Lubricants

Lubricants and lubrication Lubricant is a substance which when introduced between two moving parts reduces friction by forming a slippery film between the two surfaces and thus improving efficiency and decreasing wear So, lubrication is the process of reducing friction and wear between two moving surfaces by applying lubricating substances between the parts

Principle of lubrication All material have irregularities in form of peak (asperities) and valleys When peak of upper surface come in contact with peak of lower surface, these asperities may crush under small load, until the contact area is large enough to support the load and is known as wear And also when the upper surface and lower surface move against each other that lead to friction which further reduces the efficiency of the machine Therefore to reduce wear and to reduce friction we use lubricants that increases the efficiency of the machines

Purpose of lubrication To keep moving parts apart: by forming thin layer between the surfaces To reduce friction: lubricant to surface friction is lesser than surface to surface friction To protect against wear: lubricant prevent wear by keeping moving part apart To transfer heat: liquid lubricants are effective in heat circulation due to high specific capacity To prevent corrosion: as surface of metal do not come in contact with air (Oxygen) To seal gapes: lubricant seal the space between moving parts through capillary force To carry away contaminants and debris

Mechanism of lubrication: thick film or hydrodynamic lubrication This also known as fluid film lubricant If the surface slide over the other lead to increased friction and asperities tend to break leading to wear An excellent method to prevent this is to use lubricant film to separate the surfaces and is known as hydrodynamic lubrication (hydro= liquid, dynamic = relative motion) The lubricant film ensure that no wear is possible between moving parts due to reduced friction The hydrodynamic film is formed when the geometry, motion of moving surface and lubricant viscosity combine to increase the lubricant pressure to support the load as increased pressure forces the surface apart In hydrodynamic lubrication one surface floats over the other surface and increased fluid pressure forces the surface apart The four essential elements to consider in hydrodynamic lubrication: liquid (hydro); relative motion (dynamic); viscous properties of liquid and geometry of surfaces

Thin film of boundary lubrication Extreme pressure lubrication The heat generated between moving parts has to be dissipated effectively to decrease the temperature dependence of viscosity Delicate instruments such as watches, clocks, guns, scientific instruments are provided with hydrodynamic lubrication Hydrocarbons are considered to be best for fluid film lubrication Other mechanism: Thin film of boundary lubrication Extreme pressure lubrication

Classification of lubricants Gaseous lubricants: (lower viscosity, and higher compressibility eg air, steam or liquid metal vapors Liquid lubricants: water (used as such or in combination with other oils and used in engineering processes such as milling and lathe turning. Mineral oil (derived from crude petroleum oil) eg polyalphaolefins, naphthenics, polyalkyleneglycos Lanolin (derived from sheep wool and are corrosion inhibitor, protection against rust, salt and acid) Vegetable oil (triglyceride esters, eg castor oil, palm oil, sunflower oil, whale oil etc)

Solid lubricants: used under high temperature conditions in form of dry powder Graphite (planes of polycyclic carbon atom and hexagonal orientation. Higher the carbon atom content and higher the crystallization the better is the lubrication action Molybdenum disulphide (M0S2): hexagonal structure and lubrication performance is better than graphilte, but lubrication is limited to 400 oC as above 400 oC it undergoes degradation Boron nitride (BN): ceramic powder ( used at high temp. up to 1200 oC; exist in tow forms cubic (very hard cant used as lubricant) and hexagonal used for lubrication Teflon or poly tetrafluoroethane: PTFE does not have layered structure but the molecules slip easily along each other and hence the lubrication is acheived Note MoS2, PTFE and graphite are antifriction, antiwear additives are often mixed with polymers to give self lubrication composites

Semisolid lubricants (grease): contain thickening agents like soaps of sodium, calcium or aluminium mixed with mineral oil (80% mineral oil+10% soaps+ 10 % additives) Grease are named after soap used in manufacture eg Lithium based grease (water and dust resistance and used up to 15-30oC) Calcium based grease (water resistant and used up to 70oC) Aluminium based grease(high water and acid resistance and used up to 80oC) Sodium based grease (sparingly soluble in water and cant be used in wet condition and used up to 120oC)

Metal alloys: some metal and alloy can also be used as lubricant or grease additives eg lead, tin, zinc alloys are used in bearings and metal powders are used to lubricate sliding surfaces

Additives of lubricants Additives are employed to impart desired characteristics to lubricants and some additive used in lubricant formulation are: Antioxidants: as at elevated temperature lubricant oil get oxidized so antioxidants are used to reduce the rate of reaction Detergents: detergents additive maintain a suspension of carbon particle and extend lubricant life Antifoaming additives: to provide proper flooding of lubricant at point of contact antifoaming additives are used Alkalinity additives: certain fuels which have high sulfur content lead to SO2 and SO3 gases which lead to acid corrosion and hence to avoid that we add alkalinity additives Demulsibility (water-shedding) additives: in marine related equipments lubricant get contaminated with water and these additive force the quick separation of water from lubricants

Properties of lubricants Viscosity Flash point Fire point Cloud point Pour point

Viscosity Viscosity is the most important property of lubricants that determines the operating condition under which it is used Viscosity is the property of fluid which is a measure of its resistance to flow The steady state of flow of a liquid can be visualized to consist of series of parallel layers moving one above other The function of lubricating oil is to form a liquid film between two moving or sliding surfaces. If the viscosity of oil is too low, the resistance to flow between individual layer of lubrication is low and consequently the excessive wear take place On the other hand if the viscosity is too high resistance to flow between different layers of later is high and friction between layer increases and hence decreases efficiency Therefore viscosity of lubricant should be optimum according to need

Factors affecting viscosity Temperature: Viscosity of liquid decreases with temperature as the lubricant oil become thinner as operating temperature increases It is desirable that viscosity of a lubricant should be consistent over a range of temperature so that it can be used at different temperature and we measure it by viscosity index, higher the viscosity index, smaller the change with temperature and lower the viscosity index higher will be viscosity change with temperature Pressure: Viscosity increases with increasing temperature of operating system Molecular structure: in general an oil with high molecular weight possesses a high boiling point and high viscosity

Fire and flash points Flash point is the lowest temperature at which the lubricant oil gives off enough vapors to ignite (spark) but not burn when small flame is brought near it Fire point is the lowest temperature at which the vapors of lubricant oil burn continuously at least for 5 seconds when a small flame is brought near it Flash and fire points determine the volatility and fire resistance of lubricant Flash point is important in determining shipping, storage, and safety regularity of oil A drop in flash point shows the presence of volatile impurities in oil A good lubricant should have higher flash point than the temperature it is used (usually fire point are 8-10% higher than flash points)

Cloud and pour points Cloud point: When lubricant is slowly cooled, the temperature at which it become cloudy or hazy in appearance is called its cloud point Pour point: When the lubricant is slowly cooled, the temperature at which the oil ceases to flow or pour is called pour point Cloud and pour point indicate the suitability of lubricant in cold conditions They are also useful in identifying the source of lubricating oil as oil derived from petroleum source contain paraffin wax and other high melting fraction which solidify on cooling Lubricant used in machine working at lower temperature should posses low pour point to avoid solidification of oil.

Selection of lubrication Selection of lubrication depend upon Lubrication properties System of application and service condition requirement such as type of motion speed, temperature, load and environment In engineering domain lubricants are selected on basis of tribology (study of friction) and wear, chemical compatibility with machine parts and environmental impact The most important criteria for selection include the system on which lubricant is applied and service conditions (Application based)

Lubricant selection based on application Lubricants for cutting tools: For this application cutting fluids in combination with lubricant and coolants are used and main functions are (a) to cool cutting tool edge at appropriate temperature (b) to cool metal work piece to prevent distortion (c) to reduce power consumption by lubrication action (d) preventing rusting of machine parts For heavy cutting low viscosity mineral oil blended with fatty oils For light cutting, the emulsion of oil in aqueous soap solution is most effective For grinding and turning operation water may be used as coolant and lubricant

Lubricant for fragile equipment: For delicate machines (watches, clocks, scientific equipements which do not operate at high temp. and press. Mild lubricant are required (vegetable or mineral oil) Lubricant for combustion engines: Lubricant in internal combustion exp0se to high temperature has high temp. Therefore lubricant should posses high thermal stability and viscosity and hence petroleum oils (with additives) which are stable oxidation and temp. are used Lubricant for gears: gears are subjected to extreme pressure and centrifugal force

Lubricant for different loads and speed Lubricant for transformers Lubricant for textile industry Lubricants for refrigeration system Lubricants in food and pharmaceutical industry