Tribology Engineering Materials Chapter 3 Chapter 3: Goals

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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 1960s
• 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

• FmN
• 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

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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

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3.6 Types of Wear

• Surface Fatigue
• Pitting
• Impact Wear
• Spalling
• Brinelling
• Summary
• Material characteristics that control wear start
  at the atomic level and progress to the surface
  texture of a part
• Fig 3-27

• Sliding Contact
• Adhesive
• Archard Eqn.
• Galling
• Scuffing Scoring
• Oxidative ear
• Fretting
• Abrasion
• Low-stress Abrasion
• High-stress Abrasion
• Gouging Abrasion
• Polishing

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3.7 Bearings

• Plain two-body sliding systems
• Rolling Contact three-body tribosystems
• Bearing Life
• Rate life L10
• 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