI, the more stable the viscosity remains over temperature fluctuations.
The flash point indicates the combustion and flammable property of a liquid. Flash point is the lowest temperature at which a liquids (fuels) form vapour in the air, near its surface that will “flash,” or briefly ignite, on exposure to an open flame.
Fire point is the lowest temperature where the vapor of a liquid will initiate and sustain a combustion reaction. By definition, following ignition by an open flame the fuel must continue to burn for at least 5 seconds for the temperature to be considered the fire point.
Cloud point is the point that is reached when the oil gets cooled slowly, and the temperature at which the oil appears cloudy or hazy forms the cloud point.
Pour point refers to the temperature at which the oil that is being examined, stops to flow even after tilting the container to a horizontal position for 5 seconds.
The aniline point of an oil is defined as the minimum temperature at which equal volumes of Aniline (C6H5NH2) and lubricant oil are miscible upon mixing forming a single phase.
The value is important as it gives an approximate idea of the content of aromatic compound present in the oil, as Aniline is an aromatic compound, the miscibility of aniline shows the presence of another aromatic compound present in the mixture. The lower the aniline point, the greater is the content of aromatic compounds in the oil.
Oiliness, the concept of being covered with oil or impregnated with oil is referred to as oiliness
Saponification Value Saponification value or saponification number refers to the amount of base that is required to saponify a fat sample, saponification value can also be defined as that value which represents the number of milligrams required to saponify 1 gram of fat under the specified conditions.
In case sodium hydroxide is used for the saponification process, the saponification value must be converted from potassium to sodium by dividing the KOH values by the ratio of the molecular weights of KOH and NaOH
Acid value Acid value (or neutralization number or acid number or acidity) is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of chemical substance. The acid number is a measure of the number of carboxylic acid groups in a chemical compound, such as a fatty acid or mixture of compounds.
Fluid-Film Lubrication.
In this type of lubrication, the fluid film is interposed completely resulting in the complete separation of the sliding surfaces. The fluid is introduced intentionally into the main bearing of the as oil. Sometimes it is unintentionally used in case of water between a smooth tire and a wet pavement, though most of the time liquids are used, cases of gas can also be employed which is mostly air.
Boundary Lubrication.
This occurs during the stopping and starting of machines, it’s a condition that occurs between an unlubricated sliding and fluid filled lubrication. It is also defined as the condition of lubrication, where the friction between surfaces is determined by the properties of the surfaces and properties of the of the lubricant, other than viscosity. Boundary lubrication encompasses a significant portion of lubrication phenomena.
Extreme pressure additives, or EP additives, are additives for lubricants with a role to decrease wear of the parts of the gears exposed to very high pressures. They are also added to cutting fluids form aching of metals
Extreme pressure additives are usually used in applications such as gear boxes while antiwear additives are used with lighter load applications such as hydraulic and automotive engines.
3.3.2 Selection of lubricants for cutting tools, I.C. Engine, gears, transformers, delicate instruments & refrigeration system.
Cutting tools: A major part of the energy spent in metal cutting is converted into heat, the heat formed can damage both the cutting tool and work piece if proper cooling doesn’t take place at the cutting edge.an overheated work piece can lose its dimensional integrity and an overheating tool its hardness, therefore the solution has to be continuously cooled at the cutting edge of the tool and workpiece.
That depends on a variety of factors including:
- Material being machined;
- Type of metal cutting operation
- Speed of the machine
- Accuracy of the coolant at the cutting zone
- Type of cutting tool used.
These factors are all pertinent to making an informed cutting fluid selection.
I.C Engines
The I.C engine burns fuel to convert the chemical energy into mechanical energy, in the combustion chamber chemical energy is released when fuel-air mixture is ignited by a spark. The gas thus produced in this reaction expands rapidly and forces the piston down the cylinder on the power stroke.
The advantages of lubrication in IC engine:
- The lubrication helps to reduce the wear and tear of the moving parts.
- It damps down the vibration of the engine.
- The moving parts are cleaned.
- The gas Piston is made tight.
Gear
In order to choose the best lubricant for a gear set, the following criteria must be addressed:
- Viscosity – this is one of the important properties of lubricating oil. The viscosity of a gear lubricant is initially chosen so that they provide the desired thickness between the interacting surfaces for a given load and speed.
- Additives – the additive present in the lubricants will determine the general category and also perform key properties effectively. The gear lubricants are categorised into three parts the antiscuff, rust and oxidation and compounded, under operation condition, the best gear lubricant that fits the requirement is taken eventually.
- Base Oil Type – In most applications the mineral base oil of high quality is used. Sometimes the synthetic base oils are also preferred. The mineral base oils have higher pressure-viscosity coefficients than common synthetics that provide a greater thickness at given operating viscosities.
Transformer: A transformer lubricant provide insulation for components of the electrical systems, thereby reducing the danger of overheating and breakdowns, every sector requires power systems, with a proper transformer oil lubricant that protect the transformer and keep the power flowing.
The oil needs to reduce the cleaning and repair maintenance. Lower costs and durability are the key factors the transformer oil is for insulation and cooling of the electric transformers and switchgears of extra high voltage conforming to the technical particulars. The oil should also be suitable for oil circuit breakers and other electrical equipment’s in which oil is used as insulating medium. The tests on transformer oil shall be conducted in accordance with the relevant methods.
Delicate Instruments
Hydrodynamic lubrication is said to exist when the moving surfaces are separated by the pressure of a continuous unbroken film or layer of lubrication. In this type of lubrication, the load is taken completely by the oil film. Ynamic lubrication is the formation of an oil wedge. When the journal rotates, it creates an oil taper or wedge between the two surfaces, and the pressure
The basis of hydrod build up with the oil film supports the load.
Hydrodynamic lubrication depends on the relative speed between the surfaces, oil viscosity, load, and clearance between the moving or sliding surfaces.
In hydrodynamic lubrication the lube oil film thickness is greater than outlet, pressure at the inlet increases quickly, remains fairly steady having a maximum value a little to the outside of the bearing centre line, and then decreases quickly to zero at the outlet.
Application of hydrodynamic lubrication
- Delicate instruments.
- Light machines like watches, clocks, guns, sewing machines.
- Scientific instruments.
- Large plain bearings like pedestal bearings, main bearing of diesel engines.
Hydrocarbon oils are considered to be satisfactory lubrication for fluid film lubrication. In order to maintain the viscosity of the oil in all seasons of the year, ordinary hydrocarbon lubricants are blended with selected long chain polymers.
Refrigeration Systems
In general, large refrigerant systems, especially those that use ammonia as a refrigerant, are provided with oil separators. In these kinds of systems, using a lubricant that is immiscible or has low miscible with the refrigerant fluid is advisable.
In most other refrigeration systems, the blending of the lubricant and the refrigerant fluid is unavoidable. Therefore, it becomes necessary to select a lubricant that has the right miscibility and solubility features. For example, with systems that do not have the oil-separation capability, the lubricant that moves from the compressor into the must be appropriately miscible with the refrigerant at the temperature of the evaporator, so that the refrigerant fluid–lubricant blend rests in one phase after expansion in the evaporator and at a sufficiently low viscosity to travel through to the compressor. Consequently, if the lubricant separates itself in the evaporator due to poor miscibility with the refrigerant fluid or the blend viscosity is more, fluid is likely to get trapped in the evaporator and adversely affect the system’s cooling capacity and efficiency.
Solubility is very important at the compressor portion of the refrigeration circuit. Another important aspect of proper lubricant selection is to ensure that the viscosity of the lubricant, after absorption of gaseous refrigerant at the high compressor temperature, is sufficient for effective lubrication of the compressor.
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