Tribology by sGKO4fN

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									Tribology

Engineering Materials
Chapter 3




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Chapter 3: Goals
 Understand the importance of tribology
 Types of friction and wear
 Review of bearing basics and lubricants




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Chapter 3: Purpose
 Tribology is largely unknown, so this is
 an informative chapter
 Term coined in England in 1960’s
 Provide knowledge of the basis of
 tribology and how tribological properties
 of materials affect material selection



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Chapter 3: Introduction
  Automobile: Highlights the importance of
  friction, wear, and lubrication
  All moving systems must deal with these
  Wear is #1 cause of ‘auto death’
  Examples of wear:
     Earthquakes
     Arthritis
     Joint replacements
  5% of US GDP
  Chapter presents fundamentals of friction and
  wear
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3.1 History of Friction & Wear
  F=mN
  Devices used to study friction
     Fig 3-1
  Friction consists of forces on an object
  resisting motion when motion is
  attempted
  Static and kinetic friction


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3.2 Contact Mechanics
 Role of physicists, chemists, mechanical
 engineers, and materials scientists
 Contact mechanics (Fig 3-3)
     Form of engineering concentrating on
      stresses & deformation at the microscopic
      areas that form real areas of contact
      between solids
 Hertzian stress equations (Fig 3-4)

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3.3 Friction
   Causes
      Rugosities (Fig 3-5)
      F = Fa + Fp + Fs + Fn
   m = S/P
      Never = 0
      Rubber materials ?
   Measurement (Fig 3-6)
      Types (Fig 3-7)
      Fig 3-8 – Friction coefficients v. normal force
      Fig 3-9 – Stribeck curve
      Hydrodynamic, boundary, elastohydrodynamic
       lubrication                                       7
3.3 Friction
  Measurement
     Lambda ratio
     Film thickness
  Significance
     Value of controlled friction
     Overall objective of tribology is control of friction
      and wear
     Testing under simulated service conditions
      produces the most valid data
  Designs
     Continuous processes
     Rotating machinery
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3.4 Definition of Wear
  ASTM G 40
     Damage to a solid surface, generally involving
      progressive loss of material, due to relative motion
      between the surface and a contacting
      substance(s)
  Components wear or erode in different ways
     13 forms of wear
     5 forms of erosion




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3.5 Types of Friction
  Solid Particle – surfaces impinged by solid
  particles carried by a gas
     ASTM G 76
     Erosion (particle) = aMV2(r)(P)
  Slurry Erosion – slurry moves across a surface
     ASTM G 75 – Miller number (1 to 1000)
  Liquid Impact – impact of very high-velocity
  liquid droplets (ASTM G 73)
  Liquid Erosion – liquid impinging or moving
  along the surface of a solid
  Cavitation – implosion of bubbles near a
  surface                                        10
3.6 Types of Wear
 Sliding Contact             Surface Fatigue
     Adhesive                  Pitting
        Archard Eqn.           Impact Wear
     Galling                   Spalling
     Scuffing & Scoring        Brinelling
     Oxidative ear          Summary
     Fretting                  Material characteristics
 Abrasion                        that control wear start
                                 at the atomic level and
     Low-stress Abrasion
                                 progress to the
     High-stress Abrasion       surface texture of a
     Gouging Abrasion           part
     Polishing                 Fig 3-27
                                                     11
3.7 Bearings
  Plain – two-body sliding systems
  Rolling Contact – three-body tribosystems
  Bearing Life              16,700
                            L10        C P 
                                    k
     Rate life L10                 N
  Selection
     Precision – five grades
     Application – Fig 3-29 – 3-31
     Internal Clearance - classes


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3.7 Bearings
  Selection
     Cage materials
     Shaft and housing fits – 7 criteria
     Materials
        High-carbon, chromium steel
        Stainless steel
        Tool steel
        Silicon nitride ceramic



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3.8 Lubricants
  Definition – substances that separate rubbing
  surfaces and readily shear while adhering to surfaces
  Categories
     Oil
        Mineral v. synthetic
        Viscosity index (VI)
        Detergents
     Greases
        90% oil
        Three main categories based on thickener type
        DN value
     Solid-Film
        Organic v. inorganic
        Choose based on apparent pressure

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Summary & Critical Concepts
 Wear & friction are not developed as
 principles or laws yet
     They are system effects
 Different modes of wear
 A number of ASTM standards for testing
 Lubrication reduces friction and wear



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Case History
  Tenters used in Plastic Film Making
     Drafting and tentering films




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