AC
Unit-5Lubrication and Combustion Q1) Define the term Viscosity and Viscosity Index? A1)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. Q2) Differentiate between Flash point and Fire point?A2)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. Q3) Explain the terms Cloud point and Pour point?A3)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. Q4) What are Biodegradable Lubricants, mention its properties ? A4) 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. PropertiesBio 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. Q5) Write a note on Drop test and Consistency Test? A5) Drop testDrop 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 TestThe 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. Q6). Define Lubrication and the types of Lubrication?A6) 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. Hydrodynamic LubricationThis 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.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 metalsExtreme 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. Q7). What are the Selection features for Delicate instruments?A7)Delicate InstrumentsHydrodynamic 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 lubricationDelicate 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. Q8). What is the Criteria of using a good lubricant in gears?A8) 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. Q9) What is the function of lubricants in Transformers?A9) 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. Q10) Importance of Lubricants in Refrigerator systems? A10) Refrigeration SystemsIn 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.
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. Q10) Importance of Lubricants in Refrigerator systems? A10) Refrigeration SystemsIn 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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