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.

Definition and Scope of Tribology

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

• Conformal fluid film bearings:

Bearing Selection

• Non-Conformal fluid film 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

Bearing Selection

Bearing Selection

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

Bearing Selection

Bearing Selection

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.