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Tribology, Wear and Friction: An Overview

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Tribology is the study of the science and technology of interacting surfaces in relative motion and encompasses the study and application of friction, wear, lubrication and related design aspects. To further understand tribology, it is important to understand the definitions behind friction, wear and lubrication.

Published in: Engineering
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Tribology, Wear and Friction: An Overview

  1. 1. ©NoriaCorporation Tribology: An Overview • Define tribology. • Describe the types and results of friction. • List the three types of lubricant films. • List the six functions of a lubricating oil. • Describe the different lubrication regimes and when they occur. • Define and explain the differences between hydrodynamic and elastohydrodynamic lubrication.
  2. 2. ©NoriaCorporation Tribology Defined Tribology Ref:06006MLI Tribology The study of lubrication, friction and wear Impacted by • Motion (Sliding, Rolling) • Speed • Temperatures • Load • Operating Environment • Lubricant and Application Method
  3. 3. ©NoriaCorporation Friction – Two Surface Interaction Tribology Ref:ECF,AISE,GT,Rabinowitz05999MLI Friction: Resistance to sliding, a property of the interface between two solid bodies in contact Machines/components affected • Piston/Cylinders • Swash Plates • Journal Bearings • Gears • Cams • Rolling-element Bearings Friction can have positive and negative effects
  4. 4. ©NoriaCorporation Types of Friction Tribology Ref:Gunther,RT,GT06007MLI Sliding Friction Force that resists relative motion between sliding solid bodies whose opposing surfaces are clean and dry. Rolling Friction Force that resists relative motion between two solid bodies when one or both roll over the surface of the other. Welding (wear) Sliding Load Stationary Rolling Contact Lubricant Film Mixed Film Free-moving Body Stationary Body Free-moving Body Stationary Body Lubricant Film Boundary Contact Free-moving Body Maximum Velocity Zero Velocity Stationary Body Fluid Layers Boundary Friction Force that resists relative motion between two solid bodies whose opposing surfaces are wetted by a lubricant but barely separated by the lubricant film. Mixed-film Friction Force that resists relative motion between two solid bodies whose opposing surfaces are partially separated by a full fluid. Fluid Friction Force that resists the flow of liquids or gases. Such a force opposes the sliding action, one over the other, of the molecular layers of the fluid. (Shearing) Dry Wet
  5. 5. ©NoriaCorporation Results of Friction Tribology Ref:06008MLI Can Cause Wear The breakdown of surfaces within a machine due to surface interaction. May result in the release of wear particles in the system. Some types of wear: • Adhesive • Two-body abrasion • Three-body abrasive wear • Surface fatigue Causes Heat Generation Some impacts of excessive heat: • Loss of viscosity • Increase additive depletion • Increase oil oxidation • Increase varnish potential
  6. 6. ©NoriaCorporation Six Functions of a Lubricant Tribology Ref:JCF,GT00377MLI A Lubricant Functions to… Control Friction Reduces heat generation and energy consumption Control Wear Reduces mechanical and corrosive wear Control Temperature Absorbs and transfers heat Control Corrosion Protects surfaces from corrosive substances Transmit Power In hydraulics, transmits force and motion Control Contaminants Transports particles and other contaminants to Filters/Separators Note: Depending on the application, lubricants have other functions as well, including serving as a seal (e.g., containing combustion gases), traction medium (fluid couplings, clutching), shock dampening and control of exhaust stream emissions.
  7. 7. ©NoriaCorporation Lubricant Films Tribology Ref:JCF,Godfrey00269MLI • Hydrodynamic Lubrication Sliding • Elastohydrodynamic Lubrication Rolling - Deformation to carry the load Fluid FilmsSolid Suspension Films • Borate • Molybdenum Disulfide • Graphite • PTFE • Oiliness Agents – Fatty Acids • Zinc Dialkyldithio- phosphate (AW) • Tricresyl Phosphate (AW) • Sulfur Phosphorus (EP) Chemical Oil Films Thick Film Metal Bulk Fluid Boundary Film Surfaces protected by viscosity - no wear Friction from viscous drag only If surfaces protected by chemical oil film and/or solid lubricants - minor wear Friction from viscous drag and mechanical contact Boundary
  8. 8. ©NoriaCorporation Lubrication Regimes Tribology Ref:Noria03410MLI Boundary Lubrication Full-fluid Film Lubrication Protection essentially dependent on boundary anti-wear film Surfaces separated by bulk lubricant film, affected by viscosity, load and speed Starts, stops, slow moving conditions and shock load conditions Full-speed Conditions (most desired) Mixed-film Lubrication Both the bulk lubricant and boundary film play a role Peaks and valleys share load equallyMajority of load rests on surface peaks> 90% of load rests on surface peaks Boundary Film (anti-wear additive protection) Bulk Lubricant (viscosity protection) Legend Asperities (microscopic projections on metal surfaces)
  9. 9. ©NoriaCorporation Hydrodynamic Lubrication (Sliding) Tribology REF:JCF,Kingsbury00002MLI Contact Pressures 100 psi – 300 psi Increasing Oil Film Thickness Speed-ups, oil cooling, load reduction Decreasing Oil Film Thickness Coast-downs, oil heating, load increase Oil Film LoadLoad Rotation Oil Film Pivot Pivot • Film thicknesses vary from 5-200 microns depending on speed, viscosity and load. Typical oil films are 5-20 microns • At low speeds, starts and stops boundary conditions exist (metal-to-metal contact) • Particle and moisture contamination corrupt lubricant film • Example applications include journal bearings, piston-bore contacts, sliding cam contacts and swashplate contacts Shoe Aligning Ring Shaft Journal Bearing Bearing Shaft Load Oil wedge provides hydrodynamic lift Radial Tilting Pad Bearing
  10. 10. ©NoriaCorporation Elastohydrodynamic Lubrication - EHL/EHD (Rolling) Tribology REF:JCF00001MLI • Contact surfaces elastically deform to enlarge contact area with oil film less than 1 micron and up to 500,000 psi • Particle and moisture contamination disrupt lubricant film protection • EHL and EHD are acronyms for elastohydrodynamic lubrication and are used interchangeably • Oil film usually less than 1 micron. If you want to increase film thickness 2X, you have to increase viscosity 4X Cam Roller Load Ball Bearing Rolling Elastic Contact Oil Bearing • Highly concentrated loads create a wave of metal ahead of the load zone • Rolling contacts can occur in rolling-element bearings, meshing of gears (pitch line zone), rolling cam follower against the cam nose and sprocket and chain contacts • Rolling-element and mating surface (race) elastically deform to enlarge contact area
  11. 11. ©NoriaCorporation Stribeck Curve - Film Thickness Tribology Ref:PracticalHandbookofMachineryLubrication,p.1405802MLI FILMTHICKNESS BOUNDARY MIXED-FILM FULL-FLUID-FILM LUBRICATION No Film Due to Inadequate Speed or Viscosity Ideal • Increase the Speed • Increase the Viscosity • Decrease the Load Film Thickness Increases N = Shaft Rotational Speed P = Load Z = Viscosity ZN P X Axis= FULL-SPEED CONDITIONS STARTS, STOPS, SHOCKLOADS, DIRECTION CHANGES, SLOW TO INTERMEDIATE SPEEDS
  12. 12. ©NoriaCorporation Stribeck Curve - Coefficient of Friction Tribology Ref:PracticalHandbookofMachineryLubrication,p.1405803MLI FULL-FLUID-FILM LUBRICATIONMIXED-FILMBOUNDARY FULL-SPEED CONDITIONS STARTS, STOPS, SHOCKLOADS, DIRECTION CHANGES, SLOW TO INTERMEDIATE SPEEDS COEFFICIENTOFFRICTION N = Shaft Rotational Speed P = Load Z = Viscosity ZN P X Axis = Ideal Ratio of the force required to overcome friction to the load or pressure imposed between the surfaces of opposing bodies. Metal on Metal Contact Internal Fluid Friction
  13. 13. ©NoriaCorporation Stribeck Curve - Wear Tribology Ref:PracticalHandbookofMachineryLubrication,p.1405804MLI FULL-FLUID-FILM LUBRICATIONMIXED-FILMBOUNDARY FULL-SPEED CONDITIONS STARTS, STOPS, SHOCKLOADS, DIRECTION CHANGES, SLOW TO INTERMEDIATE SPEEDS WEAR Ideal Wear Controlled by Both Viscosity and Boundary Lubrication Film Wear Controlled by Viscous Separation • Increase the Speed • Decrease the Load • Increase the Viscosity N = Shaft Rotational Speed P = Load Z = Viscosity ZN P X Axis = Optimal place to be is slightly to right
  14. 14. ©NoriaCorporation Stribeck Curve Tribology Ref:PracticalHandbookofMachineryLubrication,p.1400037MLI Wear Coefficient of Friction Film Thickness Factors Influencing Hydrodynamic Lubrication Full-fluid-film LubricationMixed-filmBoundary Full-speed conditions Starts, stops, shockloads, direction changes, slow to intermediate speeds No film due to inadequate speed or viscosity Wear controlled by both viscosity and boundary lubrication film Wear controlled by viscous separation Z = Viscosity N = Shaft rotational speed P = Load X Axis = ZN P Ideal
  15. 15. ©NoriaCorporation Tribology • What is tribology? • What are the different types of friction? • What are the results of friction? • What are the three types of lubricant films? • List the six functions of a lubricating oil. • Describe the different lubrication regimes and when they occur. • Describe hydrodynamic and elastohydrodynamic lubrication. • What is the Stribeck curve and what does it describe? Summary

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