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week 1 review MEE 632

Kartheek Notetaker
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About this Document

includes basic introduction to tribology
Class Notes
includes basic introduction to tribology




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This 46 page Class Notes was uploaded by Kartheek Notetaker on Tuesday September 6, 2016. The Class Notes belongs to MEE 632 at Northern Illinois University taught by Dr.Thakkar in Fall 2016. Since its upload, it has received 5 views. For similar materials see Tribology in Mechanical Engineering at Northern Illinois University.

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Date Created: 09/06/16
What is tribology? Fundamentals of Tribology: • Surfaces in Contact • Friction • Lubrication • Wear •Two aspects – Science: Basic mechanisms – Technology: Design, manufacture, maintenance What is Tribology? • Tribology is defined as the science of interacting surfaces in relative motion. • The word tribology comes from the Greek tribos, meaning rubbing. • In any machine there are lots of component parts that operate by rubbing together. • Some examples are bearings, gears, cams and tappets, tires, brakes, and piston rings. All of these components have two surfaces which come into contact, support a load, and move with respect to each other. • Sometimes it is desirable to have low friction, to save energy, or high friction, as in the case of brakes. Usually we don't want the components to wear so they are lubricated. 9/7/2016 Intro_Tribology 2 Tribology is All Around Us, In Applications from Simple to Complex and Scales from Small to Large •Individual Components • Assemblies or Products • Manufacturing Processes • Construction/Exploration • Natural Phenomena When Two Surfaces Are Pressed Together •Surfaces may look smooth, but on a microscopic scale they are rough. •When two surfaces are pressed together, contact is made at the peaks of the roughness or asperities. •The real area of contact can be much less than the apparent or nominal area. •local welding, can take place.act, adhesion, or even •we have to apply a force to break those junctions.n 9/7/2016 Intro_Tribology 4 •The study of friction, wear, lubrication and contact mechanics are all important parts of tribology. •Related aspects are surface engineering (the modification of a component's surface to improve its function, for example by applying a surface coating), surface roughness, and rolling contact fatigue (where repeated contacts causes fatigue to occur). 9/7/2016 Intro_Tribology 5 Worn Surfaces Figure 33.8 Changes in original (a) wire-brushed and (b) ground-surface profiles after wear. Source: After E. Wild and K. J. Mack ManuSerope Kalpakjian and Steven R. Schmid.fth Edition, by ISBN 0-13-148965-8. © 2006 Pearson Education, Inc., Upper Real Contact Area Figure 33.5 Schematic illustration of the interface of two bodies in contact showing real areas of contact at the asperities. In engineering surfaces, the ratio of the apparent-to-real areas of contact can be as high as 4 to 5 orders of magnitude. Manufacturing, Engineering & Technology, Fifth Edition, by Serope Kalpakjian and Steven R. Schmid. ISBN 0-13-148965-8. © 2006 Pearson Education, Inc., Upper The Force of Friction •The friction force is the resistance encountered when one body moves relative to another body with which it is in contact. •The static friction force is how hard you have to push something to make it, whilst the dynamic friction force is how hard you push to keep it moving. •The ratio of the frictional force F to the normal force W is called the coeffiecient of friction and given the Greek symbol μ (pronounced mew). 9/7/2016 Intro_Tribology 8 • Usually we want low friction (in a car engine for example) so we do not waste excessive energy getting it moving. • But in same case we need high friction, in brakes for example. Friction is also important for car tires to grip the road and between shoes and the ground for walking. 9/7/2016 Intro_Tribology 9 Keeping the Surfaces Apart- Lubrication • If we put a layer of oil between the surfaces then we can separate them and easily slide one over the other with reduced friction and wear. • Mineral oils are the most common lubricants, but other low shear strength materials are also used; for graphite, PTFE, and soft metals like lead or gold. • The selection of the best lubricant and understanding the mechanism by which it acts to separate surfaces in a bearing or other machine component is a major area for study in tribology. 9/7/2016 Intro_Tribology 10 Lubrication • Lubrication: – The process of reducing friction between moving surfaces •Friction: –Resistance created when one surface rubs on another –Slows movement of parts –Generates heat –Increases wear –E9/7/2016articles -Tribology_Introduction 11 Lubrication Types Figure 33.12 Types of lubrication generally occurring in metalworking operatManufacturing, Engineering & Technology, Fifth Edition, by Serope Kalpakjian and Steven R. Schmid. ISBN 0-13-148965-8. © 2006 Pearson Education, Inc., Upper Lubrication • Forms of lubricants – Dry (powder) – Semi dry (grease) – Viscous (oil) • Function of engine oil – Lubricate – Clean – Cool – Cushion – Seal 9/7/2016 Tribology_Introduction 13 When Things Wear Out • If one surface is slid over another then the asperities come into contact and there is a possibility that wear can occur. • The breaking of all the little junction can cause material removal (called adhesive wear). Or the asperities of a hard surface can plough grooves in a soft surface (called abrasive wear). • Wear is usually unwelcome; it leads to increased clearances among moving components, increased mechanical loading and maybe even fatigue. But in grinding and polishing process the generation of high wear rates is desirable. 9/7/2016 Intro_Tribology 14 Adhesive and Abrasive Wear Figure 33.9 Schematic illustration of (a) two contracting asperities, (b) adhesion between two asperities, and (c) the formation of a wear particle. Figure 33.10 Schematic illustration of abrasive wear in sliding. Longitudinal scratches on a surface usually indicate abrasive wear. Manufacturing, Engineering & Technology, Fifth Edition, by Serope Kalpakjian and Steven R. Schmid. ISBN 0-13-148965-8. © 2006 Pearson Education, Inc., Upper • As well as adhesive and abrasive wear, there are other mechanisms whereby material can be removed from a surface. • Erosive wear occurs when particles (or even water droplet) strike a surface and break off a bit of the material. • Hard particles can become trapped in contacts and cause material to be removed from one or both of the surfaces. • One of the main reasons for frequent change of car engine oil is that it becomes contaminated with hard debris particles that can wear out the engine components. 9/7/2016 Intro_Tribology 16 • Stress and Strain at the Contact • The design of rolling bearings and gears is such that the load is supported on a small area. • This leads to high stresses (about the highest stresses we find in any branch of engineering) over small areas of the components. • This can cause high friction, wear, and contact fatigue. tribology.chanics is therefore an important part of • The analysis of contact stress is frequently difficult. Simple component geometries can be analyzed using hand calculations. • More complex component shapes frequently require analysis by numerical methods 9/7/2016 Intro_Tribology 17 The Coefficient of Friction: A simple constant of proportionality. is it? Very Simple Relation: F=µN µ = F/N = “COF” The COF is Somewhat Complicated • Surface roughness plays a role • Lubricant plays a role • Surface chemistry plays a role • Contact Stress plays a role • Contact geometry plays a role • Environment plays a role • Temperature plays a role • Sliding speed plays a role • Ring-Compression Test Figure 33.6 Ring-compression test between flat dies. (a) Effect of lubrication on type of ring-specimen barreling. (b) Test results: 1. original specimen and 2. to 4. increasing friction. Source: AfManufacturing, Engineering & Technology, Fifth Edition, by Serope Kalpakjian and Steven R. Schmid. ISBN 0-13-148965-8. © 2006 Pearson Education, Inc., Upper Friction is the resistance to relative motion between two bodies in contact. Friction Coefficient Chart Figure 33.7 Chart to determine friction coefficient from a ring-compression test. Reduction in height and change in internal diameter of the ring are measured; then μ is read directly from this chart. For example, if the ring specimen is reduced in height by 40% and its internal diameter deManufacturing, Engineering & Technology, Fifth Edition, by Serope Kalpakjian and Steven R. Schmid. ISBN 0-13-148965-8. © 2006 Pearson Education, Inc., Upper Applications and Fields which Encompass Modern Tribology Tribology is All Around Us 9/7/2016 Tribology_Introduction25 Tribology is also in Virtually every Area of Engineering and Industry • Aerospace • Agriculture • Automotive • Engine: Piston ring/cylinder, Bearings, valve seats, injectors • Brakes/clutch • Tooling/Machining/Sheet metal forming • Coatings Providers • Dental Implants • Wind • Fabric/Clothing • Highway/Transportation Depts. • Lubricant Manufacturers • Medical Implants • Pharmaceutical • Shoe Manufacturers 9/7/2016 Tribology_Introduction 26 Every Application has: Surfaces in Contact, and in Relative Motion (e.g. sliding, rolling, impacting) 9/7/2016 Tribology_Introduction 27 • All engineering surfaces have a roughness • Roughness plays an important role in tribology. • Surface Roughness comes from all prior history of the part. • Manufacturing, handling and prior use in application. 9/7/2016 Tribology_Introduction 28 Surface Characterization Variety of Methods available, if needed • Physical Characterization • Roughness • Macro – Waviness and Form • Micro – Surface Roughness • Stylus Profilometers (contact) • Optical Profilometers (non-contact) • AFM (sub-micron) • Hardness • Indent, Scratch • Chemical Characterization • Infrared, XPS, Raman, Auger • Lubricant Shear properties 9/7/2016 Tribology_Introduction 29 Aspects of a Surface Physical - Surface Roughness • Dictates Contact Area • Dictates Contact Stresses • Lubricant Paths or Reservoirs Chemical - Intervening Layers • Chemical Compatibility • Shear Strength • Lubricant Properties, e.g. Viscosity Microscopic forces of molecular Adhesion. Microscopic forces of mechanical Abrasion. When objects touch – there are forces between them. (includes electrostatic, Van der Waals, metallic bonds) Microscopic forces of mechanical Abrasion. When objects touch – there are forces between them. (includes elastic and plastic deformation) There are also “contaminants” at the interface, such as Oxides, Adsorbed films, Adsorbed gases, Foreign or “domestic” particles When assessing a system’s tribology need, we must consider: Materials, Coating, Lubricant Contact Area, Geometry, Stress Surface Roughness Sliding Speed Sliding Mode (unidirectional, reciprocating, multidirectional) Duty Cycle (continuous contact, intermittent contact) Environment Temperature, Humidity, Atmosphere (air, exhaust gases, vacuum) Friction is NOT a Material Property Friction is a “System” Property What is tribology? • Probably more failures are caused by tribological problems than fracture, fatigue, plastic deformation, etc. • Tribological problems are often related to systems issues. Economically very important --6% GDP Examples of tribological problems Assembly Failure • Drive sprockets, idlers, rollers, Grouser shoes • Pin Joints • Electrical Connectors Lubrication Fundamentals • The role of a lubricant is to: • Reduce Friction • Prevent / Minimize Wear • Transport Debris away from Interface • Provide Cooling Key Factors in Lubricant Effectiveness • Fluid Shear Properties • Viscosity • Pressure-Viscosity • Chemistry • Reactivity with the Surface • Boundary Film-Forming Properties • Extreme Pressure Constituents • Shear strength of solid lubricant or coating • Thermal Conductivity/Heat Capacity What is wear? Removal (or displacement) of material from one body when subjected to contact and relative motion with another body. Primary Wear Modes: 1. Abrasive Wear, Scratching 2. Adhesive Wear, Galling, Scuffing 3. Fretting/Fretting Corrosion 4. Erosive Wear, Cavitation, Impact, Electro-arcing 5. Rolling Contact Fatigue, Spalling, Delamination 6. Tribo-Corrosion Abrasive Wear, Scratching “The harder material scratches the softer material.” Adhesive Wear, Galling, Scuffing • Begins as “local welding” • Material “compatibility” is important for adhesive wear. Fretting/Fretting Corrosion • Experiments generally have zones of no- slip, and slip. • Small adhesive pull-outs occur at the boundary. • Often these oxidize, so sometimes called “fretting corrosion”. Small amplitude displacement (< 50 µm). Erosive Wear, Cavitation, Impact, Electro-arcing, Cavitation Damage Steam Control Valve “Fluting” Damage Dependency on particle size, shape, composition, angle of impingement, as well as ductility of “target” Rolling Contact Fatigue, Spalling, Delamination • Spalled Bearing Inner Race Propagation to surface of sub-surface-initiated cracks • Reversing sub-surface shear each time the roller or ball passes over the surface. • Accumulation of these stresses leads to subsurface crack formation, usually at a microstructural inhomogeneity. • Cracks grow toward surface and particle spalls off. • Debris typically gets rolled over, creating additional damage. Like Friction, Wear is a System Property, NOT a Materials Property • There are several distinct wear regimes, though some can operate simultaneously, or sequentially • Observed abrasive wear can results from initial adhesive wear • If you properly simulated the system and wear mode, the wear coefficient, k, can be used to predict lifetimes 1. COF is not a material property, it is a system property. 2. Wear Rate or wear resistance depends on the wear mode, which is a function of the Tribo-system. 3. If we properly characterize and understand the Tribo-sytem, the odds are better that we will succeed, because we can make the right choice for materials, contact geometry, and chemistry, and make the appropriate measurements to give us the answer we seek for our design.


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