Causes & avoidance strategies

Causes & avoidance strategies

Pitting can be translated as "pitting", but is also referred to as "pitting" or "pitting corrosion" by experts. Pitting is mainly caused by friction losses on gears and rolling bearings and can be recognised by material breakouts and the formation of microcracks on the surface of rolling elements and their raceways. When viewed with the naked eye, the areas have a matt structure.

How does pitting occur on metal surfaces?

All technical systems consisting of so-called passive layer-forming metals can be affected by pittings or micropittings. These are usually high-alloy chromium-nickel steels, aluminium and titanium alloys or nickel-based alloys. Passivation is a thin oxide layer on the surface that protects the material from further oxidation. If this passive layer is locally destroyed by tribological, abrasive, erosive or other mechanical stress, pitting occurs. Possible causes of damage to the protective oxide layer include foreign bodies in the respective lubricant, inadequate lubrication or excessive humidity.

The chemical properties of the surface environment then reduce the speed at which the affected areas heal. A pitting nucleus is formed, from which the dissolution of the metal progresses in depth. Over time, further chemical processes are then responsible for the repassivation of the protective oxide layer coming to a complete standstill and corrosion eating its way unhindered into the depths of the metal. This leads to stable hole growth, which ultimately results in irreversible damage to the gears or rolling bearing.

Pitting formation on rolling bearings and gears due to tribological stress

It is known from tribology that pitting can be caused by a localised exceeding of the material strength, defined by the Hertzian pressure. This usually occurs on the tooth flanks of gears or in rolling bearings between the rolling element and the outer or inner ring of the bearing. In the case of surface pressure, the greatest stress of the component does not occur on the surface, but at a certain depth of the material. This must be taken into account when designing and manufacturing the respective components.

Other factors for the formation and spread of pittings and micropittings are

• material-related: the surface hardness and quality, as well as the depth of hardening
• shape-related: Errors in the flank shape of gears and in the peripheral speed
• Environmental factors: insufficient oil viscosity, unsuitable additives in the oil, incorrect temperature, high humidity

Pitting caused by pitting can be seen very easily with the naked eye on railway tracks. These must be reground regularly to maintain their operational safety.

How does pitting damage manifest itself?

In rolling bearings, pitting damage changes the operating behaviour of the bearing arrangement. If you want to examine the damaged bearings more closely, you need to check not only the bearing itself, but also the components of the surrounding parts, the sealing and the lubrication. Environmental and operating conditions can also play a role, such as excessively high temperatures or excessive humidity. If gearboxes, roller bearings or gear constructions are not checked and maintained regularly, irreparable damage will occur in the long term, necessitating a complete replacement and resulting in very high costs. A similar damage pattern can occur with unusually long running times and permanently high loads.

How can pitting be prevented?

Basically, it can be said that a clean design, the right material for a specific application and precise dimensions of the individual components are the first prerequisites for preventing pitting damage. In mechanical engineering, there are recognised and permissible values for the maximum possible surface pressure. The shapes and profiles of the components must be calculated precisely to ensure that these values can actually be adhered to. Otherwise, damage can occur very quickly.
Other essential measures to prevent pitting are regular inspection, proper maintenance and servicing of rolling bearings, gears and gearboxes in accordance with the manufacturer's specifications. It can also help to optimise the sealing system, filter the lubricant more frequently to remove any foreign bodies or generally use a more suitable lubricant.

Eating tests as an optimisation measure

Eating capacity is a strange word at first glance. If you are now thinking of food intake, obesity or diets, you are unfortunately completely wrong. The term is purely technical in nature and refers to the behaviour of gears in machines and gearboxes that are under permanent high loads, as well as the suitable lubricants to prevent "seizure" in the long term.

In engineering, "galling" means the localised welding together or tearing apart of two machine parts that slide into each other - in this case gear wheels - due to inadequate or faulty lubrication. It usually occurs at roughness peaks in the tooth contact. The cause is to be found in very high temperatures, also known as "flash temperatures", which are dependent on the load, the peripheral speed and, last but not least, the temperature of the oil sump environment.

It's all about the lubrication

Proven coatings such as phosphating or metal-containing hydrocarbon alloys with tungsten have proven to be the most suitable in practice for protecting gear flanks from excessive abrasion. However, it is not only the material and design of the moving parts that are decisive for the safe operation of gear drives, but above all the lubricant used. If the composition of the base oils and additives used is not correct, the lubricant does not develop the required lubricating film thickness and therefore does not achieve the required scuffing load capacity. The result is partial damage to the gears which, if not properly maintained and checked, ultimately leads to a total failure of the gearbox and therefore the machine.

Distinction between warm and cold feeding

Seizure marks and scoring occur during hot scuffing due to a very high sliding speed and the resulting limit temperatures if the material of the gears and the lubricant are not optimally adapted. Smaller modules and the use of EP oils with chemically active additives during a gearbox overhaul can provide a remedy here. Cold seizure means scoring-like wear of the gears with very heavy material removal caused by low peripheral speeds. In this case, more precise gearing, a smoother surface of the tooth flanks or a more viscous lubricant adapted to the requirements can help.

What are food tests and where can they be carried out?

There are various institutes and research facilities in Germany that carry out scuffing tests on gears. There are test benches for this purpose, known as gear tension testing machines, which allow the loads in the gear meshes and the temperature of the respective lubricant to be set precisely in accordance with the DIN 51354 standard. The oil can be supplied as injection or splash lubrication.

The Research Centre for Gears and Transmission Engineering at the Technical University of Munich (FZG) has developed a test rig that can be used to test the viscosity and suitability of lubricants for transmissions in order to prevent the surfaces and flanks of gears from seizing. The FZG test rig has established itself as a standard test machine and is also used in this form by other institutes.

The tests themselves are carried out under different conditions. The circumferential speed, toothing and direction of rotation of the gears as well as the oil sump temperature are varied in order to define the damage force levels of different lubricants.

The FZG can check the scuffing resistance in four test procedures. The standard scuffing test is carried out in accordance with the above-mentioned DIN 51354 standard, whereby the force levels are initially set low and then slowly increased, as are the flash temperatures. In the more stringent scuffing test, the circumferential speed is doubled and the level of flash temperatures is also increased significantly. However, the run-in in the lower load levels corresponds to the standard scuffing test. In the step test, the direction of rotation of the gears is reversed and toothing with a narrower pinion is selected. The changed boundary conditions increase the pressure and also make it more difficult for the lubricant used to enter the tooth contacts. The final stage is the so-called jump test. Here, the loads are not increased in stages, but are set directly and without detours to a specific force level. The result is either "pass" or "fail" - either it works or it doesn't work. There is also no run-in during the step test, i.e. the flash temperatures that occur are higher and are caused by rough surfaces. However, when used in gearboxes, the lubricant must have certain running-in properties in order to increase the scuffing load capacity of the gearing. For this reason, the individual boundary conditions must be closely observed and documented for every scuffing test.

Innovative technologies for energy efficiency & durability

Nanotribology is a branch of tribology, the science of friction. This science or technology describes and analyses how two surfaces behave in a relative movement and which practical processes take place. Basically, it is about friction and wear and therefore about the question of how wear can be prevented or at least delayed in the long term using lubricants or other methods. In terms of industry, the main focus is on motors, gears, roller bearings, guides and other moving machine elements. Incidentally, the foundations of tribology go back to the universal genius Leonardo da Vinci (1452-1519), who also worked on the design of gears and transmissions during his lifetime.

Nanotribology can reduce the costs of industrial production

Nanotribology investigates the effects of friction in the nanometre range and at the atomic level. In addition to the development of appropriate lubricants, the focus is on research into the selection of suitable materials and the treatment and coating of surfaces. Nanotribological relationships are of particular interest to industry. They can help to increase the energy efficiency of machines and at the same time reduce the costs of material use and maintenance. This conserves important raw material resources and reduces the burden on the environment. According to expert estimates, the cost of lubricants and wear caused by friction and abrasion in machines and gearboxes worldwide is in the region of one trillion euros. Nanotribology can help to significantly reduce these costs.

What is a nano coating?

A nanocoating changes the surface structure of a material at a molecular level. On contact with the substrate material, the nanostructures form a wafer-thin, water and oil-repellent layer that is firmly bonded to the surface. This prevents contamination by liquids, dust or other dirt. These properties are also known as the "lotus effect". It can be applied to almost all materials and is particularly suitable for fine-pored materials and applications that require a high degree of precision. A nano coating can withstand temperatures of up to 350° C.

Can machines also be retrofitted with a nanocoating?

In principle, nano-coating for machine parts is also possible at a later date, for example during a gearbox repair. However, it does not last as long as if it is applied directly during the production of machine parts. The nanoparticles float in a solvent. As soon as this has evaporated due to the effect of heat and air, the nanoparticles form a net-like structure that is firmly bonded to the surface of the treated parts. The components of the coating have two effects: firstly, they protect the substrate from mechanical stress caused by frictional energy, for example in the case of highly stressed gear constructions, and secondly, they ensure that foreign bodies cannot adhere thanks to the additives added.

Are nanocoatings expensive?

The question of the cost of a nano-coating is naturally one of the most important for every technical manager and responsible person in a company. Concrete sums cannot be given at this point because this depends on a number of individual factors - the type of machine or gearbox in question, the nature of the carrier material, the daily running time and stresses on the parts moving against each other. In principle, however, it can be said that a nanocoating saves costs in the long term because it reduces the use of lubricants and increases the intervals between maintenance and repairs.

The favourable alternative - a nano-coating with lubricant

Existing systems, machines and gearboxes can also be protected against wear and tear by adding certain lubricant additives. The unique product portfolio of REWITEC® serves as an example. The products developed and manufactured in Germany are compatible with all common lubricants, but can also be supplied in the customer's lubricant on request. As an additive, they are added to the lubricant of the unit during operation and transported by the lubricant to metal surfaces subject to friction. There they utilise the frictional energy and passivate the surfaces of bearings, gears and similar units by incorporating silicates, reducing roughness and thus having a lasting and positive effect on the service life, energy consumption, operational reliability and performance of gears and rolling bearings.

What effect do nanocoatings have on health and the environment?

The question of the effects on health and the environment is a key factor in favour of the use of nanocoatings, alongside the costs. According to current research, nanocoatings are neither hazardous to health nor harmful to the environment. On the contrary, their positive properties make them safer at work because they reduce the susceptibility of machines to damage.

Side effects of tribological friction forces

The roughness of surfaces is caused by the processing of different materials during sawing, cutting, punching and other mechanical processes. If you saw a piece of wood to burn it later in the fireplace, the roughness of the surface is basically irrelevant. The function and energy efficiency of the piece of wood - it should simply burn and provide heat - is not affected. The situation is completely different for workpieces such as gears, gearboxes or roller bearings, where machine parts are constantly in motion and in continuous surface contact. If the individual parts are not as smooth and resistance-free as possible, they will "grind" or you will have "sand in the gears", so to speak. In the end, you may end up with a total loss and therefore an extensive repair or new purchase.

What is the significance of roughness in tribology?

Roughness is a term from the field of surface physics. In tribology (friction theory), roughness means that the rougher the surfaces of two objects in relative motion to each other, the more friction losses they cause. This phenomenon is particularly significant for moving machine parts such as gears and roller bearings. Roughness leads to high operating temperatures, faster wear of the individual elements and, in the worst case, to total failure if the wrong lubricants are used and maintenance is inadequate.

How can roughness be prevented?

Depending on the material used, various processes are used today in modern production technology to avoid roughness on surfaces. These include polishing and electropolishing, grinding and pickling, etching, sandblasting and honing. However, a certain degree of roughness always remains, even if perhaps only in the nano range, which is not recognisable to the naked eye or by touch. Appropriate lubricants and regular inspection and maintenance help to minimise the effects of surface roughness.

Which shape deviations of a surface are decisive for roughness?

The DIN 4760 standard divides shape deviations of surfaces into six classes, whereby the values of the first four classes can overlap:

    1st class: shape deviations
    2nd class: Waviness
    3rd class: Roughness due to grooves
    4th class: Roughness due to scoring
    5th class: Roughness in the microstructure
    6th class: Lattice structure of the respective material

Roughness of classes 1 to 4 can normally be recognised visually and haptically on the surface. For the higher classes 5 and 6, the use of a microscope or electron microscope is necessary to check the condition of the surface or to be able to detect defects.

How do you measure roughness and what roughness measuring devices are available?

There is a wide range of measuring devices for measuring the roughness of surfaces, holes and grooves. They are generally quite easy to use. The devices measure the average roughness depth (unit: Rz) and the average roughness value (unit: Ra) in micrometres (µm) and are able to transfer the values directly to a PC or notebook for more precise analysis. Depending on the workpiece, the measuring devices should comply with the DIN 4762, 4768, 4771 or 4775 standards in order to ensure that the results are unadulterated after the test.
Commercially available measuring devices basically work with three methods. The manual methods include, for example, the rugotest, in which the comparison is carried out visually or haptically by touching sample surfaces. In profile-based methods such as the stylus method, a diamond is moved over the surface at a constant speed to measure the roughness. Thirdly, there are surface-based methods in which the measurement is carried out using optical processes, for example confocal microscopy or conoscopic holography.

Can the roughness also be determined without measuring devices?

To a limited extent, the roughness of a surface can be seen with the naked eye or felt with the fingers. Whether this test is sufficient depends on the material used and the intended use of the relevant machine parts. However, only measuring devices that can recognise even the finest unevenness offer real certainty about the suitability of a surface for the planned work process.

What you should know

Grey spotting occurs as a sign of wear on the surfaces of metallic components that are permanently under high stress. The main areas affected are gear wheels in gearboxes and the sliding camshafts and tappets in valves. Damage of this type rarely occurs in vehicle gearboxes, but mainly in industrial systems such as mill drives, rolling mills and wind turbines.

How can grey spotting be recognised?

Viewed with the naked eye, the damaged parts have a matt grey appearance. Only under high magnification does it become apparent that the grey colour is caused by many tiny pores and breakouts. In the case of gears, sliding and mixed friction occurs on the contacting surfaces under permanently high loads. This causes plastic deformation and microscopic cracks appear. These can develop into deeper cracks and then into larger breakouts. This phenomenon is also known as pitting. Over time, grey spotting can have a negative impact on the flank shape of gears and change both the gear dynamics and the noise behaviour in the gears.

Causes of damage due to grey staining

Grey spotting can have various causes. It mainly occurs when components such as gear wheels are subjected to permanent heavy loads and there is a high sliding speed combined with a low lubricant film thickness. The low lubricant film thickness is often the result of the lubricant being exposed to excessively high temperatures. Another reason can be an unfavourable geometry of the gears, which results in heavy loads at individual points on the tooth flank. The condition of the surface also plays a role. If it is very rough, a lubricant film that is too thin will lead to constant contact of the metal and thus to friction damage.

The choice of lubricant is also crucial, as the gear geometry, the speeds and the load are determined by the requirements of the respective gear design. Too low a viscosity, for example, results in insufficient lubricant film thickness. In addition, certain additives in the oil can promote corrosion and thus the tendency to form grey spots. Today, high-performance lubricating oils are tested using a standard test to determine whether they are suitable for preventing grey spotting.

However, there is another theory for the development of grey spotting. In the 1970s and 1980s, the car manufacturer Mercedes-Benz noticed signs of wear on the gearwheel constructions of the rear axles. Following various metallographic tests in the company's own laboratory, the engineers came to the conclusion that the lubricating film was being destroyed by vibrations with unusually high natural frequencies of the components, resulting in seizure damage to the gear flanks. This explanation could also apply to the roller bearings in wind turbines. This would mean that the wear is not caused by external loads, but by dynamic stress inside the gear components.

Wind turbine gearboxes are particularly affected

Grey spotting has been known for many years in gearboxes. It occurs particularly frequently on wind turbines and is therefore a major problem for operators. On the one hand, it can have a lasting effect on wind turbine performance; on the other hand, gearbox damage due to grey leakage can quickly cost a six-figure sum. Above all, it is unclear who is liable for such damage.

In 2007, the German Wind Energy Association (BWE) founded a working group consisting of experts, operators, technical managers of wind turbines and lawyers to investigate the development and effects of grey spotting, as well as appropriate countermeasures. It took four years for the commission to present its final report in 2011. The conclusion: It remains unclear who can be held liable for damage caused by grey spotting. Gearbox manufacturers see the problem as a normal sign of wear and tear that is excluded from warranties. The process of grey spotting occurs very slowly and usually only leads to gearbox failures once the warranty period has expired. To date, the courts have not clarified who is liable for damages.

There is still a need for clarification

Until the causes are clarified, there will be no final decision on who has to pay for damage to the gearboxes of wind turbines. High-performance wind turbines with an output of more than one megawatt, which are constantly running at their limit, are particularly affected. The problem of grey staining can only be partially contained with conventional lubricants and improved surface structures: The older the system, the more difficult it is to find a satisfactory solution. It therefore remains to be seen whether the phenomenon of grey spotting will even be recognised as damage requiring recourse. The BWE working group has therefore not reached a final judgement in its report. So far, there is a lack of sufficient experience regarding the fatigue strength of large wind turbines and the effects of dynamic forces such as changing winds, pre-stowage or tower shadows on the drive shafts.

The very best thing to do is to protect new wind turbines from grey staining right from the start using innovative nanotribology and to rule out the dreaded pitting as consequential damage from the outset. However, this cannot be achieved even with the highest quality lubricants, as even these can only reduce friction wear sooner or later, but cannot completely eliminate it. A far more effective approach is to apply a metal-ceramic coating directly to the surfaces so that frictional forces are largely avoided. Surface ceramisation is achieved by using the oil additive DuraGear® W100. This is added to the lubricant during operation, uses it as a means of transport to friction points and deposits silicates on the metal surfaces through a chemical process, thus refining them into metal-ceramic surfaces.

Fewer gearbox changes thanks to innovative silicon coating

Wind turbine gearboxes as sensitive systems

A lack of grid capacity, strong fluctuations in grid voltage, constantly changing wind conditions, spinning operation and the intermittent operation of components: Wind turbines are extremely susceptible to faults. High gearbox wear combined with frequent gearbox changes are the result. 1 - 2 gearbox replacements per year are not uncommon for wind turbines. The negative consequences of this are machine downtime, a poorer energy balance, high maintenance costs and an extended energy-related service life. Preventive measures against premature gearbox wear therefore definitely pay off.

Surface protection from the bottle

The main causes of gearbox failures in wind turbines are damage caused by friction, such as grey spotting, roughness and pitting (link to corresponding blog article). As soon as it is possible to reduce the frictional forces and the associated wear, the number of gearbox replacements required also decreases significantly. An extremely effective approach against wear damage is offered by so-called surface passivation through enrichment with silicon atoms, which form a smooth and wear-resistant metal-ceramic layer during the friction process. The significantly smoother surfaces drastically reduce friction and wear, and the wind turbine gearbox runs much more efficiently and with less vibration. At first glance, this sounds like a rather complex protective measure, but at second glance it is quite simple thanks to the use of an innovative oil additive. The oil additive DuraGear® W100 (link on landing page), which was specially developed for wind turbines, is added to the lubricant of the wind turbine gearbox during operation and uses it to transport the active ingredient to damaged areas - without affecting the lubricant itself in any way.

Successful gearbox repair at Windkraft Simonsberg

Windkraft Simonsberg, one of Austria's largest wind power producers, made a conscious decision to increase the performance and maintenance of its wind turbine gearboxes by using DuraGear® W100 and contacted the Lahnau-based manufacturer REWITEC®. The primary objectives of the application were to improve the condition of the gearbox and extend its service life. The tooth flank load-bearing capacity was also to be optimised and the previous damage condition frozen. During an on-site visit to the Vestas V80 wind turbine in Steinberg-Prinzendorf, the wind turbine gearbox was inspected by REWITEC® engineers prior to the DuraGear® W100 application. Running marks were found on the tooth flanks and so-called run-through, as well as rust-like discolouration in the base area. The surfaces were rough, and an electrical resistance measurement revealed a value of up to 20 ohms. For a before/after comparison, high-precision surface impressions (resolution accuracy 0.1μm) were taken from a previously marked tooth flank and then analysed microscopically. The Hansen gearbox of the wind turbine was then treated with the tribological coating concentrate DuraGear® W100. The results after seven months of running with DuraGear® W100 were remarkable. A detailed examination of the gearbox revealed a slight reduction in rust-like deposits and a reduction in surface running marks, so that the surfaces felt much smoother. The electrical resistance had increased to up to 200 ohms. Analyses using a laser scanning microscope (type "Keyence VK8700") showed an improvement in the roughness values from Rz 8.899 to Rz 7.036μm and Ra 0.595 to Ra 0.411μm. The bearing surface had increased from 46.402% to 58.702%. CMS analyses showed that the trend of the measured vibrations had decreased significantly after the application.

No more gearbox failures at Marxen Bauträger GmbH

Marxen Bauträger has been using DuraGear® W100 since mid-2010 and regularly treats the gearboxes of all 25 wind turbines with the innovative oil additive. Since then, no further gearbox failures have been reported. Before the application, an average of 1-2 gearbox replacements were recorded annually. The savings over 5 years for 25 wind turbines amount to an impressive €562,500.

Flexible choice of location thanks to optimised gearbox maintenance

Turbulent regions with high wind speeds generally deliver the best wind harvest. However, the frequently changing wind conditions and often very unstable network structures have a rather unfavourable effect on the maintenance of the gearboxes. Due to the intermittent drive of the mechanical components, the tribological wear is significantly higher than in regions with more stable wind conditions. Many wind farm operators therefore deliberately forego a higher wind harvest in favour of a better grid connection and more constant wind conditions. Surface passivation with DuraGear® W100 reduces the mechanical "stress" acting on the gearbox, even under unfavourable conditions, so that wind turbine operators will be able to operate wind turbines in high-wind regions in the future without an increased risk of repair and failure.

High-performance tribology products

Lubricants for friction reduction of sliding & rolling elements

Lubricants are by no means an invention of modern industry - even if the addition of numerous additives might make it seem that way - but were already used by our ancestors as a lubricant. Friction reduction utilised. The term "smearing" also has historical origins and is derived from the Middle High German word "smer", i.e. raw animal fat.

Today, the term "lubricants" covers all products that are used to lubricate and minimise the friction of sliding and rolling elements. However, products that are similar in composition, manufacture and properties are also categorised as lubricants, even if they are used as insulating oils, corrosion inhibitors or process oils as aids for industrial processes. Lubricants account for an average of around 0.8% of total mineral oil consumption worldwide, and around 1% in industrialised countries. In economic terms, however, lubricants are far more important than their relatively small share of the mineral oil market would suggest. 30% of all energy generated in the world is consumed through friction, and billions are lost every year due to wear. For design engineers, lubricants are therefore not just necessary operating materials, but design elements that help to increase energy efficiency, avoid machine downtime and reduce the costs of spare parts and maintenance. The development of lubricants is therefore constantly being driven forward by intensive research.

What lubricants are available?

Lubricants can be divided into lubricating oils and lubricating greases, whereby lubricating oils derived from crude oil are used significantly more than lubricating greases, which are only used for very specific purposes in industry. In general, lubricants are used for a wide variety of tasks in the automotive sector or in industry. A distinction is made between

Automotive lubricants:

• Engine oils
• Gear oils
• Brake fluids

The tasks of automotive lubricants are diverse and serve to lubricate sliding parts to reduce friction and wear, to cool the engine, to seal, to protect against corrosion and to transmit pressure.

Industrial lubricants:

• Hydraulic fluids
• Gear oils
• Compressor oils
• Turbine oils
• Corrosion protection products
• Cooling lubricants
• Insulating oils and white oils
• Lubricating greases

The main tasks of industrial lubricants include the extensive reduction of friction and wear on tooth flanks, the prevention of pitting and Micro Pittings and the dissipation of frictional heat. To make matters worse, the lubricants must also guarantee the reliability of the machines even under extreme temperature influences.

Base fluids for lubricants

The starting product for all lubricants - whether mineral oil-based or synthetic - is crude oil, which is fragmented into different products in an atmospheric distillation process and then freed of excess by-products in further steps. Depending on the processing, either mineral oil of varying viscosity or the so-called hydrocracked mineral oil is produced as the base oil, whereby significantly more mineral oil raffinates are produced and used than hydrocracked mineral oils.

Hydrocracking oils

Hydrocracked oils can be based on both crude paraffin and vacuum gas oil. They are also known as HC synthetic oils and are characterised by a significantly higher viscosity index (120 to 150) and better low-temperature behaviour (pour point down to -21 degrees C) compared to mineral oil raffinates.

Synthetic base fluids (polyalphaolefins, esters)

Synthetic oils are produced in a multi-stage chemical process by linking special hydrocarbon molecules. Synthetic oils are mainly polyalphaolefins (PAO), often also called synthetic hydrocarbons, or esters. The production of synthetic base fluids is more complex than the extraction of mineral oil-based fluids, meaning that synthetic oils are considerably more expensive than mineral oils, but also offer many advantages due to their production process.

Advantages of synthetic lubricants

Synthetic oils have a higher film thickness than mineral oils, i.e. their viscosity is very high even at high temperatures: wear protection is significantly higher, the necessary addition of viscosity index improvers is reduced and shear stability is optimised at the same time. But even at extremely low temperatures, synthetic oils are superior to mineral oils due to their very good low-temperature behaviour and thus enable improved cold starting and rapid lubrication of combustion engines. The absence of unstable components ensures better oxidation and thermal stability. Their low volatility and low evaporation loss reduce oil and fuel consumption, while higher thermal resistance ensures better engine cleanliness and extended oil change intervals.

Important characteristics

• ViscosityThe most important property of lubricating oils is the so-called viscosity, which is the measure of the internal friction of an oil when flowing. It is temperature-dependent: At low oil temperatures, internal friction is high and viscosity is high. The warmer the oil temperature, the lower the internal friction and viscosity. A distinction is made between kinematic and dynamic viscosity, whereby the kinematic viscosity can be derived from the dynamic viscosity by dividing it by the density. all lubricants are categorised according to the ISO viscosity classification. The only exceptions are engine and gear oils in the automotive industry, which are divided into SAE classes. The ISO-VG (International Organisation for Standardisation - Viscosity Grade) standardises 18 viscosity classes from 2 mm2/s to 1500 mm2/s and specifies a midpoint viscosity of 40°C, from which it is permitted to deviate upwards or downwards by 10% within a viscosity class.
• Viscosity-temperature behaviour (viscosity index)Depending on the oil, viscosity changes due to rising or falling temperatures can be completely different. The viscosity-temperature behaviour is described by a dimensionless key figure, the so-called viscosity index (VI). The higher the viscosity index, the lower the drop in viscosity when the temperature increases.
• Viscosity-pressure behaviourThe viscosity of an oil depends not only on the temperature, but also on the pressure exerted on the lubricating film. This characteristic is particularly important for tribological calculations when oils of different base fluids are available.
• Shear rate/shear stabilityIn the case of multigrade oils with a viscosity index improver, the viscosity is also influenced by the shear rate. In the case of monograde oils without viscosity index improvers, so-called Newtonian fluids, the speed gradient has no influence on the viscosity.
• Pour pointThe pour point indicates the so-called yield point of the lubricating oil, which used to be described by the pour point. To determine the pour point, the pour point is still determined and then 3°C is added to this.
• Evaporation lossThe evaporation loss depends on the viscosity and the degree of refinement of mineral oils. Hydrocrack oils have a significantly lower evaporation loss than conventional mineral oils.

Lubricant additives (H3)

Additives are added to lubricants to give them certain properties. The type and quantity should be precisely matched to the respective application; the additive content can be between 1% and 30%. Depending on their mode of action, these additives can be categorised into three types:

• Surface-protecting additivesThese include detergents, dispersants, high-pressure and wear protection additives, corrosion and rust protection additives, friction modifiers and surface-refining additives such as DuraGear® and PowerShot®, which utilise the friction that occurs in aggregates to form a protective surface layer of silicon. (Link to landing page)
• Oil-improving additivesViscosity improvers reduce a potential drop in viscosity when the temperature rises. Pour point improvers are more effective in the low temperature range and delay the formation of paraffin crystals so that the oil remains free-flowing for longer even at low temperatures.
• Anti-ageing additivesAnti-ageing additives, or oxidation inhibitors, slow down oil ageing caused by oxygen ingress, high temperatures and catalytic metals. Oil ageing products are decomposed and harmful reactions are stopped. Anti-foaming agents change the surface tension of the oil and thus prevent the formation of stable surface foam.

Lubricating greases

Lubricating greases are solid lubricants, usually containing mineral oil, which are produced in a wide consistency and penetration range from liquid to sebum-like solid. They are used when liquid lubricants are not suitable due to their fluidity and run away from the lubrication point, for example in rolling and plain bearings, open gears, wire ropes or chain drives. Lubricating greases consist of 70 - 95% of a base oil, 3 - 30% of thickeners and 0 - 5% of additives.

In da Vinci's and Euler's footsteps

Earliest developments

Frictional forces and their effects have long preoccupied mankind. There are numerous indications that our ancestors were already looking for methods to utilise the power of friction very early on. The ignition of fire, levers and stone axes as the first simple tools: everything is based on friction. The first machines, fiddle drills and potter's wheels, which were created 5,000 to 6,000 years ago, are also based on the principle of friction. The use of rollers and carriages to reduce friction when transporting heavy loads are well-known tribological examples. The earliest documents on the use of wheels to reduce friction date back to 650 BC. Today's rolling bearing has its origins in the so-called "rolling elements", which were already used in Ancient Egypt to build pyramids and made it possible to transport blocks of stone weighing several tonnes over long distances. However, Leonardo da Vinci (1452-1519) was the first person to begin a targeted scientific examination of the theory of friction much later, when he carried out the first investigations into friction on horizontal and inclined planes and wear on plain bearings. Since 1966, the technical term "tribology", which is derived from the Greek terms tribein = to rub and logia = teaching, has been more commonly used in specialist circles and was first mentioned in connection with the Jost Report, an investigation into wear damage commissioned by the British government. Since then, the terms tribology and tribological systems have been used in connection with friction, wear and lubrication.

Tribology & Lubricants

Friction and lubrication have always belonged together: This is also proven by numerous early finds. The Chinese were already looking for effective lubricants to reduce friction and initially used water as a lubricant, later a mixture of vegetable oils and lead. The ancient Egyptians were also aware of the friction-reducing effect of lubricants and reduced the number of train drivers required by 50 % by lubricating the underside of the pharaoh's throne. They also lubricated their chariots with animal fats or a combination of olive oil and lime flour. However, lubricants only experienced their real breakthrough with the start of the industrial revolution, when increasing industrial development led to a rise in demand for the volume and quality of lubricants. Vegetable and animal oils were gradually replaced by mineral oils, which were obtained from crude oil, shale and coal by distillation and refining.

Leonardo da Vinci (1452-1519) as the founder of modern tribology

Leonardo da Vinci investigated the coefficient of static friction on the inclined plane and determined its value as f = 1/4. The first and second laws of friction, the so-called laws of dry friction, can also be traced back to him. These state that the frictional force is proportional to the normal force and independent of the apparent contact area and depends not only on adhesion but also on abrasion. Abrasion has a particularly great influence if the rougher friction partner is made of a harder material or if abrasion in the form of hard oxidised metal particles is present at the joint. In 1490, he almost exclusively replaced the flexible connection between two parts of the rolling bearing with balls, thus generating considerably less friction. He discovered that friction is reduced if the balls do not touch each other and subsequently developed separating elements that enabled free ball movement.

Continuation of Amonton's laws of friction

The French physicist and governor of Lille Guillaume Amontons (1663-1705) carried out research in the field of mixed friction. He discovered that the frictional force depends on the normal force and recognised surface roughness as the cause of friction. For him, frictional forces were based on mechanical-geometric interlocking of unevenness. The form fit of the micro-elevations inhibits the relative movement so that a frictional force occurs in the opposite direction to the direction of movement. Amontons defined the coefficient of friction as f = 1/3 and presented the laws of tribology discovered by Leonardo da Vinci to the Académie Royale in Paris in 1699.

Tribological explanatory model by Desaguliers

The natural philosopher John Theophilius Desaguliers (1683-1744) developed an explanatory model of friction in which he attributed frictional forces to the influence of cohesion or adhesion. He also established that a higher frictional force occurs with better polished surfaces and that two well-polished lead bodies pressed firmly together can only be separated again by very great force. He attributed the friction that occurs to the influence of cohesion and adhesion, but was not yet able to relate this idea to the quantitative tribological laws of friction.

Development of Newton's adhesion theory

The English natural scientist and civil servant Sir Isaac Newton (1643-1727) defined the material parameter of dynamic viscosity: the adhesion theory, or assumption of a molecular-mechanical cause of friction, emerged, which had already been assumed by Desaguliers as a partial cause of friction. These adhesion theories were later significantly expanded by Bowden and Tabor in the 1920s and 1930s.

Discovery of the coefficient of friction "µ" by Leonhard Euler

The Swiss mathematician and physicist Leonhard Euler (1707-1783) investigated friction on inclined planes. He discovered that static friction is about twice as high as dynamic friction and introduced the coefficient of friction "µ", which is now known as "f" in tribology. The coefficient of friction for metals is measured on polished surfaces in order to largely rule out mechanical interlocking. The decisive factors are the adhesion and cohesion forces between the two materials.

Charles Augustin de Coulomb as a worthy successor to Amonton

The French physicist Charles Augustin de Coulomb (1736-1806) further developed Amonton's fundamental ideas regarding surface roughness and mixed friction and focussed on the correlation between the force to be applied horizontally and the proportion of weight. According to Coulomb, the coefficient of friction of a surface does not depend on the load. This means that the frictional force is proportional to the weight and independent of the surface, as it is only a function of the average angle of inclination of the roughness. In his opinion, the flatter the surface, the lower the friction should be: a theory that is only partially correct according to current research.

Important technical inventions in the 18th century

Today's tribology began after the First World War, when high loads, speed and temperatures characterised increasing stress on friction pairs, making it necessary to physically adapt lubricants. Pour point and viscosity index improvers, oxidation and corrosion inhibitors were created, and at the same time the development of synthetic oils began. The use of metal-ceramics is becoming increasingly important: metal-ceramic surfaces and ceramic cutting materials ensure greater efficiency and a longer service life for industrial bearings and gears. Thanks to modern and innovative oil additives such as DuraGear® or PowerShot® (link to landing page), the surfaces of bearings, gearboxes and combustion engines can even be subsequently upgraded to metal-ceramic surfaces during the friction process.