Leonardo Centre for Tribology

What is Tribology?

Tribology is defined as the science of interacting surfaces in relative motion. The word tribology comes from the Greek tribos, meaning rubbing. In any machine there are lots of component parts that operate by rubbing together, such as bearings, gears, cams and tappets, tyres, brakes, and piston rings. All of these components have two surfaces which come into contact, support a load, and move with respect to one another. Depending on the application, it can be desirable to have either low friction; to save energy, or high friction; as in the case of brakes. Usually we don't want the components to wear so they are lubricated to avoid this.

 

The study of friction, wear, lubrication and contact mechanics are all important parts of tribology. Related aspects are surface engineering (the modification of a component's surface to improve its function, for example by applying a surface coating), surface roughness, and rolling contact fatigue (where repeated contact causes fatigue to occur).

When Two Surfaces are Pressed Together

Surfaces may look smooth, but on a microscopic scale they are rough. When two surfaces are pressed together, contact is made at the peaks of the roughness or asperities. The real area of contact can be much less than the apparent or nominal area. At the points of intimate contact, adhesion, or even local welding, can take place. If we want to slide one surface over the other then we have to apply a force to break those junctions.

The Force of Friction

Friction force is the resistance encountered when one body moves relative to another body with which it is in contact. Static friction force is how hard something must be pushed to make it move, whilst dynamic friction force is how hard it must be pushed to maintain motion. The ratio of the frictional force F to the normal force P is called the coefficient of friction.

 

Usually we want low friction (in a car engine for example) so we do not waste excessive energy getting it moving. But in the same case we need high friction, in brakes for example. Friction is also important for car tyres to grip the road and between shoes and the ground for walking.

Keeping the Surfaces Apart - Lubrication

If we put a layer of oil between two surfaces then we can separate them and easily slide one over the other with reduced friction and wear. Mineral oils are the most common lubricants, but other low shear strength materials are also used, such as graphite, PTFE, and soft metals like lead or gold.

 

The selection of the best lubricant and understanding the mechanism by which it acts to separate surfaces in a bearing or other machine component is a major area for study in tribology.

When Things Wear Out

If one surface is slid over another then the asperities come into contact and there is a possibility that wear can occur. The breaking of all these little junctions can cause material removal (adhesive wear), or the asperities of a hard surface can plough grooves in a soft surface (abrasive wear).

 

Wear is usually unwelcome; it leads to increased clearances between moving components, increased mechanical loading and maybe even fatigue. But in grinding and polishing processes the generation of high wear rates is desirable.

 

As well as adhesive and abrasive wear, there are other mechanisms whereby material can be removed from a surface. Erosive wear occurs when particles (or even water droplets) strike a surface and break off a bit of the material. Hard particles can become trapped in contacts and cause material to be removed from one or both of the surfaces. One of the main reasons for frequent change of car engine oil is that it becomes contaminated with hard debris particles that can wear out the engine components.

Stress and Strain at the Contact

The design of rolling bearings and gears is such that the load is supported on a small area. This leads to high stresses (about the highest stresses we find in any branch of engineering) over small areas of the components. This can cause high friction, wear and contact fatigue. Contact mechanics is therefore an important part of tribology.

 

The analysis of contact stress is frequently difficult. Simple component geometries can be analysed using hand calculations, but more complex component shapes frequently require analysis by numerical methods.

Tribology Through The Centuries

Early civilisations developed quite sophisticated tribological devices such as potter’s wheels, door hinges and wheeled carriages. The carvings on the tomb at Saqqara show an Egyptian tribologist bending down to lubricate the sled that carries a statue of Ti (c. 2400 BC).

Military engineers rose to prominence in the days of the Roman Empire by devising both war machinery and methods of fortification using tribological principles. War ships (c. 50 AD) recovered from Lake Nemi near Rome contained bronze balls and rollers used to support rotating platforms.

 

It was the renaissance engineer-artist, Leonardo da Vinci (1452-1519), celebrated engineer, painter, and sculptor, who discovered that the tangential force of friction between moving solid bodies is proportional to the normal force. His notebooks show many designs for moving parts and machines that show a remarkable similarity to those in use today.

 

Several of the innovative designs behind John Harrison's (1693-1776) marine chronometers were based on a tribological understanding of the moving parts. He designed and built clocks that required no lubrication. The properties of oil could not be kept constant over long periods of time or with climate changes, so the removal of the requirement for lubrication allowed the clocks to remain accurate over sea journeys lasting many months.

 

The coming of the computer age has provided new challenges for tribologists. The interface between the reading head and the magnetic disk in a computer hard disk requires careful design and lubrication to minimise friction and reduce the likelihood of disk crashes and damage.

Want to Know More?

There are several excellent books on the subject of tribology (take a look at our book list - especially the undergraduate texts section).