Tribology - Friction, Abrasion, Lubrication
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Tribology - friction, wear, lubrication When an aircraft lands and its wheels touch
the runway, there is considerable wear accompanied by clouds of abraded, burnt rubber.
Wherever there's movement, there's also friction: in ships, aircraft, motor
vehicles as well as railways. Friction costs energy and causes heavy overheads through wear.
Every car driver knows how expensive wear is when he has to change tyres or replace worn brake linings, clutch-plates or even a worn out engine. Wear can also adversary affect our environment. What happens to worn-out tyres?
How much energy is wasted through friction? The driver of this car uses the petrol mainly to overcome friction in the engine, the gearbox and the transmission gear. This wasted frictional energy is released as wasted heat. She has to foot the bill
railways are also prone to problems with friction and wear. In particular, the contact area between wheel and rail is extremely small. A train weighing several hundred tons accelerates, travels and brakes on a total surface area of little more than that of the palm of a hand. Wheel and rail are subject to repeated high intermittent loads while rolling and their surfaces rub and wear. That's why the wheel sets on rolling
stock have to be changed at regular intervals. Federal German Railways own more than a million wheel sets. The worn wheels are
either reground - or the tyres removed and replaced by new ones.
Friction and wear are highly detrimental to household appliances too. Refrigerators and washing machines often end up on the scrap heap because of worn bearings, gears or actuating pins, or simply because a link has worked loose. Wear losses of only a few milligrams of material often cause the total loss of the whole appliance.
In industry heavy losses through friction and wear are frequently due to faulty materials or assembly; inadequate maintenance, or to misuse. Under arduous running conditions gears, for example, often show damage on their tooth flanks.
A gear wheel that cost several thousand marks is then worth only its scrap value. Friction and wear all too often occur
in manufacturing engineering as well. During machining of an engine cylinder block, material is removed from the casting by boring, milling or grinding. But the drill bits, milling cutters and grinding wheels also suffer loss of materials. In spite of lubrication and cooling they wear out far too quickly. After a relatively short time the drills become blunt and have to be replaced.
The drill bits that are still long enough are re-sharpened.
Those drills that are too short after repeated sharpening, are scrapped.
The steel industry is particularly prone to problems caused
by friction and wear. They are largely due to a combination of high loads and large transmitted forces, high temperatures and speeds, as well as considerable contamination.
The high forces applied in a rolling mill can be appreciated from this roll drive coupling.
On a wide-strip hot mill, the energy lost through friction can amount to two million kilowatt hours per annum.
Wear of the rolls themselves is also enormously expensive.
The mill has to be shut down several times a day so that the rolls, whose surfaces are often worn after only six hours' operation, can be removed and replaced. The new work rolls are already on standby next to the mill stand.
They are moved over and then inserted into the mill stand using a hydraulic manipulator.
The worn rolls are taken to the roll-grinding shop.
In extreme cases the roll surface can
be heavily worn, as this 20 cm close-up view demonstrates. The score marks on the roll surface are removed by grinding.
The grinding depth along the whole length of the roll must be precisely controlled, to ensure even thickness and surface finish of the rolled strip.
Intensive maintenance is carried out every time the mill is shut down. But shut down causes production losses, up to 25,000 marks per hour. In this case the shut down is due to a faulty pinion bearing.
The bearing has now been stripped and the engineers are holding an inquest into the reasons for its failure.
Since it is a difficult engineering problem they decide to consult a tribology specialist. The tribologist is a specialist in problems associated with friction, lubrication and wear. In the steel industry friction is the cause of major losses - a consequence of the high transmission forces involved; also the high temperatures and contamination. In the transport sector the major contributors to frictional losses in vehicles are the driving and braking parts. These are responsible for 25% of all wear in the Federal Republic of Germany. In households, major losses result from incorrect operation, inadequate maintenance and faulty repair. Frictional problems arising in industrial applications can be more varied than the production processes themselves. The trend towards larger and more complex production units, higher speeds, increased automation and higher precision, place ever increasing demands on the various frictional surfaces. The sum total of losses in the Federal Republic of Germany as a consequence of friction and wear is about 40,000 million marks per annum. In order to minimize these immense losses, many scientists and engineers are tackling the problems of friction and wear in the relatively new science of tribology. By definition of the German and the British Standards Institution tribology is the "science and technology of interacting surfaces in relative motion and the practices related thereto".
The tribology specialist comes on the scene and assesses the damage. He finds that the pinion journal shows considerable wear and that the bearings are worn through. Metal transfer to the journal indicates that the
bearings have been worn to destruction. Frictional effects in engineering are so
varied and complex that many large companies are now setting up their own tribology sections.
The discussion is continued in one such department. The maintenance engineer explains the position to the Tribologist: it is the second time this year that damage of this nature has occurred to the bearing, and the downtime these failures have caused has been considerable. The Design Engineer explains that the lubrication system had been improved after the previous The tribo-system defines the conditions of friction and the factors affecting it. Friction generally occurs between very small, discrete areas of contact "asperities" when surfaces are forced into contact with one another. Such areas are theoretically defined as "tribological systems" or "tribo-systems" for short.
The elements of a tribo-system are the Primary Element and the Opposing Element. The surfaces which may come into contact with one another are of special interest. A further element may be an Intermediate Material. It may, for example, be a lubricating oil or grease, or some other fluid; a solid (such as plastic or a soft metal) or a gaseous substance. Finally, there is the environment which in most cases is gaseous, that is the surrounding air. If the Primary and Opposing elements are separated by a fluid, fluid friction occurs when the opposed surfaces move in relation to one another, the intermediate material forms a separating film. In the absence of a separating film the two interacting surfaces will be in intimate contact a state known as "unlubricated" friction. If elements of the solid bodies as well as the intermediary are present, we speak of "mixed" or "boundary" lubrication. Both dry unlubricated sliding and "mixed" lubrication produce wear. A tribo-system is subjected to various stresses, depending on the circumstances: These stresses make up the resultant stress. The resultant stress will arise from a combination of motion patterns, such as: sliding, rolling, cutting or impact. In addition, other relevant factors are speed applied loads, ambient temperature and also vibration. As a consequence of all these relevant factors the tribo-system produces friction. Friction always gives rise to the dissipation of energy and unwanted wear. The task of tribology is therefore to determine the causes and the factors responsible for energy loss For given loading conditions, a tribo-system can be influenced positively
by modifying the contacting geometry by altering the composition of the interacting surfaces or material combination as well as by the choice of a more appropriate "intermediary", to reduce friction and wear. There is a complex interdependence between these three factors influencing friction. They all have to be taken into consideration in designing machinery and plant. In addition, improved standards of maintenance should ensure that a tribo-system remains effective throughout the lifetime of a machine. Some examples will serve to show how improvements can be obtained by changing the geometry, the material combination, the intermediary; and by improving maintenance procedures. To start with, here's an instance of improved geometry: An important component in mechanical engineering is the journal or plain bearing. The sleeve represents the Primary Element, in which the shaft rotates as the Opposing Element. In our example it is the shaft of a turbine. When in position, the split bearing would enclose the shaft at the point where it is now supported by the trestle. When the turbine is in operation, the bearing and the shaft are separated by a thin supporting film of lubricating oil.
The turbine bearing must be designed to ensure the best possible load distribution.
Comprehensive computer programs serve to simulate the behaviour of bearings of different configurations.
This shaft runs in rigidly mounted bearings. When a load is applied to the centre of the shaft, it causes it to sag, and the outer edges of the bearings are subject to concentrated loading.
Flexibly mounted bearings on the other hand, avoid such load concentrations and thus the lubricant film in the bearing remains unbroken.
Another example of the importance of "Geometry": the faces of mechanical face seals are manufactured to extremely close tolerances. Such seals are used in high-pressure pumps, for example.
The sealing efficiency of the mating surfaces depends on the dimensional accuracy and the surface finish of the counterface.
In this case it has undulations only a few micrometers in depth, which facilitate the generation of a pressure profile in the lubricant film. To test the sealing effect
the mechanical free seals are put on a special test bench. The test run can begin. The experimental data are stored in a computer
and can be used to improve the design of the mechanical free seal. A few examples from the materials sector illustrate how changing material combinations can give improvements in tribological performance. In open cast coal mining, extremely large bucket-wheel excavators are used to strip the overburden and to mine the seams of brown coal. Wear is exceptionally high on the bucket teeth, which are screw mounted to facilitate easy replacement.
Here a worn bucket tooth is due for replacement. As an accident-prevention measure it is held in position by a crane. As a result of digging the hard sod the tooth is completely worn away (it is abrasive wear). In order to reduce wear, the new bucket
teeth will be hard-faced.
The finished bucket tooth. The surface coating is built up by laying down a wear-resistant alloy in a series of close-packed rows. Bucket tooth replacement may be necessary several times a year, depending on the excavator duty and the type of soil.
A further example of optimizing wear-resistance is the use of hard-faced tool bits in metal cutting operations, such as milling or turning. The tool bits in this milling cutter are uncoated and it soon gets blunt, even while milling steel at relatively low cutting speed.
Each tool bit has three cutting edges so that, when a cutting edge is worn, the bit can be indexed.
A marked improvement in cutting efficiency and cutting life of the tool is achieved by coating the tool bits with titanium carbide. By chemical deposition a thin layer of a few micrometers thickness is coated onto the tool bits,
which here are stacked in open baskets one above the other. The stack is then covered with a gas-tight protective container,
which serves as a reaction chamber into which the gaseous titanium and nitrogen compound is introduced.
A furnace is then lowered over the gas-tight container to heat up the reaction chamber, so that the gases decompose at about 1000 degrees
centigrade and deposit an ultra-hard layer of titanium carbide onto the tool bits. Finally, the furnace is lifted off again. The process is known as Chemical Vapour Deposition, or CVD for short. After the reaction chamber and its contents have cooled down, the container is removed.
The golden-coloured titanium carbide coating turns the cutting bits into much tougher tools. The milling speed can now be doubled, and
even then the cutting edges remain sharp for much longer periods. A comparison between milling with uncoated cutting bits
and again with coated tool bits.
Here is another example of enhanced wear-resistance: this time in the area of marine and stationary engines. A high-performance laser hardens the bore of a cylinder-liner of a marine diesel engine.
Conventional methods of heat treatment would not be used because they would have resulted in distortion. With the help of a mobile reflector system, a laser beam is directed at the contact surface and scanned line-by-line. The cylinder bore thus hardened enables the engine to operate for much longer periods before the liners need replacing. In reducing losses through friction and wear, the intermediate material (the "lubricant") is often more important than the materials combination.
Samples of lubricants should be taken from the workshops at regular intervals, to ascertain whether they are still doing their job properly. Depending on the application, they are required to have either high or low viscosity, as well as high shear strength, under widely different pressure and temperature conditions.
The lubricant should not deteriorate with time; it should be tolerant of contaminants. It should have a high flash-point and, of course it must be non-toxic. These are only a few of the requirements. Here a spectroscopic analysis of an oil sample is being carried out to determine the changes that have occurred in service.
State-of-the-art analytical techniques are a
prerequisite for precise quality assurance and detailed research on lubricants. These help to ensure the smooth-running of high-performance machines, ranging from turbines to industrial robots.
Displays of light-intensity as a function of wavelength give an indication of the chemical composition of oils and of the changes that have taken place in service.
One important requirement which lubricants must fulfil is that they are compatible with the materials of the tribo-system. This can be tested quite simply: Four different plastic materials are immersed in a type of oil
frequently applied in precision engineering. The results are clearly evident after only a few weeks.
Lubricating greases are the second most important class of lubricants. They consist of 80% liquid lubricants containing a thickening or gelling agent, which acts like a sponge. Lubricating grease finds applications in anti-friction bearings and in situations where movement is so slow that a supporting oil film cannot be maintained.
They can also protect bearings against ingress of dirt.
New lubricants cannot be used in practice until they have been subjected to exhaustive physical and chemical analyses and practical tests, such as for example in this rolling-element bearing test rig.
All relevant lubrication data are recorded and stored for future reference.
If boundary or (mixed) lubrication is replaced by "full-film" lubrication, LED's indicate the break in the electrical circuit.
Hydraulic fluids are an important class of lubricants. For mining applications they should be non-flammable.
The comparative flammability test between two hydraulic fluids with the same lubricating properties shows how far research has progressed in this direction. First a common type of hydraulic fluid based on a mineral oil is tested.
It is highly flammable, which could easily lead to disastrous accidents in underground mining.
In comparison a fire-resistant hydraulic fluid.
Certain constituents of the fluid do not begin to burn until the test is almost concluded.
A non-flammable and non-toxic hydraulic fluid with good lubricating properties has yet to be developed. Finally, maintenance is the best way to obtain the benefits of the improvements made in the design of the tribo-system and thus ensure
safe and reliable long-term operation of machinery and plant. Maintenance includes servicing, inspection and repair.
The main problem associated with inspection is that of being able to assess the condition of the many tribo-systems, so that action can be taken before failure occurs. Essentially, this primarily involves monitoring
lubricants in respect of their functional properties and also detecting possible
wear particles which are frequently abrasive.
Another method enables us to determine the extent of wear in an anti-friction bearing. The vibrations that occur while a
bearing is in operation give an indication of how accurately it is still running; what reasons there may be for uneven running; and when the bearing needs renewing. The test procedure relies on the principle of measuring the oscillation frequency
spectrum and comparing it with reference data. Similar diagnostic tests are applied during scheduled maintenance in aviation.
In this case service is of paramount importance for safety reasons, and exhaustive test routines have been developed to check the reliability of all moving systems.
Checks are made, for example, on engine fuel pumps to detect any wear that could hazard the reliable operation of the unit. The pumps are mounted behind the other units, and are thus not easily accessible. Previously they had to be taken out and
dismantled at every inspection. Now the fuel pumps are only removed and stripped down if the vibrational analysis, performed quickly in situ, indicates the need for a closer inspection. Long before pump failure occurs, this diagnostic procedure leads to the discovery of cavitation pits on the tooth flanks.
The fundamental component of all tribological measures, aimed at reducing initial cost and consequential losses in all four sectors, is research. It must be pursued with perseverance, in order to achieve a drastic reduction in the exorbitant loss to the Federal German economy. This loss accounts for approximately DM 40,000 million per annum. The Federal Ministry of Research and Technology is backing a whole range
of research projects. Among these are studies to investigate the possibility of coating surfaces with
extremely hard materials by the Physical Vapour Deposition process, or PVD.
Considerable wear occurs during the bulk transportation of solids, for example by conveyors. More fundamental experiments are required in order to obtain data relevant to this sort of abrasive wear.
Here is the experimental set-up designed for this purpose.
Abrasive wear scars under the scanning
Research is also directed at a number of highly stressed tribo-systems where surfaces in sliding
contact wear out too quickly under the severe operating conditions. Test bench experiments with the appropriate material samples are carried out under various extreme conditions. The knowledge gained from these experiments helps to combat wear.
For new material combinations we first have to ascertain frictional behaviour under a wide
range of different conditions. Exhaustive test programmes are necessary.
In order to ensure that new findings in every sector of tribology are applied and lead to
innovations, we need to step up technology transfer.
In order to provide assistance, in solving tribological problems, the tribology advisory service was set up.
The establishment of this advisory body was made possible by a grant from the Federal Ministry of Research and Technology and the tribology Society, which is the Federal German forum for all aspects of this new branch of science. Expert advice is providedon all aspects of tribology and solutions are offered for specific problems. Many small and medium-sized firms cannot afford their own tribology specialists and are therefore dependent on external help and advice. The tribology Advisory Service fulfils a much needed role in providing specialised information and guidance as well as putting firms in contact with experts in specific fields. There are many places where specialist know-how on particular aspects of tribology has been accumulating. This knowledge should be placed at the disposal of those who are seeking for solutions. Unwanted friction and wear are not inevitable; they are a challenge to science and technology to allow us to make fuller and more efficient use of available sources of energy and raw materials.
Proof <Graphische Technik>
Diesellokomotive Baureihe 219
|Titel||Tribology - Friction, Abrasion, Lubrication|
|Alternativer Titel||Tribologie - Reibung, Verschleiß, Schmierung|
Adolf, Helmut (Redaktion)
Matzdorf, Gerhard (Kamera und Schnitt)
Bertram, Klaus (Ton)
Kemner, Klaus (Ton)
Es gilt deutsches Urheberrecht. Der Film darf zum eigenen Gebrauch kostenfrei genutzt, aber nicht im Internet bereitgestellt oder an Außenstehende weitergegeben werden.
Gesellschaft für Tribologie e. V.
|IWF-Filmdaten||Film, 16 mm, LT, 445 m ; F, 41 min|
|Abstract||Examples taken from the traffic sector, from the household and from industry, especially heavy industry, show that movements occuring in technological processes are generally connected with friction. Friction always causes losses of energy and often also results in wear. Losses of this kind amount to about 40 billion West German Marks per annum in the Federal Republic of Germany. The obejctive of the scientific discipline of tribology is to reduce such losses. Tribology is the science and technology of surfaces moving against one another which are in contact with and interacting with one another and the processes involved in these movements (DIN 50 323). The work of a tribology staff division is shown working on the practical case of a broken down pinion mechanism on a rolling mill. That serves as an introduction to an explanation of the "Tribology System" (basic and counter body, with and without precursor, environmental medium, strain collective, loss of energy, abrasion, capability of being influenced). The film shows how the tribosystem can be carefully developed towards better and longer operability by: geometric design (calculating friction bearings, testing rotating mechanical seal); selection of tools for basic and counter body (dredging shovel with layer of welding, CVD coating on slipping beds, laser hardening on cylinder liners; choice of lubricant (spectral analysis of lubricating substances, compatibility of plastics with lubricating oils, examination of lubricating grease, poorly combustible hydraulic fluids). The operability of a tribosystem applied over a longer period of time must be maintained by: maintaince (checking lubricating substances, analysis of oscillation frequency). Research and development is done in all areas of tribology. The Federal Department for Research and Technology assists by encouraging a number of research projects of which a few are mentioned. In order to reduce the large losses caused by friction, the knowledge already available on tribology must be passed on, particularly to small and middle-scale industrial enterprises, by means of technology transfer and innovation. The task of the Society for Tribology-Consulting is to be of help here. The Society and its work are introduced at the end of the film.|