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
Mechanisms
Hydrodynamic Lubrication
This type of lubrication occurs when two rubbing surfaces are separated by a thin film lubricant. This condition is very effective and the lubrication is used to reduce the friction between the rubbing solids with the support of a liquid or semi solid lubricant. The main aim of hydrodynamic lubrication is to provide a specific lubricant that penetrates into the contact zone present between the rubbing solids and creates a thin film, this film separates the surface from direct contact and subsequently reduces friction.
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.
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
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 disulphate, 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.
Biodegradable lubricants possess molecular structures that have the ability to biologically degrade over time by processes like bacterial decomposition or by the enzyme biodegradation of other living microorganisms like yeast, fungi and protozoans. The bases of the biodegradable are mainly derived from vegetable oils, that includes castor oil, rapeseed oil, soybean oil, palm oil, corn oil, hemp oil or chemically synthesized esters.
Properties
- Bio lubricants have excellent lubricity
- These lubricants possess a high flash point and a high viscosity index.
- Compared to mineral based lubricants they are less toxic.
- The biodegradable lubricants degrade rapidly in the natural environment and can reach up to 90% in biodegradability levels.
- The lubricants are produced from renewable materials and are non-toxic in nature, that helps to overall use up the fossil fuels. Base oils used are vegetable and synthetic esters, which are 100% renewable bio-products.
- They show positive impact on the emissions caused by the reduction of carbon dioxide.
- They are designed specifically to show an increase in performance, efficiency and reliability.
Applications
- The biodegradable oils are used in two stroke outboard outboard engine oils and also in Chain saw lubricants.
- They are used in concrete mould release lubricants and also in lubricant greases.
- The lubricants are useful in leakages in mobile hydraulic equipment
- They are economical in the water industry and are treated in sewage plants locks and turbines.
- Transporting plants (ropeways, sky lifts)
- Railways, tramways (wheel flange, tracks, swathes)
- Corrosion preventatives
- Open gears, metal working and lubrication of air tools
Advantages
- The bio-lubricants reduce the risk of skin irritation in contrast to mineral lubricants that produce oil mist that release volatile organic compounds into the air and cause skin irritation and other dermatitis.
- Biodegradable lubricants are found to improve lubricity.
- These lubricants increase customer satisfaction as customers are looking for natural based products than man made ones.
- They meet the requirements of sustainability and environmental protection set by the government.
- Vegetable oils have a higher flashpoint than the mineral oil, which helps substantially to reduce the the risk of machine shop fires.
- The use of Bio-lubricant helps to reduce the dependency on imported petroleum products.
- The lubricants are made from vegetable oils, which are biodegradable and are less toxic, and better for the environment and renewable causing less damage.
- Bio-lubricants may be economically beneficial for domestic farmers. Greases and lubricants made from soybean oil can help stabilize prices of soybeans for farmers. This is because these products use surpluses of soybeans for production, stimulating demand for the product.
Disadvantages
- The vegetable oils in its natural form lack sufficient oxidative stability for lubricant use
- As the stability(oxidative) is low, the oil will oxidize quickly during use and if left untreated can become thick and polymerise to a plastic
- Any chemical modification in the vegetable oil to increase its efficiency like partial hydrogenation and fatty acid shifting can lead to cost increase
- Natural Vegetable oils cannot withstand temperatures above 80oC
- Biodegradable lubricants have a high pour point
- The presence of a minute amount of water leads to foaming and degradation problems.
Drop test
Drop point is the temperature at which, the grease becomes the fluid enough to drip. the dropping point is useful in identifying the property of the grease and establishing and making it a bench mark for quality control purposes.
The dropping point indicates the upper temperature limit at which grease retains its original structure.
The test is carried out in normal and proper conditions, the tube that contains the grease is placed in a water bath and both the water bath and the temperature inside the tube is monitored as the water bath is heated, as the process continues when the first drop of grease falls from the cup, the temperature of the test cup and the water bath is noted. The results are averaged with the temperature in the water bath and the cup and recorded as the dropping point of grease.
Consistency Test
The NLGI consistency number (National Lubricating Grease Institute) is defined as the measure of the relative hardness of grease used for lubrication, the consistency number alone does not determine the grease required for particular applications, subsequently it complements other classifications.
The Penetration test is caried out, where grease is placed in a penetration test apparatus, the apparatus consists of a dial indicator, a container and a specially designed cone. The contained is filled with grease till the top and is smothered off, the cone is placed in such a position so as it just touches the grease and at this point the indicator is set to zero position. When the test begins, the weight of the cone helps it to penetrate into the grease, after a specific time interval the depth of penetration is estimated.
Liquid lubricants typically contain 90% oil and 10% additives that helps reduce friction and wear, they show high viscosity and resistance to corrosion and other properties that are required for machine functions
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
· 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.
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.
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.
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 Mechanical System
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.
The basis of hydrodynamic 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 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.
Steam Turbines
The importance should be first given to the physical and chemical properties of turbines before the selection process.
Steam, gas and hydro turbines operate on a family of lubricating oils known as R&O oils (Rust & Oxidation inhibited oil). Compared to other lubricating oils like gasoline and diesel the turbine lubricating oil, its equipment, geometry, maintenance practises, operating temperatures and contamination system present a lot of demand.
The drive for a long-lasting lubricant oil lies in having a huge size of turbine sump capacities of size ranging from 1,000 to 20,000 gallons, and is also economical. Low turbine oils makeup rates also help to contribute the need for high quality and long-life lubricants. Without significant oil contamination issues, turbine oil life is primarily dictated by oxidation stability.
Heat and water, particulate contamination and aeration adversely affect oxidation stability the oil life is extended by adding rust inhibitors, antioxidants and demulsibility additives and a rich quality base stock Lube oil cooler, water removal systems and filters are installed in turbine lubrication systems for the same purpose.
Steam turbine oil is designed to shed water and allow the solid particles to settle, these are later removed through kidney loop filtration systems, sump drains. formulated to shed water and allow solid particles to settle where they can be removed through sump drains or kidney loop filtration systems during operation. To help in contamination separation, high levels of detergents are not added. Turbine oils are not exposed to fuel or soot and therefore do not need to be drained and replaced on a frequent basis.
The combustion of methane in atmospheric air: -
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The reactants are methane, oxygen and nitrogen and the products formed are carbon dioxide water and nitrogen
The reaction is as follows
CH4+2[ O2 +3.76N2] -----------> CO2+2H2O+7.52N2
the above equation is obtained after equating the molar coefficients from the reactants and products side.
The air-fuel ratio:
it is defined as follows.
AF = mair
mfuel
now comparing the reactants of the methane with the theoretical air we obtain:
CH4+2[ O2 +3.76N2] ‹-->Reactants
Fuel 2kmol air
AF = mair = 2(4.76) [kmol] 29[kg/kmol]
mfuel 1[kmol] (12+4) [kg/kmol]
AF= 17.3 kg-air
Kg-fuel
Key takeaway
Lubrication is the science of reducing friction between two solid bodies in relative motion by interposing a lubricant between their rubbing surfaces. Lubrication and lubricants are the most important aspect for running a machine without giving pre-matured failure. It is also used to minimize power loss, heat generation, wear and tear of mechanical components as well as for smooth running of the machine. Production and productivity are very much dependent on lubrication system
Reference:
1. Engineering Chemistry, Jain and Jain, Dhanpat Rai & Co
2. Engineering Chemistry, M. Subha Ramesh, Dr. S. Vairan-Ed.-IInd Wiley
3. Instrumental Methods of chemical analysis, Chatwal and Anand,Himalaya Pub House
4. Industrial Chemistry, B.K.Sharma,Goyal
5. Chemistry for Engineers, Rajesh Agnihotri, Wiley
