LUBRICANTSINTRODUCTION AND LUBRICATIONTHEORY 1 INTRODUCTION LubricantsaredefinedasThechemicalsubstanceswhich reducefrictionbetweentwosliding/movingmetalsurfacesand reduce wear and tear of machines. Thelubricantsformlayersbetweentwomovingsurfacesin contact,thusreducingfrictionalresistance.Thisfurtherhelpsin reducing destruction of metals. Friction:Itistheforceofresistancetotherelativemotionof two solid surfaces in contact Coefficientoffriction:Itisratioofforceoffrictionto applied force or load. Theprocessofreducingfrictionalresistancebetweenmoving/ sliding surface by the introducing of lubricants in between them is called lubrication. Functions of lubricants: Lubricantsactascoolantsastheyreducefrictionalheat. Therebycontrollingexpansionofmetalswhichhelpsin maintainingshape,sizeanddimensionsofmetallicpartsof machine. Lubricantsactassealantsastheydonotallowescapeof gases from engine under high pressure. LUBRICANTSINTRODUCTION AND LUBRICATIONTHEORY 2 Lubricantspreventtheattackofmoistureonmachine surface which helps in controlling corrosion. Theyactascleaningagentssincetheyhavetendencyto wash off solids produced due to combustion. Reduces maintainence cost of machine by reduction of wear and tear of metal surfaces The basic principle of lubrication is lowering the coefficient of friction using lubricant. 1)Theoriesoflubrication (Mechanism) Duringlubricationofamachine,weinterposefilmsof lubricantbetweenthemovingmetallicsurfacesinorderto reduce frictional resistance, wear & damage. There are mainly 2 types of mechanism by which lubrication is done. LUBRICANTSINTRODUCTION AND LUBRICATIONTHEORY 3 I.Fluidfilmorthickfilmor hydrodynamic lubrication Inthistypeoflubrication,thickfilmoflubricant (thicknessatleast1000Ao)isappliedbetween moving surfaces. This mechanism for lubrication is useful in machines where load is low and speed is not very high. Inthislubrication,thelubricantchosenshouldhave theminimumviscosityunderworkingcondition&at thesametimeitshouldremaininplace&separatethe surfaces. Insuchsystem,frictionisindependentoftheload, butdependsontheviscosity,thicknessoffilm& relative velocity & area of surfaces. Thecoefficientoffrictioninsuchcasesisaslowas 0.001 to 0.03. LUBRICANTSINTRODUCTION AND LUBRICATIONTHEORY 4 Light LoadOil molecules which are tightly held up(thick) 1000 A0 Loosely (Thin) arrange Oilmolecules Running Bearinglining Journal (Shaft) Rotating Shaft infloating lubricant. LUBRICANTSINTRODUCTION AND LUBRICATIONTHEORY 5 Uses : a. Sewing machines b. Clocks c. Scientific instruments . HydrocarbonOils:areconsideredtothesatisfactory lubricantsforfluidFilmlubrication.Inordertomaintainsuitable viscosity of the oil in all seasons of a year, ordinary hydrocarbon lubricants are blended with selected long chain polymers. Moreoverhydrocarbonpetroleumfractions,generallycontain small quantities of Unsaturated hydrocarbons, which oxides under operating conditions forming gummy product. So anti oxidant (like amino phenol) are used in journal bearing. LUBRICANTSINTRODUCTION AND LUBRICATIONTHEORY 6 II.BoundaryLubricationORThinfilm lubrication: High Load Low Velocity Adsorbed thin layer of lubricant Thistypeoflubricationisdonewhenacontinuous filmoflubricantcannotpersist&directmetalto metal contact is possible due to certain reasons. Athinfilmisapplied(whichislessthan10Ao) between two moving surfaces The machine which is operated under high load and low speed follows this type of boundary lubrication. Thishappenswhen(i)ashaft(steelrod)starts moving from rest. or (ii) the speed is very low. or (iii) the load is very high & viscosity of the oil is too low. Hence to reduce friction in such machines a thin film of lubricating il is introduced between them. Thisfilmgetadsorbed(Surfaceattached)onmetal surfaceduetoPhysicalorChemicalorboththe forces & it is retained (keep in place) there. LUBRICANTSINTRODUCTION AND LUBRICATIONTHEORY 7 The coefficient of friction in such case is usually 0.05 to0.15.Sothatmachinesurfacesremainprotected from friction. Thelubricantgenerallyappliedarethevegetable,animal oils&theirsoapspossessagreattendencyofadsorptionon surfaces. Thesoapsaresuitableatmoderatetemperature.Butat hightemp,theygetdecomposed.Hencethesearenot suitable to be used in I.C. Engines. In such cases, Mineral alls blended with fatty oils or fatty acids are used because the thermal stability of mineral oils is high. Othersubstitutesaresolidlubricantssuchasgraphiteor molybdenumdisulphides.eitheraloneortheirstable suspension in oil. Which is also suitable. Inshort,thelubricantpossessinghighviscosityindex, resistancetoheat&oxidation,goodsoiliness,lowpour paint are most suitable for boundary lubrication. LUBRICANTSINTRODUCTION AND LUBRICATIONTHEORY 8 Uses : a. Rail axle bones b. Tractors c. Rollers Whenthemoving/slidingsurfacesareunderveryhigh pressure&speed,ahighlocaltemperatureisattained& undersuchconditionsliquidlubricantsfailto stick& may decompose & even vaporize. Tomeettheseextremepressureconditions,special additivesareaddedtoMineralOils.Thesearecalled extremepressureadditives.Theseadditivesformon metalsurfacemoredurablefilms,Capableofwith standing very high loads & high temp. Importantadditivesareorganiccompoundhavingactive radicalsorgroupssuchasChlorine(Chlormatedesters), Sulphur(Sulphurisedoil)orPhosphorus(Tri-cresyl phosphate). LUBRICANTSINTRODUCTION AND LUBRICATIONTHEORY 9 Thesecompoundreadswithmetallicsurfacesathigh temptoformmetallicchlorides,sulphideorphosphides. These compound posses high M.P. (1000oC) + Serve as good lubricant under extreme pressure & temp conditions.