Friction and wear processes have played an essential role since the genesis of our planet. For example, the joint mechanisms in vertebrates or the mucus layer of fish are perfect tribological systems that originated through evolutionary processes.
As early as the Stone Age, humans were familiar with friction and tribology: If one patiently rotated a dry wooden stick back and forth in the hollow of a limb, it was possible to kindle a fire after some time. In this case, friction provided an effective means to an end.
Man began using the first tools such as levers, stone axes and similar tools as far back as 500,000 to 600,000 years ago. Much later, about 5000 to 6000 years ago, the first simple machines were invented with the advent of the fiddle drill or potter's wheel. Yet humans had confronted the phenomena of friction and wear well before that time.
Friction was used to generate
fire. The use of sledges and rollers to reduce friction for the transport of
large loads provide generally well-known examples.
Over the course of history, there are clues that some very early steps were taken to influence friction and wear in a way that benefited human beings.
Man's search for effective lubricants and lubrication technologies has a colourful past going back as far as the recorded history of humankind. As early as 3500 BC, the Chinese made use of the lubricating properties of water. Around 1400 B.C., the Egyptians used animal fats or olive oil mixed with lime powder for their chariots. By 780 B.C., the Chinese had discovered the friction-reducing properties of a mixture consisting of vegetable oils and lead, and more than 100 years ago, the first discoveries that air can serve as a suitable lubricant were made.
Necessity is the mother of invention – a fact humans were aware of as early as the Neolithic Age (4000-1800 B.C.). One can certainly assume that the Sumerians and Egyptians used “lubricants” (bitumen, animal and vegetable oils, water) to reduce friction.
There are numerous examples of this:
Thus, about 3,000 years ago, the Sumerians were already using leather loops in their carriages and inverted forks to support the rigid axles. There is evidence that these friction points were lubricated to minimise friction, thereby reducing the wear and tear as well.
The roller bearing owes its name and its origin to the “rolling bodies” such as roundwood used by the Egyptians during the age of the Pharaohs. This process of tribology made it possible to increase the operating speed, solve the problem of friction and simplify strenuous tasks.
The idea of replacing sliding
friction with rolling friction goes far back in the history of civilisation.
Several Egyptian reliefs depict the transport of huge stone blocks that were designed for the construction of monuments and were slid on tree trunks rather than rollers.
Pictorial records show how greased skids were already being used to transport an Egyptian statue to the grave of Tehuti-Hetep, El-Bersheh around 1880 B.C.:
This picture clearly shows the man pouring lubricant in front of the sliding surfaces. The caption indicates that water was used as a lubricant, while an inscription accompanying the picture mentions olive oil as a lubricant. Tribology as an aid.
Everything works better when lubricated – the ancient Egyptians knew this already.
The subsurface of the Pharaoh's throne was wetted with olive oil. By reducing friction, it was possible to
reduce the manpower needed for pulling by 50 %.
During other historical epochs, mainly vegetable oils and animal fats were used as lubricants and anti-wear agents. It is, however, fair to assume that bitumen (oil seeping out of the ground) was used for to a certain degree, just as it was in antiquity.
The earliest documents mentioning the use of wheels to reduce friction.
The Greek engineer Diades developed what was possibly one of the first roller bearing mechanisms for supporting the battering rams on warships. This was also a deveploment of tribology .
The remains of a rotating
platform originating from a ship of the Emperor Caligula and which was found in
1930 at the bottom of Lake Nemi show that rudimentary bearings have been used
since ancient times.
This platform can be viewed as one of the first examples of a thrust bearing,
i.e. a bearing designed to support direct loads and rotate around its axle.
“The mechanical sciences are among the noblest and most useful because their capacity for diversion allows bodies in motion to perform the work for which they were developed.”
A scientific examination of the topic of tribology began only in recent history, beginning with Leonardo da Vinci, who in 1500 studied the problem of friction by examining the coefficient of friction (static friction) on an inclined plane. Da Vinci determined the value of the friction coefficient f = ¼ and formulated the laws of dry friction.
Da Vinci studied friction on horizontal and inclined planes as well as wear on slide bearings. The results of these investigations were the first and second friction laws of Leonardo da Vinci.
In 1490, Leonardo da Vinci modified the rolling bearing by almost entirely replacing the moving coupling between the two parts with a mechanism with reduced rolling friction. He used balls for this purpose.
He concluded that friction is
reduced when the balls do not touch. He then developed separator elements to
allow the balls to move freely.
Drawings from the friction experiments within the field of tribology . Made by Leonardo da Vinci.
The Renaissance marks the beginning of tribology as a discrete field. A basic understanding of friction and wear, however, do not bring forth any new developments in lubricants.
Guillaume Amontonswas a French physicist and governor of Lille.
He conducted research in the area of mixed friction and found that the
friction force depends on the normal force and that the surface roughness must
be regarded as the cause of friction.
Amontons attributed friction to the mechanical-geometric causes explained in terms of the “interlocking” between irregularities. This theory of interlocking indicates that the form closure of microscopic elevations inhibits the relative motion, and a frictional force in the opposite direction of motion occurs. Amontons derived the coefficient of friction to be f = 1/3.
Amontons’ two laws form the basis of the empirical understanding of tribology (friction theory). In actuality, he rediscovered these laws and presented them in 1699 to the Académie Royale in Paris. The real discoverer was – about two hundred years earlier – Leonardo da Vinci (1452-1519).
According to these laws, the friction force is proportional to the normal force and independent of the ostensible surface area of contact. In addition, it must be noted that the friction force depends not only on adhesion but on abrasion as well. Abrasion has a particularly great effect when the rougher counterpart is made of a harder material or if there is grit in the joint in the form of hard, oxidised metallic particles.
John Theophilius Desaguliers was a French natural philosopher.
Desaguliers developed a tribology-model
to explain friction and attributed friction to the influence of cohesion and
Desaguliers presented a new aspect to the field of tribology : He found that a higher frictional force occurs on better polished surfaces and also showed that two well-polished lead bodies that are pressed firmly together could only be separated with a surprisingly large force; hence, he recognised the importance of adhesion or cohesion for the friction process but was unable to bring his idea in line with the quantitative laws of friction.
Newton defined the term viscosity.
Adhesion theory, i.e. the
assumption that friction has molecular-mechanical causes, came about when
Newton defined the material parameter of dynamic viscosity. This definition is
based on the idea of a molecular-mechanical cause for friction (which is also
an important aspect for tribology ).
Euler studied friction on inclined planes and found that static friction is approximately twice the magnitude of sliding friction. He also introduced the friction coefficient “ μ ” (nowadays in tribology, the coefficient of friction is defined by “f”).
Coulomb continued to develop the basic ideas of Amontons in terms of surface roughness and mixed friction. He also examined the relationship between the necessary horizontal force and the weight percentage. Both are important terms in tribology.
According to Coulomb's model, the
friction coefficient of a given surface does not depend on the load, i.e. the
friction force is proportional to the weight. Furthermore, friction is
independent of the surface area, because it is only a function of the mean
angle of incline of the roughnesses. The smoother the surface, the smaller the
friction coefficient should be – a consequence which was in line with the ideas
of the time and helped Coulomb's model gain acceptance. But Coulomb's model has
one critical flaw, however: It exhibits no dissipative components. The energy
that must be expended when sliding up inclined planes has to be released again
if the inclined planes on the other side glide backwards. In other
words, according to Coulomb, gliding friction is not an energy-consuming
Coulomb's roughness model for friction. The normal force FN is distributed over the surface elevations having the same angle of inclination.
The first known patent of a deep-groove ball bearing by Philip Vaughan in England.
Philip Vaughan (†1824 in Kidwelly) from Carmarthen was an iron founder who
invented the ball bearing around 1791 and patented it in 1794.
The mechanism of the rolling or anti-friction bearing was reinvented in the 18th century in England when an axle for horse carriages was fitted with a ball bearing ring in the semicircular partial grooves, which were distributed over this axle.
After a long wait, advances in the development of lubricants in tribology were finally achieved during the Industrial Revolution. These were the product of discoveries in fluid mechanics and viscous flow properties as well as the increasing demand industrial development brought with it for lubricants in greater volume and quality, and the rapid replacement of vegetable and animal oils with mineral oils. The latter were extracted through the distillation and refining of petroleum, shale and coal.
Patented tapered roller bearings by M. Cardinet in France.
This bearing can withstand a great deal of stress both in the radial and axial directions. In general, it is installed in pairs; two bearings are positioned opposite each other, as the bearing consists of two separate elements: the inner ring with rolling elements and the outer ring that forms the bearing shell.
The rolling elements on the inner ring have the shape of a truncated cone
and they are slightly inclined against the shaft axis. The amount of space is
adjustable. The cone axes of the inner ring, outer ring and tapered rollers
meet at a point on the axis of rotation; only then can the tapered rollers roll
The first oil well is drilled in Titusville.
Lubricating components experienced a breakthrough in the 19th century during the course of industrialisation when a great need arose for industrially manufactured bearings for machine elements. The main components of roller bearings are two rings that move together – the inner ring and outer ring – which are separated by rolling elements. In most cases, the rolling elements are guided in a retainer that keeps them a uniform distance from one another and prevents them from touching each other.
The period from 1850 to 1925 is regarded as a time of “technical progress”. The railways are the focal point of societal events. A transition from solid lubrication (precursors of greases) to the liquid lubricants occurs in the lubrication of bearings and slideways. Important discoveries are made that ultimately form the foundation of modern tribology . To meet the higher standards, an inexpensive lubricant is now available in practically unlimited quantities: mineral oil-based lubricant with its wide range of qualities for nearly all technical industries.
The actual age of tribology began after the First World War. High loads, speeds and temperatures were accompanied by an increasing stress on the friction pair. The limitations of the physical properties of the lubricants available at the time had to be adapted to the more extreme conditions.
Although the preliminary stage of technological development and the introduction of additives go back to the 19th century, the modern concepts for the use of lubricant additives began in the 1930s. Viscosity index improvers, pour point improvers, oxidation and corrosion inhibitors etc. emerged – a whole range of protective measures.
At the same time, the extreme stresses caused by extremely high temperatures began the development of synthetic oils. Despite their importance, synthetic lubricants in tribology made up only about five percent of conventional lubricants used at the time. After the Second World War, scientists examined the solid lubricant molybdenum disulfide (MoS2), an exceptional lubricant for exceptional conditions - the perfect product for space research.
The term tribology was mentioned for the first time in 1966 in the Jost Report, a study commissioned by the British government to investigate damage from wear. The report has since been referred to in connection with friction, wear and lubrication.
In recent decades, the requirements for lubricating oils within the field of tribology have increased dramatically: To ensure reliable lubrication, synthetic lubricants, which are also produced mainly from crude oil by way of a chemical synthesis, have been used more often. Synthetic lubricating oils are characterised by their high thermal stability, low friction coefficient, good metallic wettability, low tendency to vaporise, miscibility with water and low flammability.
Lubricating greases are close relatives of lubricating oils.
Lubrication grease is used for many lubrication points instead of oil, because oil would leak. Lubricating greases are lubricating oils thickened with agents such as lithium, calcium, aluminium soaps or inorganic thickeners (e.g. bentonite).
The range of uses for lubricants
is of utmost importance, because “nothing works without lubrication”.