Friction and Wear in Engineering

⚙️Friction and Wear in Engineering Unit 5 – Lubrication Principles in Engineering

Lubrication principles are crucial in engineering, reducing friction and wear between moving surfaces. By introducing a lubricant film, engineers can minimize energy loss, extend component life, and improve system efficiency across various applications. From automotive engines to industrial machinery, proper lubrication is essential. Understanding lubricant properties, selection criteria, and application methods enables engineers to design effective lubrication systems tailored to specific operating conditions and performance requirements.

Key Concepts and Terminology

  • Lubrication reduces friction and wear between surfaces in relative motion by introducing a lubricant film
  • Lubricant is a substance (oil, grease, or solid) that separates surfaces and reduces friction, heat, and wear
  • Viscosity measures a fluid's resistance to flow and shear stress, affecting its ability to form a lubricating film
  • Additives enhance lubricant properties (antioxidants, anti-wear agents, corrosion inhibitors)
  • Tribology studies interacting surfaces in relative motion, including friction, lubrication, and wear
  • Wear mechanisms include adhesive wear, abrasive wear, fatigue wear, and corrosive wear
  • Boundary lubrication occurs when surfaces are in direct contact with minimal lubricant film
  • Hydrodynamic lubrication occurs when surfaces are completely separated by a thick lubricant film

Types of Lubrication

  • Fluid lubrication uses a liquid or gas lubricant to separate surfaces and reduce friction
    • Hydrodynamic lubrication relies on the motion of surfaces to generate a pressurized lubricant film
    • Hydrostatic lubrication uses an external pump to supply pressurized lubricant between surfaces
  • Solid lubrication uses a solid material (graphite, molybdenum disulfide) to reduce friction and wear
  • Boundary lubrication occurs when surfaces are in direct contact, with a thin molecular layer of lubricant
  • Mixed lubrication is a combination of fluid and boundary lubrication, with partial surface contact
  • Elastohydrodynamic lubrication (EHL) occurs in non-conforming surfaces under high pressure and elastic deformation
  • Grease lubrication uses a semi-solid lubricant consisting of a base oil and thickener

Properties of Lubricants

  • Viscosity is the most important property, affecting the formation and thickness of the lubricant film
    • Kinematic viscosity is the ratio of dynamic viscosity to density, measured in centistokes (cSt)
    • Viscosity index (VI) indicates the change in viscosity with temperature
  • Pour point is the lowest temperature at which a lubricant flows, important for low-temperature applications
  • Flash point is the lowest temperature at which a lubricant's vapors ignite, indicating thermal stability
  • Oxidation stability measures a lubricant's resistance to chemical degradation and sludge formation
  • Thermal conductivity affects a lubricant's ability to dissipate heat and prevent overheating
  • Additives improve specific properties (anti-wear, extreme pressure, corrosion inhibition)
  • Compatibility with materials (seals, coatings) is crucial to prevent leakage and degradation

Lubrication Mechanisms

  • Fluid film lubrication separates surfaces with a pressurized lubricant film, reducing friction and wear
    • Hydrodynamic lubrication generates a film through the motion of surfaces and viscous forces
    • Elastohydrodynamic lubrication (EHL) occurs in non-conforming surfaces under high pressure and elastic deformation
  • Boundary lubrication relies on chemical interactions between the lubricant and surfaces to reduce friction
    • Adsorption of polar molecules forms a protective layer on surfaces
    • Extreme pressure (EP) additives react with surfaces to form a sacrificial film under high loads
  • Mixed lubrication combines fluid film and boundary lubrication, with partial surface contact
  • Stribeck curve illustrates the relationship between friction coefficient, viscosity, speed, and load
  • Squeeze film lubrication occurs when surfaces approach each other, trapping and pressurizing the lubricant

Lubricant Selection and Application

  • Consider operating conditions (temperature, speed, load) when selecting a lubricant
  • Viscosity should be high enough to maintain a lubricant film but low enough to minimize friction
  • Additives are chosen based on specific requirements (anti-wear, extreme pressure, corrosion inhibition)
  • Compatibility with materials (seals, coatings) is essential to prevent leakage and degradation
  • Application methods include oil baths, splash lubrication, forced circulation, and oil mist
    • Oil baths are simple and cost-effective for low-speed applications
    • Splash lubrication uses the motion of components to distribute the lubricant
  • Grease lubrication is suitable for low-speed, high-load applications and sealed systems
  • Solid lubricants are used in extreme temperatures, vacuum conditions, or where contamination is a concern
  • Lubricant dispensing systems (pumps, nozzles) ensure proper delivery and distribution

Lubrication System Design

  • Identify lubrication requirements based on component design, operating conditions, and maintenance needs
  • Select appropriate lubricant type (oil, grease, solid) and properties (viscosity, additives)
  • Determine lubrication method (splash, forced circulation, oil mist) and system layout
    • Reservoir stores and cools the lubricant, with filters and strainers to remove contaminants
    • Pumps circulate the lubricant through the system, with pressure relief valves for protection
  • Size and locate components (reservoir, pumps, filters) based on flow rate and pressure requirements
  • Incorporate monitoring and control systems (sensors, alarms) to ensure proper operation and maintenance
  • Consider lubricant storage and handling, including filling, draining, and disposal procedures
  • Conduct failure mode and effects analysis (FMEA) to identify and mitigate potential failures
  • Optimize system design for efficiency, reliability, and maintainability

Testing and Analysis Methods

  • Viscosity testing measures a lubricant's resistance to flow and shear stress using viscometers
    • Kinematic viscosity is measured using capillary or rotational viscometers (ASTM D445)
    • Dynamic viscosity is measured using rotational viscometers (ASTM D2983)
  • Spectroscopic analysis (infrared, atomic emission) detects contaminants, additives, and wear metals
  • Particle counting and classification (ISO 4406) assess the cleanliness of lubricants and systems
  • Ferrous density testing measures the concentration of ferrous wear particles using magnetometry
  • Acid number (AN) and base number (BN) indicate the lubricant's acidity and alkalinity, respectively
  • Fourier-transform infrared spectroscopy (FTIR) identifies chemical changes and contamination
  • Wear debris analysis examines the size, shape, and composition of wear particles to diagnose wear mechanisms
  • Filterability testing evaluates a lubricant's ability to pass through filters without clogging

Practical Applications and Case Studies

  • Automotive engines use multi-grade motor oils with additives for improved performance and fuel efficiency
    • Low-viscosity oils (0W-20, 5W-30) reduce friction and improve fuel economy
    • High-quality base oils and additives extend oil drain intervals and engine life
  • Industrial gearboxes require gear oils with high load-carrying capacity and anti-wear properties
    • Extreme pressure (EP) additives protect gears under high loads and shock loading
    • Synthetic gear oils provide improved thermal and oxidation stability for extended service life
  • Compressors use synthetic lubricants (PAOs, PAGs) for high-temperature stability and reduced deposits
  • Steam turbines require turbine oils with excellent oxidation stability, demulsibility, and air release properties
    • Rust and oxidation inhibitors protect against corrosion and sludge formation
    • Demulsifiers promote rapid water separation to prevent emulsions and corrosion
  • Hydraulic systems use hydraulic fluids with good viscosity-temperature behavior and filterability
    • Anti-wear additives (zinc dialkyldithiophosphates) reduce wear in pumps and valves
    • Fire-resistant hydraulic fluids (water-glycol, phosphate esters) are used in high-risk applications
  • Food processing equipment requires food-grade lubricants (NSF H1) to prevent contamination
  • Aerospace applications use synthetic lubricants (esters, PAOs) for low-temperature fluidity and high-temperature stability


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© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.