Download - I-Introduction to Tribology
Definition and Scope of Tribology
• Tribology is defined as the science and technology of interacting surfaces in relative motion.
- The term is derived from the Greek word tribos, meaning rubbing, and logos, meaning principle or science.
- It is the study of the friction, wear, and lubrication, involved at moving contacts.
- As an interdisciplinary field of study, it encompasses physics, chemistry, metallurgy, materials science, rheology, lubrication, elasticity, viscoelasticity, elastohydrodynamics, thermodynamics and heat transfer.
Importance of Tribology
• Usual tasks:
Reduction of friction and wear to conserve energy,
enabling faster and more precise motions,
increasing productivity, and reduced maintenance.
• It is involved in:
Lubricant formulation, industrial operations,
aerospace and transportation equipment, material
shaping and machining, computers and electronic
devices, power generation, etc..
Friction
• Friction is a tangential force that resists relative motion
between two surfaces in contact under a normal load.
• Causes: Surface roughness and adhesion.
• Effects: Wear and heat.
Wear
• Wear may be defined as the progressive removal of material from a surface in bearing under conditions of sliding, rolling or fretting.
• Types of wear:
Adhesive wear is due to shearing at points of
contact or asperities that undergo adhesion or
cold welding;
Abrasive wear is caused by a hard material
sliding and cutting grooves on a softer one;
Wear
Pitting wear is a result of surface
fatigue caused by varying local
stress over a large number of cycles.
Erosive wear is caused by the
impingement of fluids and/or solids
that removes surface material.
Corrosive wear is the result of the
removal of surface material by
chemical action or by a combination
of chemical action and relative motion.
Lubrication
• Lubrication is the process of interposing a film of any solid, liquid or gaseous substance between contacting surfaces undergoing relative motion.
• Lubrication regimes: - Dry sliding – Absence of a fluid film. Solid film or no lubricant at all. Contact
between the asperities of the rubbing surfaces. Load capacity results from the yield of the softer material high spots.
Lubrication
- Fluid-film lubrication – A fluid film completely separates the moving surfaces. The fluid pressure is normally generated by the following actions:
Hydrodynamic: A film is drawn into a converging wedge-shaped zone by the self-acting pumping action of a moving surface.
Squeeze-film: The fluid pressure is generated by oscillating loads or bearing surfaces.
Externally-pressurized: An external pumping source generates pressure in the fluid.
Lubrication
• Lubrication regimes (cont.): - Elastohydrodynamic lubrication (EHL) – A combination of hydrodynamic lubrication
with the significant elastic deformation of the surfaces caused by the very high film
pressure. The following regimes exist in EHL:
Hard EHL: Relates to materials of high elastic modulus in nonconformal contacts.
Soft EHL: Relates to soft materials having low elastic modulus in bearings under heavy loads and low speeds.
- Boundary (or mixed-film) lubrication – Insufficient oil-film pressure. The lubricant is
unable to separate the opposing surfaces completely. The reduced film thickness
may permit momentary dry contact between surfaces asperities.
Bearing Selection
• Common classes of bearings: - Fluid-film bearings;
- Dry bearings;
- Semilubricated sliding bearings;
- Rolling element bearings.
Bearing Selection
• Preliminary selection: Load, speed and size relations
using ESDU
(Engineering Sciences Data Unit).
Journal bearing type: Except for
rolling-element bearings, curves
are drawn for bearings with width
equal to diameter. A medium-
viscosity mineral oil is assumed
for hydrodynamic bearings.
Bearing Selection
• Preliminary selection: Load, speed and size relations
using ESDU
(Engineering Sciences Data Unit).
Thrust bearing type: Except for
rolling-element bearings, curves
are drawn for typical ratios of
inside diameter to outside
diameter. A medium-viscosity
mineral oil is assumed for
hydrodynamic bearings.
Bearing Selection
• Final selection: Mechanical requirements:
- Friction and power loss;
- Speed;
- Load;
- Life;
- Lubrication;
- Space requirement.
Environmental conditions:
- Temperature range;
- Moisture, dirt and corrosive atmospheres.
Economics:
- Cost;
- Maintenance and life.
Bearing Selection
• Future trends: Dictated by the needs for lower
maintenance in more compact designs operating at
higher speeds and higher temperatures. Examples:
- Dry and semilubricated bearings – Increasing use of plastics and their composites for mild operating conditions.
- Roller bearings – Innovations in greases, solid lubricant films and roller bearing cages self-contained lubricant impregnation. New developments of materials like ceramics, tool steels, and special lubricants will enable higher operating speeds and temperatures.
- Fluid-film bearings – Increasing use of gaseous and low-viscosity liquid lubricants. Development of new bearing materials and lubricants, advanced analysis and design techniques, and improved surface profiles to match extremely thin fluid films.