# 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
 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
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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
   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
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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