Tribology
What is tribology?
Tribology comprises the fields of friction, wear and lubrication. It aims at the functional, economical and ecological optimization of motion systems. The implementation of tribological knowledge results in a reduction of wear and an optimization of friction systems.
The operational reliability of machines and installations is increased, production costs are reduced, resources and energy are saved and emissions are decreased.
Tribology is the science and technology of surfaces interacting in a relative motion. It comprises the entire field of friction and wear including lubrication. It includes corresponding interfacial interactions between solids as well as between solids and liquids, consistent materials or gases. (See also DIN 50323, Part 1)
The meaning of tribology
By friction and wear, the respective national economies of the industrialized countries suffer annual losses amounting to approx. 5 % of the gross national product which are, e.g., for Germany approx. 35 billion Euros per year. When implementing the tribological knowledge already available, it would be possible to save 5 billion Euros per year of such losses. This savings capacity can even be increased by further tribological research.
The intensified consideration of tribological knowledge results in important savings regarding energy and material consumption, production and maintenance. Energy and raw material resources are saved, environmental damage is avoided and protection of labour is improved. The application of tribology to technology comprises all fields of the development, construction, fabrication and maintenance of mechanical motion systems in different branches of industry and sectors of the economy such as: mechanical engineering, production engineering, motive power engineering and materials-handling technology, automotive and motor industry, rail vehicle technology, aeronautics and space operations, basic industry, structural engineering, precision mechanics, power supply, EDP technology, medical technology.
Tribologyis the science and engineering of interacting surfaces in relativemotion. It includes the study and application of the principles offriction,lubricationandwear. Tribology is a branch ofmechanical engineering.
Fundamentals
The tribological interactions of a solid surface's exposed face with interfacing materials and environment may result in loss of material from the surface. The process leading to loss of material is known as "wear". Major types of wear includeabrasion, friction (adhesionandcohesion),erosion, andcorrosion. Wear can be minimized by modifying the surface properties of solids by one or more of "surface engineering" processes (also calledsurface finishing) or by use oflubricants(for frictional or adhesive wear).
Estimated direct and consequential annual loss to industries in the USA due to wear is approximately 1-2% of GDP. (Heinz, 1987). Engineered surfaces extend the working life of both original and recycled and resurfaced equipments, thus saving large sums of money and leading to conservation of material, energy and the environment. Methodologies to minimize wear include systematic approaches to diagnose the wear and to prescribe appropriate solutions. Important methods include:
- Terotechnology, where multidisciplinary engineering and management techniques are used to protect equipment and machinery from degradation (Peter Jost, 1972)
- Horst Czichos's systems approach, where appropriate material is selected by checking properties against tribological requirements under operating environment (H. Czichos,1978)
- Asset Management by Material Prognosis - a concept similar to terotechnology which has been introduced by the US Military (DARPA) for upkeep of equipment in good health and start-ready condition for 24 hours. Good health monitoring systems combined with appropriate remedies at maintenance and repair stages have led to improved performance, reliability and extended life cycle of the assets, such as advanced military hardware and civil aircraft.
In recent years, micro- andnanotribologyhave been gaining ground. Frictional interactions in microscopically small components are becoming increasingly important for the development of new products inelectronics, life sciences,chemistry, sensors and by extension for all modern technology.
[edit]Friction regimes
A typical Stribeck curve obtained by Martens
Friction regimes for sliding lubricated surfaces have been broadly categorized into:
- Solid/boundary friction
- Fluid friction
- Mixed friction
on the basis of the “Stribeck curve”. These curves clearly show the minimum value of friction as the demarcation between full fluid-film lubrication and some solid asperity interactions.
Stribeck and others systematically studied the variation of friction between two liquid lubricated surfaces as a function of a dimensionless lubrication parameter ηN/P, where η is the dynamic viscosity, N the speed (e.g. revolutions per minute of a bearing) and P the load projected on to the geometrical surface.[1]
The “Stribeck-curve” has been a classic teaching element in tribology classes.[2]
[edit]History
Tribological experiments suggested by Leonardo da Vinci
The history of tribology is surveyed by Duncan Dowson in his book "History of Tribology (2nd edition)".[3]This comprehensive book covers developments from prehistory, though early civilizations (Mesopotamia, Egypt) and finally the key developments up to the end of the previous century.
Historically,Leonardo da Vinci(1452–1519) was the first to enunciate two laws offriction.[4]According to da Vinci, the frictional resistance was the same for two different objects of the same weight but making contacts over different widths and lengths. He also observed that the force needed to overcome friction is doubled when the weight is doubled, but his findings remained unpublished in his notebooks.[5][6]The classic rules were rediscovered byGuillaume Amontons(1699) and were further developed byCharles-Augustin de Coulomb(1785).
The first reliable test on frictional wear was carried out byCharles Hatchett(1760–1820) using a simple reciprocating machine to evaluate wear ongold coins. He found that compared to self-mated coins, coins with grits between them wore at a faster rate.
[edit]Stribeck curve
The "Stribeck curve" or "Stribeck–Hersey curve" (named afterRichard Stribeck[7][8][9], who heavily documented and established examples of it, and Mayo D. Hersey[10][11]), used to categorize the friction properties between two surfaces, was developed in the first half of the 20th century. The research of Professor Richard Stribeck (1861–1950) was performed in Berlin at the Royal Prussian Technical Testing Institute (MPA, now BAM). Similar work was previously performed around 1885 by Prof. Adolf Martens (1850–1914) at the same Institute and in the mid 1870s by Dr. Robert H. Thurston[12][13]at theStevens Institute of Technologyin the U.S. Prof. Dr. Thurston was therefore close to establishing the “Stribeck curve”, but he presented no “Stribeck”-like graphs, as he evidently did not fully believe in the relevance of this dependency. Since that time the “Stribeck-curve” has been a classic teaching element in tribology classes.[2]
The graphs of friction force reported by Stribeck stem from a carefully conducted, wide-ranging series of experiments onjournal bearings. Stribeck systematically studied the variation of friction between two liquid lubricated surfaces.[1]His results were presented on 5 December 1901 during a public session of the railway society and published on 6 September 1902. They clearly showed the minimum value of friction as the demarcation between full fluid-film lubrication and some solid asperity interactions. Stribeck studied different bearing materials and aspect ratios D/L from 1:1 to 1:2. The maximum sliding speed was 4m/s and the geometrical contact pressure was limited to 5 MPa. (These operating conditions were related to railway wagon journal bearings.)
The reason why the form of the friction curve for liquid lubricated surfaces was later attributed to Stribeck, although both Thurston and Martens achieved their results considerably earlier, (Martens even in the same organization roughly 15 years before), may be because Stribeck published in the most important technical journal in Germany at that time,Zeitschrift des VereinsDeutscherIngenieure(VDI, Journal of German Mechanical Engineers). Martens published his results “only” in the official journal of theRoyal Prussian Technical Testing Institute, which has now become BAM. The VDI journal, as one of the most important journals for engineers, provided wide access to these data and later colleagues rationalized the results into the three classical friction regimes. Thurston however, did not have the experimental means to record a continuous graph of the coefficient of friction but only measured the friction at discrete points; this may be the reason why the minimum in the coefficient of friction was not discovered by him. Instead, Thurston's data did not indicate such a pronounced minimum of friction for a liquid lubricated journal bearing as was demonstrated by the graphs of Martens and Stribeck.
[edit]Jost Report
The termtribologybecame widely used followingThe Jost Reportin 1966, in which huge sums of money were reported to have been lost in the UK annually due to the consequences of friction, wear and corrosion. As a result several national centres for tribology were created in theUK. Since then the term has diffused into the international engineering field and many specialists now claim to be tribologists.
There are now numerous national and international societies, such as theSociety for Tribologists and Lubrication Engineers(STLE) in the USA and theInstitution of Mechanical Engineers' Tribology Group(IMechE Tribology Group) in the UK or the German Society for Tribology (GesellschaftfürTribologie,
Most technical universities have a group working on tribology, often as part of theirmechanical engineeringdepartments. The limitations in tribological interactions are however no longer mainly determined by mechanical designs, but rather by material limitations so the discipline of tribology now counts at least as many materials engineers, physicists and chemists as it does mechanical engineers.
[edit]Etymology
The word 'tribology' derives from the Greek rootτριβ-of the verbτρίβω-tribo"Ι rub", and the suffix-logy, and was coined by the British physicistDavid Tabor.
[edit]Applications
The study of tribology is commonly applied inbearingdesign but extends into almost all other aspects of modern technology, even to such unlikely areas ashair conditionersand cosmetics such aslipstick,powdersand lipgloss.
Any product where one material slides or rubs over another is affected by complex tribological interactions, whether lubricated like hip implants and other artificial prostheses, or unlubricated as in high temperature sliding wear in which conventional lubricants cannot be used but in which the formation ofcompacted oxide layer glazeshave been observed to protect against wear.
Tribology plays an important role in manufacturing. In metal-forming operations, friction increases tool wear and the power required to work a piece. This results in increased costs due to more frequent tool replacement, loss of tolerance as tool dimensions shift, and greater forces required to shape a piece. The use of lubricants which minimize direct surface contact reduces tool wear and power requirements.
What is Tribology?Tribology is the science of friction, lubrication, and wear.There is little understanding of tribology; consequently, it requires a systematic approach and a broad knowledge of Chemistry, Fluid Mechanics, Thermodynamics, Metallurgy, and Solid Mechanics.
In metalforming, tribology is the critical process control factor. Friction transmits the forces of the press to the work piece to achieve the desired strains. Lubrication is used primarily to control friction and prevent adhesion. It may also function to reduce friction and prevent corrosion.
Adhesion is the most prevalent and problematic form of wear. Build up of the work piece material on the die causes uncontrolled friction and may reduce clearances causing high local stresses and part or die fracture.