Introduction To Tribology - Bbsbec.edu.in

Introduction to TribologyTribology is defined as the science and technology ofinteracting surfaces in relative motion, having itsorigin in the Greek word tribos meaning rubbing. It isa study of the friction, lubrication, and wear ofengineering surfaces with a view to understandingsurface interactions in detail and then prescribingimprovements in given applications.

WearThe process leading to loss of material is known as"wear“Types of wear Adhesive wear Abrasive wear Surface fatigue Fretting wear

Adhesive wearAdhesive wear are caused by relative motion, "directcontact" and plastic deformation which create weardebris and material transfer from one surface toanother.Example of Adhesive Wear: A Shaft rotating in a bushing Chalk on board-while writing

Abrasive WearAbrasive wear occurs when a hard roughsurface slides across a softer surface. ASTMInternational (formerly American Society forTesting and Materials) defines it as the loss ofmaterial due to hard particles or hard protuberancesthat are forced against and move along a solidsurface.

Types of Abrasive wear Abrasive wear is commonly classified according to the type of contact andthe contact environment The two modes of abrasive wear are known as two-body and three-bodyabrasive wear Two-body wear occurs when the grits or hard particles remove materialfrom the opposite surface. Three-body wear occurs when the particles are not constrained, and are freeto roll and slide down a surface.Two-body wearThree-body wear

Erosive Wear Erosive WearErosive wear can be described as anextremely short sliding motion and is executedwithin a short time interval. Erosive wear iscaused by the impact of particles of solid orliquid against the surface of an object.

Fretting wearFretting is the repeated cyclical rubbing betweentwo surfaces, which is known as fretting, over aperiod of time which will remove material from oneor both surfaces in contact

FrictionFriction is the force resisting the relativemotion of solid surfaces, fluid layers, andmaterial elements sliding against each other.Types of friction: Dry frictionFluid frictionLubricated frictionSkin friction (Fluid and solid) andInternal friction (Solids),

LubricationLubrication is the process or technique employedto reduce wear of one or both surfaces in closeproximity, and moving relative to each another, byinterposing a substance called lubricant between thesurfaces to carry or to help carry the load (pressuregenerated) between the opposing surfaces.

Regimes of LubricationAs the load increasessurfaces three distinctobserved with respectlubrication, which arelubrication: on the contactingsituations can beto the mode ofcalled regimes ofFluid film lubrication or boundary lubricationHydrostatic lubricationHydrodynamic lubrication (thick film)Extreme pressure lubrication

Hydrodynamic Lubrication or ThickFilm Lubrication Hydrodynamic lubrication is said to exist when the moving surfacesare separated by the pressure of a continuous unbroken film orlayer of lubrication. In this type of lubrication, the load is takencompletely by the oil film. The basis of hydrodynamic lubrication is the formation of an oilwedge. When the journal rotates, it creates an oil taper or wedgebetween the two surfaces, and the pressure build up with the oilfilm supports the load. Hydrodynamic lubrication depends on the relative speed betweenthe surfaces, oil viscosity, load, and clearance between the movingor sliding surfaces. In hydrodynamic lubrication the lube oil film thickness is greaterthan outlet, pressure at the inlet increases quickly, remains fairlysteady having a maximum value a little to the outside of the bearingcenter line, and then decreases quickly to zero at the outlet.

Hydrodynamic Lubrication

Hydrostatic Lubrication Hydrostatic lubrication is essentially a form of hydrodynamiclubrication in which the metal surfaces are separated by a completefilm of oil, but instead of being self-generated, the separatingpressure is supplied by an external oil pump. Hydrostatic lubricationdepends on the inlet pressure of lube oil and clearance betweenthe metal surfaces, whereas in hydrodynamic lubrication it dependson the relative speed between the surfaces, oil viscosity, load onthe surfaces, and clearance between the moving surfaces. Example: the cross head pin bearing or gudgeon pin bearing in twostroke engines employs this hydrostatic lubrication mechanism. Inthe cross head bearing, the load is very high and the motion is notcontinuous as the bearing oscillation is fairly short

Hydrostatic Lubrication

Boundary Lubrication or Thin FilmLubrication Boundary lubrication exists when the operatingcondition are such that it is not possible to establish afull fluid condition, particularly at low relative speedsbetween the moving or sliding surfaces. The oil film thickness may be reduced to such a degreethat metal to metal contact occurs between themoving surfaces. The oil film thickness is so small thatoiliness becomes predominant for boundarylubrication. Boundary lubrication happens whenA shaft starts moving from rest.The speed is very low.

Boundary Lubrication

Extreme pressure lubrication When the moving or sliding surfaces are under veryhigh pressure and speed, a high local temperature isattained. Under such condition, liquid lubricant fails tostick to the moving parts and may decompose andeven vaporize. To meet this extreme pressurecondition, special additives are added to the mineralsoils. These are called “extreme pressure lubrication.”These additives form on the metal surfaces moredurable films capable of withstanding high loads andhigh temperature. Additives are organic compoundslike chlorine (as in chlorinated esters), sulphur (as insulphurized oils), and phosphorus (as in tricresylphosphate).

Definition : Wear is progressive damage, involving material loss , occurs on thesurface as a result of relative motion between the surfaces.TypeSliding wear(delamintationwear)Typical characteristics and definitionsobserved InWear due to localized bonding between contacting solid Sliders, bearing, gears and camshaft.surfaces leading to material transfer between the twosurfaces or the loss from either surface.Plastic deformation, crack nucleation and propagation inthe surfaceFretting wearWear arising as a result of fretting (Small amplitude Press fit parts with a small relativeoscillatory motion, usually tangential, between two solid Sliding motionsurfaces in contact).Abrasive wearWear due to hard particles or hard protuberances forced Slidingsurfacesagainst and moving along a solid surface.EquipmentErosive wearWear due to mechanical interaction between that surface Turbine, pipes for coal slurries andand a fluid, a multi component fluid, or impinging liquid helicopter bladesor solid particles(solid particleimpingement)Fatigue wearCavitation wear,earth-removingWear of a solid surface caused by fracture arising from Ball bearing, roller bearing glassy solidmaterial fatigue.sliderA form of erosion causing material to wear by the action Soft Bearing Surfacesof vapour bubbles in a very turbulent liquid.

Wear Measurement Archard wear Equation :Wαw wearw Normal Load on contactH surface hardness of the wearing materialK wear coefficient (dimensionless)W K is called Dimensional wear constantUnit (volume )/(Load/meter)

Wear DependenceFor Dry/unlubricated surfaces sliding Normal Load Relative sliding speed The initial temperature Thermal, Mechanical , chemical properties ofthe material in contact No simpler Model to explain wear

IDENTIFICATION OF WEARMECHANISM Examination of the weardebris (collected) large lumps imply- adhesivewear fine particles- oxidative wear chip like particles-abrasivewear flake like particlesdelamination wear Examination of the wornsurfaces: Heavy tearing implies adhesive wear Scratches imply -abrasive wear burnishing indicates –nonadhesive wear

Schematic diagram of loading configuration of Pin-on-Disc.

pin and disc were fitted. wear track diameter was measured. Load applied in the dead cell. Values of displacement , Time, speed, load and diameterof disk were entered. Displacement value of every second and coefficient offriction were noted from LVDT. wear values were calculated from displacement value .

Wear rate of Ti-6Al-4V under ambientcondition at 1kg,4kg and 8kgWear rate of Ti-6Al-4V undervacuum condition at 1kg,4kg and 8kg

Abrasive wearSpeed 0.1m/sec at 1kgSource :Materials engg,IISc,BangaloreSpeed 0.8m/sec at 1kg

Speed 0.1m/sec at 1kgSpeed 0.4m/sec at 1kgSpeed 0.8m/sec at 1kgSpeed 0.2m/sec at 1kgSource :Materials engg,IISc,Bangalore

SAMPLE CALCULATIONSWear Rate Calculations200 C, 0.15 m/s, 10 MPa250200Wear Rate Slope of the steady state regionin the graph (Y2 – y1)/(x2 – x1)wear (microns)150100500-50-1000204060sliding distance (m)80100

Case study from Tribology international journal Wear rate determination by oilexamination Helps to avoid secondary damageby identification of wear debris . Wear debris quantification doesnot always correlate with the realwear . Methodology used : spectrometricwear debris measurement data toobtain parameter of wearconditionReference : Analytic approch towear tare determinationForinternal combustio enginecondition monitering based on oilanalysis (Author:V Macian, BTormos,P.Olmeda, L.Montoro)

Case study from Tribologyinternational journal The spectrometer purpose is to determine theelemental content of each debris particle . Inductively Coupled Plasma (ICP) spectrometer isused , typically maximum size of 5 micron can bemeasured . Quantative information (concentration) is related tothe amount of electromagnetic radiation that isemitted while qualitative information (which elementis present is related to the wavelength at whichradiation is emitted .

Viscosity

Introduction Viscosity is a quantitative measure of a fluid’s resistance toflow.Dynamic (or Absolute) Viscosity: The dynamic viscosity(η) of a fluid is a measure of theresistance it offers to relative shearing motion.η F/ [A (u/h)]η τ /(u/h) N-s/m²Kinematic Viscosity : It is defined as the ratio of absolute viscosity to the densityof fluid.ν η/ρm²/s; ρ density of fluid

Viscosity MeasurementsCapillary Viscometers It gives the ‘kinematic viscosity’ of the fluid. It is based onPoiseuille’s law for steady viscous flow in a pipe.

Viscosity MeasurementsRotational Viscometers These viscometer give the value of the ‘dynamic viscosity’. It is based on the principle that the fluid whose viscosity isbeing measured is sheared between two surfaces. In these viscometers one of the surfaces is stationary and theother is rotated by an external drive and the fluid fills thespace in between. The measurements are conducted by applying either aconstant torque and measuring the changes in the speed ofrotation or applying a constant speed and measuring thechanges in the torque. There are two main types of these viscometers: rotatingcylinder and cone-on-plate viscometers

Viscosity MeasurementsRotating cylinder viscometer

Viscosity MeasurementsCone-on-plate viscometer

Effects of temperature The viscosity of liquids decreases with increase the temperature. The viscosity of gases increases with the increase the temperature.

Effects of temperature The lubricant oil viscosity at a specific temperature can beeither calculated from the viscosity - temperature equationor obtained from the viscosity-temperature ASTM chart.Viscosity-Temperature Equations

Effects of temperaturefig: Viscosity-temperature characteristics of selected oils

Viscosity index An entirely empirical parameter which would accurately describethe viscosity- temperature characteristics of the oils. The viscosity index is calculated by the following formula:VI (L - U)/ (L - H) * 10where ,VI is viscosity indexU is the kinematic viscosityof oil of interestL and H are the kinematicviscosity of the reference oilsFig . Shows the evaluation of viscosity index